Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

217 files changed, 63204 insertions, 0 deletions

diff --git a/services/sync/SyncComponents.manifest b/services/sync/SyncComponents.manifest
new file mode 100644
index 0000000000..68c36cd80f
--- /dev/null
+++ b/services/sync/SyncComponents.manifest
@@ -0,0 +1,3 @@
+# Register resource aliases
+# (Note, for tests these are also set up in addResourceAlias)
+resource services-sync resource://gre/modules/services-sync/
diff --git a/services/sync/Weave.sys.mjs b/services/sync/Weave.sys.mjs
new file mode 100644
index 0000000000..05a7031a73
--- /dev/null
+++ b/services/sync/Weave.sys.mjs
@@ -0,0 +1,190 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+ChromeUtils.defineESModuleGetters(lazy, {
+  CLIENT_NOT_CONFIGURED: "resource://services-sync/constants.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+});
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "syncUsername",
+  "services.sync.username"
+);
+
+/**
+ * Sync's XPCOM service.
+ *
+ * It is named "Weave" for historical reasons.
+ *
+ * It's worth noting how Sync is lazily loaded. We register a timer that
+ * loads Sync a few seconds after app startup. This is so Sync does not
+ * adversely affect application start time.
+ *
+ * If Sync is not configured, no extra Sync code is loaded. If an
+ * external component (say the UI) needs to interact with Sync, it
+ * should use the promise-base function whenLoaded() - something like the
+ * following:
+ *
+ * // 1. Grab a handle to the Sync XPCOM service.
+ * let service = Cc["@mozilla.org/weave/service;1"]
+ *                 .getService(Components.interfaces.nsISupports)
+ *                 .wrappedJSObject;
+ *
+ * // 2. Use the .then method of the promise.
+ * service.whenLoaded().then(() => {
+ *   // You are free to interact with "Weave." objects.
+ *   return;
+ * });
+ *
+ * And that's it!  However, if you really want to avoid promises and do it
+ * old-school, then
+ *
+ * // 1. Get a reference to the service as done in (1) above.
+ *
+ * // 2. Check if the service has been initialized.
+ * if (service.ready) {
+ *   // You are free to interact with "Weave." objects.
+ *   return;
+ * }
+ *
+ * // 3. Install "ready" listener.
+ * Services.obs.addObserver(function onReady() {
+ *   Services.obs.removeObserver(onReady, "weave:service:ready");
+ *
+ *   // You are free to interact with "Weave." objects.
+ * }, "weave:service:ready", false);
+ *
+ * // 4. Trigger loading of Sync.
+ * service.ensureLoaded();
+ */
+export function WeaveService() {
+  this.wrappedJSObject = this;
+  this.ready = false;
+}
+
+WeaveService.prototype = {
+  classID: Components.ID("{74b89fb0-f200-4ae8-a3ec-dd164117f6de}"),
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIObserver",
+    "nsISupportsWeakReference",
+  ]),
+
+  get Weave() {
+    const { Weave } = ChromeUtils.importESModule(
+      "resource://services-sync/main.sys.mjs"
+    );
+    return Weave;
+  },
+
+  ensureLoaded() {
+    // Side-effect of accessing the service is that it is instantiated.
+    this.Weave.Service;
+  },
+
+  whenLoaded() {
+    if (this.ready) {
+      return Promise.resolve();
+    }
+    let onReadyPromise = new Promise(resolve => {
+      Services.obs.addObserver(function onReady() {
+        Services.obs.removeObserver(onReady, "weave:service:ready");
+        resolve();
+      }, "weave:service:ready");
+    });
+    this.ensureLoaded();
+    return onReadyPromise;
+  },
+
+  init() {
+    // Force Weave service to load if it hasn't triggered from overlays
+    this.timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    this.timer.initWithCallback(
+      {
+        notify: () => {
+          let isConfigured = false;
+          // We only load more if it looks like Sync is configured.
+          if (this.enabled) {
+            // We have an associated FxAccount. So, do a more thorough check.
+            // This will import a number of modules and thus increase memory
+            // accordingly. We could potentially copy code performed by
+            // this check into this file if our above code is yielding too
+            // many false positives.
+            var { Weave } = ChromeUtils.importESModule(
+              "resource://services-sync/main.sys.mjs"
+            );
+            isConfigured =
+              Weave.Status.checkSetup() != lazy.CLIENT_NOT_CONFIGURED;
+          }
+          if (isConfigured) {
+            this.ensureLoaded();
+          }
+        },
+      },
+      10000,
+      Ci.nsITimer.TYPE_ONE_SHOT
+    );
+  },
+
+  /**
+   * Whether Sync appears to be enabled.
+   *
+   * This returns true if we have an associated FxA account and Sync is enabled.
+   *
+   * It does *not* perform a robust check to see if the client is working.
+   * For that, you'll want to check Weave.Status.checkSetup().
+   */
+  get enabled() {
+    return (
+      !!lazy.syncUsername &&
+      Services.prefs.getBoolPref("identity.fxaccounts.enabled")
+    );
+  },
+};
+
+export function AboutWeaveLog() {}
+AboutWeaveLog.prototype = {
+  classID: Components.ID("{d28f8a0b-95da-48f4-b712-caf37097be41}"),
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIAboutModule",
+    "nsISupportsWeakReference",
+  ]),
+
+  getURIFlags(aURI) {
+    return 0;
+  },
+
+  newChannel(aURI, aLoadInfo) {
+    let dir = lazy.FileUtils.getDir("ProfD", ["weave", "logs"]);
+    try {
+      dir.create(Ci.nsIFile.DIRECTORY_TYPE, lazy.FileUtils.PERMS_DIRECTORY);
+    } catch (ex) {
+      if (ex.result != Cr.NS_ERROR_FILE_ALREADY_EXISTS) {
+        throw ex;
+      }
+      // Ignore the exception due to a directory that already exists.
+    }
+    let uri = Services.io.newFileURI(dir);
+    let channel = Services.io.newChannelFromURIWithLoadInfo(uri, aLoadInfo);
+
+    channel.originalURI = aURI;
+
+    // Ensure that the about page has the same privileges as a regular directory
+    // view. That way links to files can be opened. make sure we use the correct
+    // origin attributes when creating the principal for accessing the
+    // about:sync-log data.
+    let principal = Services.scriptSecurityManager.createContentPrincipal(
+      uri,
+      aLoadInfo.originAttributes
+    );
+
+    channel.owner = principal;
+    return channel;
+  },
+};
diff --git a/services/sync/components.conf b/services/sync/components.conf
new file mode 100644
index 0000000000..e4f82b35b1
--- /dev/null
+++ b/services/sync/components.conf
@@ -0,0 +1,20 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Classes = [
+    {
+        'cid': '{74b89fb0-f200-4ae8-a3ec-dd164117f6de}',
+        'contract_ids': ['@mozilla.org/weave/service;1'],
+        'esModule': 'resource://services-sync/Weave.sys.mjs',
+        'constructor': 'WeaveService',
+    },
+    {
+        'cid': '{d28f8a0b-95da-48f4-b712-caf37097be41}',
+        'contract_ids': ['@mozilla.org/network/protocol/about;1?what=sync-log'],
+        'esModule': 'resource://services-sync/Weave.sys.mjs',
+        'constructor': 'AboutWeaveLog',
+    },
+]
diff --git a/services/sync/docs/engines.rst b/services/sync/docs/engines.rst
new file mode 100644
index 0000000000..7a4fa721af
--- /dev/null
+++ b/services/sync/docs/engines.rst
@@ -0,0 +1,133 @@
+============================
+The Sync engines in the tree
+============================
+
+Unless otherwise specified, the engine implementations can be found
+`here <https://searchfox.org/mozilla-central/source/services/sync/modules/engines>`_
+
+Please read the :doc:`overview`.
+
+Clients
+=======
+
+The ``clients`` engine is a special engine in that it's invisible to the
+user and can not be disabled - think of it as a "meta" engine. As such, it
+doesn't really have a sensible concept of ``store`` or ``tracker``.
+
+The engine is mainly responsible for keeping its own record current in the
+``clients`` collection. Some parts of Sync use this collection to know what
+other clients exist and when they last synced (although alot of this is moving
+to using the Firefox Accounts devices).
+
+Clients also has the ability to handle ``commands`` - in short, some other
+client can write to this client's ``commands``, and when this client notices,
+it will execute the command. Commands aren't arbitrary, so commands must be
+understood by both sides for them to work. There are commands to "wipe"
+collections etc. In practice, this is used only by ``bookmarks`` when a device
+restores bookmarks - in that case, the restoring device will send a ``wipe``
+command to all other clients so that they take the new bookmarks instead of
+merging them.
+
+If not for this somewhat limited ``commands`` functionality, this engine could
+be considered deprecated and subsumed by FxA devices - but because we
+can't just remove support for commands and also do not have a plan for
+replacing them, the clients engine remains important.
+
+Bookmarks
+=========
+
+The ``bookmarks`` engine has changed so that it's tightly integrated with the
+``places`` database. Instead of an external ``tracker``, the tracking is
+integrated into Places. Each bookmark has a `syncStatus` and a
+`syncChangeCounter` and these are managed internally by places. Sync then just
+queries for changed bookmarks by looking for these fields.
+
+Bookmarks is somewhat unique in that it needs to maintain a tree structure,
+which makes merging a challenge. The `dogear <https://github.com/mozilla/dogear>`_
+component (written in Rust and also used by the
+`application-services bookmarks component <https://github.com/mozilla/application-services/tree/main/components/places>`_)
+performs this merging.
+
+Bookmarks also pioneered the concept of a "mirror" - this is a database table
+which tracks exactly what is on the server. Because each sync only fetches
+changes from the server since the last sync, each sync does not supply every
+record on the server. However, the merging code does need to know what's on
+the server - so the mirror tracks this.
+
+History
+=======
+
+History is similar to bookmarks described above - it's closely integrated with
+places - but is less complex because there's no tree structure involved.
+
+One unique characteristic of history is that the engine takes steps to *not*
+upload everything - old profiles tend to have too much history to reasonably
+store and upload, so typically uploads are limited to the  last 5000 visits.
+
+Logins
+======
+
+Logins has also been upgraded to be closely integrated with `Services.logins` -
+the logins component itself manages the metadata.
+
+Tabs
+====
+
+Tabs is a special engine in that there's no underlying storage at all - it
+both saves the currently open tabs from this device (which are enumerated
+every time it's updated) and also lets other parts of Firefox know which tabs
+are open on other devices. There's no database - if we haven't synced yet we
+don't know what other tabs are open, and when we do know, the list is just
+stored in memory.
+
+The `SyncedTabs module <https://searchfox.org/mozilla-central/source/services/sync/modules/SyncedTabs.jsm>`_
+is the main interface the browser uses to get the list of tabs from other
+devices.
+
+Add-ons
+=======
+
+Addons is still an "old school" engine, with a tracker and store which aren't
+closely integrated with the addon manager. As a result it's fairly complex and
+error prone - eg, it persists the "last known" state so it can know what to
+sync, where a better model would be for the addon manager to track the changes
+on Sync's behalf.
+
+It also attempts to sync themes etc. The future of this engine isn't clear given
+it doesn't work on mobile platforms.
+
+Addresses / Credit-Cards
+========================
+
+Addresses and Credit-cards have Sync functionality tightly bound with the
+store. Unlike other engines above, this engine has always been tightly bound,
+because it was written after we realized this tight-binding was a feature and
+not a bug.
+
+Technically these are 2 separate engines and collections. However, because the
+underlying storage uses a shared implementation, the syncing also uses a
+shared implementation - ie, the same logic is used for both - so we tend to
+treat them as a single engine in practice.
+
+As a result, only a shim is in the `services/sync/modules/engines/` directory,
+while the actual logic is
+`next to the storage implementation <https://searchfox.org/mozilla-central/source/toolkit/components/formautofill/FormAutofillSync.jsm>`_.
+
+This engine has a unique twist on the "mirror" concept described above -
+whenever a change is made to a fields, the original value of the field is
+stored directly in the storage. This means that on the next sync, the value
+of the record on the server can be deduced, meaning a "3-way" merge can be
+done, so it can better tell the difference between local only, remote only, or
+conflicting changes.
+
+WebExt-Storage
+==============
+
+webext-storage is implemented in Rust and lives in
+`application services <https://github.com/mozilla/application-services/tree/main/components/webext-storage>`_
+and is vendored into the `addons code <https://searchfox.org/mozilla-central/source/toolkit/components/extensions/storage/webext_storage_bridge>`_ -
+note that this includes the storage *and* Sync code. The Sync engine itself
+is a shim in the sync directory.
+
+See the :doc:`rust-engines` document for more about how rust engines are
+integrated.
diff --git a/services/sync/docs/external.rst b/services/sync/docs/external.rst
new file mode 100644
index 0000000000..f7cebde32d
--- /dev/null
+++ b/services/sync/docs/external.rst
@@ -0,0 +1,8 @@
+==============
+External Links
+==============
+
+Some external links that might be of interest:
+
+* `Information about the server APIs <https://mozilla-services.readthedocs.io/en/latest/index.html>`_
+* `Some external Sync Client docs <https://mozilla-services.readthedocs.io/en/latest/sync/index.html>`_
diff --git a/services/sync/docs/index.rst b/services/sync/docs/index.rst
new file mode 100644
index 0000000000..37ce3c19a0
--- /dev/null
+++ b/services/sync/docs/index.rst
@@ -0,0 +1,17 @@
+====
+Sync
+====
+
+This documents the sync implementation inside mozilla-central. It assumes
+a general understanding of what Sync is and how it works at a high level - you
+can find `some external docs <https://mozilla-services.readthedocs.io/en/latest/sync/>`_
+which can help with this.
+
+.. toctree::
+   :maxdepth: 1
+
+   overview
+   engines
+   rust-engines
+   payload-evolution
+   external
diff --git a/services/sync/docs/overview.rst b/services/sync/docs/overview.rst
new file mode 100644
index 0000000000..e956090d70
--- /dev/null
+++ b/services/sync/docs/overview.rst
@@ -0,0 +1,81 @@
+====================
+Introduction to Sync
+====================
+
+This document is a brief introduction to how Sync is implemented in desktop Firefox.
+
+General, Historical, Anatomy of a Sync Engine
+=============================================
+
+This section describes how Sync used to work - and indeed, how much of it still
+does. While we discuss how this is slowly changing, this context is valuable.
+
+For any datatype which syncs, there tends to be 3 parts:
+
+Store
+-----
+
+The sync ``store`` interfaces with the actual Firefox desktop store. For example,
+in the ``passwords`` engine, the "store" is that layer that talks to
+``Services.logins``
+
+Tracker
+-------
+
+The ``tracker`` is what knows that something should be synced. For example,
+when the user creates or updates a password, it is the tracker that knows
+we should sync now, and what particular password(s) should be updated.
+
+This is typically done via "observer" notifications - ``Services.logins``,
+``places`` etc all send specific notifications when certain events happen
+(and indeed, some of these were added for Sync's benefit)
+
+Engine
+------
+
+The ``engine`` ties it all together. It works with the ``store`` and
+``tracker`` and tracks its own metadata (eg, the timestamp of the passwords on
+the server, so it knows how to grab just changed records and how to pass them
+off to the ``store`` so the actual underlying storage can be updated.
+
+All of the above parts were typically in the
+`services/sync/modules/engines directory <https://searchfox.org/mozilla-central/source/services/sync/modules/engines>`_
+directory and decoupled from the data they were syncing.
+
+
+The Future of Desktop-Specific Sync Engines
+===========================================
+
+The system described above reflects the fact that Sync was "bolted on" to
+Desktop Firefox relatively late - eg, the Sync ``store`` is decoupled from the
+actual ``store``. This has causes a number of problems - particularly around
+the ``tracker`` and the metadata used by the engine, and the fact that changes
+to the backing store would often forget that Sync existed.
+
+Over the last few years, the Sync team has come to the conclusion that Sync
+support must be integrated much closer to the store itself. For example,
+``Services.logins`` should track when something has changed that would cause
+an item to be synced. It should also track the metadata for the store so that
+if (say) a corrupt database is recovered by creating a new, empty one, the
+metadata should also vanish so Sync knows something bad has happened and can
+recover.
+
+However, this is a slow process - currently the ``bookmarks``, ``history`` and
+``passwords`` legacy engines have been improved so more responsibility is taken
+by the stores. In all cases, for implementation reasons, the Sync
+implementation still has a ``store``, but it tends to be a thin wrapper around
+the actual underlying store.
+
+The Future of Cross-Platform Sync Engines
+=========================================
+
+There are a number of Sync engines implemented in Rust and which live in the
+application-services repository. While these were often done for mobile
+platforms, the longer term hope is that they can be reused on Desktop.
+:doc:`engines` has more details on these.
+
+While no existing engines have been replaced with Rust implemented engines,
+the webext-storage engine is implemented in Rust via application-services, so
+doesn't tend to use any of the infrastructure described above.
+
+Hopefully over time we will find more Rust-implemented engines in Desktop.
diff --git a/services/sync/docs/payload-evolution.md b/services/sync/docs/payload-evolution.md
new file mode 100644
index 0000000000..e195ee545d
--- /dev/null
+++ b/services/sync/docs/payload-evolution.md
@@ -0,0 +1,168 @@
+# Handling the evolution of Sync payloads
+
+(Note that this document has been written in the format of an [application-services ADR](https://github.com/mozilla/application-services/blob/main/docs/adr/0000-use-markdown-architectural-decision-records.md)
+but the relelvant teams decided that ultimately the best home for this doc is in mozilla-central)
+
+* Status: Accepted
+* Deciders: sync team, credentials management team
+* Date: 2023-03-15
+
+Technical Story:
+* https://github.com/mozilla/application-services/pull/5434
+* https://docs.google.com/document/d/1ToLOERA5HKzEzRVZNv6Ohv_2wZaujW69pVb1Kef2jNY
+
+## Context and Problem Statement
+
+Sync exists on all platforms (Desktop, Android, iOS), all channels (Nightly, Beta, Release, ESR) and is heavily used across all Firefox features.
+Whenever there are feature changes or requests that potentially involve schema changes, there are not a lot of good options to ensure sync doesn’t break for any specific client.
+Since sync data is synced from all channels, we need to make sure each client can handle the new data and that all channels can support the new schema.
+Issues like [credit card failing on android and desktop release channels due to schema change on desktop Nightly](https://bugzilla.mozilla.org/show_bug.cgi?id=1812235)
+are examples of such cases we can run into.
+This document describes our decision on how we will support payload evolution over time.
+
+Note that even though this document exists in the application-services repository, it should
+be considered to apply to all sync implementations, whether in this repository, in mozilla-central,
+or anywhere else.
+
+## Definitions
+
+* A "new" Firefox installation is a version of Firefox which has a change to a Sync payload which
+  is not yet understood by "recent" versions. The most common example would be a Nightly version
+  of Firefox with a new feature not yet on the release channel.
+
+* A "recent" Firefox installation is a version older than a "new" version, which does not understand
+  or have support for new features in "new" versions, but which we still want to support without
+  breakage and without the user perceiving data-loss. This is typically accepted to mean the
+  current ESR version or later, but taking into account the slow update when new ESRs are released.
+
+* An "old" version is any version before what we consider "recent".
+
+
+## Decision Drivers
+
+* It must be possible to change what data is carried by Sync to meet future product requirements.
+* Both desktop and mobile platforms must be considered.
+* We must not break "recent" Firefox installations when a "new" Firefox installation syncs, and vice-versa.
+* Round-tripping data from a "new" Firefox installation through a "recent" Firefox installation must not discard any of the new data, and vice-versa.
+* Some degree of breakage for "old" Firefox installations when "new" or "recent" firefoxes sync
+  might be considered acceptable if absolutely necessary.
+* However, breakage of "new" or "recent" Firefoxes when an "old" version syncs is *not* acceptable.
+* Because such evolution should be rare, we do not want to set an up-front policy about locking out
+  "old" versions just because they might have a problem in the future. That is, we want to avoid
+  a policy that dictates versions more than (say) 2 years old will break when syncing "just in case"
+* Any solution to this must be achievable in a relatively short timeframe as we know of product
+  asks coming down the line which require this capability.
+
+## Considered Options
+
+* A backwards compatible schema policy, consisting of  (a) having engines "round trip" data they
+  do not know about and (b) never changing the semantics of existing data.
+* A policy which prevents "recent" clients from syncing, or editing data, or other restrictions.
+* A formal schema-driven process.
+* Consider the sync payloads frozen and never change them.
+* Use separate collections for new data
+
+## Decision Outcome
+
+Chosen option: A backwards compatible schema policy because it is very flexible and the only option
+meeting the decision drivers.
+
+## Pros and Cons of the Options
+
+### A backwards compatible schema policy
+
+A summary of this option is a policy by which:
+
+* Every sync engine must arrange to persist any fields from the payload which it
+  does not understand. The next time that engine needs to upload that record to the storage server,
+  it must arrange to add all such "unknown" fields back into the payload.
+
+* Different engines must identify different locations where this might happen. For example, the
+ `passwords` engine would identify the "root" of the payload, `addresses` and `creditcards` would
+ identify the `entry` sub-object in the payload, while the history engine would probably identify
+ *both* the root of the payload and the `visits` array.
+
+* Fields can not change type, nor be removed for a significant amount of time. This might mean
+  that "new" clients must support both new fields *and* fields which are considered deprecated
+  by these "new" clients because they are still used by "recent" versions.
+
+The pros and cons:
+
+* Good, because it meets the requirements.
+
+* Good, because the initial set of work identified is relatively simple to implement (that work
+  specifically is to support the round-tripping of "unknown" fields, in the hope that by the
+  time actual schema changes are proposed, this round-trip capability will then be on all "recent"
+  versions)
+
+* Bad, because the inability to deprecate or change existing fields means that
+  some evolution tasks become complicated. For example, consider a hypothetical change where
+  we wanted to change from "street/city/state" fields into a free-form "address" field. New
+  Firefox versions would need to populate *both* new and old fields when writing to the server,
+  and handle the fact that only the old versions might be updated when it sees an incoming
+  record written by a "recent" or "old" versions of Firefox. However, this should be rare.
+
+* Bad, because it's not possible to prove a proposed change meets the requirements - the policy
+  is informal and requires good judgement as changes are proposed.
+
+### A policy which prevents "recent" clients from syncing, or editing data
+
+Proposals which fit into this category might have been implemented by (say) adding
+a version number to the schema, and if clients did not fully understand the schema it would
+either prevent syncing the record, or sync it but not allow editing it, or similar.
+
+This was rejected because:
+
+* The user would certainly perceive data-loss if we ignored the incoming data entirely.
+* If we still wanted older versions to "partially" see the record (eg, but disallow editing) we'd
+  still need most of the chosen option anyway - specifically, we could still never
+  deprecate fields etc.
+* The UI/UX of trying to explain to the user why they can't edit a record was deemed impossible
+  to do in a satisfactory way.
+* This would effectively penalize users who chose to use Nightly Firefoxes in any way. Simply
+  allowing a Nightly to sync would effectively break Release/Mobile Firefox versions.
+
+### A formal schema-driven process.
+
+Ideally we could formally describe schemas, but we can't come up with anything here which
+works with the constraints of supporting older clients - we simply can't update older released
+Firefoxes so they know how to work with the new schemas. We also couldn't come up with a solution
+where a schema is downloaded dynamically which also allowed the *semantics* (as opposed to simply
+validity) of new fields to be described.
+
+### Consider the sync payloads frozen and never change them.
+
+A process where payloads are frozen was rejected because:
+
+* The most naive approach here would not meet the needs of Firefox in the future.
+
+* A complicated system where we started creating new payload and new collections
+  (ie, freezing "old" schemas but then creating "new" schemas only understood by
+  newer clients) could not be conceived in a way that still met the requirements,
+  particularly around data-loss for older clients. For example, adding a credit-card
+  on a Nightly version but having it be completely unavailable on a release firefox
+  isn't acceptable.
+
+### Use separate collections for new data
+
+We could store the new data in a separate collection. For example define a
+bookmarks2 collection where each record has the same guid as one in bookmarks alongside any new fields.
+Newer clients use both collections to sync.
+
+The pros and cons:
+
+* Good, because it allows newer clients to sync new data without affecting recent or older clients
+* Bad, because sync writes would lose atomicity without server changes.
+  We can currently write to a single collection in an atomic way, but don't have a way to write to multiple collections.
+* Bad because this number of collections grows each time we want to add fields.
+* Bad because it potentially leaks extra information to an attacker that gets access to the encrypted server records.
+  For example if we added a new collection for a single field, then the attacker could guess if that
+  field was set or not based on the size of the encrypted record.
+* Bad because it's difficult to handle nested data with this approach,
+  for example adding a field to a history record visit.
+* Bad because it has the same issue with dependent data as the chosen solution.
+
+## Links <!-- optional -->
+
+* This document was originally [brain-stormed in this google docs document](https://docs.google.com/document/d/1ToLOERA5HKzEzRVZNv6Ohv_2wZaujW69pVb1Kef2jNY),
+  which may be of interest for historical context, but should not be considered part of this ADR.
diff --git a/services/sync/docs/rust-engines.rst b/services/sync/docs/rust-engines.rst
new file mode 100644
index 0000000000..af00fd6619
--- /dev/null
+++ b/services/sync/docs/rust-engines.rst
@@ -0,0 +1,37 @@
+================================
+How Rust Engines are implemented
+================================
+
+There are 2 main components to engines implemented in Rust
+
+The bridged-engine
+==================
+
+Because Rust engines still need to work with the existing Sync infrastructure,
+there's the concept of a `bridged-engine <https://searchfox.org/mozilla-central/source/services/sync/modules/bridged_engine.js>`_.
+In short, this is just a shim between the existing
+`Sync Service <https://searchfox.org/mozilla-central/source/services/sync/modules/service.js>`_
+and the Rust code.
+
+The bridge
+==========
+
+`"Golden Gate" <https://searchfox.org/mozilla-central/source/services/sync/golden_gate>`_
+is a utility to help bridge any Rust implemented Sync engines with desktop. In
+other words, it's a "rusty bridge" - get it? Get it? Yet another of Lina's puns
+that live on!
+
+One of the key challenges with integrating a Rust Sync component with desktop
+is the different threading models. The Rust code tends to be synchronous -
+most functions block the calling thread to do the disk or network IO necessary
+to work - it assumes that the consumer will delegate this to some other thread.
+
+So golden_gate is this background thread delegation for a Rust Sync engine -
+gecko calls golden-gate on the main thread, it marshalls the call to a worker
+thread, and the result is marshalled back to the main thread.
+
+It's worth noting that golden_gate is just for the Sync engine part - other
+parts of the component (ie, the part that provides the functionality that's not
+sync related) will have its own mechanism for this. For example, the
+`webext-storage bridge <https://searchfox.org/mozilla-central/source/toolkit/components/extensions/storage/webext_storage_bridge/src>`_
+uses a similar technique `which has some in-depth documentation <../../toolkit/components/extensions/webextensions/webext-storage.html>`_.
diff --git a/services/sync/golden_gate/Cargo.toml b/services/sync/golden_gate/Cargo.toml
new file mode 100644
index 0000000000..3f94e1a1e9
--- /dev/null
+++ b/services/sync/golden_gate/Cargo.toml
@@ -0,0 +1,25 @@
+[package]
+name = "golden_gate"
+description = "A bridge for wiring up Sync engines implemented in Rust"
+version = "0.1.0"
+authors = ["The Firefox Sync Developers <sync-team@mozilla.com>"]
+edition = "2018"
+license = "MPL-2.0"
+
+[dependencies]
+anyhow = "1"
+atomic_refcell = "0.1"
+cstr = "0.2"
+interrupt-support = "0.1"
+log = "0.4"
+moz_task = { path = "../../../xpcom/rust/moz_task" }
+nserror = { path = "../../../xpcom/rust/nserror" }
+nsstring = { path = "../../../xpcom/rust/nsstring" }
+serde_json = "1"
+storage_variant = { path = "../../../storage/variant" }
+sync15 = "0.1"
+xpcom = { path = "../../../xpcom/rust/xpcom" }
+
+[dependencies.thin-vec]
+version = "0.2.1"
+features = ["gecko-ffi"]
diff --git a/services/sync/golden_gate/src/error.rs b/services/sync/golden_gate/src/error.rs
new file mode 100644
index 0000000000..373d20756e
--- /dev/null
+++ b/services/sync/golden_gate/src/error.rs
@@ -0,0 +1,71 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{error, fmt, result, str::Utf8Error};
+
+use nserror::{nsresult, NS_ERROR_INVALID_ARG, NS_ERROR_UNEXPECTED};
+use serde_json::Error as JsonError;
+
+/// A specialized `Result` type for Golden Gate.
+pub type Result<T> = result::Result<T, Error>;
+
+/// The error type for Golden Gate errors.
+#[derive(Debug)]
+pub enum Error {
+    /// A wrapped XPCOM error.
+    Nsresult(nsresult),
+
+    /// A ferry didn't run on the background task queue.
+    DidNotRun(&'static str),
+
+    /// A string contains invalid UTF-8 or JSON.
+    MalformedString(Box<dyn error::Error + Send + Sync + 'static>),
+}
+
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match self {
+            Error::MalformedString(error) => Some(error.as_ref()),
+            _ => None,
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Error::Nsresult(result) => write!(f, "Operation failed with {}", result.error_name()),
+            Error::DidNotRun(what) => write!(f, "Failed to run `{what}` on background thread"),
+            Error::MalformedString(error) => error.fmt(f),
+        }
+    }
+}
+
+impl From<nsresult> for Error {
+    fn from(result: nsresult) -> Error {
+        Error::Nsresult(result)
+    }
+}
+
+impl From<Utf8Error> for Error {
+    fn from(error: Utf8Error) -> Error {
+        Error::MalformedString(error.into())
+    }
+}
+
+impl From<JsonError> for Error {
+    fn from(error: JsonError) -> Error {
+        Error::MalformedString(error.into())
+    }
+}
+
+impl From<Error> for nsresult {
+    fn from(error: Error) -> nsresult {
+        match error {
+            Error::DidNotRun(_) => NS_ERROR_UNEXPECTED,
+            Error::Nsresult(result) => result,
+            Error::MalformedString(_) => NS_ERROR_INVALID_ARG,
+        }
+    }
+}
diff --git a/services/sync/golden_gate/src/ferry.rs b/services/sync/golden_gate/src/ferry.rs
new file mode 100644
index 0000000000..99994811ab
--- /dev/null
+++ b/services/sync/golden_gate/src/ferry.rs
@@ -0,0 +1,74 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use nsstring::nsCString;
+use storage_variant::VariantType;
+use sync15::Guid;
+use xpcom::{interfaces::nsIVariant, RefPtr};
+
+/// An operation that runs on the background thread, and optionally passes a
+/// result to its callback.
+pub enum Ferry {
+    LastSync,
+    SetLastSync(i64),
+    SyncId,
+    ResetSyncId,
+    EnsureCurrentSyncId(String),
+    SyncStarted,
+    StoreIncoming(Vec<nsCString>),
+    SetUploaded(i64, Vec<Guid>),
+    SyncFinished,
+    Reset,
+    Wipe,
+}
+
+impl Ferry {
+    /// Returns the operation name for debugging and labeling the task
+    /// runnable.
+    pub fn name(&self) -> &'static str {
+        match self {
+            Ferry::LastSync => concat!(module_path!(), "getLastSync"),
+            Ferry::SetLastSync(_) => concat!(module_path!(), "setLastSync"),
+            Ferry::SyncId => concat!(module_path!(), "getSyncId"),
+            Ferry::ResetSyncId => concat!(module_path!(), "resetSyncId"),
+            Ferry::EnsureCurrentSyncId(_) => concat!(module_path!(), "ensureCurrentSyncId"),
+            Ferry::SyncStarted => concat!(module_path!(), "syncStarted"),
+            Ferry::StoreIncoming { .. } => concat!(module_path!(), "storeIncoming"),
+            Ferry::SetUploaded { .. } => concat!(module_path!(), "setUploaded"),
+            Ferry::SyncFinished => concat!(module_path!(), "syncFinished"),
+            Ferry::Reset => concat!(module_path!(), "reset"),
+            Ferry::Wipe => concat!(module_path!(), "wipe"),
+        }
+    }
+}
+
+/// The result of a ferry task, sent from the background thread back to the
+/// main thread. Results are converted to variants, and passed as arguments to
+/// `mozIBridgedSyncEngineCallback`s.
+pub enum FerryResult {
+    LastSync(i64),
+    SyncId(Option<String>),
+    AssignedSyncId(String),
+    Null,
+}
+
+impl Default for FerryResult {
+    fn default() -> Self {
+        FerryResult::Null
+    }
+}
+
+impl FerryResult {
+    /// Converts the result to an `nsIVariant` that can be passed as an
+    /// argument to `callback.handleResult()`.
+    pub fn into_variant(self) -> RefPtr<nsIVariant> {
+        match self {
+            FerryResult::LastSync(v) => v.into_variant(),
+            FerryResult::SyncId(Some(v)) => nsCString::from(v).into_variant(),
+            FerryResult::SyncId(None) => ().into_variant(),
+            FerryResult::AssignedSyncId(v) => nsCString::from(v).into_variant(),
+            FerryResult::Null => ().into_variant(),
+        }
+    }
+}
diff --git a/services/sync/golden_gate/src/lib.rs b/services/sync/golden_gate/src/lib.rs
new file mode 100644
index 0000000000..8da6524bd7
--- /dev/null
+++ b/services/sync/golden_gate/src/lib.rs
@@ -0,0 +1,119 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! **Golden Gate** 🌉 is a crate for bridging Desktop Sync to our suite of
+//! Rust sync and storage components. It connects Sync's `BridgedEngine` class
+//! to the Rust `BridgedEngine` trait via the `mozIBridgedSyncEngine` XPCOM
+//! interface.
+//!
+//! Due to limitations in implementing XPCOM interfaces for generic types,
+//! Golden Gate doesn't implement `mozIBridgedSyncEngine` directly. Instead,
+//! it provides helpers, called "ferries", for passing Sync records between
+//! JavaScript and Rust. The ferries also handle threading and type
+//! conversions.
+//!
+//! Here's a step-by-step guide for adding a new Rust Sync engine to Firefox.
+//!
+//! ## Step 1: Create your (XPCOM) bridge
+//!
+//! In your consuming crate, define a type for your `mozIBridgedSyncEngine`
+//! implementation. We'll call this type the **brige**. The bridge is
+//! responsible for exposing your Sync engine to XPIDL [^1], in a way that lets
+//! JavaScript call it.
+//!
+//! For your bridge type, you'll need to implement an xpcom interface with the
+//! `#[xpcom(implement(mozIBridgedSyncEngine), nonatomic)]` attribute then
+//! define `xpcom_method!()` stubs for the `mozIBridgedSyncEngine` methods.  For
+//! more details about implementing XPCOM methods in Rust, check out the docs in
+//! `xpcom/rust/xpcom/src/method.rs`.
+//!
+//! You'll also need to add an entry for your bridge type to `components.conf`,
+//! and define C++ and Rust constructors for it, so that JavaScript code can
+//! create instances of it. Check out `NS_NewWebExtStorage` (and, in C++,
+//! `mozilla::extensions::storageapi::NewWebExtStorage`) and
+//! `NS_NewSyncedBookmarksMerger` (`mozilla::places::NewSyncedBookmarksMerger`
+//! in C++) for how to do this.
+//!
+//! [^1]: You can think of XPIDL as a souped-up C FFI, with richer types and a
+//! degree of type safety.
+//!
+//! ## Step 2: Add a background task queue to your bridge
+//!
+//! A task queue lets your engine do I/O, merging, and other syncing tasks on a
+//! background thread pool. This is important because database reads and writes
+//! can take an unpredictable amount of time. Doing these on the main thread can
+//! cause jank, and, in the worst case, lock up the browser UI for seconds at a
+//! time.
+//!
+//! The `moz_task` crate provides a `create_background_task_queue` function to
+//! do this. Once you have a queue, you can use it to call into your Rust
+//! engine. Golden Gate takes care of ferrying arguments back and forth across
+//! the thread boundary.
+//!
+//! Since it's a queue, ferries arrive in the order they're scheduled, so
+//! your engine's `store_incoming` method will always be called before `apply`,
+//! which is likewise called before `set_uploaded`. The thread manager scales
+//! the pool for you; you don't need to create or manage your own threads.
+//!
+//! ## Step 3: Create your Rust engine
+//!
+//! Next, you'll need to implement the Rust side of the bridge. This is a type
+//! that implements the `BridgedEngine` trait.
+//!
+//! Bridged engines handle storing incoming Sync records, merging changes,
+//! resolving conflicts, and fetching outgoing records for upload. Under the
+//! hood, your engine will hold either a database connection directly, or
+//! another object that does.
+//!
+//! Although outside the scope of Golden Gate, your engine will also likely
+//! expose a data storage API, for fetching, updating, and deleting items
+//! locally. Golden Gate provides the syncing layer on top of this local store.
+//!
+//! A `BridgedEngine` itself doesn't need to be `Send` or `Sync`, but the
+//! ferries require both, since they're calling into your bridge on the
+//! background task queue.
+//!
+//! In practice, this means your bridge will need to hold a thread-safe owned
+//! reference to the engine, via `Arc<Mutex<BridgedEngine>>`. In fact, this
+//! pattern is so common that Golden Gate implements `BridgedEngine` for any
+//! `Mutex<BridgedEngine>`, which automatically locks the mutex before calling
+//! into the engine.
+//!
+//! ## Step 4: Connect the bridge to the JavaScript and Rust sides
+//!
+//! On the JavaScript side, you'll need to subclass Sync's `BridgedEngine`
+//! class, and give it a handle to your XPCOM bridge. The base class has all the
+//! machinery for hooking up any `mozIBridgedSyncEngine` implementation so that
+//! Sync can drive it.
+//!
+//! On the Rust side, each `mozIBridgedSyncEngine` method should create a
+//! Golden Gate ferry, and dispatch it to the background task queue. The
+//! ferries correspond to the method names. For example, `ensureCurrentSyncId`
+//! should create a `Ferry::ensure_current_sync_id(...)`; `storeIncoming`, a
+//! `Ferry::store_incoming(...)`; and so on. This is mostly boilerplate.
+//!
+//! And that's it! Each ferry will, in turn, call into your Rust
+//! `BridgedEngine`, and send the results back to JavaScript.
+//!
+//! For an example of how all this works, including exposing a storage (not
+//! just syncing!) API to JS via XPIDL, check out `webext_storage::Bridge` for
+//! the `storage.sync` API!
+
+#[macro_use]
+extern crate cstr;
+
+pub mod error;
+mod ferry;
+pub mod log;
+pub mod task;
+
+pub use crate::log::LogSink;
+pub use error::{Error, Result};
+// Re-export items from `interrupt-support` and `sync15`, so that
+// consumers of `golden_gate` don't have to depend on them.
+pub use interrupt_support::{Interrupted, Interruptee};
+pub use sync15::bso::{IncomingBso, OutgoingBso};
+pub use sync15::engine::{ApplyResults, BridgedEngine};
+pub use sync15::Guid;
+pub use task::{ApplyTask, FerryTask};
diff --git a/services/sync/golden_gate/src/log.rs b/services/sync/golden_gate/src/log.rs
new file mode 100644
index 0000000000..de7fd0dfc3
--- /dev/null
+++ b/services/sync/golden_gate/src/log.rs
@@ -0,0 +1,161 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::fmt::{self, Write};
+
+use log::{Level, LevelFilter, Log, Metadata, Record};
+use moz_task::{Task, TaskRunnable, ThreadPtrHandle, ThreadPtrHolder};
+use nserror::nsresult;
+use nsstring::nsString;
+use xpcom::{interfaces::mozIServicesLogSink, RefPtr};
+
+pub struct LogSink {
+    pub max_level: LevelFilter,
+    logger: Option<ThreadPtrHandle<mozIServicesLogSink>>,
+}
+
+impl Default for LogSink {
+    fn default() -> Self {
+        LogSink {
+            max_level: LevelFilter::Off,
+            logger: None,
+        }
+    }
+}
+
+impl LogSink {
+    /// Creates a log sink that adapts the Rust `log` crate to the Sync
+    /// `Log.sys.mjs` logger.
+    ///
+    /// This is copied from `bookmark_sync::Logger`. It would be nice to share
+    /// these, but, for now, we've just duplicated it to make prototyping
+    /// easier.
+    #[inline]
+    pub fn new(max_level: LevelFilter, logger: ThreadPtrHandle<mozIServicesLogSink>) -> LogSink {
+        LogSink {
+            max_level,
+            logger: Some(logger),
+        }
+    }
+
+    /// Creates a log sink using the given Services `logger` as the
+    /// underlying implementation. The `logger` will always be called
+    /// asynchronously on its owning thread; it doesn't need to be
+    /// thread-safe.
+    pub fn with_logger(logger: Option<&mozIServicesLogSink>) -> Result<LogSink, nsresult> {
+        Ok(if let Some(logger) = logger {
+            // Fetch the maximum log level while we're on the main thread, so
+            // that `LogSink::enabled()` can check it while on the background
+            // thread. Otherwise, we'd need to dispatch a `LogTask` for every
+            // log message, only to discard most of them when the task calls
+            // into the logger on the main thread.
+            let mut raw_max_level = 0i16;
+            let rv = unsafe { logger.GetMaxLevel(&mut raw_max_level) };
+            let max_level = if rv.succeeded() {
+                match raw_max_level {
+                    mozIServicesLogSink::LEVEL_ERROR => LevelFilter::Error,
+                    mozIServicesLogSink::LEVEL_WARN => LevelFilter::Warn,
+                    mozIServicesLogSink::LEVEL_DEBUG => LevelFilter::Debug,
+                    mozIServicesLogSink::LEVEL_TRACE => LevelFilter::Trace,
+                    mozIServicesLogSink::LEVEL_INFO => LevelFilter::Info,
+                    _ => LevelFilter::Off,
+                }
+            } else {
+                LevelFilter::Off
+            };
+            LogSink::new(
+                max_level,
+                ThreadPtrHolder::new(cstr!("mozIServicesLogSink"), RefPtr::new(logger))?,
+            )
+        } else {
+            LogSink::default()
+        })
+    }
+
+    /// Returns a reference to the underlying `mozIServicesLogSink`.
+    pub fn logger(&self) -> Option<&mozIServicesLogSink> {
+        self.logger.as_ref().and_then(|l| l.get())
+    }
+
+    /// Logs a message to the Sync logger, if one is set. This would be better
+    /// implemented as a macro, as Dogear does, so that we can pass variadic
+    /// arguments without manually invoking `fmt_args!()` every time we want
+    /// to log a message.
+    ///
+    /// The `log` crate's macros aren't suitable here, because those log to the
+    /// global logger. However, we don't want to set the global logger in our
+    /// crate, because that will log _everything_ that uses the Rust `log` crate
+    /// to the Sync logs, including WebRender and audio logging.
+    pub fn debug(&self, args: fmt::Arguments) {
+        let meta = Metadata::builder()
+            .level(Level::Debug)
+            .target(module_path!())
+            .build();
+        if self.enabled(&meta) {
+            self.log(&Record::builder().args(args).metadata(meta).build());
+        }
+    }
+}
+
+impl Log for LogSink {
+    #[inline]
+    fn enabled(&self, meta: &Metadata) -> bool {
+        self.logger.is_some() && meta.level() <= self.max_level
+    }
+
+    fn log(&self, record: &Record) {
+        if !self.enabled(record.metadata()) {
+            return;
+        }
+        if let Some(logger) = &self.logger {
+            let mut message = nsString::new();
+            if write!(message, "{}", record.args()).is_ok() {
+                let task = LogTask {
+                    logger: logger.clone(),
+                    level: record.metadata().level(),
+                    message,
+                };
+                let _ = TaskRunnable::new("extension_storage_sync::Logger::log", Box::new(task))
+                    .and_then(|r| TaskRunnable::dispatch(r, logger.owning_thread()));
+            }
+        }
+    }
+
+    fn flush(&self) {}
+}
+
+/// Logs a message to the mirror logger. This task is created on the background
+/// thread queue, and dispatched to the main thread.
+struct LogTask {
+    logger: ThreadPtrHandle<mozIServicesLogSink>,
+    level: Level,
+    message: nsString,
+}
+
+impl Task for LogTask {
+    fn run(&self) {
+        let logger = self.logger.get().unwrap();
+        match self.level {
+            Level::Error => unsafe {
+                logger.Error(&*self.message);
+            },
+            Level::Warn => unsafe {
+                logger.Warn(&*self.message);
+            },
+            Level::Debug => unsafe {
+                logger.Debug(&*self.message);
+            },
+            Level::Trace => unsafe {
+                logger.Trace(&*self.message);
+            },
+            Level::Info => unsafe {
+                logger.Info(&*self.message);
+            },
+        }
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        Ok(())
+    }
+}
diff --git a/services/sync/golden_gate/src/task.rs b/services/sync/golden_gate/src/task.rs
new file mode 100644
index 0000000000..8cab21830b
--- /dev/null
+++ b/services/sync/golden_gate/src/task.rs
@@ -0,0 +1,355 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{fmt::Write, mem, result};
+
+use atomic_refcell::AtomicRefCell;
+use moz_task::{DispatchOptions, Task, TaskRunnable, ThreadPtrHandle, ThreadPtrHolder};
+use nserror::{nsresult, NS_ERROR_FAILURE};
+use nsstring::{nsACString, nsCString};
+use sync15::engine::{ApplyResults, BridgedEngine};
+use sync15::Guid;
+use thin_vec::ThinVec;
+use xpcom::{
+    interfaces::{
+        mozIBridgedSyncEngineApplyCallback, mozIBridgedSyncEngineCallback, nsIEventTarget,
+    },
+    RefPtr,
+};
+
+use crate::error::{Error, Result};
+use crate::ferry::{Ferry, FerryResult};
+
+/// A ferry task sends (or ferries) an operation to a bridged engine on a
+/// background thread or task queue, and ferries back an optional result to
+/// a callback.
+pub struct FerryTask {
+    /// We want to ensure scheduled ferries can't block finalization of the underlying
+    /// store - we want a degree of confidence that closing the database will happen when
+    /// we want even if tasks are queued up to run on another thread.
+    /// We rely on the semantics of our BridgedEngines to help here:
+    /// * A bridged engine is expected to hold a weak reference to its store.
+    /// * Our LazyStore is the only thing holding a reference to the "real" store.
+    /// Thus, when our LazyStore asks our "real" store to close, we can be confident
+    /// a close will happen (ie, we assume that the real store will be able to unwrapp
+    /// the underlying sqlite `Connection` (using `Arc::try_unwrap`) and close it.
+    /// However, note that if an operation on the bridged engine is currently running,
+    /// we will block waiting for that operation to complete, so while this isn't
+    /// guaranteed to happen immediately, it should happen "soon enough".
+    engine: Box<dyn BridgedEngine>,
+    ferry: Ferry,
+    callback: ThreadPtrHandle<mozIBridgedSyncEngineCallback>,
+    result: AtomicRefCell<anyhow::Result<FerryResult>>,
+}
+
+impl FerryTask {
+    /// Creates a task to fetch the engine's last sync time, in milliseconds.
+    #[inline]
+    pub fn for_last_sync(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::LastSync, callback)
+    }
+
+    /// Creates a task to set the engine's last sync time, in milliseconds.
+    #[inline]
+    pub fn for_set_last_sync(
+        engine: Box<dyn BridgedEngine>,
+        last_sync_millis: i64,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::SetLastSync(last_sync_millis), callback)
+    }
+
+    /// Creates a task to fetch the engine's sync ID.
+    #[inline]
+    pub fn for_sync_id(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::SyncId, callback)
+    }
+
+    /// Creates a task to reset the engine's sync ID and all its local Sync
+    /// metadata.
+    #[inline]
+    pub fn for_reset_sync_id(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::ResetSyncId, callback)
+    }
+
+    /// Creates a task to compare the bridged engine's local sync ID with
+    /// the `new_sync_id` from `meta/global`, and ferry back the final sync ID
+    /// to use.
+    #[inline]
+    pub fn for_ensure_current_sync_id(
+        engine: Box<dyn BridgedEngine>,
+        new_sync_id: &nsACString,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(
+            engine,
+            Ferry::EnsureCurrentSyncId(std::str::from_utf8(new_sync_id)?.into()),
+            callback,
+        )
+    }
+
+    /// Creates a task to signal that the engine is about to sync.
+    #[inline]
+    pub fn for_sync_started(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::SyncStarted, callback)
+    }
+
+    /// Creates a task to store incoming records.
+    pub fn for_store_incoming(
+        engine: Box<dyn BridgedEngine>,
+        incoming_envelopes_json: &[nsCString],
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(
+            engine,
+            Ferry::StoreIncoming(incoming_envelopes_json.to_vec()),
+            callback,
+        )
+    }
+
+    /// Creates a task to mark a subset of outgoing records as uploaded. This
+    /// may be called multiple times per sync, or not at all if there are no
+    /// records to upload.
+    pub fn for_set_uploaded(
+        engine: Box<dyn BridgedEngine>,
+        server_modified_millis: i64,
+        uploaded_ids: &[nsCString],
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        let uploaded_ids = uploaded_ids.iter().map(|id| Guid::from_slice(id)).collect();
+        Self::with_ferry(
+            engine,
+            Ferry::SetUploaded(server_modified_millis, uploaded_ids),
+            callback,
+        )
+    }
+
+    /// Creates a task to signal that all records have been uploaded, and
+    /// the engine has been synced. This is called even if there were no
+    /// records uploaded.
+    #[inline]
+    pub fn for_sync_finished(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::SyncFinished, callback)
+    }
+
+    /// Creates a task to reset all local Sync state for the engine, without
+    /// erasing user data.
+    #[inline]
+    pub fn for_reset(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::Reset, callback)
+    }
+
+    /// Creates a task to erase all local user data for the engine.
+    #[inline]
+    pub fn for_wipe(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        Self::with_ferry(engine, Ferry::Wipe, callback)
+    }
+
+    /// Creates a task for a ferry. The `callback` is bound to the current
+    /// thread, and will be called once, after the ferry returns from the
+    /// background thread.
+    fn with_ferry(
+        engine: Box<dyn BridgedEngine>,
+        ferry: Ferry,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<FerryTask> {
+        let name = ferry.name();
+        Ok(FerryTask {
+            engine,
+            ferry,
+            callback: ThreadPtrHolder::new(
+                cstr!("mozIBridgedSyncEngineCallback"),
+                RefPtr::new(callback),
+            )?,
+            result: AtomicRefCell::new(Err(Error::DidNotRun(name).into())),
+        })
+    }
+
+    /// Dispatches the task to the given thread `target`.
+    pub fn dispatch(self, target: &nsIEventTarget) -> Result<()> {
+        let runnable = TaskRunnable::new(self.ferry.name(), Box::new(self))?;
+        // `may_block` schedules the task on the I/O thread pool, since we
+        // expect most operations to wait on I/O.
+        TaskRunnable::dispatch_with_options(
+            runnable,
+            target,
+            DispatchOptions::default().may_block(true),
+        )?;
+        Ok(())
+    }
+
+    /// Runs the task on the background thread. This is split out into its own
+    /// method to make error handling easier.
+    fn inner_run(&self) -> anyhow::Result<FerryResult> {
+        let engine = &self.engine;
+        Ok(match &self.ferry {
+            Ferry::LastSync => FerryResult::LastSync(engine.last_sync()?),
+            Ferry::SetLastSync(last_sync_millis) => {
+                engine.set_last_sync(*last_sync_millis)?;
+                FerryResult::default()
+            }
+            Ferry::SyncId => FerryResult::SyncId(engine.sync_id()?),
+            Ferry::ResetSyncId => FerryResult::AssignedSyncId(engine.reset_sync_id()?),
+            Ferry::EnsureCurrentSyncId(new_sync_id) => {
+                FerryResult::AssignedSyncId(engine.ensure_current_sync_id(new_sync_id)?)
+            }
+            Ferry::SyncStarted => {
+                engine.sync_started()?;
+                FerryResult::default()
+            }
+            Ferry::StoreIncoming(incoming_envelopes_json) => {
+                let incoming_envelopes = incoming_envelopes_json
+                    .iter()
+                    .map(|envelope| Ok(serde_json::from_slice(envelope)?))
+                    .collect::<Result<_>>()?;
+
+                engine.store_incoming(incoming_envelopes)?;
+                FerryResult::default()
+            }
+            Ferry::SetUploaded(server_modified_millis, uploaded_ids) => {
+                engine.set_uploaded(*server_modified_millis, uploaded_ids.as_slice())?;
+                FerryResult::default()
+            }
+            Ferry::SyncFinished => {
+                engine.sync_finished()?;
+                FerryResult::default()
+            }
+            Ferry::Reset => {
+                engine.reset()?;
+                FerryResult::default()
+            }
+            Ferry::Wipe => {
+                engine.wipe()?;
+                FerryResult::default()
+            }
+        })
+    }
+}
+
+impl Task for FerryTask {
+    fn run(&self) {
+        *self.result.borrow_mut() = self.inner_run();
+    }
+
+    fn done(&self) -> result::Result<(), nsresult> {
+        let callback = self.callback.get().unwrap();
+        match mem::replace(
+            &mut *self.result.borrow_mut(),
+            Err(Error::DidNotRun(self.ferry.name()).into()),
+        ) {
+            Ok(result) => unsafe { callback.HandleSuccess(result.into_variant().coerce()) },
+            Err(err) => {
+                let mut message = nsCString::new();
+                write!(message, "{err}").unwrap();
+                unsafe { callback.HandleError(NS_ERROR_FAILURE, &*message) }
+            }
+        }
+        .to_result()
+    }
+}
+
+/// An apply task ferries incoming records to an engine on a background
+/// thread, and ferries back records to upload. It's separate from
+/// `FerryTask` because its callback type is different.
+pub struct ApplyTask {
+    engine: Box<dyn BridgedEngine>,
+    callback: ThreadPtrHandle<mozIBridgedSyncEngineApplyCallback>,
+    result: AtomicRefCell<anyhow::Result<Vec<String>>>,
+}
+
+impl ApplyTask {
+    /// Returns the task name for debugging.
+    pub fn name() -> &'static str {
+        concat!(module_path!(), "apply")
+    }
+
+    /// Runs the task on the background thread.
+    fn inner_run(&self) -> anyhow::Result<Vec<String>> {
+        let ApplyResults {
+            records: outgoing_records,
+            ..
+        } = self.engine.apply()?;
+        let outgoing_records_json = outgoing_records
+            .iter()
+            .map(|record| Ok(serde_json::to_string(record)?))
+            .collect::<Result<_>>()?;
+        Ok(outgoing_records_json)
+    }
+
+    /// Creates a task. The `callback` is bound to the current thread, and will
+    /// be called once, after the records are applied on the background thread.
+    pub fn new(
+        engine: Box<dyn BridgedEngine>,
+        callback: &mozIBridgedSyncEngineApplyCallback,
+    ) -> Result<ApplyTask> {
+        Ok(ApplyTask {
+            engine,
+            callback: ThreadPtrHolder::new(
+                cstr!("mozIBridgedSyncEngineApplyCallback"),
+                RefPtr::new(callback),
+            )?,
+            result: AtomicRefCell::new(Err(Error::DidNotRun(Self::name()).into())),
+        })
+    }
+
+    /// Dispatches the task to the given thread `target`.
+    pub fn dispatch(self, target: &nsIEventTarget) -> Result<()> {
+        let runnable = TaskRunnable::new(Self::name(), Box::new(self))?;
+        TaskRunnable::dispatch_with_options(
+            runnable,
+            target,
+            DispatchOptions::default().may_block(true),
+        )?;
+        Ok(())
+    }
+}
+
+impl Task for ApplyTask {
+    fn run(&self) {
+        *self.result.borrow_mut() = self.inner_run();
+    }
+
+    fn done(&self) -> result::Result<(), nsresult> {
+        let callback = self.callback.get().unwrap();
+        match mem::replace(
+            &mut *self.result.borrow_mut(),
+            Err(Error::DidNotRun(Self::name()).into()),
+        ) {
+            Ok(envelopes) => {
+                let result = envelopes
+                    .into_iter()
+                    .map(nsCString::from)
+                    .collect::<ThinVec<_>>();
+                unsafe { callback.HandleSuccess(&result) }
+            }
+            Err(err) => {
+                let mut message = nsCString::new();
+                write!(message, "{err}").unwrap();
+                unsafe { callback.HandleError(NS_ERROR_FAILURE, &*message) }
+            }
+        }
+        .to_result()
+    }
+}
diff --git a/services/sync/modules-testing/fakeservices.sys.mjs b/services/sync/modules-testing/fakeservices.sys.mjs
new file mode 100644
index 0000000000..4fd7534bf1
--- /dev/null
+++ b/services/sync/modules-testing/fakeservices.sys.mjs
@@ -0,0 +1,114 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { Weave } from "resource://services-sync/main.sys.mjs";
+import { RawCryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Utils } from "resource://services-sync/util.sys.mjs";
+
+export function FakeFilesystemService(contents) {
+  this.fakeContents = contents;
+  let self = this;
+
+  // Save away the unmocked versions of the functions we replace here for tests
+  // that really want the originals. As this may be called many times per test,
+  // we must be careful to not replace them with ones we previously replaced.
+  // (And why are we bothering with these mocks in the first place? Is the
+  // performance of the filesystem *really* such that it outweighs the downside
+  // of not running our real JSON functions in the tests? Eg, these mocks don't
+  // always throw exceptions when the real ones do. Anyway...)
+  for (let name of ["jsonSave", "jsonLoad", "jsonMove", "jsonRemove"]) {
+    let origName = "_real_" + name;
+    if (!Utils[origName]) {
+      Utils[origName] = Utils[name];
+    }
+  }
+
+  Utils.jsonSave = async function jsonSave(filePath, that, obj) {
+    let json = typeof obj == "function" ? obj.call(that) : obj;
+    self.fakeContents["weave/" + filePath + ".json"] = JSON.stringify(json);
+  };
+
+  Utils.jsonLoad = async function jsonLoad(filePath, that) {
+    let obj;
+    let json = self.fakeContents["weave/" + filePath + ".json"];
+    if (json) {
+      obj = JSON.parse(json);
+    }
+    return obj;
+  };
+
+  Utils.jsonMove = function jsonMove(aFrom, aTo, that) {
+    const fromPath = "weave/" + aFrom + ".json";
+    self.fakeContents["weave/" + aTo + ".json"] = self.fakeContents[fromPath];
+    delete self.fakeContents[fromPath];
+    return Promise.resolve();
+  };
+
+  Utils.jsonRemove = function jsonRemove(filePath, that) {
+    delete self.fakeContents["weave/" + filePath + ".json"];
+    return Promise.resolve();
+  };
+}
+
+export function fakeSHA256HMAC(message) {
+  message = message.substr(0, 64);
+  while (message.length < 64) {
+    message += " ";
+  }
+  return message;
+}
+
+export function FakeGUIDService() {
+  let latestGUID = 0;
+
+  Utils.makeGUID = function makeGUID() {
+    // ensure that this always returns a unique 12 character string
+    let nextGUID = "fake-guid-" + String(latestGUID++).padStart(2, "0");
+    return nextGUID.slice(nextGUID.length - 12, nextGUID.length);
+  };
+}
+
+/*
+ * Mock implementation of WeaveCrypto. It does not encrypt or
+ * decrypt, merely returning the input verbatim.
+ */
+export function FakeCryptoService() {
+  this.counter = 0;
+
+  delete Weave.Crypto; // get rid of the getter first
+  Weave.Crypto = this;
+
+  RawCryptoWrapper.prototype.ciphertextHMAC = function ciphertextHMAC(
+    keyBundle
+  ) {
+    return fakeSHA256HMAC(this.ciphertext);
+  };
+}
+
+FakeCryptoService.prototype = {
+  async encrypt(clearText, symmetricKey, iv) {
+    return clearText;
+  },
+
+  async decrypt(cipherText, symmetricKey, iv) {
+    return cipherText;
+  },
+
+  async generateRandomKey() {
+    return btoa("fake-symmetric-key-" + this.counter++);
+  },
+
+  generateRandomIV: function generateRandomIV() {
+    // A base64-encoded IV is 24 characters long
+    return btoa("fake-fake-fake-random-iv");
+  },
+
+  expandData: function expandData(data, len) {
+    return data;
+  },
+
+  generateRandomBytes: function generateRandomBytes(byteCount) {
+    return "not-so-random-now-are-we-HA-HA-HA! >:)".slice(byteCount);
+  },
+};
diff --git a/services/sync/modules-testing/fxa_utils.sys.mjs b/services/sync/modules-testing/fxa_utils.sys.mjs
new file mode 100644
index 0000000000..c953f0eaa3
--- /dev/null
+++ b/services/sync/modules-testing/fxa_utils.sys.mjs
@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Weave } from "resource://services-sync/main.sys.mjs";
+import { SyncAuthManager } from "resource://services-sync/sync_auth.sys.mjs";
+
+import { TokenServerClient } from "resource://services-common/tokenserverclient.sys.mjs";
+import { configureFxAccountIdentity } from "resource://testing-common/services/sync/utils.sys.mjs";
+
+// Create a new sync_auth object and initialize it with a
+// mocked TokenServerClient which always receives the specified response.
+export var initializeIdentityWithTokenServerResponse = function (response) {
+  // First create a mock "request" object that well' hack into the token server.
+  // A log for it
+  let requestLog = Log.repository.getLogger("testing.mock-rest");
+  if (!requestLog.appenders.length) {
+    // might as well see what it says :)
+    requestLog.addAppender(new Log.DumpAppender());
+    requestLog.level = Log.Level.Trace;
+  }
+
+  // A mock request object.
+  function MockRESTRequest(url) {}
+  MockRESTRequest.prototype = {
+    _log: requestLog,
+    setHeader() {},
+    async get() {
+      this.response = response;
+      return response;
+    },
+  };
+  // The mocked TokenServer client which will get the response.
+  function MockTSC() {}
+  MockTSC.prototype = new TokenServerClient();
+  MockTSC.prototype.constructor = MockTSC;
+  MockTSC.prototype.newRESTRequest = function (url) {
+    return new MockRESTRequest(url);
+  };
+  // Arrange for the same observerPrefix as sync_auth uses.
+  MockTSC.prototype.observerPrefix = "weave:service";
+
+  // tie it all together.
+  Weave.Status.__authManager = Weave.Service.identity = new SyncAuthManager();
+  let syncAuthManager = Weave.Service.identity;
+  // a sanity check
+  if (!(syncAuthManager instanceof SyncAuthManager)) {
+    throw new Error("sync isn't configured to use sync_auth");
+  }
+  let mockTSC = new MockTSC();
+  configureFxAccountIdentity(syncAuthManager);
+  syncAuthManager._tokenServerClient = mockTSC;
+};
diff --git a/services/sync/modules-testing/rotaryengine.sys.mjs b/services/sync/modules-testing/rotaryengine.sys.mjs
new file mode 100644
index 0000000000..d7f2165e4d
--- /dev/null
+++ b/services/sync/modules-testing/rotaryengine.sys.mjs
@@ -0,0 +1,120 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import {
+  Store,
+  SyncEngine,
+  LegacyTracker,
+} from "resource://services-sync/engines.sys.mjs";
+
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { SerializableSet, Utils } from "resource://services-sync/util.sys.mjs";
+
+/*
+ * A fake engine implementation.
+ * This is used all over the place.
+ *
+ * Complete with record, store, and tracker implementations.
+ */
+
+export function RotaryRecord(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+
+RotaryRecord.prototype = {};
+Object.setPrototypeOf(RotaryRecord.prototype, CryptoWrapper.prototype);
+Utils.deferGetSet(RotaryRecord, "cleartext", ["denomination"]);
+
+export function RotaryStore(name, engine) {
+  Store.call(this, name, engine);
+  this.items = {};
+}
+
+RotaryStore.prototype = {
+  async create(record) {
+    this.items[record.id] = record.denomination;
+  },
+
+  async remove(record) {
+    delete this.items[record.id];
+  },
+
+  async update(record) {
+    this.items[record.id] = record.denomination;
+  },
+
+  async itemExists(id) {
+    return id in this.items;
+  },
+
+  async createRecord(id, collection) {
+    let record = new RotaryRecord(collection, id);
+
+    if (!(id in this.items)) {
+      record.deleted = true;
+      return record;
+    }
+
+    record.denomination = this.items[id] || "Data for new record: " + id;
+    return record;
+  },
+
+  async changeItemID(oldID, newID) {
+    if (oldID in this.items) {
+      this.items[newID] = this.items[oldID];
+    }
+
+    delete this.items[oldID];
+  },
+
+  async getAllIDs() {
+    let ids = {};
+    for (let id in this.items) {
+      ids[id] = true;
+    }
+    return ids;
+  },
+
+  async wipe() {
+    this.items = {};
+  },
+};
+
+Object.setPrototypeOf(RotaryStore.prototype, Store.prototype);
+
+export function RotaryTracker(name, engine) {
+  LegacyTracker.call(this, name, engine);
+}
+
+RotaryTracker.prototype = {};
+Object.setPrototypeOf(RotaryTracker.prototype, LegacyTracker.prototype);
+
+export function RotaryEngine(service) {
+  SyncEngine.call(this, "Rotary", service);
+  // Ensure that the engine starts with a clean slate.
+  this.toFetch = new SerializableSet();
+  this.previousFailed = new SerializableSet();
+}
+
+RotaryEngine.prototype = {
+  _storeObj: RotaryStore,
+  _trackerObj: RotaryTracker,
+  _recordObj: RotaryRecord,
+
+  async _findDupe(item) {
+    // This is a Special Value® used for testing proper reconciling on dupe
+    // detection.
+    if (item.id == "DUPE_INCOMING") {
+      return "DUPE_LOCAL";
+    }
+
+    for (let [id, value] of Object.entries(this._store.items)) {
+      if (item.denomination == value) {
+        return id;
+      }
+    }
+    return null;
+  },
+};
+Object.setPrototypeOf(RotaryEngine.prototype, SyncEngine.prototype);
diff --git a/services/sync/modules-testing/utils.sys.mjs b/services/sync/modules-testing/utils.sys.mjs
new file mode 100644
index 0000000000..498bf9872a
--- /dev/null
+++ b/services/sync/modules-testing/utils.sys.mjs
@@ -0,0 +1,319 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+
+import { Assert } from "resource://testing-common/Assert.sys.mjs";
+
+import { initTestLogging } from "resource://testing-common/services/common/logging.sys.mjs";
+import {
+  FakeCryptoService,
+  FakeFilesystemService,
+  FakeGUIDService,
+  fakeSHA256HMAC,
+} from "resource://testing-common/services/sync/fakeservices.sys.mjs";
+
+import {
+  FxAccounts,
+  AccountState,
+} from "resource://gre/modules/FxAccounts.sys.mjs";
+import { FxAccountsClient } from "resource://gre/modules/FxAccountsClient.sys.mjs";
+
+import { SCOPE_OLD_SYNC } from "resource://gre/modules/FxAccountsCommon.sys.mjs";
+
+// A mock "storage manager" for FxAccounts that doesn't actually write anywhere.
+export function MockFxaStorageManager() {}
+
+MockFxaStorageManager.prototype = {
+  promiseInitialized: Promise.resolve(),
+
+  initialize(accountData) {
+    this.accountData = accountData;
+  },
+
+  finalize() {
+    return Promise.resolve();
+  },
+
+  getAccountData(fields = null) {
+    let result;
+    if (!this.accountData) {
+      result = null;
+    } else if (fields == null) {
+      // can't use cloneInto as the keys get upset...
+      result = {};
+      for (let field of Object.keys(this.accountData)) {
+        result[field] = this.accountData[field];
+      }
+    } else {
+      if (!Array.isArray(fields)) {
+        fields = [fields];
+      }
+      result = {};
+      for (let field of fields) {
+        result[field] = this.accountData[field];
+      }
+    }
+    return Promise.resolve(result);
+  },
+
+  updateAccountData(updatedFields) {
+    for (let [name, value] of Object.entries(updatedFields)) {
+      if (value == null) {
+        delete this.accountData[name];
+      } else {
+        this.accountData[name] = value;
+      }
+    }
+    return Promise.resolve();
+  },
+
+  deleteAccountData() {
+    this.accountData = null;
+    return Promise.resolve();
+  },
+};
+
+/**
+ * First wait >100ms (nsITimers can take up to that much time to fire, so
+ * we can account for the timer in delayedAutoconnect) and then two event
+ * loop ticks (to account for the CommonUtils.nextTick() in autoConnect).
+ */
+export function waitForZeroTimer(callback) {
+  let ticks = 2;
+  function wait() {
+    if (ticks) {
+      ticks -= 1;
+      CommonUtils.nextTick(wait);
+      return;
+    }
+    callback();
+  }
+  CommonUtils.namedTimer(wait, 150, {}, "timer");
+}
+
+export var promiseZeroTimer = function () {
+  return new Promise(resolve => {
+    waitForZeroTimer(resolve);
+  });
+};
+
+export var promiseNamedTimer = function (wait, thisObj, name) {
+  return new Promise(resolve => {
+    CommonUtils.namedTimer(resolve, wait, thisObj, name);
+  });
+};
+
+// Return an identity configuration suitable for testing with our identity
+// providers.  |overrides| can specify overrides for any default values.
+// |server| is optional, but if specified, will be used to form the cluster
+// URL for the FxA identity.
+export var makeIdentityConfig = function (overrides) {
+  // first setup the defaults.
+  let result = {
+    // Username used in both fxaccount and sync identity configs.
+    username: "foo",
+    // fxaccount specific credentials.
+    fxaccount: {
+      user: {
+        email: "foo",
+        scopedKeys: {
+          [SCOPE_OLD_SYNC]: {
+            kid: "1234567890123-u7u7u7u7u7u7u7u7u7u7uw",
+            k: "qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqg",
+            kty: "oct",
+          },
+        },
+        sessionToken: "sessionToken",
+        uid: "a".repeat(32),
+        verified: true,
+      },
+      token: {
+        endpoint: null,
+        duration: 300,
+        id: "id",
+        key: "key",
+        hashed_fxa_uid: "f".repeat(32), // used during telemetry validation
+        // uid will be set to the username.
+      },
+    },
+  };
+
+  // Now handle any specified overrides.
+  if (overrides) {
+    if (overrides.username) {
+      result.username = overrides.username;
+    }
+    if (overrides.fxaccount) {
+      // TODO: allow just some attributes to be specified
+      result.fxaccount = overrides.fxaccount;
+    }
+    if (overrides.node_type) {
+      result.fxaccount.token.node_type = overrides.node_type;
+    }
+  }
+  return result;
+};
+
+export var makeFxAccountsInternalMock = function (config) {
+  return {
+    newAccountState(credentials) {
+      // We only expect this to be called with null indicating the (mock)
+      // storage should be read.
+      if (credentials) {
+        throw new Error("Not expecting to have credentials passed");
+      }
+      let storageManager = new MockFxaStorageManager();
+      storageManager.initialize(config.fxaccount.user);
+      let accountState = new AccountState(storageManager);
+      return accountState;
+    },
+    getOAuthToken: () => Promise.resolve("some-access-token"),
+    destroyOAuthToken: () => Promise.resolve(),
+    keys: {
+      getScopedKeys: () =>
+        Promise.resolve({
+          "https://identity.mozilla.com/apps/oldsync": {
+            identifier: "https://identity.mozilla.com/apps/oldsync",
+            keyRotationSecret:
+              "0000000000000000000000000000000000000000000000000000000000000000",
+            keyRotationTimestamp: 1510726317123,
+          },
+        }),
+    },
+    profile: {
+      getProfile() {
+        return null;
+      },
+    },
+  };
+};
+
+// Configure an instance of an FxAccount identity provider with the specified
+// config (or the default config if not specified).
+export var configureFxAccountIdentity = function (
+  authService,
+  config = makeIdentityConfig(),
+  fxaInternal = makeFxAccountsInternalMock(config)
+) {
+  // until we get better test infrastructure for bid_identity, we set the
+  // signedin user's "email" to the username, simply as many tests rely on this.
+  config.fxaccount.user.email = config.username;
+
+  let fxa = new FxAccounts(fxaInternal);
+
+  let MockFxAccountsClient = function () {
+    FxAccountsClient.apply(this);
+  };
+  MockFxAccountsClient.prototype = {
+    accountStatus() {
+      return Promise.resolve(true);
+    },
+  };
+  Object.setPrototypeOf(
+    MockFxAccountsClient.prototype,
+    FxAccountsClient.prototype
+  );
+  let mockFxAClient = new MockFxAccountsClient();
+  fxa._internal._fxAccountsClient = mockFxAClient;
+
+  let mockTSC = {
+    // TokenServerClient
+    async getTokenUsingOAuth(url, oauthToken) {
+      Assert.equal(
+        url,
+        Services.prefs.getStringPref("identity.sync.tokenserver.uri")
+      );
+      Assert.ok(oauthToken, "oauth token present");
+      config.fxaccount.token.uid = config.username;
+      return config.fxaccount.token;
+    },
+  };
+  authService._fxaService = fxa;
+  authService._tokenServerClient = mockTSC;
+  // Set the "account" of the sync auth manager to be the "email" of the
+  // logged in user of the mockFXA service.
+  authService._signedInUser = config.fxaccount.user;
+  authService._account = config.fxaccount.user.email;
+};
+
+export var configureIdentity = async function (identityOverrides, server) {
+  let config = makeIdentityConfig(identityOverrides, server);
+  // Must be imported after the identity configuration is set up.
+  let { Service } = ChromeUtils.importESModule(
+    "resource://services-sync/service.sys.mjs"
+  );
+
+  // If a server was specified, ensure FxA has a correct cluster URL available.
+  if (server && !config.fxaccount.token.endpoint) {
+    let ep = server.baseURI;
+    if (!ep.endsWith("/")) {
+      ep += "/";
+    }
+    ep += "1.1/" + config.username + "/";
+    config.fxaccount.token.endpoint = ep;
+  }
+
+  configureFxAccountIdentity(Service.identity, config);
+  Services.prefs.setStringPref("services.sync.username", config.username);
+  // many of these tests assume all the auth stuff is setup and don't hit
+  // a path which causes that auth to magically happen - so do it now.
+  await Service.identity._ensureValidToken();
+
+  // and cheat to avoid requiring each test do an explicit login - give it
+  // a cluster URL.
+  if (config.fxaccount.token.endpoint) {
+    Service.clusterURL = config.fxaccount.token.endpoint;
+  }
+};
+
+export function syncTestLogging(level = "Trace") {
+  let logStats = initTestLogging(level);
+  Services.prefs.setStringPref("services.sync.log.logger", level);
+  Services.prefs.setStringPref("services.sync.log.logger.engine", "");
+  return logStats;
+}
+
+export var SyncTestingInfrastructure = async function (server, username) {
+  let config = makeIdentityConfig({ username });
+  await configureIdentity(config, server);
+  return {
+    logStats: syncTestLogging(),
+    fakeFilesystem: new FakeFilesystemService({}),
+    fakeGUIDService: new FakeGUIDService(),
+    fakeCryptoService: new FakeCryptoService(),
+  };
+};
+
+/**
+ * Turn WBO cleartext into fake "encrypted" payload as it goes over the wire.
+ */
+export function encryptPayload(cleartext) {
+  if (typeof cleartext == "object") {
+    cleartext = JSON.stringify(cleartext);
+  }
+
+  return {
+    ciphertext: cleartext, // ciphertext == cleartext with fake crypto
+    IV: "irrelevant",
+    hmac: fakeSHA256HMAC(cleartext),
+  };
+}
+
+export var sumHistogram = function (name, options = {}) {
+  let histogram = options.key
+    ? Services.telemetry.getKeyedHistogramById(name)
+    : Services.telemetry.getHistogramById(name);
+  let snapshot = histogram.snapshot();
+  let sum = -Infinity;
+  if (snapshot) {
+    if (options.key && snapshot[options.key]) {
+      sum = snapshot[options.key].sum;
+    } else {
+      sum = snapshot.sum;
+    }
+  }
+  histogram.clear();
+  return sum;
+};
diff --git a/services/sync/modules/SyncDisconnect.sys.mjs b/services/sync/modules/SyncDisconnect.sys.mjs
new file mode 100644
index 0000000000..2206a462ac
--- /dev/null
+++ b/services/sync/modules/SyncDisconnect.sys.mjs
@@ -0,0 +1,235 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// This module provides a facility for disconnecting Sync and FxA, optionally
+// sanitizing profile data as part of the process.
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AsyncShutdown: "resource://gre/modules/AsyncShutdown.sys.mjs",
+  Log: "resource://gre/modules/Log.sys.mjs",
+  PREF_LAST_FXA_USER: "resource://gre/modules/FxAccountsCommon.sys.mjs",
+  Sanitizer: "resource:///modules/Sanitizer.sys.mjs",
+  Utils: "resource://services-sync/util.sys.mjs",
+  setTimeout: "resource://gre/modules/Timer.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "fxAccounts", () => {
+  return ChromeUtils.importESModule(
+    "resource://gre/modules/FxAccounts.sys.mjs"
+  ).getFxAccountsSingleton();
+});
+
+export const SyncDisconnectInternal = {
+  lockRetryInterval: 1000, // wait 1 seconds before trying for the lock again.
+  lockRetryCount: 120, // Try 120 times (==2 mins) before giving up in disgust.
+  promiseDisconnectFinished: null, // If we are sanitizing, a promise for completion.
+
+  // mocked by tests.
+  getWeave() {
+    return ChromeUtils.importESModule("resource://services-sync/main.sys.mjs")
+      .Weave;
+  },
+
+  // Returns a promise that resolves when we are not syncing, waiting until
+  // a current Sync completes if necessary. Resolves with true if we
+  // successfully waited, in which case the sync lock will have been taken to
+  // ensure future syncs don't state, or resolves with false if we gave up
+  // waiting for the sync to complete (in which case we didn't take a lock -
+  // but note that Sync probably remains locked in this case regardless.)
+  async promiseNotSyncing(abortController) {
+    let weave = this.getWeave();
+    let log = lazy.Log.repository.getLogger("Sync.Service");
+    // We might be syncing - poll for up to 2 minutes waiting for the lock.
+    // (2 minutes seems extreme, but should be very rare.)
+    return new Promise(resolve => {
+      abortController.signal.onabort = () => {
+        resolve(false);
+      };
+
+      let attempts = 0;
+      let checkLock = () => {
+        if (abortController.signal.aborted) {
+          // We've already resolved, so don't want a new timer to ever start.
+          return;
+        }
+        if (weave.Service.lock()) {
+          resolve(true);
+          return;
+        }
+        attempts += 1;
+        if (attempts >= this.lockRetryCount) {
+          log.error(
+            "Gave up waiting for the sync lock - going ahead with sanitize anyway"
+          );
+          resolve(false);
+          return;
+        }
+        log.debug("Waiting a couple of seconds to get the sync lock");
+        lazy.setTimeout(checkLock, this.lockRetryInterval);
+      };
+      checkLock();
+    });
+  },
+
+  // Sanitize Sync-related data.
+  async doSanitizeSyncData() {
+    let weave = this.getWeave();
+    // Get the sync logger - if stuff goes wrong it can be useful to have that
+    // recorded in the sync logs.
+    let log = lazy.Log.repository.getLogger("Sync.Service");
+    log.info("Starting santitize of Sync data");
+    try {
+      // We clobber data for all Sync engines that are enabled.
+      await weave.Service.promiseInitialized;
+      weave.Service.enabled = false;
+
+      log.info("starting actual sanitization");
+      for (let engine of weave.Service.engineManager.getAll()) {
+        if (engine.enabled) {
+          try {
+            log.info("Wiping engine", engine.name);
+            await engine.wipeClient();
+          } catch (ex) {
+            log.error("Failed to wipe engine", ex);
+          }
+        }
+      }
+      // Reset the pref which is used to show a warning when a different user
+      // signs in - this is no longer a concern now that we've removed the
+      // data from the profile.
+      Services.prefs.clearUserPref(lazy.PREF_LAST_FXA_USER);
+
+      log.info("Finished wiping sync data");
+    } catch (ex) {
+      log.error("Failed to sanitize Sync data", ex);
+      console.error("Failed to sanitize Sync data", ex);
+    }
+    try {
+      // ensure any logs we wrote are flushed to disk.
+      await weave.Service.errorHandler.resetFileLog();
+    } catch (ex) {
+      console.log("Failed to flush the Sync log", ex);
+    }
+  },
+
+  // Sanitize all Browser data.
+  async doSanitizeBrowserData() {
+    try {
+      // sanitize everything other than "open windows" (and we don't do that
+      // because it may confuse the user - they probably want to see
+      // about:prefs with the disconnection reflected.
+      let itemsToClear = Object.keys(lazy.Sanitizer.items).filter(
+        k => k != "openWindows"
+      );
+      await lazy.Sanitizer.sanitize(itemsToClear);
+    } catch (ex) {
+      console.error("Failed to sanitize other data", ex);
+    }
+  },
+
+  async doSyncAndAccountDisconnect(shouldUnlock) {
+    // We do a startOver of Sync first - if we do the account first we end
+    // up with Sync configured but FxA not configured, which causes the browser
+    // UI to briefly enter a "needs reauth" state.
+    let Weave = this.getWeave();
+    await Weave.Service.promiseInitialized;
+    await Weave.Service.startOver();
+    await lazy.fxAccounts.signOut();
+    // Sync may have been disabled if we santized, so re-enable it now or
+    // else the user will be unable to resync should they sign in before a
+    // restart.
+    Weave.Service.enabled = true;
+
+    // and finally, if we managed to get the lock before, we should unlock it
+    // now.
+    if (shouldUnlock) {
+      Weave.Service.unlock();
+    }
+  },
+
+  // Start the sanitization process. Returns a promise that resolves when
+  // the sanitize is complete, and an AbortController which can be used to
+  // abort the process of waiting for a sync to complete.
+  async _startDisconnect(abortController, sanitizeData = false) {
+    // This is a bit convoluted - we want to wait for a sync to finish before
+    // sanitizing, but want to abort that wait if the browser shuts down while
+    // we are waiting (in which case we'll charge ahead anyway).
+    // So we do this by using an AbortController and passing that to the
+    // function that waits for the sync lock - it will immediately resolve
+    // if the abort controller is aborted.
+    let log = lazy.Log.repository.getLogger("Sync.Service");
+
+    // If the master-password is locked then we will fail to fully sanitize,
+    // so prompt for that now. If canceled, we just abort now.
+    log.info("checking master-password state");
+    if (!lazy.Utils.ensureMPUnlocked()) {
+      log.warn(
+        "The master-password needs to be unlocked to fully disconnect from sync"
+      );
+      return;
+    }
+
+    log.info("waiting for any existing syncs to complete");
+    let locked = await this.promiseNotSyncing(abortController);
+
+    if (sanitizeData) {
+      await this.doSanitizeSyncData();
+
+      // We disconnect before sanitizing the browser data - in a worst-case
+      // scenario where the sanitize takes so long that even the shutdown
+      // blocker doesn't allow it to finish, we should still at least be in
+      // a disconnected state on the next startup.
+      log.info("disconnecting account");
+      await this.doSyncAndAccountDisconnect(locked);
+
+      await this.doSanitizeBrowserData();
+    } else {
+      log.info("disconnecting account");
+      await this.doSyncAndAccountDisconnect(locked);
+    }
+  },
+
+  async disconnect(sanitizeData) {
+    if (this.promiseDisconnectFinished) {
+      throw new Error("A disconnect is already in progress");
+    }
+    let abortController = new AbortController();
+    let promiseDisconnectFinished = this._startDisconnect(
+      abortController,
+      sanitizeData
+    );
+    this.promiseDisconnectFinished = promiseDisconnectFinished;
+    let shutdownBlocker = () => {
+      // oh dear - we are sanitizing (probably stuck waiting for a sync to
+      // complete) and the browser is shutting down. Let's avoid the wait
+      // for sync to complete and continue the process anyway.
+      abortController.abort();
+      return promiseDisconnectFinished;
+    };
+    lazy.AsyncShutdown.quitApplicationGranted.addBlocker(
+      "SyncDisconnect: removing requested data",
+      shutdownBlocker
+    );
+
+    // wait for it to finish - hopefully without the blocker being called.
+    await promiseDisconnectFinished;
+    this.promiseDisconnectFinished = null;
+
+    // sanitize worked so remove our blocker - it's a noop if the blocker
+    // did call us.
+    lazy.AsyncShutdown.quitApplicationGranted.removeBlocker(shutdownBlocker);
+  },
+};
+
+export const SyncDisconnect = {
+  get promiseDisconnectFinished() {
+    return SyncDisconnectInternal.promiseDisconnectFinished;
+  },
+
+  disconnect(sanitizeData) {
+    return SyncDisconnectInternal.disconnect(sanitizeData);
+  },
+};
diff --git a/services/sync/modules/SyncedTabs.sys.mjs b/services/sync/modules/SyncedTabs.sys.mjs
new file mode 100644
index 0000000000..410244413e
--- /dev/null
+++ b/services/sync/modules/SyncedTabs.sys.mjs
@@ -0,0 +1,348 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  CLIENT_NOT_CONFIGURED: "resource://services-sync/constants.sys.mjs",
+  Weave: "resource://services-sync/main.sys.mjs",
+});
+
+// The Sync XPCOM service
+ChromeUtils.defineLazyGetter(lazy, "weaveXPCService", function () {
+  return Cc["@mozilla.org/weave/service;1"].getService(
+    Ci.nsISupports
+  ).wrappedJSObject;
+});
+
+// from MDN...
+function escapeRegExp(string) {
+  return string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+// A topic we fire whenever we have new tabs available. This might be due
+// to a request made by this module to refresh the tab list, or as the result
+// of a regularly scheduled sync. The intent is that consumers just listen
+// for this notification and update their UI in response.
+const TOPIC_TABS_CHANGED = "services.sync.tabs.changed";
+
+// The interval, in seconds, before which we consider the existing list
+// of tabs "fresh enough" and don't force a new sync.
+const TABS_FRESH_ENOUGH_INTERVAL_SECONDS = 30;
+
+ChromeUtils.defineLazyGetter(lazy, "log", () => {
+  const { Log } = ChromeUtils.importESModule(
+    "resource://gre/modules/Log.sys.mjs"
+  );
+  let log = Log.repository.getLogger("Sync.RemoteTabs");
+  log.manageLevelFromPref("services.sync.log.logger.tabs");
+  return log;
+});
+
+// A private singleton that does the work.
+let SyncedTabsInternal = {
+  /* Make a "tab" record. Returns a promise */
+  async _makeTab(client, tab, url, showRemoteIcons) {
+    let icon;
+    if (showRemoteIcons) {
+      icon = tab.icon;
+    }
+    if (!icon) {
+      // By not specifying a size the favicon service will pick the default,
+      // that is usually set through setDefaultIconURIPreferredSize by the
+      // first browser window. Commonly it's 16px at current dpi.
+      icon = "page-icon:" + url;
+    }
+    return {
+      type: "tab",
+      title: tab.title || url,
+      url,
+      icon,
+      client: client.id,
+      lastUsed: tab.lastUsed,
+      inactive: tab.inactive,
+    };
+  },
+
+  /* Make a "client" record. Returns a promise for consistency with _makeTab */
+  async _makeClient(client) {
+    return {
+      id: client.id,
+      type: "client",
+      name: lazy.Weave.Service.clientsEngine.getClientName(client.id),
+      clientType: lazy.Weave.Service.clientsEngine.getClientType(client.id),
+      lastModified: client.lastModified * 1000, // sec to ms
+      tabs: [],
+    };
+  },
+
+  _tabMatchesFilter(tab, filter) {
+    let reFilter = new RegExp(escapeRegExp(filter), "i");
+    return reFilter.test(tab.url) || reFilter.test(tab.title);
+  },
+
+  _createRecentTabsList(
+    clients,
+    maxCount,
+    extraParams = { removeAllDupes: true, removeDeviceDupes: false }
+  ) {
+    let tabs = [];
+
+    for (let client of clients) {
+      if (extraParams.removeDeviceDupes) {
+        client.tabs = this._filterRecentTabsDupes(client.tabs);
+      }
+      for (let tab of client.tabs) {
+        tab.device = client.name;
+        tab.deviceType = client.clientType;
+      }
+      tabs = [...tabs, ...client.tabs.reverse()];
+    }
+    if (extraParams.removeAllDupes) {
+      tabs = this._filterRecentTabsDupes(tabs);
+    }
+    tabs = tabs.sort((a, b) => b.lastUsed - a.lastUsed).slice(0, maxCount);
+    return tabs;
+  },
+
+  _filterRecentTabsDupes(tabs) {
+    // Filter out any tabs with duplicate URLs preserving
+    // the duplicate with the most recent lastUsed value
+    return tabs.filter(tab => {
+      return !tabs.some(t => {
+        return t.url === tab.url && tab.lastUsed < t.lastUsed;
+      });
+    });
+  },
+
+  async getTabClients(filter) {
+    lazy.log.info("Generating tab list with filter", filter);
+    let result = [];
+
+    // If Sync isn't ready, don't try and get anything.
+    if (!lazy.weaveXPCService.ready) {
+      lazy.log.debug("Sync isn't yet ready, so returning an empty tab list");
+      return result;
+    }
+
+    // A boolean that controls whether we should show the icon from the remote tab.
+    const showRemoteIcons = Services.prefs.getBoolPref(
+      "services.sync.syncedTabs.showRemoteIcons",
+      true
+    );
+
+    let engine = lazy.Weave.Service.engineManager.get("tabs");
+
+    let ntabs = 0;
+    let clientTabList = await engine.getAllClients();
+    for (let client of clientTabList) {
+      if (!lazy.Weave.Service.clientsEngine.remoteClientExists(client.id)) {
+        continue;
+      }
+      let clientRepr = await this._makeClient(client);
+      lazy.log.debug("Processing client", clientRepr);
+
+      for (let tab of client.tabs) {
+        let url = tab.urlHistory[0];
+        lazy.log.trace("remote tab", url);
+
+        if (!url) {
+          continue;
+        }
+        let tabRepr = await this._makeTab(client, tab, url, showRemoteIcons);
+        if (filter && !this._tabMatchesFilter(tabRepr, filter)) {
+          continue;
+        }
+        clientRepr.tabs.push(tabRepr);
+      }
+      // We return all clients, even those without tabs - the consumer should
+      // filter it if they care.
+      ntabs += clientRepr.tabs.length;
+      result.push(clientRepr);
+    }
+    lazy.log.info(
+      `Final tab list has ${result.length} clients with ${ntabs} tabs.`
+    );
+    return result;
+  },
+
+  async syncTabs(force) {
+    if (!force) {
+      // Don't bother refetching tabs if we already did so recently
+      let lastFetch = Services.prefs.getIntPref(
+        "services.sync.lastTabFetch",
+        0
+      );
+      let now = Math.floor(Date.now() / 1000);
+      if (now - lastFetch < TABS_FRESH_ENOUGH_INTERVAL_SECONDS) {
+        lazy.log.info("_refetchTabs was done recently, do not doing it again");
+        return false;
+      }
+    }
+
+    // If Sync isn't configured don't try and sync, else we will get reports
+    // of a login failure.
+    if (lazy.Weave.Status.checkSetup() === lazy.CLIENT_NOT_CONFIGURED) {
+      lazy.log.info(
+        "Sync client is not configured, so not attempting a tab sync"
+      );
+      return false;
+    }
+    // If the primary pass is locked, we should not try to sync
+    if (lazy.Weave.Utils.mpLocked()) {
+      lazy.log.info(
+        "Can't sync tabs due to the primary password being locked",
+        lazy.Weave.Status.login
+      );
+      return false;
+    }
+    // Ask Sync to just do the tabs engine if it can.
+    try {
+      lazy.log.info("Doing a tab sync.");
+      await lazy.Weave.Service.sync({ why: "tabs", engines: ["tabs"] });
+      return true;
+    } catch (ex) {
+      lazy.log.error("Sync failed", ex);
+      throw ex;
+    }
+  },
+
+  observe(subject, topic, data) {
+    lazy.log.trace(`observed topic=${topic}, data=${data}, subject=${subject}`);
+    switch (topic) {
+      case "weave:engine:sync:finish":
+        if (data != "tabs") {
+          return;
+        }
+        // The tabs engine just finished syncing
+        // Set our lastTabFetch pref here so it tracks both explicit sync calls
+        // and normally scheduled ones.
+        Services.prefs.setIntPref(
+          "services.sync.lastTabFetch",
+          Math.floor(Date.now() / 1000)
+        );
+        Services.obs.notifyObservers(null, TOPIC_TABS_CHANGED);
+        break;
+      case "weave:service:start-over":
+        // start-over needs to notify so consumers find no tabs.
+        Services.prefs.clearUserPref("services.sync.lastTabFetch");
+        Services.obs.notifyObservers(null, TOPIC_TABS_CHANGED);
+        break;
+      case "nsPref:changed":
+        Services.obs.notifyObservers(null, TOPIC_TABS_CHANGED);
+        break;
+      default:
+        break;
+    }
+  },
+
+  // Returns true if Sync is configured to Sync tabs, false otherwise
+  get isConfiguredToSyncTabs() {
+    if (!lazy.weaveXPCService.ready) {
+      lazy.log.debug("Sync isn't yet ready; assuming tab engine is enabled");
+      return true;
+    }
+
+    let engine = lazy.Weave.Service.engineManager.get("tabs");
+    return engine && engine.enabled;
+  },
+
+  get hasSyncedThisSession() {
+    let engine = lazy.Weave.Service.engineManager.get("tabs");
+    return engine && engine.hasSyncedThisSession;
+  },
+};
+
+Services.obs.addObserver(SyncedTabsInternal, "weave:engine:sync:finish");
+Services.obs.addObserver(SyncedTabsInternal, "weave:service:start-over");
+// Observe the pref the indicates the state of the tabs engine has changed.
+// This will force consumers to re-evaluate the state of sync and update
+// accordingly.
+Services.prefs.addObserver("services.sync.engine.tabs", SyncedTabsInternal);
+
+// The public interface.
+export var SyncedTabs = {
+  // A mock-point for tests.
+  _internal: SyncedTabsInternal,
+
+  // We make the topic for the observer notification public.
+  TOPIC_TABS_CHANGED,
+
+  // Expose the interval used to determine if synced tabs data needs a new sync
+  TABS_FRESH_ENOUGH_INTERVAL_SECONDS,
+
+  // Returns true if Sync is configured to Sync tabs, false otherwise
+  get isConfiguredToSyncTabs() {
+    return this._internal.isConfiguredToSyncTabs;
+  },
+
+  // Returns true if a tab sync has completed once this session. If this
+  // returns false, then getting back no clients/tabs possibly just means we
+  // are waiting for that first sync to complete.
+  get hasSyncedThisSession() {
+    return this._internal.hasSyncedThisSession;
+  },
+
+  // Return a promise that resolves with an array of client records, each with
+  // a .tabs array. Note that part of the contract for this module is that the
+  // returned objects are not shared between invocations, so callers are free
+  // to mutate the returned objects (eg, sort, truncate) however they see fit.
+  getTabClients(query) {
+    return this._internal.getTabClients(query);
+  },
+
+  // Starts a background request to start syncing tabs. Returns a promise that
+  // resolves when the sync is complete, but there's no resolved value -
+  // callers should be listening for TOPIC_TABS_CHANGED.
+  // If |force| is true we always sync. If false, we only sync if the most
+  // recent sync wasn't "recently".
+  syncTabs(force) {
+    return this._internal.syncTabs(force);
+  },
+
+  createRecentTabsList(clients, maxCount, extraParams) {
+    return this._internal._createRecentTabsList(clients, maxCount, extraParams);
+  },
+
+  sortTabClientsByLastUsed(clients) {
+    // First sort the list of tabs for each client. Note that
+    // this module promises that the objects it returns are never
+    // shared, so we are free to mutate those objects directly.
+    for (let client of clients) {
+      let tabs = client.tabs;
+      tabs.sort((a, b) => b.lastUsed - a.lastUsed);
+    }
+    // Now sort the clients - the clients are sorted in the order of the
+    // most recent tab for that client (ie, it is important the tabs for
+    // each client are already sorted.)
+    clients.sort((a, b) => {
+      if (!a.tabs.length) {
+        return 1; // b comes first.
+      }
+      if (!b.tabs.length) {
+        return -1; // a comes first.
+      }
+      return b.tabs[0].lastUsed - a.tabs[0].lastUsed;
+    });
+  },
+
+  recordSyncedTabsTelemetry(object, tabEvent, extraOptions) {
+    Services.telemetry.setEventRecordingEnabled("synced_tabs", true);
+    Services.telemetry.recordEvent(
+      "synced_tabs",
+      tabEvent,
+      object,
+      null,
+      extraOptions
+    );
+  },
+
+  // Get list of synced tabs across all devices/clients
+  // truncated by value of maxCount param, sorted by
+  // lastUsed value, and filtered for duplicate URLs
+  async getRecentTabs(maxCount, extraParams) {
+    let clients = await this.getTabClients();
+    return this._internal._createRecentTabsList(clients, maxCount, extraParams);
+  },
+};
diff --git a/services/sync/modules/UIState.sys.mjs b/services/sync/modules/UIState.sys.mjs
new file mode 100644
index 0000000000..8981d81f7d
--- /dev/null
+++ b/services/sync/modules/UIState.sys.mjs
@@ -0,0 +1,285 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * @typedef {Object} UIState
+ * @property {string} status The Sync/FxA status, see STATUS_* constants.
+ * @property {string} [email] The FxA email configured to log-in with Sync.
+ * @property {string} [displayName] The user's FxA display name.
+ * @property {string} [avatarURL] The user's FxA avatar URL.
+ * @property {Date} [lastSync] The last sync time.
+ * @property {boolean} [syncing] Whether or not we are currently syncing.
+ */
+
+const lazy = {};
+ChromeUtils.defineESModuleGetters(lazy, {
+  LOGIN_FAILED_LOGIN_REJECTED: "resource://services-sync/constants.sys.mjs",
+  Weave: "resource://services-sync/main.sys.mjs",
+});
+
+const TOPICS = [
+  "weave:connected",
+  "weave:service:login:got-hashed-id",
+  "weave:service:login:error",
+  "weave:service:ready",
+  "weave:service:sync:start",
+  "weave:service:sync:finish",
+  "weave:service:sync:error",
+  "weave:service:start-over:finish",
+  "fxaccounts:onverified",
+  "fxaccounts:onlogin", // Defined in FxAccountsCommon, pulling it is expensive.
+  "fxaccounts:onlogout",
+  "fxaccounts:profilechange",
+  "fxaccounts:statechange",
+];
+
+const ON_UPDATE = "sync-ui-state:update";
+
+const STATUS_NOT_CONFIGURED = "not_configured";
+const STATUS_LOGIN_FAILED = "login_failed";
+const STATUS_NOT_VERIFIED = "not_verified";
+const STATUS_SIGNED_IN = "signed_in";
+
+const DEFAULT_STATE = {
+  status: STATUS_NOT_CONFIGURED,
+};
+
+const UIStateInternal = {
+  _initialized: false,
+  _state: null,
+
+  // We keep _syncing out of the state object because we can only track it
+  // using sync events and we can't determine it at any point in time.
+  _syncing: false,
+
+  get state() {
+    if (!this._state) {
+      return DEFAULT_STATE;
+    }
+    return Object.assign({}, this._state, { syncing: this._syncing });
+  },
+
+  isReady() {
+    if (!this._initialized) {
+      this.init();
+      return false;
+    }
+    return true;
+  },
+
+  init() {
+    this._initialized = true;
+    // Because the FxA toolbar is usually visible, this module gets loaded at
+    // browser startup, and we want to avoid pulling in all of FxA or Sync at
+    // that time, so we refresh the state after the browser has settled.
+    Services.tm.idleDispatchToMainThread(() => {
+      this.refreshState().catch(e => {
+        console.error(e);
+      });
+    }, 2000);
+  },
+
+  // Used for testing.
+  reset() {
+    this._state = null;
+    this._syncing = false;
+    this._initialized = false;
+  },
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "weave:service:sync:start":
+        this.toggleSyncActivity(true);
+        break;
+      case "weave:service:sync:finish":
+      case "weave:service:sync:error":
+        this.toggleSyncActivity(false);
+        break;
+      default:
+        this.refreshState().catch(e => {
+          console.error(e);
+        });
+        break;
+    }
+  },
+
+  // Builds a new state from scratch.
+  async refreshState() {
+    const newState = {};
+    await this._refreshFxAState(newState);
+    // Optimize the "not signed in" case to avoid refreshing twice just after
+    // startup - if there's currently no _state, and we still aren't configured,
+    // just early exit.
+    if (this._state == null && newState.status == DEFAULT_STATE.status) {
+      return this.state;
+    }
+    if (newState.syncEnabled) {
+      this._setLastSyncTime(newState); // We want this in case we change accounts.
+    }
+    this._state = newState;
+
+    this.notifyStateUpdated();
+    return this.state;
+  },
+
+  // Update the current state with the last sync time/currently syncing status.
+  toggleSyncActivity(syncing) {
+    this._syncing = syncing;
+    this._setLastSyncTime(this._state);
+
+    this.notifyStateUpdated();
+  },
+
+  notifyStateUpdated() {
+    Services.obs.notifyObservers(null, ON_UPDATE);
+  },
+
+  async _refreshFxAState(newState) {
+    let userData = await this._getUserData();
+    await this._populateWithUserData(newState, userData);
+  },
+
+  async _populateWithUserData(state, userData) {
+    let status;
+    let syncUserName = Services.prefs.getStringPref(
+      "services.sync.username",
+      ""
+    );
+    if (!userData) {
+      // If Sync thinks it is configured but there's no FxA user, then we
+      // want to enter the "login failed" state so the user can get
+      // reconfigured.
+      if (syncUserName) {
+        state.email = syncUserName;
+        status = STATUS_LOGIN_FAILED;
+      } else {
+        // everyone agrees nothing is configured.
+        status = STATUS_NOT_CONFIGURED;
+      }
+    } else {
+      let loginFailed = await this._loginFailed();
+      if (loginFailed) {
+        status = STATUS_LOGIN_FAILED;
+      } else if (!userData.verified) {
+        status = STATUS_NOT_VERIFIED;
+      } else {
+        status = STATUS_SIGNED_IN;
+      }
+      state.uid = userData.uid;
+      state.email = userData.email;
+      state.displayName = userData.displayName;
+      // for better or worse, this module renames these attribues.
+      state.avatarURL = userData.avatar;
+      state.avatarIsDefault = userData.avatarDefault;
+      state.syncEnabled = !!syncUserName;
+    }
+    state.status = status;
+  },
+
+  async _getUserData() {
+    try {
+      return await this.fxAccounts.getSignedInUser();
+    } catch (e) {
+      // This is most likely in tests, where we quickly log users in and out.
+      // The most likely scenario is a user logged out, so reflect that.
+      // Bug 995134 calls for better errors so we could retry if we were
+      // sure this was the failure reason.
+      console.error("Error updating FxA account info:", e);
+      return null;
+    }
+  },
+
+  _setLastSyncTime(state) {
+    if (state?.status == UIState.STATUS_SIGNED_IN) {
+      const lastSync = Services.prefs.getStringPref(
+        "services.sync.lastSync",
+        null
+      );
+      state.lastSync = lastSync ? new Date(lastSync) : null;
+    }
+  },
+
+  async _loginFailed() {
+    // First ask FxA if it thinks the user needs re-authentication. In practice,
+    // this check is probably canonical (ie, we probably don't really need
+    // the check below at all as we drop local session info on the first sign
+    // of a problem) - but we keep it for now to keep the risk down.
+    let hasLocalSession = await this.fxAccounts.hasLocalSession();
+    if (!hasLocalSession) {
+      return true;
+    }
+
+    // Referencing Weave.Service will implicitly initialize sync, and we don't
+    // want to force that - so first check if it is ready.
+    let service = Cc["@mozilla.org/weave/service;1"].getService(
+      Ci.nsISupports
+    ).wrappedJSObject;
+    if (!service.ready) {
+      return false;
+    }
+    // LOGIN_FAILED_LOGIN_REJECTED explicitly means "you must log back in".
+    // All other login failures are assumed to be transient and should go
+    // away by themselves, so aren't reflected here.
+    return lazy.Weave.Status.login == lazy.LOGIN_FAILED_LOGIN_REJECTED;
+  },
+
+  set fxAccounts(mockFxAccounts) {
+    delete this.fxAccounts;
+    this.fxAccounts = mockFxAccounts;
+  },
+};
+
+ChromeUtils.defineLazyGetter(UIStateInternal, "fxAccounts", () => {
+  return ChromeUtils.importESModule(
+    "resource://gre/modules/FxAccounts.sys.mjs"
+  ).getFxAccountsSingleton();
+});
+
+for (let topic of TOPICS) {
+  Services.obs.addObserver(UIStateInternal, topic);
+}
+
+export var UIState = {
+  _internal: UIStateInternal,
+
+  ON_UPDATE,
+
+  STATUS_NOT_CONFIGURED,
+  STATUS_LOGIN_FAILED,
+  STATUS_NOT_VERIFIED,
+  STATUS_SIGNED_IN,
+
+  /**
+   * Returns true if the module has been initialized and the state set.
+   * If not, return false and trigger an init in the background.
+   */
+  isReady() {
+    return this._internal.isReady();
+  },
+
+  /**
+   * @returns {UIState} The current Sync/FxA UI State.
+   */
+  get() {
+    return this._internal.state;
+  },
+
+  /**
+   * Refresh the state. Used for testing, don't call this directly since
+   * UIState already listens to Sync/FxA notifications to determine if the state
+   * needs to be refreshed. ON_UPDATE will be fired once the state is refreshed.
+   *
+   * @returns {Promise<UIState>} Resolved once the state is refreshed.
+   */
+  refresh() {
+    return this._internal.refreshState();
+  },
+
+  /**
+   * Reset the state of the whole module. Used for testing.
+   */
+  reset() {
+    this._internal.reset();
+  },
+};
diff --git a/services/sync/modules/addonsreconciler.sys.mjs b/services/sync/modules/addonsreconciler.sys.mjs
new file mode 100644
index 0000000000..902f57348e
--- /dev/null
+++ b/services/sync/modules/addonsreconciler.sys.mjs
@@ -0,0 +1,584 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This file contains middleware to reconcile state of AddonManager for
+ * purposes of tracking events for Sync. The content in this file exists
+ * because AddonManager does not have a getChangesSinceX() API and adding
+ * that functionality properly was deemed too time-consuming at the time
+ * add-on sync was originally written. If/when AddonManager adds this API,
+ * this file can go away and the add-ons engine can be rewritten to use it.
+ *
+ * It was decided to have this tracking functionality exist in a separate
+ * standalone file so it could be more easily understood, tested, and
+ * hopefully ported.
+ */
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import { AddonManager } from "resource://gre/modules/AddonManager.sys.mjs";
+
+const DEFAULT_STATE_FILE = "addonsreconciler";
+
+export var CHANGE_INSTALLED = 1;
+export var CHANGE_UNINSTALLED = 2;
+export var CHANGE_ENABLED = 3;
+export var CHANGE_DISABLED = 4;
+
+/**
+ * Maintains state of add-ons.
+ *
+ * State is maintained in 2 data structures, an object mapping add-on IDs
+ * to metadata and an array of changes over time. The object mapping can be
+ * thought of as a minimal copy of data from AddonManager which is needed for
+ * Sync. The array is effectively a log of changes over time.
+ *
+ * The data structures are persisted to disk by serializing to a JSON file in
+ * the current profile. The data structures are updated by 2 mechanisms. First,
+ * they can be refreshed from the global state of the AddonManager. This is a
+ * sure-fire way of ensuring the reconciler is up to date. Second, the
+ * reconciler adds itself as an AddonManager listener. When it receives change
+ * notifications, it updates its internal state incrementally.
+ *
+ * The internal state is persisted to a JSON file in the profile directory.
+ *
+ * An instance of this is bound to an AddonsEngine instance. In reality, it
+ * likely exists as a singleton. To AddonsEngine, it functions as a store and
+ * an entity which emits events for tracking.
+ *
+ * The usage pattern for instances of this class is:
+ *
+ *   let reconciler = new AddonsReconciler(...);
+ *   await reconciler.ensureStateLoaded();
+ *
+ *   // At this point, your instance should be ready to use.
+ *
+ * When you are finished with the instance, please call:
+ *
+ *   reconciler.stopListening();
+ *   await reconciler.saveState(...);
+ *
+ * This class uses the AddonManager AddonListener interface.
+ * When an add-on is installed, listeners are called in the following order:
+ *  AL.onInstalling, AL.onInstalled
+ *
+ * For uninstalls, we see AL.onUninstalling then AL.onUninstalled.
+ *
+ * Enabling and disabling work by sending:
+ *
+ *   AL.onEnabling, AL.onEnabled
+ *   AL.onDisabling, AL.onDisabled
+ *
+ * Actions can be undone. All undoable actions notify the same
+ * AL.onOperationCancelled event. We treat this event like any other.
+ *
+ * When an add-on is uninstalled from about:addons, the user is offered an
+ * "Undo" option, which leads to the following sequence of events as
+ * observed by an AddonListener:
+ * Add-ons are first disabled then they are actually uninstalled. So, we will
+ * see AL.onDisabling and AL.onDisabled. The onUninstalling and onUninstalled
+ * events only come after the Addon Manager is closed or another view is
+ * switched to. In the case of Sync performing the uninstall, the uninstall
+ * events will occur immediately. However, we still see disabling events and
+ * heed them like they were normal. In the end, the state is proper.
+ */
+export function AddonsReconciler(queueCaller) {
+  this._log = Log.repository.getLogger("Sync.AddonsReconciler");
+  this._log.manageLevelFromPref("services.sync.log.logger.addonsreconciler");
+  this.queueCaller = queueCaller;
+
+  Svc.Obs.add("xpcom-shutdown", this.stopListening, this);
+}
+
+AddonsReconciler.prototype = {
+  /** Flag indicating whether we are listening to AddonManager events. */
+  _listening: false,
+
+  /**
+   * Define this as false if the reconciler should not persist state
+   * to disk when handling events.
+   *
+   * This allows test code to avoid spinning to write during observer
+   * notifications and xpcom shutdown, which appears to cause hangs on WinXP
+   * (Bug 873861).
+   */
+  _shouldPersist: true,
+
+  /** Log logger instance */
+  _log: null,
+
+  /**
+   * Container for add-on metadata.
+   *
+   * Keys are add-on IDs. Values are objects which describe the state of the
+   * add-on. This is a minimal mirror of data that can be queried from
+   * AddonManager. In some cases, we retain data longer than AddonManager.
+   */
+  _addons: {},
+
+  /**
+   * List of add-on changes over time.
+   *
+   * Each element is an array of [time, change, id].
+   */
+  _changes: [],
+
+  /**
+   * Objects subscribed to changes made to this instance.
+   */
+  _listeners: [],
+
+  /**
+   * Accessor for add-ons in this object.
+   *
+   * Returns an object mapping add-on IDs to objects containing metadata.
+   */
+  get addons() {
+    return this._addons;
+  },
+
+  async ensureStateLoaded() {
+    if (!this._promiseStateLoaded) {
+      this._promiseStateLoaded = this.loadState();
+    }
+    return this._promiseStateLoaded;
+  },
+
+  /**
+   * Load reconciler state from a file.
+   *
+   * The path is relative to the weave directory in the profile. If no
+   * path is given, the default one is used.
+   *
+   * If the file does not exist or there was an error parsing the file, the
+   * state will be transparently defined as empty.
+   *
+   * @param file
+   *        Path to load. ".json" is appended automatically. If not defined,
+   *        a default path will be consulted.
+   */
+  async loadState(file = DEFAULT_STATE_FILE) {
+    let json = await Utils.jsonLoad(file, this);
+    this._addons = {};
+    this._changes = [];
+
+    if (!json) {
+      this._log.debug("No data seen in loaded file: " + file);
+      return false;
+    }
+
+    let version = json.version;
+    if (!version || version != 1) {
+      this._log.error(
+        "Could not load JSON file because version not " +
+          "supported: " +
+          version
+      );
+      return false;
+    }
+
+    this._addons = json.addons;
+    for (let id in this._addons) {
+      let record = this._addons[id];
+      record.modified = new Date(record.modified);
+    }
+
+    for (let [time, change, id] of json.changes) {
+      this._changes.push([new Date(time), change, id]);
+    }
+
+    return true;
+  },
+
+  /**
+   * Saves the current state to a file in the local profile.
+   *
+   * @param  file
+   *         String path in profile to save to. If not defined, the default
+   *         will be used.
+   */
+  async saveState(file = DEFAULT_STATE_FILE) {
+    let state = { version: 1, addons: {}, changes: [] };
+
+    for (let [id, record] of Object.entries(this._addons)) {
+      state.addons[id] = {};
+      for (let [k, v] of Object.entries(record)) {
+        if (k == "modified") {
+          state.addons[id][k] = v.getTime();
+        } else {
+          state.addons[id][k] = v;
+        }
+      }
+    }
+
+    for (let [time, change, id] of this._changes) {
+      state.changes.push([time.getTime(), change, id]);
+    }
+
+    this._log.info("Saving reconciler state to file: " + file);
+    await Utils.jsonSave(file, this, state);
+  },
+
+  /**
+   * Registers a change listener with this instance.
+   *
+   * Change listeners are called every time a change is recorded. The listener
+   * is an object with the function "changeListener" that takes 3 arguments,
+   * the Date at which the change happened, the type of change (a CHANGE_*
+   * constant), and the add-on state object reflecting the current state of
+   * the add-on at the time of the change.
+   *
+   * @param listener
+   *        Object containing changeListener function.
+   */
+  addChangeListener: function addChangeListener(listener) {
+    if (!this._listeners.includes(listener)) {
+      this._log.debug("Adding change listener.");
+      this._listeners.push(listener);
+    }
+  },
+
+  /**
+   * Removes a previously-installed change listener from the instance.
+   *
+   * @param listener
+   *        Listener instance to remove.
+   */
+  removeChangeListener: function removeChangeListener(listener) {
+    this._listeners = this._listeners.filter(element => {
+      if (element == listener) {
+        this._log.debug("Removing change listener.");
+        return false;
+      }
+      return true;
+    });
+  },
+
+  /**
+   * Tells the instance to start listening for AddonManager changes.
+   *
+   * This is typically called automatically when Sync is loaded.
+   */
+  startListening: function startListening() {
+    if (this._listening) {
+      return;
+    }
+
+    this._log.info("Registering as Add-on Manager listener.");
+    AddonManager.addAddonListener(this);
+    this._listening = true;
+  },
+
+  /**
+   * Tells the instance to stop listening for AddonManager changes.
+   *
+   * The reconciler should always be listening. This should only be called when
+   * the instance is being destroyed.
+   *
+   * This function will get called automatically on XPCOM shutdown. However, it
+   * is a best practice to call it yourself.
+   */
+  stopListening: function stopListening() {
+    if (!this._listening) {
+      return;
+    }
+
+    this._log.debug("Stopping listening and removing AddonManager listener.");
+    AddonManager.removeAddonListener(this);
+    this._listening = false;
+  },
+
+  /**
+   * Refreshes the global state of add-ons by querying the AddonManager.
+   */
+  async refreshGlobalState() {
+    this._log.info("Refreshing global state from AddonManager.");
+
+    let installs;
+    let addons = await AddonManager.getAllAddons();
+
+    let ids = {};
+
+    for (let addon of addons) {
+      ids[addon.id] = true;
+      await this.rectifyStateFromAddon(addon);
+    }
+
+    // Look for locally-defined add-ons that no longer exist and update their
+    // record.
+    for (let [id, addon] of Object.entries(this._addons)) {
+      if (id in ids) {
+        continue;
+      }
+
+      // If the id isn't in ids, it means that the add-on has been deleted or
+      // the add-on is in the process of being installed. We detect the
+      // latter by seeing if an AddonInstall is found for this add-on.
+
+      if (!installs) {
+        installs = await AddonManager.getAllInstalls();
+      }
+
+      let installFound = false;
+      for (let install of installs) {
+        if (
+          install.addon &&
+          install.addon.id == id &&
+          install.state == AddonManager.STATE_INSTALLED
+        ) {
+          installFound = true;
+          break;
+        }
+      }
+
+      if (installFound) {
+        continue;
+      }
+
+      if (addon.installed) {
+        addon.installed = false;
+        this._log.debug(
+          "Adding change because add-on not present in " +
+            "Add-on Manager: " +
+            id
+        );
+        await this._addChange(new Date(), CHANGE_UNINSTALLED, addon);
+      }
+    }
+
+    // See note for _shouldPersist.
+    if (this._shouldPersist) {
+      await this.saveState();
+    }
+  },
+
+  /**
+   * Rectifies the state of an add-on from an Addon instance.
+   *
+   * This basically says "given an Addon instance, assume it is truth and
+   * apply changes to the local state to reflect it."
+   *
+   * This function could result in change listeners being called if the local
+   * state differs from the passed add-on's state.
+   *
+   * @param addon
+   *        Addon instance being updated.
+   */
+  async rectifyStateFromAddon(addon) {
+    this._log.debug(
+      `Rectifying state for addon ${addon.name} (version=${addon.version}, id=${addon.id})`
+    );
+
+    let id = addon.id;
+    let enabled = !addon.userDisabled;
+    let guid = addon.syncGUID;
+    let now = new Date();
+
+    if (!(id in this._addons)) {
+      let record = {
+        id,
+        guid,
+        enabled,
+        installed: true,
+        modified: now,
+        type: addon.type,
+        scope: addon.scope,
+        foreignInstall: addon.foreignInstall,
+        isSyncable: addon.isSyncable,
+      };
+      this._addons[id] = record;
+      this._log.debug(
+        "Adding change because add-on not present locally: " + id
+      );
+      await this._addChange(now, CHANGE_INSTALLED, record);
+      return;
+    }
+
+    let record = this._addons[id];
+    record.isSyncable = addon.isSyncable;
+
+    if (!record.installed) {
+      // It is possible the record is marked as uninstalled because an
+      // uninstall is pending.
+      if (!(addon.pendingOperations & AddonManager.PENDING_UNINSTALL)) {
+        record.installed = true;
+        record.modified = now;
+      }
+    }
+
+    if (record.enabled != enabled) {
+      record.enabled = enabled;
+      record.modified = now;
+      let change = enabled ? CHANGE_ENABLED : CHANGE_DISABLED;
+      this._log.debug("Adding change because enabled state changed: " + id);
+      await this._addChange(new Date(), change, record);
+    }
+
+    if (record.guid != guid) {
+      record.guid = guid;
+      // We don't record a change because the Sync engine rectifies this on its
+      // own. This is tightly coupled with Sync. If this code is ever lifted
+      // outside of Sync, this exception should likely be removed.
+    }
+  },
+
+  /**
+   * Record a change in add-on state.
+   *
+   * @param date
+   *        Date at which the change occurred.
+   * @param change
+   *        The type of the change. A CHANGE_* constant.
+   * @param state
+   *        The new state of the add-on. From this.addons.
+   */
+  async _addChange(date, change, state) {
+    this._log.info("Change recorded for " + state.id);
+    this._changes.push([date, change, state.id]);
+
+    for (let listener of this._listeners) {
+      try {
+        await listener.changeListener(date, change, state);
+      } catch (ex) {
+        this._log.error("Exception calling change listener", ex);
+      }
+    }
+  },
+
+  /**
+   * Obtain the set of changes to add-ons since the date passed.
+   *
+   * This will return an array of arrays. Each entry in the array has the
+   * elements [date, change_type, id], where
+   *
+   *   date - Date instance representing when the change occurred.
+   *   change_type - One of CHANGE_* constants.
+   *   id - ID of add-on that changed.
+   */
+  getChangesSinceDate(date) {
+    let length = this._changes.length;
+    for (let i = 0; i < length; i++) {
+      if (this._changes[i][0] >= date) {
+        return this._changes.slice(i);
+      }
+    }
+
+    return [];
+  },
+
+  /**
+   * Prunes all recorded changes from before the specified Date.
+   *
+   * @param date
+   *        Entries older than this Date will be removed.
+   */
+  pruneChangesBeforeDate(date) {
+    this._changes = this._changes.filter(function test_age(change) {
+      return change[0] >= date;
+    });
+  },
+
+  /**
+   * Obtains the set of all known Sync GUIDs for add-ons.
+   */
+  getAllSyncGUIDs() {
+    let result = {};
+    for (let id in this.addons) {
+      result[id] = true;
+    }
+
+    return result;
+  },
+
+  /**
+   * Obtain the add-on state record for an add-on by Sync GUID.
+   *
+   * If the add-on could not be found, returns null.
+   *
+   * @param  guid
+   *         Sync GUID of add-on to retrieve.
+   */
+  getAddonStateFromSyncGUID(guid) {
+    for (let id in this.addons) {
+      let addon = this.addons[id];
+      if (addon.guid == guid) {
+        return addon;
+      }
+    }
+
+    return null;
+  },
+
+  /**
+   * Handler that is invoked as part of the AddonManager listeners.
+   */
+  async _handleListener(action, addon) {
+    // Since this is called as an observer, we explicitly trap errors and
+    // log them to ourselves so we don't see errors reported elsewhere.
+    try {
+      let id = addon.id;
+      this._log.debug("Add-on change: " + action + " to " + id);
+
+      switch (action) {
+        case "onEnabled":
+        case "onDisabled":
+        case "onInstalled":
+        case "onInstallEnded":
+        case "onOperationCancelled":
+          await this.rectifyStateFromAddon(addon);
+          break;
+
+        case "onUninstalled":
+          let id = addon.id;
+          let addons = this.addons;
+          if (id in addons) {
+            let now = new Date();
+            let record = addons[id];
+            record.installed = false;
+            record.modified = now;
+            this._log.debug(
+              "Adding change because of uninstall listener: " + id
+            );
+            await this._addChange(now, CHANGE_UNINSTALLED, record);
+          }
+      }
+
+      // See note for _shouldPersist.
+      if (this._shouldPersist) {
+        await this.saveState();
+      }
+    } catch (ex) {
+      this._log.warn("Exception", ex);
+    }
+  },
+
+  // AddonListeners
+  onEnabled: function onEnabled(addon) {
+    this.queueCaller.enqueueCall(() =>
+      this._handleListener("onEnabled", addon)
+    );
+  },
+  onDisabled: function onDisabled(addon) {
+    this.queueCaller.enqueueCall(() =>
+      this._handleListener("onDisabled", addon)
+    );
+  },
+  onInstalled: function onInstalled(addon) {
+    this.queueCaller.enqueueCall(() =>
+      this._handleListener("onInstalled", addon)
+    );
+  },
+  onUninstalled: function onUninstalled(addon) {
+    this.queueCaller.enqueueCall(() =>
+      this._handleListener("onUninstalled", addon)
+    );
+  },
+  onOperationCancelled: function onOperationCancelled(addon) {
+    this.queueCaller.enqueueCall(() =>
+      this._handleListener("onOperationCancelled", addon)
+    );
+  },
+};
diff --git a/services/sync/modules/addonutils.sys.mjs b/services/sync/modules/addonutils.sys.mjs
new file mode 100644
index 0000000000..08a8c5b5f0
--- /dev/null
+++ b/services/sync/modules/addonutils.sys.mjs
@@ -0,0 +1,391 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Svc } from "resource://services-sync/util.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+  AddonRepository: "resource://gre/modules/addons/AddonRepository.sys.mjs",
+});
+
+function AddonUtilsInternal() {
+  this._log = Log.repository.getLogger("Sync.AddonUtils");
+  this._log.Level =
+    Log.Level[Svc.PrefBranch.getStringPref("log.logger.addonutils", null)];
+}
+AddonUtilsInternal.prototype = {
+  /**
+   * Obtain an AddonInstall object from an AddonSearchResult instance.
+   *
+   * The returned promise will be an AddonInstall on success or null (failure or
+   * addon not found)
+   *
+   * @param addon
+   *        AddonSearchResult to obtain install from.
+   */
+  getInstallFromSearchResult(addon) {
+    this._log.debug("Obtaining install for " + addon.id);
+
+    // We should theoretically be able to obtain (and use) addon.install if
+    // it is available. However, the addon.sourceURI rewriting won't be
+    // reflected in the AddonInstall, so we can't use it. If we ever get rid
+    // of sourceURI rewriting, we can avoid having to reconstruct the
+    // AddonInstall.
+    return lazy.AddonManager.getInstallForURL(addon.sourceURI.spec, {
+      name: addon.name,
+      icons: addon.iconURL,
+      version: addon.version,
+      telemetryInfo: { source: "sync" },
+    });
+  },
+
+  /**
+   * Installs an add-on from an AddonSearchResult instance.
+   *
+   * The options argument defines extra options to control the install.
+   * Recognized keys in this map are:
+   *
+   *   syncGUID - Sync GUID to use for the new add-on.
+   *   enabled - Boolean indicating whether the add-on should be enabled upon
+   *             install.
+   *
+   * The result object has the following keys:
+   *
+   *   id      ID of add-on that was installed.
+   *   install AddonInstall that was installed.
+   *   addon   Addon that was installed.
+   *
+   * @param addon
+   *        AddonSearchResult to install add-on from.
+   * @param options
+   *        Object with additional metadata describing how to install add-on.
+   */
+  async installAddonFromSearchResult(addon, options) {
+    this._log.info("Trying to install add-on from search result: " + addon.id);
+
+    const install = await this.getInstallFromSearchResult(addon);
+    if (!install) {
+      throw new Error("AddonInstall not available: " + addon.id);
+    }
+
+    try {
+      this._log.info("Installing " + addon.id);
+      let log = this._log;
+
+      return new Promise((res, rej) => {
+        let listener = {
+          onInstallStarted: function onInstallStarted(install) {
+            if (!options) {
+              return;
+            }
+
+            if (options.syncGUID) {
+              log.info(
+                "Setting syncGUID of " + install.name + ": " + options.syncGUID
+              );
+              install.addon.syncGUID = options.syncGUID;
+            }
+
+            // We only need to change userDisabled if it is disabled because
+            // enabled is the default.
+            if ("enabled" in options && !options.enabled) {
+              log.info(
+                "Marking add-on as disabled for install: " + install.name
+              );
+              install.addon.disable();
+            }
+          },
+          onInstallEnded(install, addon) {
+            install.removeListener(listener);
+
+            res({ id: addon.id, install, addon });
+          },
+          onInstallFailed(install) {
+            install.removeListener(listener);
+
+            rej(new Error("Install failed: " + install.error));
+          },
+          onDownloadFailed(install) {
+            install.removeListener(listener);
+
+            rej(new Error("Download failed: " + install.error));
+          },
+        };
+        install.addListener(listener);
+        install.install();
+      });
+    } catch (ex) {
+      this._log.error("Error installing add-on", ex);
+      throw ex;
+    }
+  },
+
+  /**
+   * Uninstalls the addon instance.
+   *
+   * @param addon
+   *        Addon instance to uninstall.
+   */
+  async uninstallAddon(addon) {
+    return new Promise(res => {
+      let listener = {
+        onUninstalling(uninstalling, needsRestart) {
+          if (addon.id != uninstalling.id) {
+            return;
+          }
+
+          // We assume restartless add-ons will send the onUninstalled event
+          // soon.
+          if (!needsRestart) {
+            return;
+          }
+
+          // For non-restartless add-ons, we issue the callback on uninstalling
+          // because we will likely never see the uninstalled event.
+          lazy.AddonManager.removeAddonListener(listener);
+          res(addon);
+        },
+        onUninstalled(uninstalled) {
+          if (addon.id != uninstalled.id) {
+            return;
+          }
+
+          lazy.AddonManager.removeAddonListener(listener);
+          res(addon);
+        },
+      };
+      lazy.AddonManager.addAddonListener(listener);
+      addon.uninstall();
+    });
+  },
+
+  /**
+   * Installs multiple add-ons specified by metadata.
+   *
+   * The first argument is an array of objects. Each object must have the
+   * following keys:
+   *
+   *   id - public ID of the add-on to install.
+   *   syncGUID - syncGUID for new add-on.
+   *   enabled - boolean indicating whether the add-on should be enabled.
+   *   requireSecureURI - Boolean indicating whether to require a secure
+   *     URI when installing from a remote location. This defaults to
+   *     true.
+   *
+   * The callback will be called when activity on all add-ons is complete. The
+   * callback receives 2 arguments, error and result.
+   *
+   * If error is truthy, it contains a string describing the overall error.
+   *
+   * The 2nd argument to the callback is always an object with details on the
+   * overall execution state. It contains the following keys:
+   *
+   *   installedIDs  Array of add-on IDs that were installed.
+   *   installs      Array of AddonInstall instances that were installed.
+   *   addons        Array of Addon instances that were installed.
+   *   errors        Array of errors encountered. Only has elements if error is
+   *                 truthy.
+   *
+   * @param installs
+   *        Array of objects describing add-ons to install.
+   */
+  async installAddons(installs) {
+    let ids = [];
+    for (let addon of installs) {
+      ids.push(addon.id);
+    }
+
+    let addons = await lazy.AddonRepository.getAddonsByIDs(ids);
+    this._log.info(
+      `Found ${addons.length} / ${ids.length}` +
+        " add-ons during repository search."
+    );
+
+    let ourResult = {
+      installedIDs: [],
+      installs: [],
+      addons: [],
+      skipped: [],
+      errors: [],
+    };
+
+    let toInstall = [];
+
+    // Rewrite the "src" query string parameter of the source URI to note
+    // that the add-on was installed by Sync and not something else so
+    // server-side metrics aren't skewed (bug 708134). The server should
+    // ideally send proper URLs, but this solution was deemed too
+    // complicated at the time the functionality was implemented.
+    for (let addon of addons) {
+      // Find the specified options for this addon.
+      let options;
+      for (let install of installs) {
+        if (install.id == addon.id) {
+          options = install;
+          break;
+        }
+      }
+      if (!this.canInstallAddon(addon, options)) {
+        ourResult.skipped.push(addon.id);
+        continue;
+      }
+
+      // We can go ahead and attempt to install it.
+      toInstall.push(addon);
+
+      // We should always be able to QI the nsIURI to nsIURL. If not, we
+      // still try to install the add-on, but we don't rewrite the URL,
+      // potentially skewing metrics.
+      try {
+        addon.sourceURI.QueryInterface(Ci.nsIURL);
+      } catch (ex) {
+        this._log.warn(
+          "Unable to QI sourceURI to nsIURL: " + addon.sourceURI.spec
+        );
+        continue;
+      }
+
+      let params = addon.sourceURI.query
+        .split("&")
+        .map(function rewrite(param) {
+          if (param.indexOf("src=") == 0) {
+            return "src=sync";
+          }
+          return param;
+        });
+
+      addon.sourceURI = addon.sourceURI
+        .mutate()
+        .setQuery(params.join("&"))
+        .finalize();
+    }
+
+    if (!toInstall.length) {
+      return ourResult;
+    }
+
+    const installPromises = [];
+    // Start all the installs asynchronously. They will report back to us
+    // as they finish, eventually triggering the global callback.
+    for (let addon of toInstall) {
+      let options = {};
+      for (let install of installs) {
+        if (install.id == addon.id) {
+          options = install;
+          break;
+        }
+      }
+
+      installPromises.push(
+        (async () => {
+          try {
+            const result = await this.installAddonFromSearchResult(
+              addon,
+              options
+            );
+            ourResult.installedIDs.push(result.id);
+            ourResult.installs.push(result.install);
+            ourResult.addons.push(result.addon);
+          } catch (error) {
+            ourResult.errors.push(error);
+          }
+        })()
+      );
+    }
+
+    await Promise.all(installPromises);
+
+    if (ourResult.errors.length) {
+      throw new Error("1 or more add-ons failed to install");
+    }
+    return ourResult;
+  },
+
+  /**
+   * Returns true if we are able to install the specified addon, false
+   * otherwise. It is expected that this will log the reason if it returns
+   * false.
+   *
+   * @param addon
+   *        (Addon) Add-on instance to check.
+   * @param options
+   *        (object) The options specified for this addon. See installAddons()
+   *        for the valid elements.
+   */
+  canInstallAddon(addon, options) {
+    // sourceURI presence isn't enforced by AddonRepository. So, we skip
+    // add-ons without a sourceURI.
+    if (!addon.sourceURI) {
+      this._log.info(
+        "Skipping install of add-on because missing sourceURI: " + addon.id
+      );
+      return false;
+    }
+    // Verify that the source URI uses TLS. We don't allow installs from
+    // insecure sources for security reasons. The Addon Manager ensures
+    // that cert validation etc is performed.
+    // (We should also consider just dropping this entirely and calling
+    // XPIProvider.isInstallAllowed, but that has additional semantics we might
+    // need to think through...)
+    let requireSecureURI = true;
+    if (options && options.requireSecureURI !== undefined) {
+      requireSecureURI = options.requireSecureURI;
+    }
+
+    if (requireSecureURI) {
+      let scheme = addon.sourceURI.scheme;
+      if (scheme != "https") {
+        this._log.info(
+          `Skipping install of add-on "${addon.id}" because sourceURI's scheme of "${scheme}" is not trusted`
+        );
+        return false;
+      }
+    }
+
+    // Policy prevents either installing this addon or any addon
+    if (
+      Services.policies &&
+      (!Services.policies.mayInstallAddon(addon) ||
+        !Services.policies.isAllowed("xpinstall"))
+    ) {
+      this._log.info(
+        `Skipping install of "${addon.id}" due to enterprise policy`
+      );
+      return false;
+    }
+
+    this._log.info(`Add-on "${addon.id}" is able to be installed`);
+    return true;
+  },
+
+  /**
+   * Update the user disabled flag for an add-on.
+   *
+   * If the new flag matches the existing or if the add-on
+   * isn't currently active, the function will return immediately.
+   *
+   * @param addon
+   *        (Addon) Add-on instance to operate on.
+   * @param value
+   *        (bool) New value for add-on's userDisabled property.
+   */
+  updateUserDisabled(addon, value) {
+    if (addon.userDisabled == value) {
+      return;
+    }
+
+    this._log.info("Updating userDisabled flag: " + addon.id + " -> " + value);
+    if (value) {
+      addon.disable();
+    } else {
+      addon.enable();
+    }
+  },
+};
+
+export const AddonUtils = new AddonUtilsInternal();
diff --git a/services/sync/modules/bridged_engine.sys.mjs b/services/sync/modules/bridged_engine.sys.mjs
new file mode 100644
index 0000000000..45e5f685cd
--- /dev/null
+++ b/services/sync/modules/bridged_engine.sys.mjs
@@ -0,0 +1,499 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This file has all the machinery for hooking up bridged engines implemented
+ * in Rust. It's the JavaScript side of the Golden Gate bridge that connects
+ * Desktop Sync to a Rust `BridgedEngine`, via the `mozIBridgedSyncEngine`
+ * XPCOM interface.
+ *
+ * Creating a bridged engine only takes a few lines of code, since most of the
+ * hard work is done on the Rust side. On the JS side, you'll need to subclass
+ * `BridgedEngine` (instead of `SyncEngine`), supply a `mozIBridgedSyncEngine`
+ * for your subclass to wrap, and optionally implement and override the tracker.
+ */
+
+import { SyncEngine, Tracker } from "resource://services-sync/engines.sys.mjs";
+import { RawCryptoWrapper } from "resource://services-sync/record.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "resource://gre/modules/Log.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+});
+
+/**
+ * A stub store that converts between raw decrypted incoming records and
+ * envelopes. Since the interface we need is so minimal, this class doesn't
+ * inherit from the base `Store` implementation...it would take more code to
+ * override all those behaviors!
+ *
+ * This class isn't meant to be subclassed, because bridged engines shouldn't
+ * override their store classes in `_storeObj`.
+ */
+class BridgedStore {
+  constructor(name, engine) {
+    if (!engine) {
+      throw new Error("Store must be associated with an Engine instance.");
+    }
+    this.engine = engine;
+    this._log = lazy.Log.repository.getLogger(`Sync.Engine.${name}.Store`);
+    this._batchChunkSize = 500;
+  }
+
+  async applyIncomingBatch(records, countTelemetry) {
+    for (let chunk of lazy.PlacesUtils.chunkArray(
+      records,
+      this._batchChunkSize
+    )) {
+      let incomingEnvelopesAsJSON = chunk.map(record =>
+        JSON.stringify(record.toIncomingBso())
+      );
+      this._log.trace("incoming envelopes", incomingEnvelopesAsJSON);
+      await this.engine._bridge.storeIncoming(incomingEnvelopesAsJSON);
+    }
+    // Array of failed records.
+    return [];
+  }
+
+  async wipe() {
+    await this.engine._bridge.wipe();
+  }
+}
+
+/**
+ * A wrapper class to convert between BSOs on the JS side, and envelopes on the
+ * Rust side. This class intentionally subclasses `RawCryptoWrapper`, because we
+ * don't want the stringification and parsing machinery in `CryptoWrapper`.
+ *
+ * This class isn't meant to be subclassed, because bridged engines shouldn't
+ * override their record classes in `_recordObj`.
+ */
+class BridgedRecord extends RawCryptoWrapper {
+  /**
+   * Creates an outgoing record from a BSO returned by a bridged engine.
+   *
+   * @param  {String} collection The collection name.
+   * @param  {Object} bso   The outgoing bso (ie, a sync15::bso::OutgoingBso) returned from
+   *                        `mozIBridgedSyncEngine::apply`.
+   * @return {BridgedRecord}     A Sync record ready to encrypt and upload.
+   */
+  static fromOutgoingBso(collection, bso) {
+    // The BSO has already been JSON serialized coming out of Rust, so the
+    // envelope has been flattened.
+    if (typeof bso.id != "string") {
+      throw new TypeError("Outgoing BSO missing ID");
+    }
+    if (typeof bso.payload != "string") {
+      throw new TypeError("Outgoing BSO missing payload");
+    }
+    let record = new BridgedRecord(collection, bso.id);
+    record.cleartext = bso.payload;
+    return record;
+  }
+
+  transformBeforeEncrypt(cleartext) {
+    if (typeof cleartext != "string") {
+      throw new TypeError("Outgoing bridged engine records must be strings");
+    }
+    return cleartext;
+  }
+
+  transformAfterDecrypt(cleartext) {
+    if (typeof cleartext != "string") {
+      throw new TypeError("Incoming bridged engine records must be strings");
+    }
+    return cleartext;
+  }
+
+  /*
+   * Converts this incoming record into an envelope to pass to a bridged engine.
+   * This object must be kept in sync with `sync15::IncomingBso`.
+   *
+   * @return {Object} The incoming envelope, to pass to
+   *                  `mozIBridgedSyncEngine::storeIncoming`.
+   */
+  toIncomingBso() {
+    return {
+      id: this.data.id,
+      modified: this.data.modified,
+      payload: this.cleartext,
+    };
+  }
+}
+
+class BridgeError extends Error {
+  constructor(code, message) {
+    super(message);
+    this.name = "BridgeError";
+    // TODO: We may want to use a different name for this, since errors with
+    // a `result` property are treated specially by telemetry, discarding the
+    // message...but, unlike other `nserror`s, the message is actually useful,
+    // and we still want to capture it.
+    this.result = code;
+  }
+}
+
+class InterruptedError extends Error {
+  constructor(message) {
+    super(message);
+    this.name = "InterruptedError";
+  }
+}
+
+/**
+ * Adapts a `Log.sys.mjs` logger to a `mozIServicesLogSink`. This class is copied
+ * from `SyncedBookmarksMirror.jsm`.
+ */
+export class LogAdapter {
+  constructor(log) {
+    this.log = log;
+  }
+
+  get maxLevel() {
+    let level = this.log.level;
+    if (level <= lazy.Log.Level.All) {
+      return Ci.mozIServicesLogSink.LEVEL_TRACE;
+    }
+    if (level <= lazy.Log.Level.Info) {
+      return Ci.mozIServicesLogSink.LEVEL_DEBUG;
+    }
+    if (level <= lazy.Log.Level.Warn) {
+      return Ci.mozIServicesLogSink.LEVEL_WARN;
+    }
+    if (level <= lazy.Log.Level.Error) {
+      return Ci.mozIServicesLogSink.LEVEL_ERROR;
+    }
+    return Ci.mozIServicesLogSink.LEVEL_OFF;
+  }
+
+  trace(message) {
+    this.log.trace(message);
+  }
+
+  debug(message) {
+    this.log.debug(message);
+  }
+
+  warn(message) {
+    this.log.warn(message);
+  }
+
+  error(message) {
+    this.log.error(message);
+  }
+}
+
+// This converts the XPCOM-defined, callback-based mozIBridgedSyncEngine to
+// a promise-based implementation.
+export class BridgeWrapperXPCOM {
+  constructor(component) {
+    this.comp = component;
+  }
+
+  // A few sync, non-callback based attributes.
+  get storageVersion() {
+    return this.comp.storageVersion;
+  }
+
+  get allowSkippedRecord() {
+    return this.comp.allowSkippedRecord;
+  }
+
+  get logger() {
+    return this.comp.logger;
+  }
+
+  // And the async functions we promisify.
+  // Note this is `lastSync` via uniffi but `getLastSync` via xpcom
+  lastSync() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.getLastSync);
+  }
+
+  setLastSync(lastSyncMillis) {
+    return BridgeWrapperXPCOM.#promisify(this.comp.setLastSync, lastSyncMillis);
+  }
+
+  getSyncId() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.getSyncId);
+  }
+
+  resetSyncId() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.resetSyncId);
+  }
+
+  ensureCurrentSyncId(newSyncId) {
+    return BridgeWrapperXPCOM.#promisify(
+      this.comp.ensureCurrentSyncId,
+      newSyncId
+    );
+  }
+
+  syncStarted() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.syncStarted);
+  }
+
+  storeIncoming(incomingEnvelopesAsJSON) {
+    return BridgeWrapperXPCOM.#promisify(
+      this.comp.storeIncoming,
+      incomingEnvelopesAsJSON
+    );
+  }
+
+  apply() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.apply);
+  }
+
+  setUploaded(newTimestampMillis, uploadedIds) {
+    return BridgeWrapperXPCOM.#promisify(
+      this.comp.setUploaded,
+      newTimestampMillis,
+      uploadedIds
+    );
+  }
+
+  syncFinished() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.syncFinished);
+  }
+
+  reset() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.reset);
+  }
+
+  wipe() {
+    return BridgeWrapperXPCOM.#promisify(this.comp.wipe);
+  }
+
+  // Converts a XPCOM bridged function that takes a callback into one that returns a
+  // promise.
+  static #promisify(func, ...params) {
+    return new Promise((resolve, reject) => {
+      func(...params, {
+        // This object implicitly implements all three callback interfaces
+        // (`mozIBridgedSyncEngine{Apply, Result}Callback`), because they have
+        // the same methods. The only difference is the type of the argument
+        // passed to `handleSuccess`, which doesn't matter in JS.
+        handleSuccess: resolve,
+        handleError(code, message) {
+          reject(transformError(code, message));
+        },
+      });
+    });
+  }
+}
+
+/**
+ * A base class used to plug a Rust engine into Sync, and have it work like any
+ * other engine. The constructor takes a bridge as its first argument, which is
+ * a "bridged sync engine", as defined by UniFFI in the application-services
+ * crate.
+ * For backwards compatibility, this can also be an instance of an XPCOM
+ * component class that implements `mozIBridgedSyncEngine`, wrapped in
+ * a `BridgeWrapperXPCOM` wrapper.
+ * (Note that at time of writing, the above is slightly aspirational; the
+ * actual definition of the UniFFI shared bridged engine is still in flux.)
+ *
+ * This class inherits from `SyncEngine`, which has a lot of machinery that we
+ * don't need, but that's fairly easy to override. It would be harder to
+ * reimplement the machinery that we _do_ need here. However, because of that,
+ * this class has lots of methods that do nothing, or return empty data. The
+ * docs above each method explain what it's overriding, and why.
+ *
+ * This class is designed to be subclassed, but the only part that your engine
+ * may want to override is `_trackerObj`. Even then, using the default (no-op)
+ * tracker is fine, because the shape of the `Tracker` interface may not make
+ * sense for all engines.
+ */
+export function BridgedEngine(name, service) {
+  SyncEngine.call(this, name, service);
+}
+
+BridgedEngine.prototype = {
+  /**
+   * The Rust implemented bridge. Must be set by the engine which subclasses us.
+   */
+  _bridge: null,
+  /**
+   * The tracker class for this engine. Subclasses may want to override this
+   * with their own tracker, though using the default `Tracker` is fine.
+   */
+  _trackerObj: Tracker,
+
+  /** Returns the record class for all bridged engines. */
+  get _recordObj() {
+    return BridgedRecord;
+  },
+
+  set _recordObj(obj) {
+    throw new TypeError("Don't override the record class for bridged engines");
+  },
+
+  /** Returns the store class for all bridged engines. */
+  get _storeObj() {
+    return BridgedStore;
+  },
+
+  set _storeObj(obj) {
+    throw new TypeError("Don't override the store class for bridged engines");
+  },
+
+  /** Returns the storage version for this engine. */
+  get version() {
+    return this._bridge.storageVersion;
+  },
+
+  // Legacy engines allow sync to proceed if some records are too large to
+  // upload (eg, a payload that's bigger than the server's published limits).
+  // If this returns true, we will just skip the record without even attempting
+  // to upload. If this is false, we'll abort the entire batch.
+  // If the engine allows this, it will need to detect this scenario by noticing
+  // the ID is not in the 'success' records reported to `setUploaded`.
+  // (Note that this is not to be confused with the fact server's can currently
+  // reject records as part of a POST - but we hope to remove this ability from
+  // the server API. Note also that this is not bullet-proof - if the count of
+  // records is high, it's possible that we will have committed a previous
+  // batch before we hit the relevant limits, so things might have been written.
+  // We hope to fix this by ensuring batch limits are such that this is
+  // impossible)
+  get allowSkippedRecord() {
+    return this._bridge.allowSkippedRecord;
+  },
+
+  /**
+   * Returns the sync ID for this engine. This is exposed for tests, but
+   * Sync code always calls `resetSyncID()` and `ensureCurrentSyncID()`,
+   * not this.
+   *
+   * @returns {String?} The sync ID, or `null` if one isn't set.
+   */
+  async getSyncID() {
+    // Note that all methods on an XPCOM class instance are automatically bound,
+    // so we don't need to write `this._bridge.getSyncId.bind(this._bridge)`.
+    let syncID = await this._bridge.getSyncId();
+    return syncID;
+  },
+
+  async resetSyncID() {
+    await this._deleteServerCollection();
+    let newSyncID = await this.resetLocalSyncID();
+    return newSyncID;
+  },
+
+  async resetLocalSyncID() {
+    let newSyncID = await this._bridge.resetSyncId();
+    return newSyncID;
+  },
+
+  async ensureCurrentSyncID(newSyncID) {
+    let assignedSyncID = await this._bridge.ensureCurrentSyncId(newSyncID);
+    return assignedSyncID;
+  },
+
+  async getLastSync() {
+    // The bridge defines lastSync as integer ms, but sync itself wants to work
+    // in a float seconds with 2 decimal places.
+    let lastSyncMS = await this._bridge.lastSync();
+    return Math.round(lastSyncMS / 10) / 100;
+  },
+
+  async setLastSync(lastSyncSeconds) {
+    await this._bridge.setLastSync(Math.round(lastSyncSeconds * 1000));
+  },
+
+  /**
+   * Returns the initial changeset for the sync. Bridged engines handle
+   * reconciliation internally, so we don't know what changed until after we've
+   * stored and applied all incoming records. So we return an empty changeset
+   * here, and replace it with the real one in `_processIncoming`.
+   */
+  async pullChanges() {
+    return {};
+  },
+
+  async trackRemainingChanges() {
+    await this._bridge.syncFinished();
+  },
+
+  /**
+   * Marks a record for a hard-`DELETE` at the end of the sync. The base method
+   * also removes it from the tracker, but we don't use the tracker for that,
+   * so we override the method to just mark.
+   */
+  _deleteId(id) {
+    this._noteDeletedId(id);
+  },
+
+  /**
+   * Always stage incoming records, bypassing the base engine's reconciliation
+   * machinery.
+   */
+  async _reconcile() {
+    return true;
+  },
+
+  async _syncStartup() {
+    await super._syncStartup();
+    await this._bridge.syncStarted();
+  },
+
+  async _processIncoming(newitems) {
+    await super._processIncoming(newitems);
+
+    let outgoingBsosAsJSON = await this._bridge.apply();
+    let changeset = {};
+    for (let bsoAsJSON of outgoingBsosAsJSON) {
+      this._log.trace("outgoing bso", bsoAsJSON);
+      let record = BridgedRecord.fromOutgoingBso(
+        this.name,
+        JSON.parse(bsoAsJSON)
+      );
+      changeset[record.id] = {
+        synced: false,
+        record,
+      };
+    }
+    this._modified.replace(changeset);
+  },
+
+  /**
+   * Notify the bridged engine that we've successfully uploaded a batch, so
+   * that it can update its local state. For example, if the engine uses a
+   * mirror and a temp table for outgoing records, it can write the uploaded
+   * records from the outgoing table back to the mirror.
+   */
+  async _onRecordsWritten(succeeded, failed, serverModifiedTime) {
+    // JS uses seconds but Rust uses milliseconds so we'll need to convert
+    let serverModifiedMS = Math.round(serverModifiedTime * 1000);
+    await this._bridge.setUploaded(Math.floor(serverModifiedMS), succeeded);
+  },
+
+  async _createTombstone() {
+    throw new Error("Bridged engines don't support weak uploads");
+  },
+
+  async _createRecord(id) {
+    let change = this._modified.changes[id];
+    if (!change) {
+      throw new TypeError("Can't create record for unchanged item");
+    }
+    return change.record;
+  },
+
+  async _resetClient() {
+    await super._resetClient();
+    await this._bridge.reset();
+  },
+};
+Object.setPrototypeOf(BridgedEngine.prototype, SyncEngine.prototype);
+
+function transformError(code, message) {
+  switch (code) {
+    case Cr.NS_ERROR_ABORT:
+      return new InterruptedError(message);
+
+    default:
+      return new BridgeError(code, message);
+  }
+}
diff --git a/services/sync/modules/collection_validator.sys.mjs b/services/sync/modules/collection_validator.sys.mjs
new file mode 100644
index 0000000000..a64ede10e9
--- /dev/null
+++ b/services/sync/modules/collection_validator.sys.mjs
@@ -0,0 +1,267 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Async: "resource://services-common/async.sys.mjs",
+});
+
+export class CollectionProblemData {
+  constructor() {
+    this.missingIDs = 0;
+    this.clientDuplicates = [];
+    this.duplicates = [];
+    this.clientMissing = [];
+    this.serverMissing = [];
+    this.serverDeleted = [];
+    this.serverUnexpected = [];
+    this.differences = [];
+  }
+
+  /**
+   * Produce a list summarizing problems found. Each entry contains {name, count},
+   * where name is the field name for the problem, and count is the number of times
+   * the problem was encountered.
+   *
+   * Validation has failed if all counts are not 0.
+   */
+  getSummary() {
+    return [
+      { name: "clientMissing", count: this.clientMissing.length },
+      { name: "serverMissing", count: this.serverMissing.length },
+      { name: "serverDeleted", count: this.serverDeleted.length },
+      { name: "serverUnexpected", count: this.serverUnexpected.length },
+      { name: "differences", count: this.differences.length },
+      { name: "missingIDs", count: this.missingIDs },
+      { name: "clientDuplicates", count: this.clientDuplicates.length },
+      { name: "duplicates", count: this.duplicates.length },
+    ];
+  }
+}
+
+export class CollectionValidator {
+  // Construct a generic collection validator. This is intended to be called by
+  // subclasses.
+  // - name: Name of the engine
+  // - idProp: Property that identifies a record. That is, if a client and server
+  //   record have the same value for the idProp property, they should be
+  //   compared against eachother.
+  // - props: Array of properties that should be compared
+  constructor(name, idProp, props) {
+    this.name = name;
+    this.props = props;
+    this.idProp = idProp;
+
+    // This property deals with the fact that form history records are never
+    // deleted from the server. The FormValidator subclass needs to ignore the
+    // client missing records, and it uses this property to achieve it -
+    // (Bug 1354016).
+    this.ignoresMissingClients = false;
+  }
+
+  // Should a custom ProblemData type be needed, return it here.
+  emptyProblemData() {
+    return new CollectionProblemData();
+  }
+
+  async getServerItems(engine) {
+    let collection = engine.itemSource();
+    let collectionKey = engine.service.collectionKeys.keyForCollection(
+      engine.name
+    );
+    collection.full = true;
+    let result = await collection.getBatched();
+    if (!result.response.success) {
+      throw result.response;
+    }
+    let cleartexts = [];
+
+    await lazy.Async.yieldingForEach(result.records, async record => {
+      await record.decrypt(collectionKey);
+      cleartexts.push(record.cleartext);
+    });
+
+    return cleartexts;
+  }
+
+  // Should return a promise that resolves to an array of client items.
+  getClientItems() {
+    return Promise.reject("Must implement");
+  }
+
+  /**
+   * Can we guarantee validation will fail with a reason that isn't actually a
+   * problem? For example, if we know there are pending changes left over from
+   * the last sync, this should resolve to false. By default resolves to true.
+   */
+  async canValidate() {
+    return true;
+  }
+
+  // Turn the client item into something that can be compared with the server item,
+  // and is also safe to mutate.
+  normalizeClientItem(item) {
+    return Cu.cloneInto(item, {});
+  }
+
+  // Turn the server item into something that can be easily compared with the client
+  // items.
+  async normalizeServerItem(item) {
+    return item;
+  }
+
+  // Return whether or not a server item should be present on the client. Expected
+  // to be overridden.
+  clientUnderstands(item) {
+    return true;
+  }
+
+  // Return whether or not a client item should be present on the server. Expected
+  // to be overridden
+  async syncedByClient(item) {
+    return true;
+  }
+
+  // Compare the server item and the client item, and return a list of property
+  // names that are different. Can be overridden if needed.
+  getDifferences(client, server) {
+    let differences = [];
+    for (let prop of this.props) {
+      let clientProp = client[prop];
+      let serverProp = server[prop];
+      if ((clientProp || "") !== (serverProp || "")) {
+        differences.push(prop);
+      }
+    }
+    return differences;
+  }
+
+  // Returns an object containing
+  //   problemData: an instance of the class returned by emptyProblemData(),
+  //   clientRecords: Normalized client records
+  //   records: Normalized server records,
+  //   deletedRecords: Array of ids that were marked as deleted by the server.
+  async compareClientWithServer(clientItems, serverItems) {
+    const yieldState = lazy.Async.yieldState();
+
+    const clientRecords = [];
+
+    await lazy.Async.yieldingForEach(
+      clientItems,
+      item => {
+        clientRecords.push(this.normalizeClientItem(item));
+      },
+      yieldState
+    );
+
+    const serverRecords = [];
+    await lazy.Async.yieldingForEach(
+      serverItems,
+      async item => {
+        serverRecords.push(await this.normalizeServerItem(item));
+      },
+      yieldState
+    );
+
+    let problems = this.emptyProblemData();
+    let seenServer = new Map();
+    let serverDeleted = new Set();
+    let allRecords = new Map();
+
+    for (let record of serverRecords) {
+      let id = record[this.idProp];
+      if (!id) {
+        ++problems.missingIDs;
+        continue;
+      }
+      if (record.deleted) {
+        serverDeleted.add(record);
+      } else {
+        let serverHasPossibleDupe = seenServer.has(id);
+        if (serverHasPossibleDupe) {
+          problems.duplicates.push(id);
+        } else {
+          seenServer.set(id, record);
+          allRecords.set(id, { server: record, client: null });
+        }
+        record.understood = this.clientUnderstands(record);
+      }
+    }
+
+    let seenClient = new Map();
+    for (let record of clientRecords) {
+      let id = record[this.idProp];
+      record.shouldSync = await this.syncedByClient(record);
+      let clientHasPossibleDupe = seenClient.has(id);
+      if (clientHasPossibleDupe && record.shouldSync) {
+        // Only report duplicate client IDs for syncable records.
+        problems.clientDuplicates.push(id);
+        continue;
+      }
+      seenClient.set(id, record);
+      let combined = allRecords.get(id);
+      if (combined) {
+        combined.client = record;
+      } else {
+        allRecords.set(id, { client: record, server: null });
+      }
+    }
+
+    for (let [id, { server, client }] of allRecords) {
+      if (!client && !server) {
+        throw new Error("Impossible: no client or server record for " + id);
+      } else if (server && !client) {
+        if (!this.ignoresMissingClients && server.understood) {
+          problems.clientMissing.push(id);
+        }
+      } else if (client && !server) {
+        if (client.shouldSync) {
+          problems.serverMissing.push(id);
+        }
+      } else {
+        if (!client.shouldSync) {
+          if (!problems.serverUnexpected.includes(id)) {
+            problems.serverUnexpected.push(id);
+          }
+          continue;
+        }
+        let differences = this.getDifferences(client, server);
+        if (differences && differences.length) {
+          problems.differences.push({ id, differences });
+        }
+      }
+    }
+    return {
+      problemData: problems,
+      clientRecords,
+      records: serverRecords,
+      deletedRecords: [...serverDeleted],
+    };
+  }
+
+  async validate(engine) {
+    let start = Cu.now();
+    let clientItems = await this.getClientItems();
+    let serverItems = await this.getServerItems(engine);
+    let serverRecordCount = serverItems.length;
+    let result = await this.compareClientWithServer(clientItems, serverItems);
+    let end = Cu.now();
+    let duration = end - start;
+    engine._log.debug(`Validated ${this.name} in ${duration}ms`);
+    engine._log.debug(`Problem summary`);
+    for (let { name, count } of result.problemData.getSummary()) {
+      engine._log.debug(`  ${name}: ${count}`);
+    }
+    return {
+      duration,
+      version: this.version,
+      problems: result.problemData,
+      recordCount: serverRecordCount,
+    };
+  }
+}
+
+// Default to 0, some engines may override.
+CollectionValidator.prototype.version = 0;
diff --git a/services/sync/modules/constants.sys.mjs b/services/sync/modules/constants.sys.mjs
new file mode 100644
index 0000000000..35c0ac2f0b
--- /dev/null
+++ b/services/sync/modules/constants.sys.mjs
@@ -0,0 +1,133 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Don't manually modify this line, as it is automatically replaced on merge day
+// by the gecko_migration.py script.
+export const WEAVE_VERSION = "1.126.0";
+
+// Sync Server API version that the client supports.
+export const SYNC_API_VERSION = "1.5";
+
+// Version of the data format this client supports. The data format describes
+// how records are packaged; this is separate from the Server API version and
+// the per-engine cleartext formats.
+export const STORAGE_VERSION = 5;
+export const PREFS_BRANCH = "services.sync.";
+
+// Put in [] because those aren't allowed in a collection name.
+export const DEFAULT_KEYBUNDLE_NAME = "[default]";
+
+// Key dimensions.
+export const SYNC_KEY_ENCODED_LENGTH = 26;
+export const SYNC_KEY_DECODED_LENGTH = 16;
+
+export const NO_SYNC_NODE_INTERVAL = 10 * 60 * 1000; // 10 minutes
+
+export const MAX_ERROR_COUNT_BEFORE_BACKOFF = 3;
+
+// Backoff intervals
+export const MINIMUM_BACKOFF_INTERVAL = 15 * 60 * 1000; // 15 minutes
+export const MAXIMUM_BACKOFF_INTERVAL = 8 * 60 * 60 * 1000; // 8 hours
+
+// HMAC event handling timeout.
+// 10 minutes = a compromise between the multi-desktop sync interval
+// and the mobile sync interval.
+export const HMAC_EVENT_INTERVAL = 600000;
+
+// How long to wait between sync attempts if the Master Password is locked.
+export const MASTER_PASSWORD_LOCKED_RETRY_INTERVAL = 15 * 60 * 1000; // 15 minutes
+
+// 50 is hardcoded here because of URL length restrictions.
+// (GUIDs can be up to 64 chars long.)
+// Individual engines can set different values for their limit if their
+// identifiers are shorter.
+export const DEFAULT_GUID_FETCH_BATCH_SIZE = 50;
+
+// Default batch size for download batching
+// (how many records are fetched at a time from the server when batching is used).
+export const DEFAULT_DOWNLOAD_BATCH_SIZE = 1000;
+
+// score thresholds for early syncs
+export const SINGLE_USER_THRESHOLD = 1000;
+export const MULTI_DEVICE_THRESHOLD = 300;
+
+// Other score increment constants
+export const SCORE_INCREMENT_SMALL = 1;
+export const SCORE_INCREMENT_MEDIUM = 10;
+
+// Instant sync score increment
+export const SCORE_INCREMENT_XLARGE = 300 + 1; //MULTI_DEVICE_THRESHOLD + 1
+
+// Delay before incrementing global score
+export const SCORE_UPDATE_DELAY = 100;
+
+// Delay for the back observer debouncer. This is chosen to be longer than any
+// observed spurious idle/back events and short enough to pre-empt user activity.
+export const IDLE_OBSERVER_BACK_DELAY = 100;
+
+// Duplicate URI_LENGTH_MAX from Places (from nsNavHistory.h), used to discard
+// tabs with huge uris during tab sync.
+export const URI_LENGTH_MAX = 65536;
+
+export const MAX_HISTORY_UPLOAD = 5000;
+export const MAX_HISTORY_DOWNLOAD = 5000;
+
+// Top-level statuses
+export const STATUS_OK = "success.status_ok";
+export const SYNC_FAILED = "error.sync.failed";
+export const LOGIN_FAILED = "error.login.failed";
+export const SYNC_FAILED_PARTIAL = "error.sync.failed_partial";
+export const CLIENT_NOT_CONFIGURED = "service.client_not_configured";
+export const STATUS_DISABLED = "service.disabled";
+export const MASTER_PASSWORD_LOCKED = "service.master_password_locked";
+
+// success states
+export const LOGIN_SUCCEEDED = "success.login";
+export const SYNC_SUCCEEDED = "success.sync";
+export const ENGINE_SUCCEEDED = "success.engine";
+
+// login failure status codes
+export const LOGIN_FAILED_NO_USERNAME = "error.login.reason.no_username";
+export const LOGIN_FAILED_NO_PASSPHRASE = "error.login.reason.no_recoverykey";
+export const LOGIN_FAILED_NETWORK_ERROR = "error.login.reason.network";
+export const LOGIN_FAILED_SERVER_ERROR = "error.login.reason.server";
+export const LOGIN_FAILED_INVALID_PASSPHRASE = "error.login.reason.recoverykey";
+export const LOGIN_FAILED_LOGIN_REJECTED = "error.login.reason.account";
+
+// sync failure status codes
+export const METARECORD_DOWNLOAD_FAIL =
+  "error.sync.reason.metarecord_download_fail";
+export const VERSION_OUT_OF_DATE = "error.sync.reason.version_out_of_date";
+export const CREDENTIALS_CHANGED = "error.sync.reason.credentials_changed";
+export const ABORT_SYNC_COMMAND = "aborting sync, process commands said so";
+export const NO_SYNC_NODE_FOUND = "error.sync.reason.no_node_found";
+export const OVER_QUOTA = "error.sync.reason.over_quota";
+export const SERVER_MAINTENANCE = "error.sync.reason.serverMaintenance";
+
+export const RESPONSE_OVER_QUOTA = "14";
+
+// engine failure status codes
+export const ENGINE_UPLOAD_FAIL = "error.engine.reason.record_upload_fail";
+export const ENGINE_DOWNLOAD_FAIL = "error.engine.reason.record_download_fail";
+export const ENGINE_UNKNOWN_FAIL = "error.engine.reason.unknown_fail";
+export const ENGINE_APPLY_FAIL = "error.engine.reason.apply_fail";
+// an upload failure where the batch was interrupted with a 412
+export const ENGINE_BATCH_INTERRUPTED = "error.engine.reason.batch_interrupted";
+
+// Ways that a sync can be disabled (messages only to be printed in debug log)
+export const kSyncMasterPasswordLocked =
+  "User elected to leave Primary Password locked";
+export const kSyncWeaveDisabled = "Weave is disabled";
+export const kSyncNetworkOffline = "Network is offline";
+export const kSyncBackoffNotMet =
+  "Trying to sync before the server said it's okay";
+export const kFirstSyncChoiceNotMade =
+  "User has not selected an action for first sync";
+export const kSyncNotConfigured = "Sync is not configured";
+export const kFirefoxShuttingDown = "Firefox is about to shut down";
+
+export const DEVICE_TYPE_DESKTOP = "desktop";
+export const DEVICE_TYPE_MOBILE = "mobile";
+
+export const SQLITE_MAX_VARIABLE_NUMBER = 999;
diff --git a/services/sync/modules/doctor.sys.mjs b/services/sync/modules/doctor.sys.mjs
new file mode 100644
index 0000000000..ebcf38e2a2
--- /dev/null
+++ b/services/sync/modules/doctor.sys.mjs
@@ -0,0 +1,201 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// A doctor for our collections. She can be asked to make a consultation, and
+// may just diagnose an issue without attempting to cure it, may diagnose and
+// attempt to cure, or may decide she is overworked and underpaid.
+// Or something - naming is hard :)
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { Observers } from "resource://services-common/observers.sys.mjs";
+import { Service } from "resource://services-sync/service.sys.mjs";
+import { Resource } from "resource://services-sync/resource.sys.mjs";
+import { Svc } from "resource://services-sync/util.sys.mjs";
+
+const log = Log.repository.getLogger("Sync.Doctor");
+
+export var Doctor = {
+  async consult(recentlySyncedEngines) {
+    if (!Services.telemetry.canRecordBase) {
+      log.info("Skipping consultation: telemetry reporting is disabled");
+      return;
+    }
+
+    let engineInfos = this._getEnginesToValidate(recentlySyncedEngines);
+
+    await this._runValidators(engineInfos);
+  },
+
+  _getEnginesToValidate(recentlySyncedEngines) {
+    let result = {};
+    for (let e of recentlySyncedEngines) {
+      let prefPrefix = `engine.${e.name}.`;
+      if (
+        !Svc.PrefBranch.getBoolPref(prefPrefix + "validation.enabled", false)
+      ) {
+        log.info(`Skipping check of ${e.name} - disabled via preferences`);
+        continue;
+      }
+      // Check the last validation time for the engine.
+      let lastValidation = Svc.PrefBranch.getIntPref(
+        prefPrefix + "validation.lastTime",
+        0
+      );
+      let validationInterval = Svc.PrefBranch.getIntPref(
+        prefPrefix + "validation.interval"
+      );
+      let nowSeconds = this._now();
+
+      if (nowSeconds - lastValidation < validationInterval) {
+        log.info(
+          `Skipping validation of ${e.name}: too recent since last validation attempt`
+        );
+        continue;
+      }
+      // Update the time now, even if we decline to actually perform a
+      // validation. We don't want to check the rest of these more frequently
+      // than once a day.
+      Svc.PrefBranch.setIntPref(
+        prefPrefix + "validation.lastTime",
+        Math.floor(nowSeconds)
+      );
+
+      // Validation only occurs a certain percentage of the time.
+      let validationProbability =
+        Svc.PrefBranch.getIntPref(
+          prefPrefix + "validation.percentageChance",
+          0
+        ) / 100.0;
+      if (validationProbability < Math.random()) {
+        log.info(
+          `Skipping validation of ${e.name}: Probability threshold not met`
+        );
+        continue;
+      }
+
+      let maxRecords = Svc.PrefBranch.getIntPref(
+        prefPrefix + "validation.maxRecords"
+      );
+      if (!maxRecords) {
+        log.info(`Skipping validation of ${e.name}: No maxRecords specified`);
+        continue;
+      }
+      // OK, so this is a candidate - the final decision will be based on the
+      // number of records actually found.
+      result[e.name] = { engine: e, maxRecords };
+    }
+    return result;
+  },
+
+  async _runValidators(engineInfos) {
+    if (!Object.keys(engineInfos).length) {
+      log.info("Skipping validation: no engines qualify");
+      return;
+    }
+
+    if (Object.values(engineInfos).filter(i => i.maxRecords != -1).length) {
+      // at least some of the engines have maxRecord restrictions which require
+      // us to ask the server for the counts.
+      let countInfo = await this._fetchCollectionCounts();
+      for (let [engineName, recordCount] of Object.entries(countInfo)) {
+        if (engineName in engineInfos) {
+          engineInfos[engineName].recordCount = recordCount;
+        }
+      }
+    }
+
+    for (let [
+      engineName,
+      { engine, maxRecords, recordCount },
+    ] of Object.entries(engineInfos)) {
+      // maxRecords of -1 means "any number", so we can skip asking the server.
+      // Used for tests.
+      if (maxRecords >= 0 && recordCount > maxRecords) {
+        log.debug(
+          `Skipping validation for ${engineName} because ` +
+            `the number of records (${recordCount}) is greater ` +
+            `than the maximum allowed (${maxRecords}).`
+        );
+        continue;
+      }
+      let validator = engine.getValidator();
+      if (!validator) {
+        // This is probably only possible in profile downgrade cases.
+        log.warn(
+          `engine.getValidator returned null for ${engineName} but the pref that controls validation is enabled.`
+        );
+        continue;
+      }
+
+      if (!(await validator.canValidate())) {
+        log.debug(
+          `Skipping validation for ${engineName} because validator.canValidate() is false`
+        );
+        continue;
+      }
+
+      // Let's do it!
+      Services.console.logStringMessage(
+        `Sync is about to run a consistency check of ${engine.name}. This may be slow, and ` +
+          `can be controlled using the pref "services.sync.${engine.name}.validation.enabled".\n` +
+          `If you encounter any problems because of this, please file a bug.`
+      );
+
+      try {
+        log.info(`Running validator for ${engine.name}`);
+        let result = await validator.validate(engine);
+        let { problems, version, duration, recordCount } = result;
+        Observers.notify(
+          "weave:engine:validate:finish",
+          {
+            version,
+            checked: recordCount,
+            took: duration,
+            problems: problems ? problems.getSummary(true) : null,
+          },
+          engine.name
+        );
+      } catch (ex) {
+        if (Async.isShutdownException(ex)) {
+          throw ex;
+        }
+        log.error(`Failed to run validation on ${engine.name}!`, ex);
+        Observers.notify("weave:engine:validate:error", ex, engine.name);
+        // Keep validating -- there's no reason to think that a failure for one
+        // validator would mean the others will fail.
+      }
+    }
+  },
+
+  // mainly for mocking.
+  async _fetchCollectionCounts() {
+    let collectionCountsURL = Service.userBaseURL + "info/collection_counts";
+    try {
+      let infoResp = await Service._fetchInfo(collectionCountsURL);
+      if (!infoResp.success) {
+        log.error(
+          "Can't fetch collection counts: request to info/collection_counts responded with " +
+            infoResp.status
+        );
+        return {};
+      }
+      return infoResp.obj; // might throw because obj is a getter which parses json.
+    } catch (ex) {
+      if (Async.isShutdownException(ex)) {
+        throw ex;
+      }
+      // Not running validation is totally fine, so we just write an error log and return.
+      log.error("Caught error when fetching counts", ex);
+      return {};
+    }
+  },
+
+  // functions used so tests can mock them
+  _now() {
+    // We use the server time, which is SECONDS
+    return Resource.serverTime;
+  },
+};
diff --git a/services/sync/modules/engines.sys.mjs b/services/sync/modules/engines.sys.mjs
new file mode 100644
index 0000000000..0d490ac4b3
--- /dev/null
+++ b/services/sync/modules/engines.sys.mjs
@@ -0,0 +1,2274 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import { JSONFile } from "resource://gre/modules/JSONFile.sys.mjs";
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { Observers } from "resource://services-common/observers.sys.mjs";
+
+import {
+  DEFAULT_DOWNLOAD_BATCH_SIZE,
+  DEFAULT_GUID_FETCH_BATCH_SIZE,
+  ENGINE_BATCH_INTERRUPTED,
+  ENGINE_DOWNLOAD_FAIL,
+  ENGINE_UPLOAD_FAIL,
+  VERSION_OUT_OF_DATE,
+  PREFS_BRANCH,
+} from "resource://services-sync/constants.sys.mjs";
+
+import {
+  Collection,
+  CryptoWrapper,
+} from "resource://services-sync/record.sys.mjs";
+import { Resource } from "resource://services-sync/resource.sys.mjs";
+import {
+  SerializableSet,
+  Svc,
+  Utils,
+} from "resource://services-sync/util.sys.mjs";
+import { SyncedRecordsTelemetry } from "resource://services-sync/telemetry.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+});
+
+function ensureDirectory(path) {
+  return IOUtils.makeDirectory(PathUtils.parent(path), {
+    createAncestors: true,
+  });
+}
+
+/**
+ * Trackers are associated with a single engine and deal with
+ * listening for changes to their particular data type.
+ *
+ * The base `Tracker` only supports listening for changes, and bumping the score
+ * to indicate how urgently the engine wants to sync. It does not persist any
+ * data. Engines that track changes directly in the storage layer (like
+ * bookmarks, bridged engines, addresses, and credit cards) or only upload a
+ * single record (tabs and preferences) should subclass `Tracker`.
+ */
+export function Tracker(name, engine) {
+  if (!engine) {
+    throw new Error("Tracker must be associated with an Engine instance.");
+  }
+
+  name = name || "Unnamed";
+  this.name = name.toLowerCase();
+  this.engine = engine;
+
+  this._log = Log.repository.getLogger(`Sync.Engine.${name}.Tracker`);
+
+  this._score = 0;
+
+  this.asyncObserver = Async.asyncObserver(this, this._log);
+}
+
+Tracker.prototype = {
+  // New-style trackers use change sources to filter out changes made by Sync in
+  // observer notifications, so we don't want to let the engine ignore all
+  // changes during a sync.
+  get ignoreAll() {
+    return false;
+  },
+
+  // Define an empty setter so that the engine doesn't throw a `TypeError`
+  // setting a read-only property.
+  set ignoreAll(value) {},
+
+  /*
+   * Score can be called as often as desired to decide which engines to sync
+   *
+   * Valid values for score:
+   * -1: Do not sync unless the user specifically requests it (almost disabled)
+   * 0: Nothing has changed
+   * 100: Please sync me ASAP!
+   *
+   * Setting it to other values should (but doesn't currently) throw an exception
+   */
+  get score() {
+    return this._score;
+  },
+
+  set score(value) {
+    this._score = value;
+    Observers.notify("weave:engine:score:updated", this.name);
+  },
+
+  // Should be called by service everytime a sync has been done for an engine
+  resetScore() {
+    this._score = 0;
+  },
+
+  // Unsupported, and throws a more descriptive error to ensure callers aren't
+  // accidentally using persistence.
+  async getChangedIDs() {
+    throw new TypeError("This tracker doesn't store changed IDs");
+  },
+
+  // Also unsupported.
+  async addChangedID(id, when) {
+    throw new TypeError("Can't add changed ID to this tracker");
+  },
+
+  // Ditto.
+  async removeChangedID(...ids) {
+    throw new TypeError("Can't remove changed IDs from this tracker");
+  },
+
+  // This method is called at various times, so we override with a no-op
+  // instead of throwing.
+  clearChangedIDs() {},
+
+  _now() {
+    return Date.now() / 1000;
+  },
+
+  _isTracking: false,
+
+  start() {
+    if (!this.engineIsEnabled()) {
+      return;
+    }
+    this._log.trace("start().");
+    if (!this._isTracking) {
+      this.onStart();
+      this._isTracking = true;
+    }
+  },
+
+  async stop() {
+    this._log.trace("stop().");
+    if (this._isTracking) {
+      await this.asyncObserver.promiseObserversComplete();
+      this.onStop();
+      this._isTracking = false;
+    }
+  },
+
+  // Override these in your subclasses.
+  onStart() {},
+  onStop() {},
+  async observe(subject, topic, data) {},
+
+  engineIsEnabled() {
+    if (!this.engine) {
+      // Can't tell -- we must be running in a test!
+      return true;
+    }
+    return this.engine.enabled;
+  },
+
+  /**
+   * Starts or stops listening for changes depending on the associated engine's
+   * enabled state.
+   *
+   * @param {Boolean} engineEnabled Whether the engine was enabled.
+   */
+  async onEngineEnabledChanged(engineEnabled) {
+    if (engineEnabled == this._isTracking) {
+      return;
+    }
+
+    if (engineEnabled) {
+      this.start();
+    } else {
+      await this.stop();
+      this.clearChangedIDs();
+    }
+  },
+
+  async finalize() {
+    await this.stop();
+  },
+};
+
+/*
+ * A tracker that persists a list of IDs for all changed items that need to be
+ * synced. This is 🚨 _extremely deprecated_ 🚨 and only kept around for current
+ * engines. ⚠️ Please **don't use it** for new engines! ⚠️
+ *
+ * Why is this kind of external change tracking deprecated? Because it causes
+ * consistency issues due to missed notifications, interrupted syncs, and the
+ * tracker's view of what changed diverging from the data store's.
+ */
+export function LegacyTracker(name, engine) {
+  Tracker.call(this, name, engine);
+
+  this._ignored = [];
+  this.file = this.name;
+  this._storage = new JSONFile({
+    path: Utils.jsonFilePath("changes", this.file),
+    dataPostProcessor: json => this._dataPostProcessor(json),
+    beforeSave: () => this._beforeSave(),
+  });
+  this._ignoreAll = false;
+}
+
+LegacyTracker.prototype = {
+  get ignoreAll() {
+    return this._ignoreAll;
+  },
+
+  set ignoreAll(value) {
+    this._ignoreAll = value;
+  },
+
+  // Default to an empty object if the file doesn't exist.
+  _dataPostProcessor(json) {
+    return (typeof json == "object" && json) || {};
+  },
+
+  // Ensure the Weave storage directory exists before writing the file.
+  _beforeSave() {
+    return ensureDirectory(this._storage.path);
+  },
+
+  async getChangedIDs() {
+    await this._storage.load();
+    return this._storage.data;
+  },
+
+  _saveChangedIDs() {
+    this._storage.saveSoon();
+  },
+
+  // ignore/unignore specific IDs.  Useful for ignoring items that are
+  // being processed, or that shouldn't be synced.
+  // But note: not persisted to disk
+
+  ignoreID(id) {
+    this.unignoreID(id);
+    this._ignored.push(id);
+  },
+
+  unignoreID(id) {
+    let index = this._ignored.indexOf(id);
+    if (index != -1) {
+      this._ignored.splice(index, 1);
+    }
+  },
+
+  async _saveChangedID(id, when) {
+    this._log.trace(`Adding changed ID: ${id}, ${JSON.stringify(when)}`);
+    const changedIDs = await this.getChangedIDs();
+    changedIDs[id] = when;
+    this._saveChangedIDs();
+  },
+
+  async addChangedID(id, when) {
+    if (!id) {
+      this._log.warn("Attempted to add undefined ID to tracker");
+      return false;
+    }
+
+    if (this.ignoreAll || this._ignored.includes(id)) {
+      return false;
+    }
+
+    // Default to the current time in seconds if no time is provided.
+    if (when == null) {
+      when = this._now();
+    }
+
+    const changedIDs = await this.getChangedIDs();
+    // Add/update the entry if we have a newer time.
+    if ((changedIDs[id] || -Infinity) < when) {
+      await this._saveChangedID(id, when);
+    }
+
+    return true;
+  },
+
+  async removeChangedID(...ids) {
+    if (!ids.length || this.ignoreAll) {
+      return false;
+    }
+    for (let id of ids) {
+      if (!id) {
+        this._log.warn("Attempted to remove undefined ID from tracker");
+        continue;
+      }
+      if (this._ignored.includes(id)) {
+        this._log.debug(`Not removing ignored ID ${id} from tracker`);
+        continue;
+      }
+      const changedIDs = await this.getChangedIDs();
+      if (changedIDs[id] != null) {
+        this._log.trace("Removing changed ID " + id);
+        delete changedIDs[id];
+      }
+    }
+    this._saveChangedIDs();
+    return true;
+  },
+
+  clearChangedIDs() {
+    this._log.trace("Clearing changed ID list");
+    this._storage.data = {};
+    this._saveChangedIDs();
+  },
+
+  async finalize() {
+    // Persist all pending tracked changes to disk, and wait for the final write
+    // to finish.
+    await super.finalize();
+    this._saveChangedIDs();
+    await this._storage.finalize();
+  },
+};
+Object.setPrototypeOf(LegacyTracker.prototype, Tracker.prototype);
+
+/**
+ * The Store serves as the interface between Sync and stored data.
+ *
+ * The name "store" is slightly a misnomer because it doesn't actually "store"
+ * anything. Instead, it serves as a gateway to something that actually does
+ * the "storing."
+ *
+ * The store is responsible for record management inside an engine. It tells
+ * Sync what items are available for Sync, converts items to and from Sync's
+ * record format, and applies records from Sync into changes on the underlying
+ * store.
+ *
+ * Store implementations require a number of functions to be implemented. These
+ * are all documented below.
+ *
+ * For stores that deal with many records or which have expensive store access
+ * routines, it is highly recommended to implement a custom applyIncomingBatch
+ * and/or applyIncoming function on top of the basic APIs.
+ */
+
+export function Store(name, engine) {
+  if (!engine) {
+    throw new Error("Store must be associated with an Engine instance.");
+  }
+
+  name = name || "Unnamed";
+  this.name = name.toLowerCase();
+  this.engine = engine;
+
+  this._log = Log.repository.getLogger(`Sync.Engine.${name}.Store`);
+
+  ChromeUtils.defineLazyGetter(this, "_timer", function () {
+    return Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+  });
+}
+
+Store.prototype = {
+  /**
+   * Apply multiple incoming records against the store.
+   *
+   * This is called with a set of incoming records to process. The function
+   * should look at each record, reconcile with the current local state, and
+   * make the local changes required to bring its state in alignment with the
+   * record.
+   *
+   * The default implementation simply iterates over all records and calls
+   * applyIncoming(). Store implementations may overwrite this function
+   * if desired.
+   *
+   * @param  records Array of records to apply
+   * @param  a SyncedRecordsTelemetry obj that will keep track of failed reasons
+   * @return Array of record IDs which did not apply cleanly
+   */
+  async applyIncomingBatch(records, countTelemetry) {
+    let failed = [];
+
+    await Async.yieldingForEach(records, async record => {
+      try {
+        await this.applyIncoming(record);
+      } catch (ex) {
+        if (ex.code == SyncEngine.prototype.eEngineAbortApplyIncoming) {
+          // This kind of exception should have a 'cause' attribute, which is an
+          // originating exception.
+          // ex.cause will carry its stack with it when rethrown.
+          throw ex.cause;
+        }
+        if (Async.isShutdownException(ex)) {
+          throw ex;
+        }
+        this._log.warn("Failed to apply incoming record " + record.id, ex);
+        failed.push(record.id);
+        countTelemetry.addIncomingFailedReason(ex.message);
+      }
+    });
+
+    return failed;
+  },
+
+  /**
+   * Apply a single record against the store.
+   *
+   * This takes a single record and makes the local changes required so the
+   * local state matches what's in the record.
+   *
+   * The default implementation calls one of remove(), create(), or update()
+   * depending on the state obtained from the store itself. Store
+   * implementations may overwrite this function if desired.
+   *
+   * @param record
+   *        Record to apply
+   */
+  async applyIncoming(record) {
+    if (record.deleted) {
+      await this.remove(record);
+    } else if (!(await this.itemExists(record.id))) {
+      await this.create(record);
+    } else {
+      await this.update(record);
+    }
+  },
+
+  // override these in derived objects
+
+  /**
+   * Create an item in the store from a record.
+   *
+   * This is called by the default implementation of applyIncoming(). If using
+   * applyIncomingBatch(), this won't be called unless your store calls it.
+   *
+   * @param record
+   *        The store record to create an item from
+   */
+  async create(record) {
+    throw new Error("override create in a subclass");
+  },
+
+  /**
+   * Remove an item in the store from a record.
+   *
+   * This is called by the default implementation of applyIncoming(). If using
+   * applyIncomingBatch(), this won't be called unless your store calls it.
+   *
+   * @param record
+   *        The store record to delete an item from
+   */
+  async remove(record) {
+    throw new Error("override remove in a subclass");
+  },
+
+  /**
+   * Update an item from a record.
+   *
+   * This is called by the default implementation of applyIncoming(). If using
+   * applyIncomingBatch(), this won't be called unless your store calls it.
+   *
+   * @param record
+   *        The record to use to update an item from
+   */
+  async update(record) {
+    throw new Error("override update in a subclass");
+  },
+
+  /**
+   * Determine whether a record with the specified ID exists.
+   *
+   * Takes a string record ID and returns a booleans saying whether the record
+   * exists.
+   *
+   * @param  id
+   *         string record ID
+   * @return boolean indicating whether record exists locally
+   */
+  async itemExists(id) {
+    throw new Error("override itemExists in a subclass");
+  },
+
+  /**
+   * Create a record from the specified ID.
+   *
+   * If the ID is known, the record should be populated with metadata from
+   * the store. If the ID is not known, the record should be created with the
+   * delete field set to true.
+   *
+   * @param  id
+   *         string record ID
+   * @param  collection
+   *         Collection to add record to. This is typically passed into the
+   *         constructor for the newly-created record.
+   * @return record type for this engine
+   */
+  async createRecord(id, collection) {
+    throw new Error("override createRecord in a subclass");
+  },
+
+  /**
+   * Change the ID of a record.
+   *
+   * @param  oldID
+   *         string old/current record ID
+   * @param  newID
+   *         string new record ID
+   */
+  async changeItemID(oldID, newID) {
+    throw new Error("override changeItemID in a subclass");
+  },
+
+  /**
+   * Obtain the set of all known record IDs.
+   *
+   * @return Object with ID strings as keys and values of true. The values
+   *         are ignored.
+   */
+  async getAllIDs() {
+    throw new Error("override getAllIDs in a subclass");
+  },
+
+  /**
+   * Wipe all data in the store.
+   *
+   * This function is called during remote wipes or when replacing local data
+   * with remote data.
+   *
+   * This function should delete all local data that the store is managing. It
+   * can be thought of as clearing out all state and restoring the "new
+   * browser" state.
+   */
+  async wipe() {
+    throw new Error("override wipe in a subclass");
+  },
+};
+
+export function EngineManager(service) {
+  this.service = service;
+
+  this._engines = {};
+
+  this._altEngineInfo = {};
+
+  // This will be populated by Service on startup.
+  this._declined = new Set();
+  this._log = Log.repository.getLogger("Sync.EngineManager");
+  this._log.manageLevelFromPref("services.sync.log.logger.service.engines");
+  // define the default level for all engine logs here (although each engine
+  // allows its level to be controlled via a specific, non-default pref)
+  Log.repository
+    .getLogger(`Sync.Engine`)
+    .manageLevelFromPref("services.sync.log.logger.engine");
+}
+
+EngineManager.prototype = {
+  get(name) {
+    // Return an array of engines if we have an array of names
+    if (Array.isArray(name)) {
+      let engines = [];
+      name.forEach(function (name) {
+        let engine = this.get(name);
+        if (engine) {
+          engines.push(engine);
+        }
+      }, this);
+      return engines;
+    }
+
+    return this._engines[name]; // Silently returns undefined for unknown names.
+  },
+
+  getAll() {
+    let engines = [];
+    for (let [, engine] of Object.entries(this._engines)) {
+      engines.push(engine);
+    }
+    return engines;
+  },
+
+  /**
+   * If a user has changed a pref that controls which variant of a sync engine
+   * for a given collection we use, unregister the old engine and register the
+   * new one.
+   *
+   * This is called by EngineSynchronizer before every sync.
+   */
+  async switchAlternatives() {
+    for (let [name, info] of Object.entries(this._altEngineInfo)) {
+      let prefValue = info.prefValue;
+      if (prefValue === info.lastValue) {
+        this._log.trace(
+          `No change for engine ${name} (${info.pref} is still ${prefValue})`
+        );
+        continue;
+      }
+      // Unregister the old engine, register the new one.
+      this._log.info(
+        `Switching ${name} engine ("${info.pref}" went from ${info.lastValue} => ${prefValue})`
+      );
+      try {
+        await this._removeAndFinalize(name);
+      } catch (e) {
+        this._log.warn(`Failed to remove previous ${name} engine...`, e);
+      }
+      let engineType = prefValue ? info.whenTrue : info.whenFalse;
+      try {
+        // If register throws, we'll try again next sync, but until then there
+        // won't be an engine registered for this collection.
+        await this.register(engineType);
+        info.lastValue = prefValue;
+        // Note: engineType.name is using Function.prototype.name.
+        this._log.info(`Switched the ${name} engine to use ${engineType.name}`);
+      } catch (e) {
+        this._log.warn(
+          `Switching the ${name} engine to use ${engineType.name} failed (couldn't register)`,
+          e
+        );
+      }
+    }
+  },
+
+  async registerAlternatives(name, pref, whenTrue, whenFalse) {
+    let info = { name, pref, whenTrue, whenFalse };
+
+    XPCOMUtils.defineLazyPreferenceGetter(info, "prefValue", pref, false);
+
+    let chosen = info.prefValue ? info.whenTrue : info.whenFalse;
+    info.lastValue = info.prefValue;
+    this._altEngineInfo[name] = info;
+
+    await this.register(chosen);
+  },
+
+  /**
+   * N.B., does not pay attention to the declined list.
+   */
+  getEnabled() {
+    return this.getAll()
+      .filter(engine => engine.enabled)
+      .sort((a, b) => a.syncPriority - b.syncPriority);
+  },
+
+  get enabledEngineNames() {
+    return this.getEnabled().map(e => e.name);
+  },
+
+  persistDeclined() {
+    Svc.PrefBranch.setStringPref(
+      "declinedEngines",
+      [...this._declined].join(",")
+    );
+  },
+
+  /**
+   * Returns an array.
+   */
+  getDeclined() {
+    return [...this._declined];
+  },
+
+  setDeclined(engines) {
+    this._declined = new Set(engines);
+    this.persistDeclined();
+  },
+
+  isDeclined(engineName) {
+    return this._declined.has(engineName);
+  },
+
+  /**
+   * Accepts a Set or an array.
+   */
+  decline(engines) {
+    for (let e of engines) {
+      this._declined.add(e);
+    }
+    this.persistDeclined();
+  },
+
+  undecline(engines) {
+    for (let e of engines) {
+      this._declined.delete(e);
+    }
+    this.persistDeclined();
+  },
+
+  /**
+   * Register an Engine to the service. Alternatively, give an array of engine
+   * objects to register.
+   *
+   * @param engineObject
+   *        Engine object used to get an instance of the engine
+   * @return The engine object if anything failed
+   */
+  async register(engineObject) {
+    if (Array.isArray(engineObject)) {
+      for (const e of engineObject) {
+        await this.register(e);
+      }
+      return;
+    }
+
+    try {
+      let engine = new engineObject(this.service);
+      let name = engine.name;
+      if (name in this._engines) {
+        this._log.error("Engine '" + name + "' is already registered!");
+      } else {
+        if (engine.initialize) {
+          await engine.initialize();
+        }
+        this._engines[name] = engine;
+      }
+    } catch (ex) {
+      let name = engineObject || "";
+      name = name.prototype || "";
+      name = name.name || "";
+
+      this._log.error(`Could not initialize engine ${name}`, ex);
+    }
+  },
+
+  async unregister(val) {
+    let name = val;
+    if (val instanceof SyncEngine) {
+      name = val.name;
+    }
+    await this._removeAndFinalize(name);
+    delete this._altEngineInfo[name];
+  },
+
+  // Common code for disabling an engine by name, that doesn't complain if the
+  // engine doesn't exist. Doesn't touch the engine's alternative info (if any
+  // exists).
+  async _removeAndFinalize(name) {
+    if (name in this._engines) {
+      let engine = this._engines[name];
+      delete this._engines[name];
+      await engine.finalize();
+    }
+  },
+
+  async clear() {
+    for (let name in this._engines) {
+      let engine = this._engines[name];
+      delete this._engines[name];
+      await engine.finalize();
+    }
+    this._altEngineInfo = {};
+  },
+};
+
+export function SyncEngine(name, service) {
+  if (!service) {
+    throw new Error("SyncEngine must be associated with a Service instance.");
+  }
+
+  this.Name = name || "Unnamed";
+  this.name = name.toLowerCase();
+  this.service = service;
+
+  this._notify = Utils.notify("weave:engine:");
+  this._log = Log.repository.getLogger("Sync.Engine." + this.Name);
+  this._log.manageLevelFromPref(`services.sync.log.logger.engine.${this.name}`);
+
+  this._modified = this.emptyChangeset();
+  this._tracker; // initialize tracker to load previously changed IDs
+  this._log.debug("Engine constructed");
+
+  this._toFetchStorage = new JSONFile({
+    path: Utils.jsonFilePath("toFetch", this.name),
+    dataPostProcessor: json => this._metadataPostProcessor(json),
+    beforeSave: () => this._beforeSaveMetadata(),
+  });
+
+  this._previousFailedStorage = new JSONFile({
+    path: Utils.jsonFilePath("failed", this.name),
+    dataPostProcessor: json => this._metadataPostProcessor(json),
+    beforeSave: () => this._beforeSaveMetadata(),
+  });
+
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_enabled",
+    `services.sync.engine.${this.prefName}`,
+    false
+  );
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_syncID",
+    `services.sync.${this.name}.syncID`,
+    ""
+  );
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_lastSync",
+    `services.sync.${this.name}.lastSync`,
+    "0",
+    null,
+    v => parseFloat(v)
+  );
+  // Async initializations can be made in the initialize() method.
+
+  this.asyncObserver = Async.asyncObserver(this, this._log);
+}
+
+// Enumeration to define approaches to handling bad records.
+// Attached to the constructor to allow use as a kind of static enumeration.
+SyncEngine.kRecoveryStrategy = {
+  ignore: "ignore",
+  retry: "retry",
+  error: "error",
+};
+
+SyncEngine.prototype = {
+  _recordObj: CryptoWrapper,
+  // _storeObj, and _trackerObj should to be overridden in subclasses
+  _storeObj: Store,
+  _trackerObj: Tracker,
+  version: 1,
+
+  // Local 'constant'.
+  // Signal to the engine that processing further records is pointless.
+  eEngineAbortApplyIncoming: "error.engine.abort.applyincoming",
+
+  // Should we keep syncing if we find a record that cannot be uploaded (ever)?
+  // If this is false, we'll throw, otherwise, we'll ignore the record and
+  // continue. This currently can only happen due to the record being larger
+  // than the record upload limit.
+  allowSkippedRecord: true,
+
+  // Which sortindex to use when retrieving records for this engine.
+  _defaultSort: undefined,
+
+  _hasSyncedThisSession: false,
+
+  _metadataPostProcessor(json) {
+    if (Array.isArray(json)) {
+      // Pre-`JSONFile` storage stored an array, but `JSONFile` defaults to
+      // an object, so we wrap the array for consistency.
+      json = { ids: json };
+    }
+    if (!json.ids) {
+      json.ids = [];
+    }
+    // The set serializes the same way as an array, but offers more efficient
+    // methods of manipulation.
+    json.ids = new SerializableSet(json.ids);
+    return json;
+  },
+
+  async _beforeSaveMetadata() {
+    await ensureDirectory(this._toFetchStorage.path);
+    await ensureDirectory(this._previousFailedStorage.path);
+  },
+
+  // A relative priority to use when computing an order
+  // for engines to be synced. Higher-priority engines
+  // (lower numbers) are synced first.
+  // It is recommended that a unique value be used for each engine,
+  // in order to guarantee a stable sequence.
+  syncPriority: 0,
+
+  // How many records to pull in a single sync. This is primarily to avoid very
+  // long first syncs against profiles with many history records.
+  downloadLimit: null,
+
+  // How many records to pull at one time when specifying IDs. This is to avoid
+  // URI length limitations.
+  guidFetchBatchSize: DEFAULT_GUID_FETCH_BATCH_SIZE,
+
+  downloadBatchSize: DEFAULT_DOWNLOAD_BATCH_SIZE,
+
+  async initialize() {
+    await this._toFetchStorage.load();
+    await this._previousFailedStorage.load();
+    Services.prefs.addObserver(
+      `${PREFS_BRANCH}engine.${this.prefName}`,
+      this.asyncObserver,
+      true
+    );
+    this._log.debug("SyncEngine initialized", this.name);
+  },
+
+  get prefName() {
+    return this.name;
+  },
+
+  get enabled() {
+    return this._enabled;
+  },
+
+  set enabled(val) {
+    if (!!val != this._enabled) {
+      Svc.PrefBranch.setBoolPref("engine." + this.prefName, !!val);
+    }
+  },
+
+  get score() {
+    return this._tracker.score;
+  },
+
+  get _store() {
+    let store = new this._storeObj(this.Name, this);
+    this.__defineGetter__("_store", () => store);
+    return store;
+  },
+
+  get _tracker() {
+    let tracker = new this._trackerObj(this.Name, this);
+    this.__defineGetter__("_tracker", () => tracker);
+    return tracker;
+  },
+
+  get storageURL() {
+    return this.service.storageURL;
+  },
+
+  get engineURL() {
+    return this.storageURL + this.name;
+  },
+
+  get cryptoKeysURL() {
+    return this.storageURL + "crypto/keys";
+  },
+
+  get metaURL() {
+    return this.storageURL + "meta/global";
+  },
+
+  startTracking() {
+    this._tracker.start();
+  },
+
+  // Returns a promise
+  stopTracking() {
+    return this._tracker.stop();
+  },
+
+  // Listens for engine enabled state changes, and updates the tracker's state.
+  // This is an async observer because the tracker waits on all its async
+  // observers to finish when it's stopped.
+  async observe(subject, topic, data) {
+    if (
+      topic == "nsPref:changed" &&
+      data == `services.sync.engine.${this.prefName}`
+    ) {
+      await this._tracker.onEngineEnabledChanged(this._enabled);
+    }
+  },
+
+  async sync() {
+    if (!this.enabled) {
+      return false;
+    }
+
+    if (!this._sync) {
+      throw new Error("engine does not implement _sync method");
+    }
+
+    return this._notify("sync", this.name, this._sync)();
+  },
+
+  // Override this method to return a new changeset type.
+  emptyChangeset() {
+    return new Changeset();
+  },
+
+  /**
+   * Returns the local sync ID for this engine, or `""` if the engine hasn't
+   * synced for the first time. This is exposed for tests.
+   *
+   * @return the current sync ID.
+   */
+  async getSyncID() {
+    return this._syncID;
+  },
+
+  /**
+   * Ensures that the local sync ID for the engine matches the sync ID for the
+   * collection on the server. A mismatch indicates that another client wiped
+   * the collection; we're syncing after a node reassignment, and another
+   * client synced before us; or the store was replaced since the last sync.
+   * In case of a mismatch, we need to reset all local Sync state and start
+   * over as a first sync.
+   *
+   * In most cases, this method should return the new sync ID as-is. However, an
+   * engine may ignore the given ID and assign a different one, if it determines
+   * that the sync ID on the server is out of date. The bookmarks engine uses
+   * this to wipe the server and other clients on the first sync after the user
+   * restores from a backup.
+   *
+   * @param  newSyncID
+   *         The new sync ID for the collection from `meta/global`.
+   * @return The assigned sync ID. If this doesn't match `newSyncID`, we'll
+   *         replace the sync ID in `meta/global` with the assigned ID.
+   */
+  async ensureCurrentSyncID(newSyncID) {
+    let existingSyncID = this._syncID;
+    if (existingSyncID == newSyncID) {
+      return existingSyncID;
+    }
+    this._log.debug(
+      `Engine syncIDs differ (old="${existingSyncID}", new="${newSyncID}") - resetting the engine`
+    );
+    await this.resetClient();
+    Svc.PrefBranch.setStringPref(this.name + ".syncID", newSyncID);
+    Svc.PrefBranch.setStringPref(this.name + ".lastSync", "0");
+    return newSyncID;
+  },
+
+  /**
+   * Resets the local sync ID for the engine, wipes the server, and resets all
+   * local Sync state to start over as a first sync.
+   *
+   * @return the new sync ID.
+   */
+  async resetSyncID() {
+    let newSyncID = await this.resetLocalSyncID();
+    await this.wipeServer();
+    return newSyncID;
+  },
+
+  /**
+   * Resets the local sync ID for the engine, signaling that we're starting over
+   * as a first sync.
+   *
+   * @return the new sync ID.
+   */
+  async resetLocalSyncID() {
+    return this.ensureCurrentSyncID(Utils.makeGUID());
+  },
+
+  /**
+   * Allows overriding scheduler logic -- added to help reduce kinto server
+   * getting hammered because our scheduler never got tuned for it.
+   *
+   * Note: Overriding engines must take resyncs into account -- score will not
+   * be cleared.
+   */
+  shouldSkipSync(syncReason) {
+    return false;
+  },
+
+  /*
+   * lastSync is a timestamp in server time.
+   */
+  async getLastSync() {
+    return this._lastSync;
+  },
+  async setLastSync(lastSync) {
+    // Store the value as a string to keep floating point precision
+    Svc.PrefBranch.setStringPref(this.name + ".lastSync", lastSync.toString());
+  },
+  async resetLastSync() {
+    this._log.debug("Resetting " + this.name + " last sync time");
+    await this.setLastSync(0);
+  },
+
+  get hasSyncedThisSession() {
+    return this._hasSyncedThisSession;
+  },
+
+  set hasSyncedThisSession(hasSynced) {
+    this._hasSyncedThisSession = hasSynced;
+  },
+
+  get toFetch() {
+    this._toFetchStorage.ensureDataReady();
+    return this._toFetchStorage.data.ids;
+  },
+
+  set toFetch(ids) {
+    if (ids.constructor.name != "SerializableSet") {
+      throw new Error(
+        "Bug: Attempted to set toFetch to something that isn't a SerializableSet"
+      );
+    }
+    this._toFetchStorage.data = { ids };
+    this._toFetchStorage.saveSoon();
+  },
+
+  get previousFailed() {
+    this._previousFailedStorage.ensureDataReady();
+    return this._previousFailedStorage.data.ids;
+  },
+
+  set previousFailed(ids) {
+    if (ids.constructor.name != "SerializableSet") {
+      throw new Error(
+        "Bug: Attempted to set previousFailed to something that isn't a SerializableSet"
+      );
+    }
+    this._previousFailedStorage.data = { ids };
+    this._previousFailedStorage.saveSoon();
+  },
+
+  /*
+   * Returns a changeset for this sync. Engine implementations can override this
+   * method to bypass the tracker for certain or all changed items.
+   */
+  async getChangedIDs() {
+    return this._tracker.getChangedIDs();
+  },
+
+  // Create a new record using the store and add in metadata.
+  async _createRecord(id) {
+    let record = await this._store.createRecord(id, this.name);
+    record.id = id;
+    record.collection = this.name;
+    return record;
+  },
+
+  // Creates a tombstone Sync record with additional metadata.
+  _createTombstone(id) {
+    let tombstone = new this._recordObj(this.name, id);
+    tombstone.id = id;
+    tombstone.collection = this.name;
+    tombstone.deleted = true;
+    return tombstone;
+  },
+
+  // Any setup that needs to happen at the beginning of each sync.
+  async _syncStartup() {
+    // Determine if we need to wipe on outdated versions
+    let metaGlobal = await this.service.recordManager.get(this.metaURL);
+    let engines = metaGlobal.payload.engines || {};
+    let engineData = engines[this.name] || {};
+
+    // Assume missing versions are 0 and wipe the server
+    if ((engineData.version || 0) < this.version) {
+      this._log.debug("Old engine data: " + [engineData.version, this.version]);
+
+      // Clear the server and reupload everything on bad version or missing
+      // meta. Note that we don't regenerate per-collection keys here.
+      let newSyncID = await this.resetSyncID();
+
+      // Set the newer version and newly generated syncID
+      engineData.version = this.version;
+      engineData.syncID = newSyncID;
+
+      // Put the new data back into meta/global and mark for upload
+      engines[this.name] = engineData;
+      metaGlobal.payload.engines = engines;
+      metaGlobal.changed = true;
+    } else if (engineData.version > this.version) {
+      // Don't sync this engine if the server has newer data
+
+      let error = new Error("New data: " + [engineData.version, this.version]);
+      error.failureCode = VERSION_OUT_OF_DATE;
+      throw error;
+    } else {
+      // Changes to syncID mean we'll need to upload everything
+      let assignedSyncID = await this.ensureCurrentSyncID(engineData.syncID);
+      if (assignedSyncID != engineData.syncID) {
+        engineData.syncID = assignedSyncID;
+        metaGlobal.changed = true;
+      }
+    }
+
+    // Save objects that need to be uploaded in this._modified. As we
+    // successfully upload objects we remove them from this._modified. If an
+    // error occurs or any objects fail to upload, they will remain in
+    // this._modified. At the end of a sync, or after an error, we add all
+    // objects remaining in this._modified to the tracker.
+    let initialChanges = await this.pullChanges();
+    this._modified.replace(initialChanges);
+    // Clear the tracker now. If the sync fails we'll add the ones we failed
+    // to upload back.
+    this._tracker.clearChangedIDs();
+    this._tracker.resetScore();
+
+    // Keep track of what to delete at the end of sync
+    this._delete = {};
+  },
+
+  async pullChanges() {
+    let lastSync = await this.getLastSync();
+    if (lastSync) {
+      return this.pullNewChanges();
+    }
+    this._log.debug("First sync, uploading all items");
+    return this.pullAllChanges();
+  },
+
+  /**
+   * A tiny abstraction to make it easier to test incoming record
+   * application.
+   */
+  itemSource() {
+    return new Collection(this.engineURL, this._recordObj, this.service);
+  },
+
+  /**
+   * Download and apply remote records changed since the last sync. This
+   * happens in three stages.
+   *
+   * In the first stage, we fetch full records for all changed items, newest
+   * first, up to the download limit. The limit lets us make progress for large
+   * collections, where the sync is likely to be interrupted before we
+   * can fetch everything.
+   *
+   * In the second stage, we fetch the IDs of any remaining records changed
+   * since the last sync, add them to our backlog, and fast-forward our last
+   * sync time.
+   *
+   * In the third stage, we fetch and apply records for all backlogged IDs,
+   * as well as any records that failed to apply during the last sync. We
+   * request records for the IDs in chunks, to avoid exceeding URL length
+   * limits, then remove successfully applied records from the backlog, and
+   * record IDs of any records that failed to apply to retry on the next sync.
+   */
+  async _processIncoming() {
+    this._log.trace("Downloading & applying server changes");
+
+    let newitems = this.itemSource();
+    let lastSync = await this.getLastSync();
+
+    newitems.newer = lastSync;
+    newitems.full = true;
+
+    let downloadLimit = Infinity;
+    if (this.downloadLimit) {
+      // Fetch new records up to the download limit. Currently, only the history
+      // engine sets a limit, since the history collection has the highest volume
+      // of changed records between syncs. The other engines fetch all records
+      // changed since the last sync.
+      if (this._defaultSort) {
+        // A download limit with a sort order doesn't make sense: we won't know
+        // which records to backfill.
+        throw new Error("Can't specify download limit with default sort order");
+      }
+      newitems.sort = "newest";
+      downloadLimit = newitems.limit = this.downloadLimit;
+    } else if (this._defaultSort) {
+      // The bookmarks engine fetches records by sort index; other engines leave
+      // the order unspecified. We can remove `_defaultSort` entirely after bug
+      // 1305563: the sort index won't matter because we'll buffer all bookmarks
+      // before applying.
+      newitems.sort = this._defaultSort;
+    }
+
+    // applied    => number of items that should be applied.
+    // failed     => number of items that failed in this sync.
+    // newFailed  => number of items that failed for the first time in this sync.
+    // reconciled => number of items that were reconciled.
+    // failedReasons => {name, count} of reasons a record failed
+    let countTelemetry = new SyncedRecordsTelemetry();
+    let count = countTelemetry.incomingCounts;
+    let recordsToApply = [];
+    let failedInCurrentSync = new SerializableSet();
+
+    let oldestModified = this.lastModified;
+    let downloadedIDs = new Set();
+
+    // Stage 1: Fetch new records from the server, up to the download limit.
+    if (this.lastModified == null || this.lastModified > lastSync) {
+      let { response, records } = await newitems.getBatched(
+        this.downloadBatchSize
+      );
+      if (!response.success) {
+        response.failureCode = ENGINE_DOWNLOAD_FAIL;
+        throw response;
+      }
+
+      await Async.yieldingForEach(records, async record => {
+        downloadedIDs.add(record.id);
+
+        if (record.modified < oldestModified) {
+          oldestModified = record.modified;
+        }
+
+        let { shouldApply, error } = await this._maybeReconcile(record);
+        if (error) {
+          failedInCurrentSync.add(record.id);
+          count.failed++;
+          countTelemetry.addIncomingFailedReason(error.message);
+          return;
+        }
+        if (!shouldApply) {
+          count.reconciled++;
+          return;
+        }
+        recordsToApply.push(record);
+      });
+
+      let failedToApply = await this._applyRecords(
+        recordsToApply,
+        countTelemetry
+      );
+      Utils.setAddAll(failedInCurrentSync, failedToApply);
+
+      // `applied` is a bit of a misnomer: it counts records that *should* be
+      // applied, so it also includes records that we tried to apply and failed.
+      // `recordsToApply.length - failedToApply.length` is the number of records
+      // that we *successfully* applied.
+      count.failed += failedToApply.length;
+      count.applied += recordsToApply.length;
+    }
+
+    // Stage 2: If we reached our download limit, we might still have records
+    // on the server that changed since the last sync. Fetch the IDs for the
+    // remaining records, and add them to the backlog. Note that this stage
+    // only runs for engines that set a download limit.
+    if (downloadedIDs.size == downloadLimit) {
+      let guidColl = this.itemSource();
+
+      guidColl.newer = lastSync;
+      guidColl.older = oldestModified;
+      guidColl.sort = "oldest";
+
+      let guids = await guidColl.get();
+      if (!guids.success) {
+        throw guids;
+      }
+
+      // Filtering out already downloaded IDs here isn't necessary. We only do
+      // that in case the Sync server doesn't support `older` (bug 1316110).
+      let remainingIDs = guids.obj.filter(id => !downloadedIDs.has(id));
+      if (remainingIDs.length) {
+        this.toFetch = Utils.setAddAll(this.toFetch, remainingIDs);
+      }
+    }
+
+    // Fast-foward the lastSync timestamp since we have backlogged the
+    // remaining items.
+    if (lastSync < this.lastModified) {
+      lastSync = this.lastModified;
+      await this.setLastSync(lastSync);
+    }
+
+    // Stage 3: Backfill records from the backlog, and those that failed to
+    // decrypt or apply during the last sync. We only backfill up to the
+    // download limit, to prevent a large backlog for one engine from blocking
+    // the others. We'll keep processing the backlog on subsequent engine syncs.
+    let failedInPreviousSync = this.previousFailed;
+    let idsToBackfill = Array.from(
+      Utils.setAddAll(
+        Utils.subsetOfSize(this.toFetch, downloadLimit),
+        failedInPreviousSync
+      )
+    );
+
+    // Note that we intentionally overwrite the previously failed list here.
+    // Records that fail to decrypt or apply in two consecutive syncs are likely
+    // corrupt; we remove them from the list because retrying and failing on
+    // every subsequent sync just adds noise.
+    this.previousFailed = failedInCurrentSync;
+
+    let backfilledItems = this.itemSource();
+
+    backfilledItems.sort = "newest";
+    backfilledItems.full = true;
+
+    // `getBatched` includes the list of IDs as a query parameter, so we need to fetch
+    // records in chunks to avoid exceeding URI length limits.
+    if (this.guidFetchBatchSize) {
+      for (let ids of lazy.PlacesUtils.chunkArray(
+        idsToBackfill,
+        this.guidFetchBatchSize
+      )) {
+        backfilledItems.ids = ids;
+
+        let { response, records } = await backfilledItems.getBatched(
+          this.downloadBatchSize
+        );
+        if (!response.success) {
+          response.failureCode = ENGINE_DOWNLOAD_FAIL;
+          throw response;
+        }
+
+        let backfilledRecordsToApply = [];
+        let failedInBackfill = [];
+
+        await Async.yieldingForEach(records, async record => {
+          let { shouldApply, error } = await this._maybeReconcile(record);
+          if (error) {
+            failedInBackfill.push(record.id);
+            count.failed++;
+            countTelemetry.addIncomingFailedReason(error.message);
+            return;
+          }
+          if (!shouldApply) {
+            count.reconciled++;
+            return;
+          }
+          backfilledRecordsToApply.push(record);
+        });
+
+        let failedToApply = await this._applyRecords(
+          backfilledRecordsToApply,
+          countTelemetry
+        );
+        failedInBackfill.push(...failedToApply);
+
+        count.failed += failedToApply.length;
+        count.applied += backfilledRecordsToApply.length;
+
+        this.toFetch = Utils.setDeleteAll(this.toFetch, ids);
+        this.previousFailed = Utils.setAddAll(
+          this.previousFailed,
+          failedInBackfill
+        );
+
+        if (lastSync < this.lastModified) {
+          lastSync = this.lastModified;
+          await this.setLastSync(lastSync);
+        }
+      }
+    }
+
+    count.newFailed = 0;
+    for (let item of this.previousFailed) {
+      // Anything that failed in the current sync that also failed in
+      // the previous sync means there is likely something wrong with
+      // the record, we remove it from trying again to prevent
+      // infinitely syncing corrupted records
+      if (failedInPreviousSync.has(item)) {
+        this.previousFailed.delete(item);
+      } else {
+        // otherwise it's a new failed and we count it as so
+        ++count.newFailed;
+      }
+    }
+
+    count.succeeded = Math.max(0, count.applied - count.failed);
+    this._log.info(
+      [
+        "Records:",
+        count.applied,
+        "applied,",
+        count.succeeded,
+        "successfully,",
+        count.failed,
+        "failed to apply,",
+        count.newFailed,
+        "newly failed to apply,",
+        count.reconciled,
+        "reconciled.",
+      ].join(" ")
+    );
+    Observers.notify("weave:engine:sync:applied", count, this.name);
+  },
+
+  async _maybeReconcile(item) {
+    let key = this.service.collectionKeys.keyForCollection(this.name);
+
+    // Grab a later last modified if possible
+    if (this.lastModified == null || item.modified > this.lastModified) {
+      this.lastModified = item.modified;
+    }
+
+    try {
+      try {
+        await item.decrypt(key);
+      } catch (ex) {
+        if (!Utils.isHMACMismatch(ex)) {
+          throw ex;
+        }
+        let strategy = await this.handleHMACMismatch(item, true);
+        if (strategy == SyncEngine.kRecoveryStrategy.retry) {
+          // You only get one retry.
+          try {
+            // Try decrypting again, typically because we've got new keys.
+            this._log.info("Trying decrypt again...");
+            key = this.service.collectionKeys.keyForCollection(this.name);
+            await item.decrypt(key);
+            strategy = null;
+          } catch (ex) {
+            if (!Utils.isHMACMismatch(ex)) {
+              throw ex;
+            }
+            strategy = await this.handleHMACMismatch(item, false);
+          }
+        }
+
+        switch (strategy) {
+          case null:
+            // Retry succeeded! No further handling.
+            break;
+          case SyncEngine.kRecoveryStrategy.retry:
+            this._log.debug("Ignoring second retry suggestion.");
+          // Fall through to error case.
+          case SyncEngine.kRecoveryStrategy.error:
+            this._log.warn("Error decrypting record", ex);
+            return { shouldApply: false, error: ex };
+          case SyncEngine.kRecoveryStrategy.ignore:
+            this._log.debug(
+              "Ignoring record " + item.id + " with bad HMAC: already handled."
+            );
+            return { shouldApply: false, error: null };
+        }
+      }
+    } catch (ex) {
+      if (Async.isShutdownException(ex)) {
+        throw ex;
+      }
+      this._log.warn("Error decrypting record", ex);
+      return { shouldApply: false, error: ex };
+    }
+
+    if (this._shouldDeleteRemotely(item)) {
+      this._log.trace("Deleting item from server without applying", item);
+      await this._deleteId(item.id);
+      return { shouldApply: false, error: null };
+    }
+
+    let shouldApply;
+    try {
+      shouldApply = await this._reconcile(item);
+    } catch (ex) {
+      if (ex.code == SyncEngine.prototype.eEngineAbortApplyIncoming) {
+        this._log.warn("Reconciliation failed: aborting incoming processing.");
+        throw ex.cause;
+      } else if (!Async.isShutdownException(ex)) {
+        this._log.warn("Failed to reconcile incoming record " + item.id, ex);
+        return { shouldApply: false, error: ex };
+      } else {
+        throw ex;
+      }
+    }
+
+    if (!shouldApply) {
+      this._log.trace("Skipping reconciled incoming item " + item.id);
+    }
+
+    return { shouldApply, error: null };
+  },
+
+  async _applyRecords(records, countTelemetry) {
+    this._tracker.ignoreAll = true;
+    try {
+      let failedIDs = await this._store.applyIncomingBatch(
+        records,
+        countTelemetry
+      );
+      return failedIDs;
+    } catch (ex) {
+      // Catch any error that escapes from applyIncomingBatch. At present
+      // those will all be abort events.
+      this._log.warn("Got exception, aborting processIncoming", ex);
+      throw ex;
+    } finally {
+      this._tracker.ignoreAll = false;
+    }
+  },
+
+  // Indicates whether an incoming item should be deleted from the server at
+  // the end of the sync. Engines can override this method to clean up records
+  // that shouldn't be on the server.
+  _shouldDeleteRemotely(remoteItem) {
+    return false;
+  },
+
+  /**
+   * Find a GUID of an item that is a duplicate of the incoming item but happens
+   * to have a different GUID
+   *
+   * @return GUID of the similar item; falsy otherwise
+   */
+  async _findDupe(item) {
+    // By default, assume there's no dupe items for the engine
+  },
+
+  /**
+   * Called before a remote record is discarded due to failed reconciliation.
+   * Used by bookmark sync to merge folder child orders.
+   */
+  beforeRecordDiscard(localRecord, remoteRecord, remoteIsNewer) {},
+
+  // Called when the server has a record marked as deleted, but locally we've
+  // changed it more recently than the deletion. If we return false, the
+  // record will be deleted locally. If we return true, we'll reupload the
+  // record to the server -- any extra work that's needed as part of this
+  // process should be done at this point (such as mark the record's parent
+  // for reuploading in the case of bookmarks).
+  async _shouldReviveRemotelyDeletedRecord(remoteItem) {
+    return true;
+  },
+
+  async _deleteId(id) {
+    await this._tracker.removeChangedID(id);
+    this._noteDeletedId(id);
+  },
+
+  // Marks an ID for deletion at the end of the sync.
+  _noteDeletedId(id) {
+    if (this._delete.ids == null) {
+      this._delete.ids = [id];
+    } else {
+      this._delete.ids.push(id);
+    }
+  },
+
+  async _switchItemToDupe(localDupeGUID, incomingItem) {
+    // The local, duplicate ID is always deleted on the server.
+    await this._deleteId(localDupeGUID);
+
+    // We unconditionally change the item's ID in case the engine knows of
+    // an item but doesn't expose it through itemExists. If the API
+    // contract were stronger, this could be changed.
+    this._log.debug(
+      "Switching local ID to incoming: " +
+        localDupeGUID +
+        " -> " +
+        incomingItem.id
+    );
+    return this._store.changeItemID(localDupeGUID, incomingItem.id);
+  },
+
+  /**
+   * Reconcile incoming record with local state.
+   *
+   * This function essentially determines whether to apply an incoming record.
+   *
+   * @param  item
+   *         Record from server to be tested for application.
+   * @return boolean
+   *         Truthy if incoming record should be applied. False if not.
+   */
+  async _reconcile(item) {
+    if (this._log.level <= Log.Level.Trace) {
+      this._log.trace("Incoming: " + item);
+    }
+
+    // We start reconciling by collecting a bunch of state. We do this here
+    // because some state may change during the course of this function and we
+    // need to operate on the original values.
+    let existsLocally = await this._store.itemExists(item.id);
+    let locallyModified = this._modified.has(item.id);
+
+    // TODO Handle clock drift better. Tracked in bug 721181.
+    let remoteAge = Resource.serverTime - item.modified;
+    let localAge = locallyModified
+      ? Date.now() / 1000 - this._modified.getModifiedTimestamp(item.id)
+      : null;
+    let remoteIsNewer = remoteAge < localAge;
+
+    this._log.trace(
+      "Reconciling " +
+        item.id +
+        ". exists=" +
+        existsLocally +
+        "; modified=" +
+        locallyModified +
+        "; local age=" +
+        localAge +
+        "; incoming age=" +
+        remoteAge
+    );
+
+    // We handle deletions first so subsequent logic doesn't have to check
+    // deleted flags.
+    if (item.deleted) {
+      // If the item doesn't exist locally, there is nothing for us to do. We
+      // can't check for duplicates because the incoming record has no data
+      // which can be used for duplicate detection.
+      if (!existsLocally) {
+        this._log.trace(
+          "Ignoring incoming item because it was deleted and " +
+            "the item does not exist locally."
+        );
+        return false;
+      }
+
+      // We decide whether to process the deletion by comparing the record
+      // ages. If the item is not modified locally, the remote side wins and
+      // the deletion is processed. If it is modified locally, we take the
+      // newer record.
+      if (!locallyModified) {
+        this._log.trace(
+          "Applying incoming delete because the local item " +
+            "exists and isn't modified."
+        );
+        return true;
+      }
+      this._log.trace("Incoming record is deleted but we had local changes.");
+
+      if (remoteIsNewer) {
+        this._log.trace("Remote record is newer -- deleting local record.");
+        return true;
+      }
+      // If the local record is newer, we defer to individual engines for
+      // how to handle this. By default, we revive the record.
+      let willRevive = await this._shouldReviveRemotelyDeletedRecord(item);
+      this._log.trace("Local record is newer -- reviving? " + willRevive);
+
+      return !willRevive;
+    }
+
+    // At this point the incoming record is not for a deletion and must have
+    // data. If the incoming record does not exist locally, we check for a local
+    // duplicate existing under a different ID. The default implementation of
+    // _findDupe() is empty, so engines have to opt in to this functionality.
+    //
+    // If we find a duplicate, we change the local ID to the incoming ID and we
+    // refresh the metadata collected above. See bug 710448 for the history
+    // of this logic.
+    if (!existsLocally) {
+      let localDupeGUID = await this._findDupe(item);
+      if (localDupeGUID) {
+        this._log.trace(
+          "Local item " +
+            localDupeGUID +
+            " is a duplicate for " +
+            "incoming item " +
+            item.id
+        );
+
+        // The current API contract does not mandate that the ID returned by
+        // _findDupe() actually exists. Therefore, we have to perform this
+        // check.
+        existsLocally = await this._store.itemExists(localDupeGUID);
+
+        // If the local item was modified, we carry its metadata forward so
+        // appropriate reconciling can be performed.
+        if (this._modified.has(localDupeGUID)) {
+          locallyModified = true;
+          localAge =
+            this._tracker._now() -
+            this._modified.getModifiedTimestamp(localDupeGUID);
+          remoteIsNewer = remoteAge < localAge;
+
+          this._modified.changeID(localDupeGUID, item.id);
+        } else {
+          locallyModified = false;
+          localAge = null;
+        }
+
+        // Tell the engine to do whatever it needs to switch the items.
+        await this._switchItemToDupe(localDupeGUID, item);
+
+        this._log.debug(
+          "Local item after duplication: age=" +
+            localAge +
+            "; modified=" +
+            locallyModified +
+            "; exists=" +
+            existsLocally
+        );
+      } else {
+        this._log.trace("No duplicate found for incoming item: " + item.id);
+      }
+    }
+
+    // At this point we've performed duplicate detection. But, nothing here
+    // should depend on duplicate detection as the above should have updated
+    // state seamlessly.
+
+    if (!existsLocally) {
+      // If the item doesn't exist locally and we have no local modifications
+      // to the item (implying that it was not deleted), always apply the remote
+      // item.
+      if (!locallyModified) {
+        this._log.trace(
+          "Applying incoming because local item does not exist " +
+            "and was not deleted."
+        );
+        return true;
+      }
+
+      // If the item was modified locally but isn't present, it must have
+      // been deleted. If the incoming record is younger, we restore from
+      // that record.
+      if (remoteIsNewer) {
+        this._log.trace(
+          "Applying incoming because local item was deleted " +
+            "before the incoming item was changed."
+        );
+        this._modified.delete(item.id);
+        return true;
+      }
+
+      this._log.trace(
+        "Ignoring incoming item because the local item's " +
+          "deletion is newer."
+      );
+      return false;
+    }
+
+    // If the remote and local records are the same, there is nothing to be
+    // done, so we don't do anything. In the ideal world, this logic wouldn't
+    // be here and the engine would take a record and apply it. The reason we
+    // want to defer this logic is because it would avoid a redundant and
+    // possibly expensive dip into the storage layer to query item state.
+    // This should get addressed in the async rewrite, so we ignore it for now.
+    let localRecord = await this._createRecord(item.id);
+    let recordsEqual = Utils.deepEquals(item.cleartext, localRecord.cleartext);
+
+    // If the records are the same, we don't need to do anything. This does
+    // potentially throw away a local modification time. But, if the records
+    // are the same, does it matter?
+    if (recordsEqual) {
+      this._log.trace(
+        "Ignoring incoming item because the local item is identical."
+      );
+
+      this._modified.delete(item.id);
+      return false;
+    }
+
+    // At this point the records are different.
+
+    // If we have no local modifications, always take the server record.
+    if (!locallyModified) {
+      this._log.trace("Applying incoming record because no local conflicts.");
+      return true;
+    }
+
+    // At this point, records are different and the local record is modified.
+    // We resolve conflicts by record age, where the newest one wins. This does
+    // result in data loss and should be handled by giving the engine an
+    // opportunity to merge the records. Bug 720592 tracks this feature.
+    this._log.warn(
+      "DATA LOSS: Both local and remote changes to record: " + item.id
+    );
+    if (!remoteIsNewer) {
+      this.beforeRecordDiscard(localRecord, item, remoteIsNewer);
+    }
+    return remoteIsNewer;
+  },
+
+  // Upload outgoing records.
+  async _uploadOutgoing() {
+    this._log.trace("Uploading local changes to server.");
+
+    // collection we'll upload
+    let up = new Collection(this.engineURL, null, this.service);
+    let modifiedIDs = new Set(this._modified.ids());
+    let countTelemetry = new SyncedRecordsTelemetry();
+    let counts = countTelemetry.outgoingCounts;
+    this._log.info(`Uploading ${modifiedIDs.size} outgoing records`);
+    if (modifiedIDs.size) {
+      counts.sent = modifiedIDs.size;
+
+      let failed = [];
+      let successful = [];
+      let lastSync = await this.getLastSync();
+      let handleResponse = async (postQueue, resp, batchOngoing) => {
+        // Note: We don't want to update this.lastSync, or this._modified until
+        // the batch is complete, however we want to remember success/failure
+        // indicators for when that happens.
+        if (!resp.success) {
+          this._log.debug(`Uploading records failed: ${resp.status}`);
+          resp.failureCode =
+            resp.status == 412 ? ENGINE_BATCH_INTERRUPTED : ENGINE_UPLOAD_FAIL;
+          throw resp;
+        }
+
+        // Update server timestamp from the upload.
+        failed = failed.concat(Object.keys(resp.obj.failed));
+        successful = successful.concat(resp.obj.success);
+
+        if (batchOngoing) {
+          // Nothing to do yet
+          return;
+        }
+
+        if (failed.length && this._log.level <= Log.Level.Debug) {
+          this._log.debug(
+            "Records that will be uploaded again because " +
+              "the server couldn't store them: " +
+              failed.join(", ")
+          );
+        }
+
+        counts.failed += failed.length;
+        Object.values(failed).forEach(message => {
+          countTelemetry.addOutgoingFailedReason(message);
+        });
+
+        for (let id of successful) {
+          this._modified.delete(id);
+        }
+
+        await this._onRecordsWritten(
+          successful,
+          failed,
+          postQueue.lastModified
+        );
+
+        // Advance lastSync since we've finished the batch.
+        if (postQueue.lastModified > lastSync) {
+          lastSync = postQueue.lastModified;
+          await this.setLastSync(lastSync);
+        }
+
+        // clear for next batch
+        failed.length = 0;
+        successful.length = 0;
+      };
+
+      let postQueue = up.newPostQueue(this._log, lastSync, handleResponse);
+
+      for (let id of modifiedIDs) {
+        let out;
+        let ok = false;
+        try {
+          out = await this._createRecord(id);
+          if (this._log.level <= Log.Level.Trace) {
+            this._log.trace("Outgoing: " + out);
+          }
+          await out.encrypt(
+            this.service.collectionKeys.keyForCollection(this.name)
+          );
+          ok = true;
+        } catch (ex) {
+          this._log.warn("Error creating record", ex);
+          ++counts.failed;
+          countTelemetry.addOutgoingFailedReason(ex.message);
+          if (Async.isShutdownException(ex) || !this.allowSkippedRecord) {
+            if (!this.allowSkippedRecord) {
+              // Don't bother for shutdown errors
+              Observers.notify("weave:engine:sync:uploaded", counts, this.name);
+            }
+            throw ex;
+          }
+        }
+        if (ok) {
+          let { enqueued, error } = await postQueue.enqueue(out);
+          if (!enqueued) {
+            ++counts.failed;
+            countTelemetry.addOutgoingFailedReason(error.message);
+            if (!this.allowSkippedRecord) {
+              Observers.notify("weave:engine:sync:uploaded", counts, this.name);
+              this._log.warn(
+                `Failed to enqueue record "${id}" (aborting)`,
+                error
+              );
+              throw error;
+            }
+            this._modified.delete(id);
+            this._log.warn(
+              `Failed to enqueue record "${id}" (skipping)`,
+              error
+            );
+          }
+        }
+        await Async.promiseYield();
+      }
+      await postQueue.flush(true);
+    }
+
+    if (counts.sent || counts.failed) {
+      Observers.notify("weave:engine:sync:uploaded", counts, this.name);
+    }
+  },
+
+  async _onRecordsWritten(succeeded, failed, serverModifiedTime) {
+    // Implement this method to take specific actions against successfully
+    // uploaded records and failed records.
+  },
+
+  // Any cleanup necessary.
+  // Save the current snapshot so as to calculate changes at next sync
+  async _syncFinish() {
+    this._log.trace("Finishing up sync");
+
+    let doDelete = async (key, val) => {
+      let coll = new Collection(this.engineURL, this._recordObj, this.service);
+      coll[key] = val;
+      await coll.delete();
+    };
+
+    for (let [key, val] of Object.entries(this._delete)) {
+      // Remove the key for future uses
+      delete this._delete[key];
+
+      this._log.trace("doing post-sync deletions", { key, val });
+      // Send a simple delete for the property
+      if (key != "ids" || val.length <= 100) {
+        await doDelete(key, val);
+      } else {
+        // For many ids, split into chunks of at most 100
+        while (val.length) {
+          await doDelete(key, val.slice(0, 100));
+          val = val.slice(100);
+        }
+      }
+    }
+    this.hasSyncedThisSession = true;
+    await this._tracker.asyncObserver.promiseObserversComplete();
+  },
+
+  async _syncCleanup() {
+    try {
+      // Mark failed WBOs as changed again so they are reuploaded next time.
+      await this.trackRemainingChanges();
+    } finally {
+      this._modified.clear();
+    }
+  },
+
+  async _sync() {
+    try {
+      Async.checkAppReady();
+      await this._syncStartup();
+      Async.checkAppReady();
+      Observers.notify("weave:engine:sync:status", "process-incoming");
+      await this._processIncoming();
+      Async.checkAppReady();
+      Observers.notify("weave:engine:sync:status", "upload-outgoing");
+      try {
+        await this._uploadOutgoing();
+        Async.checkAppReady();
+        await this._syncFinish();
+      } catch (ex) {
+        if (!ex.status || ex.status != 412) {
+          throw ex;
+        }
+        // a 412 posting just means another client raced - but we don't want
+        // to treat that as a sync error - the next sync is almost certain
+        // to work.
+        this._log.warn("412 error during sync - will retry.");
+      }
+    } finally {
+      await this._syncCleanup();
+    }
+  },
+
+  async canDecrypt() {
+    // Report failure even if there's nothing to decrypt
+    let canDecrypt = false;
+
+    // Fetch the most recently uploaded record and try to decrypt it
+    let test = new Collection(this.engineURL, this._recordObj, this.service);
+    test.limit = 1;
+    test.sort = "newest";
+    test.full = true;
+
+    let key = this.service.collectionKeys.keyForCollection(this.name);
+
+    // Any failure fetching/decrypting will just result in false
+    try {
+      this._log.trace("Trying to decrypt a record from the server..");
+      let json = (await test.get()).obj[0];
+      let record = new this._recordObj();
+      record.deserialize(json);
+      await record.decrypt(key);
+      canDecrypt = true;
+    } catch (ex) {
+      if (Async.isShutdownException(ex)) {
+        throw ex;
+      }
+      this._log.debug("Failed test decrypt", ex);
+    }
+
+    return canDecrypt;
+  },
+
+  /**
+   * Deletes the collection for this engine on the server, and removes all local
+   * Sync metadata for this engine. This does *not* remove any existing data on
+   * other clients. This is called when we reset the sync ID.
+   */
+  async wipeServer() {
+    await this._deleteServerCollection();
+    await this._resetClient();
+  },
+
+  /**
+   * Deletes the collection for this engine on the server, without removing
+   * any local Sync metadata or user data. Deleting the collection will not
+   * remove any user data on other clients, but will force other clients to
+   * start over as a first sync.
+   */
+  async _deleteServerCollection() {
+    let response = await this.service.resource(this.engineURL).delete();
+    if (response.status != 200 && response.status != 404) {
+      throw response;
+    }
+  },
+
+  async removeClientData() {
+    // Implement this method in engines that store client specific data
+    // on the server.
+  },
+
+  /*
+   * Decide on (and partially effect) an error-handling strategy.
+   *
+   * Asks the Service to respond to an HMAC error, which might result in keys
+   * being downloaded. That call returns true if an action which might allow a
+   * retry to occur.
+   *
+   * If `mayRetry` is truthy, and the Service suggests a retry,
+   * handleHMACMismatch returns kRecoveryStrategy.retry. Otherwise, it returns
+   * kRecoveryStrategy.error.
+   *
+   * Subclasses of SyncEngine can override this method to allow for different
+   * behavior -- e.g., to delete and ignore erroneous entries.
+   *
+   * All return values will be part of the kRecoveryStrategy enumeration.
+   */
+  async handleHMACMismatch(item, mayRetry) {
+    // By default we either try again, or bail out noisily.
+    return (await this.service.handleHMACEvent()) && mayRetry
+      ? SyncEngine.kRecoveryStrategy.retry
+      : SyncEngine.kRecoveryStrategy.error;
+  },
+
+  /**
+   * Returns a changeset containing all items in the store. The default
+   * implementation returns a changeset with timestamps from long ago, to
+   * ensure we always use the remote version if one exists.
+   *
+   * This function is only called for the first sync. Subsequent syncs call
+   * `pullNewChanges`.
+   *
+   * @return A `Changeset` object.
+   */
+  async pullAllChanges() {
+    let changes = {};
+    let ids = await this._store.getAllIDs();
+    for (let id in ids) {
+      changes[id] = 0;
+    }
+    return changes;
+  },
+
+  /*
+   * Returns a changeset containing entries for all currently tracked items.
+   * The default implementation returns a changeset with timestamps indicating
+   * when the item was added to the tracker.
+   *
+   * @return A `Changeset` object.
+   */
+  async pullNewChanges() {
+    await this._tracker.asyncObserver.promiseObserversComplete();
+    return this.getChangedIDs();
+  },
+
+  /**
+   * Adds all remaining changeset entries back to the tracker, typically for
+   * items that failed to upload. This method is called at the end of each sync.
+   *
+   */
+  async trackRemainingChanges() {
+    for (let [id, change] of this._modified.entries()) {
+      await this._tracker.addChangedID(id, change);
+    }
+  },
+
+  /**
+   * Removes all local Sync metadata for this engine, but keeps all existing
+   * local user data.
+   */
+  async resetClient() {
+    return this._notify("reset-client", this.name, this._resetClient)();
+  },
+
+  async _resetClient() {
+    await this.resetLastSync();
+    this.hasSyncedThisSession = false;
+    this.previousFailed = new SerializableSet();
+    this.toFetch = new SerializableSet();
+  },
+
+  /**
+   * Removes all local Sync metadata and user data for this engine.
+   */
+  async wipeClient() {
+    return this._notify("wipe-client", this.name, this._wipeClient)();
+  },
+
+  async _wipeClient() {
+    await this.resetClient();
+    this._log.debug("Deleting all local data");
+    this._tracker.ignoreAll = true;
+    await this._store.wipe();
+    this._tracker.ignoreAll = false;
+    this._tracker.clearChangedIDs();
+  },
+
+  /**
+   * If one exists, initialize and return a validator for this engine (which
+   * must have a `validate(engine)` method that returns a promise to an object
+   * with a getSummary method). Otherwise return null.
+   */
+  getValidator() {
+    return null;
+  },
+
+  async finalize() {
+    Services.prefs.removeObserver(
+      `${PREFS_BRANCH}engine.${this.prefName}`,
+      this.asyncObserver
+    );
+    await this.asyncObserver.promiseObserversComplete();
+    await this._tracker.finalize();
+    await this._toFetchStorage.finalize();
+    await this._previousFailedStorage.finalize();
+  },
+
+  // Returns a new watchdog. Exposed for tests.
+  _newWatchdog() {
+    return Async.watchdog();
+  },
+};
+
+/**
+ * A changeset is created for each sync in `Engine::get{Changed, All}IDs`,
+ * and stores opaque change data for tracked IDs. The default implementation
+ * only records timestamps, though engines can extend this to store additional
+ * data for each entry.
+ */
+export class Changeset {
+  // Creates an empty changeset.
+  constructor() {
+    this.changes = {};
+  }
+
+  // Returns the last modified time, in seconds, for an entry in the changeset.
+  // `id` is guaranteed to be in the set.
+  getModifiedTimestamp(id) {
+    return this.changes[id];
+  }
+
+  // Adds a change for a tracked ID to the changeset.
+  set(id, change) {
+    this.changes[id] = change;
+  }
+
+  // Adds multiple entries to the changeset, preserving existing entries.
+  insert(changes) {
+    Object.assign(this.changes, changes);
+  }
+
+  // Overwrites the existing set of tracked changes with new entries.
+  replace(changes) {
+    this.changes = changes;
+  }
+
+  // Indicates whether an entry is in the changeset.
+  has(id) {
+    return id in this.changes;
+  }
+
+  // Deletes an entry from the changeset. Used to clean up entries for
+  // reconciled and successfully uploaded records.
+  delete(id) {
+    delete this.changes[id];
+  }
+
+  // Changes the ID of an entry in the changeset. Used when reconciling
+  // duplicates that have local changes.
+  changeID(oldID, newID) {
+    this.changes[newID] = this.changes[oldID];
+    delete this.changes[oldID];
+  }
+
+  // Returns an array of all tracked IDs in this changeset.
+  ids() {
+    return Object.keys(this.changes);
+  }
+
+  // Returns an array of `[id, change]` tuples. Used to repopulate the tracker
+  // with entries for failed uploads at the end of a sync.
+  entries() {
+    return Object.entries(this.changes);
+  }
+
+  // Returns the number of entries in this changeset.
+  count() {
+    return this.ids().length;
+  }
+
+  // Clears the changeset.
+  clear() {
+    this.changes = {};
+  }
+}
diff --git a/services/sync/modules/engines/addons.sys.mjs b/services/sync/modules/engines/addons.sys.mjs
new file mode 100644
index 0000000000..782d23239e
--- /dev/null
+++ b/services/sync/modules/engines/addons.sys.mjs
@@ -0,0 +1,818 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * This file defines the add-on sync functionality.
+ *
+ * There are currently a number of known limitations:
+ *  - We only sync XPI extensions and themes available from addons.mozilla.org.
+ *    We hope to expand support for other add-ons eventually.
+ *  - We only attempt syncing of add-ons between applications of the same type.
+ *    This means add-ons will not synchronize between Firefox desktop and
+ *    Firefox mobile, for example. This is because of significant add-on
+ *    incompatibility between application types.
+ *
+ * Add-on records exist for each known {add-on, app-id} pair in the Sync client
+ * set. Each record has a randomly chosen GUID. The records then contain
+ * basic metadata about the add-on.
+ *
+ * We currently synchronize:
+ *
+ *  - Installations
+ *  - Uninstallations
+ *  - User enabling and disabling
+ *
+ * Synchronization is influenced by the following preferences:
+ *
+ *  - services.sync.addons.ignoreUserEnabledChanges
+ *  - services.sync.addons.trustedSourceHostnames
+ *
+ *  and also influenced by whether addons have repository caching enabled and
+ *  whether they allow installation of addons from insecure options (both of
+ *  which are themselves influenced by the "extensions." pref branch)
+ *
+ * See the documentation in all.js for the behavior of these prefs.
+ */
+
+import { AddonUtils } from "resource://services-sync/addonutils.sys.mjs";
+import { AddonsReconciler } from "resource://services-sync/addonsreconciler.sys.mjs";
+import {
+  Store,
+  SyncEngine,
+  LegacyTracker,
+} from "resource://services-sync/engines.sys.mjs";
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import { SCORE_INCREMENT_XLARGE } from "resource://services-sync/constants.sys.mjs";
+import { CollectionValidator } from "resource://services-sync/collection_validator.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+  AddonRepository: "resource://gre/modules/addons/AddonRepository.sys.mjs",
+});
+
+// 7 days in milliseconds.
+const PRUNE_ADDON_CHANGES_THRESHOLD = 60 * 60 * 24 * 7 * 1000;
+
+/**
+ * AddonRecord represents the state of an add-on in an application.
+ *
+ * Each add-on has its own record for each application ID it is installed
+ * on.
+ *
+ * The ID of add-on records is a randomly-generated GUID. It is random instead
+ * of deterministic so the URIs of the records cannot be guessed and so
+ * compromised server credentials won't result in disclosure of the specific
+ * add-ons present in a Sync account.
+ *
+ * The record contains the following fields:
+ *
+ *  addonID
+ *    ID of the add-on. This correlates to the "id" property on an Addon type.
+ *
+ *  applicationID
+ *    The application ID this record is associated with.
+ *
+ *  enabled
+ *    Boolean stating whether add-on is enabled or disabled by the user.
+ *
+ *  source
+ *    String indicating where an add-on is from. Currently, we only support
+ *    the value "amo" which indicates that the add-on came from the official
+ *    add-ons repository, addons.mozilla.org. In the future, we may support
+ *    installing add-ons from other sources. This provides a future-compatible
+ *    mechanism for clients to only apply records they know how to handle.
+ */
+function AddonRecord(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+AddonRecord.prototype = {
+  _logName: "Record.Addon",
+};
+Object.setPrototypeOf(AddonRecord.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(AddonRecord, "cleartext", [
+  "addonID",
+  "applicationID",
+  "enabled",
+  "source",
+]);
+
+/**
+ * The AddonsEngine handles synchronization of add-ons between clients.
+ *
+ * The engine maintains an instance of an AddonsReconciler, which is the entity
+ * maintaining state for add-ons. It provides the history and tracking APIs
+ * that AddonManager doesn't.
+ *
+ * The engine instance overrides a handful of functions on the base class. The
+ * rationale for each is documented by that function.
+ */
+export function AddonsEngine(service) {
+  SyncEngine.call(this, "Addons", service);
+
+  this._reconciler = new AddonsReconciler(this._tracker.asyncObserver);
+}
+
+AddonsEngine.prototype = {
+  _storeObj: AddonsStore,
+  _trackerObj: AddonsTracker,
+  _recordObj: AddonRecord,
+  version: 1,
+
+  syncPriority: 5,
+
+  _reconciler: null,
+
+  async initialize() {
+    await SyncEngine.prototype.initialize.call(this);
+    await this._reconciler.ensureStateLoaded();
+  },
+
+  /**
+   * Override parent method to find add-ons by their public ID, not Sync GUID.
+   */
+  async _findDupe(item) {
+    let id = item.addonID;
+
+    // The reconciler should have been updated at the top of the sync, so we
+    // can assume it is up to date when this function is called.
+    let addons = this._reconciler.addons;
+    if (!(id in addons)) {
+      return null;
+    }
+
+    let addon = addons[id];
+    if (addon.guid != item.id) {
+      return addon.guid;
+    }
+
+    return null;
+  },
+
+  /**
+   * Override getChangedIDs to pull in tracker changes plus changes from the
+   * reconciler log.
+   */
+  async getChangedIDs() {
+    let changes = {};
+    const changedIDs = await this._tracker.getChangedIDs();
+    for (let [id, modified] of Object.entries(changedIDs)) {
+      changes[id] = modified;
+    }
+
+    let lastSync = await this.getLastSync();
+    let lastSyncDate = new Date(lastSync * 1000);
+
+    // The reconciler should have been refreshed at the beginning of a sync and
+    // we assume this function is only called from within a sync.
+    let reconcilerChanges = this._reconciler.getChangesSinceDate(lastSyncDate);
+    let addons = this._reconciler.addons;
+    for (let change of reconcilerChanges) {
+      let changeTime = change[0];
+      let id = change[2];
+
+      if (!(id in addons)) {
+        continue;
+      }
+
+      // Keep newest modified time.
+      if (id in changes && changeTime < changes[id]) {
+        continue;
+      }
+
+      if (!(await this.isAddonSyncable(addons[id]))) {
+        continue;
+      }
+
+      this._log.debug("Adding changed add-on from changes log: " + id);
+      let addon = addons[id];
+      changes[addon.guid] = changeTime.getTime() / 1000;
+    }
+
+    return changes;
+  },
+
+  /**
+   * Override start of sync function to refresh reconciler.
+   *
+   * Many functions in this class assume the reconciler is refreshed at the
+   * top of a sync. If this ever changes, those functions should be revisited.
+   *
+   * Technically speaking, we don't need to refresh the reconciler on every
+   * sync since it is installed as an AddonManager listener. However, add-ons
+   * are complicated and we force a full refresh, just in case the listeners
+   * missed something.
+   */
+  async _syncStartup() {
+    // We refresh state before calling parent because syncStartup in the parent
+    // looks for changed IDs, which is dependent on add-on state being up to
+    // date.
+    await this._refreshReconcilerState();
+    return SyncEngine.prototype._syncStartup.call(this);
+  },
+
+  /**
+   * Override end of sync to perform a little housekeeping on the reconciler.
+   *
+   * We prune changes to prevent the reconciler state from growing without
+   * bound. Even if it grows unbounded, there would have to be many add-on
+   * changes (thousands) for it to slow things down significantly. This is
+   * highly unlikely to occur. Still, we exercise defense just in case.
+   */
+  async _syncCleanup() {
+    let lastSync = await this.getLastSync();
+    let ms = 1000 * lastSync - PRUNE_ADDON_CHANGES_THRESHOLD;
+    this._reconciler.pruneChangesBeforeDate(new Date(ms));
+    return SyncEngine.prototype._syncCleanup.call(this);
+  },
+
+  /**
+   * Helper function to ensure reconciler is up to date.
+   *
+   * This will load the reconciler's state from the file
+   * system (if needed) and refresh the state of the reconciler.
+   */
+  async _refreshReconcilerState() {
+    this._log.debug("Refreshing reconciler state");
+    return this._reconciler.refreshGlobalState();
+  },
+
+  // Returns a promise
+  isAddonSyncable(addon, ignoreRepoCheck) {
+    return this._store.isAddonSyncable(addon, ignoreRepoCheck);
+  },
+};
+Object.setPrototypeOf(AddonsEngine.prototype, SyncEngine.prototype);
+
+/**
+ * This is the primary interface between Sync and the Addons Manager.
+ *
+ * In addition to the core store APIs, we provide convenience functions to wrap
+ * Add-on Manager APIs with Sync-specific semantics.
+ */
+function AddonsStore(name, engine) {
+  Store.call(this, name, engine);
+}
+AddonsStore.prototype = {
+  // Define the add-on types (.type) that we support.
+  _syncableTypes: ["extension", "theme"],
+
+  _extensionsPrefs: Services.prefs.getBranch("extensions."),
+
+  get reconciler() {
+    return this.engine._reconciler;
+  },
+
+  /**
+   * Override applyIncoming to filter out records we can't handle.
+   */
+  async applyIncoming(record) {
+    // The fields we look at aren't present when the record is deleted.
+    if (!record.deleted) {
+      // Ignore records not belonging to our application ID because that is the
+      // current policy.
+      if (record.applicationID != Services.appinfo.ID) {
+        this._log.info(
+          "Ignoring incoming record from other App ID: " + record.id
+        );
+        return;
+      }
+
+      // Ignore records that aren't from the official add-on repository, as that
+      // is our current policy.
+      if (record.source != "amo") {
+        this._log.info(
+          "Ignoring unknown add-on source (" +
+            record.source +
+            ")" +
+            " for " +
+            record.id
+        );
+        return;
+      }
+    }
+
+    // Ignore incoming records for which an existing non-syncable addon
+    // exists. Note that we do not insist that the addon manager already have
+    // metadata for this addon - it's possible our reconciler previously saw the
+    // addon but the addon-manager cache no longer has it - which is fine for a
+    // new incoming addon.
+    // (Note that most other cases where the addon-manager cache is invalid
+    // doesn't get this treatment because that cache self-repairs after some
+    // time - but it only re-populates addons which are currently installed.)
+    let existingMeta = this.reconciler.addons[record.addonID];
+    if (
+      existingMeta &&
+      !(await this.isAddonSyncable(existingMeta, /* ignoreRepoCheck */ true))
+    ) {
+      this._log.info(
+        "Ignoring incoming record for an existing but non-syncable addon",
+        record.addonID
+      );
+      return;
+    }
+
+    await Store.prototype.applyIncoming.call(this, record);
+  },
+
+  /**
+   * Provides core Store API to create/install an add-on from a record.
+   */
+  async create(record) {
+    // This will throw if there was an error. This will get caught by the sync
+    // engine and the record will try to be applied later.
+    const results = await AddonUtils.installAddons([
+      {
+        id: record.addonID,
+        syncGUID: record.id,
+        enabled: record.enabled,
+        requireSecureURI: this._extensionsPrefs.getBoolPref(
+          "install.requireSecureOrigin",
+          true
+        ),
+      },
+    ]);
+
+    if (results.skipped.includes(record.addonID)) {
+      this._log.info("Add-on skipped: " + record.addonID);
+      // Just early-return for skipped addons - we don't want to arrange to
+      // try again next time because the condition that caused up to skip
+      // will remain true for this addon forever.
+      return;
+    }
+
+    let addon;
+    for (let a of results.addons) {
+      if (a.id == record.addonID) {
+        addon = a;
+        break;
+      }
+    }
+
+    // This should never happen, but is present as a fail-safe.
+    if (!addon) {
+      throw new Error("Add-on not found after install: " + record.addonID);
+    }
+
+    this._log.info("Add-on installed: " + record.addonID);
+  },
+
+  /**
+   * Provides core Store API to remove/uninstall an add-on from a record.
+   */
+  async remove(record) {
+    // If this is called, the payload is empty, so we have to find by GUID.
+    let addon = await this.getAddonByGUID(record.id);
+    if (!addon) {
+      // We don't throw because if the add-on could not be found then we assume
+      // it has already been uninstalled and there is nothing for this function
+      // to do.
+      return;
+    }
+
+    this._log.info("Uninstalling add-on: " + addon.id);
+    await AddonUtils.uninstallAddon(addon);
+  },
+
+  /**
+   * Provides core Store API to update an add-on from a record.
+   */
+  async update(record) {
+    let addon = await this.getAddonByID(record.addonID);
+
+    // update() is called if !this.itemExists. And, since itemExists consults
+    // the reconciler only, we need to take care of some corner cases.
+    //
+    // First, the reconciler could know about an add-on that was uninstalled
+    // and no longer present in the add-ons manager.
+    if (!addon) {
+      await this.create(record);
+      return;
+    }
+
+    // It's also possible that the add-on is non-restartless and has pending
+    // install/uninstall activity.
+    //
+    // We wouldn't get here if the incoming record was for a deletion. So,
+    // check for pending uninstall and cancel if necessary.
+    if (addon.pendingOperations & lazy.AddonManager.PENDING_UNINSTALL) {
+      addon.cancelUninstall();
+
+      // We continue with processing because there could be state or ID change.
+    }
+
+    await this.updateUserDisabled(addon, !record.enabled);
+  },
+
+  /**
+   * Provide core Store API to determine if a record exists.
+   */
+  async itemExists(guid) {
+    let addon = this.reconciler.getAddonStateFromSyncGUID(guid);
+
+    return !!addon;
+  },
+
+  /**
+   * Create an add-on record from its GUID.
+   *
+   * @param guid
+   *        Add-on GUID (from extensions DB)
+   * @param collection
+   *        Collection to add record to.
+   *
+   * @return AddonRecord instance
+   */
+  async createRecord(guid, collection) {
+    let record = new AddonRecord(collection, guid);
+    record.applicationID = Services.appinfo.ID;
+
+    let addon = this.reconciler.getAddonStateFromSyncGUID(guid);
+
+    // If we don't know about this GUID or if it has been uninstalled, we mark
+    // the record as deleted.
+    if (!addon || !addon.installed) {
+      record.deleted = true;
+      return record;
+    }
+
+    record.modified = addon.modified.getTime() / 1000;
+
+    record.addonID = addon.id;
+    record.enabled = addon.enabled;
+
+    // This needs to be dynamic when add-ons don't come from AddonRepository.
+    record.source = "amo";
+
+    return record;
+  },
+
+  /**
+   * Changes the id of an add-on.
+   *
+   * This implements a core API of the store.
+   */
+  async changeItemID(oldID, newID) {
+    // We always update the GUID in the reconciler because it will be
+    // referenced later in the sync process.
+    let state = this.reconciler.getAddonStateFromSyncGUID(oldID);
+    if (state) {
+      state.guid = newID;
+      await this.reconciler.saveState();
+    }
+
+    let addon = await this.getAddonByGUID(oldID);
+    if (!addon) {
+      this._log.debug(
+        "Cannot change item ID (" +
+          oldID +
+          ") in Add-on " +
+          "Manager because old add-on not present: " +
+          oldID
+      );
+      return;
+    }
+
+    addon.syncGUID = newID;
+  },
+
+  /**
+   * Obtain the set of all syncable add-on Sync GUIDs.
+   *
+   * This implements a core Store API.
+   */
+  async getAllIDs() {
+    let ids = {};
+
+    let addons = this.reconciler.addons;
+    for (let id in addons) {
+      let addon = addons[id];
+      if (await this.isAddonSyncable(addon)) {
+        ids[addon.guid] = true;
+      }
+    }
+
+    return ids;
+  },
+
+  /**
+   * Wipe engine data.
+   *
+   * This uninstalls all syncable addons from the application. In case of
+   * error, it logs the error and keeps trying with other add-ons.
+   */
+  async wipe() {
+    this._log.info("Processing wipe.");
+
+    await this.engine._refreshReconcilerState();
+
+    // We only wipe syncable add-ons. Wipe is a Sync feature not a security
+    // feature.
+    let ids = await this.getAllIDs();
+    for (let guid in ids) {
+      let addon = await this.getAddonByGUID(guid);
+      if (!addon) {
+        this._log.debug(
+          "Ignoring add-on because it couldn't be obtained: " + guid
+        );
+        continue;
+      }
+
+      this._log.info("Uninstalling add-on as part of wipe: " + addon.id);
+      await Utils.catch.call(this, () => addon.uninstall())();
+    }
+  },
+
+  /** *************************************************************************
+   * Functions below are unique to this store and not part of the Store API  *
+   ***************************************************************************/
+
+  /**
+   * Obtain an add-on from its public ID.
+   *
+   * @param id
+   *        Add-on ID
+   * @return Addon or undefined if not found
+   */
+  async getAddonByID(id) {
+    return lazy.AddonManager.getAddonByID(id);
+  },
+
+  /**
+   * Obtain an add-on from its Sync GUID.
+   *
+   * @param  guid
+   *         Add-on Sync GUID
+   * @return DBAddonInternal or null
+   */
+  async getAddonByGUID(guid) {
+    return lazy.AddonManager.getAddonBySyncGUID(guid);
+  },
+
+  /**
+   * Determines whether an add-on is suitable for Sync.
+   *
+   * @param  addon
+   *         Addon instance
+   * @param ignoreRepoCheck
+   *         Should we skip checking the Addons repository (primarially useful
+   *         for testing and validation).
+   * @return Boolean indicating whether it is appropriate for Sync
+   */
+  async isAddonSyncable(addon, ignoreRepoCheck = false) {
+    // Currently, we limit syncable add-ons to those that are:
+    //   1) In a well-defined set of types
+    //   2) Installed in the current profile
+    //   3) Not installed by a foreign entity (i.e. installed by the app)
+    //      since they act like global extensions.
+    //   4) Is not a hotfix.
+    //   5) The addons XPIProvider doesn't veto it (i.e not being installed in
+    //      the profile directory, or any other reasons it says the addon can't
+    //      be synced)
+    //   6) Are installed from AMO
+
+    // We could represent the test as a complex boolean expression. We go the
+    // verbose route so the failure reason is logged.
+    if (!addon) {
+      this._log.debug("Null object passed to isAddonSyncable.");
+      return false;
+    }
+
+    if (!this._syncableTypes.includes(addon.type)) {
+      this._log.debug(
+        addon.id + " not syncable: type not in allowed list: " + addon.type
+      );
+      return false;
+    }
+
+    if (!(addon.scope & lazy.AddonManager.SCOPE_PROFILE)) {
+      this._log.debug(addon.id + " not syncable: not installed in profile.");
+      return false;
+    }
+
+    // If the addon manager says it's not syncable, we skip it.
+    if (!addon.isSyncable) {
+      this._log.debug(addon.id + " not syncable: vetoed by the addon manager.");
+      return false;
+    }
+
+    // This may be too aggressive. If an add-on is downloaded from AMO and
+    // manually placed in the profile directory, foreignInstall will be set.
+    // Arguably, that add-on should be syncable.
+    // TODO Address the edge case and come up with more robust heuristics.
+    if (addon.foreignInstall) {
+      this._log.debug(addon.id + " not syncable: is foreign install.");
+      return false;
+    }
+
+    // If the AddonRepository's cache isn't enabled (which it typically isn't
+    // in tests), getCachedAddonByID always returns null - so skip the check
+    // in that case. We also provide a way to specifically opt-out of the check
+    // even if the cache is enabled, which is used by the validators.
+    if (ignoreRepoCheck || !lazy.AddonRepository.cacheEnabled) {
+      return true;
+    }
+
+    let result = await new Promise(res => {
+      lazy.AddonRepository.getCachedAddonByID(addon.id, res);
+    });
+
+    if (!result) {
+      this._log.debug(
+        addon.id + " not syncable: add-on not found in add-on repository."
+      );
+      return false;
+    }
+
+    return this.isSourceURITrusted(result.sourceURI);
+  },
+
+  /**
+   * Determine whether an add-on's sourceURI field is trusted and the add-on
+   * can be installed.
+   *
+   * This function should only ever be called from isAddonSyncable(). It is
+   * exposed as a separate function to make testing easier.
+   *
+   * @param  uri
+   *         nsIURI instance to validate
+   * @return bool
+   */
+  isSourceURITrusted: function isSourceURITrusted(uri) {
+    // For security reasons, we currently limit synced add-ons to those
+    // installed from trusted hostname(s). We additionally require TLS with
+    // the add-ons site to help prevent forgeries.
+    let trustedHostnames = Svc.PrefBranch.getStringPref(
+      "addons.trustedSourceHostnames",
+      ""
+    ).split(",");
+
+    if (!uri) {
+      this._log.debug("Undefined argument to isSourceURITrusted().");
+      return false;
+    }
+
+    // Scheme is validated before the hostname because uri.host may not be
+    // populated for certain schemes. It appears to always be populated for
+    // https, so we avoid the potential NS_ERROR_FAILURE on field access.
+    if (uri.scheme != "https") {
+      this._log.debug("Source URI not HTTPS: " + uri.spec);
+      return false;
+    }
+
+    if (!trustedHostnames.includes(uri.host)) {
+      this._log.debug("Source hostname not trusted: " + uri.host);
+      return false;
+    }
+
+    return true;
+  },
+
+  /**
+   * Update the userDisabled flag on an add-on.
+   *
+   * This will enable or disable an add-on. It has no return value and does
+   * not catch or handle exceptions thrown by the addon manager. If no action
+   * is needed it will return immediately.
+   *
+   * @param addon
+   *        Addon instance to manipulate.
+   * @param value
+   *        Boolean to which to set userDisabled on the passed Addon.
+   */
+  async updateUserDisabled(addon, value) {
+    if (addon.userDisabled == value) {
+      return;
+    }
+
+    // A pref allows changes to the enabled flag to be ignored.
+    if (Svc.PrefBranch.getBoolPref("addons.ignoreUserEnabledChanges", false)) {
+      this._log.info(
+        "Ignoring enabled state change due to preference: " + addon.id
+      );
+      return;
+    }
+
+    AddonUtils.updateUserDisabled(addon, value);
+    // updating this flag doesn't send a notification for appDisabled addons,
+    // meaning the reconciler will not update its state and may resync the
+    // addon - so explicitly rectify the state (bug 1366994)
+    if (addon.appDisabled) {
+      await this.reconciler.rectifyStateFromAddon(addon);
+    }
+  },
+};
+
+Object.setPrototypeOf(AddonsStore.prototype, Store.prototype);
+
+/**
+ * The add-ons tracker keeps track of real-time changes to add-ons.
+ *
+ * It hooks up to the reconciler and receives notifications directly from it.
+ */
+function AddonsTracker(name, engine) {
+  LegacyTracker.call(this, name, engine);
+}
+AddonsTracker.prototype = {
+  get reconciler() {
+    return this.engine._reconciler;
+  },
+
+  get store() {
+    return this.engine._store;
+  },
+
+  /**
+   * This callback is executed whenever the AddonsReconciler sends out a change
+   * notification. See AddonsReconciler.addChangeListener().
+   */
+  async changeListener(date, change, addon) {
+    this._log.debug("changeListener invoked: " + change + " " + addon.id);
+    // Ignore changes that occur during sync.
+    if (this.ignoreAll) {
+      return;
+    }
+
+    if (!(await this.store.isAddonSyncable(addon))) {
+      this._log.debug(
+        "Ignoring change because add-on isn't syncable: " + addon.id
+      );
+      return;
+    }
+
+    const added = await this.addChangedID(addon.guid, date.getTime() / 1000);
+    if (added) {
+      this.score += SCORE_INCREMENT_XLARGE;
+    }
+  },
+
+  onStart() {
+    this.reconciler.startListening();
+    this.reconciler.addChangeListener(this);
+  },
+
+  onStop() {
+    this.reconciler.removeChangeListener(this);
+    this.reconciler.stopListening();
+  },
+};
+
+Object.setPrototypeOf(AddonsTracker.prototype, LegacyTracker.prototype);
+
+export class AddonValidator extends CollectionValidator {
+  constructor(engine = null) {
+    super("addons", "id", ["addonID", "enabled", "applicationID", "source"]);
+    this.engine = engine;
+  }
+
+  async getClientItems() {
+    return lazy.AddonManager.getAllAddons();
+  }
+
+  normalizeClientItem(item) {
+    let enabled = !item.userDisabled;
+    if (item.pendingOperations & lazy.AddonManager.PENDING_ENABLE) {
+      enabled = true;
+    } else if (item.pendingOperations & lazy.AddonManager.PENDING_DISABLE) {
+      enabled = false;
+    }
+    return {
+      enabled,
+      id: item.syncGUID,
+      addonID: item.id,
+      applicationID: Services.appinfo.ID,
+      source: "amo", // check item.foreignInstall?
+      original: item,
+    };
+  }
+
+  async normalizeServerItem(item) {
+    let guid = await this.engine._findDupe(item);
+    if (guid) {
+      item.id = guid;
+    }
+    return item;
+  }
+
+  clientUnderstands(item) {
+    return item.applicationID === Services.appinfo.ID;
+  }
+
+  async syncedByClient(item) {
+    return (
+      !item.original.hidden &&
+      !item.original.isSystem &&
+      !(
+        item.original.pendingOperations & lazy.AddonManager.PENDING_UNINSTALL
+      ) &&
+      // No need to await the returned promise explicitely:
+      // |expr1 && expr2| evaluates to expr2 if expr1 is true.
+      this.engine.isAddonSyncable(item.original, true)
+    );
+  }
+}
diff --git a/services/sync/modules/engines/bookmarks.sys.mjs b/services/sync/modules/engines/bookmarks.sys.mjs
new file mode 100644
index 0000000000..3c1396f67d
--- /dev/null
+++ b/services/sync/modules/engines/bookmarks.sys.mjs
@@ -0,0 +1,950 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { SCORE_INCREMENT_XLARGE } from "resource://services-sync/constants.sys.mjs";
+import {
+  Changeset,
+  Store,
+  SyncEngine,
+  Tracker,
+} from "resource://services-sync/engines.sys.mjs";
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Async: "resource://services-common/async.sys.mjs",
+  Observers: "resource://services-common/observers.sys.mjs",
+  PlacesBackups: "resource://gre/modules/PlacesBackups.sys.mjs",
+  PlacesDBUtils: "resource://gre/modules/PlacesDBUtils.sys.mjs",
+  PlacesSyncUtils: "resource://gre/modules/PlacesSyncUtils.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+  Resource: "resource://services-sync/resource.sys.mjs",
+  SyncedBookmarksMirror: "resource://gre/modules/SyncedBookmarksMirror.sys.mjs",
+});
+
+const PLACES_MAINTENANCE_INTERVAL_SECONDS = 4 * 60 * 60; // 4 hours.
+
+const FOLDER_SORTINDEX = 1000000;
+
+// Roots that should be deleted from the server, instead of applied locally.
+// This matches `AndroidBrowserBookmarksRepositorySession::forbiddenGUID`,
+// but allows tags because we don't want to reparent tag folders or tag items
+// to "unfiled".
+const FORBIDDEN_INCOMING_IDS = ["pinned", "places", "readinglist"];
+
+// Items with these parents should be deleted from the server. We allow
+// children of the Places root, to avoid orphaning left pane queries and other
+// descendants of custom roots.
+const FORBIDDEN_INCOMING_PARENT_IDS = ["pinned", "readinglist"];
+
+// The tracker ignores changes made by import and restore, to avoid bumping the
+// score and triggering syncs during the process, as well as changes made by
+// Sync.
+ChromeUtils.defineLazyGetter(lazy, "IGNORED_SOURCES", () => [
+  lazy.PlacesUtils.bookmarks.SOURCES.SYNC,
+  lazy.PlacesUtils.bookmarks.SOURCES.IMPORT,
+  lazy.PlacesUtils.bookmarks.SOURCES.RESTORE,
+  lazy.PlacesUtils.bookmarks.SOURCES.RESTORE_ON_STARTUP,
+  lazy.PlacesUtils.bookmarks.SOURCES.SYNC_REPARENT_REMOVED_FOLDER_CHILDREN,
+]);
+
+// The validation telemetry version for the engine. Version 1 is collected
+// by `bookmark_validator.js`, and checks value as well as structure
+// differences. Version 2 is collected by the engine as part of building the
+// remote tree, and checks structure differences only.
+const BOOKMARK_VALIDATOR_VERSION = 2;
+
+// The maximum time that the engine should wait before aborting a bookmark
+// merge.
+const BOOKMARK_APPLY_TIMEOUT_MS = 5 * 60 * 60 * 1000; // 5 minutes
+
+// The default frecency value to use when not known.
+const FRECENCY_UNKNOWN = -1;
+
+// Returns the constructor for a bookmark record type.
+function getTypeObject(type) {
+  switch (type) {
+    case "bookmark":
+      return Bookmark;
+    case "query":
+      return BookmarkQuery;
+    case "folder":
+      return BookmarkFolder;
+    case "livemark":
+      return Livemark;
+    case "separator":
+      return BookmarkSeparator;
+    case "item":
+      return PlacesItem;
+  }
+  return null;
+}
+
+export function PlacesItem(collection, id, type) {
+  CryptoWrapper.call(this, collection, id);
+  this.type = type || "item";
+}
+
+PlacesItem.prototype = {
+  async decrypt(keyBundle) {
+    // Do the normal CryptoWrapper decrypt, but change types before returning
+    let clear = await CryptoWrapper.prototype.decrypt.call(this, keyBundle);
+
+    // Convert the abstract places item to the actual object type
+    if (!this.deleted) {
+      Object.setPrototypeOf(this, this.getTypeObject(this.type).prototype);
+    }
+
+    return clear;
+  },
+
+  getTypeObject: function PlacesItem_getTypeObject(type) {
+    let recordObj = getTypeObject(type);
+    if (!recordObj) {
+      throw new Error("Unknown places item object type: " + type);
+    }
+    return recordObj;
+  },
+
+  _logName: "Sync.Record.PlacesItem",
+
+  // Converts the record to a Sync bookmark object that can be passed to
+  // `PlacesSyncUtils.bookmarks.{insert, update}`.
+  toSyncBookmark() {
+    let result = {
+      kind: this.type,
+      recordId: this.id,
+      parentRecordId: this.parentid,
+    };
+    let dateAdded = lazy.PlacesSyncUtils.bookmarks.ratchetTimestampBackwards(
+      this.dateAdded,
+      +this.modified * 1000
+    );
+    if (dateAdded > 0) {
+      result.dateAdded = dateAdded;
+    }
+    return result;
+  },
+
+  // Populates the record from a Sync bookmark object returned from
+  // `PlacesSyncUtils.bookmarks.fetch`.
+  fromSyncBookmark(item) {
+    this.parentid = item.parentRecordId;
+    this.parentName = item.parentTitle;
+    if (item.dateAdded) {
+      this.dateAdded = item.dateAdded;
+    }
+  },
+};
+
+Object.setPrototypeOf(PlacesItem.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(PlacesItem, "cleartext", [
+  "hasDupe",
+  "parentid",
+  "parentName",
+  "type",
+  "dateAdded",
+]);
+
+export function Bookmark(collection, id, type) {
+  PlacesItem.call(this, collection, id, type || "bookmark");
+}
+
+Bookmark.prototype = {
+  _logName: "Sync.Record.Bookmark",
+
+  toSyncBookmark() {
+    let info = PlacesItem.prototype.toSyncBookmark.call(this);
+    info.title = this.title;
+    info.url = this.bmkUri;
+    info.description = this.description;
+    info.tags = this.tags;
+    info.keyword = this.keyword;
+    return info;
+  },
+
+  fromSyncBookmark(item) {
+    PlacesItem.prototype.fromSyncBookmark.call(this, item);
+    this.title = item.title;
+    this.bmkUri = item.url.href;
+    this.description = item.description;
+    this.tags = item.tags;
+    this.keyword = item.keyword;
+  },
+};
+
+Object.setPrototypeOf(Bookmark.prototype, PlacesItem.prototype);
+
+Utils.deferGetSet(Bookmark, "cleartext", [
+  "title",
+  "bmkUri",
+  "description",
+  "tags",
+  "keyword",
+]);
+
+export function BookmarkQuery(collection, id) {
+  Bookmark.call(this, collection, id, "query");
+}
+
+BookmarkQuery.prototype = {
+  _logName: "Sync.Record.BookmarkQuery",
+
+  toSyncBookmark() {
+    let info = Bookmark.prototype.toSyncBookmark.call(this);
+    info.folder = this.folderName || undefined; // empty string -> undefined
+    info.query = this.queryId;
+    return info;
+  },
+
+  fromSyncBookmark(item) {
+    Bookmark.prototype.fromSyncBookmark.call(this, item);
+    this.folderName = item.folder || undefined; // empty string -> undefined
+    this.queryId = item.query;
+  },
+};
+
+Object.setPrototypeOf(BookmarkQuery.prototype, Bookmark.prototype);
+
+Utils.deferGetSet(BookmarkQuery, "cleartext", ["folderName", "queryId"]);
+
+export function BookmarkFolder(collection, id, type) {
+  PlacesItem.call(this, collection, id, type || "folder");
+}
+
+BookmarkFolder.prototype = {
+  _logName: "Sync.Record.Folder",
+
+  toSyncBookmark() {
+    let info = PlacesItem.prototype.toSyncBookmark.call(this);
+    info.description = this.description;
+    info.title = this.title;
+    return info;
+  },
+
+  fromSyncBookmark(item) {
+    PlacesItem.prototype.fromSyncBookmark.call(this, item);
+    this.title = item.title;
+    this.description = item.description;
+    this.children = item.childRecordIds;
+  },
+};
+
+Object.setPrototypeOf(BookmarkFolder.prototype, PlacesItem.prototype);
+
+Utils.deferGetSet(BookmarkFolder, "cleartext", [
+  "description",
+  "title",
+  "children",
+]);
+
+export function Livemark(collection, id) {
+  BookmarkFolder.call(this, collection, id, "livemark");
+}
+
+Livemark.prototype = {
+  _logName: "Sync.Record.Livemark",
+
+  toSyncBookmark() {
+    let info = BookmarkFolder.prototype.toSyncBookmark.call(this);
+    info.feed = this.feedUri;
+    info.site = this.siteUri;
+    return info;
+  },
+
+  fromSyncBookmark(item) {
+    BookmarkFolder.prototype.fromSyncBookmark.call(this, item);
+    this.feedUri = item.feed.href;
+    if (item.site) {
+      this.siteUri = item.site.href;
+    }
+  },
+};
+
+Object.setPrototypeOf(Livemark.prototype, BookmarkFolder.prototype);
+
+Utils.deferGetSet(Livemark, "cleartext", ["siteUri", "feedUri"]);
+
+export function BookmarkSeparator(collection, id) {
+  PlacesItem.call(this, collection, id, "separator");
+}
+
+BookmarkSeparator.prototype = {
+  _logName: "Sync.Record.Separator",
+
+  fromSyncBookmark(item) {
+    PlacesItem.prototype.fromSyncBookmark.call(this, item);
+    this.pos = item.index;
+  },
+};
+
+Object.setPrototypeOf(BookmarkSeparator.prototype, PlacesItem.prototype);
+
+Utils.deferGetSet(BookmarkSeparator, "cleartext", "pos");
+
+/**
+ * The bookmarks engine uses a different store that stages downloaded bookmarks
+ * in a separate database, instead of writing directly to Places. The buffer
+ * handles reconciliation, so we stub out `_reconcile`, and wait to pull changes
+ * until we're ready to upload.
+ */
+export function BookmarksEngine(service) {
+  SyncEngine.call(this, "Bookmarks", service);
+}
+
+BookmarksEngine.prototype = {
+  _recordObj: PlacesItem,
+  _trackerObj: BookmarksTracker,
+  _storeObj: BookmarksStore,
+  version: 2,
+  // Used to override the engine name in telemetry, so that we can distinguish
+  // this engine from the old, now removed non-buffered engine.
+  overrideTelemetryName: "bookmarks-buffered",
+
+  // Needed to ensure we don't miss items when resuming a sync that failed or
+  // aborted early.
+  _defaultSort: "oldest",
+
+  syncPriority: 4,
+  allowSkippedRecord: false,
+
+  async _ensureCurrentSyncID(newSyncID) {
+    await lazy.PlacesSyncUtils.bookmarks.ensureCurrentSyncId(newSyncID);
+    let buf = await this._store.ensureOpenMirror();
+    await buf.ensureCurrentSyncId(newSyncID);
+  },
+
+  async ensureCurrentSyncID(newSyncID) {
+    let shouldWipeRemote =
+      await lazy.PlacesSyncUtils.bookmarks.shouldWipeRemote();
+    if (!shouldWipeRemote) {
+      this._log.debug(
+        "Checking if server sync ID ${newSyncID} matches existing",
+        { newSyncID }
+      );
+      await this._ensureCurrentSyncID(newSyncID);
+      return newSyncID;
+    }
+    // We didn't take the new sync ID because we need to wipe the server
+    // and other clients after a restore. Send the command, wipe the
+    // server, and reset our sync ID to reupload everything.
+    this._log.debug(
+      "Ignoring server sync ID ${newSyncID} after restore; " +
+        "wiping server and resetting sync ID",
+      { newSyncID }
+    );
+    await this.service.clientsEngine.sendCommand(
+      "wipeEngine",
+      [this.name],
+      null,
+      { reason: "bookmark-restore" }
+    );
+    let assignedSyncID = await this.resetSyncID();
+    return assignedSyncID;
+  },
+
+  async getSyncID() {
+    return lazy.PlacesSyncUtils.bookmarks.getSyncId();
+  },
+
+  async resetSyncID() {
+    await this._deleteServerCollection();
+    return this.resetLocalSyncID();
+  },
+
+  async resetLocalSyncID() {
+    let newSyncID = await lazy.PlacesSyncUtils.bookmarks.resetSyncId();
+    this._log.debug("Assigned new sync ID ${newSyncID}", { newSyncID });
+    let buf = await this._store.ensureOpenMirror();
+    await buf.ensureCurrentSyncId(newSyncID);
+    return newSyncID;
+  },
+
+  async getLastSync() {
+    let mirror = await this._store.ensureOpenMirror();
+    return mirror.getCollectionHighWaterMark();
+  },
+
+  async setLastSync(lastSync) {
+    let mirror = await this._store.ensureOpenMirror();
+    await mirror.setCollectionLastModified(lastSync);
+    // Update the last sync time in Places so that reverting to the original
+    // bookmarks engine doesn't download records we've already applied.
+    await lazy.PlacesSyncUtils.bookmarks.setLastSync(lastSync);
+  },
+
+  async _syncStartup() {
+    await super._syncStartup();
+
+    try {
+      // For first syncs, back up the user's bookmarks.
+      let lastSync = await this.getLastSync();
+      if (!lastSync) {
+        this._log.debug("Bookmarks backup starting");
+        await lazy.PlacesBackups.create(null, true);
+        this._log.debug("Bookmarks backup done");
+      }
+    } catch (ex) {
+      // Failure to create a backup is somewhat bad, but probably not bad
+      // enough to prevent syncing of bookmarks - so just log the error and
+      // continue.
+      this._log.warn(
+        "Error while backing up bookmarks, but continuing with sync",
+        ex
+      );
+    }
+  },
+
+  async _sync() {
+    try {
+      await super._sync();
+      if (this._ranMaintenanceOnLastSync) {
+        // If the last sync failed, we ran maintenance, and this sync succeeded,
+        // maintenance likely fixed the issue.
+        this._ranMaintenanceOnLastSync = false;
+        this.service.recordTelemetryEvent("maintenance", "fix", "bookmarks");
+      }
+    } catch (ex) {
+      if (
+        lazy.Async.isShutdownException(ex) ||
+        ex.status > 0 ||
+        ex.name == "InterruptedError"
+      ) {
+        // Don't run maintenance on shutdown or HTTP errors, or if we aborted
+        // the sync because the user changed their bookmarks during merging.
+        throw ex;
+      }
+      if (ex.name == "MergeConflictError") {
+        this._log.warn(
+          "Bookmark syncing ran into a merge conflict error...will retry later"
+        );
+        return;
+      }
+      // Run Places maintenance periodically to try to recover from corruption
+      // that might have caused the sync to fail. We cap the interval because
+      // persistent failures likely indicate a problem that won't be fixed by
+      // running maintenance after every failed sync.
+      let elapsedSinceMaintenance =
+        Date.now() / 1000 -
+        Services.prefs.getIntPref("places.database.lastMaintenance", 0);
+      if (elapsedSinceMaintenance >= PLACES_MAINTENANCE_INTERVAL_SECONDS) {
+        this._log.error(
+          "Bookmark sync failed, ${elapsedSinceMaintenance}s " +
+            "elapsed since last run; running Places maintenance",
+          { elapsedSinceMaintenance }
+        );
+        await lazy.PlacesDBUtils.maintenanceOnIdle();
+        this._ranMaintenanceOnLastSync = true;
+        this.service.recordTelemetryEvent("maintenance", "run", "bookmarks");
+      } else {
+        this._ranMaintenanceOnLastSync = false;
+      }
+      throw ex;
+    }
+  },
+
+  async _syncFinish() {
+    await SyncEngine.prototype._syncFinish.call(this);
+    await lazy.PlacesSyncUtils.bookmarks.ensureMobileQuery();
+  },
+
+  async pullAllChanges() {
+    return this.pullNewChanges();
+  },
+
+  async trackRemainingChanges() {
+    let changes = this._modified.changes;
+    await lazy.PlacesSyncUtils.bookmarks.pushChanges(changes);
+  },
+
+  _deleteId(id) {
+    this._noteDeletedId(id);
+  },
+
+  // The bookmarks engine rarely calls this method directly, except in tests or
+  // when handling a `reset{All, Engine}` command from another client. We
+  // usually reset local Sync metadata on a sync ID mismatch, which both engines
+  // override with logic that lives in Places and the mirror.
+  async _resetClient() {
+    await super._resetClient();
+    await lazy.PlacesSyncUtils.bookmarks.reset();
+    let buf = await this._store.ensureOpenMirror();
+    await buf.reset();
+  },
+
+  // Cleans up the Places root, reading list items (ignored in bug 762118,
+  // removed in bug 1155684), and pinned sites.
+  _shouldDeleteRemotely(incomingItem) {
+    return (
+      FORBIDDEN_INCOMING_IDS.includes(incomingItem.id) ||
+      FORBIDDEN_INCOMING_PARENT_IDS.includes(incomingItem.parentid)
+    );
+  },
+
+  emptyChangeset() {
+    return new BookmarksChangeset();
+  },
+
+  async _apply() {
+    let buf = await this._store.ensureOpenMirror();
+    let watchdog = this._newWatchdog();
+    watchdog.start(BOOKMARK_APPLY_TIMEOUT_MS);
+
+    try {
+      let recordsToUpload = await buf.apply({
+        remoteTimeSeconds: lazy.Resource.serverTime,
+        signal: watchdog.signal,
+      });
+      this._modified.replace(recordsToUpload);
+    } finally {
+      watchdog.stop();
+      if (watchdog.abortReason) {
+        this._log.warn(`Aborting bookmark merge: ${watchdog.abortReason}`);
+      }
+    }
+  },
+
+  async _processIncoming(newitems) {
+    await super._processIncoming(newitems);
+    await this._apply();
+  },
+
+  async _reconcile(item) {
+    return true;
+  },
+
+  async _createRecord(id) {
+    let record = await this._doCreateRecord(id);
+    if (!record.deleted) {
+      // Set hasDupe on all (non-deleted) records since we don't use it and we
+      // want to minimize the risk of older clients corrupting records. Note
+      // that the SyncedBookmarksMirror sets it for all records that it created,
+      // but we would like to ensure that weakly uploaded records are marked as
+      // hasDupe as well.
+      record.hasDupe = true;
+    }
+    return record;
+  },
+
+  async _doCreateRecord(id) {
+    let change = this._modified.changes[id];
+    if (!change) {
+      this._log.error(
+        "Creating record for item ${id} not in strong changeset",
+        { id }
+      );
+      throw new TypeError("Can't create record for unchanged item");
+    }
+    let record = this._recordFromCleartext(id, change.cleartext);
+    record.sortindex = await this._store._calculateIndex(record);
+    return record;
+  },
+
+  _recordFromCleartext(id, cleartext) {
+    let recordObj = getTypeObject(cleartext.type);
+    if (!recordObj) {
+      this._log.warn(
+        "Creating record for item ${id} with unknown type ${type}",
+        { id, type: cleartext.type }
+      );
+      recordObj = PlacesItem;
+    }
+    let record = new recordObj(this.name, id);
+    record.cleartext = cleartext;
+    return record;
+  },
+
+  async pullChanges() {
+    return {};
+  },
+
+  /**
+   * Writes successfully uploaded records back to the mirror, so that the
+   * mirror matches the server. We update the mirror before updating Places,
+   * which has implications for interrupted syncs.
+   *
+   * 1. Sync interrupted during upload; server doesn't support atomic uploads.
+   *    We'll download and reapply everything that we uploaded before the
+   *    interruption. All locally changed items retain their change counters.
+   * 2. Sync interrupted during upload; atomic uploads enabled. The server
+   *    discards the batch. All changed local items retain their change
+   *    counters, so the next sync resumes cleanly.
+   * 3. Sync interrupted during upload; outgoing records can't fit in a single
+   *    batch. We'll download and reapply all records through the most recent
+   *    committed batch. This is a variation of (1).
+   * 4. Sync interrupted after we update the mirror, but before cleanup. The
+   *    mirror matches the server, but locally changed items retain their change
+   *    counters. Reuploading them on the next sync should be idempotent, though
+   *    unnecessary. If another client makes a conflicting remote change before
+   *    we sync again, we may incorrectly prefer the local state.
+   * 5. Sync completes successfully. We'll update the mirror, and reset the
+   *    change counters for all items.
+   */
+  async _onRecordsWritten(succeeded, failed, serverModifiedTime) {
+    let records = [];
+    for (let id of succeeded) {
+      let change = this._modified.changes[id];
+      if (!change) {
+        // TODO (Bug 1433178): Write weakly uploaded records back to the mirror.
+        this._log.info("Uploaded record not in strong changeset", id);
+        continue;
+      }
+      if (!change.synced) {
+        this._log.info("Record in strong changeset not uploaded", id);
+        continue;
+      }
+      let cleartext = change.cleartext;
+      if (!cleartext) {
+        this._log.error(
+          "Missing Sync record cleartext for ${id} in ${change}",
+          { id, change }
+        );
+        throw new TypeError("Missing cleartext for uploaded Sync record");
+      }
+      let record = this._recordFromCleartext(id, cleartext);
+      record.modified = serverModifiedTime;
+      records.push(record);
+    }
+    let buf = await this._store.ensureOpenMirror();
+    await buf.store(records, { needsMerge: false });
+  },
+
+  async finalize() {
+    await super.finalize();
+    await this._store.finalize();
+  },
+};
+
+Object.setPrototypeOf(BookmarksEngine.prototype, SyncEngine.prototype);
+
+/**
+ * The bookmarks store delegates to the mirror for staging and applying
+ * records. Most `Store` methods intentionally remain abstract, so you can't use
+ * this store to create or update bookmarks in Places. All changes must go
+ * through the mirror, which takes care of merging and producing a valid tree.
+ */
+function BookmarksStore(name, engine) {
+  Store.call(this, name, engine);
+}
+
+BookmarksStore.prototype = {
+  _openMirrorPromise: null,
+
+  // For tests.
+  _batchChunkSize: 500,
+
+  // Create a record starting from the weave id (places guid)
+  async createRecord(id, collection) {
+    let item = await lazy.PlacesSyncUtils.bookmarks.fetch(id);
+    if (!item) {
+      // deleted item
+      let record = new PlacesItem(collection, id);
+      record.deleted = true;
+      return record;
+    }
+
+    let recordObj = getTypeObject(item.kind);
+    if (!recordObj) {
+      this._log.warn("Unknown item type, cannot serialize: " + item.kind);
+      recordObj = PlacesItem;
+    }
+    let record = new recordObj(collection, id);
+    record.fromSyncBookmark(item);
+
+    record.sortindex = await this._calculateIndex(record);
+
+    return record;
+  },
+
+  async _calculateIndex(record) {
+    // Ensure folders have a very high sort index so they're not synced last.
+    if (record.type == "folder") {
+      return FOLDER_SORTINDEX;
+    }
+
+    // For anything directly under the toolbar, give it a boost of more than an
+    // unvisited bookmark
+    let index = 0;
+    if (record.parentid == "toolbar") {
+      index += 150;
+    }
+
+    // Add in the bookmark's frecency if we have something.
+    if (record.bmkUri != null) {
+      let frecency = FRECENCY_UNKNOWN;
+      try {
+        frecency = await lazy.PlacesSyncUtils.history.fetchURLFrecency(
+          record.bmkUri
+        );
+      } catch (ex) {
+        this._log.warn(
+          `Failed to fetch frecency for ${record.id}; assuming default`,
+          ex
+        );
+        this._log.trace("Record {id} has invalid URL ${bmkUri}", record);
+      }
+      if (frecency != FRECENCY_UNKNOWN) {
+        index += frecency;
+      }
+    }
+
+    return index;
+  },
+
+  async wipe() {
+    // Save a backup before clearing out all bookmarks.
+    await lazy.PlacesBackups.create(null, true);
+    await lazy.PlacesSyncUtils.bookmarks.wipe();
+  },
+
+  ensureOpenMirror() {
+    if (!this._openMirrorPromise) {
+      this._openMirrorPromise = this._openMirror().catch(err => {
+        // We may have failed to open the mirror temporarily; for example, if
+        // the database is locked. Clear the promise so that subsequent
+        // `ensureOpenMirror` calls can try to open the mirror again.
+        this._openMirrorPromise = null;
+        throw err;
+      });
+    }
+    return this._openMirrorPromise;
+  },
+
+  async _openMirror() {
+    let mirrorPath = PathUtils.join(
+      PathUtils.profileDir,
+      "weave",
+      "bookmarks.sqlite"
+    );
+    await IOUtils.makeDirectory(PathUtils.parent(mirrorPath), {
+      createAncestors: true,
+    });
+
+    return lazy.SyncedBookmarksMirror.open({
+      path: mirrorPath,
+      recordStepTelemetry: (name, took, counts) => {
+        lazy.Observers.notify(
+          "weave:engine:sync:step",
+          {
+            name,
+            took,
+            counts,
+          },
+          this.name
+        );
+      },
+      recordValidationTelemetry: (took, checked, problems) => {
+        lazy.Observers.notify(
+          "weave:engine:validate:finish",
+          {
+            version: BOOKMARK_VALIDATOR_VERSION,
+            took,
+            checked,
+            problems,
+          },
+          this.name
+        );
+      },
+    });
+  },
+
+  async applyIncomingBatch(records, countTelemetry) {
+    let buf = await this.ensureOpenMirror();
+    for (let chunk of lazy.PlacesUtils.chunkArray(
+      records,
+      this._batchChunkSize
+    )) {
+      await buf.store(chunk);
+    }
+    // Array of failed records.
+    return [];
+  },
+
+  async applyIncoming(record) {
+    let buf = await this.ensureOpenMirror();
+    await buf.store([record]);
+  },
+
+  async finalize() {
+    if (!this._openMirrorPromise) {
+      return;
+    }
+    let buf = await this._openMirrorPromise;
+    await buf.finalize();
+  },
+};
+
+Object.setPrototypeOf(BookmarksStore.prototype, Store.prototype);
+
+// The bookmarks tracker is a special flower. Instead of listening for changes
+// via observer notifications, it queries Places for the set of items that have
+// changed since the last sync. Because it's a "pull-based" tracker, it ignores
+// all concepts of "add a changed ID." However, it still registers an observer
+// to bump the score, so that changed bookmarks are synced immediately.
+function BookmarksTracker(name, engine) {
+  Tracker.call(this, name, engine);
+}
+BookmarksTracker.prototype = {
+  onStart() {
+    this._placesListener = new PlacesWeakCallbackWrapper(
+      this.handlePlacesEvents.bind(this)
+    );
+    lazy.PlacesUtils.observers.addListener(
+      [
+        "bookmark-added",
+        "bookmark-removed",
+        "bookmark-moved",
+        "bookmark-guid-changed",
+        "bookmark-keyword-changed",
+        "bookmark-tags-changed",
+        "bookmark-time-changed",
+        "bookmark-title-changed",
+        "bookmark-url-changed",
+      ],
+      this._placesListener
+    );
+    Svc.Obs.add("bookmarks-restore-begin", this);
+    Svc.Obs.add("bookmarks-restore-success", this);
+    Svc.Obs.add("bookmarks-restore-failed", this);
+  },
+
+  onStop() {
+    lazy.PlacesUtils.observers.removeListener(
+      [
+        "bookmark-added",
+        "bookmark-removed",
+        "bookmark-moved",
+        "bookmark-guid-changed",
+        "bookmark-keyword-changed",
+        "bookmark-tags-changed",
+        "bookmark-time-changed",
+        "bookmark-title-changed",
+        "bookmark-url-changed",
+      ],
+      this._placesListener
+    );
+    Svc.Obs.remove("bookmarks-restore-begin", this);
+    Svc.Obs.remove("bookmarks-restore-success", this);
+    Svc.Obs.remove("bookmarks-restore-failed", this);
+  },
+
+  async getChangedIDs() {
+    return lazy.PlacesSyncUtils.bookmarks.pullChanges();
+  },
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "bookmarks-restore-begin":
+        this._log.debug("Ignoring changes from importing bookmarks.");
+        break;
+      case "bookmarks-restore-success":
+        this._log.debug("Tracking all items on successful import.");
+
+        if (data == "json") {
+          this._log.debug(
+            "Restore succeeded: wiping server and other clients."
+          );
+          // Trigger an immediate sync. `ensureCurrentSyncID` will notice we
+          // restored, wipe the server and other clients, reset the sync ID, and
+          // upload the restored tree.
+          this.score += SCORE_INCREMENT_XLARGE;
+        } else {
+          // "html", "html-initial", or "json-append"
+          this._log.debug("Import succeeded.");
+        }
+        break;
+      case "bookmarks-restore-failed":
+        this._log.debug("Tracking all items on failed import.");
+        break;
+    }
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsISupportsWeakReference"]),
+
+  /* Every add/remove/change will trigger a sync for MULTI_DEVICE */
+  _upScore: function BMT__upScore() {
+    this.score += SCORE_INCREMENT_XLARGE;
+  },
+
+  handlePlacesEvents(events) {
+    for (let event of events) {
+      switch (event.type) {
+        case "bookmark-added":
+        case "bookmark-removed":
+        case "bookmark-moved":
+        case "bookmark-keyword-changed":
+        case "bookmark-tags-changed":
+        case "bookmark-time-changed":
+        case "bookmark-title-changed":
+        case "bookmark-url-changed":
+          if (lazy.IGNORED_SOURCES.includes(event.source)) {
+            continue;
+          }
+
+          this._log.trace(`'${event.type}': ${event.id}`);
+          this._upScore();
+          break;
+        case "bookmark-guid-changed":
+          if (event.source !== lazy.PlacesUtils.bookmarks.SOURCES.SYNC) {
+            this._log.warn(
+              "The source of bookmark-guid-changed event shoud be sync."
+            );
+            continue;
+          }
+
+          this._log.trace(`'${event.type}': ${event.id}`);
+          this._upScore();
+          break;
+        case "purge-caches":
+          this._log.trace("purge-caches");
+          this._upScore();
+          break;
+      }
+    }
+  },
+};
+
+Object.setPrototypeOf(BookmarksTracker.prototype, Tracker.prototype);
+
+/**
+ * A changeset that stores extra metadata in a change record for each ID. The
+ * engine updates this metadata when uploading Sync records, and writes it back
+ * to Places in `BookmarksEngine#trackRemainingChanges`.
+ *
+ * The `synced` property on a change record means its corresponding item has
+ * been uploaded, and we should pretend it doesn't exist in the changeset.
+ */
+class BookmarksChangeset extends Changeset {
+  // Only `_reconcile` calls `getModifiedTimestamp` and `has`, and the engine
+  // does its own reconciliation.
+  getModifiedTimestamp(id) {
+    throw new Error("Don't use timestamps to resolve bookmark conflicts");
+  }
+
+  has(id) {
+    throw new Error("Don't use the changeset to resolve bookmark conflicts");
+  }
+
+  delete(id) {
+    let change = this.changes[id];
+    if (change) {
+      // Mark the change as synced without removing it from the set. We do this
+      // so that we can update Places in `trackRemainingChanges`.
+      change.synced = true;
+    }
+  }
+
+  ids() {
+    let results = new Set();
+    for (let id in this.changes) {
+      if (!this.changes[id].synced) {
+        results.add(id);
+      }
+    }
+    return [...results];
+  }
+}
diff --git a/services/sync/modules/engines/clients.sys.mjs b/services/sync/modules/engines/clients.sys.mjs
new file mode 100644
index 0000000000..eda92bd75b
--- /dev/null
+++ b/services/sync/modules/engines/clients.sys.mjs
@@ -0,0 +1,1122 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * How does the clients engine work?
+ *
+ * - We use 2 files - commands.json and commands-syncing.json.
+ *
+ * - At sync upload time, we attempt a rename of commands.json to
+ *   commands-syncing.json, and ignore errors (helps for crash during sync!).
+ * - We load commands-syncing.json and stash the contents in
+ *   _currentlySyncingCommands which lives for the duration of the upload process.
+ * - We use _currentlySyncingCommands to build the outgoing records
+ * - Immediately after successful upload, we delete commands-syncing.json from
+ *   disk (and clear _currentlySyncingCommands). We reconcile our local records
+ *   with what we just wrote in the server, and add failed IDs commands
+ *   back in commands.json
+ * - Any time we need to "save" a command for future syncs, we load
+ *   commands.json, update it, and write it back out.
+ */
+
+import { Async } from "resource://services-common/async.sys.mjs";
+
+import {
+  DEVICE_TYPE_DESKTOP,
+  DEVICE_TYPE_MOBILE,
+  SINGLE_USER_THRESHOLD,
+  SYNC_API_VERSION,
+} from "resource://services-sync/constants.sys.mjs";
+
+import {
+  Store,
+  SyncEngine,
+  LegacyTracker,
+} from "resource://services-sync/engines.sys.mjs";
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Resource } from "resource://services-sync/resource.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineLazyGetter(lazy, "fxAccounts", () => {
+  return ChromeUtils.importESModule(
+    "resource://gre/modules/FxAccounts.sys.mjs"
+  ).getFxAccountsSingleton();
+});
+
+import { PREF_ACCOUNT_ROOT } from "resource://gre/modules/FxAccountsCommon.sys.mjs";
+
+const CLIENTS_TTL = 15552000; // 180 days
+const CLIENTS_TTL_REFRESH = 604800; // 7 days
+const STALE_CLIENT_REMOTE_AGE = 604800; // 7 days
+
+// TTL of the message sent to another device when sending a tab
+const NOTIFY_TAB_SENT_TTL_SECS = 1 * 3600; // 1 hour
+
+// How often we force a refresh of the FxA device list.
+const REFRESH_FXA_DEVICE_INTERVAL_MS = 2 * 60 * 60 * 1000; // 2 hours
+
+// Reasons behind sending collection_changed push notifications.
+const COLLECTION_MODIFIED_REASON_SENDTAB = "sendtab";
+const COLLECTION_MODIFIED_REASON_FIRSTSYNC = "firstsync";
+
+const SUPPORTED_PROTOCOL_VERSIONS = [SYNC_API_VERSION];
+const LAST_MODIFIED_ON_PROCESS_COMMAND_PREF =
+  "services.sync.clients.lastModifiedOnProcessCommands";
+
+function hasDupeCommand(commands, action) {
+  if (!commands) {
+    return false;
+  }
+  return commands.some(
+    other =>
+      other.command == action.command &&
+      Utils.deepEquals(other.args, action.args)
+  );
+}
+
+export function ClientsRec(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+
+ClientsRec.prototype = {
+  _logName: "Sync.Record.Clients",
+  ttl: CLIENTS_TTL,
+};
+Object.setPrototypeOf(ClientsRec.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(ClientsRec, "cleartext", [
+  "name",
+  "type",
+  "commands",
+  "version",
+  "protocols",
+  "formfactor",
+  "os",
+  "appPackage",
+  "application",
+  "device",
+  "fxaDeviceId",
+]);
+
+export function ClientEngine(service) {
+  SyncEngine.call(this, "Clients", service);
+
+  this.fxAccounts = lazy.fxAccounts;
+  this.addClientCommandQueue = Async.asyncQueueCaller(this._log);
+  Utils.defineLazyIDProperty(this, "localID", "services.sync.client.GUID");
+}
+
+ClientEngine.prototype = {
+  _storeObj: ClientStore,
+  _recordObj: ClientsRec,
+  _trackerObj: ClientsTracker,
+  allowSkippedRecord: false,
+  _knownStaleFxADeviceIds: null,
+  _lastDeviceCounts: null,
+  _lastFxaDeviceRefresh: 0,
+
+  async initialize() {
+    // Reset the last sync timestamp on every startup so that we fetch all clients
+    await this.resetLastSync();
+  },
+
+  // These two properties allow us to avoid replaying the same commands
+  // continuously if we cannot manage to upload our own record.
+  _localClientLastModified: 0,
+  get _lastModifiedOnProcessCommands() {
+    return Services.prefs.getIntPref(LAST_MODIFIED_ON_PROCESS_COMMAND_PREF, -1);
+  },
+
+  set _lastModifiedOnProcessCommands(value) {
+    Services.prefs.setIntPref(LAST_MODIFIED_ON_PROCESS_COMMAND_PREF, value);
+  },
+
+  get isFirstSync() {
+    return !this.lastRecordUpload;
+  },
+
+  // Always sync client data as it controls other sync behavior
+  get enabled() {
+    return true;
+  },
+
+  get lastRecordUpload() {
+    return Svc.PrefBranch.getIntPref(this.name + ".lastRecordUpload", 0);
+  },
+  set lastRecordUpload(value) {
+    Svc.PrefBranch.setIntPref(
+      this.name + ".lastRecordUpload",
+      Math.floor(value)
+    );
+  },
+
+  get remoteClients() {
+    // return all non-stale clients for external consumption.
+    return Object.values(this._store._remoteClients).filter(v => !v.stale);
+  },
+
+  remoteClient(id) {
+    let client = this._store._remoteClients[id];
+    return client && !client.stale ? client : null;
+  },
+
+  remoteClientExists(id) {
+    return !!this.remoteClient(id);
+  },
+
+  // Aggregate some stats on the composition of clients on this account
+  get stats() {
+    let stats = {
+      hasMobile: this.localType == DEVICE_TYPE_MOBILE,
+      names: [this.localName],
+      numClients: 1,
+    };
+
+    for (let id in this._store._remoteClients) {
+      let { name, type, stale } = this._store._remoteClients[id];
+      if (!stale) {
+        stats.hasMobile = stats.hasMobile || type == DEVICE_TYPE_MOBILE;
+        stats.names.push(name);
+        stats.numClients++;
+      }
+    }
+
+    return stats;
+  },
+
+  /**
+   * Obtain information about device types.
+   *
+   * Returns a Map of device types to integer counts. Guaranteed to include
+   * "desktop" (which will have at least 1 - this device) and "mobile" (which
+   * may have zero) counts. It almost certainly will include only these 2.
+   */
+  get deviceTypes() {
+    let counts = new Map();
+
+    counts.set(this.localType, 1); // currently this must be DEVICE_TYPE_DESKTOP
+    counts.set(DEVICE_TYPE_MOBILE, 0);
+
+    for (let id in this._store._remoteClients) {
+      let record = this._store._remoteClients[id];
+      if (record.stale) {
+        continue; // pretend "stale" records don't exist.
+      }
+      let type = record.type;
+      if (!counts.has(type)) {
+        counts.set(type, 0);
+      }
+
+      counts.set(type, counts.get(type) + 1);
+    }
+
+    return counts;
+  },
+
+  get brandName() {
+    let brand = Services.strings.createBundle(
+      "chrome://branding/locale/brand.properties"
+    );
+    return brand.GetStringFromName("brandShortName");
+  },
+
+  get localName() {
+    return this.fxAccounts.device.getLocalName();
+  },
+  set localName(value) {
+    this.fxAccounts.device.setLocalName(value);
+  },
+
+  get localType() {
+    return this.fxAccounts.device.getLocalType();
+  },
+
+  getClientName(id) {
+    if (id == this.localID) {
+      return this.localName;
+    }
+    let client = this._store._remoteClients[id];
+    if (!client) {
+      return "";
+    }
+    // Sometimes the sync clients don't always correctly update the device name
+    // However FxA always does, so try to pull the name from there first
+    let fxaDevice = this.fxAccounts.device.recentDeviceList?.find(
+      device => device.id === client.fxaDeviceId
+    );
+
+    // should be very rare, but could happen if we have yet to fetch devices,
+    // or the client recently disconnected
+    if (!fxaDevice) {
+      this._log.warn(
+        "Couldn't find associated FxA device, falling back to client name"
+      );
+      return client.name;
+    }
+    return fxaDevice.name;
+  },
+
+  getClientFxaDeviceId(id) {
+    if (this._store._remoteClients[id]) {
+      return this._store._remoteClients[id].fxaDeviceId;
+    }
+    return null;
+  },
+
+  getClientByFxaDeviceId(fxaDeviceId) {
+    for (let id in this._store._remoteClients) {
+      let client = this._store._remoteClients[id];
+      if (client.stale) {
+        continue;
+      }
+      if (client.fxaDeviceId == fxaDeviceId) {
+        return client;
+      }
+    }
+    return null;
+  },
+
+  getClientType(id) {
+    const client = this._store._remoteClients[id];
+    if (client.type == DEVICE_TYPE_DESKTOP) {
+      return "desktop";
+    }
+    if (client.formfactor && client.formfactor.includes("tablet")) {
+      return "tablet";
+    }
+    return "phone";
+  },
+
+  async _readCommands() {
+    let commands = await Utils.jsonLoad("commands", this);
+    return commands || {};
+  },
+
+  /**
+   * Low level function, do not use directly (use _addClientCommand instead).
+   */
+  async _saveCommands(commands) {
+    try {
+      await Utils.jsonSave("commands", this, commands);
+    } catch (error) {
+      this._log.error("Failed to save JSON outgoing commands", error);
+    }
+  },
+
+  async _prepareCommandsForUpload() {
+    try {
+      await Utils.jsonMove("commands", "commands-syncing", this);
+    } catch (e) {
+      // Ignore errors
+    }
+    let commands = await Utils.jsonLoad("commands-syncing", this);
+    return commands || {};
+  },
+
+  async _deleteUploadedCommands() {
+    delete this._currentlySyncingCommands;
+    try {
+      await Utils.jsonRemove("commands-syncing", this);
+    } catch (err) {
+      this._log.error("Failed to delete syncing-commands file", err);
+    }
+  },
+
+  // Gets commands for a client we are yet to write to the server. Doesn't
+  // include commands for that client which are already on the server.
+  // We should rename this!
+  async getClientCommands(clientId) {
+    const allCommands = await this._readCommands();
+    return allCommands[clientId] || [];
+  },
+
+  async removeLocalCommand(command) {
+    // the implementation of this engine is such that adding a command to
+    // the local client is how commands are deleted! ¯\_(ツ)_/¯
+    await this._addClientCommand(this.localID, command);
+  },
+
+  async _addClientCommand(clientId, command) {
+    this.addClientCommandQueue.enqueueCall(async () => {
+      try {
+        const localCommands = await this._readCommands();
+        const localClientCommands = localCommands[clientId] || [];
+        const remoteClient = this._store._remoteClients[clientId];
+        let remoteClientCommands = [];
+        if (remoteClient && remoteClient.commands) {
+          remoteClientCommands = remoteClient.commands;
+        }
+        const clientCommands = localClientCommands.concat(remoteClientCommands);
+        if (hasDupeCommand(clientCommands, command)) {
+          return false;
+        }
+        localCommands[clientId] = localClientCommands.concat(command);
+        await this._saveCommands(localCommands);
+        return true;
+      } catch (e) {
+        // Failing to save a command should not "break the queue" of pending operations.
+        this._log.error(e);
+        return false;
+      }
+    });
+
+    return this.addClientCommandQueue.promiseCallsComplete();
+  },
+
+  async _removeClientCommands(clientId) {
+    const allCommands = await this._readCommands();
+    delete allCommands[clientId];
+    await this._saveCommands(allCommands);
+  },
+
+  async updateKnownStaleClients() {
+    this._log.debug("Updating the known stale clients");
+    // _fetchFxADevices side effect updates this._knownStaleFxADeviceIds.
+    await this._fetchFxADevices();
+    let localFxADeviceId = await lazy.fxAccounts.device.getLocalId();
+    // Process newer records first, so that if we hit a record with a device ID
+    // we've seen before, we can mark it stale immediately.
+    let clientList = Object.values(this._store._remoteClients).sort(
+      (a, b) => b.serverLastModified - a.serverLastModified
+    );
+    let seenDeviceIds = new Set([localFxADeviceId]);
+    for (let client of clientList) {
+      // Clients might not have an `fxaDeviceId` if they fail the FxA
+      // registration process.
+      if (!client.fxaDeviceId) {
+        continue;
+      }
+      if (this._knownStaleFxADeviceIds.includes(client.fxaDeviceId)) {
+        this._log.info(
+          `Hiding stale client ${client.id} - in known stale clients list`
+        );
+        client.stale = true;
+      } else if (seenDeviceIds.has(client.fxaDeviceId)) {
+        this._log.info(
+          `Hiding stale client ${client.id}` +
+            ` - duplicate device id ${client.fxaDeviceId}`
+        );
+        client.stale = true;
+      } else {
+        seenDeviceIds.add(client.fxaDeviceId);
+      }
+    }
+  },
+
+  async _fetchFxADevices() {
+    // We only force a refresh periodically to keep the load on the servers
+    // down, and because we expect FxA to have received a push message in
+    // most cases when the FxA device list would have changed. For this reason
+    // we still go ahead and check the stale list even if we didn't force a
+    // refresh.
+    let now = this.fxAccounts._internal.now(); // tests mock this .now() impl.
+    if (now - REFRESH_FXA_DEVICE_INTERVAL_MS > this._lastFxaDeviceRefresh) {
+      this._lastFxaDeviceRefresh = now;
+      try {
+        await this.fxAccounts.device.refreshDeviceList();
+      } catch (e) {
+        this._log.error("Could not refresh the FxA device list", e);
+      }
+    }
+
+    // We assume that clients not present in the FxA Device Manager list have been
+    // disconnected and so are stale
+    this._log.debug("Refreshing the known stale clients list");
+    let localClients = Object.values(this._store._remoteClients)
+      .filter(client => client.fxaDeviceId) // iOS client records don't have fxaDeviceId
+      .map(client => client.fxaDeviceId);
+    const fxaClients = this.fxAccounts.device.recentDeviceList
+      ? this.fxAccounts.device.recentDeviceList.map(device => device.id)
+      : [];
+    this._knownStaleFxADeviceIds = Utils.arraySub(localClients, fxaClients);
+  },
+
+  async _syncStartup() {
+    // Reupload new client record periodically.
+    if (Date.now() / 1000 - this.lastRecordUpload > CLIENTS_TTL_REFRESH) {
+      await this._tracker.addChangedID(this.localID);
+    }
+    return SyncEngine.prototype._syncStartup.call(this);
+  },
+
+  async _processIncoming() {
+    // Fetch all records from the server.
+    await this.resetLastSync();
+    this._incomingClients = {};
+    try {
+      await SyncEngine.prototype._processIncoming.call(this);
+      // Update FxA Device list.
+      await this._fetchFxADevices();
+      // Since clients are synced unconditionally, any records in the local store
+      // that don't exist on the server must be for disconnected clients. Remove
+      // them, so that we don't upload records with commands for clients that will
+      // never see them. We also do this to filter out stale clients from the
+      // tabs collection, since showing their list of tabs is confusing.
+      for (let id in this._store._remoteClients) {
+        if (!this._incomingClients[id]) {
+          this._log.info(`Removing local state for deleted client ${id}`);
+          await this._removeRemoteClient(id);
+        }
+      }
+      let localFxADeviceId = await lazy.fxAccounts.device.getLocalId();
+      // Bug 1264498: Mobile clients don't remove themselves from the clients
+      // collection when the user disconnects Sync, so we mark as stale clients
+      // with the same name that haven't synced in over a week.
+      // (Note we can't simply delete them, or we re-apply them next sync - see
+      // bug 1287687)
+      this._localClientLastModified = Math.round(
+        this._incomingClients[this.localID]
+      );
+      delete this._incomingClients[this.localID];
+      let names = new Set([this.localName]);
+      let seenDeviceIds = new Set([localFxADeviceId]);
+      let idToLastModifiedList = Object.entries(this._incomingClients).sort(
+        (a, b) => b[1] - a[1]
+      );
+      for (let [id, serverLastModified] of idToLastModifiedList) {
+        let record = this._store._remoteClients[id];
+        // stash the server last-modified time on the record.
+        record.serverLastModified = serverLastModified;
+        if (
+          record.fxaDeviceId &&
+          this._knownStaleFxADeviceIds.includes(record.fxaDeviceId)
+        ) {
+          this._log.info(
+            `Hiding stale client ${id} - in known stale clients list`
+          );
+          record.stale = true;
+        }
+        if (!names.has(record.name)) {
+          if (record.fxaDeviceId) {
+            seenDeviceIds.add(record.fxaDeviceId);
+          }
+          names.add(record.name);
+          continue;
+        }
+        let remoteAge = Resource.serverTime - this._incomingClients[id];
+        if (remoteAge > STALE_CLIENT_REMOTE_AGE) {
+          this._log.info(`Hiding stale client ${id} with age ${remoteAge}`);
+          record.stale = true;
+          continue;
+        }
+        if (record.fxaDeviceId && seenDeviceIds.has(record.fxaDeviceId)) {
+          this._log.info(
+            `Hiding stale client ${record.id}` +
+              ` - duplicate device id ${record.fxaDeviceId}`
+          );
+          record.stale = true;
+        } else if (record.fxaDeviceId) {
+          seenDeviceIds.add(record.fxaDeviceId);
+        }
+      }
+    } finally {
+      this._incomingClients = null;
+    }
+  },
+
+  async _uploadOutgoing() {
+    this._currentlySyncingCommands = await this._prepareCommandsForUpload();
+    const clientWithPendingCommands = Object.keys(
+      this._currentlySyncingCommands
+    );
+    for (let clientId of clientWithPendingCommands) {
+      if (this._store._remoteClients[clientId] || this.localID == clientId) {
+        this._modified.set(clientId, 0);
+      }
+    }
+    let updatedIDs = this._modified.ids();
+    await SyncEngine.prototype._uploadOutgoing.call(this);
+    // Record the response time as the server time for each item we uploaded.
+    let lastSync = await this.getLastSync();
+    for (let id of updatedIDs) {
+      if (id == this.localID) {
+        this.lastRecordUpload = lastSync;
+      } else {
+        this._store._remoteClients[id].serverLastModified = lastSync;
+      }
+    }
+  },
+
+  async _onRecordsWritten(succeeded, failed) {
+    // Reconcile the status of the local records with what we just wrote on the
+    // server
+    for (let id of succeeded) {
+      const commandChanges = this._currentlySyncingCommands[id];
+      if (id == this.localID) {
+        if (this.isFirstSync) {
+          this._log.info(
+            "Uploaded our client record for the first time, notifying other clients."
+          );
+          this._notifyClientRecordUploaded();
+        }
+        if (this.localCommands) {
+          this.localCommands = this.localCommands.filter(
+            command => !hasDupeCommand(commandChanges, command)
+          );
+        }
+      } else {
+        const clientRecord = this._store._remoteClients[id];
+        if (!commandChanges || !clientRecord) {
+          // should be impossible, else we wouldn't have been writing it.
+          this._log.warn(
+            "No command/No record changes for a client we uploaded"
+          );
+          continue;
+        }
+        // fixup the client record, so our copy of _remoteClients matches what we uploaded.
+        this._store._remoteClients[id] = await this._store.createRecord(id);
+        // we could do better and pass the reference to the record we just uploaded,
+        // but this will do for now
+      }
+    }
+
+    // Re-add failed commands
+    for (let id of failed) {
+      const commandChanges = this._currentlySyncingCommands[id];
+      if (!commandChanges) {
+        continue;
+      }
+      await this._addClientCommand(id, commandChanges);
+    }
+
+    await this._deleteUploadedCommands();
+
+    // Notify other devices that their own client collection changed
+    const idsToNotify = succeeded.reduce((acc, id) => {
+      if (id == this.localID) {
+        return acc;
+      }
+      const fxaDeviceId = this.getClientFxaDeviceId(id);
+      return fxaDeviceId ? acc.concat(fxaDeviceId) : acc;
+    }, []);
+    if (idsToNotify.length) {
+      this._notifyOtherClientsModified(idsToNotify);
+    }
+  },
+
+  _notifyOtherClientsModified(ids) {
+    // We are not waiting on this promise on purpose.
+    this._notifyCollectionChanged(
+      ids,
+      NOTIFY_TAB_SENT_TTL_SECS,
+      COLLECTION_MODIFIED_REASON_SENDTAB
+    );
+  },
+
+  _notifyClientRecordUploaded() {
+    // We are not waiting on this promise on purpose.
+    this._notifyCollectionChanged(
+      null,
+      0,
+      COLLECTION_MODIFIED_REASON_FIRSTSYNC
+    );
+  },
+
+  /**
+   * @param {?string[]} ids FxA Client IDs to notify. null means everyone else.
+   * @param {number} ttl TTL of the push notification.
+   * @param {string} reason Reason for sending this push notification.
+   */
+  async _notifyCollectionChanged(ids, ttl, reason) {
+    const message = {
+      version: 1,
+      command: "sync:collection_changed",
+      data: {
+        collections: ["clients"],
+        reason,
+      },
+    };
+    let excludedIds = null;
+    if (!ids) {
+      const localFxADeviceId = await lazy.fxAccounts.device.getLocalId();
+      excludedIds = [localFxADeviceId];
+    }
+    try {
+      await this.fxAccounts.notifyDevices(ids, excludedIds, message, ttl);
+    } catch (e) {
+      this._log.error("Could not notify of changes in the collection", e);
+    }
+  },
+
+  async _syncFinish() {
+    // Record histograms for our device types, and also write them to a pref
+    // so non-histogram telemetry (eg, UITelemetry) and the sync scheduler
+    // has easy access to them, and so they are accurate even before we've
+    // successfully synced the first time after startup.
+    let deviceTypeCounts = this.deviceTypes;
+    for (let [deviceType, count] of deviceTypeCounts) {
+      let hid;
+      let prefName = this.name + ".devices.";
+      switch (deviceType) {
+        case DEVICE_TYPE_DESKTOP:
+          hid = "WEAVE_DEVICE_COUNT_DESKTOP";
+          prefName += "desktop";
+          break;
+        case DEVICE_TYPE_MOBILE:
+          hid = "WEAVE_DEVICE_COUNT_MOBILE";
+          prefName += "mobile";
+          break;
+        default:
+          this._log.warn(
+            `Unexpected deviceType "${deviceType}" recording device telemetry.`
+          );
+          continue;
+      }
+      Services.telemetry.getHistogramById(hid).add(count);
+      // Optimization: only write the pref if it changed since our last sync.
+      if (
+        this._lastDeviceCounts == null ||
+        this._lastDeviceCounts.get(prefName) != count
+      ) {
+        Svc.PrefBranch.setIntPref(prefName, count);
+      }
+    }
+    this._lastDeviceCounts = deviceTypeCounts;
+    return SyncEngine.prototype._syncFinish.call(this);
+  },
+
+  async _reconcile(item) {
+    // Every incoming record is reconciled, so we use this to track the
+    // contents of the collection on the server.
+    this._incomingClients[item.id] = item.modified;
+
+    if (!(await this._store.itemExists(item.id))) {
+      return true;
+    }
+    // Clients are synced unconditionally, so we'll always have new records.
+    // Unfortunately, this will cause the scheduler to use the immediate sync
+    // interval for the multi-device case, instead of the active interval. We
+    // work around this by updating the record during reconciliation, and
+    // returning false to indicate that the record doesn't need to be applied
+    // later.
+    await this._store.update(item);
+    return false;
+  },
+
+  // Treat reset the same as wiping for locally cached clients
+  async _resetClient() {
+    await this._wipeClient();
+  },
+
+  async _wipeClient() {
+    await SyncEngine.prototype._resetClient.call(this);
+    this._knownStaleFxADeviceIds = null;
+    delete this.localCommands;
+    await this._store.wipe();
+    try {
+      await Utils.jsonRemove("commands", this);
+    } catch (err) {
+      this._log.warn("Could not delete commands.json", err);
+    }
+    try {
+      await Utils.jsonRemove("commands-syncing", this);
+    } catch (err) {
+      this._log.warn("Could not delete commands-syncing.json", err);
+    }
+  },
+
+  async removeClientData() {
+    let res = this.service.resource(this.engineURL + "/" + this.localID);
+    await res.delete();
+  },
+
+  // Override the default behavior to delete bad records from the server.
+  async handleHMACMismatch(item, mayRetry) {
+    this._log.debug("Handling HMAC mismatch for " + item.id);
+
+    let base = await SyncEngine.prototype.handleHMACMismatch.call(
+      this,
+      item,
+      mayRetry
+    );
+    if (base != SyncEngine.kRecoveryStrategy.error) {
+      return base;
+    }
+
+    // It's a bad client record. Save it to be deleted at the end of the sync.
+    this._log.debug("Bad client record detected. Scheduling for deletion.");
+    await this._deleteId(item.id);
+
+    // Neither try again nor error; we're going to delete it.
+    return SyncEngine.kRecoveryStrategy.ignore;
+  },
+
+  /**
+   * A hash of valid commands that the client knows about. The key is a command
+   * and the value is a hash containing information about the command such as
+   * number of arguments, description, and importance (lower importance numbers
+   * indicate higher importance.
+   */
+  _commands: {
+    resetAll: {
+      args: 0,
+      importance: 0,
+      desc: "Clear temporary local data for all engines",
+    },
+    resetEngine: {
+      args: 1,
+      importance: 0,
+      desc: "Clear temporary local data for engine",
+    },
+    wipeEngine: {
+      args: 1,
+      importance: 0,
+      desc: "Delete all client data for engine",
+    },
+    logout: { args: 0, importance: 0, desc: "Log out client" },
+  },
+
+  /**
+   * Sends a command+args pair to a specific client.
+   *
+   * @param command Command string
+   * @param args Array of arguments/data for command
+   * @param clientId Client to send command to
+   */
+  async _sendCommandToClient(command, args, clientId, telemetryExtra) {
+    this._log.trace("Sending " + command + " to " + clientId);
+
+    let client = this._store._remoteClients[clientId];
+    if (!client) {
+      throw new Error("Unknown remote client ID: '" + clientId + "'.");
+    }
+    if (client.stale) {
+      throw new Error("Stale remote client ID: '" + clientId + "'.");
+    }
+
+    let action = {
+      command,
+      args,
+      // We send the flowID to the other client so *it* can report it in its
+      // telemetry - we record it in ours below.
+      flowID: telemetryExtra.flowID,
+    };
+
+    if (await this._addClientCommand(clientId, action)) {
+      this._log.trace(`Client ${clientId} got a new action`, [command, args]);
+      await this._tracker.addChangedID(clientId);
+      try {
+        telemetryExtra.deviceID =
+          this.service.identity.hashedDeviceID(clientId);
+      } catch (_) {}
+
+      this.service.recordTelemetryEvent(
+        "sendcommand",
+        command,
+        undefined,
+        telemetryExtra
+      );
+    } else {
+      this._log.trace(`Client ${clientId} got a duplicate action`, [
+        command,
+        args,
+      ]);
+    }
+  },
+
+  /**
+   * Check if the local client has any remote commands and perform them.
+   *
+   * @return false to abort sync
+   */
+  async processIncomingCommands() {
+    return this._notify("clients:process-commands", "", async function () {
+      if (
+        !this.localCommands ||
+        (this._lastModifiedOnProcessCommands == this._localClientLastModified &&
+          !this.ignoreLastModifiedOnProcessCommands)
+      ) {
+        return true;
+      }
+      this._lastModifiedOnProcessCommands = this._localClientLastModified;
+
+      const clearedCommands = await this._readCommands()[this.localID];
+      const commands = this.localCommands.filter(
+        command => !hasDupeCommand(clearedCommands, command)
+      );
+      let didRemoveCommand = false;
+      // Process each command in order.
+      for (let rawCommand of commands) {
+        let shouldRemoveCommand = true; // most commands are auto-removed.
+        let { command, args, flowID } = rawCommand;
+        this._log.debug("Processing command " + command, args);
+
+        this.service.recordTelemetryEvent(
+          "processcommand",
+          command,
+          undefined,
+          { flowID }
+        );
+
+        let engines = [args[0]];
+        switch (command) {
+          case "resetAll":
+            engines = null;
+          // Fallthrough
+          case "resetEngine":
+            await this.service.resetClient(engines);
+            break;
+          case "wipeEngine":
+            await this.service.wipeClient(engines);
+            break;
+          case "logout":
+            this.service.logout();
+            return false;
+          default:
+            this._log.warn("Received an unknown command: " + command);
+            break;
+        }
+        // Add the command to the "cleared" commands list
+        if (shouldRemoveCommand) {
+          await this.removeLocalCommand(rawCommand);
+          didRemoveCommand = true;
+        }
+      }
+      if (didRemoveCommand) {
+        await this._tracker.addChangedID(this.localID);
+      }
+
+      return true;
+    })();
+  },
+
+  /**
+   * Validates and sends a command to a client or all clients.
+   *
+   * Calling this does not actually sync the command data to the server. If the
+   * client already has the command/args pair, it won't receive a duplicate
+   * command.
+   * This method is async since it writes the command to a file.
+   *
+   * @param command
+   *        Command to invoke on remote clients
+   * @param args
+   *        Array of arguments to give to the command
+   * @param clientId
+   *        Client ID to send command to. If undefined, send to all remote
+   *        clients.
+   * @param flowID
+   *        A unique identifier used to track success for this operation across
+   *        devices.
+   */
+  async sendCommand(command, args, clientId = null, telemetryExtra = {}) {
+    let commandData = this._commands[command];
+    // Don't send commands that we don't know about.
+    if (!commandData) {
+      this._log.error("Unknown command to send: " + command);
+      return;
+    } else if (!args || args.length != commandData.args) {
+      // Don't send a command with the wrong number of arguments.
+      this._log.error(
+        "Expected " +
+          commandData.args +
+          " args for '" +
+          command +
+          "', but got " +
+          args
+      );
+      return;
+    }
+
+    // We allocate a "flowID" here, so it is used for each client.
+    telemetryExtra = Object.assign({}, telemetryExtra); // don't clobber the caller's object
+    if (!telemetryExtra.flowID) {
+      telemetryExtra.flowID = Utils.makeGUID();
+    }
+
+    if (clientId) {
+      await this._sendCommandToClient(command, args, clientId, telemetryExtra);
+    } else {
+      for (let [id, record] of Object.entries(this._store._remoteClients)) {
+        if (!record.stale) {
+          await this._sendCommandToClient(command, args, id, telemetryExtra);
+        }
+      }
+    }
+  },
+
+  async _removeRemoteClient(id) {
+    delete this._store._remoteClients[id];
+    await this._tracker.removeChangedID(id);
+    await this._removeClientCommands(id);
+    this._modified.delete(id);
+  },
+};
+Object.setPrototypeOf(ClientEngine.prototype, SyncEngine.prototype);
+
+function ClientStore(name, engine) {
+  Store.call(this, name, engine);
+}
+ClientStore.prototype = {
+  _remoteClients: {},
+
+  async create(record) {
+    await this.update(record);
+  },
+
+  async update(record) {
+    if (record.id == this.engine.localID) {
+      // Only grab commands from the server; local name/type always wins
+      this.engine.localCommands = record.commands;
+    } else {
+      this._remoteClients[record.id] = record.cleartext;
+    }
+  },
+
+  async createRecord(id, collection) {
+    let record = new ClientsRec(collection, id);
+
+    const commandsChanges = this.engine._currentlySyncingCommands
+      ? this.engine._currentlySyncingCommands[id]
+      : [];
+
+    // Package the individual components into a record for the local client
+    if (id == this.engine.localID) {
+      try {
+        record.fxaDeviceId = await this.engine.fxAccounts.device.getLocalId();
+      } catch (error) {
+        this._log.warn("failed to get fxa device id", error);
+      }
+      record.name = this.engine.localName;
+      record.type = this.engine.localType;
+      record.version = Services.appinfo.version;
+      record.protocols = SUPPORTED_PROTOCOL_VERSIONS;
+
+      // Substract the commands we recorded that we've already executed
+      if (
+        commandsChanges &&
+        commandsChanges.length &&
+        this.engine.localCommands &&
+        this.engine.localCommands.length
+      ) {
+        record.commands = this.engine.localCommands.filter(
+          command => !hasDupeCommand(commandsChanges, command)
+        );
+      }
+
+      // Optional fields.
+      record.os = Services.appinfo.OS; // "Darwin"
+      record.appPackage = Services.appinfo.ID;
+      record.application = this.engine.brandName; // "Nightly"
+
+      // We can't compute these yet.
+      // record.device = "";            // Bug 1100723
+      // record.formfactor = "";        // Bug 1100722
+    } else {
+      record.cleartext = Object.assign({}, this._remoteClients[id]);
+      delete record.cleartext.serverLastModified; // serverLastModified is a local only attribute.
+
+      // Add the commands we have to send
+      if (commandsChanges && commandsChanges.length) {
+        const recordCommands = record.cleartext.commands || [];
+        const newCommands = commandsChanges.filter(
+          command => !hasDupeCommand(recordCommands, command)
+        );
+        record.cleartext.commands = recordCommands.concat(newCommands);
+      }
+
+      if (record.cleartext.stale) {
+        // It's almost certainly a logic error for us to upload a record we
+        // consider stale, so make log noise, but still remove the flag.
+        this._log.error(
+          `Preparing to upload record ${id} that we consider stale`
+        );
+        delete record.cleartext.stale;
+      }
+    }
+    if (record.commands) {
+      const maxPayloadSize =
+        this.engine.service.getMemcacheMaxRecordPayloadSize();
+      let origOrder = new Map(record.commands.map((c, i) => [c, i]));
+      // we sort first by priority, and second by age (indicated by order in the
+      // original list)
+      let commands = record.commands.slice().sort((a, b) => {
+        let infoA = this.engine._commands[a.command];
+        let infoB = this.engine._commands[b.command];
+        // Treat unknown command types as highest priority, to allow us to add
+        // high priority commands in the future without worrying about clients
+        // removing them on each-other unnecessarially.
+        let importA = infoA ? infoA.importance : 0;
+        let importB = infoB ? infoB.importance : 0;
+        // Higher importantance numbers indicate that we care less, so they
+        // go to the end of the list where they'll be popped off.
+        let importDelta = importA - importB;
+        if (importDelta != 0) {
+          return importDelta;
+        }
+        let origIdxA = origOrder.get(a);
+        let origIdxB = origOrder.get(b);
+        // Within equivalent priorities, we put older entries near the end
+        // of the list, so that they are removed first.
+        return origIdxB - origIdxA;
+      });
+      let truncatedCommands = Utils.tryFitItems(commands, maxPayloadSize);
+      if (truncatedCommands.length != record.commands.length) {
+        this._log.warn(
+          `Removing commands from client ${id} (from ${record.commands.length} to ${truncatedCommands.length})`
+        );
+        // Restore original order.
+        record.commands = truncatedCommands.sort(
+          (a, b) => origOrder.get(a) - origOrder.get(b)
+        );
+      }
+    }
+    return record;
+  },
+
+  async itemExists(id) {
+    return id in (await this.getAllIDs());
+  },
+
+  async getAllIDs() {
+    let ids = {};
+    ids[this.engine.localID] = true;
+    for (let id in this._remoteClients) {
+      ids[id] = true;
+    }
+    return ids;
+  },
+
+  async wipe() {
+    this._remoteClients = {};
+  },
+};
+Object.setPrototypeOf(ClientStore.prototype, Store.prototype);
+
+function ClientsTracker(name, engine) {
+  LegacyTracker.call(this, name, engine);
+}
+ClientsTracker.prototype = {
+  _enabled: false,
+
+  onStart() {
+    Svc.Obs.add("fxaccounts:new_device_id", this.asyncObserver);
+    Services.prefs.addObserver(
+      PREF_ACCOUNT_ROOT + "device.name",
+      this.asyncObserver
+    );
+  },
+  onStop() {
+    Services.prefs.removeObserver(
+      PREF_ACCOUNT_ROOT + "device.name",
+      this.asyncObserver
+    );
+    Svc.Obs.remove("fxaccounts:new_device_id", this.asyncObserver);
+  },
+
+  async observe(subject, topic, data) {
+    switch (topic) {
+      case "nsPref:changed":
+        this._log.debug("client.name preference changed");
+      // Fallthrough intended.
+      case "fxaccounts:new_device_id":
+        await this.addChangedID(this.engine.localID);
+        this.score += SINGLE_USER_THRESHOLD + 1; // ALWAYS SYNC NOW.
+        break;
+    }
+  },
+};
+Object.setPrototypeOf(ClientsTracker.prototype, LegacyTracker.prototype);
diff --git a/services/sync/modules/engines/extension-storage.sys.mjs b/services/sync/modules/engines/extension-storage.sys.mjs
new file mode 100644
index 0000000000..d2671978c8
--- /dev/null
+++ b/services/sync/modules/engines/extension-storage.sys.mjs
@@ -0,0 +1,308 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import {
+  BridgedEngine,
+  BridgeWrapperXPCOM,
+  LogAdapter,
+} from "resource://services-sync/bridged_engine.sys.mjs";
+import { SyncEngine, Tracker } from "resource://services-sync/engines.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  MULTI_DEVICE_THRESHOLD: "resource://services-sync/constants.sys.mjs",
+  Observers: "resource://services-common/observers.sys.mjs",
+  SCORE_INCREMENT_MEDIUM: "resource://services-sync/constants.sys.mjs",
+  Svc: "resource://services-sync/util.sys.mjs",
+  extensionStorageSync: "resource://gre/modules/ExtensionStorageSync.sys.mjs",
+
+  extensionStorageSyncKinto:
+    "resource://gre/modules/ExtensionStorageSyncKinto.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "StorageSyncService",
+  "@mozilla.org/extensions/storage/sync;1",
+  "nsIInterfaceRequestor"
+);
+
+const PREF_FORCE_ENABLE = "engine.extension-storage.force";
+
+// A helper to indicate whether extension-storage is enabled - it's based on
+// the "addons" pref. The same logic is shared between both engine impls.
+function getEngineEnabled() {
+  // By default, we sync extension storage if we sync addons. This
+  // lets us simplify the UX since users probably don't consider
+  // "extension preferences" a separate category of syncing.
+  // However, we also respect engine.extension-storage.force, which
+  // can be set to true or false, if a power user wants to customize
+  // the behavior despite the lack of UI.
+  if (
+    lazy.Svc.PrefBranch.getPrefType(PREF_FORCE_ENABLE) !=
+    Ci.nsIPrefBranch.PREF_INVALID
+  ) {
+    return lazy.Svc.PrefBranch.getBoolPref(PREF_FORCE_ENABLE);
+  }
+  return lazy.Svc.PrefBranch.getBoolPref("engine.addons", false);
+}
+
+function setEngineEnabled(enabled) {
+  // This will be called by the engine manager when declined on another device.
+  // Things will go a bit pear-shaped if the engine manager tries to end up
+  // with 'addons' and 'extension-storage' in different states - however, this
+  // *can* happen given we support the `engine.extension-storage.force`
+  // preference. So if that pref exists, we set it to this value. If that pref
+  // doesn't exist, we just ignore it and hope that the 'addons' engine is also
+  // going to be set to the same state.
+  if (
+    lazy.Svc.PrefBranch.getPrefType(PREF_FORCE_ENABLE) !=
+    Ci.nsIPrefBranch.PREF_INVALID
+  ) {
+    lazy.Svc.PrefBranch.setBoolPref(PREF_FORCE_ENABLE, enabled);
+  }
+}
+
+// A "bridged engine" to our webext-storage component.
+export function ExtensionStorageEngineBridge(service) {
+  this.component = lazy.StorageSyncService.getInterface(
+    Ci.mozIBridgedSyncEngine
+  );
+  BridgedEngine.call(this, "Extension-Storage", service);
+  this._bridge = new BridgeWrapperXPCOM(this.component);
+
+  let app_services_logger = Cc["@mozilla.org/appservices/logger;1"].getService(
+    Ci.mozIAppServicesLogger
+  );
+  let logger_target = "app-services:webext_storage:sync";
+  app_services_logger.register(logger_target, new LogAdapter(this._log));
+}
+
+ExtensionStorageEngineBridge.prototype = {
+  syncPriority: 10,
+
+  // Used to override the engine name in telemetry, so that we can distinguish .
+  overrideTelemetryName: "rust-webext-storage",
+
+  _notifyPendingChanges() {
+    return new Promise(resolve => {
+      this.component
+        .QueryInterface(Ci.mozISyncedExtensionStorageArea)
+        .fetchPendingSyncChanges({
+          QueryInterface: ChromeUtils.generateQI([
+            "mozIExtensionStorageListener",
+            "mozIExtensionStorageCallback",
+          ]),
+          onChanged: (extId, json) => {
+            try {
+              lazy.extensionStorageSync.notifyListeners(
+                extId,
+                JSON.parse(json)
+              );
+            } catch (ex) {
+              this._log.warn(
+                `Error notifying change listeners for ${extId}`,
+                ex
+              );
+            }
+          },
+          handleSuccess: resolve,
+          handleError: (code, message) => {
+            this._log.warn(
+              "Error fetching pending synced changes",
+              message,
+              code
+            );
+            resolve();
+          },
+        });
+    });
+  },
+
+  _takeMigrationInfo() {
+    return new Promise((resolve, reject) => {
+      this.component
+        .QueryInterface(Ci.mozIExtensionStorageArea)
+        .takeMigrationInfo({
+          QueryInterface: ChromeUtils.generateQI([
+            "mozIExtensionStorageCallback",
+          ]),
+          handleSuccess: result => {
+            resolve(result ? JSON.parse(result) : null);
+          },
+          handleError: (code, message) => {
+            this._log.warn("Error fetching migration info", message, code);
+            // `takeMigrationInfo` doesn't actually perform the migration,
+            // just reads (and clears) any data stored in the DB from the
+            // previous migration.
+            //
+            // Any errors here are very likely occurring a good while
+            // after the migration ran, so we just warn and pretend
+            // nothing was there.
+            resolve(null);
+          },
+        });
+    });
+  },
+
+  async _syncStartup() {
+    let result = await super._syncStartup();
+    let info = await this._takeMigrationInfo();
+    if (info) {
+      lazy.Observers.notify(
+        "weave:telemetry:migration",
+        info,
+        "webext-storage"
+      );
+    }
+    return result;
+  },
+
+  async _processIncoming() {
+    await super._processIncoming();
+    try {
+      await this._notifyPendingChanges();
+    } catch (ex) {
+      // Failing to notify `storage.onChanged` observers is bad, but shouldn't
+      // interrupt syncing.
+      this._log.warn("Error notifying about synced changes", ex);
+    }
+  },
+
+  get enabled() {
+    return getEngineEnabled();
+  },
+  set enabled(enabled) {
+    setEngineEnabled(enabled);
+  },
+};
+Object.setPrototypeOf(
+  ExtensionStorageEngineBridge.prototype,
+  BridgedEngine.prototype
+);
+
+/**
+ *****************************************************************************
+ *
+ * Deprecated support for Kinto
+ *
+ *****************************************************************************
+ */
+
+/**
+ * The Engine that manages syncing for the web extension "storage"
+ * API, and in particular ext.storage.sync.
+ *
+ * ext.storage.sync is implemented using Kinto, so it has mechanisms
+ * for syncing that we do not need to integrate in the Firefox Sync
+ * framework, so this is something of a stub.
+ */
+export function ExtensionStorageEngineKinto(service) {
+  SyncEngine.call(this, "Extension-Storage", service);
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_skipPercentageChance",
+    "services.sync.extension-storage.skipPercentageChance",
+    0
+  );
+}
+
+ExtensionStorageEngineKinto.prototype = {
+  _trackerObj: ExtensionStorageTracker,
+  // we don't need these since we implement our own sync logic
+  _storeObj: undefined,
+  _recordObj: undefined,
+
+  syncPriority: 10,
+  allowSkippedRecord: false,
+
+  async _sync() {
+    return lazy.extensionStorageSyncKinto.syncAll();
+  },
+
+  get enabled() {
+    return getEngineEnabled();
+  },
+  // We only need the enabled setter for the edge-case where info/collections
+  // has `extension-storage` - which could happen if the pref to flip the new
+  // engine on was once set but no longer is.
+  set enabled(enabled) {
+    setEngineEnabled(enabled);
+  },
+
+  _wipeClient() {
+    return lazy.extensionStorageSyncKinto.clearAll();
+  },
+
+  shouldSkipSync(syncReason) {
+    if (syncReason == "user" || syncReason == "startup") {
+      this._log.info(
+        `Not skipping extension storage sync: reason == ${syncReason}`
+      );
+      // Always sync if a user clicks the button, or if we're starting up.
+      return false;
+    }
+    // Ensure this wouldn't cause a resync...
+    if (this._tracker.score >= lazy.MULTI_DEVICE_THRESHOLD) {
+      this._log.info(
+        "Not skipping extension storage sync: Would trigger resync anyway"
+      );
+      return false;
+    }
+
+    let probability = this._skipPercentageChance / 100.0;
+    // Math.random() returns a value in the interval [0, 1), so `>` is correct:
+    // if `probability` is 1 skip every time, and if it's 0, never skip.
+    let shouldSkip = probability > Math.random();
+
+    this._log.info(
+      `Skipping extension-storage sync with a chance of ${probability}: ${shouldSkip}`
+    );
+    return shouldSkip;
+  },
+};
+Object.setPrototypeOf(
+  ExtensionStorageEngineKinto.prototype,
+  SyncEngine.prototype
+);
+
+function ExtensionStorageTracker(name, engine) {
+  Tracker.call(this, name, engine);
+  this._ignoreAll = false;
+}
+ExtensionStorageTracker.prototype = {
+  get ignoreAll() {
+    return this._ignoreAll;
+  },
+
+  set ignoreAll(value) {
+    this._ignoreAll = value;
+  },
+
+  onStart() {
+    lazy.Svc.Obs.add("ext.storage.sync-changed", this.asyncObserver);
+  },
+
+  onStop() {
+    lazy.Svc.Obs.remove("ext.storage.sync-changed", this.asyncObserver);
+  },
+
+  async observe(subject, topic, data) {
+    if (this.ignoreAll) {
+      return;
+    }
+
+    if (topic !== "ext.storage.sync-changed") {
+      return;
+    }
+
+    // Single adds, removes and changes are not so important on their
+    // own, so let's just increment score a bit.
+    this.score += lazy.SCORE_INCREMENT_MEDIUM;
+  },
+};
+Object.setPrototypeOf(ExtensionStorageTracker.prototype, Tracker.prototype);
diff --git a/services/sync/modules/engines/forms.sys.mjs b/services/sync/modules/engines/forms.sys.mjs
new file mode 100644
index 0000000000..3516327659
--- /dev/null
+++ b/services/sync/modules/engines/forms.sys.mjs
@@ -0,0 +1,298 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import {
+  Store,
+  SyncEngine,
+  LegacyTracker,
+} from "resource://services-sync/engines.sys.mjs";
+
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import { SCORE_INCREMENT_MEDIUM } from "resource://services-sync/constants.sys.mjs";
+import {
+  CollectionProblemData,
+  CollectionValidator,
+} from "resource://services-sync/collection_validator.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+const lazy = {};
+ChromeUtils.defineESModuleGetters(lazy, {
+  FormHistory: "resource://gre/modules/FormHistory.sys.mjs",
+});
+
+const FORMS_TTL = 3 * 365 * 24 * 60 * 60; // Three years in seconds.
+
+export function FormRec(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+
+FormRec.prototype = {
+  _logName: "Sync.Record.Form",
+  ttl: FORMS_TTL,
+};
+Object.setPrototypeOf(FormRec.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(FormRec, "cleartext", ["name", "value"]);
+
+var FormWrapper = {
+  _log: Log.repository.getLogger("Sync.Engine.Forms"),
+
+  _getEntryCols: ["fieldname", "value"],
+  _guidCols: ["guid"],
+
+  _search(terms, searchData) {
+    return lazy.FormHistory.search(terms, searchData);
+  },
+
+  async _update(changes) {
+    if (!lazy.FormHistory.enabled) {
+      return; // update isn't going to do anything.
+    }
+    await lazy.FormHistory.update(changes).catch(console.error);
+  },
+
+  async getEntry(guid) {
+    let results = await this._search(this._getEntryCols, { guid });
+    if (!results.length) {
+      return null;
+    }
+    return { name: results[0].fieldname, value: results[0].value };
+  },
+
+  async getGUID(name, value) {
+    // Query for the provided entry.
+    let query = { fieldname: name, value };
+    let results = await this._search(this._guidCols, query);
+    return results.length ? results[0].guid : null;
+  },
+
+  async hasGUID(guid) {
+    // We could probably use a count function here, but search exists...
+    let results = await this._search(this._guidCols, { guid });
+    return !!results.length;
+  },
+
+  async replaceGUID(oldGUID, newGUID) {
+    let changes = {
+      op: "update",
+      guid: oldGUID,
+      newGuid: newGUID,
+    };
+    await this._update(changes);
+  },
+};
+
+export function FormEngine(service) {
+  SyncEngine.call(this, "Forms", service);
+}
+
+FormEngine.prototype = {
+  _storeObj: FormStore,
+  _trackerObj: FormTracker,
+  _recordObj: FormRec,
+
+  syncPriority: 6,
+
+  get prefName() {
+    return "history";
+  },
+
+  async _findDupe(item) {
+    return FormWrapper.getGUID(item.name, item.value);
+  },
+};
+Object.setPrototypeOf(FormEngine.prototype, SyncEngine.prototype);
+
+function FormStore(name, engine) {
+  Store.call(this, name, engine);
+}
+FormStore.prototype = {
+  async _processChange(change) {
+    // If this._changes is defined, then we are applying a batch, so we
+    // can defer it.
+    if (this._changes) {
+      this._changes.push(change);
+      return;
+    }
+
+    // Otherwise we must handle the change right now.
+    await FormWrapper._update(change);
+  },
+
+  async applyIncomingBatch(records, countTelemetry) {
+    Async.checkAppReady();
+    // We collect all the changes to be made then apply them all at once.
+    this._changes = [];
+    let failures = await Store.prototype.applyIncomingBatch.call(
+      this,
+      records,
+      countTelemetry
+    );
+    if (this._changes.length) {
+      await FormWrapper._update(this._changes);
+    }
+    delete this._changes;
+    return failures;
+  },
+
+  async getAllIDs() {
+    let results = await FormWrapper._search(["guid"], []);
+    let guids = {};
+    for (let result of results) {
+      guids[result.guid] = true;
+    }
+    return guids;
+  },
+
+  async changeItemID(oldID, newID) {
+    await FormWrapper.replaceGUID(oldID, newID);
+  },
+
+  async itemExists(id) {
+    return FormWrapper.hasGUID(id);
+  },
+
+  async createRecord(id, collection) {
+    let record = new FormRec(collection, id);
+    let entry = await FormWrapper.getEntry(id);
+    if (entry != null) {
+      record.name = entry.name;
+      record.value = entry.value;
+    } else {
+      record.deleted = true;
+    }
+    return record;
+  },
+
+  async create(record) {
+    this._log.trace("Adding form record for " + record.name);
+    let change = {
+      op: "add",
+      guid: record.id,
+      fieldname: record.name,
+      value: record.value,
+    };
+    await this._processChange(change);
+  },
+
+  async remove(record) {
+    this._log.trace("Removing form record: " + record.id);
+    let change = {
+      op: "remove",
+      guid: record.id,
+    };
+    await this._processChange(change);
+  },
+
+  async update(record) {
+    this._log.trace("Ignoring form record update request!");
+  },
+
+  async wipe() {
+    let change = {
+      op: "remove",
+    };
+    await FormWrapper._update(change);
+  },
+};
+Object.setPrototypeOf(FormStore.prototype, Store.prototype);
+
+function FormTracker(name, engine) {
+  LegacyTracker.call(this, name, engine);
+}
+FormTracker.prototype = {
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIObserver",
+    "nsISupportsWeakReference",
+  ]),
+
+  onStart() {
+    Svc.Obs.add("satchel-storage-changed", this.asyncObserver);
+  },
+
+  onStop() {
+    Svc.Obs.remove("satchel-storage-changed", this.asyncObserver);
+  },
+
+  async observe(subject, topic, data) {
+    if (this.ignoreAll) {
+      return;
+    }
+    switch (topic) {
+      case "satchel-storage-changed":
+        if (data == "formhistory-add" || data == "formhistory-remove") {
+          let guid = subject.QueryInterface(Ci.nsISupportsString).toString();
+          await this.trackEntry(guid);
+        }
+        break;
+    }
+  },
+
+  async trackEntry(guid) {
+    const added = await this.addChangedID(guid);
+    if (added) {
+      this.score += SCORE_INCREMENT_MEDIUM;
+    }
+  },
+};
+Object.setPrototypeOf(FormTracker.prototype, LegacyTracker.prototype);
+
+class FormsProblemData extends CollectionProblemData {
+  getSummary() {
+    // We don't support syncing deleted form data, so "clientMissing" isn't a problem
+    return super.getSummary().filter(entry => entry.name !== "clientMissing");
+  }
+}
+
+export class FormValidator extends CollectionValidator {
+  constructor() {
+    super("forms", "id", ["name", "value"]);
+    this.ignoresMissingClients = true;
+  }
+
+  emptyProblemData() {
+    return new FormsProblemData();
+  }
+
+  async getClientItems() {
+    return FormWrapper._search(["guid", "fieldname", "value"], {});
+  }
+
+  normalizeClientItem(item) {
+    return {
+      id: item.guid,
+      guid: item.guid,
+      name: item.fieldname,
+      fieldname: item.fieldname,
+      value: item.value,
+      original: item,
+    };
+  }
+
+  async normalizeServerItem(item) {
+    let res = Object.assign(
+      {
+        guid: item.id,
+        fieldname: item.name,
+        original: item,
+      },
+      item
+    );
+    // Missing `name` or `value` causes the getGUID call to throw
+    if (item.name !== undefined && item.value !== undefined) {
+      let guid = await FormWrapper.getGUID(item.name, item.value);
+      if (guid) {
+        res.guid = guid;
+        res.id = guid;
+        res.duped = true;
+      }
+    }
+
+    return res;
+  }
+}
diff --git a/services/sync/modules/engines/history.sys.mjs b/services/sync/modules/engines/history.sys.mjs
new file mode 100644
index 0000000000..44014e4d9e
--- /dev/null
+++ b/services/sync/modules/engines/history.sys.mjs
@@ -0,0 +1,654 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const HISTORY_TTL = 5184000; // 60 days in milliseconds
+const THIRTY_DAYS_IN_MS = 2592000000; // 30 days in milliseconds
+// Sync may bring new fields from other clients, not yet understood by our engine.
+// Unknown fields outside these fields are aggregated into 'unknownFields' and
+// safely synced to prevent data loss.
+const VALID_HISTORY_FIELDS = ["id", "title", "histUri", "visits"];
+const VALID_VISIT_FIELDS = ["date", "type", "transition"];
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+
+import {
+  MAX_HISTORY_DOWNLOAD,
+  MAX_HISTORY_UPLOAD,
+  SCORE_INCREMENT_SMALL,
+  SCORE_INCREMENT_XLARGE,
+} from "resource://services-sync/constants.sys.mjs";
+
+import {
+  Store,
+  SyncEngine,
+  LegacyTracker,
+} from "resource://services-sync/engines.sys.mjs";
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Utils } from "resource://services-sync/util.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  PlacesSyncUtils: "resource://gre/modules/PlacesSyncUtils.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+});
+
+export function HistoryRec(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+
+HistoryRec.prototype = {
+  _logName: "Sync.Record.History",
+  ttl: HISTORY_TTL,
+};
+Object.setPrototypeOf(HistoryRec.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(HistoryRec, "cleartext", ["histUri", "title", "visits"]);
+
+export function HistoryEngine(service) {
+  SyncEngine.call(this, "History", service);
+}
+
+HistoryEngine.prototype = {
+  _recordObj: HistoryRec,
+  _storeObj: HistoryStore,
+  _trackerObj: HistoryTracker,
+  downloadLimit: MAX_HISTORY_DOWNLOAD,
+
+  syncPriority: 7,
+
+  async getSyncID() {
+    return lazy.PlacesSyncUtils.history.getSyncId();
+  },
+
+  async ensureCurrentSyncID(newSyncID) {
+    this._log.debug(
+      "Checking if server sync ID ${newSyncID} matches existing",
+      { newSyncID }
+    );
+    await lazy.PlacesSyncUtils.history.ensureCurrentSyncId(newSyncID);
+    return newSyncID;
+  },
+
+  async resetSyncID() {
+    // First, delete the collection on the server. It's fine if we're
+    // interrupted here: on the next sync, we'll detect that our old sync ID is
+    // now stale, and start over as a first sync.
+    await this._deleteServerCollection();
+    // Then, reset our local sync ID.
+    return this.resetLocalSyncID();
+  },
+
+  async resetLocalSyncID() {
+    let newSyncID = await lazy.PlacesSyncUtils.history.resetSyncId();
+    this._log.debug("Assigned new sync ID ${newSyncID}", { newSyncID });
+    return newSyncID;
+  },
+
+  async getLastSync() {
+    let lastSync = await lazy.PlacesSyncUtils.history.getLastSync();
+    return lastSync;
+  },
+
+  async setLastSync(lastSync) {
+    await lazy.PlacesSyncUtils.history.setLastSync(lastSync);
+  },
+
+  shouldSyncURL(url) {
+    return !url.startsWith("file:");
+  },
+
+  async pullNewChanges() {
+    const changedIDs = await this._tracker.getChangedIDs();
+    let modifiedGUIDs = Object.keys(changedIDs);
+    if (!modifiedGUIDs.length) {
+      return {};
+    }
+
+    let guidsToRemove =
+      await lazy.PlacesSyncUtils.history.determineNonSyncableGuids(
+        modifiedGUIDs
+      );
+    await this._tracker.removeChangedID(...guidsToRemove);
+    return changedIDs;
+  },
+
+  async _resetClient() {
+    await super._resetClient();
+    await lazy.PlacesSyncUtils.history.reset();
+  },
+};
+Object.setPrototypeOf(HistoryEngine.prototype, SyncEngine.prototype);
+
+function HistoryStore(name, engine) {
+  Store.call(this, name, engine);
+}
+
+HistoryStore.prototype = {
+  // We try and only update this many visits at one time.
+  MAX_VISITS_PER_INSERT: 500,
+
+  // Some helper functions to handle GUIDs
+  async setGUID(uri, guid) {
+    if (!guid) {
+      guid = Utils.makeGUID();
+    }
+
+    try {
+      await lazy.PlacesSyncUtils.history.changeGuid(uri, guid);
+    } catch (e) {
+      this._log.error("Error setting GUID ${guid} for URI ${uri}", guid, uri);
+    }
+
+    return guid;
+  },
+
+  async GUIDForUri(uri, create) {
+    // Use the existing GUID if it exists
+    let guid;
+    try {
+      guid = await lazy.PlacesSyncUtils.history.fetchGuidForURL(uri);
+    } catch (e) {
+      this._log.error("Error fetching GUID for URL ${uri}", uri);
+    }
+
+    // If the URI has an existing GUID, return it.
+    if (guid) {
+      return guid;
+    }
+
+    // If the URI doesn't have a GUID and we were indicated to create one.
+    if (create) {
+      return this.setGUID(uri);
+    }
+
+    // If the URI doesn't have a GUID and we didn't create one for it.
+    return null;
+  },
+
+  async changeItemID(oldID, newID) {
+    let info = await lazy.PlacesSyncUtils.history.fetchURLInfoForGuid(oldID);
+    if (!info) {
+      throw new Error(`Can't change ID for nonexistent history entry ${oldID}`);
+    }
+    this.setGUID(info.url, newID);
+  },
+
+  async getAllIDs() {
+    let urls = await lazy.PlacesSyncUtils.history.getAllURLs({
+      since: new Date(Date.now() - THIRTY_DAYS_IN_MS),
+      limit: MAX_HISTORY_UPLOAD,
+    });
+
+    let urlsByGUID = {};
+    for (let url of urls) {
+      if (!this.engine.shouldSyncURL(url)) {
+        continue;
+      }
+      let guid = await this.GUIDForUri(url, true);
+      urlsByGUID[guid] = url;
+    }
+    return urlsByGUID;
+  },
+
+  async applyIncomingBatch(records, countTelemetry) {
+    // Convert incoming records to mozIPlaceInfo objects which are applied as
+    // either history additions or removals.
+    let failed = [];
+    let toAdd = [];
+    let toRemove = [];
+    let pageGuidsWithUnknownFields = new Map();
+    let visitTimesWithUnknownFields = new Map();
+    await Async.yieldingForEach(records, async record => {
+      if (record.deleted) {
+        toRemove.push(record);
+      } else {
+        try {
+          let pageInfo = await this._recordToPlaceInfo(record);
+          if (pageInfo) {
+            toAdd.push(pageInfo);
+
+            // Pull any unknown fields that may have come from other clients
+            let unknownFields = lazy.PlacesSyncUtils.extractUnknownFields(
+              record.cleartext,
+              VALID_HISTORY_FIELDS
+            );
+            if (unknownFields) {
+              pageGuidsWithUnknownFields.set(pageInfo.guid, { unknownFields });
+            }
+
+            // Visits themselves could also contain unknown fields
+            for (const visit of pageInfo.visits) {
+              let unknownVisitFields =
+                lazy.PlacesSyncUtils.extractUnknownFields(
+                  visit,
+                  VALID_VISIT_FIELDS
+                );
+              if (unknownVisitFields) {
+                // Visits don't have an id at the time of sync so we'll need
+                // to use the time instead until it's inserted in the DB
+                visitTimesWithUnknownFields.set(visit.date.getTime(), {
+                  unknownVisitFields,
+                });
+              }
+            }
+          }
+        } catch (ex) {
+          if (Async.isShutdownException(ex)) {
+            throw ex;
+          }
+          this._log.error("Failed to create a place info", ex);
+          this._log.trace("The record that failed", record);
+          failed.push(record.id);
+          countTelemetry.addIncomingFailedReason(ex.message);
+        }
+      }
+    });
+    if (toAdd.length || toRemove.length) {
+      if (toRemove.length) {
+        // PlacesUtils.history.remove takes an array of visits to remove,
+        // but the error semantics are tricky - a single "bad" entry will cause
+        // an exception before anything is removed. So we do remove them one at
+        // a time.
+        await Async.yieldingForEach(toRemove, async record => {
+          try {
+            await this.remove(record);
+          } catch (ex) {
+            if (Async.isShutdownException(ex)) {
+              throw ex;
+            }
+            this._log.error("Failed to delete a place info", ex);
+            this._log.trace("The record that failed", record);
+            failed.push(record.id);
+            countTelemetry.addIncomingFailedReason(ex.message);
+          }
+        });
+      }
+      for (let chunk of this._generateChunks(toAdd)) {
+        // Per bug 1415560, we ignore any exceptions returned by insertMany
+        // as they are likely to be spurious. We do supply an onError handler
+        // and log the exceptions seen there as they are likely to be
+        // informative, but we still never abort the sync based on them.
+        let unknownFieldsToInsert = [];
+        try {
+          await lazy.PlacesUtils.history.insertMany(
+            chunk,
+            result => {
+              const placeToUpdate = pageGuidsWithUnknownFields.get(result.guid);
+              // Extract the placeId from this result so we can add the unknownFields
+              // to the proper table
+              if (placeToUpdate) {
+                unknownFieldsToInsert.push({
+                  placeId: result.placeId,
+                  unknownFields: placeToUpdate.unknownFields,
+                });
+              }
+              // same for visits
+              result.visits.forEach(visit => {
+                let visitToUpdate = visitTimesWithUnknownFields.get(
+                  visit.date.getTime()
+                );
+                if (visitToUpdate) {
+                  unknownFieldsToInsert.push({
+                    visitId: visit.visitId,
+                    unknownFields: visitToUpdate.unknownVisitFields,
+                  });
+                }
+              });
+            },
+            failedVisit => {
+              this._log.info(
+                "Failed to insert a history record",
+                failedVisit.guid
+              );
+              this._log.trace("The record that failed", failedVisit);
+              failed.push(failedVisit.guid);
+            }
+          );
+        } catch (ex) {
+          this._log.info("Failed to insert history records", ex);
+          countTelemetry.addIncomingFailedReason(ex.message);
+        }
+
+        // All the top level places or visits that had unknown fields are sent
+        // to be added to the appropiate tables
+        await lazy.PlacesSyncUtils.history.updateUnknownFieldsBatch(
+          unknownFieldsToInsert
+        );
+      }
+    }
+
+    return failed;
+  },
+
+  /**
+   * Returns a generator that splits records into sanely sized chunks suitable
+   * for passing to places to prevent places doing bad things at shutdown.
+   */
+  *_generateChunks(records) {
+    // We chunk based on the number of *visits* inside each record. However,
+    // we do not split a single record into multiple records, because at some
+    // time in the future, we intend to ensure these records are ordered by
+    // lastModified, and advance the engine's timestamp as we process them,
+    // meaning we can resume exactly where we left off next sync - although
+    // currently that's not done, so we will retry the entire batch next sync
+    // if interrupted.
+    // ie, this means that if a single record has more than MAX_VISITS_PER_INSERT
+    // visits, we will call insertMany() with exactly 1 record, but with
+    // more than MAX_VISITS_PER_INSERT visits.
+    let curIndex = 0;
+    this._log.debug(`adding ${records.length} records to history`);
+    while (curIndex < records.length) {
+      Async.checkAppReady(); // may throw if we are shutting down.
+      let toAdd = []; // what we are going to insert.
+      let count = 0; // a counter which tells us when toAdd is full.
+      do {
+        let record = records[curIndex];
+        curIndex += 1;
+        toAdd.push(record);
+        count += record.visits.length;
+      } while (
+        curIndex < records.length &&
+        count + records[curIndex].visits.length <= this.MAX_VISITS_PER_INSERT
+      );
+      this._log.trace(`adding ${toAdd.length} items in this chunk`);
+      yield toAdd;
+    }
+  },
+
+  /* An internal helper to determine if we can add an entry to places.
+     Exists primarily so tests can override it.
+   */
+  _canAddURI(uri) {
+    return lazy.PlacesUtils.history.canAddURI(uri);
+  },
+
+  /**
+   * Converts a Sync history record to a mozIPlaceInfo.
+   *
+   * Throws if an invalid record is encountered (invalid URI, etc.),
+   * returns a new PageInfo object if the record is to be applied, null
+   * otherwise (no visits to add, etc.),
+   */
+  async _recordToPlaceInfo(record) {
+    // Sort out invalid URIs and ones Places just simply doesn't want.
+    record.url = lazy.PlacesUtils.normalizeToURLOrGUID(record.histUri);
+    record.uri = CommonUtils.makeURI(record.histUri);
+
+    if (!Utils.checkGUID(record.id)) {
+      this._log.warn("Encountered record with invalid GUID: " + record.id);
+      return null;
+    }
+    record.guid = record.id;
+
+    if (
+      !this._canAddURI(record.uri) ||
+      !this.engine.shouldSyncURL(record.uri.spec)
+    ) {
+      this._log.trace(
+        "Ignoring record " +
+          record.id +
+          " with URI " +
+          record.uri.spec +
+          ": can't add this URI."
+      );
+      return null;
+    }
+
+    // We dupe visits by date and type. So an incoming visit that has
+    // the same timestamp and type as a local one won't get applied.
+    // To avoid creating new objects, we rewrite the query result so we
+    // can simply check for containment below.
+    let curVisitsAsArray = [];
+    let curVisits = new Set();
+    try {
+      curVisitsAsArray = await lazy.PlacesSyncUtils.history.fetchVisitsForURL(
+        record.histUri
+      );
+    } catch (e) {
+      this._log.error(
+        "Error while fetching visits for URL ${record.histUri}",
+        record.histUri
+      );
+    }
+    let oldestAllowed =
+      lazy.PlacesSyncUtils.bookmarks.EARLIEST_BOOKMARK_TIMESTAMP;
+    if (curVisitsAsArray.length == 20) {
+      let oldestVisit = curVisitsAsArray[curVisitsAsArray.length - 1];
+      oldestAllowed = lazy.PlacesSyncUtils.history.clampVisitDate(
+        lazy.PlacesUtils.toDate(oldestVisit.date).getTime()
+      );
+    }
+
+    let i, k;
+    for (i = 0; i < curVisitsAsArray.length; i++) {
+      // Same logic as used in the loop below to generate visitKey.
+      let { date, type } = curVisitsAsArray[i];
+      let dateObj = lazy.PlacesUtils.toDate(date);
+      let millis = lazy.PlacesSyncUtils.history
+        .clampVisitDate(dateObj)
+        .getTime();
+      curVisits.add(`${millis},${type}`);
+    }
+
+    // Walk through the visits, make sure we have sound data, and eliminate
+    // dupes. The latter is done by rewriting the array in-place.
+    for (i = 0, k = 0; i < record.visits.length; i++) {
+      let visit = (record.visits[k] = record.visits[i]);
+
+      if (
+        !visit.date ||
+        typeof visit.date != "number" ||
+        !Number.isInteger(visit.date)
+      ) {
+        this._log.warn(
+          "Encountered record with invalid visit date: " + visit.date
+        );
+        continue;
+      }
+
+      if (
+        !visit.type ||
+        !Object.values(lazy.PlacesUtils.history.TRANSITIONS).includes(
+          visit.type
+        )
+      ) {
+        this._log.warn(
+          "Encountered record with invalid visit type: " +
+            visit.type +
+            "; ignoring."
+        );
+        continue;
+      }
+
+      // Dates need to be integers. Future and far past dates are clamped to the
+      // current date and earliest sensible date, respectively.
+      let originalVisitDate = lazy.PlacesUtils.toDate(Math.round(visit.date));
+      visit.date =
+        lazy.PlacesSyncUtils.history.clampVisitDate(originalVisitDate);
+
+      if (visit.date.getTime() < oldestAllowed) {
+        // Visit is older than the oldest visit we have, and we have so many
+        // visits for this uri that we hit our limit when inserting.
+        continue;
+      }
+      let visitKey = `${visit.date.getTime()},${visit.type}`;
+      if (curVisits.has(visitKey)) {
+        // Visit is a dupe, don't increment 'k' so the element will be
+        // overwritten.
+        continue;
+      }
+
+      // Note the visit key, so that we don't add duplicate visits with
+      // clamped timestamps.
+      curVisits.add(visitKey);
+
+      visit.transition = visit.type;
+      k += 1;
+    }
+    record.visits.length = k; // truncate array
+
+    // No update if there aren't any visits to apply.
+    // History wants at least one visit.
+    // In any case, the only thing we could change would be the title
+    // and that shouldn't change without a visit.
+    if (!record.visits.length) {
+      this._log.trace(
+        "Ignoring record " +
+          record.id +
+          " with URI " +
+          record.uri.spec +
+          ": no visits to add."
+      );
+      return null;
+    }
+
+    // PageInfo is validated using validateItemProperties which does a shallow
+    // copy of the properties. Since record uses getters some of the properties
+    // are not copied over. Thus we create and return a new object.
+    let pageInfo = {
+      title: record.title,
+      url: record.url,
+      guid: record.guid,
+      visits: record.visits,
+    };
+
+    return pageInfo;
+  },
+
+  async remove(record) {
+    this._log.trace("Removing page: " + record.id);
+    let removed = await lazy.PlacesUtils.history.remove(record.id);
+    if (removed) {
+      this._log.trace("Removed page: " + record.id);
+    } else {
+      this._log.debug("Page already removed: " + record.id);
+    }
+  },
+
+  async itemExists(id) {
+    return !!(await lazy.PlacesSyncUtils.history.fetchURLInfoForGuid(id));
+  },
+
+  async createRecord(id, collection) {
+    let foo = await lazy.PlacesSyncUtils.history.fetchURLInfoForGuid(id);
+    let record = new HistoryRec(collection, id);
+    if (foo) {
+      record.histUri = foo.url;
+      record.title = foo.title;
+      record.sortindex = foo.frecency;
+
+      // If we had any unknown fields, ensure we put it back on the
+      // top-level record
+      if (foo.unknownFields) {
+        let unknownFields = JSON.parse(foo.unknownFields);
+        Object.assign(record.cleartext, unknownFields);
+      }
+
+      try {
+        record.visits = await lazy.PlacesSyncUtils.history.fetchVisitsForURL(
+          record.histUri
+        );
+      } catch (e) {
+        this._log.error(
+          "Error while fetching visits for URL ${record.histUri}",
+          record.histUri
+        );
+        record.visits = [];
+      }
+    } else {
+      record.deleted = true;
+    }
+
+    return record;
+  },
+
+  async wipe() {
+    return lazy.PlacesSyncUtils.history.wipe();
+  },
+};
+Object.setPrototypeOf(HistoryStore.prototype, Store.prototype);
+
+function HistoryTracker(name, engine) {
+  LegacyTracker.call(this, name, engine);
+}
+HistoryTracker.prototype = {
+  onStart() {
+    this._log.info("Adding Places observer.");
+    this._placesObserver = new PlacesWeakCallbackWrapper(
+      this.handlePlacesEvents.bind(this)
+    );
+    PlacesObservers.addListener(
+      ["page-visited", "history-cleared", "page-removed"],
+      this._placesObserver
+    );
+  },
+
+  onStop() {
+    this._log.info("Removing Places observer.");
+    if (this._placesObserver) {
+      PlacesObservers.removeListener(
+        ["page-visited", "history-cleared", "page-removed"],
+        this._placesObserver
+      );
+    }
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsISupportsWeakReference"]),
+
+  handlePlacesEvents(aEvents) {
+    this.asyncObserver.enqueueCall(() => this._handlePlacesEvents(aEvents));
+  },
+
+  async _handlePlacesEvents(aEvents) {
+    if (this.ignoreAll) {
+      this._log.trace(
+        "ignoreAll: ignoring visits [" +
+          aEvents.map(v => v.guid).join(",") +
+          "]"
+      );
+      return;
+    }
+    for (let event of aEvents) {
+      switch (event.type) {
+        case "page-visited": {
+          this._log.trace("'page-visited': " + event.url);
+          if (
+            this.engine.shouldSyncURL(event.url) &&
+            (await this.addChangedID(event.pageGuid))
+          ) {
+            this.score += SCORE_INCREMENT_SMALL;
+          }
+          break;
+        }
+        case "history-cleared": {
+          this._log.trace("history-cleared");
+          // Note that we're going to trigger a sync, but none of the cleared
+          // pages are tracked, so the deletions will not be propagated.
+          // See Bug 578694.
+          this.score += SCORE_INCREMENT_XLARGE;
+          break;
+        }
+        case "page-removed": {
+          if (event.reason === PlacesVisitRemoved.REASON_EXPIRED) {
+            return;
+          }
+
+          this._log.trace(
+            "page-removed: " + event.url + ", reason " + event.reason
+          );
+          const added = await this.addChangedID(event.pageGuid);
+          if (added) {
+            this.score += event.isRemovedFromStore
+              ? SCORE_INCREMENT_XLARGE
+              : SCORE_INCREMENT_SMALL;
+          }
+          break;
+        }
+      }
+    }
+  },
+};
+Object.setPrototypeOf(HistoryTracker.prototype, LegacyTracker.prototype);
diff --git a/services/sync/modules/engines/passwords.sys.mjs b/services/sync/modules/engines/passwords.sys.mjs
new file mode 100644
index 0000000000..8dea5664be
--- /dev/null
+++ b/services/sync/modules/engines/passwords.sys.mjs
@@ -0,0 +1,546 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+
+import { SCORE_INCREMENT_XLARGE } from "resource://services-sync/constants.sys.mjs";
+import { CollectionValidator } from "resource://services-sync/collection_validator.sys.mjs";
+import {
+  Changeset,
+  Store,
+  SyncEngine,
+  Tracker,
+} from "resource://services-sync/engines.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+// These are valid fields the server could have for a logins record
+// we mainly use this to detect if there are any unknownFields and
+// store (but don't process) those fields to roundtrip them back
+const VALID_LOGIN_FIELDS = [
+  "id",
+  "displayOrigin",
+  "formSubmitURL",
+  "formActionOrigin",
+  "httpRealm",
+  "hostname",
+  "origin",
+  "password",
+  "passwordField",
+  "timeCreated",
+  "timeLastUsed",
+  "timePasswordChanged",
+  "timesUsed",
+  "username",
+  "usernameField",
+  "everSynced",
+  "syncCounter",
+  "unknownFields",
+];
+
+import { LoginManagerStorage } from "resource://passwordmgr/passwordstorage.sys.mjs";
+
+// Sync and many tests rely on having an time that is rounded to the nearest
+// 100th of a second otherwise tests can fail intermittently.
+function roundTimeForSync(time) {
+  return Math.round(time / 10) / 100;
+}
+
+export function LoginRec(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+
+LoginRec.prototype = {
+  _logName: "Sync.Record.Login",
+
+  cleartextToString() {
+    let o = Object.assign({}, this.cleartext);
+    if (o.password) {
+      o.password = "X".repeat(o.password.length);
+    }
+    return JSON.stringify(o);
+  },
+};
+Object.setPrototypeOf(LoginRec.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(LoginRec, "cleartext", [
+  "hostname",
+  "formSubmitURL",
+  "httpRealm",
+  "username",
+  "password",
+  "usernameField",
+  "passwordField",
+  "timeCreated",
+  "timePasswordChanged",
+]);
+
+export function PasswordEngine(service) {
+  SyncEngine.call(this, "Passwords", service);
+}
+
+PasswordEngine.prototype = {
+  _storeObj: PasswordStore,
+  _trackerObj: PasswordTracker,
+  _recordObj: LoginRec,
+
+  syncPriority: 2,
+
+  emptyChangeset() {
+    return new PasswordsChangeset();
+  },
+
+  async ensureCurrentSyncID(newSyncID) {
+    return Services.logins.ensureCurrentSyncID(newSyncID);
+  },
+
+  async getLastSync() {
+    let legacyValue = await super.getLastSync();
+    if (legacyValue) {
+      await this.setLastSync(legacyValue);
+      Svc.PrefBranch.clearUserPref(this.name + ".lastSync");
+      this._log.debug(
+        `migrated timestamp of ${legacyValue} to the logins store`
+      );
+      return legacyValue;
+    }
+    return this._store.storage.getLastSync();
+  },
+
+  async setLastSync(timestamp) {
+    await this._store.storage.setLastSync(timestamp);
+  },
+
+  // Testing function to emulate that a login has been synced.
+  async markSynced(guid) {
+    this._store.storage.resetSyncCounter(guid, 0);
+  },
+
+  async pullAllChanges() {
+    return this._getChangedIDs(true);
+  },
+
+  async getChangedIDs() {
+    return this._getChangedIDs(false);
+  },
+
+  async _getChangedIDs(getAll) {
+    let changes = {};
+
+    let logins = await this._store.storage.getAllLogins(true);
+    for (let login of logins) {
+      if (getAll || login.syncCounter > 0) {
+        if (Utils.getSyncCredentialsHosts().has(login.origin)) {
+          continue;
+        }
+
+        changes[login.guid] = {
+          counter: login.syncCounter, // record the initial counter value
+          modified: roundTimeForSync(login.timePasswordChanged),
+          deleted: this._store.storage.loginIsDeleted(login.guid),
+        };
+      }
+    }
+
+    return changes;
+  },
+
+  async trackRemainingChanges() {
+    // Reset the syncCounter on the items that were changed.
+    for (let [guid, { counter, synced }] of Object.entries(
+      this._modified.changes
+    )) {
+      if (synced) {
+        this._store.storage.resetSyncCounter(guid, counter);
+      }
+    }
+  },
+
+  async _findDupe(item) {
+    let login = this._store._nsLoginInfoFromRecord(item);
+    if (!login) {
+      return null;
+    }
+
+    let logins = await this._store.storage.searchLoginsAsync({
+      origin: login.origin,
+      formActionOrigin: login.formActionOrigin,
+      httpRealm: login.httpRealm,
+    });
+
+    // Look for existing logins that match the origin, but ignore the password.
+    for (let local of logins) {
+      if (login.matches(local, true) && local instanceof Ci.nsILoginMetaInfo) {
+        return local.guid;
+      }
+    }
+
+    return null;
+  },
+
+  _deleteId(id) {
+    this._noteDeletedId(id);
+  },
+
+  getValidator() {
+    return new PasswordValidator();
+  },
+};
+Object.setPrototypeOf(PasswordEngine.prototype, SyncEngine.prototype);
+
+function PasswordStore(name, engine) {
+  Store.call(this, name, engine);
+  this._nsLoginInfo = new Components.Constructor(
+    "@mozilla.org/login-manager/loginInfo;1",
+    Ci.nsILoginInfo,
+    "init"
+  );
+  this.storage = LoginManagerStorage.create();
+}
+PasswordStore.prototype = {
+  _newPropertyBag() {
+    return Cc["@mozilla.org/hash-property-bag;1"].createInstance(
+      Ci.nsIWritablePropertyBag2
+    );
+  },
+
+  // Returns an stringified object of any fields not "known" by this client
+  // mainly used to to prevent data loss for other clients by roundtripping
+  // these fields without processing them
+  _processUnknownFields(record) {
+    let unknownFields = {};
+    let keys = Object.keys(record);
+    keys
+      .filter(key => !VALID_LOGIN_FIELDS.includes(key))
+      .forEach(key => {
+        unknownFields[key] = record[key];
+      });
+    // If we found some unknown fields, we stringify it to be able
+    // to properly encrypt it for roundtripping since we can't know if
+    // it contained sensitive fields or not
+    if (Object.keys(unknownFields).length) {
+      return JSON.stringify(unknownFields);
+    }
+    return null;
+  },
+
+  /**
+   * Return an instance of nsILoginInfo (and, implicitly, nsILoginMetaInfo).
+   */
+  _nsLoginInfoFromRecord(record) {
+    function nullUndefined(x) {
+      return x == undefined ? null : x;
+    }
+
+    function stringifyNullUndefined(x) {
+      return x == undefined || x == null ? "" : x;
+    }
+
+    if (record.formSubmitURL && record.httpRealm) {
+      this._log.warn(
+        "Record " +
+          record.id +
+          " has both formSubmitURL and httpRealm. Skipping."
+      );
+      return null;
+    }
+
+    // Passing in "undefined" results in an empty string, which later
+    // counts as a value. Explicitly `|| null` these fields according to JS
+    // truthiness. Records with empty strings or null will be unmolested.
+    let info = new this._nsLoginInfo(
+      record.hostname,
+      nullUndefined(record.formSubmitURL),
+      nullUndefined(record.httpRealm),
+      stringifyNullUndefined(record.username),
+      record.password,
+      record.usernameField,
+      record.passwordField
+    );
+
+    info.QueryInterface(Ci.nsILoginMetaInfo);
+    info.guid = record.id;
+    if (record.timeCreated && !isNaN(new Date(record.timeCreated).getTime())) {
+      info.timeCreated = record.timeCreated;
+    }
+    if (
+      record.timePasswordChanged &&
+      !isNaN(new Date(record.timePasswordChanged).getTime())
+    ) {
+      info.timePasswordChanged = record.timePasswordChanged;
+    }
+
+    // Check the record if there are any unknown fields from other clients
+    // that we want to roundtrip during sync to prevent data loss
+    let unknownFields = this._processUnknownFields(record.cleartext);
+    if (unknownFields) {
+      info.unknownFields = unknownFields;
+    }
+    return info;
+  },
+
+  async _getLoginFromGUID(guid) {
+    let logins = await this.storage.searchLoginsAsync({ guid }, true);
+    if (logins.length) {
+      this._log.trace(logins.length + " items matching " + guid + " found.");
+      return logins[0];
+    }
+
+    this._log.trace("No items matching " + guid + " found. Ignoring");
+    return null;
+  },
+
+  async applyIncoming(record) {
+    if (record.deleted) {
+      // Need to supply the sourceSync flag.
+      await this.remove(record, { sourceSync: true });
+      return;
+    }
+
+    await super.applyIncoming(record);
+  },
+
+  async getAllIDs() {
+    let items = {};
+    let logins = await this.storage.getAllLogins(true);
+
+    for (let i = 0; i < logins.length; i++) {
+      // Skip over Weave password/passphrase entries.
+      let metaInfo = logins[i].QueryInterface(Ci.nsILoginMetaInfo);
+      if (Utils.getSyncCredentialsHosts().has(metaInfo.origin)) {
+        continue;
+      }
+
+      items[metaInfo.guid] = metaInfo;
+    }
+
+    return items;
+  },
+
+  async changeItemID(oldID, newID) {
+    this._log.trace("Changing item ID: " + oldID + " to " + newID);
+
+    if (!(await this.itemExists(oldID))) {
+      this._log.trace("Can't change item ID: item doesn't exist");
+      return;
+    }
+    if (await this._getLoginFromGUID(newID)) {
+      this._log.trace("Can't change item ID: new ID already in use");
+      return;
+    }
+
+    let prop = this._newPropertyBag();
+    prop.setPropertyAsAUTF8String("guid", newID);
+
+    let oldLogin = await this._getLoginFromGUID(oldID);
+    this.storage.modifyLogin(oldLogin, prop, true);
+  },
+
+  async itemExists(id) {
+    let login = await this._getLoginFromGUID(id);
+    return login && !this.storage.loginIsDeleted(id);
+  },
+
+  async createRecord(id, collection) {
+    let record = new LoginRec(collection, id);
+    let login = await this._getLoginFromGUID(id);
+
+    if (!login || this.storage.loginIsDeleted(id)) {
+      record.deleted = true;
+      return record;
+    }
+
+    record.hostname = login.origin;
+    record.formSubmitURL = login.formActionOrigin;
+    record.httpRealm = login.httpRealm;
+    record.username = login.username;
+    record.password = login.password;
+    record.usernameField = login.usernameField;
+    record.passwordField = login.passwordField;
+
+    // Optional fields.
+    login.QueryInterface(Ci.nsILoginMetaInfo);
+    record.timeCreated = login.timeCreated;
+    record.timePasswordChanged = login.timePasswordChanged;
+
+    // put the unknown fields back to the top-level record
+    // during upload
+    if (login.unknownFields) {
+      let unknownFields = JSON.parse(login.unknownFields);
+      if (unknownFields) {
+        Object.keys(unknownFields).forEach(key => {
+          // We have to manually add it to the cleartext since that's
+          // what gets processed during upload
+          record.cleartext[key] = unknownFields[key];
+        });
+      }
+    }
+
+    return record;
+  },
+
+  async create(record) {
+    let login = this._nsLoginInfoFromRecord(record);
+    if (!login) {
+      return;
+    }
+
+    login.everSynced = true;
+
+    this._log.trace("Adding login for " + record.hostname);
+    this._log.trace(
+      "httpRealm: " +
+        JSON.stringify(login.httpRealm) +
+        "; " +
+        "formSubmitURL: " +
+        JSON.stringify(login.formActionOrigin)
+    );
+    await Services.logins.addLoginAsync(login);
+  },
+
+  async remove(record, { sourceSync = false } = {}) {
+    this._log.trace("Removing login " + record.id);
+
+    let loginItem = await this._getLoginFromGUID(record.id);
+    if (!loginItem) {
+      this._log.trace("Asked to remove record that doesn't exist, ignoring");
+      return;
+    }
+
+    this.storage.removeLogin(loginItem, sourceSync);
+  },
+
+  async update(record) {
+    let loginItem = await this._getLoginFromGUID(record.id);
+    if (!loginItem || this.storage.loginIsDeleted(record.id)) {
+      this._log.trace("Skipping update for unknown item: " + record.hostname);
+      return;
+    }
+
+    this._log.trace("Updating " + record.hostname);
+    let newinfo = this._nsLoginInfoFromRecord(record);
+    if (!newinfo) {
+      return;
+    }
+
+    loginItem.everSynced = true;
+
+    this.storage.modifyLogin(loginItem, newinfo, true);
+  },
+
+  async wipe() {
+    this.storage.removeAllUserFacingLogins(true);
+  },
+};
+Object.setPrototypeOf(PasswordStore.prototype, Store.prototype);
+
+function PasswordTracker(name, engine) {
+  Tracker.call(this, name, engine);
+}
+PasswordTracker.prototype = {
+  onStart() {
+    Svc.Obs.add("passwordmgr-storage-changed", this.asyncObserver);
+  },
+
+  onStop() {
+    Svc.Obs.remove("passwordmgr-storage-changed", this.asyncObserver);
+  },
+
+  async observe(subject, topic, data) {
+    if (this.ignoreAll) {
+      return;
+    }
+
+    switch (data) {
+      case "modifyLogin":
+        // The syncCounter should have been incremented only for
+        // those items that need to be sycned.
+        if (
+          subject.QueryInterface(Ci.nsIArrayExtensions).GetElementAt(1)
+            .syncCounter > 0
+        ) {
+          this.score += SCORE_INCREMENT_XLARGE;
+        }
+        break;
+
+      case "addLogin":
+      case "removeLogin":
+      case "importLogins":
+        this.score += SCORE_INCREMENT_XLARGE;
+        break;
+
+      case "removeAllLogins":
+        this.score +=
+          SCORE_INCREMENT_XLARGE *
+          (subject.QueryInterface(Ci.nsIArrayExtensions).Count() + 1);
+        break;
+    }
+  },
+};
+Object.setPrototypeOf(PasswordTracker.prototype, Tracker.prototype);
+
+export class PasswordValidator extends CollectionValidator {
+  constructor() {
+    super("passwords", "id", [
+      "hostname",
+      "formSubmitURL",
+      "httpRealm",
+      "password",
+      "passwordField",
+      "username",
+      "usernameField",
+    ]);
+  }
+
+  async getClientItems() {
+    let logins = await Services.logins.getAllLogins();
+    let syncHosts = Utils.getSyncCredentialsHosts();
+    let result = logins
+      .map(l => l.QueryInterface(Ci.nsILoginMetaInfo))
+      .filter(l => !syncHosts.has(l.origin));
+    return Promise.resolve(result);
+  }
+
+  normalizeClientItem(item) {
+    return {
+      id: item.guid,
+      guid: item.guid,
+      hostname: item.hostname,
+      formSubmitURL: item.formSubmitURL,
+      httpRealm: item.httpRealm,
+      password: item.password,
+      passwordField: item.passwordField,
+      username: item.username,
+      usernameField: item.usernameField,
+      original: item,
+    };
+  }
+
+  async normalizeServerItem(item) {
+    return Object.assign({ guid: item.id }, item);
+  }
+}
+
+export class PasswordsChangeset extends Changeset {
+  getModifiedTimestamp(id) {
+    return this.changes[id].modified;
+  }
+
+  has(id) {
+    let change = this.changes[id];
+    if (change) {
+      return !change.synced;
+    }
+    return false;
+  }
+
+  delete(id) {
+    let change = this.changes[id];
+    if (change) {
+      // Mark the change as synced without removing it from the set.
+      // This allows the sync counter to be reset when sync is complete
+      // within trackRemainingChanges.
+      change.synced = true;
+    }
+  }
+}
diff --git a/services/sync/modules/engines/prefs.sys.mjs b/services/sync/modules/engines/prefs.sys.mjs
new file mode 100644
index 0000000000..f29a9e7b59
--- /dev/null
+++ b/services/sync/modules/engines/prefs.sys.mjs
@@ -0,0 +1,503 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Prefs which start with this prefix are our "control" prefs - they indicate
+// which preferences should be synced.
+const PREF_SYNC_PREFS_PREFIX = "services.sync.prefs.sync.";
+
+// Prefs which have a default value are usually not synced - however, if the
+// preference exists under this prefix and the value is:
+// * `true`, then we do sync default values.
+// * `false`, then as soon as we ever sync a non-default value out, or sync
+//    any value in, then we toggle the value to `true`.
+//
+// We never explicitly set this pref back to false, so it's one-shot.
+// Some preferences which are known to have a different default value on
+// different platforms have this preference with a default value of `false`,
+// so they don't sync until one device changes to the non-default value, then
+// that value forever syncs, even if it gets reset back to the default.
+// Note that preferences handled this way *must also* have the "normal"
+// control pref set.
+// A possible future enhancement would be to sync these prefs so that
+// other distributions can flag them if they change the default, but that
+// doesn't seem worthwhile until we can be confident they'd actually create
+// this special control pref at the same time they flip the default.
+const PREF_SYNC_SEEN_PREFIX = "services.sync.prefs.sync-seen.";
+
+import {
+  Store,
+  SyncEngine,
+  Tracker,
+} from "resource://services-sync/engines.sys.mjs";
+import { CryptoWrapper } from "resource://services-sync/record.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+import { SCORE_INCREMENT_XLARGE } from "resource://services-sync/constants.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineLazyGetter(lazy, "PREFS_GUID", () =>
+  CommonUtils.encodeBase64URL(Services.appinfo.ID)
+);
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+});
+
+// In bug 1538015, we decided that it isn't always safe to allow all "incoming"
+// preferences to be applied locally. So we introduced another preference to control
+// this for backward compatibility. We removed that capability in bug 1854698, but in the
+// interests of working well between different versions of Firefox, we still forever
+// want to prevent this preference from syncing.
+// This was the name of the "control" pref.
+const PREF_SYNC_PREFS_ARBITRARY =
+  "services.sync.prefs.dangerously_allow_arbitrary";
+
+// Check for a local control pref or PREF_SYNC_PREFS_ARBITRARY
+function isAllowedPrefName(prefName) {
+  if (prefName == PREF_SYNC_PREFS_ARBITRARY) {
+    return false; // never allow this.
+  }
+  // The pref must already have a control pref set, although it doesn't matter
+  // here whether that value is true or false. We can't use prefHasUserValue
+  // here because we also want to check prefs still with default values.
+  try {
+    Services.prefs.getBoolPref(PREF_SYNC_PREFS_PREFIX + prefName);
+    // pref exists!
+    return true;
+  } catch (_) {
+    return false;
+  }
+}
+
+export function PrefRec(collection, id) {
+  CryptoWrapper.call(this, collection, id);
+}
+
+PrefRec.prototype = {
+  _logName: "Sync.Record.Pref",
+};
+Object.setPrototypeOf(PrefRec.prototype, CryptoWrapper.prototype);
+
+Utils.deferGetSet(PrefRec, "cleartext", ["value"]);
+
+export function PrefsEngine(service) {
+  SyncEngine.call(this, "Prefs", service);
+}
+
+PrefsEngine.prototype = {
+  _storeObj: PrefStore,
+  _trackerObj: PrefTracker,
+  _recordObj: PrefRec,
+  version: 2,
+
+  syncPriority: 1,
+  allowSkippedRecord: false,
+
+  async getChangedIDs() {
+    // No need for a proper timestamp (no conflict resolution needed).
+    let changedIDs = {};
+    if (this._tracker.modified) {
+      changedIDs[lazy.PREFS_GUID] = 0;
+    }
+    return changedIDs;
+  },
+
+  async _wipeClient() {
+    await SyncEngine.prototype._wipeClient.call(this);
+    this.justWiped = true;
+  },
+
+  async _reconcile(item) {
+    // Apply the incoming item if we don't care about the local data
+    if (this.justWiped) {
+      this.justWiped = false;
+      return true;
+    }
+    return SyncEngine.prototype._reconcile.call(this, item);
+  },
+
+  async _uploadOutgoing() {
+    try {
+      await SyncEngine.prototype._uploadOutgoing.call(this);
+    } finally {
+      this._store._incomingPrefs = null;
+    }
+  },
+
+  async trackRemainingChanges() {
+    if (this._modified.count() > 0) {
+      this._tracker.modified = true;
+    }
+  },
+};
+Object.setPrototypeOf(PrefsEngine.prototype, SyncEngine.prototype);
+
+// We don't use services.sync.engine.tabs.filteredSchemes since it includes
+// about: pages and the like, which we want to be syncable in preferences.
+// Blob, moz-extension, data and file uris are never safe to sync,
+// so we limit our check to those.
+const UNSYNCABLE_URL_REGEXP = /^(moz-extension|blob|data|file):/i;
+function isUnsyncableURLPref(prefName) {
+  if (Services.prefs.getPrefType(prefName) != Ci.nsIPrefBranch.PREF_STRING) {
+    return false;
+  }
+  const prefValue = Services.prefs.getStringPref(prefName, "");
+  return UNSYNCABLE_URL_REGEXP.test(prefValue);
+}
+
+function PrefStore(name, engine) {
+  Store.call(this, name, engine);
+  Svc.Obs.add(
+    "profile-before-change",
+    function () {
+      this.__prefs = null;
+    },
+    this
+  );
+}
+PrefStore.prototype = {
+  __prefs: null,
+  // used just for logging so we can work out why we chose to re-upload
+  _incomingPrefs: null,
+  get _prefs() {
+    if (!this.__prefs) {
+      this.__prefs = Services.prefs.getBranch("");
+    }
+    return this.__prefs;
+  },
+
+  _getSyncPrefs() {
+    let syncPrefs = Services.prefs
+      .getBranch(PREF_SYNC_PREFS_PREFIX)
+      .getChildList("")
+      .filter(pref => isAllowedPrefName(pref) && !isUnsyncableURLPref(pref));
+    // Also sync preferences that determine which prefs get synced.
+    let controlPrefs = syncPrefs.map(pref => PREF_SYNC_PREFS_PREFIX + pref);
+    return controlPrefs.concat(syncPrefs);
+  },
+
+  _isSynced(pref) {
+    if (pref.startsWith(PREF_SYNC_PREFS_PREFIX)) {
+      // this is an incoming control pref, which is ignored if there's not already
+      // a local control pref for the preference.
+      let controlledPref = pref.slice(PREF_SYNC_PREFS_PREFIX.length);
+      return isAllowedPrefName(controlledPref);
+    }
+
+    // This is the pref itself - it must be both allowed, and have a control
+    // pref which is true.
+    if (!this._prefs.getBoolPref(PREF_SYNC_PREFS_PREFIX + pref, false)) {
+      return false;
+    }
+    return isAllowedPrefName(pref);
+  },
+
+  // Given a preference name, returns either a string, bool, number or null.
+  _getPrefValue(pref) {
+    switch (this._prefs.getPrefType(pref)) {
+      case Ci.nsIPrefBranch.PREF_STRING:
+        return this._prefs.getStringPref(pref);
+      case Ci.nsIPrefBranch.PREF_INT:
+        return this._prefs.getIntPref(pref);
+      case Ci.nsIPrefBranch.PREF_BOOL:
+        return this._prefs.getBoolPref(pref);
+      //  case Ci.nsIPrefBranch.PREF_INVALID: handled by the fallthrough
+    }
+    return null;
+  },
+
+  _getAllPrefs() {
+    let values = {};
+    for (let pref of this._getSyncPrefs()) {
+      // Note: _isSynced doesn't call isUnsyncableURLPref since it would cause
+      // us not to apply (syncable) changes to preferences that are set locally
+      // which have unsyncable urls.
+      if (this._isSynced(pref) && !isUnsyncableURLPref(pref)) {
+        let isSet = this._prefs.prefHasUserValue(pref);
+        // Missing and default prefs get the null value, unless that `seen`
+        // pref is set, in which case it always gets the value.
+        let forceValue = this._prefs.getBoolPref(
+          PREF_SYNC_SEEN_PREFIX + pref,
+          false
+        );
+        if (isSet || forceValue) {
+          values[pref] = this._getPrefValue(pref);
+        } else {
+          values[pref] = null;
+        }
+        // If incoming and outgoing don't match then either the user toggled a
+        // pref that doesn't match an incoming non-default value for that pref
+        // during a sync (unlikely!) or it refused to stick and is behaving oddly.
+        if (this._incomingPrefs) {
+          let inValue = this._incomingPrefs[pref];
+          let outValue = values[pref];
+          if (inValue != null && outValue != null && inValue != outValue) {
+            this._log.debug(`Incoming pref '${pref}' refused to stick?`);
+            this._log.trace(`Incoming: '${inValue}', outgoing: '${outValue}'`);
+          }
+        }
+        // If this is a special "sync-seen" pref, and it's not the default value,
+        // set the seen pref to true.
+        if (
+          isSet &&
+          this._prefs.getBoolPref(PREF_SYNC_SEEN_PREFIX + pref, false) === false
+        ) {
+          this._log.trace(`toggling sync-seen pref for '${pref}' to true`);
+          this._prefs.setBoolPref(PREF_SYNC_SEEN_PREFIX + pref, true);
+        }
+      }
+    }
+    return values;
+  },
+
+  _maybeLogPrefChange(pref, incomingValue, existingValue) {
+    if (incomingValue != existingValue) {
+      this._log.debug(`Adjusting preference "${pref}" to the incoming value`);
+      // values are PII, so must only be logged at trace.
+      this._log.trace(`Existing: ${existingValue}. Incoming: ${incomingValue}`);
+    }
+  },
+
+  _setAllPrefs(values) {
+    const selectedThemeIDPref = "extensions.activeThemeID";
+    let selectedThemeIDBefore = this._prefs.getStringPref(
+      selectedThemeIDPref,
+      null
+    );
+    let selectedThemeIDAfter = selectedThemeIDBefore;
+
+    // Update 'services.sync.prefs.sync.foo.pref' before 'foo.pref', otherwise
+    // _isSynced returns false when 'foo.pref' doesn't exist (e.g., on a new device).
+    let prefs = Object.keys(values).sort(
+      a => -a.indexOf(PREF_SYNC_PREFS_PREFIX)
+    );
+    for (let pref of prefs) {
+      let value = values[pref];
+      if (!this._isSynced(pref)) {
+        // It's unusual for us to find an incoming preference (ie, a pref some other
+        // instance thinks is syncable) which we don't think is syncable.
+        this._log.trace(`Ignoring incoming unsyncable preference "${pref}"`);
+        continue;
+      }
+
+      if (typeof value == "string" && UNSYNCABLE_URL_REGEXP.test(value)) {
+        this._log.trace(`Skipping incoming unsyncable url for pref: ${pref}`);
+        continue;
+      }
+
+      switch (pref) {
+        // Some special prefs we don't want to set directly.
+        case selectedThemeIDPref:
+          selectedThemeIDAfter = value;
+          break;
+
+        // default is to just set the pref
+        default:
+          if (value == null) {
+            // Pref has gone missing. The best we can do is reset it.
+            if (this._prefs.prefHasUserValue(pref)) {
+              this._log.debug(`Clearing existing local preference "${pref}"`);
+              this._log.trace(
+                `Existing local value for preference: ${this._getPrefValue(
+                  pref
+                )}`
+              );
+            }
+            this._prefs.clearUserPref(pref);
+          } else {
+            try {
+              switch (typeof value) {
+                case "string":
+                  this._maybeLogPrefChange(
+                    pref,
+                    value,
+                    this._prefs.getStringPref(pref, undefined)
+                  );
+                  this._prefs.setStringPref(pref, value);
+                  break;
+                case "number":
+                  this._maybeLogPrefChange(
+                    pref,
+                    value,
+                    this._prefs.getIntPref(pref, undefined)
+                  );
+                  this._prefs.setIntPref(pref, value);
+                  break;
+                case "boolean":
+                  this._maybeLogPrefChange(
+                    pref,
+                    value,
+                    this._prefs.getBoolPref(pref, undefined)
+                  );
+                  this._prefs.setBoolPref(pref, value);
+                  break;
+              }
+            } catch (ex) {
+              this._log.trace(`Failed to set pref: ${pref}`, ex);
+            }
+          }
+          // If there's a "sync-seen" pref for this it gets toggled to true
+          // regardless of the value.
+          let seenPref = PREF_SYNC_SEEN_PREFIX + pref;
+          if (
+            this._prefs.getPrefType(seenPref) != Ci.nsIPrefBranch.PREF_INVALID
+          ) {
+            this._prefs.setBoolPref(PREF_SYNC_SEEN_PREFIX + pref, true);
+          }
+      }
+    }
+    // Themes are a little messy. Themes which have been installed are handled
+    // by the addons engine - but default themes aren't seen by that engine.
+    // So if there's a new default theme ID and that ID corresponds to a
+    // system addon, then we arrange to enable that addon here.
+    if (selectedThemeIDBefore != selectedThemeIDAfter) {
+      this._maybeEnableBuiltinTheme(selectedThemeIDAfter).catch(e => {
+        this._log.error("Failed to maybe update the default theme", e);
+      });
+    }
+  },
+
+  async _maybeEnableBuiltinTheme(themeId) {
+    let addon = null;
+    try {
+      addon = await lazy.AddonManager.getAddonByID(themeId);
+    } catch (ex) {
+      this._log.trace(
+        `There's no addon with ID '${themeId} - it can't be a builtin theme`
+      );
+      return;
+    }
+    if (addon && addon.isBuiltin && addon.type == "theme") {
+      this._log.trace(`Enabling builtin theme '${themeId}'`);
+      await addon.enable();
+    } else {
+      this._log.trace(
+        `Have incoming theme ID of '${themeId}' but it's not a builtin theme`
+      );
+    }
+  },
+
+  async getAllIDs() {
+    /* We store all prefs in just one WBO, with just one GUID */
+    let allprefs = {};
+    allprefs[lazy.PREFS_GUID] = true;
+    return allprefs;
+  },
+
+  async changeItemID(oldID, newID) {
+    this._log.trace("PrefStore GUID is constant!");
+  },
+
+  async itemExists(id) {
+    return id === lazy.PREFS_GUID;
+  },
+
+  async createRecord(id, collection) {
+    let record = new PrefRec(collection, id);
+
+    if (id == lazy.PREFS_GUID) {
+      record.value = this._getAllPrefs();
+    } else {
+      record.deleted = true;
+    }
+
+    return record;
+  },
+
+  async create(record) {
+    this._log.trace("Ignoring create request");
+  },
+
+  async remove(record) {
+    this._log.trace("Ignoring remove request");
+  },
+
+  async update(record) {
+    // Silently ignore pref updates that are for other apps.
+    if (record.id != lazy.PREFS_GUID) {
+      return;
+    }
+
+    this._log.trace("Received pref updates, applying...");
+    this._incomingPrefs = record.value;
+    this._setAllPrefs(record.value);
+  },
+
+  async wipe() {
+    this._log.trace("Ignoring wipe request");
+  },
+};
+Object.setPrototypeOf(PrefStore.prototype, Store.prototype);
+
+function PrefTracker(name, engine) {
+  Tracker.call(this, name, engine);
+  this._ignoreAll = false;
+  Svc.Obs.add("profile-before-change", this.asyncObserver);
+}
+PrefTracker.prototype = {
+  get ignoreAll() {
+    return this._ignoreAll;
+  },
+
+  set ignoreAll(value) {
+    this._ignoreAll = value;
+  },
+
+  get modified() {
+    return Svc.PrefBranch.getBoolPref("engine.prefs.modified", false);
+  },
+  set modified(value) {
+    Svc.PrefBranch.setBoolPref("engine.prefs.modified", value);
+  },
+
+  clearChangedIDs: function clearChangedIDs() {
+    this.modified = false;
+  },
+
+  __prefs: null,
+  get _prefs() {
+    if (!this.__prefs) {
+      this.__prefs = Services.prefs.getBranch("");
+    }
+    return this.__prefs;
+  },
+
+  onStart() {
+    Services.prefs.addObserver("", this.asyncObserver);
+  },
+
+  onStop() {
+    this.__prefs = null;
+    Services.prefs.removeObserver("", this.asyncObserver);
+  },
+
+  async observe(subject, topic, data) {
+    switch (topic) {
+      case "profile-before-change":
+        await this.stop();
+        break;
+      case "nsPref:changed":
+        if (this.ignoreAll) {
+          break;
+        }
+        // Trigger a sync for MULTI-DEVICE for a change that determines
+        // which prefs are synced or a regular pref change.
+        if (
+          data.indexOf(PREF_SYNC_PREFS_PREFIX) == 0 ||
+          this._prefs.getBoolPref(PREF_SYNC_PREFS_PREFIX + data, false)
+        ) {
+          this.score += SCORE_INCREMENT_XLARGE;
+          this.modified = true;
+          this._log.trace("Preference " + data + " changed");
+        }
+        break;
+    }
+  },
+};
+Object.setPrototypeOf(PrefTracker.prototype, Tracker.prototype);
+
+export function getPrefsGUIDForTest() {
+  return lazy.PREFS_GUID;
+}
diff --git a/services/sync/modules/engines/tabs.sys.mjs b/services/sync/modules/engines/tabs.sys.mjs
new file mode 100644
index 0000000000..861e051d1a
--- /dev/null
+++ b/services/sync/modules/engines/tabs.sys.mjs
@@ -0,0 +1,625 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const STORAGE_VERSION = 1; // This needs to be kept in-sync with the rust storage version
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+import { SyncEngine, Tracker } from "resource://services-sync/engines.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+import {
+  SCORE_INCREMENT_SMALL,
+  STATUS_OK,
+  URI_LENGTH_MAX,
+} from "resource://services-sync/constants.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+import { Async } from "resource://services-common/async.sys.mjs";
+import {
+  SyncRecord,
+  SyncTelemetry,
+} from "resource://services-sync/telemetry.sys.mjs";
+import { BridgedEngine } from "resource://services-sync/bridged_engine.sys.mjs";
+
+const FAR_FUTURE = 4102405200000; // 2100/01/01
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+  ReaderMode: "resource://gre/modules/ReaderMode.sys.mjs",
+  TabsStore: "resource://gre/modules/RustTabs.sys.mjs",
+  RemoteTabRecord: "resource://gre/modules/RustTabs.sys.mjs",
+});
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "TABS_FILTERED_SCHEMES",
+  "services.sync.engine.tabs.filteredSchemes",
+  "",
+  null,
+  val => {
+    return new Set(val.split("|"));
+  }
+);
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "SYNC_AFTER_DELAY_MS",
+  "services.sync.syncedTabs.syncDelayAfterTabChange",
+  0
+);
+
+// A "bridged engine" to our tabs component.
+export function TabEngine(service) {
+  BridgedEngine.call(this, "Tabs", service);
+}
+
+TabEngine.prototype = {
+  _trackerObj: TabTracker,
+  syncPriority: 3,
+
+  async prepareTheBridge(isQuickWrite) {
+    let clientsEngine = this.service.clientsEngine;
+    // Tell the bridged engine about clients.
+    // This is the same shape as ClientData in app-services.
+    // schema: https://github.com/mozilla/application-services/blob/a1168751231ed4e88c44d85f6dccc09c3b412bd2/components/sync15/src/client_types.rs#L14
+    let clientData = {
+      local_client_id: clientsEngine.localID,
+      recent_clients: {},
+    };
+
+    // We shouldn't upload tabs past what the server will accept
+    let tabs = await this.getTabsWithinPayloadSize();
+    await this._rustStore.setLocalTabs(
+      tabs.map(tab => {
+        // rust wants lastUsed in MS but the provider gives it in seconds
+        tab.lastUsed = tab.lastUsed * 1000;
+        return new lazy.RemoteTabRecord(tab);
+      })
+    );
+
+    for (let remoteClient of clientsEngine.remoteClients) {
+      let id = remoteClient.id;
+      if (!id) {
+        throw new Error("Remote client somehow did not have an id");
+      }
+      let client = {
+        fxa_device_id: remoteClient.fxaDeviceId,
+        // device_name and device_type are soft-deprecated - every client
+        // prefers what's in the FxA record. But fill them correctly anyway.
+        device_name: clientsEngine.getClientName(id) ?? "",
+        device_type: clientsEngine.getClientType(id),
+      };
+      clientData.recent_clients[id] = client;
+    }
+
+    // put ourself in there too so we record the correct device info in our sync record.
+    clientData.recent_clients[clientsEngine.localID] = {
+      fxa_device_id: await clientsEngine.fxAccounts.device.getLocalId(),
+      device_name: clientsEngine.localName,
+      device_type: clientsEngine.localType,
+    };
+
+    // Quick write needs to adjust the lastSync so we can POST to the server
+    // see quickWrite() for details
+    if (isQuickWrite) {
+      await this.setLastSync(FAR_FUTURE);
+      await this._bridge.prepareForSync(JSON.stringify(clientData));
+      return;
+    }
+
+    // Just incase we crashed while the lastSync timestamp was FAR_FUTURE, we
+    // reset it to zero
+    if ((await this.getLastSync()) === FAR_FUTURE) {
+      await this._bridge.setLastSync(0);
+    }
+    await this._bridge.prepareForSync(JSON.stringify(clientData));
+  },
+
+  async _syncStartup() {
+    await super._syncStartup();
+    await this.prepareTheBridge();
+  },
+
+  async initialize() {
+    await SyncEngine.prototype.initialize.call(this);
+
+    let path = PathUtils.join(PathUtils.profileDir, "synced-tabs.db");
+    this._rustStore = await lazy.TabsStore.init(path);
+    this._bridge = await this._rustStore.bridgedEngine();
+
+    // Uniffi doesn't currently only support async methods, so we'll need to hardcode
+    // these values for now (which is fine for now as these hardly ever change)
+    this._bridge.storageVersion = STORAGE_VERSION;
+    this._bridge.allowSkippedRecord = true;
+
+    this._log.info("Got a bridged engine!");
+    this._tracker.modified = true;
+  },
+
+  async getChangedIDs() {
+    // No need for a proper timestamp (no conflict resolution needed).
+    let changedIDs = {};
+    if (this._tracker.modified) {
+      changedIDs[this.service.clientsEngine.localID] = 0;
+    }
+    return changedIDs;
+  },
+
+  // API for use by Sync UI code to give user choices of tabs to open.
+  async getAllClients() {
+    let remoteTabs = await this._rustStore.getAll();
+    let remoteClientTabs = [];
+    for (let remoteClient of this.service.clientsEngine.remoteClients) {
+      // We get the some client info from the rust tabs engine and some from
+      // the clients engine.
+      let rustClient = remoteTabs.find(
+        x => x.clientId === remoteClient.fxaDeviceId
+      );
+      if (!rustClient) {
+        continue;
+      }
+      let client = {
+        // rust gives us ms but js uses seconds, so fix them up.
+        tabs: rustClient.remoteTabs.map(tab => {
+          tab.lastUsed = tab.lastUsed / 1000;
+          return tab;
+        }),
+        lastModified: rustClient.lastModified / 1000,
+        ...remoteClient,
+      };
+      remoteClientTabs.push(client);
+    }
+    return remoteClientTabs;
+  },
+
+  async removeClientData() {
+    let url = this.engineURL + "/" + this.service.clientsEngine.localID;
+    await this.service.resource(url).delete();
+  },
+
+  async trackRemainingChanges() {
+    if (this._modified.count() > 0) {
+      this._tracker.modified = true;
+    }
+  },
+
+  async getTabsWithinPayloadSize() {
+    const maxPayloadSize = this.service.getMaxRecordPayloadSize();
+    // See bug 535326 comment 8 for an explanation of the estimation
+    const maxSerializedSize = (maxPayloadSize / 4) * 3 - 1500;
+    return TabProvider.getAllTabsWithEstimatedMax(true, maxSerializedSize);
+  },
+
+  // Support for "quick writes"
+  _engineLock: Utils.lock,
+  _engineLocked: false,
+
+  // Tabs has a special lock to help support its "quick write"
+  get locked() {
+    return this._engineLocked;
+  },
+  lock() {
+    if (this._engineLocked) {
+      return false;
+    }
+    this._engineLocked = true;
+    return true;
+  },
+  unlock() {
+    this._engineLocked = false;
+  },
+
+  // Quickly do a POST of our current tabs if possible.
+  // This does things that would be dangerous for other engines - eg, posting
+  // without checking what's on the server could cause data-loss for other
+  // engines, but because each device exclusively owns exactly 1 tabs record
+  // with a known ID, it's safe here.
+  // Returns true if we successfully synced, false otherwise (either on error
+  // or because we declined to sync for any reason.) The return value is
+  // primarily for tests.
+  async quickWrite() {
+    if (!this.enabled) {
+      // this should be very rare, and only if tabs are disabled after the
+      // timer is created.
+      this._log.info("Can't do a quick-sync as tabs is disabled");
+      return false;
+    }
+    // This quick-sync doesn't drive the login state correctly, so just
+    // decline to sync if out status is bad
+    if (this.service.status.checkSetup() != STATUS_OK) {
+      this._log.info(
+        "Can't do a quick-sync due to the service status",
+        this.service.status.toString()
+      );
+      return false;
+    }
+    if (!this.service.serverConfiguration) {
+      this._log.info("Can't do a quick sync before the first full sync");
+      return false;
+    }
+    try {
+      return await this._engineLock("tabs.js: quickWrite", async () => {
+        // We want to restore the lastSync timestamp when complete so next sync
+        // takes tabs written by other devices since our last real sync.
+        // And for this POST we don't want the protections offered by
+        // X-If-Unmodified-Since - we want the POST to work even if the remote
+        // has moved on and we will catch back up next full sync.
+        const origLastSync = await this.getLastSync();
+        try {
+          return this._doQuickWrite();
+        } finally {
+          // set the lastSync to it's original value for regular sync
+          await this.setLastSync(origLastSync);
+        }
+      })();
+    } catch (ex) {
+      if (!Utils.isLockException(ex)) {
+        throw ex;
+      }
+      this._log.info(
+        "Can't do a quick-write as another tab sync is in progress"
+      );
+      return false;
+    }
+  },
+
+  // The guts of the quick-write sync, after we've taken the lock, checked
+  // the service status etc.
+  async _doQuickWrite() {
+    // We need to track telemetry for these syncs too!
+    const name = "tabs";
+    let telemetryRecord = new SyncRecord(
+      SyncTelemetry.allowedEngines,
+      "quick-write"
+    );
+    telemetryRecord.onEngineStart(name);
+    try {
+      Async.checkAppReady();
+      // We need to prep the bridge before we try to POST since it grabs
+      // the most recent local client id and properly sets a lastSync
+      // which is needed for a proper POST request
+      await this.prepareTheBridge(true);
+      this._tracker.clearChangedIDs();
+      this._tracker.resetScore();
+
+      Async.checkAppReady();
+      // now just the "upload" part of a sync,
+      // which for a rust engine is  not obvious.
+      // We need to do is ask the rust engine for the changes. Although
+      // this is kinda abusing the bridged-engine interface, we know the tabs
+      // implementation of it works ok
+      let outgoing = await this._bridge.apply();
+      // We know we always have exactly 1 record.
+      let mine = outgoing[0];
+      this._log.trace("outgoing bso", mine);
+      // `this._recordObj` is a `BridgedRecord`, which isn't exported.
+      let record = this._recordObj.fromOutgoingBso(this.name, JSON.parse(mine));
+      let changeset = {};
+      changeset[record.id] = { synced: false, record };
+      this._modified.replace(changeset);
+
+      Async.checkAppReady();
+      await this._uploadOutgoing();
+      telemetryRecord.onEngineStop(name, null);
+      return true;
+    } catch (ex) {
+      this._log.warn("quicksync sync failed", ex);
+      telemetryRecord.onEngineStop(name, ex);
+      return false;
+    } finally {
+      // The top-level sync is never considered to fail here, just the engine
+      telemetryRecord.finished(null);
+      SyncTelemetry.takeTelemetryRecord(telemetryRecord);
+    }
+  },
+
+  async _sync() {
+    try {
+      await this._engineLock("tabs.js: fullSync", async () => {
+        await super._sync();
+      })();
+    } catch (ex) {
+      if (!Utils.isLockException(ex)) {
+        throw ex;
+      }
+      this._log.info(
+        "Can't do full tabs sync as a quick-write is currently running"
+      );
+    }
+  },
+};
+Object.setPrototypeOf(TabEngine.prototype, BridgedEngine.prototype);
+
+export const TabProvider = {
+  getWindowEnumerator() {
+    return Services.wm.getEnumerator("navigator:browser");
+  },
+
+  shouldSkipWindow(win) {
+    return win.closed || lazy.PrivateBrowsingUtils.isWindowPrivate(win);
+  },
+
+  getAllBrowserTabs() {
+    let tabs = [];
+    for (let win of this.getWindowEnumerator()) {
+      if (this.shouldSkipWindow(win)) {
+        continue;
+      }
+      // Get all the tabs from the browser
+      for (let tab of win.gBrowser.tabs) {
+        tabs.push(tab);
+      }
+    }
+
+    return tabs.sort(function (a, b) {
+      return b.lastAccessed - a.lastAccessed;
+    });
+  },
+
+  // This function creates tabs records up to a specified amount of bytes
+  // It is an "estimation" since we don't accurately calculate how much the
+  // favicon and JSON overhead is and give a rough estimate (for optimization purposes)
+  async getAllTabsWithEstimatedMax(filter, bytesMax) {
+    let log = Log.repository.getLogger(`Sync.Engine.Tabs.Provider`);
+    let tabRecords = [];
+    let iconPromises = [];
+    let runningByteLength = 0;
+    let encoder = new TextEncoder();
+
+    // Fetch all the tabs the user has open
+    let winTabs = this.getAllBrowserTabs();
+
+    for (let tab of winTabs) {
+      // We don't want to process any more tabs than we can sync
+      if (runningByteLength >= bytesMax) {
+        log.warn(
+          `Can't fit all tabs in sync payload: have ${winTabs.length},
+             but can only fit ${tabRecords.length}.`
+        );
+        break;
+      }
+
+      // Note that we used to sync "tab history" (ie, the "back button") state,
+      // but in practice this hasn't been used - only the current URI is of
+      // interest to clients.
+      // We stopped recording this in bug 1783991.
+      if (!tab?.linkedBrowser) {
+        continue;
+      }
+      let acceptable = !filter
+        ? url => url
+        : url =>
+            url &&
+            !lazy.TABS_FILTERED_SCHEMES.has(Services.io.extractScheme(url));
+
+      let url = tab.linkedBrowser.currentURI?.spec;
+      // Special case for reader mode.
+      if (url && url.startsWith("about:reader?")) {
+        url = lazy.ReaderMode.getOriginalUrl(url);
+      }
+      // We ignore the tab completely if the current entry url is
+      // not acceptable (we need something accurate to open).
+      if (!acceptable(url)) {
+        continue;
+      }
+
+      if (url.length > URI_LENGTH_MAX) {
+        log.trace("Skipping over-long URL.");
+        continue;
+      }
+
+      let thisTab = new lazy.RemoteTabRecord({
+        title: tab.linkedBrowser.contentTitle || "",
+        urlHistory: [url],
+        icon: "",
+        lastUsed: Math.floor((tab.lastAccessed || 0) / 1000),
+      });
+      tabRecords.push(thisTab);
+
+      // we don't want to wait for each favicon to resolve to get the bytes
+      // so we estimate a conservative 100 chars for the favicon and json overhead
+      // Rust will further optimize and trim if we happened to be wildly off
+      runningByteLength +=
+        encoder.encode(thisTab.title + thisTab.lastUsed + url).byteLength + 100;
+
+      // Use the favicon service for the icon url - we can wait for the promises at the end.
+      let iconPromise = lazy.PlacesUtils.promiseFaviconData(url)
+        .then(iconData => {
+          thisTab.icon = iconData.uri.spec;
+        })
+        .catch(ex => {
+          log.trace(
+            `Failed to fetch favicon for ${url}`,
+            thisTab.urlHistory[0]
+          );
+        });
+      iconPromises.push(iconPromise);
+    }
+
+    await Promise.allSettled(iconPromises);
+    return tabRecords;
+  },
+};
+
+function TabTracker(name, engine) {
+  Tracker.call(this, name, engine);
+
+  // Make sure "this" pointer is always set correctly for event listeners.
+  this.onTab = Utils.bind2(this, this.onTab);
+  this._unregisterListeners = Utils.bind2(this, this._unregisterListeners);
+}
+TabTracker.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["nsIObserver"]),
+
+  clearChangedIDs() {
+    this.modified = false;
+  },
+
+  // We do not track TabSelect because that almost always triggers
+  // the web progress listeners (onLocationChange), which we already track
+  _topics: ["TabOpen", "TabClose"],
+
+  _registerListenersForWindow(window) {
+    this._log.trace("Registering tab listeners in window");
+    for (let topic of this._topics) {
+      window.addEventListener(topic, this.onTab);
+    }
+    window.addEventListener("unload", this._unregisterListeners);
+    // If it's got a tab browser we can listen for things like navigation.
+    if (window.gBrowser) {
+      window.gBrowser.addProgressListener(this);
+    }
+  },
+
+  _unregisterListeners(event) {
+    this._unregisterListenersForWindow(event.target);
+  },
+
+  _unregisterListenersForWindow(window) {
+    this._log.trace("Removing tab listeners in window");
+    window.removeEventListener("unload", this._unregisterListeners);
+    for (let topic of this._topics) {
+      window.removeEventListener(topic, this.onTab);
+    }
+    if (window.gBrowser) {
+      window.gBrowser.removeProgressListener(this);
+    }
+  },
+
+  onStart() {
+    Svc.Obs.add("domwindowopened", this.asyncObserver);
+    for (let win of Services.wm.getEnumerator("navigator:browser")) {
+      this._registerListenersForWindow(win);
+    }
+  },
+
+  onStop() {
+    Svc.Obs.remove("domwindowopened", this.asyncObserver);
+    for (let win of Services.wm.getEnumerator("navigator:browser")) {
+      this._unregisterListenersForWindow(win);
+    }
+  },
+
+  async observe(subject, topic, data) {
+    switch (topic) {
+      case "domwindowopened":
+        let onLoad = () => {
+          subject.removeEventListener("load", onLoad);
+          // Only register after the window is done loading to avoid unloads.
+          this._registerListenersForWindow(subject);
+        };
+
+        // Add tab listeners now that a window has opened.
+        subject.addEventListener("load", onLoad);
+        break;
+    }
+  },
+
+  onTab(event) {
+    if (event.originalTarget.linkedBrowser) {
+      let browser = event.originalTarget.linkedBrowser;
+      if (
+        lazy.PrivateBrowsingUtils.isBrowserPrivate(browser) &&
+        !lazy.PrivateBrowsingUtils.permanentPrivateBrowsing
+      ) {
+        this._log.trace("Ignoring tab event from private browsing.");
+        return;
+      }
+    }
+    this._log.trace("onTab event: " + event.type);
+
+    switch (event.type) {
+      case "TabOpen":
+        /* We do not have a reliable way of checking the URI on the TabOpen
+         * so we will rely on the other methods (onLocationChange, getAllTabsWithEstimatedMax)
+         * to filter these when going through sync
+         */
+        this.callScheduleSync(SCORE_INCREMENT_SMALL);
+        break;
+      case "TabClose":
+        // If event target has `linkedBrowser`, the event target can be assumed <tab> element.
+        // Else, event target is assumed <browser> element, use the target as it is.
+        const tab = event.target.linkedBrowser || event.target;
+
+        // TabClose means the tab has already loaded and we can check the URI
+        // and ignore if it's a scheme we don't care about
+        if (lazy.TABS_FILTERED_SCHEMES.has(tab.currentURI.scheme)) {
+          return;
+        }
+        this.callScheduleSync(SCORE_INCREMENT_SMALL);
+        break;
+    }
+  },
+
+  // web progress listeners.
+  onLocationChange(webProgress, request, locationURI, flags) {
+    // We only care about top-level location changes. We do want location changes in the
+    // same document because if a page uses the `pushState()` API, they *appear* as though
+    // they are in the same document even if the URL changes. It also doesn't hurt to accurately
+    // reflect the fragment changing - so we allow LOCATION_CHANGE_SAME_DOCUMENT
+    if (
+      flags & Ci.nsIWebProgressListener.LOCATION_CHANGE_RELOAD ||
+      !webProgress.isTopLevel ||
+      !locationURI
+    ) {
+      return;
+    }
+
+    // We can't filter out tabs that we don't sync here, because we might be
+    // navigating from a tab that we *did* sync to one we do not, and that
+    // tab we *did* sync should no longer be synced.
+    this.callScheduleSync();
+  },
+
+  callScheduleSync(scoreIncrement) {
+    this.modified = true;
+    let { scheduler } = this.engine.service;
+    let delayInMs = lazy.SYNC_AFTER_DELAY_MS;
+
+    // Schedule a sync once we detect a tab change
+    // to ensure the server always has the most up to date tabs
+    if (
+      delayInMs > 0 &&
+      scheduler.numClients > 1 // Only schedule quick syncs for multi client users
+    ) {
+      if (this.tabsQuickWriteTimer) {
+        this._log.debug(
+          "Detected a tab change, but a quick-write is already scheduled"
+        );
+        return;
+      }
+      this._log.debug(
+        "Detected a tab change: scheduling a quick-write in " + delayInMs + "ms"
+      );
+      CommonUtils.namedTimer(
+        () => {
+          this._log.trace("tab quick-sync timer fired.");
+          this.engine
+            .quickWrite()
+            .then(() => {
+              this._log.trace("tab quick-sync done.");
+            })
+            .catch(ex => {
+              this._log.error("tab quick-sync failed.", ex);
+            });
+        },
+        delayInMs,
+        this,
+        "tabsQuickWriteTimer"
+      );
+    } else if (scoreIncrement) {
+      this._log.debug(
+        "Detected a tab change, but conditions aren't met for a quick write - bumping score"
+      );
+      this.score += scoreIncrement;
+    } else {
+      this._log.debug(
+        "Detected a tab change, but conditions aren't met for a quick write or a score bump"
+      );
+    }
+  },
+};
+Object.setPrototypeOf(TabTracker.prototype, Tracker.prototype);
diff --git a/services/sync/modules/keys.sys.mjs b/services/sync/modules/keys.sys.mjs
new file mode 100644
index 0000000000..b6a1dce19a
--- /dev/null
+++ b/services/sync/modules/keys.sys.mjs
@@ -0,0 +1,166 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Weave } from "resource://services-sync/main.sys.mjs";
+
+/**
+ * Represents a pair of keys.
+ *
+ * Each key stored in a key bundle is 256 bits. One key is used for symmetric
+ * encryption. The other is used for HMAC.
+ *
+ * A KeyBundle by itself is just an anonymous pair of keys. Other types
+ * deriving from this one add semantics, such as associated collections or
+ * generating a key bundle via HKDF from another key.
+ */
+function KeyBundle() {
+  this._encrypt = null;
+  this._encryptB64 = null;
+  this._hmac = null;
+  this._hmacB64 = null;
+}
+KeyBundle.prototype = {
+  _encrypt: null,
+  _encryptB64: null,
+  _hmac: null,
+  _hmacB64: null,
+
+  equals: function equals(bundle) {
+    return (
+      bundle &&
+      bundle.hmacKey == this.hmacKey &&
+      bundle.encryptionKey == this.encryptionKey
+    );
+  },
+
+  /*
+   * Accessors for the two keys.
+   */
+  get encryptionKey() {
+    return this._encrypt;
+  },
+
+  set encryptionKey(value) {
+    if (!value || typeof value != "string") {
+      throw new Error("Encryption key can only be set to string values.");
+    }
+
+    if (value.length < 16) {
+      throw new Error("Encryption key must be at least 128 bits long.");
+    }
+
+    this._encrypt = value;
+    this._encryptB64 = btoa(value);
+  },
+
+  get encryptionKeyB64() {
+    return this._encryptB64;
+  },
+
+  get hmacKey() {
+    return this._hmac;
+  },
+
+  set hmacKey(value) {
+    if (!value || typeof value != "string") {
+      throw new Error("HMAC key can only be set to string values.");
+    }
+
+    if (value.length < 16) {
+      throw new Error("HMAC key must be at least 128 bits long.");
+    }
+
+    this._hmac = value;
+    this._hmacB64 = btoa(value);
+  },
+
+  get hmacKeyB64() {
+    return this._hmacB64;
+  },
+
+  /**
+   * Populate this key pair with 2 new, randomly generated keys.
+   */
+  async generateRandom() {
+    // Compute both at that same time
+    let [generatedHMAC, generatedEncr] = await Promise.all([
+      Weave.Crypto.generateRandomKey(),
+      Weave.Crypto.generateRandomKey(),
+    ]);
+    this.keyPairB64 = [generatedEncr, generatedHMAC];
+  },
+};
+
+/**
+ * Represents a KeyBundle associated with a collection.
+ *
+ * This is just a KeyBundle with a collection attached.
+ */
+export function BulkKeyBundle(collection) {
+  let log = Log.repository.getLogger("Sync.BulkKeyBundle");
+  log.info("BulkKeyBundle being created for " + collection);
+  KeyBundle.call(this);
+
+  this._collection = collection;
+}
+
+BulkKeyBundle.fromHexKey = function (hexKey) {
+  let key = CommonUtils.hexToBytes(hexKey);
+  let bundle = new BulkKeyBundle();
+  // [encryptionKey, hmacKey]
+  bundle.keyPair = [key.slice(0, 32), key.slice(32, 64)];
+  return bundle;
+};
+
+BulkKeyBundle.fromJWK = function (jwk) {
+  if (!jwk || !jwk.k || jwk.kty !== "oct") {
+    throw new Error("Invalid JWK provided to BulkKeyBundle.fromJWK");
+  }
+  return BulkKeyBundle.fromHexKey(CommonUtils.base64urlToHex(jwk.k));
+};
+
+BulkKeyBundle.prototype = {
+  get collection() {
+    return this._collection;
+  },
+
+  /**
+   * Obtain the key pair in this key bundle.
+   *
+   * The returned keys are represented as raw byte strings.
+   */
+  get keyPair() {
+    return [this.encryptionKey, this.hmacKey];
+  },
+
+  set keyPair(value) {
+    if (!Array.isArray(value) || value.length != 2) {
+      throw new Error("BulkKeyBundle.keyPair value must be array of 2 keys.");
+    }
+
+    this.encryptionKey = value[0];
+    this.hmacKey = value[1];
+  },
+
+  get keyPairB64() {
+    return [this.encryptionKeyB64, this.hmacKeyB64];
+  },
+
+  set keyPairB64(value) {
+    if (!Array.isArray(value) || value.length != 2) {
+      throw new Error(
+        "BulkKeyBundle.keyPairB64 value must be an array of 2 keys."
+      );
+    }
+
+    this.encryptionKey = CommonUtils.safeAtoB(value[0]);
+    this.hmacKey = CommonUtils.safeAtoB(value[1]);
+  },
+};
+
+Object.setPrototypeOf(BulkKeyBundle.prototype, KeyBundle.prototype);
diff --git a/services/sync/modules/main.sys.mjs b/services/sync/modules/main.sys.mjs
new file mode 100644
index 0000000000..838da15742
--- /dev/null
+++ b/services/sync/modules/main.sys.mjs
@@ -0,0 +1,23 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+export { lazy as Weave };
+
+const lazy = {};
+
+// We want these to be lazily loaded, which helps performance and also tests
+// to not have these loaded before they are ready.
+ChromeUtils.defineESModuleGetters(lazy, {
+  Service: "resource://services-sync/service.sys.mjs",
+  Status: "resource://services-sync/status.sys.mjs",
+  Svc: "resource://services-sync/util.sys.mjs",
+  Utils: "resource://services-sync/util.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "Crypto", () => {
+  let { WeaveCrypto } = ChromeUtils.importESModule(
+    "resource://services-crypto/WeaveCrypto.sys.mjs"
+  );
+  return new WeaveCrypto();
+});
diff --git a/services/sync/modules/policies.sys.mjs b/services/sync/modules/policies.sys.mjs
new file mode 100644
index 0000000000..290e793b8e
--- /dev/null
+++ b/services/sync/modules/policies.sys.mjs
@@ -0,0 +1,1055 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import {
+  CREDENTIALS_CHANGED,
+  ENGINE_APPLY_FAIL,
+  ENGINE_UNKNOWN_FAIL,
+  IDLE_OBSERVER_BACK_DELAY,
+  LOGIN_FAILED_INVALID_PASSPHRASE,
+  LOGIN_FAILED_LOGIN_REJECTED,
+  LOGIN_FAILED_NETWORK_ERROR,
+  LOGIN_FAILED_NO_PASSPHRASE,
+  LOGIN_SUCCEEDED,
+  MASTER_PASSWORD_LOCKED,
+  MASTER_PASSWORD_LOCKED_RETRY_INTERVAL,
+  MAX_ERROR_COUNT_BEFORE_BACKOFF,
+  MINIMUM_BACKOFF_INTERVAL,
+  MULTI_DEVICE_THRESHOLD,
+  NO_SYNC_NODE_FOUND,
+  NO_SYNC_NODE_INTERVAL,
+  OVER_QUOTA,
+  RESPONSE_OVER_QUOTA,
+  SCORE_UPDATE_DELAY,
+  SERVER_MAINTENANCE,
+  SINGLE_USER_THRESHOLD,
+  STATUS_OK,
+  SYNC_FAILED_PARTIAL,
+  SYNC_SUCCEEDED,
+  kSyncBackoffNotMet,
+  kSyncMasterPasswordLocked,
+} from "resource://services-sync/constants.sys.mjs";
+
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import { logManager } from "resource://gre/modules/FxAccountsCommon.sys.mjs";
+import { Async } from "resource://services-common/async.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+  Status: "resource://services-sync/status.sys.mjs",
+});
+ChromeUtils.defineLazyGetter(lazy, "fxAccounts", () => {
+  return ChromeUtils.importESModule(
+    "resource://gre/modules/FxAccounts.sys.mjs"
+  ).getFxAccountsSingleton();
+});
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "IdleService",
+  "@mozilla.org/widget/useridleservice;1",
+  "nsIUserIdleService"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "CaptivePortalService",
+  "@mozilla.org/network/captive-portal-service;1",
+  "nsICaptivePortalService"
+);
+
+// Get the value for an interval that's stored in preferences. To save users
+// from themselves (and us from them!) the minimum time they can specify
+// is 60s.
+function getThrottledIntervalPreference(prefName) {
+  return Math.max(Svc.PrefBranch.getIntPref(prefName), 60) * 1000;
+}
+
+export function SyncScheduler(service) {
+  this.service = service;
+  this.init();
+}
+
+SyncScheduler.prototype = {
+  _log: Log.repository.getLogger("Sync.SyncScheduler"),
+
+  _fatalLoginStatus: [
+    LOGIN_FAILED_NO_PASSPHRASE,
+    LOGIN_FAILED_INVALID_PASSPHRASE,
+    LOGIN_FAILED_LOGIN_REJECTED,
+  ],
+
+  /**
+   * The nsITimer object that schedules the next sync. See scheduleNextSync().
+   */
+  syncTimer: null,
+
+  setDefaults: function setDefaults() {
+    this._log.trace("Setting SyncScheduler policy values to defaults.");
+
+    this.singleDeviceInterval = getThrottledIntervalPreference(
+      "scheduler.fxa.singleDeviceInterval"
+    );
+    this.idleInterval = getThrottledIntervalPreference(
+      "scheduler.idleInterval"
+    );
+    this.activeInterval = getThrottledIntervalPreference(
+      "scheduler.activeInterval"
+    );
+    this.immediateInterval = getThrottledIntervalPreference(
+      "scheduler.immediateInterval"
+    );
+
+    // A user is non-idle on startup by default.
+    this.idle = false;
+
+    this.hasIncomingItems = false;
+    // This is the last number of clients we saw when previously updating the
+    // client mode. If this != currentNumClients (obtained from prefs written
+    // by the clients engine) then we need to transition to and from
+    // single and multi-device mode.
+    this.numClientsLastSync = 0;
+
+    this._resyncs = 0;
+
+    this.clearSyncTriggers();
+  },
+
+  // nextSync is in milliseconds, but prefs can't hold that much
+  get nextSync() {
+    return Svc.PrefBranch.getIntPref("nextSync", 0) * 1000;
+  },
+  set nextSync(value) {
+    Svc.PrefBranch.setIntPref("nextSync", Math.floor(value / 1000));
+  },
+
+  get missedFxACommandsFetchInterval() {
+    return Services.prefs.getIntPref(
+      "identity.fxaccounts.commands.missed.fetch_interval"
+    );
+  },
+
+  get missedFxACommandsLastFetch() {
+    return Services.prefs.getIntPref(
+      "identity.fxaccounts.commands.missed.last_fetch",
+      0
+    );
+  },
+
+  set missedFxACommandsLastFetch(val) {
+    Services.prefs.setIntPref(
+      "identity.fxaccounts.commands.missed.last_fetch",
+      val
+    );
+  },
+
+  get syncInterval() {
+    return this._syncInterval;
+  },
+  set syncInterval(value) {
+    if (value != this._syncInterval) {
+      Services.prefs.setIntPref("services.sync.syncInterval", value);
+    }
+  },
+
+  get syncThreshold() {
+    return this._syncThreshold;
+  },
+  set syncThreshold(value) {
+    if (value != this._syncThreshold) {
+      Services.prefs.setIntPref("services.sync.syncThreshold", value);
+    }
+  },
+
+  get globalScore() {
+    return this._globalScore;
+  },
+  set globalScore(value) {
+    if (this._globalScore != value) {
+      Services.prefs.setIntPref("services.sync.globalScore", value);
+    }
+  },
+
+  // Managed by the clients engine (by way of prefs)
+  get numClients() {
+    return this.numDesktopClients + this.numMobileClients;
+  },
+  set numClients(value) {
+    throw new Error("Don't set numClients - the clients engine manages it.");
+  },
+
+  get offline() {
+    // Services.io.offline has slowly become fairly useless over the years - it
+    // no longer attempts to track the actual network state by default, but one
+    // thing stays true: if it says we're offline then we are definitely not online.
+    //
+    // We also ask the captive portal service if we are behind a locked captive
+    // portal.
+    //
+    // We don't check on the NetworkLinkService however, because it gave us
+    // false positives in the past in a vm environment.
+    try {
+      if (
+        Services.io.offline ||
+        lazy.CaptivePortalService.state ==
+          lazy.CaptivePortalService.LOCKED_PORTAL
+      ) {
+        return true;
+      }
+    } catch (ex) {
+      this._log.warn("Could not determine network status.", ex);
+    }
+    return false;
+  },
+
+  _initPrefGetters() {
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "idleTime",
+      "services.sync.scheduler.idleTime"
+    );
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "maxResyncs",
+      "services.sync.maxResyncs",
+      0
+    );
+
+    // The number of clients we have is maintained in preferences via the
+    // clients engine, and only updated after a successsful sync.
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "numDesktopClients",
+      "services.sync.clients.devices.desktop",
+      0
+    );
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "numMobileClients",
+      "services.sync.clients.devices.mobile",
+      0
+    );
+
+    // Scheduler state that seems to be read more often than it's written.
+    // We also check if the value has changed before writing in the setters.
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "_syncThreshold",
+      "services.sync.syncThreshold",
+      SINGLE_USER_THRESHOLD
+    );
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "_syncInterval",
+      "services.sync.syncInterval",
+      this.singleDeviceInterval
+    );
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "_globalScore",
+      "services.sync.globalScore",
+      0
+    );
+  },
+
+  init: function init() {
+    this._log.manageLevelFromPref("services.sync.log.logger.service.main");
+    this.setDefaults();
+    this._initPrefGetters();
+    Svc.Obs.add("weave:engine:score:updated", this);
+    Svc.Obs.add("network:offline-status-changed", this);
+    Svc.Obs.add("network:link-status-changed", this);
+    Svc.Obs.add("captive-portal-detected", this);
+    Svc.Obs.add("weave:service:sync:start", this);
+    Svc.Obs.add("weave:service:sync:finish", this);
+    Svc.Obs.add("weave:engine:sync:finish", this);
+    Svc.Obs.add("weave:engine:sync:error", this);
+    Svc.Obs.add("weave:service:login:error", this);
+    Svc.Obs.add("weave:service:logout:finish", this);
+    Svc.Obs.add("weave:service:sync:error", this);
+    Svc.Obs.add("weave:service:backoff:interval", this);
+    Svc.Obs.add("weave:engine:sync:applied", this);
+    Svc.Obs.add("weave:service:setup-complete", this);
+    Svc.Obs.add("weave:service:start-over", this);
+    Svc.Obs.add("FxA:hawk:backoff:interval", this);
+
+    if (lazy.Status.checkSetup() == STATUS_OK) {
+      Svc.Obs.add("wake_notification", this);
+      Svc.Obs.add("captive-portal-login-success", this);
+      Svc.Obs.add("sleep_notification", this);
+      lazy.IdleService.addIdleObserver(this, this.idleTime);
+    }
+  },
+
+  // eslint-disable-next-line complexity
+  observe: function observe(subject, topic, data) {
+    this._log.trace("Handling " + topic);
+    switch (topic) {
+      case "weave:engine:score:updated":
+        if (lazy.Status.login == LOGIN_SUCCEEDED) {
+          CommonUtils.namedTimer(
+            this.calculateScore,
+            SCORE_UPDATE_DELAY,
+            this,
+            "_scoreTimer"
+          );
+        }
+        break;
+      case "network:link-status-changed":
+        // Note: NetworkLinkService is unreliable, we get false negatives for it
+        // in cases such as VMs (bug 1420802), so we don't want to use it in
+        // `get offline`, but we assume that it's probably reliable if we're
+        // getting status changed events. (We might be wrong about this, but
+        // if that's true, then the only downside is that we won't sync as
+        // promptly).
+        let isOffline = this.offline;
+        this._log.debug(
+          `Network link status changed to "${data}". Offline?`,
+          isOffline
+        );
+        // Data may be one of `up`, `down`, `change`, or `unknown`. We only want
+        // to sync if it's "up".
+        if (data == "up" && !isOffline) {
+          this._log.debug("Network link looks up. Syncing.");
+          this.scheduleNextSync(0, { why: topic });
+        } else if (data == "down") {
+          // Unschedule pending syncs if we know we're going down. We don't do
+          // this via `checkSyncStatus`, since link status isn't reflected in
+          // `this.offline`.
+          this.clearSyncTriggers();
+        }
+        break;
+      case "network:offline-status-changed":
+      case "captive-portal-detected":
+        // Whether online or offline, we'll reschedule syncs
+        this._log.trace("Network offline status change: " + data);
+        this.checkSyncStatus();
+        break;
+      case "weave:service:sync:start":
+        // Clear out any potentially pending syncs now that we're syncing
+        this.clearSyncTriggers();
+
+        // reset backoff info, if the server tells us to continue backing off,
+        // we'll handle that later
+        lazy.Status.resetBackoff();
+
+        this.globalScore = 0;
+        break;
+      case "weave:service:sync:finish":
+        this.nextSync = 0;
+        this.adjustSyncInterval();
+
+        if (
+          lazy.Status.service == SYNC_FAILED_PARTIAL &&
+          this.requiresBackoff
+        ) {
+          this.requiresBackoff = false;
+          this.handleSyncError();
+          return;
+        }
+
+        let sync_interval;
+        let nextSyncReason = "schedule";
+        this.updateGlobalScore();
+        if (
+          this.globalScore > this.syncThreshold &&
+          lazy.Status.service == STATUS_OK
+        ) {
+          // The global score should be 0 after a sync. If it's not, either
+          // items were changed during the last sync (and we should schedule an
+          // immediate follow-up sync), or an engine skipped
+          this._resyncs++;
+          if (this._resyncs <= this.maxResyncs) {
+            sync_interval = 0;
+            nextSyncReason = "resync";
+          } else {
+            this._log.warn(
+              `Resync attempt ${this._resyncs} exceeded ` +
+                `maximum ${this.maxResyncs}`
+            );
+            Svc.Obs.notify("weave:service:resyncs-finished");
+          }
+        } else {
+          this._resyncs = 0;
+          Svc.Obs.notify("weave:service:resyncs-finished");
+        }
+
+        this._syncErrors = 0;
+        if (lazy.Status.sync == NO_SYNC_NODE_FOUND) {
+          // If we don't have a Sync node, override the interval, even if we've
+          // scheduled a follow-up sync.
+          this._log.trace("Scheduling a sync at interval NO_SYNC_NODE_FOUND.");
+          sync_interval = NO_SYNC_NODE_INTERVAL;
+        }
+        this.scheduleNextSync(sync_interval, { why: nextSyncReason });
+        break;
+      case "weave:engine:sync:finish":
+        if (data == "clients") {
+          // Update the client mode because it might change what we sync.
+          this.updateClientMode();
+        }
+        break;
+      case "weave:engine:sync:error":
+        // `subject` is the exception thrown by an engine's sync() method.
+        let exception = subject;
+        if (exception.status >= 500 && exception.status <= 504) {
+          this.requiresBackoff = true;
+        }
+        break;
+      case "weave:service:login:error":
+        this.clearSyncTriggers();
+
+        if (lazy.Status.login == MASTER_PASSWORD_LOCKED) {
+          // Try again later, just as if we threw an error... only without the
+          // error count.
+          this._log.debug("Couldn't log in: master password is locked.");
+          this._log.trace(
+            "Scheduling a sync at MASTER_PASSWORD_LOCKED_RETRY_INTERVAL"
+          );
+          this.scheduleAtInterval(MASTER_PASSWORD_LOCKED_RETRY_INTERVAL);
+        } else if (!this._fatalLoginStatus.includes(lazy.Status.login)) {
+          // Not a fatal login error, just an intermittent network or server
+          // issue. Keep on syncin'.
+          this.checkSyncStatus();
+        }
+        break;
+      case "weave:service:logout:finish":
+        // Start or cancel the sync timer depending on if
+        // logged in or logged out
+        this.checkSyncStatus();
+        break;
+      case "weave:service:sync:error":
+        // There may be multiple clients but if the sync fails, client mode
+        // should still be updated so that the next sync has a correct interval.
+        this.updateClientMode();
+        this.adjustSyncInterval();
+        this.nextSync = 0;
+        this.handleSyncError();
+        break;
+      case "FxA:hawk:backoff:interval":
+      case "weave:service:backoff:interval":
+        let requested_interval = subject * 1000;
+        this._log.debug(
+          "Got backoff notification: " + requested_interval + "ms"
+        );
+        // Leave up to 25% more time for the back off.
+        let interval = requested_interval * (1 + Math.random() * 0.25);
+        lazy.Status.backoffInterval = interval;
+        lazy.Status.minimumNextSync = Date.now() + requested_interval;
+        this._log.debug(
+          "Fuzzed minimum next sync: " + lazy.Status.minimumNextSync
+        );
+        break;
+      case "weave:engine:sync:applied":
+        let numItems = subject.succeeded;
+        this._log.trace(
+          "Engine " + data + " successfully applied " + numItems + " items."
+        );
+        // Bug 1800186 - the tabs engine always reports incoming items, so we don't
+        // want special scheduling in this scenario.
+        // (However, even when we fix the underlying cause of that, we probably still can
+        // ignore tabs here - new incoming tabs don't need to trigger the extra syncs we do
+        // based on this flag.)
+        if (data != "tabs" && numItems) {
+          this.hasIncomingItems = true;
+        }
+        if (subject.newFailed) {
+          this._log.error(
+            `Engine ${data} found ${subject.newFailed} new records that failed to apply`
+          );
+        }
+        break;
+      case "weave:service:setup-complete":
+        Services.prefs.savePrefFile(null);
+        lazy.IdleService.addIdleObserver(this, this.idleTime);
+        Svc.Obs.add("wake_notification", this);
+        Svc.Obs.add("captive-portal-login-success", this);
+        Svc.Obs.add("sleep_notification", this);
+        break;
+      case "weave:service:start-over":
+        this.setDefaults();
+        try {
+          lazy.IdleService.removeIdleObserver(this, this.idleTime);
+        } catch (ex) {
+          if (ex.result != Cr.NS_ERROR_FAILURE) {
+            throw ex;
+          }
+          // In all likelihood we didn't have an idle observer registered yet.
+          // It's all good.
+        }
+        break;
+      case "idle":
+        this._log.trace("We're idle.");
+        this.idle = true;
+        // Adjust the interval for future syncs. This won't actually have any
+        // effect until the next pending sync (which will happen soon since we
+        // were just active.)
+        this.adjustSyncInterval();
+        break;
+      case "active":
+        this._log.trace("Received notification that we're back from idle.");
+        this.idle = false;
+        CommonUtils.namedTimer(
+          function onBack() {
+            if (this.idle) {
+              this._log.trace(
+                "... and we're idle again. " +
+                  "Ignoring spurious back notification."
+              );
+              return;
+            }
+
+            this._log.trace("Genuine return from idle. Syncing.");
+            // Trigger a sync if we have multiple clients.
+            if (this.numClients > 1) {
+              this.scheduleNextSync(0, { why: topic });
+            }
+          },
+          IDLE_OBSERVER_BACK_DELAY,
+          this,
+          "idleDebouncerTimer"
+        );
+        break;
+      case "wake_notification":
+        this._log.debug("Woke from sleep.");
+        CommonUtils.nextTick(() => {
+          // Trigger a sync if we have multiple clients. We give it 2 seconds
+          // so the browser can recover from the wake and do more important
+          // operations first (timers etc).
+          if (this.numClients > 1) {
+            if (!this.offline) {
+              this._log.debug("Online, will sync in 2s.");
+              this.scheduleNextSync(2000, { why: topic });
+            }
+          }
+        });
+        break;
+      case "captive-portal-login-success":
+        this._log.debug("Captive portal login success. Scheduling a sync.");
+        CommonUtils.nextTick(() => {
+          this.scheduleNextSync(3000, { why: topic });
+        });
+        break;
+      case "sleep_notification":
+        if (this.service.engineManager.get("tabs")._tracker.modified) {
+          this._log.debug("Going to sleep, doing a quick sync.");
+          this.scheduleNextSync(0, { engines: ["tabs"], why: "sleep" });
+        }
+        break;
+    }
+  },
+
+  adjustSyncInterval: function adjustSyncInterval() {
+    if (this.numClients <= 1) {
+      this._log.trace("Adjusting syncInterval to singleDeviceInterval.");
+      this.syncInterval = this.singleDeviceInterval;
+      return;
+    }
+
+    // Only MULTI_DEVICE clients will enter this if statement
+    // since SINGLE_USER clients will be handled above.
+    if (this.idle) {
+      this._log.trace("Adjusting syncInterval to idleInterval.");
+      this.syncInterval = this.idleInterval;
+      return;
+    }
+
+    if (this.hasIncomingItems) {
+      this._log.trace("Adjusting syncInterval to immediateInterval.");
+      this.hasIncomingItems = false;
+      this.syncInterval = this.immediateInterval;
+    } else {
+      this._log.trace("Adjusting syncInterval to activeInterval.");
+      this.syncInterval = this.activeInterval;
+    }
+  },
+
+  updateGlobalScore() {
+    let engines = [this.service.clientsEngine].concat(
+      this.service.engineManager.getEnabled()
+    );
+    let globalScore = this.globalScore;
+    for (let i = 0; i < engines.length; i++) {
+      this._log.trace(engines[i].name + ": score: " + engines[i].score);
+      globalScore += engines[i].score;
+      engines[i]._tracker.resetScore();
+    }
+    this.globalScore = globalScore;
+    this._log.trace("Global score updated: " + globalScore);
+  },
+
+  calculateScore() {
+    this.updateGlobalScore();
+    this.checkSyncStatus();
+  },
+
+  /**
+   * Query the number of known clients to figure out what mode to be in
+   */
+  updateClientMode: function updateClientMode() {
+    // Nothing to do if it's the same amount
+    let numClients = this.numClients;
+    if (numClients == this.numClientsLastSync) {
+      return;
+    }
+
+    this._log.debug(
+      `Client count: ${this.numClientsLastSync} -> ${numClients}`
+    );
+    this.numClientsLastSync = numClients;
+
+    if (numClients <= 1) {
+      this._log.trace("Adjusting syncThreshold to SINGLE_USER_THRESHOLD");
+      this.syncThreshold = SINGLE_USER_THRESHOLD;
+    } else {
+      this._log.trace("Adjusting syncThreshold to MULTI_DEVICE_THRESHOLD");
+      this.syncThreshold = MULTI_DEVICE_THRESHOLD;
+    }
+    this.adjustSyncInterval();
+  },
+
+  /**
+   * Check if we should be syncing and schedule the next sync, if it's not scheduled
+   */
+  checkSyncStatus: function checkSyncStatus() {
+    // Should we be syncing now, if not, cancel any sync timers and return
+    // if we're in backoff, we'll schedule the next sync.
+    let ignore = [kSyncBackoffNotMet, kSyncMasterPasswordLocked];
+    let skip = this.service._checkSync(ignore);
+    this._log.trace('_checkSync returned "' + skip + '".');
+    if (skip) {
+      this.clearSyncTriggers();
+      return;
+    }
+
+    let why = "schedule";
+    // Only set the wait time to 0 if we need to sync right away
+    let wait;
+    if (this.globalScore > this.syncThreshold) {
+      this._log.debug("Global Score threshold hit, triggering sync.");
+      wait = 0;
+      why = "score";
+    }
+    this.scheduleNextSync(wait, { why });
+  },
+
+  /**
+   * Call sync() if Master Password is not locked.
+   *
+   * Otherwise, reschedule a sync for later.
+   */
+  syncIfMPUnlocked(engines, why) {
+    // No point if we got kicked out by the master password dialog.
+    if (lazy.Status.login == MASTER_PASSWORD_LOCKED && Utils.mpLocked()) {
+      this._log.debug(
+        "Not initiating sync: Login status is " + lazy.Status.login
+      );
+
+      // If we're not syncing now, we need to schedule the next one.
+      this._log.trace(
+        "Scheduling a sync at MASTER_PASSWORD_LOCKED_RETRY_INTERVAL"
+      );
+      this.scheduleAtInterval(MASTER_PASSWORD_LOCKED_RETRY_INTERVAL);
+      return;
+    }
+
+    if (!Async.isAppReady()) {
+      this._log.debug("Not initiating sync: app is shutting down");
+      return;
+    }
+    Services.tm.dispatchToMainThread(() => {
+      this.service.sync({ engines, why });
+      const now = Math.round(new Date().getTime() / 1000);
+      // Only fetch missed messages in a "scheduled" sync so we don't race against
+      // the Push service reconnecting on a network link change for example.
+      if (
+        why == "schedule" &&
+        now >=
+          this.missedFxACommandsLastFetch + this.missedFxACommandsFetchInterval
+      ) {
+        lazy.fxAccounts.commands
+          .pollDeviceCommands()
+          .then(() => {
+            this.missedFxACommandsLastFetch = now;
+          })
+          .catch(e => {
+            this._log.error("Fetching missed remote commands failed.", e);
+          });
+      }
+    });
+  },
+
+  /**
+   * Set a timer for the next sync
+   */
+  scheduleNextSync(interval, { engines = null, why = null } = {}) {
+    // If no interval was specified, use the current sync interval.
+    if (interval == null) {
+      interval = this.syncInterval;
+    }
+
+    // Ensure the interval is set to no less than the backoff.
+    if (lazy.Status.backoffInterval && interval < lazy.Status.backoffInterval) {
+      this._log.trace(
+        "Requested interval " +
+          interval +
+          " ms is smaller than the backoff interval. " +
+          "Using backoff interval " +
+          lazy.Status.backoffInterval +
+          " ms instead."
+      );
+      interval = lazy.Status.backoffInterval;
+    }
+    let nextSync = this.nextSync;
+    if (nextSync != 0) {
+      // There's already a sync scheduled. Don't reschedule if there's already
+      // a timer scheduled for sooner than requested.
+      let currentInterval = nextSync - Date.now();
+      this._log.trace(
+        "There's already a sync scheduled in " + currentInterval + " ms."
+      );
+      if (currentInterval < interval && this.syncTimer) {
+        this._log.trace(
+          "Ignoring scheduling request for next sync in " + interval + " ms."
+        );
+        return;
+      }
+    }
+
+    // Start the sync right away if we're already late.
+    if (interval <= 0) {
+      this._log.trace(`Requested sync should happen right away. (why=${why})`);
+      this.syncIfMPUnlocked(engines, why);
+      return;
+    }
+
+    this._log.debug(`Next sync in ${interval} ms. (why=${why})`);
+    CommonUtils.namedTimer(
+      () => {
+        this.syncIfMPUnlocked(engines, why);
+      },
+      interval,
+      this,
+      "syncTimer"
+    );
+
+    // Save the next sync time in-case sync is disabled (logout/offline/etc.)
+    this.nextSync = Date.now() + interval;
+  },
+
+  /**
+   * Incorporates the backoff/retry logic used in error handling and elective
+   * non-syncing.
+   */
+  scheduleAtInterval: function scheduleAtInterval(minimumInterval) {
+    let interval = Utils.calculateBackoff(
+      this._syncErrors,
+      MINIMUM_BACKOFF_INTERVAL,
+      lazy.Status.backoffInterval
+    );
+    if (minimumInterval) {
+      interval = Math.max(minimumInterval, interval);
+    }
+
+    this._log.debug(
+      "Starting client-initiated backoff. Next sync in " + interval + " ms."
+    );
+    this.scheduleNextSync(interval, { why: "client-backoff-schedule" });
+  },
+
+  autoConnect: function autoConnect() {
+    if (this.service._checkSetup() == STATUS_OK && !this.service._checkSync()) {
+      // Schedule a sync based on when a previous sync was scheduled.
+      // scheduleNextSync() will do the right thing if that time lies in
+      // the past.
+      this.scheduleNextSync(this.nextSync - Date.now(), { why: "startup" });
+    }
+  },
+
+  _syncErrors: 0,
+  /**
+   * Deal with sync errors appropriately
+   */
+  handleSyncError: function handleSyncError() {
+    this._log.trace("In handleSyncError. Error count: " + this._syncErrors);
+    this._syncErrors++;
+
+    // Do nothing on the first couple of failures, if we're not in
+    // backoff due to 5xx errors.
+    if (!lazy.Status.enforceBackoff) {
+      if (this._syncErrors < MAX_ERROR_COUNT_BEFORE_BACKOFF) {
+        this.scheduleNextSync(null, { why: "reschedule" });
+        return;
+      }
+      this._log.debug(
+        "Sync error count has exceeded " +
+          MAX_ERROR_COUNT_BEFORE_BACKOFF +
+          "; enforcing backoff."
+      );
+      lazy.Status.enforceBackoff = true;
+    }
+
+    this.scheduleAtInterval();
+  },
+
+  /**
+   * Remove any timers/observers that might trigger a sync
+   */
+  clearSyncTriggers: function clearSyncTriggers() {
+    this._log.debug("Clearing sync triggers and the global score.");
+    this.globalScore = this.nextSync = 0;
+
+    // Clear out any scheduled syncs
+    if (this.syncTimer) {
+      this.syncTimer.clear();
+    }
+  },
+};
+
+export function ErrorHandler(service) {
+  this.service = service;
+  this.init();
+}
+
+ErrorHandler.prototype = {
+  init() {
+    Svc.Obs.add("weave:engine:sync:applied", this);
+    Svc.Obs.add("weave:engine:sync:error", this);
+    Svc.Obs.add("weave:service:login:error", this);
+    Svc.Obs.add("weave:service:sync:error", this);
+    Svc.Obs.add("weave:service:sync:finish", this);
+    Svc.Obs.add("weave:service:start-over:finish", this);
+
+    this.initLogs();
+  },
+
+  initLogs: function initLogs() {
+    // Set the root Sync logger level based on a pref. All other logs will
+    // inherit this level unless they specifically override it.
+    Log.repository
+      .getLogger("Sync")
+      .manageLevelFromPref(`services.sync.log.logger`);
+    // And allow our specific log to have a custom level via a pref.
+    this._log = Log.repository.getLogger("Sync.ErrorHandler");
+    this._log.manageLevelFromPref("services.sync.log.logger.service.main");
+  },
+
+  observe(subject, topic, data) {
+    this._log.trace("Handling " + topic);
+    switch (topic) {
+      case "weave:engine:sync:applied":
+        if (subject.newFailed) {
+          // An engine isn't able to apply one or more incoming records.
+          // We don't fail hard on this, but it usually indicates a bug,
+          // so for now treat it as sync error (c.f. Service._syncEngine())
+          lazy.Status.engines = [data, ENGINE_APPLY_FAIL];
+          this._log.debug(data + " failed to apply some records.");
+        }
+        break;
+      case "weave:engine:sync:error": {
+        let exception = subject; // exception thrown by engine's sync() method
+        let engine_name = data; // engine name that threw the exception
+
+        this.checkServerError(exception);
+
+        lazy.Status.engines = [
+          engine_name,
+          exception.failureCode || ENGINE_UNKNOWN_FAIL,
+        ];
+        if (Async.isShutdownException(exception)) {
+          this._log.debug(
+            engine_name +
+              " was interrupted due to the application shutting down"
+          );
+        } else {
+          this._log.debug(engine_name + " failed", exception);
+        }
+        break;
+      }
+      case "weave:service:login:error":
+        this._log.error("Sync encountered a login error");
+        this.resetFileLog();
+        break;
+      case "weave:service:sync:error": {
+        if (lazy.Status.sync == CREDENTIALS_CHANGED) {
+          this.service.logout();
+        }
+
+        let exception = subject;
+        if (Async.isShutdownException(exception)) {
+          // If we are shutting down we just log the fact, attempt to flush
+          // the log file and get out of here!
+          this._log.error(
+            "Sync was interrupted due to the application shutting down"
+          );
+          this.resetFileLog();
+          break;
+        }
+
+        // Not a shutdown related exception...
+        this._log.error("Sync encountered an error", exception);
+        this.resetFileLog();
+        break;
+      }
+      case "weave:service:sync:finish":
+        this._log.trace("Status.service is " + lazy.Status.service);
+
+        // Check both of these status codes: in the event of a failure in one
+        // engine, Status.service will be SYNC_FAILED_PARTIAL despite
+        // Status.sync being SYNC_SUCCEEDED.
+        // *facepalm*
+        if (
+          lazy.Status.sync == SYNC_SUCCEEDED &&
+          lazy.Status.service == STATUS_OK
+        ) {
+          // Great. Let's clear our mid-sync 401 note.
+          this._log.trace("Clearing lastSyncReassigned.");
+          Svc.PrefBranch.clearUserPref("lastSyncReassigned");
+        }
+
+        if (lazy.Status.service == SYNC_FAILED_PARTIAL) {
+          this._log.error("Some engines did not sync correctly.");
+        }
+        this.resetFileLog();
+        break;
+      case "weave:service:start-over:finish":
+        // ensure we capture any logs between the last sync and the reset completing.
+        this.resetFileLog()
+          .then(() => {
+            // although for privacy reasons we also delete all logs (but we allow
+            // a preference to avoid this to help with debugging.)
+            if (!Svc.PrefBranch.getBoolPref("log.keepLogsOnReset", false)) {
+              return logManager.removeAllLogs().then(() => {
+                Svc.Obs.notify("weave:service:remove-file-log");
+              });
+            }
+            return null;
+          })
+          .catch(err => {
+            // So we failed to delete the logs - take the ironic option of
+            // writing this error to the logs we failed to delete!
+            this._log.error("Failed to delete logs on reset", err);
+          });
+        break;
+    }
+  },
+
+  async _dumpAddons() {
+    // Just dump the items that sync may be concerned with. Specifically,
+    // active extensions that are not hidden.
+    let addons = [];
+    try {
+      addons = await lazy.AddonManager.getAddonsByTypes(["extension"]);
+    } catch (e) {
+      this._log.warn("Failed to dump addons", e);
+    }
+
+    let relevantAddons = addons.filter(x => x.isActive && !x.hidden);
+    this._log.trace("Addons installed", relevantAddons.length);
+    for (let addon of relevantAddons) {
+      this._log.trace(" - ${name}, version ${version}, id ${id}", addon);
+    }
+  },
+
+  /**
+   * Generate a log file for the sync that just completed
+   * and refresh the input & output streams.
+   */
+  async resetFileLog() {
+    // If we're writing an error log, dump extensions that may be causing problems.
+    if (logManager.sawError) {
+      await this._dumpAddons();
+    }
+    const logType = await logManager.resetFileLog();
+    if (logType == logManager.ERROR_LOG_WRITTEN) {
+      console.error(
+        "Sync encountered an error - see about:sync-log for the log file."
+      );
+    }
+    Svc.Obs.notify("weave:service:reset-file-log");
+  },
+
+  /**
+   * Handle HTTP response results or exceptions and set the appropriate
+   * Status.* bits.
+   *
+   * This method also looks for "side-channel" warnings.
+   */
+  checkServerError(resp) {
+    // In this case we were passed a resolved value of Resource#_doRequest.
+    switch (resp.status) {
+      case 400:
+        if (resp == RESPONSE_OVER_QUOTA) {
+          lazy.Status.sync = OVER_QUOTA;
+        }
+        break;
+
+      case 401:
+        this.service.logout();
+        this._log.info("Got 401 response; resetting clusterURL.");
+        this.service.clusterURL = null;
+
+        let delay = 0;
+        if (Svc.PrefBranch.getBoolPref("lastSyncReassigned", false)) {
+          // We got a 401 in the middle of the previous sync, and we just got
+          // another. Login must have succeeded in order for us to get here, so
+          // the password should be correct.
+          // This is likely to be an intermittent server issue, so back off and
+          // give it time to recover.
+          this._log.warn("Last sync also failed for 401. Delaying next sync.");
+          delay = MINIMUM_BACKOFF_INTERVAL;
+        } else {
+          this._log.debug("New mid-sync 401 failure. Making a note.");
+          Svc.PrefBranch.setBoolPref("lastSyncReassigned", true);
+        }
+        this._log.info("Attempting to schedule another sync.");
+        this.service.scheduler.scheduleNextSync(delay, { why: "reschedule" });
+        break;
+
+      case 500:
+      case 502:
+      case 503:
+      case 504:
+        lazy.Status.enforceBackoff = true;
+        if (resp.status == 503 && resp.headers["retry-after"]) {
+          let retryAfter = resp.headers["retry-after"];
+          this._log.debug("Got Retry-After: " + retryAfter);
+          if (this.service.isLoggedIn) {
+            lazy.Status.sync = SERVER_MAINTENANCE;
+          } else {
+            lazy.Status.login = SERVER_MAINTENANCE;
+          }
+          Svc.Obs.notify(
+            "weave:service:backoff:interval",
+            parseInt(retryAfter, 10)
+          );
+        }
+        break;
+    }
+
+    // In this other case we were passed a rejection value.
+    switch (resp.result) {
+      case Cr.NS_ERROR_UNKNOWN_HOST:
+      case Cr.NS_ERROR_CONNECTION_REFUSED:
+      case Cr.NS_ERROR_NET_TIMEOUT:
+      case Cr.NS_ERROR_NET_RESET:
+      case Cr.NS_ERROR_NET_INTERRUPT:
+      case Cr.NS_ERROR_PROXY_CONNECTION_REFUSED:
+        // The constant says it's about login, but in fact it just
+        // indicates general network error.
+        if (this.service.isLoggedIn) {
+          lazy.Status.sync = LOGIN_FAILED_NETWORK_ERROR;
+        } else {
+          lazy.Status.login = LOGIN_FAILED_NETWORK_ERROR;
+        }
+        break;
+    }
+  },
+};
diff --git a/services/sync/modules/record.sys.mjs b/services/sync/modules/record.sys.mjs
new file mode 100644
index 0000000000..7d5918a8ca
--- /dev/null
+++ b/services/sync/modules/record.sys.mjs
@@ -0,0 +1,1335 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const CRYPTO_COLLECTION = "crypto";
+const KEYS_WBO = "keys";
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import {
+  DEFAULT_DOWNLOAD_BATCH_SIZE,
+  DEFAULT_KEYBUNDLE_NAME,
+} from "resource://services-sync/constants.sys.mjs";
+import { BulkKeyBundle } from "resource://services-sync/keys.sys.mjs";
+import { Weave } from "resource://services-sync/main.sys.mjs";
+import { Resource } from "resource://services-sync/resource.sys.mjs";
+import { Utils } from "resource://services-sync/util.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+import { CryptoUtils } from "resource://services-crypto/utils.sys.mjs";
+
+/**
+ * The base class for all Sync basic storage objects (BSOs). This is the format
+ * used to store all records on the Sync server. In an earlier version of the
+ * Sync protocol, BSOs used to be called WBOs, or Weave Basic Objects. This
+ * class retains the old name.
+ *
+ * @class
+ * @param {String} collection The collection name for this BSO.
+ * @param {String} id         The ID of this BSO.
+ */
+export function WBORecord(collection, id) {
+  this.data = {};
+  this.payload = {};
+  this.collection = collection; // Optional.
+  this.id = id; // Optional.
+}
+
+WBORecord.prototype = {
+  _logName: "Sync.Record.WBO",
+
+  get sortindex() {
+    if (this.data.sortindex) {
+      return this.data.sortindex;
+    }
+    return 0;
+  },
+
+  // Get thyself from your URI, then deserialize.
+  // Set thine 'response' field.
+  async fetch(resource) {
+    if (!(resource instanceof Resource)) {
+      throw new Error("First argument must be a Resource instance.");
+    }
+
+    let r = await resource.get();
+    if (r.success) {
+      this.deserialize(r.obj); // Warning! Muffles exceptions!
+    }
+    this.response = r;
+    return this;
+  },
+
+  upload(resource) {
+    if (!(resource instanceof Resource)) {
+      throw new Error("First argument must be a Resource instance.");
+    }
+
+    return resource.put(this);
+  },
+
+  // Take a base URI string, with trailing slash, and return the URI of this
+  // WBO based on collection and ID.
+  uri(base) {
+    if (this.collection && this.id) {
+      let url = CommonUtils.makeURI(base + this.collection + "/" + this.id);
+      url.QueryInterface(Ci.nsIURL);
+      return url;
+    }
+    return null;
+  },
+
+  deserialize: function deserialize(json) {
+    if (!json || typeof json !== "object") {
+      throw new TypeError("Can't deserialize record from: " + json);
+    }
+    this.data = json;
+    try {
+      // The payload is likely to be JSON, but if not, keep it as a string
+      this.payload = JSON.parse(this.payload);
+    } catch (ex) {}
+  },
+
+  toJSON: function toJSON() {
+    // Copy fields from data to be stringified, making sure payload is a string
+    let obj = {};
+    for (let [key, val] of Object.entries(this.data)) {
+      obj[key] = key == "payload" ? JSON.stringify(val) : val;
+    }
+    if (this.ttl) {
+      obj.ttl = this.ttl;
+    }
+    return obj;
+  },
+
+  toString: function toString() {
+    return (
+      "{ " +
+      "id: " +
+      this.id +
+      "  " +
+      "index: " +
+      this.sortindex +
+      "  " +
+      "modified: " +
+      this.modified +
+      "  " +
+      "ttl: " +
+      this.ttl +
+      "  " +
+      "payload: " +
+      JSON.stringify(this.payload) +
+      " }"
+    );
+  },
+};
+
+Utils.deferGetSet(WBORecord, "data", [
+  "id",
+  "modified",
+  "sortindex",
+  "payload",
+]);
+
+/**
+ * An encrypted BSO record. This subclass handles encrypting and decrypting the
+ * BSO payload, but doesn't parse or interpret the cleartext string. Subclasses
+ * must override `transformBeforeEncrypt` and `transformAfterDecrypt` to process
+ * the cleartext.
+ *
+ * This class is only exposed for bridged engines, which handle serialization
+ * and deserialization in Rust. Sync engines implemented in JS should subclass
+ * `CryptoWrapper` instead, which takes care of transforming the cleartext into
+ * an object, and ensuring its contents are valid.
+ *
+ * @class
+ * @template Cleartext
+ * @param    {String} collection The collection name for this BSO.
+ * @param    {String} id         The ID of this BSO.
+ */
+export function RawCryptoWrapper(collection, id) {
+  // Setting properties before calling the superclass constructor isn't allowed
+  // in new-style classes (`class MyRecord extends RawCryptoWrapper`), but
+  // allowed with plain functions. This is also why `defaultCleartext` is a
+  // method, and not simply set in the subclass constructor.
+  this.cleartext = this.defaultCleartext();
+  WBORecord.call(this, collection, id);
+  this.ciphertext = null;
+}
+
+RawCryptoWrapper.prototype = {
+  _logName: "Sync.Record.RawCryptoWrapper",
+
+  /**
+   * Returns the default empty cleartext for this record type. This is exposed
+   * as a method so that subclasses can override it, and access the default
+   * cleartext in their constructors. `CryptoWrapper`, for example, overrides
+   * this to return an empty object, so that initializing the `id` in its
+   * constructor calls its overridden `id` setter.
+   *
+   * @returns {Cleartext} An empty cleartext.
+   */
+  defaultCleartext() {
+    return null;
+  },
+
+  /**
+   * Transforms the cleartext into a string that can be encrypted and wrapped
+   * in a BSO payload. This is called before uploading the record to the server.
+   *
+   * @param   {Cleartext} outgoingCleartext The cleartext to upload.
+   * @returns {String}                      The serialized cleartext.
+   */
+  transformBeforeEncrypt(outgoingCleartext) {
+    throw new TypeError("Override to stringify outgoing records");
+  },
+
+  /**
+   * Transforms an incoming cleartext string into an instance of the
+   * `Cleartext` type. This is called when fetching the record from the
+   * server.
+   *
+   * @param   {String} incomingCleartext The decrypted cleartext string.
+   * @returns {Cleartext}                The parsed cleartext.
+   */
+  transformAfterDecrypt(incomingCleartext) {
+    throw new TypeError("Override to parse incoming records");
+  },
+
+  ciphertextHMAC: async function ciphertextHMAC(keyBundle) {
+    let hmacKeyByteString = keyBundle.hmacKey;
+    if (!hmacKeyByteString) {
+      throw new Error("Cannot compute HMAC without an HMAC key.");
+    }
+    let hmacKey = CommonUtils.byteStringToArrayBuffer(hmacKeyByteString);
+    // NB: this.ciphertext is a base64-encoded string. For some reason this
+    // implementation computes the HMAC on the encoded value.
+    let data = CommonUtils.byteStringToArrayBuffer(this.ciphertext);
+    let hmac = await CryptoUtils.hmac("SHA-256", hmacKey, data);
+    return CommonUtils.bytesAsHex(CommonUtils.arrayBufferToByteString(hmac));
+  },
+
+  /*
+   * Don't directly use the sync key. Instead, grab a key for this
+   * collection, which is decrypted with the sync key.
+   *
+   * Cache those keys; invalidate the cache if the time on the keys collection
+   * changes, or other auth events occur.
+   *
+   * Optional key bundle overrides the collection key lookup.
+   */
+  async encrypt(keyBundle) {
+    if (!keyBundle) {
+      throw new Error("A key bundle must be supplied to encrypt.");
+    }
+
+    this.IV = Weave.Crypto.generateRandomIV();
+    this.ciphertext = await Weave.Crypto.encrypt(
+      this.transformBeforeEncrypt(this.cleartext),
+      keyBundle.encryptionKeyB64,
+      this.IV
+    );
+    this.hmac = await this.ciphertextHMAC(keyBundle);
+    this.cleartext = null;
+  },
+
+  // Optional key bundle.
+  async decrypt(keyBundle) {
+    if (!this.ciphertext) {
+      throw new Error("No ciphertext: nothing to decrypt?");
+    }
+
+    if (!keyBundle) {
+      throw new Error("A key bundle must be supplied to decrypt.");
+    }
+
+    // Authenticate the encrypted blob with the expected HMAC
+    let computedHMAC = await this.ciphertextHMAC(keyBundle);
+
+    if (computedHMAC != this.hmac) {
+      Utils.throwHMACMismatch(this.hmac, computedHMAC);
+    }
+
+    let cleartext = await Weave.Crypto.decrypt(
+      this.ciphertext,
+      keyBundle.encryptionKeyB64,
+      this.IV
+    );
+    this.cleartext = this.transformAfterDecrypt(cleartext);
+    this.ciphertext = null;
+
+    return this.cleartext;
+  },
+};
+
+Object.setPrototypeOf(RawCryptoWrapper.prototype, WBORecord.prototype);
+
+Utils.deferGetSet(RawCryptoWrapper, "payload", ["ciphertext", "IV", "hmac"]);
+
+/**
+ * An encrypted BSO record with a JSON payload. All engines implemented in JS
+ * should subclass this class to describe their own record types.
+ *
+ * @class
+ * @param {String} collection The collection name for this BSO.
+ * @param {String} id         The ID of this BSO.
+ */
+export function CryptoWrapper(collection, id) {
+  RawCryptoWrapper.call(this, collection, id);
+}
+
+CryptoWrapper.prototype = {
+  _logName: "Sync.Record.CryptoWrapper",
+
+  defaultCleartext() {
+    return {};
+  },
+
+  transformBeforeEncrypt(cleartext) {
+    return JSON.stringify(cleartext);
+  },
+
+  transformAfterDecrypt(cleartext) {
+    // Handle invalid data here. Elsewhere we assume that cleartext is an object.
+    let json_result = JSON.parse(cleartext);
+
+    if (!(json_result && json_result instanceof Object)) {
+      throw new Error(
+        `Decryption failed: result is <${json_result}>, not an object.`
+      );
+    }
+
+    // If the payload has an encrypted id ensure it matches the requested record's id.
+    if (json_result.id && json_result.id != this.id) {
+      throw new Error(`Record id mismatch: ${json_result.id} != ${this.id}`);
+    }
+
+    return json_result;
+  },
+
+  cleartextToString() {
+    return JSON.stringify(this.cleartext);
+  },
+
+  toString: function toString() {
+    let payload = this.deleted ? "DELETED" : this.cleartextToString();
+
+    return (
+      "{ " +
+      "id: " +
+      this.id +
+      "  " +
+      "index: " +
+      this.sortindex +
+      "  " +
+      "modified: " +
+      this.modified +
+      "  " +
+      "ttl: " +
+      this.ttl +
+      "  " +
+      "payload: " +
+      payload +
+      "  " +
+      "collection: " +
+      (this.collection || "undefined") +
+      " }"
+    );
+  },
+
+  // The custom setter below masks the parent's getter, so explicitly call it :(
+  get id() {
+    return super.id;
+  },
+
+  // Keep both plaintext and encrypted versions of the id to verify integrity
+  set id(val) {
+    super.id = val;
+    this.cleartext.id = val;
+  },
+};
+
+Object.setPrototypeOf(CryptoWrapper.prototype, RawCryptoWrapper.prototype);
+
+Utils.deferGetSet(CryptoWrapper, "cleartext", "deleted");
+
+/**
+ * An interface and caching layer for records.
+ */
+export function RecordManager(service) {
+  this.service = service;
+
+  this._log = Log.repository.getLogger(this._logName);
+  this._records = {};
+}
+
+RecordManager.prototype = {
+  _recordType: CryptoWrapper,
+  _logName: "Sync.RecordManager",
+
+  async import(url) {
+    this._log.trace("Importing record: " + (url.spec ? url.spec : url));
+    try {
+      // Clear out the last response with empty object if GET fails
+      this.response = {};
+      this.response = await this.service.resource(url).get();
+
+      // Don't parse and save the record on failure
+      if (!this.response.success) {
+        return null;
+      }
+
+      let record = new this._recordType(url);
+      record.deserialize(this.response.obj);
+
+      return this.set(url, record);
+    } catch (ex) {
+      if (Async.isShutdownException(ex)) {
+        throw ex;
+      }
+      this._log.debug("Failed to import record", ex);
+      return null;
+    }
+  },
+
+  get(url) {
+    // Use a url string as the key to the hash
+    let spec = url.spec ? url.spec : url;
+    if (spec in this._records) {
+      return Promise.resolve(this._records[spec]);
+    }
+    return this.import(url);
+  },
+
+  set: function RecordMgr_set(url, record) {
+    let spec = url.spec ? url.spec : url;
+    return (this._records[spec] = record);
+  },
+
+  contains: function RecordMgr_contains(url) {
+    if ((url.spec || url) in this._records) {
+      return true;
+    }
+    return false;
+  },
+
+  clearCache: function recordMgr_clearCache() {
+    this._records = {};
+  },
+
+  del: function RecordMgr_del(url) {
+    delete this._records[url];
+  },
+};
+
+/**
+ * Keeps track of mappings between collection names ('tabs') and KeyBundles.
+ *
+ * You can update this thing simply by giving it /info/collections. It'll
+ * use the last modified time to bring itself up to date.
+ */
+export function CollectionKeyManager(lastModified, default_, collections) {
+  this.lastModified = lastModified || 0;
+  this._default = default_ || null;
+  this._collections = collections || {};
+
+  this._log = Log.repository.getLogger("Sync.CollectionKeyManager");
+}
+
+// TODO: persist this locally as an Identity. Bug 610913.
+// Note that the last modified time needs to be preserved.
+CollectionKeyManager.prototype = {
+  /**
+   * Generate a new CollectionKeyManager that has the same attributes
+   * as this one.
+   */
+  clone() {
+    const newCollections = {};
+    for (let c in this._collections) {
+      newCollections[c] = this._collections[c];
+    }
+
+    return new CollectionKeyManager(
+      this.lastModified,
+      this._default,
+      newCollections
+    );
+  },
+
+  // Return information about old vs new keys:
+  // * same: true if two collections are equal
+  // * changed: an array of collection names that changed.
+  _compareKeyBundleCollections: function _compareKeyBundleCollections(m1, m2) {
+    let changed = [];
+
+    function process(m1, m2) {
+      for (let k1 in m1) {
+        let v1 = m1[k1];
+        let v2 = m2[k1];
+        if (!(v1 && v2 && v1.equals(v2))) {
+          changed.push(k1);
+        }
+      }
+    }
+
+    // Diffs both ways.
+    process(m1, m2);
+    process(m2, m1);
+
+    // Return a sorted, unique array.
+    changed.sort();
+    let last;
+    changed = changed.filter(x => x != last && (last = x));
+    return { same: !changed.length, changed };
+  },
+
+  get isClear() {
+    return !this._default;
+  },
+
+  clear: function clear() {
+    this._log.info("Clearing collection keys...");
+    this.lastModified = 0;
+    this._collections = {};
+    this._default = null;
+  },
+
+  keyForCollection(collection) {
+    if (collection && this._collections[collection]) {
+      return this._collections[collection];
+    }
+
+    return this._default;
+  },
+
+  /**
+   * If `collections` (an array of strings) is provided, iterate
+   * over it and generate random keys for each collection.
+   * Create a WBO for the given data.
+   */
+  _makeWBO(collections, defaultBundle) {
+    let wbo = new CryptoWrapper(CRYPTO_COLLECTION, KEYS_WBO);
+    let c = {};
+    for (let k in collections) {
+      c[k] = collections[k].keyPairB64;
+    }
+    wbo.cleartext = {
+      default: defaultBundle ? defaultBundle.keyPairB64 : null,
+      collections: c,
+      collection: CRYPTO_COLLECTION,
+      id: KEYS_WBO,
+    };
+    return wbo;
+  },
+
+  /**
+   * Create a WBO for the current keys.
+   */
+  asWBO(collection, id) {
+    return this._makeWBO(this._collections, this._default);
+  },
+
+  /**
+   * Compute a new default key, and new keys for any specified collections.
+   */
+  async newKeys(collections) {
+    let newDefaultKeyBundle = await this.newDefaultKeyBundle();
+
+    let newColls = {};
+    if (collections) {
+      for (let c of collections) {
+        let b = new BulkKeyBundle(c);
+        await b.generateRandom();
+        newColls[c] = b;
+      }
+    }
+    return [newDefaultKeyBundle, newColls];
+  },
+
+  /**
+   * Generates new keys, but does not replace our local copy. Use this to
+   * verify an upload before storing.
+   */
+  async generateNewKeysWBO(collections) {
+    let newDefaultKey, newColls;
+    [newDefaultKey, newColls] = await this.newKeys(collections);
+
+    return this._makeWBO(newColls, newDefaultKey);
+  },
+
+  /**
+   * Create a new default key.
+   *
+   * @returns {BulkKeyBundle}
+   */
+  async newDefaultKeyBundle() {
+    const key = new BulkKeyBundle(DEFAULT_KEYBUNDLE_NAME);
+    await key.generateRandom();
+    return key;
+  },
+
+  /**
+   * Create a new default key and store it as this._default, since without one you cannot use setContents.
+   */
+  async generateDefaultKey() {
+    this._default = await this.newDefaultKeyBundle();
+  },
+
+  /**
+   * Return true if keys are already present for each of the given
+   * collections.
+   */
+  hasKeysFor(collections) {
+    // We can't use filter() here because sometimes collections is an iterator.
+    for (let collection of collections) {
+      if (!this._collections[collection]) {
+        return false;
+      }
+    }
+    return true;
+  },
+
+  /**
+   * Return a new CollectionKeyManager that has keys for each of the
+   * given collections (creating new ones for collections where we
+   * don't already have keys).
+   */
+  async ensureKeysFor(collections) {
+    const newKeys = Object.assign({}, this._collections);
+    for (let c of collections) {
+      if (newKeys[c]) {
+        continue; // don't replace existing keys
+      }
+
+      const b = new BulkKeyBundle(c);
+      await b.generateRandom();
+      newKeys[c] = b;
+    }
+    return new CollectionKeyManager(this.lastModified, this._default, newKeys);
+  },
+
+  // Take the fetched info/collections WBO, checking the change
+  // time of the crypto collection.
+  updateNeeded(info_collections) {
+    this._log.info(
+      "Testing for updateNeeded. Last modified: " + this.lastModified
+    );
+
+    // No local record of modification time? Need an update.
+    if (!this.lastModified) {
+      return true;
+    }
+
+    // No keys on the server? We need an update, though our
+    // update handling will be a little more drastic...
+    if (!(CRYPTO_COLLECTION in info_collections)) {
+      return true;
+    }
+
+    // Otherwise, we need an update if our modification time is stale.
+    return info_collections[CRYPTO_COLLECTION] > this.lastModified;
+  },
+
+  //
+  // Set our keys and modified time to the values fetched from the server.
+  // Returns one of three values:
+  //
+  // * If the default key was modified, return true.
+  // * If the default key was not modified, but per-collection keys were,
+  //   return an array of such.
+  // * Otherwise, return false -- we were up-to-date.
+  //
+  setContents: function setContents(payload, modified) {
+    let self = this;
+
+    this._log.info(
+      "Setting collection keys contents. Our last modified: " +
+        this.lastModified +
+        ", input modified: " +
+        modified +
+        "."
+    );
+
+    if (!payload) {
+      throw new Error("No payload in CollectionKeyManager.setContents().");
+    }
+
+    if (!payload.default) {
+      this._log.warn("No downloaded default key: this should not occur.");
+      this._log.warn("Not clearing local keys.");
+      throw new Error(
+        "No default key in CollectionKeyManager.setContents(). Cannot proceed."
+      );
+    }
+
+    // Process the incoming default key.
+    let b = new BulkKeyBundle(DEFAULT_KEYBUNDLE_NAME);
+    b.keyPairB64 = payload.default;
+    let newDefault = b;
+
+    // Process the incoming collections.
+    let newCollections = {};
+    if ("collections" in payload) {
+      this._log.info("Processing downloaded per-collection keys.");
+      let colls = payload.collections;
+      for (let k in colls) {
+        let v = colls[k];
+        if (v) {
+          let keyObj = new BulkKeyBundle(k);
+          keyObj.keyPairB64 = v;
+          newCollections[k] = keyObj;
+        }
+      }
+    }
+
+    // Check to see if these are already our keys.
+    let sameDefault = this._default && this._default.equals(newDefault);
+    let collComparison = this._compareKeyBundleCollections(
+      newCollections,
+      this._collections
+    );
+    let sameColls = collComparison.same;
+
+    if (sameDefault && sameColls) {
+      self._log.info("New keys are the same as our old keys!");
+      if (modified) {
+        self._log.info("Bumped local modified time.");
+        self.lastModified = modified;
+      }
+      return false;
+    }
+
+    // Make sure things are nice and tidy before we set.
+    this.clear();
+
+    this._log.info("Saving downloaded keys.");
+    this._default = newDefault;
+    this._collections = newCollections;
+
+    // Always trust the server.
+    if (modified) {
+      self._log.info("Bumping last modified to " + modified);
+      self.lastModified = modified;
+    }
+
+    return sameDefault ? collComparison.changed : true;
+  },
+
+  async updateContents(syncKeyBundle, storage_keys) {
+    let log = this._log;
+    log.info("Updating collection keys...");
+
+    // storage_keys is a WBO, fetched from storage/crypto/keys.
+    // Its payload is the default key, and a map of collections to keys.
+    // We lazily compute the key objects from the strings we're given.
+
+    let payload;
+    try {
+      payload = await storage_keys.decrypt(syncKeyBundle);
+    } catch (ex) {
+      log.warn("Got exception decrypting storage keys with sync key.", ex);
+      log.info("Aborting updateContents. Rethrowing.");
+      throw ex;
+    }
+
+    let r = this.setContents(payload, storage_keys.modified);
+    log.info("Collection keys updated.");
+    return r;
+  },
+};
+
+export function Collection(uri, recordObj, service) {
+  if (!service) {
+    throw new Error("Collection constructor requires a service.");
+  }
+
+  Resource.call(this, uri);
+
+  // This is a bit hacky, but gets the job done.
+  let res = service.resource(uri);
+  this.authenticator = res.authenticator;
+
+  this._recordObj = recordObj;
+  this._service = service;
+
+  this._full = false;
+  this._ids = null;
+  this._limit = 0;
+  this._older = 0;
+  this._newer = 0;
+  this._data = [];
+  // optional members used by batch upload operations.
+  this._batch = null;
+  this._commit = false;
+  // Used for batch download operations -- note that this is explicitly an
+  // opaque value and not (necessarily) a number.
+  this._offset = null;
+}
+
+Collection.prototype = {
+  _logName: "Sync.Collection",
+
+  _rebuildURL: function Coll__rebuildURL() {
+    // XXX should consider what happens if it's not a URL...
+    this.uri.QueryInterface(Ci.nsIURL);
+
+    let args = [];
+    if (this.older) {
+      args.push("older=" + this.older);
+    }
+    if (this.newer) {
+      args.push("newer=" + this.newer);
+    }
+    if (this.full) {
+      args.push("full=1");
+    }
+    if (this.sort) {
+      args.push("sort=" + this.sort);
+    }
+    if (this.ids != null) {
+      args.push("ids=" + this.ids);
+    }
+    if (this.limit > 0 && this.limit != Infinity) {
+      args.push("limit=" + this.limit);
+    }
+    if (this._batch) {
+      args.push("batch=" + encodeURIComponent(this._batch));
+    }
+    if (this._commit) {
+      args.push("commit=true");
+    }
+    if (this._offset) {
+      args.push("offset=" + encodeURIComponent(this._offset));
+    }
+
+    this.uri = this.uri
+      .mutate()
+      .setQuery(args.length ? "?" + args.join("&") : "")
+      .finalize();
+  },
+
+  // get full items
+  get full() {
+    return this._full;
+  },
+  set full(value) {
+    this._full = value;
+    this._rebuildURL();
+  },
+
+  // Apply the action to a certain set of ids
+  get ids() {
+    return this._ids;
+  },
+  set ids(value) {
+    this._ids = value;
+    this._rebuildURL();
+  },
+
+  // Limit how many records to get
+  get limit() {
+    return this._limit;
+  },
+  set limit(value) {
+    this._limit = value;
+    this._rebuildURL();
+  },
+
+  // get only items modified before some date
+  get older() {
+    return this._older;
+  },
+  set older(value) {
+    this._older = value;
+    this._rebuildURL();
+  },
+
+  // get only items modified since some date
+  get newer() {
+    return this._newer;
+  },
+  set newer(value) {
+    this._newer = value;
+    this._rebuildURL();
+  },
+
+  // get items sorted by some criteria. valid values:
+  // oldest (oldest first)
+  // newest (newest first)
+  // index
+  get sort() {
+    return this._sort;
+  },
+  set sort(value) {
+    if (value && value != "oldest" && value != "newest" && value != "index") {
+      throw new TypeError(
+        `Illegal value for sort: "${value}" (should be "oldest", "newest", or "index").`
+      );
+    }
+    this._sort = value;
+    this._rebuildURL();
+  },
+
+  get offset() {
+    return this._offset;
+  },
+  set offset(value) {
+    this._offset = value;
+    this._rebuildURL();
+  },
+
+  // Set information about the batch for this request.
+  get batch() {
+    return this._batch;
+  },
+  set batch(value) {
+    this._batch = value;
+    this._rebuildURL();
+  },
+
+  get commit() {
+    return this._commit;
+  },
+  set commit(value) {
+    this._commit = value && true;
+    this._rebuildURL();
+  },
+
+  // Similar to get(), but will page through the items `batchSize` at a time,
+  // deferring calling the record handler until we've gotten them all.
+  //
+  // Returns the last response processed, and doesn't run the record handler
+  // on any items if a non-success status is received while downloading the
+  // records (or if a network error occurs).
+  async getBatched(batchSize = DEFAULT_DOWNLOAD_BATCH_SIZE) {
+    let totalLimit = Number(this.limit) || Infinity;
+    if (batchSize <= 0 || batchSize >= totalLimit) {
+      throw new Error("Invalid batch size");
+    }
+
+    if (!this.full) {
+      throw new Error("getBatched is unimplemented for guid-only GETs");
+    }
+
+    // _onComplete and _onProgress are reset after each `get` by Resource.
+    let { _onComplete, _onProgress } = this;
+    let recordBuffer = [];
+    let resp;
+    try {
+      let lastModifiedTime;
+      this.limit = batchSize;
+
+      do {
+        this._onProgress = _onProgress;
+        this._onComplete = _onComplete;
+        if (batchSize + recordBuffer.length > totalLimit) {
+          this.limit = totalLimit - recordBuffer.length;
+        }
+        this._log.trace("Performing batched GET", {
+          limit: this.limit,
+          offset: this.offset,
+        });
+        // Actually perform the request
+        resp = await this.get();
+        if (!resp.success) {
+          recordBuffer = [];
+          break;
+        }
+        for (let json of resp.obj) {
+          let record = new this._recordObj();
+          record.deserialize(json);
+          recordBuffer.push(record);
+        }
+
+        // Initialize last modified, or check that something broken isn't happening.
+        let lastModified = resp.headers["x-last-modified"];
+        if (!lastModifiedTime) {
+          lastModifiedTime = lastModified;
+          this.setHeader("X-If-Unmodified-Since", lastModified);
+        } else if (lastModified != lastModifiedTime) {
+          // Should be impossible -- We'd get a 412 in this case.
+          throw new Error(
+            "X-Last-Modified changed in the middle of a download batch! " +
+              `${lastModified} => ${lastModifiedTime}`
+          );
+        }
+
+        // If this is missing, we're finished.
+        this.offset = resp.headers["x-weave-next-offset"];
+      } while (this.offset && totalLimit > recordBuffer.length);
+    } finally {
+      // Ensure we undo any temporary state so that subsequent calls to get()
+      // or getBatched() work properly. We do this before calling the record
+      // handler so that we can more convincingly pretend to be a normal get()
+      // call. Note: we're resetting these to the values they had before this
+      // function was called.
+      this._limit = totalLimit;
+      this._offset = null;
+      delete this._headers["x-if-unmodified-since"];
+      this._rebuildURL();
+    }
+    return { response: resp, records: recordBuffer };
+  },
+
+  // This object only supports posting via the postQueue object.
+  post() {
+    throw new Error(
+      "Don't directly post to a collection - use newPostQueue instead"
+    );
+  },
+
+  newPostQueue(log, timestamp, postCallback) {
+    let poster = (data, headers, batch, commit) => {
+      this.batch = batch;
+      this.commit = commit;
+      for (let [header, value] of headers) {
+        this.setHeader(header, value);
+      }
+      return Resource.prototype.post.call(this, data);
+    };
+    return new PostQueue(
+      poster,
+      timestamp,
+      this._service.serverConfiguration || {},
+      log,
+      postCallback
+    );
+  },
+};
+
+Object.setPrototypeOf(Collection.prototype, Resource.prototype);
+
+// These are limits for requests provided by the server at the
+// info/configuration endpoint -- server documentation is available here:
+// http://moz-services-docs.readthedocs.io/en/latest/storage/apis-1.5.html#api-instructions
+//
+// All are optional, however we synthesize (non-infinite) default values for the
+// "max_request_bytes" and "max_record_payload_bytes" options. For the others,
+// we ignore them (we treat the limit is infinite) if they're missing.
+//
+// These are also the only ones that all servers (even batching-disabled
+// servers) should support, at least once this sync-serverstorage patch is
+// everywhere https://github.com/mozilla-services/server-syncstorage/pull/74
+//
+// Batching enabled servers also limit the amount of payload data and number
+// of and records we can send in a single post as well as in the whole batch.
+// Note that the byte limits for these there are just with respect to the
+// *payload* data, e.g. the data appearing in the payload property (a
+// string) of the object.
+//
+// Note that in practice, these limits should be sensible, but the code makes
+// no assumptions about this. If we hit any of the limits, we perform the
+// corresponding action (e.g. submit a request, possibly committing the
+// current batch).
+const DefaultPostQueueConfig = Object.freeze({
+  // Number of total bytes allowed in a request
+  max_request_bytes: 260 * 1024,
+
+  // Maximum number of bytes allowed in the "payload" property of a record.
+  max_record_payload_bytes: 256 * 1024,
+
+  // The limit for how many bytes worth of data appearing in "payload"
+  // properties are allowed in a single post.
+  max_post_bytes: Infinity,
+
+  // The limit for the number of records allowed in a single post.
+  max_post_records: Infinity,
+
+  // The limit for how many bytes worth of data appearing in "payload"
+  // properties are allowed in a batch. (Same as max_post_bytes, but for
+  // batches).
+  max_total_bytes: Infinity,
+
+  // The limit for the number of records allowed in a single post. (Same
+  // as max_post_records, but for batches).
+  max_total_records: Infinity,
+});
+
+// Manages a pair of (byte, count) limits for a PostQueue, such as
+// (max_post_bytes, max_post_records) or (max_total_bytes, max_total_records).
+class LimitTracker {
+  constructor(maxBytes, maxRecords) {
+    this.maxBytes = maxBytes;
+    this.maxRecords = maxRecords;
+    this.curBytes = 0;
+    this.curRecords = 0;
+  }
+
+  clear() {
+    this.curBytes = 0;
+    this.curRecords = 0;
+  }
+
+  canAddRecord(payloadSize) {
+    // The record counts are inclusive, but depending on the version of the
+    // server, the byte counts may or may not be inclusive (See
+    // https://github.com/mozilla-services/server-syncstorage/issues/73).
+    return (
+      this.curRecords + 1 <= this.maxRecords &&
+      this.curBytes + payloadSize < this.maxBytes
+    );
+  }
+
+  canNeverAdd(recordSize) {
+    return recordSize >= this.maxBytes;
+  }
+
+  didAddRecord(recordSize) {
+    if (!this.canAddRecord(recordSize)) {
+      // This is a bug, caller is expected to call canAddRecord first.
+      throw new Error(
+        "LimitTracker.canAddRecord must be checked before adding record"
+      );
+    }
+    this.curRecords += 1;
+    this.curBytes += recordSize;
+  }
+}
+
+/* A helper to manage the posting of records while respecting the various
+   size limits.
+
+   This supports the concept of a server-side "batch". The general idea is:
+   * We queue as many records as allowed in memory, then make a single POST.
+   * This first POST (optionally) gives us a batch ID, which we use for
+     all subsequent posts, until...
+   * At some point we hit a batch-maximum, and jump through a few hoops to
+     commit the current batch (ie, all previous POSTs) and start a new one.
+   * Eventually commit the final batch.
+
+  In most cases we expect there to be exactly 1 batch consisting of possibly
+  multiple POSTs.
+*/
+export function PostQueue(poster, timestamp, serverConfig, log, postCallback) {
+  // The "post" function we should use when it comes time to do the post.
+  this.poster = poster;
+  this.log = log;
+
+  let config = Object.assign({}, DefaultPostQueueConfig, serverConfig);
+
+  if (!serverConfig.max_request_bytes && serverConfig.max_post_bytes) {
+    // Use max_post_bytes for max_request_bytes if it's missing. Only needed
+    // until server-syncstorage/pull/74 is everywhere, and even then it's
+    // unnecessary if the server limits are configured sanely (there's no
+    // guarantee of -- at least before that is fully deployed)
+    config.max_request_bytes = serverConfig.max_post_bytes;
+  }
+
+  this.log.trace("new PostQueue config (after defaults): ", config);
+
+  // The callback we make with the response when we do get around to making the
+  // post (which could be during any of the enqueue() calls or the final flush())
+  // This callback may be called multiple times and must not add new items to
+  // the queue.
+  // The second argument passed to this callback is a boolean value that is true
+  // if we're in the middle of a batch, and false if either the batch is
+  // complete, or it's a post to a server that does not understand batching.
+  this.postCallback = postCallback;
+
+  // Tracks the count and combined payload size for the records we've queued
+  // so far but are yet to POST.
+  this.postLimits = new LimitTracker(
+    config.max_post_bytes,
+    config.max_post_records
+  );
+
+  // As above, but for the batch size.
+  this.batchLimits = new LimitTracker(
+    config.max_total_bytes,
+    config.max_total_records
+  );
+
+  // Limit for the size of `this.queued` before we do a post.
+  this.maxRequestBytes = config.max_request_bytes;
+
+  // Limit for the size of incoming record payloads.
+  this.maxPayloadBytes = config.max_record_payload_bytes;
+
+  // The string where we are capturing the stringified version of the records
+  // queued so far. It will always be invalid JSON as it is always missing the
+  // closing bracket. It's also used to track whether or not we've gone past
+  // maxRequestBytes.
+  this.queued = "";
+
+  // The ID of our current batch. Can be undefined (meaning we are yet to make
+  // the first post of a patch, so don't know if we have a batch), null (meaning
+  // we've made the first post but the server response indicated no batching
+  // semantics), otherwise we have made the first post and it holds the batch ID
+  // returned from the server.
+  this.batchID = undefined;
+
+  // Time used for X-If-Unmodified-Since -- should be the timestamp from the last GET.
+  this.lastModified = timestamp;
+}
+
+PostQueue.prototype = {
+  async enqueue(record) {
+    // We want to ensure the record has a .toJSON() method defined - even
+    // though JSON.stringify() would implicitly call it, the stringify might
+    // still work even if it isn't defined, which isn't what we want.
+    let jsonRepr = record.toJSON();
+    if (!jsonRepr) {
+      throw new Error(
+        "You must only call this with objects that explicitly support JSON"
+      );
+    }
+
+    let bytes = JSON.stringify(jsonRepr);
+
+    // We use the payload size for the LimitTrackers, since that's what the
+    // byte limits other than max_request_bytes refer to.
+    let payloadLength = jsonRepr.payload.length;
+
+    // The `+ 2` is to account for the 2-byte (maximum) overhead (one byte for
+    // the leading comma or "[", which all records will have, and the other for
+    // the final trailing "]", only present for the last record).
+    let encodedLength = bytes.length + 2;
+
+    // Check first if there's some limit that indicates we cannot ever enqueue
+    // this record.
+    let isTooBig =
+      this.postLimits.canNeverAdd(payloadLength) ||
+      this.batchLimits.canNeverAdd(payloadLength) ||
+      encodedLength >= this.maxRequestBytes ||
+      payloadLength >= this.maxPayloadBytes;
+
+    if (isTooBig) {
+      return {
+        enqueued: false,
+        error: new Error("Single record too large to submit to server"),
+      };
+    }
+
+    let canPostRecord = this.postLimits.canAddRecord(payloadLength);
+    let canBatchRecord = this.batchLimits.canAddRecord(payloadLength);
+    let canSendRecord =
+      this.queued.length + encodedLength < this.maxRequestBytes;
+
+    if (!canPostRecord || !canBatchRecord || !canSendRecord) {
+      this.log.trace("PostQueue flushing: ", {
+        canPostRecord,
+        canSendRecord,
+        canBatchRecord,
+      });
+      // We need to write the queue out before handling this one, but we only
+      // commit the batch (and thus start a new one) if the record couldn't fit
+      // inside the batch.
+      await this.flush(!canBatchRecord);
+    }
+
+    this.postLimits.didAddRecord(payloadLength);
+    this.batchLimits.didAddRecord(payloadLength);
+
+    // Either a ',' or a '[' depending on whether this is the first record.
+    this.queued += this.queued.length ? "," : "[";
+    this.queued += bytes;
+    return { enqueued: true };
+  },
+
+  async flush(finalBatchPost) {
+    if (!this.queued) {
+      // nothing queued - we can't be in a batch, and something has gone very
+      // bad if we think we are.
+      if (this.batchID) {
+        throw new Error(
+          `Flush called when no queued records but we are in a batch ${this.batchID}`
+        );
+      }
+      return;
+    }
+    // the batch query-param and headers we'll send.
+    let batch;
+    let headers = [];
+    if (this.batchID === undefined) {
+      // First commit in a (possible) batch.
+      batch = "true";
+    } else if (this.batchID) {
+      // We have an existing batch.
+      batch = this.batchID;
+    } else {
+      // Not the first post and we know we have no batch semantics.
+      batch = null;
+    }
+
+    headers.push(["x-if-unmodified-since", this.lastModified]);
+
+    let numQueued = this.postLimits.curRecords;
+    this.log.info(
+      `Posting ${numQueued} records of ${
+        this.queued.length + 1
+      } bytes with batch=${batch}`
+    );
+    let queued = this.queued + "]";
+    if (finalBatchPost) {
+      this.batchLimits.clear();
+    }
+    this.postLimits.clear();
+    this.queued = "";
+    let response = await this.poster(
+      queued,
+      headers,
+      batch,
+      !!(finalBatchPost && this.batchID !== null)
+    );
+
+    if (!response.success) {
+      this.log.trace("Server error response during a batch", response);
+      // not clear what we should do here - we expect the consumer of this to
+      // abort by throwing in the postCallback below.
+      await this.postCallback(this, response, !finalBatchPost);
+      return;
+    }
+
+    if (finalBatchPost) {
+      this.log.trace("Committed batch", this.batchID);
+      this.batchID = undefined; // we are now in "first post for the batch" state.
+      this.lastModified = response.headers["x-last-modified"];
+      await this.postCallback(this, response, false);
+      return;
+    }
+
+    if (response.status != 202) {
+      if (this.batchID) {
+        throw new Error(
+          "Server responded non-202 success code while a batch was in progress"
+        );
+      }
+      this.batchID = null; // no batch semantics are in place.
+      this.lastModified = response.headers["x-last-modified"];
+      await this.postCallback(this, response, false);
+      return;
+    }
+
+    // this response is saying the server has batch semantics - we should
+    // always have a batch ID in the response.
+    let responseBatchID = response.obj.batch;
+    this.log.trace("Server responsed 202 with batch", responseBatchID);
+    if (!responseBatchID) {
+      this.log.error(
+        "Invalid server response: 202 without a batch ID",
+        response
+      );
+      throw new Error("Invalid server response: 202 without a batch ID");
+    }
+
+    if (this.batchID === undefined) {
+      this.batchID = responseBatchID;
+      if (!this.lastModified) {
+        this.lastModified = response.headers["x-last-modified"];
+        if (!this.lastModified) {
+          throw new Error("Batch response without x-last-modified");
+        }
+      }
+    }
+
+    if (this.batchID != responseBatchID) {
+      throw new Error(
+        `Invalid client/server batch state - client has ${this.batchID}, server has ${responseBatchID}`
+      );
+    }
+
+    await this.postCallback(this, response, true);
+  },
+};
diff --git a/services/sync/modules/resource.sys.mjs b/services/sync/modules/resource.sys.mjs
new file mode 100644
index 0000000000..537ffd5219
--- /dev/null
+++ b/services/sync/modules/resource.sys.mjs
@@ -0,0 +1,292 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Observers } from "resource://services-common/observers.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+import { Utils } from "resource://services-sync/util.sys.mjs";
+import { setTimeout, clearTimeout } from "resource://gre/modules/Timer.sys.mjs";
+
+/* global AbortController */
+
+/*
+ * Resource represents a remote network resource, identified by a URI.
+ * Create an instance like so:
+ *
+ *   let resource = new Resource("http://foobar.com/path/to/resource");
+ *
+ * The 'resource' object has the following methods to issue HTTP requests
+ * of the corresponding HTTP methods:
+ *
+ *   get(callback)
+ *   put(data, callback)
+ *   post(data, callback)
+ *   delete(callback)
+ */
+export function Resource(uri) {
+  this._log = Log.repository.getLogger(this._logName);
+  this._log.manageLevelFromPref("services.sync.log.logger.network.resources");
+  this.uri = uri;
+  this._headers = {};
+}
+
+// (static) Caches the latest server timestamp (X-Weave-Timestamp header).
+Resource.serverTime = null;
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  Resource,
+  "SEND_VERSION_INFO",
+  "services.sync.sendVersionInfo",
+  true
+);
+Resource.prototype = {
+  _logName: "Sync.Resource",
+
+  /**
+   * Callback to be invoked at request time to add authentication details.
+   * If the callback returns a promise, it will be awaited upon.
+   *
+   * By default, a global authenticator is provided. If this is set, it will
+   * be used instead of the global one.
+   */
+  authenticator: null,
+
+  // Wait 5 minutes before killing a request.
+  ABORT_TIMEOUT: 300000,
+
+  // Headers to be included when making a request for the resource.
+  // Note: Header names should be all lower case, there's no explicit
+  // check for duplicates due to case!
+  get headers() {
+    return this._headers;
+  },
+  set headers(_) {
+    throw new Error("headers can't be mutated directly. Please use setHeader.");
+  },
+  setHeader(header, value) {
+    this._headers[header.toLowerCase()] = value;
+  },
+
+  // URI representing this resource.
+  get uri() {
+    return this._uri;
+  },
+  set uri(value) {
+    if (typeof value == "string") {
+      this._uri = CommonUtils.makeURI(value);
+    } else {
+      this._uri = value;
+    }
+  },
+
+  // Get the string representation of the URI.
+  get spec() {
+    if (this._uri) {
+      return this._uri.spec;
+    }
+    return null;
+  },
+
+  /**
+   * @param {string} method HTTP method
+   * @returns {Headers}
+   */
+  async _buildHeaders(method) {
+    const headers = new Headers(this._headers);
+
+    if (Resource.SEND_VERSION_INFO) {
+      headers.append("user-agent", Utils.userAgent);
+    }
+
+    if (this.authenticator) {
+      const result = await this.authenticator(this, method);
+      if (result && result.headers) {
+        for (const [k, v] of Object.entries(result.headers)) {
+          headers.append(k.toLowerCase(), v);
+        }
+      }
+    } else {
+      this._log.debug("No authenticator found.");
+    }
+
+    // PUT and POST are treated differently because they have payload data.
+    if (("PUT" == method || "POST" == method) && !headers.has("content-type")) {
+      headers.append("content-type", "text/plain");
+    }
+
+    if (this._log.level <= Log.Level.Trace) {
+      for (const [k, v] of headers) {
+        if (k == "authorization" || k == "x-client-state") {
+          this._log.trace(`HTTP Header ${k}: ***** (suppressed)`);
+        } else {
+          this._log.trace(`HTTP Header ${k}: ${v}`);
+        }
+      }
+    }
+
+    if (!headers.has("accept")) {
+      headers.append("accept", "application/json;q=0.9,*/*;q=0.2");
+    }
+
+    return headers;
+  },
+
+  /**
+   * @param {string} method HTTP method
+   * @param {string} data HTTP body
+   * @param {object} signal AbortSignal instance
+   * @returns {Request}
+   */
+  async _createRequest(method, data, signal) {
+    const headers = await this._buildHeaders(method);
+    const init = {
+      cache: "no-store", // No cache.
+      headers,
+      method,
+      signal,
+      mozErrors: true, // Return nsresult error codes instead of a generic
+      // NetworkError when fetch rejects.
+    };
+
+    if (data) {
+      if (!(typeof data == "string" || data instanceof String)) {
+        data = JSON.stringify(data);
+      }
+      this._log.debug(`${method} Length: ${data.length}`);
+      this._log.trace(`${method} Body: ${data}`);
+      init.body = data;
+    }
+    return new Request(this.uri.spec, init);
+  },
+
+  /**
+   * @param {string} method HTTP method
+   * @param {string} [data] HTTP body
+   * @returns {Response}
+   */
+  async _doRequest(method, data = null) {
+    const controller = new AbortController();
+    const request = await this._createRequest(method, data, controller.signal);
+    const responsePromise = fetch(request); // Rejects on network failure.
+    let didTimeout = false;
+    const timeoutId = setTimeout(() => {
+      didTimeout = true;
+      this._log.error(
+        `Request timed out after ${this.ABORT_TIMEOUT}ms. Aborting.`
+      );
+      controller.abort();
+    }, this.ABORT_TIMEOUT);
+    let response;
+    try {
+      response = await responsePromise;
+    } catch (e) {
+      this._log.warn(`${method} request to ${this.uri.spec} failed`, e);
+      if (!didTimeout) {
+        throw e;
+      }
+      throw Components.Exception(
+        "Request aborted (timeout)",
+        Cr.NS_ERROR_NET_TIMEOUT
+      );
+    } finally {
+      clearTimeout(timeoutId);
+    }
+    return this._processResponse(response, method);
+  },
+
+  async _processResponse(response, method) {
+    const data = await response.text();
+    this._logResponse(response, method, data);
+    this._processResponseHeaders(response);
+
+    const ret = {
+      data,
+      url: response.url,
+      status: response.status,
+      success: response.ok,
+      headers: {},
+    };
+    for (const [k, v] of response.headers) {
+      ret.headers[k] = v;
+    }
+
+    // Make a lazy getter to convert the json response into an object.
+    // Note that this can cause a parse error to be thrown far away from the
+    // actual fetch, so be warned!
+    ChromeUtils.defineLazyGetter(ret, "obj", () => {
+      try {
+        return JSON.parse(ret.data);
+      } catch (ex) {
+        this._log.warn("Got exception parsing response body", ex);
+        // Stringify to avoid possibly printing non-printable characters.
+        this._log.debug(
+          "Parse fail: Response body starts",
+          (ret.data + "").slice(0, 100)
+        );
+        throw ex;
+      }
+    });
+
+    return ret;
+  },
+
+  _logResponse(response, method, data) {
+    const { status, ok: success, url } = response;
+
+    // Log the status of the request.
+    this._log.debug(
+      `${method} ${success ? "success" : "fail"} ${status} ${url}`
+    );
+
+    // Additionally give the full response body when Trace logging.
+    if (this._log.level <= Log.Level.Trace) {
+      this._log.trace(`${method} body`, data);
+    }
+
+    if (!success) {
+      this._log.warn(
+        `${method} request to ${url} failed with status ${status}`
+      );
+    }
+  },
+
+  _processResponseHeaders({ headers, ok: success }) {
+    if (headers.has("x-weave-timestamp")) {
+      Resource.serverTime = parseFloat(headers.get("x-weave-timestamp"));
+    }
+    // This is a server-side safety valve to allow slowing down
+    // clients without hurting performance.
+    if (headers.has("x-weave-backoff")) {
+      let backoff = headers.get("x-weave-backoff");
+      this._log.debug(`Got X-Weave-Backoff: ${backoff}`);
+      Observers.notify("weave:service:backoff:interval", parseInt(backoff, 10));
+    }
+
+    if (success && headers.has("x-weave-quota-remaining")) {
+      Observers.notify(
+        "weave:service:quota:remaining",
+        parseInt(headers.get("x-weave-quota-remaining"), 10)
+      );
+    }
+  },
+
+  get() {
+    return this._doRequest("GET");
+  },
+
+  put(data) {
+    return this._doRequest("PUT", data);
+  },
+
+  post(data) {
+    return this._doRequest("POST", data);
+  },
+
+  delete() {
+    return this._doRequest("DELETE");
+  },
+};
diff --git a/services/sync/modules/service.sys.mjs b/services/sync/modules/service.sys.mjs
new file mode 100644
index 0000000000..97ba0d32cd
--- /dev/null
+++ b/services/sync/modules/service.sys.mjs
@@ -0,0 +1,1643 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const CRYPTO_COLLECTION = "crypto";
+const KEYS_WBO = "keys";
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+
+import {
+  CLIENT_NOT_CONFIGURED,
+  CREDENTIALS_CHANGED,
+  HMAC_EVENT_INTERVAL,
+  LOGIN_FAILED,
+  LOGIN_FAILED_INVALID_PASSPHRASE,
+  LOGIN_FAILED_NETWORK_ERROR,
+  LOGIN_FAILED_NO_PASSPHRASE,
+  LOGIN_FAILED_NO_USERNAME,
+  LOGIN_FAILED_SERVER_ERROR,
+  LOGIN_SUCCEEDED,
+  MASTER_PASSWORD_LOCKED,
+  METARECORD_DOWNLOAD_FAIL,
+  NO_SYNC_NODE_FOUND,
+  PREFS_BRANCH,
+  STATUS_DISABLED,
+  STATUS_OK,
+  STORAGE_VERSION,
+  VERSION_OUT_OF_DATE,
+  WEAVE_VERSION,
+  kFirefoxShuttingDown,
+  kFirstSyncChoiceNotMade,
+  kSyncBackoffNotMet,
+  kSyncMasterPasswordLocked,
+  kSyncNetworkOffline,
+  kSyncNotConfigured,
+  kSyncWeaveDisabled,
+} from "resource://services-sync/constants.sys.mjs";
+
+import { EngineManager } from "resource://services-sync/engines.sys.mjs";
+import { ClientEngine } from "resource://services-sync/engines/clients.sys.mjs";
+import { Weave } from "resource://services-sync/main.sys.mjs";
+import {
+  ErrorHandler,
+  SyncScheduler,
+} from "resource://services-sync/policies.sys.mjs";
+import {
+  CollectionKeyManager,
+  CryptoWrapper,
+  RecordManager,
+  WBORecord,
+} from "resource://services-sync/record.sys.mjs";
+import { Resource } from "resource://services-sync/resource.sys.mjs";
+import { EngineSynchronizer } from "resource://services-sync/stages/enginesync.sys.mjs";
+import { DeclinedEngines } from "resource://services-sync/stages/declined.sys.mjs";
+import { Status } from "resource://services-sync/status.sys.mjs";
+
+ChromeUtils.importESModule("resource://services-sync/telemetry.sys.mjs");
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import { getFxAccountsSingleton } from "resource://gre/modules/FxAccounts.sys.mjs";
+
+const fxAccounts = getFxAccountsSingleton();
+
+function getEngineModules() {
+  let result = {
+    Addons: { module: "addons.sys.mjs", symbol: "AddonsEngine" },
+    Password: { module: "passwords.sys.mjs", symbol: "PasswordEngine" },
+    Prefs: { module: "prefs.sys.mjs", symbol: "PrefsEngine" },
+  };
+  if (AppConstants.MOZ_APP_NAME != "thunderbird") {
+    result.Bookmarks = {
+      module: "bookmarks.sys.mjs",
+      symbol: "BookmarksEngine",
+    };
+    result.Form = { module: "forms.sys.mjs", symbol: "FormEngine" };
+    result.History = { module: "history.sys.mjs", symbol: "HistoryEngine" };
+    result.Tab = { module: "tabs.sys.mjs", symbol: "TabEngine" };
+  }
+  if (Svc.PrefBranch.getBoolPref("engine.addresses.available", false)) {
+    result.Addresses = {
+      module: "resource://autofill/FormAutofillSync.sys.mjs",
+      symbol: "AddressesEngine",
+    };
+  }
+  if (Svc.PrefBranch.getBoolPref("engine.creditcards.available", false)) {
+    result.CreditCards = {
+      module: "resource://autofill/FormAutofillSync.sys.mjs",
+      symbol: "CreditCardsEngine",
+    };
+  }
+  result["Extension-Storage"] = {
+    module: "extension-storage.sys.mjs",
+    controllingPref: "webextensions.storage.sync.kinto",
+    whenTrue: "ExtensionStorageEngineKinto",
+    whenFalse: "ExtensionStorageEngineBridge",
+  };
+  return result;
+}
+
+const lazy = {};
+
+// A unique identifier for this browser session. Used for logging so
+// we can easily see whether 2 logs are in the same browser session or
+// after the browser restarted.
+ChromeUtils.defineLazyGetter(lazy, "browserSessionID", Utils.makeGUID);
+
+function Sync11Service() {
+  this._notify = Utils.notify("weave:service:");
+  Utils.defineLazyIDProperty(this, "syncID", "services.sync.client.syncID");
+}
+Sync11Service.prototype = {
+  _lock: Utils.lock,
+  _locked: false,
+  _loggedIn: false,
+
+  infoURL: null,
+  storageURL: null,
+  metaURL: null,
+  cryptoKeyURL: null,
+  // The cluster URL comes via the identity object, which in the FxA
+  // world is ebbedded in the token returned from the token server.
+  _clusterURL: null,
+
+  get clusterURL() {
+    return this._clusterURL || "";
+  },
+  set clusterURL(value) {
+    if (value != null && typeof value != "string") {
+      throw new Error("cluster must be a string, got " + typeof value);
+    }
+    this._clusterURL = value;
+    this._updateCachedURLs();
+  },
+
+  get isLoggedIn() {
+    return this._loggedIn;
+  },
+
+  get locked() {
+    return this._locked;
+  },
+  lock: function lock() {
+    if (this._locked) {
+      return false;
+    }
+    this._locked = true;
+    return true;
+  },
+  unlock: function unlock() {
+    this._locked = false;
+  },
+
+  // A specialized variant of Utils.catch.
+  // This provides a more informative error message when we're already syncing:
+  // see Bug 616568.
+  _catch(func) {
+    function lockExceptions(ex) {
+      if (Utils.isLockException(ex)) {
+        // This only happens if we're syncing already.
+        this._log.info("Cannot start sync: already syncing?");
+      }
+    }
+
+    return Utils.catch.call(this, func, lockExceptions);
+  },
+
+  get userBaseURL() {
+    // The user URL is the cluster URL.
+    return this.clusterURL;
+  },
+
+  _updateCachedURLs: function _updateCachedURLs() {
+    // Nothing to cache yet if we don't have the building blocks
+    if (!this.clusterURL) {
+      // Also reset all other URLs used by Sync to ensure we aren't accidentally
+      // using one cached earlier - if there's no cluster URL any cached ones
+      // are invalid.
+      this.infoURL = undefined;
+      this.storageURL = undefined;
+      this.metaURL = undefined;
+      this.cryptoKeysURL = undefined;
+      return;
+    }
+
+    this._log.debug(
+      "Caching URLs under storage user base: " + this.userBaseURL
+    );
+
+    // Generate and cache various URLs under the storage API for this user
+    this.infoURL = this.userBaseURL + "info/collections";
+    this.storageURL = this.userBaseURL + "storage/";
+    this.metaURL = this.storageURL + "meta/global";
+    this.cryptoKeysURL = this.storageURL + CRYPTO_COLLECTION + "/" + KEYS_WBO;
+  },
+
+  _checkCrypto: function _checkCrypto() {
+    let ok = false;
+
+    try {
+      let iv = Weave.Crypto.generateRandomIV();
+      if (iv.length == 24) {
+        ok = true;
+      }
+    } catch (e) {
+      this._log.debug("Crypto check failed: " + e);
+    }
+
+    return ok;
+  },
+
+  /**
+   * Here is a disgusting yet reasonable way of handling HMAC errors deep in
+   * the guts of Sync. The astute reader will note that this is a hacky way of
+   * implementing something like continuable conditions.
+   *
+   * A handler function is glued to each engine. If the engine discovers an
+   * HMAC failure, we fetch keys from the server and update our keys, just as
+   * we would on startup.
+   *
+   * If our key collection changed, we signal to the engine (via our return
+   * value) that it should retry decryption.
+   *
+   * If our key collection did not change, it means that we already had the
+   * correct keys... and thus a different client has the wrong ones. Reupload
+   * the bundle that we fetched, which will bump the modified time on the
+   * server and (we hope) prompt a broken client to fix itself.
+   *
+   * We keep track of the time at which we last applied this reasoning, because
+   * thrashing doesn't solve anything. We keep a reasonable interval between
+   * these remedial actions.
+   */
+  lastHMACEvent: 0,
+
+  /*
+   * Returns whether to try again.
+   */
+  async handleHMACEvent() {
+    let now = Date.now();
+
+    // Leave a sizable delay between HMAC recovery attempts. This gives us
+    // time for another client to fix themselves if we touch the record.
+    if (now - this.lastHMACEvent < HMAC_EVENT_INTERVAL) {
+      return false;
+    }
+
+    this._log.info(
+      "Bad HMAC event detected. Attempting recovery " +
+        "or signaling to other clients."
+    );
+
+    // Set the last handled time so that we don't act again.
+    this.lastHMACEvent = now;
+
+    // Fetch keys.
+    let cryptoKeys = new CryptoWrapper(CRYPTO_COLLECTION, KEYS_WBO);
+    try {
+      let cryptoResp = (
+        await cryptoKeys.fetch(this.resource(this.cryptoKeysURL))
+      ).response;
+
+      // Save out the ciphertext for when we reupload. If there's a bug in
+      // CollectionKeyManager, this will prevent us from uploading junk.
+      let cipherText = cryptoKeys.ciphertext;
+
+      if (!cryptoResp.success) {
+        this._log.warn("Failed to download keys.");
+        return false;
+      }
+
+      let keysChanged = await this.handleFetchedKeys(
+        this.identity.syncKeyBundle,
+        cryptoKeys,
+        true
+      );
+      if (keysChanged) {
+        // Did they change? If so, carry on.
+        this._log.info("Suggesting retry.");
+        return true; // Try again.
+      }
+
+      // If not, reupload them and continue the current sync.
+      cryptoKeys.ciphertext = cipherText;
+      cryptoKeys.cleartext = null;
+
+      let uploadResp = await this._uploadCryptoKeys(
+        cryptoKeys,
+        cryptoResp.obj.modified
+      );
+      if (uploadResp.success) {
+        this._log.info("Successfully re-uploaded keys. Continuing sync.");
+      } else {
+        this._log.warn(
+          "Got error response re-uploading keys. " +
+            "Continuing sync; let's try again later."
+        );
+      }
+
+      return false; // Don't try again: same keys.
+    } catch (ex) {
+      this._log.warn(
+        "Got exception fetching and handling crypto keys. " +
+          "Will try again later.",
+        ex
+      );
+      return false;
+    }
+  },
+
+  async handleFetchedKeys(syncKey, cryptoKeys, skipReset) {
+    // Don't want to wipe if we're just starting up!
+    let wasBlank = this.collectionKeys.isClear;
+    let keysChanged = await this.collectionKeys.updateContents(
+      syncKey,
+      cryptoKeys
+    );
+
+    if (keysChanged && !wasBlank) {
+      this._log.debug("Keys changed: " + JSON.stringify(keysChanged));
+
+      if (!skipReset) {
+        this._log.info("Resetting client to reflect key change.");
+
+        if (keysChanged.length) {
+          // Collection keys only. Reset individual engines.
+          await this.resetClient(keysChanged);
+        } else {
+          // Default key changed: wipe it all.
+          await this.resetClient();
+        }
+
+        this._log.info("Downloaded new keys, client reset. Proceeding.");
+      }
+      return true;
+    }
+    return false;
+  },
+
+  /**
+   * Prepare to initialize the rest of Weave after waiting a little bit
+   */
+  async onStartup() {
+    this.status = Status;
+    this.identity = Status._authManager;
+    this.collectionKeys = new CollectionKeyManager();
+
+    this.scheduler = new SyncScheduler(this);
+    this.errorHandler = new ErrorHandler(this);
+
+    this._log = Log.repository.getLogger("Sync.Service");
+    this._log.manageLevelFromPref("services.sync.log.logger.service.main");
+
+    this._log.info("Loading Weave " + WEAVE_VERSION);
+
+    this.recordManager = new RecordManager(this);
+
+    this.enabled = true;
+
+    await this._registerEngines();
+
+    let ua = Cc["@mozilla.org/network/protocol;1?name=http"].getService(
+      Ci.nsIHttpProtocolHandler
+    ).userAgent;
+    this._log.info(ua);
+
+    if (!this._checkCrypto()) {
+      this.enabled = false;
+      this._log.info(
+        "Could not load the Weave crypto component. Disabling " +
+          "Weave, since it will not work correctly."
+      );
+    }
+
+    Svc.Obs.add("weave:service:setup-complete", this);
+    Svc.Obs.add("sync:collection_changed", this); // Pulled from FxAccountsCommon
+    Svc.Obs.add("fxaccounts:device_disconnected", this);
+    Services.prefs.addObserver(PREFS_BRANCH + "engine.", this);
+
+    if (!this.enabled) {
+      this._log.info("Firefox Sync disabled.");
+    }
+
+    this._updateCachedURLs();
+
+    let status = this._checkSetup();
+    if (status != STATUS_DISABLED && status != CLIENT_NOT_CONFIGURED) {
+      this._startTracking();
+    }
+
+    // Send an event now that Weave service is ready.  We don't do this
+    // synchronously so that observers can import this module before
+    // registering an observer.
+    CommonUtils.nextTick(() => {
+      this.status.ready = true;
+
+      // UI code uses the flag on the XPCOM service so it doesn't have
+      // to load a bunch of modules.
+      let xps = Cc["@mozilla.org/weave/service;1"].getService(
+        Ci.nsISupports
+      ).wrappedJSObject;
+      xps.ready = true;
+
+      Svc.Obs.notify("weave:service:ready");
+    });
+  },
+
+  _checkSetup: function _checkSetup() {
+    if (!this.enabled) {
+      return (this.status.service = STATUS_DISABLED);
+    }
+    return this.status.checkSetup();
+  },
+
+  /**
+   * Register the built-in engines for certain applications
+   */
+  async _registerEngines() {
+    this.engineManager = new EngineManager(this);
+
+    let engineModules = getEngineModules();
+
+    let engines = [];
+    // We allow a pref, which has no default value, to limit the engines
+    // which are registered. We expect only tests will use this.
+    if (
+      Svc.PrefBranch.getPrefType("registerEngines") !=
+      Ci.nsIPrefBranch.PREF_INVALID
+    ) {
+      engines = Svc.PrefBranch.getStringPref("registerEngines").split(",");
+      this._log.info("Registering custom set of engines", engines);
+    } else {
+      // default is all engines.
+      engines = Object.keys(engineModules);
+    }
+
+    let declined = [];
+    let pref = Svc.PrefBranch.getStringPref("declinedEngines", null);
+    if (pref) {
+      declined = pref.split(",");
+    }
+
+    let clientsEngine = new ClientEngine(this);
+    // Ideally clientsEngine should not exist
+    // (or be a promise that calls initialize() before returning the engine)
+    await clientsEngine.initialize();
+    this.clientsEngine = clientsEngine;
+
+    for (let name of engines) {
+      if (!(name in engineModules)) {
+        this._log.info("Do not know about engine: " + name);
+        continue;
+      }
+      let modInfo = engineModules[name];
+      if (!modInfo.module.includes(":")) {
+        modInfo.module = "resource://services-sync/engines/" + modInfo.module;
+      }
+      try {
+        let ns = ChromeUtils.importESModule(modInfo.module);
+        if (modInfo.symbol) {
+          let symbol = modInfo.symbol;
+          if (!(symbol in ns)) {
+            this._log.warn(
+              "Could not find exported engine instance: " + symbol
+            );
+            continue;
+          }
+          await this.engineManager.register(ns[symbol]);
+        } else {
+          let { whenTrue, whenFalse, controllingPref } = modInfo;
+          if (!(whenTrue in ns) || !(whenFalse in ns)) {
+            this._log.warn("Could not find all exported engine instances", {
+              whenTrue,
+              whenFalse,
+            });
+            continue;
+          }
+          await this.engineManager.registerAlternatives(
+            name.toLowerCase(),
+            controllingPref,
+            ns[whenTrue],
+            ns[whenFalse]
+          );
+        }
+      } catch (ex) {
+        this._log.warn("Could not register engine " + name, ex);
+      }
+    }
+
+    this.engineManager.setDeclined(declined);
+  },
+
+  /**
+   * This method updates the local engines state from an existing meta/global
+   * when Sync is disabled.
+   * Running this code if sync is enabled would end up in very weird results
+   * (but we're nice and we check before doing anything!).
+   */
+  async updateLocalEnginesState() {
+    await this.promiseInitialized;
+
+    // Sanity check, this method is not meant to be run if Sync is enabled!
+    if (Svc.PrefBranch.getStringPref("username", "")) {
+      throw new Error("Sync is enabled!");
+    }
+
+    // For historical reasons the behaviour of setCluster() is bizarre,
+    // so just check what we care about - the meta URL.
+    if (!this.metaURL) {
+      await this.identity.setCluster();
+      if (!this.metaURL) {
+        this._log.warn("Could not find a cluster.");
+        return;
+      }
+    }
+    // Clear the cache so we always fetch the latest meta/global.
+    this.recordManager.clearCache();
+    let meta = await this.recordManager.get(this.metaURL);
+    if (!meta) {
+      this._log.info("Meta record is null, aborting engine state update.");
+      return;
+    }
+    const declinedEngines = meta.payload.declined;
+    const allEngines = this.engineManager.getAll().map(e => e.name);
+    // We don't want our observer of the enabled prefs to treat the change as
+    // a user-change, otherwise we will do the wrong thing with declined etc.
+    this._ignorePrefObserver = true;
+    try {
+      for (const engine of allEngines) {
+        Svc.PrefBranch.setBoolPref(
+          `engine.${engine}`,
+          !declinedEngines.includes(engine)
+        );
+      }
+    } finally {
+      this._ignorePrefObserver = false;
+    }
+  },
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIObserver",
+    "nsISupportsWeakReference",
+  ]),
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      // Ideally this observer should be in the SyncScheduler, but it would require
+      // some work to know about the sync specific engines. We should move this there once it does.
+      case "sync:collection_changed":
+        // We check if we're running TPS here to avoid TPS failing because it
+        // couldn't get to get the sync lock, due to us currently syncing the
+        // clients engine.
+        if (
+          data.includes("clients") &&
+          !Svc.PrefBranch.getBoolPref("testing.tps", false)
+        ) {
+          // Sync in the background (it's fine not to wait on the returned promise
+          // because sync() has a lock).
+          // [] = clients collection only
+          this.sync({ why: "collection_changed", engines: [] }).catch(e => {
+            this._log.error(e);
+          });
+        }
+        break;
+      case "fxaccounts:device_disconnected":
+        data = JSON.parse(data);
+        if (!data.isLocalDevice) {
+          // Refresh the known stale clients list in the background.
+          this.clientsEngine.updateKnownStaleClients().catch(e => {
+            this._log.error(e);
+          });
+        }
+        break;
+      case "weave:service:setup-complete":
+        let status = this._checkSetup();
+        if (status != STATUS_DISABLED && status != CLIENT_NOT_CONFIGURED) {
+          this._startTracking();
+        }
+        break;
+      case "nsPref:changed":
+        if (this._ignorePrefObserver) {
+          return;
+        }
+        const engine = data.slice((PREFS_BRANCH + "engine.").length);
+        if (engine.includes(".")) {
+          // A sub-preference of the engine was changed. For example
+          // `services.sync.engine.bookmarks.validation.percentageChance`.
+          return;
+        }
+        this._handleEngineStatusChanged(engine);
+        break;
+    }
+  },
+
+  _handleEngineStatusChanged(engine) {
+    this._log.trace("Status for " + engine + " engine changed.");
+    if (Svc.PrefBranch.getBoolPref("engineStatusChanged." + engine, false)) {
+      // The enabled status being changed back to what it was before.
+      Svc.PrefBranch.clearUserPref("engineStatusChanged." + engine);
+    } else {
+      // Remember that the engine status changed locally until the next sync.
+      Svc.PrefBranch.setBoolPref("engineStatusChanged." + engine, true);
+    }
+  },
+
+  _startTracking() {
+    const engines = [this.clientsEngine, ...this.engineManager.getAll()];
+    for (let engine of engines) {
+      try {
+        engine.startTracking();
+      } catch (e) {
+        this._log.error(`Could not start ${engine.name} engine tracker`, e);
+      }
+    }
+    // This is for TPS. We should try to do better.
+    Svc.Obs.notify("weave:service:tracking-started");
+  },
+
+  async _stopTracking() {
+    const engines = [this.clientsEngine, ...this.engineManager.getAll()];
+    for (let engine of engines) {
+      try {
+        await engine.stopTracking();
+      } catch (e) {
+        this._log.error(`Could not stop ${engine.name} engine tracker`, e);
+      }
+    }
+    Svc.Obs.notify("weave:service:tracking-stopped");
+  },
+
+  /**
+   * Obtain a Resource instance with authentication credentials.
+   */
+  resource: function resource(url) {
+    let res = new Resource(url);
+    res.authenticator = this.identity.getResourceAuthenticator();
+
+    return res;
+  },
+
+  /**
+   * Perform the info fetch as part of a login or key fetch, or
+   * inside engine sync.
+   */
+  async _fetchInfo(url) {
+    let infoURL = url || this.infoURL;
+
+    this._log.trace("In _fetchInfo: " + infoURL);
+    let info;
+    try {
+      info = await this.resource(infoURL).get();
+    } catch (ex) {
+      this.errorHandler.checkServerError(ex);
+      throw ex;
+    }
+
+    // Always check for errors.
+    this.errorHandler.checkServerError(info);
+    if (!info.success) {
+      this._log.error("Aborting sync: failed to get collections.");
+      throw info;
+    }
+    return info;
+  },
+
+  async verifyAndFetchSymmetricKeys(infoResponse) {
+    this._log.debug(
+      "Fetching and verifying -- or generating -- symmetric keys."
+    );
+
+    let syncKeyBundle = this.identity.syncKeyBundle;
+    if (!syncKeyBundle) {
+      this.status.login = LOGIN_FAILED_NO_PASSPHRASE;
+      this.status.sync = CREDENTIALS_CHANGED;
+      return false;
+    }
+
+    try {
+      if (!infoResponse) {
+        infoResponse = await this._fetchInfo(); // Will throw an exception on failure.
+      }
+
+      // This only applies when the server is already at version 4.
+      if (infoResponse.status != 200) {
+        this._log.warn(
+          "info/collections returned non-200 response. Failing key fetch."
+        );
+        this.status.login = LOGIN_FAILED_SERVER_ERROR;
+        this.errorHandler.checkServerError(infoResponse);
+        return false;
+      }
+
+      let infoCollections = infoResponse.obj;
+
+      this._log.info(
+        "Testing info/collections: " + JSON.stringify(infoCollections)
+      );
+
+      if (this.collectionKeys.updateNeeded(infoCollections)) {
+        this._log.info("collection keys reports that a key update is needed.");
+
+        // Don't always set to CREDENTIALS_CHANGED -- we will probably take care of this.
+
+        // Fetch storage/crypto/keys.
+        let cryptoKeys;
+
+        if (infoCollections && CRYPTO_COLLECTION in infoCollections) {
+          try {
+            cryptoKeys = new CryptoWrapper(CRYPTO_COLLECTION, KEYS_WBO);
+            let cryptoResp = (
+              await cryptoKeys.fetch(this.resource(this.cryptoKeysURL))
+            ).response;
+
+            if (cryptoResp.success) {
+              await this.handleFetchedKeys(syncKeyBundle, cryptoKeys);
+              return true;
+            } else if (cryptoResp.status == 404) {
+              // On failure, ask to generate new keys and upload them.
+              // Fall through to the behavior below.
+              this._log.warn(
+                "Got 404 for crypto/keys, but 'crypto' in info/collections. Regenerating."
+              );
+              cryptoKeys = null;
+            } else {
+              // Some other problem.
+              this.status.login = LOGIN_FAILED_SERVER_ERROR;
+              this.errorHandler.checkServerError(cryptoResp);
+              this._log.warn(
+                "Got status " + cryptoResp.status + " fetching crypto keys."
+              );
+              return false;
+            }
+          } catch (ex) {
+            this._log.warn("Got exception fetching cryptoKeys.", ex);
+            // TODO: Um, what exceptions might we get here? Should we re-throw any?
+
+            // One kind of exception: HMAC failure.
+            if (Utils.isHMACMismatch(ex)) {
+              this.status.login = LOGIN_FAILED_INVALID_PASSPHRASE;
+              this.status.sync = CREDENTIALS_CHANGED;
+            } else {
+              // In the absence of further disambiguation or more precise
+              // failure constants, just report failure.
+              this.status.login = LOGIN_FAILED;
+            }
+            return false;
+          }
+        } else {
+          this._log.info(
+            "... 'crypto' is not a reported collection. Generating new keys."
+          );
+        }
+
+        if (!cryptoKeys) {
+          this._log.info("No keys! Generating new ones.");
+
+          // Better make some and upload them, and wipe the server to ensure
+          // consistency. This is all achieved via _freshStart.
+          // If _freshStart fails to clear the server or upload keys, it will
+          // throw.
+          await this._freshStart();
+          return true;
+        }
+
+        // Last-ditch case.
+        return false;
+      }
+      // No update needed: we're good!
+      return true;
+    } catch (ex) {
+      // This means no keys are present, or there's a network error.
+      this._log.debug("Failed to fetch and verify keys", ex);
+      this.errorHandler.checkServerError(ex);
+      return false;
+    }
+  },
+
+  getMaxRecordPayloadSize() {
+    let config = this.serverConfiguration;
+    if (!config || !config.max_record_payload_bytes) {
+      this._log.warn(
+        "No config or incomplete config in getMaxRecordPayloadSize." +
+          " Are we running tests?"
+      );
+      return 256 * 1024;
+    }
+    let payloadMax = config.max_record_payload_bytes;
+    if (config.max_post_bytes && payloadMax <= config.max_post_bytes) {
+      return config.max_post_bytes - 4096;
+    }
+    return payloadMax;
+  },
+
+  getMemcacheMaxRecordPayloadSize() {
+    // Collections stored in memcached ("tabs", "clients" or "meta") have a
+    // different max size than ones stored in the normal storage server db.
+    // In practice, the real limit here is 1M (bug 1300451 comment 40), but
+    // there's overhead involved that is hard to calculate on the client, so we
+    // use 512k to be safe (at the recommendation of the server team). Note
+    // that if the server reports a lower limit (via info/configuration), we
+    // respect that limit instead. See also bug 1403052.
+    return Math.min(512 * 1024, this.getMaxRecordPayloadSize());
+  },
+
+  async verifyLogin(allow40XRecovery = true) {
+    // Attaching auth credentials to a request requires access to
+    // passwords, which means that Resource.get can throw MP-related
+    // exceptions!
+    // So we ask the identity to verify the login state after unlocking the
+    // master password (ie, this call is expected to prompt for MP unlock
+    // if necessary) while we still have control.
+    this.status.login = await this.identity.unlockAndVerifyAuthState();
+    this._log.debug(
+      "Fetching unlocked auth state returned " + this.status.login
+    );
+    if (this.status.login != STATUS_OK) {
+      return false;
+    }
+
+    try {
+      // Make sure we have a cluster to verify against.
+      // This is a little weird, if we don't get a node we pretend
+      // to succeed, since that probably means we just don't have storage.
+      if (this.clusterURL == "" && !(await this.identity.setCluster())) {
+        this.status.sync = NO_SYNC_NODE_FOUND;
+        return true;
+      }
+
+      // Fetch collection info on every startup.
+      let test = await this.resource(this.infoURL).get();
+
+      switch (test.status) {
+        case 200:
+          // The user is authenticated.
+
+          // We have no way of verifying the passphrase right now,
+          // so wait until remoteSetup to do so.
+          // Just make the most trivial checks.
+          if (!this.identity.syncKeyBundle) {
+            this._log.warn("No passphrase in verifyLogin.");
+            this.status.login = LOGIN_FAILED_NO_PASSPHRASE;
+            return false;
+          }
+
+          // Go ahead and do remote setup, so that we can determine
+          // conclusively that our passphrase is correct.
+          if (await this._remoteSetup(test)) {
+            // Username/password verified.
+            this.status.login = LOGIN_SUCCEEDED;
+            return true;
+          }
+
+          this._log.warn("Remote setup failed.");
+          // Remote setup must have failed.
+          return false;
+
+        case 401:
+          this._log.warn("401: login failed.");
+        // Fall through to the 404 case.
+
+        case 404:
+          // Check that we're verifying with the correct cluster
+          if (allow40XRecovery && (await this.identity.setCluster())) {
+            return await this.verifyLogin(false);
+          }
+
+          // We must have the right cluster, but the server doesn't expect us.
+          // For FxA this almost certainly means "transient error fetching token".
+          this.status.login = LOGIN_FAILED_NETWORK_ERROR;
+          return false;
+
+        default:
+          // Server didn't respond with something that we expected
+          this.status.login = LOGIN_FAILED_SERVER_ERROR;
+          this.errorHandler.checkServerError(test);
+          return false;
+      }
+    } catch (ex) {
+      // Must have failed on some network issue
+      this._log.debug("verifyLogin failed", ex);
+      this.status.login = LOGIN_FAILED_NETWORK_ERROR;
+      this.errorHandler.checkServerError(ex);
+      return false;
+    }
+  },
+
+  async generateNewSymmetricKeys() {
+    this._log.info("Generating new keys WBO...");
+    let wbo = await this.collectionKeys.generateNewKeysWBO();
+    this._log.info("Encrypting new key bundle.");
+    await wbo.encrypt(this.identity.syncKeyBundle);
+
+    let uploadRes = await this._uploadCryptoKeys(wbo, 0);
+    if (uploadRes.status != 200) {
+      this._log.warn(
+        "Got status " +
+          uploadRes.status +
+          " uploading new keys. What to do? Throw!"
+      );
+      this.errorHandler.checkServerError(uploadRes);
+      throw new Error("Unable to upload symmetric keys.");
+    }
+    this._log.info("Got status " + uploadRes.status + " uploading keys.");
+    let serverModified = uploadRes.obj; // Modified timestamp according to server.
+    this._log.debug("Server reports crypto modified: " + serverModified);
+
+    // Now verify that info/collections shows them!
+    this._log.debug("Verifying server collection records.");
+    let info = await this._fetchInfo();
+    this._log.debug("info/collections is: " + info.data);
+
+    if (info.status != 200) {
+      this._log.warn("Non-200 info/collections response. Aborting.");
+      throw new Error("Unable to upload symmetric keys.");
+    }
+
+    info = info.obj;
+    if (!(CRYPTO_COLLECTION in info)) {
+      this._log.error(
+        "Consistency failure: info/collections excludes " +
+          "crypto after successful upload."
+      );
+      throw new Error("Symmetric key upload failed.");
+    }
+
+    // Can't check against local modified: clock drift.
+    if (info[CRYPTO_COLLECTION] < serverModified) {
+      this._log.error(
+        "Consistency failure: info/collections crypto entry " +
+          "is stale after successful upload."
+      );
+      throw new Error("Symmetric key upload failed.");
+    }
+
+    // Doesn't matter if the timestamp is ahead.
+
+    // Download and install them.
+    let cryptoKeys = new CryptoWrapper(CRYPTO_COLLECTION, KEYS_WBO);
+    let cryptoResp = (await cryptoKeys.fetch(this.resource(this.cryptoKeysURL)))
+      .response;
+    if (cryptoResp.status != 200) {
+      this._log.warn("Failed to download keys.");
+      throw new Error("Symmetric key download failed.");
+    }
+    let keysChanged = await this.handleFetchedKeys(
+      this.identity.syncKeyBundle,
+      cryptoKeys,
+      true
+    );
+    if (keysChanged) {
+      this._log.info("Downloaded keys differed, as expected.");
+    }
+  },
+
+  // configures/enabled/turns-on sync. There must be an FxA user signed in.
+  async configure() {
+    // We don't, and must not, throw if sync is already configured, because we
+    // might end up being called as part of a "reconnect" flow. We also want to
+    // avoid checking the FxA user is the same as the pref because the email
+    // address for the FxA account can change - we'd need to use the uid.
+    let user = await fxAccounts.getSignedInUser();
+    if (!user) {
+      throw new Error("No FxA user is signed in");
+    }
+    this._log.info("Configuring sync with current FxA user");
+    Svc.PrefBranch.setStringPref("username", user.email);
+    Svc.Obs.notify("weave:connected");
+  },
+
+  // resets/turns-off sync.
+  async startOver() {
+    this._log.trace("Invoking Service.startOver.");
+    await this._stopTracking();
+    this.status.resetSync();
+
+    // Deletion doesn't make sense if we aren't set up yet!
+    if (this.clusterURL != "") {
+      // Clear client-specific data from the server, including disabled engines.
+      const engines = [this.clientsEngine, ...this.engineManager.getAll()];
+      for (let engine of engines) {
+        try {
+          await engine.removeClientData();
+        } catch (ex) {
+          this._log.warn(`Deleting client data for ${engine.name} failed`, ex);
+        }
+      }
+      this._log.debug("Finished deleting client data.");
+    } else {
+      this._log.debug("Skipping client data removal: no cluster URL.");
+    }
+
+    this.identity.resetCredentials();
+    this.status.login = LOGIN_FAILED_NO_USERNAME;
+    this.logout();
+    Svc.Obs.notify("weave:service:start-over");
+
+    // Reset all engines and clear keys.
+    await this.resetClient();
+    this.collectionKeys.clear();
+    this.status.resetBackoff();
+
+    // Reset Weave prefs.
+    this._ignorePrefObserver = true;
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    this._ignorePrefObserver = false;
+    this.clusterURL = null;
+
+    Svc.PrefBranch.setStringPref("lastversion", WEAVE_VERSION);
+
+    try {
+      this.identity.finalize();
+      this.status.__authManager = null;
+      this.identity = Status._authManager;
+      Svc.Obs.notify("weave:service:start-over:finish");
+    } catch (err) {
+      this._log.error(
+        "startOver failed to re-initialize the identity manager",
+        err
+      );
+      // Still send the observer notification so the current state is
+      // reflected in the UI.
+      Svc.Obs.notify("weave:service:start-over:finish");
+    }
+  },
+
+  async login() {
+    async function onNotify() {
+      this._loggedIn = false;
+      if (this.scheduler.offline) {
+        this.status.login = LOGIN_FAILED_NETWORK_ERROR;
+        throw new Error("Application is offline, login should not be called");
+      }
+
+      this._log.info("User logged in successfully - verifying login.");
+      if (!(await this.verifyLogin())) {
+        // verifyLogin sets the failure states here.
+        throw new Error(`Login failed: ${this.status.login}`);
+      }
+
+      this._updateCachedURLs();
+
+      this._loggedIn = true;
+
+      return true;
+    }
+
+    let notifier = this._notify("login", "", onNotify.bind(this));
+    return this._catch(this._lock("service.js: login", notifier))();
+  },
+
+  logout: function logout() {
+    // If we failed during login, we aren't going to have this._loggedIn set,
+    // but we still want to ask the identity to logout, so it doesn't try and
+    // reuse any old credentials next time we sync.
+    this._log.info("Logging out");
+    this.identity.logout();
+    this._loggedIn = false;
+
+    Svc.Obs.notify("weave:service:logout:finish");
+  },
+
+  // Note: returns false if we failed for a reason other than the server not yet
+  // supporting the api.
+  async _fetchServerConfiguration() {
+    // This is similar to _fetchInfo, but with different error handling.
+
+    let infoURL = this.userBaseURL + "info/configuration";
+    this._log.debug("Fetching server configuration", infoURL);
+    let configResponse;
+    try {
+      configResponse = await this.resource(infoURL).get();
+    } catch (ex) {
+      // This is probably a network or similar error.
+      this._log.warn("Failed to fetch info/configuration", ex);
+      this.errorHandler.checkServerError(ex);
+      return false;
+    }
+
+    if (configResponse.status == 404) {
+      // This server doesn't support the URL yet - that's OK.
+      this._log.debug(
+        "info/configuration returned 404 - using default upload semantics"
+      );
+    } else if (configResponse.status != 200) {
+      this._log.warn(
+        `info/configuration returned ${configResponse.status} - using default configuration`
+      );
+      this.errorHandler.checkServerError(configResponse);
+      return false;
+    } else {
+      this.serverConfiguration = configResponse.obj;
+    }
+    this._log.trace(
+      "info/configuration for this server",
+      this.serverConfiguration
+    );
+    return true;
+  },
+
+  // Stuff we need to do after login, before we can really do
+  // anything (e.g. key setup).
+  async _remoteSetup(infoResponse, fetchConfig = true) {
+    if (fetchConfig && !(await this._fetchServerConfiguration())) {
+      return false;
+    }
+
+    this._log.debug("Fetching global metadata record");
+    let meta = await this.recordManager.get(this.metaURL);
+
+    // Checking modified time of the meta record.
+    if (
+      infoResponse &&
+      infoResponse.obj.meta != this.metaModified &&
+      (!meta || !meta.isNew)
+    ) {
+      // Delete the cached meta record...
+      this._log.debug(
+        "Clearing cached meta record. metaModified is " +
+          JSON.stringify(this.metaModified) +
+          ", setting to " +
+          JSON.stringify(infoResponse.obj.meta)
+      );
+
+      this.recordManager.del(this.metaURL);
+
+      // ... fetch the current record from the server, and COPY THE FLAGS.
+      let newMeta = await this.recordManager.get(this.metaURL);
+
+      // If we got a 401, we do not want to create a new meta/global - we
+      // should be able to get the existing meta after we get a new node.
+      if (this.recordManager.response.status == 401) {
+        this._log.debug(
+          "Fetching meta/global record on the server returned 401."
+        );
+        this.errorHandler.checkServerError(this.recordManager.response);
+        return false;
+      }
+
+      if (this.recordManager.response.status == 404) {
+        this._log.debug("No meta/global record on the server. Creating one.");
+        try {
+          await this._uploadNewMetaGlobal();
+        } catch (uploadRes) {
+          this._log.warn(
+            "Unable to upload new meta/global. Failing remote setup."
+          );
+          this.errorHandler.checkServerError(uploadRes);
+          return false;
+        }
+      } else if (!newMeta) {
+        this._log.warn("Unable to get meta/global. Failing remote setup.");
+        this.errorHandler.checkServerError(this.recordManager.response);
+        return false;
+      } else {
+        // If newMeta, then it stands to reason that meta != null.
+        newMeta.isNew = meta.isNew;
+        newMeta.changed = meta.changed;
+      }
+
+      // Switch in the new meta object and record the new time.
+      meta = newMeta;
+      this.metaModified = infoResponse.obj.meta;
+    }
+
+    let remoteVersion =
+      meta && meta.payload.storageVersion ? meta.payload.storageVersion : "";
+
+    this._log.debug(
+      [
+        "Weave Version:",
+        WEAVE_VERSION,
+        "Local Storage:",
+        STORAGE_VERSION,
+        "Remote Storage:",
+        remoteVersion,
+      ].join(" ")
+    );
+
+    // Check for cases that require a fresh start. When comparing remoteVersion,
+    // we need to convert it to a number as older clients used it as a string.
+    if (
+      !meta ||
+      !meta.payload.storageVersion ||
+      !meta.payload.syncID ||
+      STORAGE_VERSION > parseFloat(remoteVersion)
+    ) {
+      this._log.info(
+        "One of: no meta, no meta storageVersion, or no meta syncID. Fresh start needed."
+      );
+
+      // abort the server wipe if the GET status was anything other than 404 or 200
+      let status = this.recordManager.response.status;
+      if (status != 200 && status != 404) {
+        this.status.sync = METARECORD_DOWNLOAD_FAIL;
+        this.errorHandler.checkServerError(this.recordManager.response);
+        this._log.warn(
+          "Unknown error while downloading metadata record. Aborting sync."
+        );
+        return false;
+      }
+
+      if (!meta) {
+        this._log.info("No metadata record, server wipe needed");
+      }
+      if (meta && !meta.payload.syncID) {
+        this._log.warn("No sync id, server wipe needed");
+      }
+
+      this._log.info("Wiping server data");
+      await this._freshStart();
+
+      if (status == 404) {
+        this._log.info(
+          "Metadata record not found, server was wiped to ensure " +
+            "consistency."
+        );
+      } else {
+        // 200
+        this._log.info("Wiped server; incompatible metadata: " + remoteVersion);
+      }
+      return true;
+    } else if (remoteVersion > STORAGE_VERSION) {
+      this.status.sync = VERSION_OUT_OF_DATE;
+      this._log.warn("Upgrade required to access newer storage version.");
+      return false;
+    } else if (meta.payload.syncID != this.syncID) {
+      this._log.info(
+        "Sync IDs differ. Local is " +
+          this.syncID +
+          ", remote is " +
+          meta.payload.syncID
+      );
+      await this.resetClient();
+      this.collectionKeys.clear();
+      this.syncID = meta.payload.syncID;
+      this._log.debug("Clear cached values and take syncId: " + this.syncID);
+
+      if (!(await this.verifyAndFetchSymmetricKeys(infoResponse))) {
+        this._log.warn("Failed to fetch symmetric keys. Failing remote setup.");
+        return false;
+      }
+
+      // bug 545725 - re-verify creds and fail sanely
+      if (!(await this.verifyLogin())) {
+        this.status.sync = CREDENTIALS_CHANGED;
+        this._log.info(
+          "Credentials have changed, aborting sync and forcing re-login."
+        );
+        return false;
+      }
+
+      return true;
+    }
+    if (!(await this.verifyAndFetchSymmetricKeys(infoResponse))) {
+      this._log.warn("Failed to fetch symmetric keys. Failing remote setup.");
+      return false;
+    }
+
+    return true;
+  },
+
+  /**
+   * Return whether we should attempt login at the start of a sync.
+   *
+   * Note that this function has strong ties to _checkSync: callers
+   * of this function should typically use _checkSync to verify that
+   * any necessary login took place.
+   */
+  _shouldLogin: function _shouldLogin() {
+    return (
+      this.enabled &&
+      !this.scheduler.offline &&
+      !this.isLoggedIn &&
+      Async.isAppReady()
+    );
+  },
+
+  /**
+   * Determine if a sync should run.
+   *
+   * @param ignore [optional]
+   *        array of reasons to ignore when checking
+   *
+   * @return Reason for not syncing; not-truthy if sync should run
+   */
+  _checkSync: function _checkSync(ignore) {
+    let reason = "";
+    // Ideally we'd call _checkSetup() here but that has too many side-effects.
+    if (Status.service == CLIENT_NOT_CONFIGURED) {
+      reason = kSyncNotConfigured;
+    } else if (Status.service == STATUS_DISABLED || !this.enabled) {
+      reason = kSyncWeaveDisabled;
+    } else if (this.scheduler.offline) {
+      reason = kSyncNetworkOffline;
+    } else if (this.status.minimumNextSync > Date.now()) {
+      reason = kSyncBackoffNotMet;
+    } else if (
+      this.status.login == MASTER_PASSWORD_LOCKED &&
+      Utils.mpLocked()
+    ) {
+      reason = kSyncMasterPasswordLocked;
+    } else if (Svc.PrefBranch.getStringPref("firstSync", null) == "notReady") {
+      reason = kFirstSyncChoiceNotMade;
+    } else if (!Async.isAppReady()) {
+      reason = kFirefoxShuttingDown;
+    }
+
+    if (ignore && ignore.includes(reason)) {
+      return "";
+    }
+
+    return reason;
+  },
+
+  async sync({ engines, why } = {}) {
+    let dateStr = Utils.formatTimestamp(new Date());
+    this._log.debug("User-Agent: " + Utils.userAgent);
+    await this.promiseInitialized;
+    this._log.info(
+      `Starting sync at ${dateStr} in browser session ${lazy.browserSessionID}`
+    );
+    return this._catch(async function () {
+      // Make sure we're logged in.
+      if (this._shouldLogin()) {
+        this._log.debug("In sync: should login.");
+        if (!(await this.login())) {
+          this._log.debug("Not syncing: login returned false.");
+          return;
+        }
+      } else {
+        this._log.trace("In sync: no need to login.");
+      }
+      await this._lockedSync(engines, why);
+    })();
+  },
+
+  /**
+   * Sync up engines with the server.
+   */
+  async _lockedSync(engineNamesToSync, why) {
+    return this._lock(
+      "service.js: sync",
+      this._notify("sync", JSON.stringify({ why }), async function onNotify() {
+        let histogram =
+          Services.telemetry.getHistogramById("WEAVE_START_COUNT");
+        histogram.add(1);
+
+        let synchronizer = new EngineSynchronizer(this);
+        await synchronizer.sync(engineNamesToSync, why); // Might throw!
+
+        histogram = Services.telemetry.getHistogramById(
+          "WEAVE_COMPLETE_SUCCESS_COUNT"
+        );
+        histogram.add(1);
+
+        // We successfully synchronized.
+        // Check if the identity wants to pre-fetch a migration sentinel from
+        // the server.
+        // If we have no clusterURL, we are probably doing a node reassignment
+        // so don't attempt to get it in that case.
+        if (this.clusterURL) {
+          this.identity.prefetchMigrationSentinel(this);
+        }
+
+        // Now let's update our declined engines
+        await this._maybeUpdateDeclined();
+      })
+    )();
+  },
+
+  /**
+   * Update the "declined" information in meta/global if necessary.
+   */
+  async _maybeUpdateDeclined() {
+    // if Sync failed due to no node we will not have a meta URL, so can't
+    // update anything.
+    if (!this.metaURL) {
+      return;
+    }
+    let meta = await this.recordManager.get(this.metaURL);
+    if (!meta) {
+      this._log.warn("No meta/global; can't update declined state.");
+      return;
+    }
+
+    let declinedEngines = new DeclinedEngines(this);
+    let didChange = declinedEngines.updateDeclined(meta, this.engineManager);
+    if (!didChange) {
+      this._log.info(
+        "No change to declined engines. Not reuploading meta/global."
+      );
+      return;
+    }
+
+    await this.uploadMetaGlobal(meta);
+  },
+
+  /**
+   * Upload a fresh meta/global record
+   * @throws the response object if the upload request was not a success
+   */
+  async _uploadNewMetaGlobal() {
+    let meta = new WBORecord("meta", "global");
+    meta.payload.syncID = this.syncID;
+    meta.payload.storageVersion = STORAGE_VERSION;
+    meta.payload.declined = this.engineManager.getDeclined();
+    meta.modified = 0;
+    meta.isNew = true;
+
+    await this.uploadMetaGlobal(meta);
+  },
+
+  /**
+   * Upload meta/global, throwing the response on failure
+   * @param {WBORecord} meta meta/global record
+   * @throws the response object if the request was not a success
+   */
+  async uploadMetaGlobal(meta) {
+    this._log.debug("Uploading meta/global", meta);
+    let res = this.resource(this.metaURL);
+    res.setHeader("X-If-Unmodified-Since", meta.modified);
+    let response = await res.put(meta);
+    if (!response.success) {
+      throw response;
+    }
+    // From https://docs.services.mozilla.com/storage/apis-1.5.html:
+    // "Successful responses will return the new last-modified time for the collection."
+    meta.modified = response.obj;
+    this.recordManager.set(this.metaURL, meta);
+  },
+
+  /**
+   * Upload crypto/keys
+   * @param {WBORecord} cryptoKeys crypto/keys record
+   * @param {Number} lastModified known last modified timestamp (in decimal seconds),
+   *                 will be used to set the X-If-Unmodified-Since header
+   */
+  async _uploadCryptoKeys(cryptoKeys, lastModified) {
+    this._log.debug(`Uploading crypto/keys (lastModified: ${lastModified})`);
+    let res = this.resource(this.cryptoKeysURL);
+    res.setHeader("X-If-Unmodified-Since", lastModified);
+    return res.put(cryptoKeys);
+  },
+
+  async _freshStart() {
+    this._log.info("Fresh start. Resetting client.");
+    await this.resetClient();
+    this.collectionKeys.clear();
+
+    // Wipe the server.
+    await this.wipeServer();
+
+    // Upload a new meta/global record.
+    // _uploadNewMetaGlobal throws on failure -- including race conditions.
+    // If we got into a race condition, we'll abort the sync this way, too.
+    // That's fine. We'll just wait till the next sync. The client that we're
+    // racing is probably busy uploading stuff right now anyway.
+    await this._uploadNewMetaGlobal();
+
+    // Wipe everything we know about except meta because we just uploaded it
+    // TODO: there's a bug here. We should be calling resetClient, no?
+
+    // Generate, upload, and download new keys. Do this last so we don't wipe
+    // them...
+    await this.generateNewSymmetricKeys();
+  },
+
+  /**
+   * Wipe user data from the server.
+   *
+   * @param collections [optional]
+   *        Array of collections to wipe. If not given, all collections are
+   *        wiped by issuing a DELETE request for `storageURL`.
+   *
+   * @return the server's timestamp of the (last) DELETE.
+   */
+  async wipeServer(collections) {
+    let response;
+    let histogram = Services.telemetry.getHistogramById(
+      "WEAVE_WIPE_SERVER_SUCCEEDED"
+    );
+    if (!collections) {
+      // Strip the trailing slash.
+      let res = this.resource(this.storageURL.slice(0, -1));
+      res.setHeader("X-Confirm-Delete", "1");
+      try {
+        response = await res.delete();
+      } catch (ex) {
+        this._log.debug("Failed to wipe server", ex);
+        histogram.add(false);
+        throw ex;
+      }
+      if (response.status != 200 && response.status != 404) {
+        this._log.debug(
+          "Aborting wipeServer. Server responded with " +
+            response.status +
+            " response for " +
+            this.storageURL
+        );
+        histogram.add(false);
+        throw response;
+      }
+      histogram.add(true);
+      return response.headers["x-weave-timestamp"];
+    }
+
+    let timestamp;
+    for (let name of collections) {
+      let url = this.storageURL + name;
+      try {
+        response = await this.resource(url).delete();
+      } catch (ex) {
+        this._log.debug("Failed to wipe '" + name + "' collection", ex);
+        histogram.add(false);
+        throw ex;
+      }
+
+      if (response.status != 200 && response.status != 404) {
+        this._log.debug(
+          "Aborting wipeServer. Server responded with " +
+            response.status +
+            " response for " +
+            url
+        );
+        histogram.add(false);
+        throw response;
+      }
+
+      if ("x-weave-timestamp" in response.headers) {
+        timestamp = response.headers["x-weave-timestamp"];
+      }
+    }
+    histogram.add(true);
+    return timestamp;
+  },
+
+  /**
+   * Wipe all local user data.
+   *
+   * @param engines [optional]
+   *        Array of engine names to wipe. If not given, all engines are used.
+   */
+  async wipeClient(engines) {
+    // If we don't have any engines, reset the service and wipe all engines
+    if (!engines) {
+      // Clear out any service data
+      await this.resetService();
+
+      engines = [this.clientsEngine, ...this.engineManager.getAll()];
+    } else {
+      // Convert the array of names into engines
+      engines = this.engineManager.get(engines);
+    }
+
+    // Fully wipe each engine if it's able to decrypt data
+    for (let engine of engines) {
+      if (await engine.canDecrypt()) {
+        await engine.wipeClient();
+      }
+    }
+  },
+
+  /**
+   * Wipe all remote user data by wiping the server then telling each remote
+   * client to wipe itself.
+   *
+   * @param engines
+   *        Array of engine names to wipe.
+   */
+  async wipeRemote(engines) {
+    try {
+      // Make sure stuff gets uploaded.
+      await this.resetClient(engines);
+
+      // Clear out any server data.
+      await this.wipeServer(engines);
+
+      // Only wipe the engines provided.
+      let extra = { reason: "wipe-remote" };
+      for (const e of engines) {
+        await this.clientsEngine.sendCommand("wipeEngine", [e], null, extra);
+      }
+
+      // Make sure the changed clients get updated.
+      await this.clientsEngine.sync();
+    } catch (ex) {
+      this.errorHandler.checkServerError(ex);
+      throw ex;
+    }
+  },
+
+  /**
+   * Reset local service information like logs, sync times, caches.
+   */
+  async resetService() {
+    return this._catch(async function reset() {
+      this._log.info("Service reset.");
+
+      // Pretend we've never synced to the server and drop cached data
+      this.syncID = "";
+      this.recordManager.clearCache();
+    })();
+  },
+
+  /**
+   * Reset the client by getting rid of any local server data and client data.
+   *
+   * @param engines [optional]
+   *        Array of engine names to reset. If not given, all engines are used.
+   */
+  async resetClient(engines) {
+    return this._catch(async function doResetClient() {
+      // If we don't have any engines, reset everything including the service
+      if (!engines) {
+        // Clear out any service data
+        await this.resetService();
+
+        engines = [this.clientsEngine, ...this.engineManager.getAll()];
+      } else {
+        // Convert the array of names into engines
+        engines = this.engineManager.get(engines);
+      }
+
+      // Have each engine drop any temporary meta data
+      for (let engine of engines) {
+        await engine.resetClient();
+      }
+    })();
+  },
+
+  recordTelemetryEvent(object, method, value, extra = undefined) {
+    Svc.Obs.notify("weave:telemetry:event", { object, method, value, extra });
+  },
+};
+
+export var Service = new Sync11Service();
+Service.promiseInitialized = new Promise(resolve => {
+  Service.onStartup().then(resolve);
+});
diff --git a/services/sync/modules/stages/declined.sys.mjs b/services/sync/modules/stages/declined.sys.mjs
new file mode 100644
index 0000000000..2c74aab117
--- /dev/null
+++ b/services/sync/modules/stages/declined.sys.mjs
@@ -0,0 +1,78 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This file contains code for maintaining the set of declined engines,
+ * in conjunction with EngineManager.
+ */
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+import { Observers } from "resource://services-common/observers.sys.mjs";
+
+export var DeclinedEngines = function (service) {
+  this._log = Log.repository.getLogger("Sync.Declined");
+  this._log.manageLevelFromPref("services.sync.log.logger.declined");
+
+  this.service = service;
+};
+
+DeclinedEngines.prototype = {
+  updateDeclined(meta, engineManager = this.service.engineManager) {
+    let enabled = new Set(engineManager.getEnabled().map(e => e.name));
+    let known = new Set(engineManager.getAll().map(e => e.name));
+    let remoteDeclined = new Set(meta.payload.declined || []);
+    let localDeclined = new Set(engineManager.getDeclined());
+
+    this._log.debug(
+      "Handling remote declined: " + JSON.stringify([...remoteDeclined])
+    );
+    this._log.debug(
+      "Handling local declined: " + JSON.stringify([...localDeclined])
+    );
+
+    // Any engines that are locally enabled should be removed from the remote
+    // declined list.
+    //
+    // Any engines that are locally declined should be added to the remote
+    // declined list.
+    let newDeclined = CommonUtils.union(
+      localDeclined,
+      CommonUtils.difference(remoteDeclined, enabled)
+    );
+
+    // If our declined set has changed, put it into the meta object and mark
+    // it as changed.
+    let declinedChanged = !CommonUtils.setEqual(newDeclined, remoteDeclined);
+    this._log.debug("Declined changed? " + declinedChanged);
+    if (declinedChanged) {
+      meta.changed = true;
+      meta.payload.declined = [...newDeclined];
+    }
+
+    // Update the engine manager regardless.
+    engineManager.setDeclined(newDeclined);
+
+    // Any engines that are locally known, locally disabled, and not remotely
+    // or locally declined, are candidates for enablement.
+    let undecided = CommonUtils.difference(
+      CommonUtils.difference(known, enabled),
+      newDeclined
+    );
+    if (undecided.size) {
+      let subject = {
+        declined: newDeclined,
+        enabled,
+        known,
+        undecided,
+      };
+      CommonUtils.nextTick(() => {
+        Observers.notify("weave:engines:notdeclined", subject);
+      });
+    }
+
+    return declinedChanged;
+  },
+};
diff --git a/services/sync/modules/stages/enginesync.sys.mjs b/services/sync/modules/stages/enginesync.sys.mjs
new file mode 100644
index 0000000000..190fd1f5fa
--- /dev/null
+++ b/services/sync/modules/stages/enginesync.sys.mjs
@@ -0,0 +1,412 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This file contains code for synchronizing engines.
+ */
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import {
+  ABORT_SYNC_COMMAND,
+  LOGIN_FAILED_NETWORK_ERROR,
+  NO_SYNC_NODE_FOUND,
+  STATUS_OK,
+  SYNC_FAILED_PARTIAL,
+  SYNC_SUCCEEDED,
+  WEAVE_VERSION,
+  kSyncNetworkOffline,
+} from "resource://services-sync/constants.sys.mjs";
+
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+
+const lazy = {};
+ChromeUtils.defineESModuleGetters(lazy, {
+  Doctor: "resource://services-sync/doctor.sys.mjs",
+});
+
+/**
+ * Perform synchronization of engines.
+ *
+ * This was originally split out of service.js. The API needs lots of love.
+ */
+export function EngineSynchronizer(service) {
+  this._log = Log.repository.getLogger("Sync.Synchronizer");
+  this._log.manageLevelFromPref("services.sync.log.logger.synchronizer");
+
+  this.service = service;
+}
+
+EngineSynchronizer.prototype = {
+  async sync(engineNamesToSync, why) {
+    let fastSync = why && why == "sleep";
+    let startTime = Date.now();
+
+    this.service.status.resetSync();
+
+    // Make sure we should sync or record why we shouldn't.
+    let reason = this.service._checkSync();
+    if (reason) {
+      if (reason == kSyncNetworkOffline) {
+        this.service.status.sync = LOGIN_FAILED_NETWORK_ERROR;
+      }
+
+      // this is a purposeful abort rather than a failure, so don't set
+      // any status bits
+      reason = "Can't sync: " + reason;
+      throw new Error(reason);
+    }
+
+    // If we don't have a node, get one. If that fails, retry in 10 minutes.
+    if (
+      !this.service.clusterURL &&
+      !(await this.service.identity.setCluster())
+    ) {
+      this.service.status.sync = NO_SYNC_NODE_FOUND;
+      this._log.info("No cluster URL found. Cannot sync.");
+      return;
+    }
+
+    // Ping the server with a special info request once a day.
+    let infoURL = this.service.infoURL;
+    let now = Math.floor(Date.now() / 1000);
+    let lastPing = Svc.PrefBranch.getIntPref("lastPing", 0);
+    if (now - lastPing > 86400) {
+      // 60 * 60 * 24
+      infoURL += "?v=" + WEAVE_VERSION;
+      Svc.PrefBranch.setIntPref("lastPing", now);
+    }
+
+    let engineManager = this.service.engineManager;
+
+    // Figure out what the last modified time is for each collection
+    let info = await this.service._fetchInfo(infoURL);
+
+    // Convert the response to an object and read out the modified times
+    for (let engine of [this.service.clientsEngine].concat(
+      engineManager.getAll()
+    )) {
+      engine.lastModified = info.obj[engine.name] || 0;
+    }
+
+    if (!(await this.service._remoteSetup(info, !fastSync))) {
+      throw new Error("Aborting sync, remote setup failed");
+    }
+
+    if (!fastSync) {
+      // Make sure we have an up-to-date list of clients before sending commands
+      this._log.debug("Refreshing client list.");
+      if (!(await this._syncEngine(this.service.clientsEngine))) {
+        // Clients is an engine like any other; it can fail with a 401,
+        // and we can elect to abort the sync.
+        this._log.warn("Client engine sync failed. Aborting.");
+        return;
+      }
+    }
+
+    // We only honor the "hint" of what engines to Sync if this isn't
+    // a first sync.
+    let allowEnginesHint = false;
+    // Wipe data in the desired direction if necessary
+    switch (Svc.PrefBranch.getStringPref("firstSync", null)) {
+      case "resetClient":
+        await this.service.resetClient(engineManager.enabledEngineNames);
+        break;
+      case "wipeClient":
+        await this.service.wipeClient(engineManager.enabledEngineNames);
+        break;
+      case "wipeRemote":
+        await this.service.wipeRemote(engineManager.enabledEngineNames);
+        break;
+      default:
+        allowEnginesHint = true;
+        break;
+    }
+
+    if (!fastSync && this.service.clientsEngine.localCommands) {
+      try {
+        if (!(await this.service.clientsEngine.processIncomingCommands())) {
+          this.service.status.sync = ABORT_SYNC_COMMAND;
+          throw new Error("Processed command aborted sync.");
+        }
+
+        // Repeat remoteSetup in-case the commands forced us to reset
+        if (!(await this.service._remoteSetup(info))) {
+          throw new Error("Remote setup failed after processing commands.");
+        }
+      } finally {
+        // Always immediately attempt to push back the local client (now
+        // without commands).
+        // Note that we don't abort here; if there's a 401 because we've
+        // been reassigned, we'll handle it around another engine.
+        await this._syncEngine(this.service.clientsEngine);
+      }
+    }
+
+    // Update engines because it might change what we sync.
+    try {
+      await this._updateEnabledEngines();
+    } catch (ex) {
+      this._log.debug("Updating enabled engines failed", ex);
+      this.service.errorHandler.checkServerError(ex);
+      throw ex;
+    }
+
+    await this.service.engineManager.switchAlternatives();
+
+    // If the engines to sync has been specified, we sync in the order specified.
+    let enginesToSync;
+    if (allowEnginesHint && engineNamesToSync) {
+      this._log.info("Syncing specified engines", engineNamesToSync);
+      enginesToSync = engineManager
+        .get(engineNamesToSync)
+        .filter(e => e.enabled);
+    } else {
+      this._log.info("Syncing all enabled engines.");
+      enginesToSync = engineManager.getEnabled();
+    }
+    try {
+      // We don't bother validating engines that failed to sync.
+      let enginesToValidate = [];
+      for (let engine of enginesToSync) {
+        if (engine.shouldSkipSync(why)) {
+          this._log.info(`Engine ${engine.name} asked to be skipped`);
+          continue;
+        }
+        // If there's any problems with syncing the engine, report the failure
+        if (
+          !(await this._syncEngine(engine)) ||
+          this.service.status.enforceBackoff
+        ) {
+          this._log.info("Aborting sync for failure in " + engine.name);
+          break;
+        }
+        enginesToValidate.push(engine);
+      }
+
+      // If _syncEngine fails for a 401, we might not have a cluster URL here.
+      // If that's the case, break out of this immediately, rather than
+      // throwing an exception when trying to fetch metaURL.
+      if (!this.service.clusterURL) {
+        this._log.debug(
+          "Aborting sync, no cluster URL: not uploading new meta/global."
+        );
+        return;
+      }
+
+      // Upload meta/global if any engines changed anything.
+      let meta = await this.service.recordManager.get(this.service.metaURL);
+      if (meta.isNew || meta.changed) {
+        this._log.info("meta/global changed locally: reuploading.");
+        try {
+          await this.service.uploadMetaGlobal(meta);
+          delete meta.isNew;
+          delete meta.changed;
+        } catch (error) {
+          this._log.error(
+            "Unable to upload meta/global. Leaving marked as new."
+          );
+        }
+      }
+
+      if (!fastSync) {
+        await lazy.Doctor.consult(enginesToValidate);
+      }
+
+      // If there were no sync engine failures
+      if (this.service.status.service != SYNC_FAILED_PARTIAL) {
+        this.service.status.sync = SYNC_SUCCEEDED;
+      }
+
+      // Even if there were engine failures, bump lastSync even on partial since
+      // it's reflected in the UI (bug 1439777).
+      if (
+        this.service.status.service == SYNC_FAILED_PARTIAL ||
+        this.service.status.service == STATUS_OK
+      ) {
+        Svc.PrefBranch.setStringPref("lastSync", new Date().toString());
+      }
+    } finally {
+      Svc.PrefBranch.clearUserPref("firstSync");
+
+      let syncTime = ((Date.now() - startTime) / 1000).toFixed(2);
+      let dateStr = Utils.formatTimestamp(new Date());
+      this._log.info(
+        "Sync completed at " + dateStr + " after " + syncTime + " secs."
+      );
+    }
+  },
+
+  // Returns true if sync should proceed.
+  // false / no return value means sync should be aborted.
+  async _syncEngine(engine) {
+    try {
+      await engine.sync();
+    } catch (e) {
+      if (e.status == 401) {
+        // Maybe a 401, cluster update perhaps needed?
+        // We rely on ErrorHandler observing the sync failure notification to
+        // schedule another sync and clear node assignment values.
+        // Here we simply want to muffle the exception and return an
+        // appropriate value.
+        return false;
+      }
+      // Note that policies.js has already logged info about the exception...
+      if (Async.isShutdownException(e)) {
+        // Failure due to a shutdown exception should prevent other engines
+        // trying to start and immediately failing.
+        this._log.info(
+          `${engine.name} was interrupted by shutdown; no other engines will sync`
+        );
+        return false;
+      }
+    }
+
+    return true;
+  },
+
+  async _updateEnabledFromMeta(
+    meta,
+    numClients,
+    engineManager = this.service.engineManager
+  ) {
+    this._log.info("Updating enabled engines: " + numClients + " clients.");
+
+    if (meta.isNew || !meta.payload.engines) {
+      this._log.debug(
+        "meta/global isn't new, or is missing engines. Not updating enabled state."
+      );
+      return;
+    }
+
+    // If we're the only client, and no engines are marked as enabled,
+    // thumb our noses at the server data: it can't be right.
+    // Belt-and-suspenders approach to Bug 615926.
+    let hasEnabledEngines = false;
+    for (let e in meta.payload.engines) {
+      if (e != "clients") {
+        hasEnabledEngines = true;
+        break;
+      }
+    }
+
+    if (numClients <= 1 && !hasEnabledEngines) {
+      this._log.info(
+        "One client and no enabled engines: not touching local engine status."
+      );
+      return;
+    }
+
+    this.service._ignorePrefObserver = true;
+
+    let enabled = engineManager.enabledEngineNames;
+
+    let toDecline = new Set();
+    let toUndecline = new Set();
+
+    for (let engineName in meta.payload.engines) {
+      if (engineName == "clients") {
+        // Clients is special.
+        continue;
+      }
+      let index = enabled.indexOf(engineName);
+      if (index != -1) {
+        // The engine is enabled locally. Nothing to do.
+        enabled.splice(index, 1);
+        continue;
+      }
+      let engine = engineManager.get(engineName);
+      if (!engine) {
+        // The engine doesn't exist locally. Nothing to do.
+        continue;
+      }
+
+      let attemptedEnable = false;
+      // If the engine was enabled remotely, enable it locally.
+      if (
+        !Svc.PrefBranch.getBoolPref(
+          "engineStatusChanged." + engine.prefName,
+          false
+        )
+      ) {
+        this._log.trace(
+          "Engine " + engineName + " was enabled. Marking as non-declined."
+        );
+        toUndecline.add(engineName);
+        this._log.trace(engineName + " engine was enabled remotely.");
+        engine.enabled = true;
+        // Note that setting engine.enabled to true might not have worked for
+        // the password engine if a master-password is enabled.  However, it's
+        // still OK that we added it to undeclined - the user *tried* to enable
+        // it remotely - so it still winds up as not being flagged as declined
+        // even though it's disabled remotely.
+        attemptedEnable = true;
+      }
+
+      // If either the engine was disabled locally or enabling the engine
+      // failed (see above re master-password) then wipe server data and
+      // disable it everywhere.
+      if (!engine.enabled) {
+        this._log.trace("Wiping data for " + engineName + " engine.");
+        await engine.wipeServer();
+        delete meta.payload.engines[engineName];
+        meta.changed = true; // the new enabled state must propagate
+        // We also here mark the engine as declined, because the pref
+        // was explicitly changed to false - unless we tried, and failed,
+        // to enable it - in which case we leave the declined state alone.
+        if (!attemptedEnable) {
+          // This will be reflected in meta/global in the next stage.
+          this._log.trace(
+            "Engine " +
+              engineName +
+              " was disabled locally. Marking as declined."
+          );
+          toDecline.add(engineName);
+        }
+      }
+    }
+
+    // Any remaining engines were either enabled locally or disabled remotely.
+    for (let engineName of enabled) {
+      let engine = engineManager.get(engineName);
+      if (
+        Svc.PrefBranch.getBoolPref(
+          "engineStatusChanged." + engine.prefName,
+          false
+        )
+      ) {
+        this._log.trace("The " + engineName + " engine was enabled locally.");
+        toUndecline.add(engineName);
+      } else {
+        this._log.trace("The " + engineName + " engine was disabled remotely.");
+
+        // Don't automatically mark it as declined!
+        try {
+          engine.enabled = false;
+        } catch (e) {
+          this._log.trace("Failed to disable engine " + engineName);
+        }
+      }
+    }
+
+    engineManager.decline(toDecline);
+    engineManager.undecline(toUndecline);
+
+    for (const pref of Svc.PrefBranch.getChildList("engineStatusChanged.")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    this.service._ignorePrefObserver = false;
+  },
+
+  async _updateEnabledEngines() {
+    let meta = await this.service.recordManager.get(this.service.metaURL);
+    let numClients = this.service.scheduler.numClients;
+    let engineManager = this.service.engineManager;
+
+    await this._updateEnabledFromMeta(meta, numClients, engineManager);
+  },
+};
+Object.freeze(EngineSynchronizer.prototype);
diff --git a/services/sync/modules/status.sys.mjs b/services/sync/modules/status.sys.mjs
new file mode 100644
index 0000000000..429dbda7b6
--- /dev/null
+++ b/services/sync/modules/status.sys.mjs
@@ -0,0 +1,135 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import {
+  CLIENT_NOT_CONFIGURED,
+  ENGINE_SUCCEEDED,
+  LOGIN_FAILED,
+  LOGIN_FAILED_NO_PASSPHRASE,
+  LOGIN_FAILED_NO_USERNAME,
+  LOGIN_SUCCEEDED,
+  STATUS_OK,
+  SYNC_FAILED,
+  SYNC_FAILED_PARTIAL,
+  SYNC_SUCCEEDED,
+} from "resource://services-sync/constants.sys.mjs";
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { SyncAuthManager } from "resource://services-sync/sync_auth.sys.mjs";
+
+export var Status = {
+  _log: Log.repository.getLogger("Sync.Status"),
+  __authManager: null,
+  ready: false,
+
+  get _authManager() {
+    if (this.__authManager) {
+      return this.__authManager;
+    }
+    this.__authManager = new SyncAuthManager();
+    return this.__authManager;
+  },
+
+  get service() {
+    return this._service;
+  },
+
+  set service(code) {
+    this._log.debug(
+      "Status.service: " + (this._service || undefined) + " => " + code
+    );
+    this._service = code;
+  },
+
+  get login() {
+    return this._login;
+  },
+
+  set login(code) {
+    this._log.debug("Status.login: " + this._login + " => " + code);
+    this._login = code;
+
+    if (
+      code == LOGIN_FAILED_NO_USERNAME ||
+      code == LOGIN_FAILED_NO_PASSPHRASE
+    ) {
+      this.service = CLIENT_NOT_CONFIGURED;
+    } else if (code != LOGIN_SUCCEEDED) {
+      this.service = LOGIN_FAILED;
+    } else {
+      this.service = STATUS_OK;
+    }
+  },
+
+  get sync() {
+    return this._sync;
+  },
+
+  set sync(code) {
+    this._log.debug("Status.sync: " + this._sync + " => " + code);
+    this._sync = code;
+    this.service = code == SYNC_SUCCEEDED ? STATUS_OK : SYNC_FAILED;
+  },
+
+  get engines() {
+    return this._engines;
+  },
+
+  set engines([name, code]) {
+    this._log.debug("Status for engine " + name + ": " + code);
+    this._engines[name] = code;
+
+    if (code != ENGINE_SUCCEEDED) {
+      this.service = SYNC_FAILED_PARTIAL;
+    }
+  },
+
+  // Implement toString because adding a logger introduces a cyclic object
+  // value, so we can't trivially debug-print Status as JSON.
+  toString: function toString() {
+    return (
+      "<Status" +
+      ": login: " +
+      Status.login +
+      ", service: " +
+      Status.service +
+      ", sync: " +
+      Status.sync +
+      ">"
+    );
+  },
+
+  checkSetup: function checkSetup() {
+    if (!this._authManager.username) {
+      Status.login = LOGIN_FAILED_NO_USERNAME;
+      Status.service = CLIENT_NOT_CONFIGURED;
+    } else if (Status.login == STATUS_OK) {
+      Status.service = STATUS_OK;
+    }
+    return Status.service;
+  },
+
+  resetBackoff: function resetBackoff() {
+    this.enforceBackoff = false;
+    this.backoffInterval = 0;
+    this.minimumNextSync = 0;
+  },
+
+  resetSync: function resetSync() {
+    // Logger setup.
+    this._log.manageLevelFromPref("services.sync.log.logger.status");
+
+    this._log.info("Resetting Status.");
+    this.service = STATUS_OK;
+    this._login = LOGIN_SUCCEEDED;
+    this._sync = SYNC_SUCCEEDED;
+    this._engines = {};
+    this.partial = false;
+  },
+};
+
+// Initialize various status values.
+Status.resetBackoff();
+Status.resetSync();
diff --git a/services/sync/modules/sync_auth.sys.mjs b/services/sync/modules/sync_auth.sys.mjs
new file mode 100644
index 0000000000..6b8da4061c
--- /dev/null
+++ b/services/sync/modules/sync_auth.sys.mjs
@@ -0,0 +1,655 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+import { Async } from "resource://services-common/async.sys.mjs";
+import { TokenServerClient } from "resource://services-common/tokenserverclient.sys.mjs";
+import { CryptoUtils } from "resource://services-crypto/utils.sys.mjs";
+import { Svc, Utils } from "resource://services-sync/util.sys.mjs";
+
+import {
+  LOGIN_FAILED_LOGIN_REJECTED,
+  LOGIN_FAILED_NETWORK_ERROR,
+  LOGIN_FAILED_NO_USERNAME,
+  LOGIN_SUCCEEDED,
+  MASTER_PASSWORD_LOCKED,
+  STATUS_OK,
+} from "resource://services-sync/constants.sys.mjs";
+
+const lazy = {};
+
+// Lazy imports to prevent unnecessary load on startup.
+ChromeUtils.defineESModuleGetters(lazy, {
+  BulkKeyBundle: "resource://services-sync/keys.sys.mjs",
+  Weave: "resource://services-sync/main.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "fxAccounts", () => {
+  return ChromeUtils.importESModule(
+    "resource://gre/modules/FxAccounts.sys.mjs"
+  ).getFxAccountsSingleton();
+});
+
+ChromeUtils.defineLazyGetter(lazy, "log", function () {
+  let log = Log.repository.getLogger("Sync.SyncAuthManager");
+  log.manageLevelFromPref("services.sync.log.logger.identity");
+  return log;
+});
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "IGNORE_CACHED_AUTH_CREDENTIALS",
+  "services.sync.debug.ignoreCachedAuthCredentials"
+);
+
+// FxAccountsCommon.js doesn't use a "namespace", so create one here.
+import * as fxAccountsCommon from "resource://gre/modules/FxAccountsCommon.sys.mjs";
+
+const SCOPE_OLD_SYNC = fxAccountsCommon.SCOPE_OLD_SYNC;
+
+const OBSERVER_TOPICS = [
+  fxAccountsCommon.ONLOGIN_NOTIFICATION,
+  fxAccountsCommon.ONVERIFIED_NOTIFICATION,
+  fxAccountsCommon.ONLOGOUT_NOTIFICATION,
+  fxAccountsCommon.ON_ACCOUNT_STATE_CHANGE_NOTIFICATION,
+  "weave:connected",
+];
+
+/*
+  General authentication error for abstracting authentication
+  errors from multiple sources (e.g., from FxAccounts, TokenServer).
+  details is additional details about the error - it might be a string, or
+  some other error object (which should do the right thing when toString() is
+  called on it)
+*/
+export function AuthenticationError(details, source) {
+  this.details = details;
+  this.source = source;
+}
+
+AuthenticationError.prototype = {
+  toString() {
+    return "AuthenticationError(" + this.details + ")";
+  },
+};
+
+// The `SyncAuthManager` coordinates access authorization to the Sync server.
+// Its job is essentially to get us from having a signed-in Firefox Accounts user,
+// to knowing the user's sync storage node and having the necessary short-lived
+// credentials in order to access it.
+//
+
+export function SyncAuthManager() {
+  // NOTE: _fxaService and _tokenServerClient are replaced with mocks by
+  // the test suite.
+  this._fxaService = lazy.fxAccounts;
+  this._tokenServerClient = new TokenServerClient();
+  this._tokenServerClient.observerPrefix = "weave:service";
+  this._log = lazy.log;
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "_username",
+    "services.sync.username"
+  );
+
+  this.asyncObserver = Async.asyncObserver(this, lazy.log);
+  for (let topic of OBSERVER_TOPICS) {
+    Services.obs.addObserver(this.asyncObserver, topic);
+  }
+}
+
+SyncAuthManager.prototype = {
+  _fxaService: null,
+  _tokenServerClient: null,
+  // https://docs.services.mozilla.com/token/apis.html
+  _token: null,
+  // protection against the user changing underneath us - the uid
+  // of the current user.
+  _userUid: null,
+
+  hashedUID() {
+    const id = this._fxaService.telemetry.getSanitizedUID();
+    if (!id) {
+      throw new Error("hashedUID: Don't seem to have previously seen a token");
+    }
+    return id;
+  },
+
+  // Return a hashed version of a deviceID, suitable for telemetry.
+  hashedDeviceID(deviceID) {
+    const id = this._fxaService.telemetry.sanitizeDeviceId(deviceID);
+    if (!id) {
+      throw new Error("hashedUID: Don't seem to have previously seen a token");
+    }
+    return id;
+  },
+
+  // The "node type" reported to telemetry or null if not specified.
+  get telemetryNodeType() {
+    return this._token && this._token.node_type ? this._token.node_type : null;
+  },
+
+  finalize() {
+    // After this is called, we can expect Service.identity != this.
+    for (let topic of OBSERVER_TOPICS) {
+      Services.obs.removeObserver(this.asyncObserver, topic);
+    }
+    this.resetCredentials();
+    this._userUid = null;
+  },
+
+  async getSignedInUser() {
+    let data = await this._fxaService.getSignedInUser();
+    if (!data) {
+      this._userUid = null;
+      return null;
+    }
+    if (this._userUid == null) {
+      this._userUid = data.uid;
+    } else if (this._userUid != data.uid) {
+      throw new Error("The signed in user has changed");
+    }
+    return data;
+  },
+
+  logout() {
+    // This will be called when sync fails (or when the account is being
+    // unlinked etc).  It may have failed because we got a 401 from a sync
+    // server, so we nuke the token.  Next time sync runs and wants an
+    // authentication header, we will notice the lack of the token and fetch a
+    // new one.
+    this._token = null;
+  },
+
+  async observe(subject, topic, data) {
+    this._log.debug("observed " + topic);
+    if (!this.username) {
+      this._log.info("Sync is not configured, so ignoring the notification");
+      return;
+    }
+    switch (topic) {
+      case "weave:connected":
+      case fxAccountsCommon.ONLOGIN_NOTIFICATION: {
+        this._log.info("Sync has been connected to a logged in user");
+        this.resetCredentials();
+        let accountData = await this.getSignedInUser();
+
+        if (!accountData.verified) {
+          // wait for a verified notification before we kick sync off.
+          this._log.info("The user is not verified");
+          break;
+        }
+      }
+      // We've been configured with an already verified user, so fall-through.
+      // intentional fall-through - the user is verified.
+      case fxAccountsCommon.ONVERIFIED_NOTIFICATION: {
+        this._log.info("The user became verified");
+        lazy.Weave.Status.login = LOGIN_SUCCEEDED;
+
+        // And actually sync. If we've never synced before, we force a full sync.
+        // If we have, then we are probably just reauthenticating so it's a normal sync.
+        // We can use any pref that must be set if we've synced before, and check
+        // the sync lock state because we might already be doing that first sync.
+        let isFirstSync =
+          !lazy.Weave.Service.locked &&
+          !Svc.PrefBranch.getStringPref("client.syncID", null);
+        if (isFirstSync) {
+          this._log.info("Doing initial sync actions");
+          Svc.PrefBranch.setStringPref("firstSync", "resetClient");
+          Services.obs.notifyObservers(null, "weave:service:setup-complete");
+        }
+        // There's no need to wait for sync to complete and it would deadlock
+        // our AsyncObserver.
+        if (!Svc.PrefBranch.getBoolPref("testing.tps", false)) {
+          lazy.Weave.Service.sync({ why: "login" });
+        }
+        break;
+      }
+
+      case fxAccountsCommon.ONLOGOUT_NOTIFICATION:
+        lazy.Weave.Service.startOver()
+          .then(() => {
+            this._log.trace("startOver completed");
+          })
+          .catch(err => {
+            this._log.warn("Failed to reset sync", err);
+          });
+        // startOver will cause this instance to be thrown away, so there's
+        // nothing else to do.
+        break;
+
+      case fxAccountsCommon.ON_ACCOUNT_STATE_CHANGE_NOTIFICATION:
+        // throw away token forcing us to fetch a new one later.
+        this.resetCredentials();
+        break;
+    }
+  },
+
+  /**
+   * Provide override point for testing token expiration.
+   */
+  _now() {
+    return this._fxaService._internal.now();
+  },
+
+  get _localtimeOffsetMsec() {
+    return this._fxaService._internal.localtimeOffsetMsec;
+  },
+
+  get syncKeyBundle() {
+    return this._syncKeyBundle;
+  },
+
+  get username() {
+    return this._username;
+  },
+
+  /**
+   * Set the username value.
+   *
+   * Changing the username has the side-effect of wiping credentials.
+   */
+  set username(value) {
+    // setting .username is an old throwback, but it should no longer happen.
+    throw new Error("don't set the username");
+  },
+
+  /**
+   * Resets all calculated credentials we hold for the current user. This will
+   * *not* force the user to reauthenticate, but instead will force us to
+   * calculate a new key bundle, fetch a new token, etc.
+   */
+  resetCredentials() {
+    this._syncKeyBundle = null;
+    this._token = null;
+    // The cluster URL comes from the token, so resetting it to empty will
+    // force Sync to not accidentally use a value from an earlier token.
+    lazy.Weave.Service.clusterURL = null;
+  },
+
+  /**
+   * Pre-fetches any information that might help with migration away from this
+   * identity.  Called after every sync and is really just an optimization that
+   * allows us to avoid a network request for when we actually need the
+   * migration info.
+   */
+  prefetchMigrationSentinel(service) {
+    // nothing to do here until we decide to migrate away from FxA.
+  },
+
+  /**
+   * Verify the current auth state, unlocking the master-password if necessary.
+   *
+   * Returns a promise that resolves with the current auth state after
+   * attempting to unlock.
+   */
+  async unlockAndVerifyAuthState() {
+    let data = await this.getSignedInUser();
+    const fxa = this._fxaService;
+    if (!data) {
+      lazy.log.debug("unlockAndVerifyAuthState has no FxA user");
+      return LOGIN_FAILED_NO_USERNAME;
+    }
+    if (!this.username) {
+      lazy.log.debug(
+        "unlockAndVerifyAuthState finds that sync isn't configured"
+      );
+      return LOGIN_FAILED_NO_USERNAME;
+    }
+    if (!data.verified) {
+      // Treat not verified as if the user needs to re-auth, so the browser
+      // UI reflects the state.
+      lazy.log.debug("unlockAndVerifyAuthState has an unverified user");
+      return LOGIN_FAILED_LOGIN_REJECTED;
+    }
+    if (await fxa.keys.canGetKeyForScope(SCOPE_OLD_SYNC)) {
+      lazy.log.debug(
+        "unlockAndVerifyAuthState already has (or can fetch) sync keys"
+      );
+      return STATUS_OK;
+    }
+    // so no keys - ensure MP unlocked.
+    if (!Utils.ensureMPUnlocked()) {
+      // user declined to unlock, so we don't know if they are stored there.
+      lazy.log.debug(
+        "unlockAndVerifyAuthState: user declined to unlock master-password"
+      );
+      return MASTER_PASSWORD_LOCKED;
+    }
+    // If we still can't get keys it probably means the user authenticated
+    // without unlocking the MP or cleared the saved logins, so we've now
+    // lost them - the user will need to reauth before continuing.
+    let result;
+    if (await fxa.keys.canGetKeyForScope(SCOPE_OLD_SYNC)) {
+      result = STATUS_OK;
+    } else {
+      result = LOGIN_FAILED_LOGIN_REJECTED;
+    }
+    lazy.log.debug(
+      "unlockAndVerifyAuthState re-fetched credentials and is returning",
+      result
+    );
+    return result;
+  },
+
+  /**
+   * Do we have a non-null, not yet expired token for the user currently
+   * signed in?
+   */
+  _hasValidToken() {
+    // If pref is set to ignore cached authentication credentials for debugging,
+    // then return false to force the fetching of a new token.
+    if (lazy.IGNORE_CACHED_AUTH_CREDENTIALS) {
+      return false;
+    }
+    if (!this._token) {
+      return false;
+    }
+    if (this._token.expiration < this._now()) {
+      return false;
+    }
+    return true;
+  },
+
+  // Get our tokenServerURL - a private helper. Returns a string.
+  get _tokenServerUrl() {
+    // We used to support services.sync.tokenServerURI but this was a
+    // pain-point for people using non-default servers as Sync may auto-reset
+    // all services.sync prefs. So if that still exists, it wins.
+    let url = Svc.PrefBranch.getStringPref("tokenServerURI", null); // Svc.PrefBranch "root" is services.sync
+    if (!url) {
+      url = Services.prefs.getStringPref("identity.sync.tokenserver.uri");
+    }
+    while (url.endsWith("/")) {
+      // trailing slashes cause problems...
+      url = url.slice(0, -1);
+    }
+    return url;
+  },
+
+  // Refresh the sync token for our user. Returns a promise that resolves
+  // with a token, or rejects with an error.
+  async _fetchTokenForUser() {
+    const fxa = this._fxaService;
+    // We need keys for things to work.  If we don't have them, just
+    // return null for the token - sync calling unlockAndVerifyAuthState()
+    // before actually syncing will setup the error states if necessary.
+    if (!(await fxa.keys.canGetKeyForScope(SCOPE_OLD_SYNC))) {
+      this._log.info(
+        "Unable to fetch keys (master-password locked?), so aborting token fetch"
+      );
+      throw new Error("Can't fetch a token as we can't get keys");
+    }
+
+    // Do the token dance, with a retry in case of transient auth failure.
+    // We need to prove that we know the sync key in order to get a token
+    // from the tokenserver.
+    let getToken = async key => {
+      this._log.info("Getting a sync token from", this._tokenServerUrl);
+      let token = await this._fetchTokenUsingOAuth(key);
+      this._log.trace("Successfully got a token");
+      return token;
+    };
+
+    try {
+      let token, key;
+      try {
+        this._log.info("Getting sync key");
+        key = await fxa.keys.getKeyForScope(SCOPE_OLD_SYNC);
+        if (!key) {
+          throw new Error("browser does not have the sync key, cannot sync");
+        }
+        token = await getToken(key);
+      } catch (err) {
+        // If we get a 401 fetching the token it may be that our auth tokens needed
+        // to be regenerated; retry exactly once.
+        if (!err.response || err.response.status !== 401) {
+          throw err;
+        }
+        this._log.warn(
+          "Token server returned 401, retrying token fetch with fresh credentials"
+        );
+        key = await fxa.keys.getKeyForScope(SCOPE_OLD_SYNC);
+        token = await getToken(key);
+      }
+      // TODO: Make it be only 80% of the duration, so refresh the token
+      // before it actually expires. This is to avoid sync storage errors
+      // otherwise, we may briefly enter a "needs reauthentication" state.
+      // (XXX - the above may no longer be true - someone should check ;)
+      token.expiration = this._now() + token.duration * 1000 * 0.8;
+      if (!this._syncKeyBundle) {
+        this._syncKeyBundle = lazy.BulkKeyBundle.fromJWK(key);
+      }
+      lazy.Weave.Status.login = LOGIN_SUCCEEDED;
+      this._token = token;
+      return token;
+    } catch (caughtErr) {
+      let err = caughtErr; // The error we will rethrow.
+
+      // TODO: unify these errors - we need to handle errors thrown by
+      // both tokenserverclient and hawkclient.
+      // A tokenserver error thrown based on a bad response.
+      if (err.response && err.response.status === 401) {
+        err = new AuthenticationError(err, "tokenserver");
+        // A hawkclient error.
+      } else if (err.code && err.code === 401) {
+        err = new AuthenticationError(err, "hawkclient");
+        // An FxAccounts.jsm error.
+      } else if (err.message == fxAccountsCommon.ERROR_AUTH_ERROR) {
+        err = new AuthenticationError(err, "fxaccounts");
+      }
+
+      // TODO: write tests to make sure that different auth error cases are handled here
+      // properly: auth error getting oauth token, auth error getting sync token (invalid
+      // generation or client-state error)
+      if (err instanceof AuthenticationError) {
+        this._log.error("Authentication error in _fetchTokenForUser", err);
+        // set it to the "fatal" LOGIN_FAILED_LOGIN_REJECTED reason.
+        lazy.Weave.Status.login = LOGIN_FAILED_LOGIN_REJECTED;
+      } else {
+        this._log.error("Non-authentication error in _fetchTokenForUser", err);
+        // for now assume it is just a transient network related problem
+        // (although sadly, it might also be a regular unhandled exception)
+        lazy.Weave.Status.login = LOGIN_FAILED_NETWORK_ERROR;
+      }
+      throw err;
+    }
+  },
+
+  /**
+   * Generates an OAuth access_token using the OLD_SYNC scope and exchanges it
+   * for a TokenServer token.
+   *
+   * @returns {Promise}
+   * @private
+   */
+  async _fetchTokenUsingOAuth(key) {
+    this._log.debug("Getting a token using OAuth");
+    const fxa = this._fxaService;
+    const ttl = fxAccountsCommon.OAUTH_TOKEN_FOR_SYNC_LIFETIME_SECONDS;
+    const accessToken = await fxa.getOAuthToken({ scope: SCOPE_OLD_SYNC, ttl });
+    const headers = {
+      "X-KeyId": key.kid,
+    };
+
+    return this._tokenServerClient
+      .getTokenUsingOAuth(this._tokenServerUrl, accessToken, headers)
+      .catch(async err => {
+        if (err.response && err.response.status === 401) {
+          // remove the cached token if we cannot authorize with it.
+          // we have to do this here because we know which `token` to remove
+          // from cache.
+          await fxa.removeCachedOAuthToken({ token: accessToken });
+        }
+
+        // continue the error chain, so other handlers can deal with the error.
+        throw err;
+      });
+  },
+
+  // Returns a promise that is resolved with a valid token for the current
+  // user, or rejects if one can't be obtained.
+  // NOTE: This does all the authentication for Sync - it both sets the
+  // key bundle (ie, decryption keys) and does the token fetch. These 2
+  // concepts could be decoupled, but there doesn't seem any value in that
+  // currently.
+  async _ensureValidToken(forceNewToken = false) {
+    let signedInUser = await this.getSignedInUser();
+    if (!signedInUser) {
+      throw new Error("no user is logged in");
+    }
+    if (!signedInUser.verified) {
+      throw new Error("user is not verified");
+    }
+
+    await this.asyncObserver.promiseObserversComplete();
+
+    if (!forceNewToken && this._hasValidToken()) {
+      this._log.trace("_ensureValidToken already has one");
+      return this._token;
+    }
+
+    // We are going to grab a new token - re-use the same promise if we are
+    // already fetching one.
+    if (!this._ensureValidTokenPromise) {
+      this._ensureValidTokenPromise = this.__ensureValidToken().finally(() => {
+        this._ensureValidTokenPromise = null;
+      });
+    }
+    return this._ensureValidTokenPromise;
+  },
+
+  async __ensureValidToken() {
+    // reset this._token as a safety net to reduce the possibility of us
+    // repeatedly attempting to use an invalid token if _fetchTokenForUser throws.
+    this._token = null;
+    try {
+      let token = await this._fetchTokenForUser();
+      this._token = token;
+      // This is a little bit of a hack. The tokenserver tells us a HMACed version
+      // of the FxA uid which we can use for metrics purposes without revealing the
+      // user's true uid. It conceptually belongs to FxA but we get it from tokenserver
+      // for legacy reasons. Hand it back to the FxA client code to deal with.
+      this._fxaService.telemetry._setHashedUID(token.hashed_fxa_uid);
+      return token;
+    } finally {
+      Services.obs.notifyObservers(null, "weave:service:login:got-hashed-id");
+    }
+  },
+
+  getResourceAuthenticator() {
+    return this._getAuthenticationHeader.bind(this);
+  },
+
+  /**
+   * @return a Hawk HTTP Authorization Header, lightly wrapped, for the .uri
+   * of a RESTRequest or AsyncResponse object.
+   */
+  async _getAuthenticationHeader(httpObject, method) {
+    // Note that in failure states we return null, causing the request to be
+    // made without authorization headers, thereby presumably causing a 401,
+    // which causes Sync to log out. If we throw, this may not happen as
+    // expected.
+    try {
+      await this._ensureValidToken();
+    } catch (ex) {
+      this._log.error("Failed to fetch a token for authentication", ex);
+      return null;
+    }
+    if (!this._token) {
+      return null;
+    }
+    let credentials = { id: this._token.id, key: this._token.key };
+    method = method || httpObject.method;
+
+    // Get the local clock offset from the Firefox Accounts server.  This should
+    // be close to the offset from the storage server.
+    let options = {
+      now: this._now(),
+      localtimeOffsetMsec: this._localtimeOffsetMsec,
+      credentials,
+    };
+
+    let headerValue = await CryptoUtils.computeHAWK(
+      httpObject.uri,
+      method,
+      options
+    );
+    return { headers: { authorization: headerValue.field } };
+  },
+
+  /**
+   * Determine the cluster for the current user and update state.
+   * Returns true if a new cluster URL was found and it is different from
+   * the existing cluster URL, false otherwise.
+   */
+  async setCluster() {
+    // Make sure we didn't get some unexpected response for the cluster.
+    let cluster = await this._findCluster();
+    this._log.debug("Cluster value = " + cluster);
+    if (cluster == null) {
+      return false;
+    }
+
+    // Convert from the funky "String object with additional properties" that
+    // resource.js returns to a plain-old string.
+    cluster = cluster.toString();
+    // Don't update stuff if we already have the right cluster
+    if (cluster == lazy.Weave.Service.clusterURL) {
+      return false;
+    }
+
+    this._log.debug("Setting cluster to " + cluster);
+    lazy.Weave.Service.clusterURL = cluster;
+
+    return true;
+  },
+
+  async _findCluster() {
+    try {
+      // Ensure we are ready to authenticate and have a valid token.
+      // We need to handle node reassignment here.  If we are being asked
+      // for a clusterURL while the service already has a clusterURL, then
+      // it's likely a 401 was received using the existing token - in which
+      // case we just discard the existing token and fetch a new one.
+      let forceNewToken = false;
+      if (lazy.Weave.Service.clusterURL) {
+        this._log.debug(
+          "_findCluster has a pre-existing clusterURL, so fetching a new token token"
+        );
+        forceNewToken = true;
+      }
+      let token = await this._ensureValidToken(forceNewToken);
+      let endpoint = token.endpoint;
+      // For Sync 1.5 storage endpoints, we use the base endpoint verbatim.
+      // However, it should end in "/" because we will extend it with
+      // well known path components. So we add a "/" if it's missing.
+      if (!endpoint.endsWith("/")) {
+        endpoint += "/";
+      }
+      this._log.debug("_findCluster returning " + endpoint);
+      return endpoint;
+    } catch (err) {
+      this._log.info("Failed to fetch the cluster URL", err);
+      // service.js's verifyLogin() method will attempt to fetch a cluster
+      // URL when it sees a 401.  If it gets null, it treats it as a "real"
+      // auth error and sets Status.login to LOGIN_FAILED_LOGIN_REJECTED, which
+      // in turn causes a notification bar to appear informing the user they
+      // need to re-authenticate.
+      // On the other hand, if fetching the cluster URL fails with an exception,
+      // verifyLogin() assumes it is a transient error, and thus doesn't show
+      // the notification bar under the assumption the issue will resolve
+      // itself.
+      // Thus:
+      // * On a real 401, we must return null.
+      // * On any other problem we must let an exception bubble up.
+      if (err instanceof AuthenticationError) {
+        return null;
+      }
+      throw err;
+    }
+  },
+};
diff --git a/services/sync/modules/telemetry.sys.mjs b/services/sync/modules/telemetry.sys.mjs
new file mode 100644
index 0000000000..c08f405b0e
--- /dev/null
+++ b/services/sync/modules/telemetry.sys.mjs
@@ -0,0 +1,1279 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Support for Sync-and-FxA-related telemetry, which is submitted in a special-purpose
+// telemetry ping called the "sync ping", documented here:
+//
+//  ../../../toolkit/components/telemetry/docs/data/sync-ping.rst
+//
+// The sync ping contains identifiers that are linked to the user's Firefox Account
+// and are separate from the main telemetry client_id, so this file is also responsible
+// for ensuring that we can delete those pings upon user request, by plumbing its
+// identifiers into the "deletion-request" ping.
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Async: "resource://services-common/async.sys.mjs",
+  AuthenticationError: "resource://services-sync/sync_auth.sys.mjs",
+  FxAccounts: "resource://gre/modules/FxAccounts.sys.mjs",
+  ObjectUtils: "resource://gre/modules/ObjectUtils.sys.mjs",
+  Observers: "resource://services-common/observers.sys.mjs",
+  Resource: "resource://services-sync/resource.sys.mjs",
+  Status: "resource://services-sync/status.sys.mjs",
+  Svc: "resource://services-sync/util.sys.mjs",
+  TelemetryController: "resource://gre/modules/TelemetryController.sys.mjs",
+  TelemetryEnvironment: "resource://gre/modules/TelemetryEnvironment.sys.mjs",
+  TelemetryUtils: "resource://gre/modules/TelemetryUtils.sys.mjs",
+  Weave: "resource://services-sync/main.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "fxAccounts", () => {
+  return ChromeUtils.importESModule(
+    "resource://gre/modules/FxAccounts.sys.mjs"
+  ).getFxAccountsSingleton();
+});
+
+import * as constants from "resource://services-sync/constants.sys.mjs";
+
+ChromeUtils.defineLazyGetter(
+  lazy,
+  "WeaveService",
+  () => Cc["@mozilla.org/weave/service;1"].getService().wrappedJSObject
+);
+const log = Log.repository.getLogger("Sync.Telemetry");
+
+const TOPICS = [
+  // For tracking change to account/device identifiers.
+  "fxaccounts:new_device_id",
+  "fxaccounts:onlogout",
+  "weave:service:ready",
+  "weave:service:login:got-hashed-id",
+
+  // For whole-of-sync metrics.
+  "weave:service:sync:start",
+  "weave:service:sync:finish",
+  "weave:service:sync:error",
+
+  // For individual engine metrics.
+  "weave:engine:sync:start",
+  "weave:engine:sync:finish",
+  "weave:engine:sync:error",
+  "weave:engine:sync:applied",
+  "weave:engine:sync:step",
+  "weave:engine:sync:uploaded",
+  "weave:engine:validate:finish",
+  "weave:engine:validate:error",
+
+  // For ad-hoc telemetry events.
+  "weave:telemetry:event",
+  "weave:telemetry:histogram",
+  "fxa:telemetry:event",
+
+  "weave:telemetry:migration",
+];
+
+const PING_FORMAT_VERSION = 1;
+
+const EMPTY_UID = "0".repeat(32);
+
+// The set of engines we record telemetry for - any other engines are ignored.
+const ENGINES = new Set([
+  "addons",
+  "bookmarks",
+  "clients",
+  "forms",
+  "history",
+  "passwords",
+  "prefs",
+  "tabs",
+  "extension-storage",
+  "addresses",
+  "creditcards",
+]);
+
+function tryGetMonotonicTimestamp() {
+  try {
+    return Services.telemetry.msSinceProcessStart();
+  } catch (e) {
+    log.warn("Unable to get a monotonic timestamp!");
+    return -1;
+  }
+}
+
+function timeDeltaFrom(monotonicStartTime) {
+  let now = tryGetMonotonicTimestamp();
+  if (monotonicStartTime !== -1 && now !== -1) {
+    return Math.round(now - monotonicStartTime);
+  }
+  return -1;
+}
+
+const NS_ERROR_MODULE_BASE_OFFSET = 0x45;
+const NS_ERROR_MODULE_NETWORK = 6;
+
+// A reimplementation of NS_ERROR_GET_MODULE, which surprisingly doesn't seem
+// to exist anywhere in .js code in a way that can be reused.
+// This is taken from DownloadCore.sys.mjs.
+function NS_ERROR_GET_MODULE(code) {
+  return ((code & 0x7fff0000) >> 16) - NS_ERROR_MODULE_BASE_OFFSET;
+}
+
+// Converts extra integer fields to strings, rounds floats to three
+// decimal places (nanosecond precision for timings), and removes profile
+// directory paths and URLs from potential error messages.
+function normalizeExtraTelemetryFields(extra) {
+  let result = {};
+  for (let key in extra) {
+    let value = extra[key];
+    let type = typeof value;
+    if (type == "string") {
+      result[key] = ErrorSanitizer.cleanErrorMessage(value);
+    } else if (type == "number") {
+      result[key] = Number.isInteger(value)
+        ? value.toString(10)
+        : value.toFixed(3);
+    } else if (type != "undefined") {
+      throw new TypeError(
+        `Invalid type ${type} for extra telemetry field ${key}`
+      );
+    }
+  }
+  return lazy.ObjectUtils.isEmpty(result) ? undefined : result;
+}
+
+// Keps track of the counts of individual records fate during a sync cycle
+// The main reason this is a class is to keep track of reasons individual records
+// failure reasons without huge memory overhead.
+export class SyncedRecordsTelemetry {
+  // applied    => number of items that should be applied.
+  // failed     => number of items that failed in this sync.
+  // newFailed  => number of items that failed for the first time in this sync.
+  // reconciled => number of items that were reconciled.
+  // failedReasons => {name, count} of reasons a record failed
+  incomingCounts = {
+    applied: 0,
+    failed: 0,
+    newFailed: 0,
+    reconciled: 0,
+    failedReasons: null,
+  };
+  outgoingCounts = { failed: 0, sent: 0, failedReasons: null };
+
+  addIncomingFailedReason(reason) {
+    if (!this.incomingCounts.failedReasons) {
+      this.incomingCounts.failedReasons = [];
+    }
+    let transformedReason = SyncTelemetry.transformError(reason);
+    // Some errors like http/nss errors don't have an error object
+    // those will be caught by the higher level telemetry
+    if (!transformedReason.error) {
+      return;
+    }
+
+    let index = this.incomingCounts.failedReasons.findIndex(
+      reasons => reasons.name === transformedReason.error
+    );
+
+    if (index >= 0) {
+      this.incomingCounts.failedReasons[index].count += 1;
+    } else {
+      this.incomingCounts.failedReasons.push({
+        name: transformedReason.error,
+        count: 1,
+      });
+    }
+  }
+
+  addOutgoingFailedReason(reason) {
+    if (!this.outgoingCounts.failedReasons) {
+      this.outgoingCounts.failedReasons = [];
+    }
+    let transformedReason = SyncTelemetry.transformError(reason);
+    // Some errors like http/nss errors don't have an error object
+    // those will be caught by the higher level telemetry
+    if (!transformedReason.error) {
+      return;
+    }
+    let index = this.outgoingCounts.failedReasons.findIndex(
+      reasons => reasons.name === transformedReason.error
+    );
+    if (index >= 0) {
+      this.outgoingCounts.failedReasons[index].count += 1;
+    } else {
+      this.outgoingCounts.failedReasons.push({
+        name: transformedReason.error,
+        count: 1,
+      });
+    }
+  }
+}
+
+// The `ErrorSanitizer` has 2 main jobs:
+// * Remove PII from errors, such as the profile dir or URLs the user might
+//   have visited.
+// * Normalize errors so different locales or operating systems etc don't
+//   generate different messages for the same underlying error.
+// * [TODO] Normalize errors so environmental factors don't influence message.
+//   For example, timestamps or GUIDs should be replaced with something static.
+export class ErrorSanitizer {
+  // Things we normalize - this isn't exhaustive, but covers the common error messages we see.
+  // Eg:
+  // > Win error 112 during operation write on file [profileDir]\weave\addonsreconciler.json (Espacio en disco insuficiente. )
+  // > Win error 112 during operation write on file [profileDir]\weave\addonsreconciler.json (Diskte yeterli yer yok. )
+  // > <snip many other translations of the error>
+  // > Unix error 28 during operation write on file [profileDir]/weave/addonsreconciler.json (No space left on device)
+  // These tend to crowd out other errors we might care about (eg, top 5 errors for some engines are
+  // variations of the "no space left on device")
+
+  // Note that only errors that have same-but-different errors on Windows and Unix are here - we
+  // still sanitize ones that aren't in these maps to remove the translations etc - eg,
+  // `ERROR_SHARING_VIOLATION` doesn't really have a unix equivalent, so doesn't appear here, but
+  // we still strip the translations to avoid the variants.
+  static E_PERMISSION_DENIED = "OS error [Permission denied]";
+  static E_NO_FILE_OR_DIR = "OS error [File/Path not found]";
+
+  static DOMErrorSubstitutions = {
+    NotFoundError: this.E_NO_FILE_OR_DIR,
+    NotAllowedError: this.E_PERMISSION_DENIED,
+  };
+
+  static #cleanOSErrorMessage(message, error = undefined) {
+    if (DOMException.isInstance(error)) {
+      const sub = this.DOMErrorSubstitutions[error.name];
+      message = message.replaceAll("\\", "/");
+      if (sub) {
+        return `${sub} ${message}`;
+      }
+    }
+
+    return message;
+  }
+
+  // A regex we can use to replace the profile dir in error messages. We use a
+  // regexp so we can simply replace all case-insensitive occurences.
+  // This escaping function is from:
+  // https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions
+  static reProfileDir = new RegExp(
+    PathUtils.profileDir.replace(/[.*+?^${}()|[\]\\]/g, "\\$&"),
+    "gi"
+  );
+
+  /**
+   * Clean an error message, removing PII and normalizing OS-specific messages.
+   *
+   * @param {string} message The error message
+   * @param {Error?} error The error class instance, if any.
+   */
+  static cleanErrorMessage(message, error = undefined) {
+    // There's a chance the profiledir is in the error string which is PII we
+    // want to avoid including in the ping.
+    message = message.replace(this.reProfileDir, "[profileDir]");
+    // MSG_INVALID_URL from /dom/bindings/Errors.msg -- no way to access this
+    // directly from JS.
+    if (message.endsWith("is not a valid URL.")) {
+      message = "<URL> is not a valid URL.";
+    }
+    // Try to filter things that look somewhat like a URL (in that they contain a
+    // colon in the middle of non-whitespace), in case anything else is including
+    // these in error messages. Note that JSON.stringified stuff comes through
+    // here, so we explicitly ignore double-quotes as well.
+    message = message.replace(/[^\s"]+:[^\s"]+/g, "<URL>");
+
+    // Anywhere that's normalized the guid in errors we can easily filter
+    // to make it easier to aggregate these types of errors
+    message = message.replace(/<guid: ([^>]+)>/g, "<GUID>");
+
+    return this.#cleanOSErrorMessage(message, error);
+  }
+}
+
+// This function validates the payload of a telemetry "event" - this can be
+// removed once there are APIs available for the telemetry modules to collect
+// these events (bug 1329530) - but for now we simulate that planned API as
+// best we can.
+function validateTelemetryEvent(eventDetails) {
+  let { object, method, value, extra } = eventDetails;
+  // Do do basic validation of the params - everything except "extra" must
+  // be a string. method and object are required.
+  if (
+    typeof method != "string" ||
+    typeof object != "string" ||
+    (value && typeof value != "string") ||
+    (extra && typeof extra != "object")
+  ) {
+    log.warn("Invalid event parameters - wrong types", eventDetails);
+    return false;
+  }
+  // length checks.
+  if (
+    method.length > 20 ||
+    object.length > 20 ||
+    (value && value.length > 80)
+  ) {
+    log.warn("Invalid event parameters - wrong lengths", eventDetails);
+    return false;
+  }
+
+  // extra can be falsey, or an object with string names and values.
+  if (extra) {
+    if (Object.keys(extra).length > 10) {
+      log.warn("Invalid event parameters - too many extra keys", eventDetails);
+      return false;
+    }
+    for (let [ename, evalue] of Object.entries(extra)) {
+      if (
+        typeof ename != "string" ||
+        ename.length > 15 ||
+        typeof evalue != "string" ||
+        evalue.length > 85
+      ) {
+        log.warn(
+          `Invalid event parameters: extra item "${ename} is invalid`,
+          eventDetails
+        );
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
+class EngineRecord {
+  constructor(name) {
+    // startTime is in ms from process start, but is monotonic (unlike Date.now())
+    // so we need to keep both it and when.
+    this.startTime = tryGetMonotonicTimestamp();
+    this.name = name;
+
+    // This allows cases like bookmarks-buffered to have a separate name from
+    // the bookmarks engine.
+    let engineImpl = lazy.Weave.Service.engineManager.get(name);
+    if (engineImpl && engineImpl.overrideTelemetryName) {
+      this.overrideTelemetryName = engineImpl.overrideTelemetryName;
+    }
+  }
+
+  toJSON() {
+    let result = { name: this.overrideTelemetryName || this.name };
+    let properties = [
+      "took",
+      "status",
+      "failureReason",
+      "incoming",
+      "outgoing",
+      "validation",
+      "steps",
+    ];
+    for (let property of properties) {
+      result[property] = this[property];
+    }
+    return result;
+  }
+
+  finished(error) {
+    let took = timeDeltaFrom(this.startTime);
+    if (took > 0) {
+      this.took = took;
+    }
+    if (error) {
+      this.failureReason = SyncTelemetry.transformError(error);
+    }
+  }
+
+  recordApplied(counts) {
+    if (this.incoming) {
+      log.error(
+        `Incoming records applied multiple times for engine ${this.name}!`
+      );
+      return;
+    }
+    if (this.name === "clients" && !counts.failed) {
+      // ignore successful application of client records
+      // since otherwise they show up every time and are meaningless.
+      return;
+    }
+
+    let incomingData = {};
+
+    if (counts.failedReasons) {
+      // sort the failed reasons in desc by count, then take top 10
+      counts.failedReasons = counts.failedReasons
+        .sort((a, b) => b.count - a.count)
+        .slice(0, 10);
+    }
+    // Counts has extra stuff used for logging, but we only care about a few
+    let properties = ["applied", "failed", "failedReasons"];
+    // Only record non-zero properties and only record incoming at all if
+    // there's at least one property we care about.
+    for (let property of properties) {
+      if (counts[property]) {
+        incomingData[property] = counts[property];
+        this.incoming = incomingData;
+      }
+    }
+  }
+
+  recordStep(stepResult) {
+    let step = {
+      name: stepResult.name,
+    };
+    if (stepResult.took > 0) {
+      step.took = Math.round(stepResult.took);
+    }
+    if (stepResult.counts) {
+      let counts = stepResult.counts.filter(({ count }) => count > 0);
+      if (counts.length) {
+        step.counts = counts;
+      }
+    }
+    if (this.steps) {
+      this.steps.push(step);
+    } else {
+      this.steps = [step];
+    }
+  }
+
+  recordValidation(validationResult) {
+    if (this.validation) {
+      log.error(`Multiple validations occurred for engine ${this.name}!`);
+      return;
+    }
+    let { problems, version, took, checked } = validationResult;
+    let validation = {
+      version: version || 0,
+      checked: checked || 0,
+    };
+    if (took > 0) {
+      validation.took = Math.round(took);
+    }
+    let summarized = problems.filter(({ count }) => count > 0);
+    if (summarized.length) {
+      validation.problems = summarized;
+    }
+    this.validation = validation;
+  }
+
+  recordValidationError(e) {
+    if (this.validation) {
+      log.error(`Multiple validations occurred for engine ${this.name}!`);
+      return;
+    }
+
+    this.validation = {
+      failureReason: SyncTelemetry.transformError(e),
+    };
+  }
+
+  recordUploaded(counts) {
+    if (counts.sent || counts.failed) {
+      if (!this.outgoing) {
+        this.outgoing = [];
+      }
+      if (counts.failedReasons) {
+        // sort the failed reasons in desc by count, then take top 10
+        counts.failedReasons = counts.failedReasons
+          .sort((a, b) => b.count - a.count)
+          .slice(0, 10);
+      }
+      this.outgoing.push({
+        sent: counts.sent || undefined,
+        failed: counts.failed || undefined,
+        failedReasons: counts.failedReasons || undefined,
+      });
+    }
+  }
+}
+
+// The record of a single "sync" - typically many of these are submitted in
+// a single ping (ie, as a 'syncs' array)
+export class SyncRecord {
+  constructor(allowedEngines, why) {
+    this.allowedEngines = allowedEngines;
+    // Our failure reason. This property only exists in the generated ping if an
+    // error actually occurred.
+    this.failureReason = undefined;
+    this.syncNodeType = null;
+    this.when = Date.now();
+    this.startTime = tryGetMonotonicTimestamp();
+    this.took = 0; // will be set later.
+    this.why = why;
+
+    // All engines that have finished (ie, does not include the "current" one)
+    // We omit this from the ping if it's empty.
+    this.engines = [];
+    // The engine that has started but not yet stopped.
+    this.currentEngine = null;
+  }
+
+  toJSON() {
+    let result = {
+      when: this.when,
+      took: this.took,
+      failureReason: this.failureReason,
+      status: this.status,
+    };
+    if (this.why) {
+      result.why = this.why;
+    }
+    let engines = [];
+    for (let engine of this.engines) {
+      engines.push(engine.toJSON());
+    }
+    if (engines.length) {
+      result.engines = engines;
+    }
+    return result;
+  }
+
+  finished(error) {
+    this.took = timeDeltaFrom(this.startTime);
+    if (this.currentEngine != null) {
+      log.error(
+        "Finished called for the sync before the current engine finished"
+      );
+      this.currentEngine.finished(null);
+      this.onEngineStop(this.currentEngine.name);
+    }
+    if (error) {
+      this.failureReason = SyncTelemetry.transformError(error);
+    }
+
+    this.syncNodeType = lazy.Weave.Service.identity.telemetryNodeType;
+
+    // Check for engine statuses. -- We do this now, and not in engine.finished
+    // to make sure any statuses that get set "late" are recorded
+    for (let engine of this.engines) {
+      let status = lazy.Status.engines[engine.name];
+      if (status && status !== constants.ENGINE_SUCCEEDED) {
+        engine.status = status;
+      }
+    }
+
+    let statusObject = {};
+
+    let serviceStatus = lazy.Status.service;
+    if (serviceStatus && serviceStatus !== constants.STATUS_OK) {
+      statusObject.service = serviceStatus;
+      this.status = statusObject;
+    }
+    let syncStatus = lazy.Status.sync;
+    if (syncStatus && syncStatus !== constants.SYNC_SUCCEEDED) {
+      statusObject.sync = syncStatus;
+      this.status = statusObject;
+    }
+  }
+
+  onEngineStart(engineName) {
+    if (this._shouldIgnoreEngine(engineName, false)) {
+      return;
+    }
+
+    if (this.currentEngine) {
+      log.error(
+        `Being told that engine ${engineName} has started, but current engine ${this.currentEngine.name} hasn't stopped`
+      );
+      // Just discard the current engine rather than making up data for it.
+    }
+    this.currentEngine = new EngineRecord(engineName);
+  }
+
+  onEngineStop(engineName, error) {
+    // We only care if it's the current engine if we have a current engine.
+    if (this._shouldIgnoreEngine(engineName, !!this.currentEngine)) {
+      return;
+    }
+    if (!this.currentEngine) {
+      // It's possible for us to get an error before the start message of an engine
+      // (somehow), in which case we still want to record that error.
+      if (!error) {
+        return;
+      }
+      log.error(
+        `Error triggered on ${engineName} when no current engine exists: ${error}`
+      );
+      this.currentEngine = new EngineRecord(engineName);
+    }
+    this.currentEngine.finished(error);
+    this.engines.push(this.currentEngine);
+    this.currentEngine = null;
+  }
+
+  onEngineApplied(engineName, counts) {
+    if (this._shouldIgnoreEngine(engineName)) {
+      return;
+    }
+    this.currentEngine.recordApplied(counts);
+  }
+
+  onEngineStep(engineName, step) {
+    if (this._shouldIgnoreEngine(engineName)) {
+      return;
+    }
+    this.currentEngine.recordStep(step);
+  }
+
+  onEngineValidated(engineName, validationData) {
+    if (this._shouldIgnoreEngine(engineName, false)) {
+      return;
+    }
+    let engine = this.engines.find(e => e.name === engineName);
+    if (
+      !engine &&
+      this.currentEngine &&
+      engineName === this.currentEngine.name
+    ) {
+      engine = this.currentEngine;
+    }
+    if (engine) {
+      engine.recordValidation(validationData);
+    } else {
+      log.warn(
+        `Validation event triggered for engine ${engineName}, which hasn't been synced!`
+      );
+    }
+  }
+
+  onEngineValidateError(engineName, error) {
+    if (this._shouldIgnoreEngine(engineName, false)) {
+      return;
+    }
+    let engine = this.engines.find(e => e.name === engineName);
+    if (
+      !engine &&
+      this.currentEngine &&
+      engineName === this.currentEngine.name
+    ) {
+      engine = this.currentEngine;
+    }
+    if (engine) {
+      engine.recordValidationError(error);
+    } else {
+      log.warn(
+        `Validation failure event triggered for engine ${engineName}, which hasn't been synced!`
+      );
+    }
+  }
+
+  onEngineUploaded(engineName, counts) {
+    if (this._shouldIgnoreEngine(engineName)) {
+      return;
+    }
+    this.currentEngine.recordUploaded(counts);
+  }
+
+  _shouldIgnoreEngine(engineName, shouldBeCurrent = true) {
+    if (!this.allowedEngines.has(engineName)) {
+      log.info(
+        `Notification for engine ${engineName}, but we aren't recording telemetry for it`
+      );
+      return true;
+    }
+    if (shouldBeCurrent) {
+      if (!this.currentEngine || engineName != this.currentEngine.name) {
+        log.info(`Notification for engine ${engineName} but it isn't current`);
+        return true;
+      }
+    }
+    return false;
+  }
+}
+
+// The entire "sync ping" - it includes all the syncs, events etc recorded in
+// the ping.
+class SyncTelemetryImpl {
+  constructor(allowedEngines) {
+    log.manageLevelFromPref("services.sync.log.logger.telemetry");
+    // This is accessible so we can enable custom engines during tests.
+    this.allowedEngines = allowedEngines;
+    this.current = null;
+    this.setupObservers();
+
+    this.payloads = [];
+    this.discarded = 0;
+    this.events = [];
+    this.histograms = {};
+    this.migrations = [];
+    this.maxEventsCount = lazy.Svc.PrefBranch.getIntPref(
+      "telemetry.maxEventsCount",
+      1000
+    );
+    this.maxPayloadCount = lazy.Svc.PrefBranch.getIntPref(
+      "telemetry.maxPayloadCount"
+    );
+    this.submissionInterval =
+      lazy.Svc.PrefBranch.getIntPref("telemetry.submissionInterval") * 1000;
+    this.lastSubmissionTime = Services.telemetry.msSinceProcessStart();
+    this.lastUID = EMPTY_UID;
+    this.lastSyncNodeType = null;
+    this.currentSyncNodeType = null;
+    // Note that the sessionStartDate is somewhat arbitrary - the telemetry
+    // modules themselves just use `new Date()`. This means that our startDate
+    // isn't going to be the same as the sessionStartDate in the main pings,
+    // but that's OK for now - if it's a problem we'd need to change the
+    // telemetry modules to expose what it thinks the sessionStartDate is.
+    let sessionStartDate = new Date();
+    this.sessionStartDate = lazy.TelemetryUtils.toLocalTimeISOString(
+      lazy.TelemetryUtils.truncateToHours(sessionStartDate)
+    );
+    lazy.TelemetryController.registerSyncPingShutdown(() => this.shutdown());
+  }
+
+  sanitizeFxaDeviceId(deviceId) {
+    return lazy.fxAccounts.telemetry.sanitizeDeviceId(deviceId);
+  }
+
+  prepareFxaDevices(devices) {
+    // For non-sync users, the data per device is limited -- just an id and a
+    // type (and not even the id yet). For sync users, if we can correctly map
+    // the fxaDevice to a sync device, then we can get os and version info,
+    // which would be quite unfortunate to lose.
+    let extraInfoMap = new Map();
+    if (this.syncIsEnabled()) {
+      for (let client of this.getClientsEngineRecords()) {
+        if (client.fxaDeviceId) {
+          extraInfoMap.set(client.fxaDeviceId, {
+            os: client.os,
+            version: client.version,
+            syncID: this.sanitizeFxaDeviceId(client.id),
+          });
+        }
+      }
+    }
+    // Finally, sanitize and convert to the proper format.
+    return devices.map(d => {
+      let { os, version, syncID } = extraInfoMap.get(d.id) || {
+        os: undefined,
+        version: undefined,
+        syncID: undefined,
+      };
+      return {
+        id: this.sanitizeFxaDeviceId(d.id) || EMPTY_UID,
+        type: d.type,
+        os,
+        version,
+        syncID,
+      };
+    });
+  }
+
+  syncIsEnabled() {
+    return lazy.WeaveService.enabled && lazy.WeaveService.ready;
+  }
+
+  // Separate for testing.
+  getClientsEngineRecords() {
+    if (!this.syncIsEnabled()) {
+      throw new Error("Bug: syncIsEnabled() must be true, check it first");
+    }
+    return lazy.Weave.Service.clientsEngine.remoteClients;
+  }
+
+  updateFxaDevices(devices) {
+    if (!devices) {
+      return {};
+    }
+    let me = devices.find(d => d.isCurrentDevice);
+    let id = me ? this.sanitizeFxaDeviceId(me.id) : undefined;
+    let cleanDevices = this.prepareFxaDevices(devices);
+    return { deviceID: id, devices: cleanDevices };
+  }
+
+  getFxaDevices() {
+    return lazy.fxAccounts.device.recentDeviceList;
+  }
+
+  getPingJSON(reason) {
+    let { devices, deviceID } = this.updateFxaDevices(this.getFxaDevices());
+    return {
+      os: lazy.TelemetryEnvironment.currentEnvironment.system.os,
+      why: reason,
+      devices,
+      discarded: this.discarded || undefined,
+      version: PING_FORMAT_VERSION,
+      syncs: this.payloads.slice(),
+      uid: this.lastUID,
+      syncNodeType: this.lastSyncNodeType || undefined,
+      deviceID,
+      sessionStartDate: this.sessionStartDate,
+      events: !this.events.length ? undefined : this.events,
+      migrations: !this.migrations.length ? undefined : this.migrations,
+      histograms: !Object.keys(this.histograms).length
+        ? undefined
+        : this.histograms,
+    };
+  }
+
+  _addMigrationRecord(type, info) {
+    log.debug("Saw telemetry migration info", type, info);
+    // Updates to this need to be documented in `sync-ping.rst`
+    switch (type) {
+      case "webext-storage":
+        this.migrations.push({
+          type: "webext-storage",
+          entries: +info.entries,
+          entriesSuccessful: +info.entries_successful,
+          extensions: +info.extensions,
+          extensionsSuccessful: +info.extensions_successful,
+          openFailure: !!info.open_failure,
+        });
+        break;
+      default:
+        throw new Error("Bug: Unknown migration record type " + type);
+    }
+  }
+
+  finish(reason) {
+    // Note that we might be in the middle of a sync right now, and so we don't
+    // want to touch this.current.
+    let result = this.getPingJSON(reason);
+    this.payloads = [];
+    this.discarded = 0;
+    this.events = [];
+    this.migrations = [];
+    this.histograms = {};
+    this.submit(result);
+  }
+
+  setupObservers() {
+    for (let topic of TOPICS) {
+      lazy.Observers.add(topic, this, this);
+    }
+  }
+
+  shutdown() {
+    this.finish("shutdown");
+    for (let topic of TOPICS) {
+      lazy.Observers.remove(topic, this, this);
+    }
+  }
+
+  submit(record) {
+    if (!this.isProductionSyncUser()) {
+      return false;
+    }
+    // We still call submit() with possibly illegal payloads so that tests can
+    // know that the ping was built. We don't end up submitting them, however.
+    let numEvents = record.events ? record.events.length : 0;
+    let numMigrations = record.migrations ? record.migrations.length : 0;
+    if (record.syncs.length || numEvents || numMigrations) {
+      log.trace(
+        `submitting ${record.syncs.length} sync record(s) and ` +
+          `${numEvents} event(s) to telemetry`
+      );
+      lazy.TelemetryController.submitExternalPing("sync", record, {
+        usePingSender: true,
+      }).catch(err => {
+        log.error("failed to submit ping", err);
+      });
+      return true;
+    }
+    return false;
+  }
+
+  isProductionSyncUser() {
+    // If FxA isn't production then we treat sync as not being production.
+    // Further, there's the deprecated "services.sync.tokenServerURI" pref we
+    // need to consider - fxa doesn't consider that as if that's the only
+    // pref set, they *are* running a production fxa, just not production sync.
+    if (
+      !lazy.FxAccounts.config.isProductionConfig() ||
+      Services.prefs.prefHasUserValue("services.sync.tokenServerURI")
+    ) {
+      log.trace(`Not sending telemetry ping for self-hosted Sync user`);
+      return false;
+    }
+    return true;
+  }
+
+  onSyncStarted(data) {
+    const why = data && JSON.parse(data).why;
+    if (this.current) {
+      log.warn(
+        "Observed weave:service:sync:start, but we're already recording a sync!"
+      );
+      // Just discard the old record, consistent with our handling of engines, above.
+      this.current = null;
+    }
+    this.current = new SyncRecord(this.allowedEngines, why);
+  }
+
+  // We need to ensure that the telemetry `deletion-request` ping always contains the user's
+  // current sync device ID, because if the user opts out of telemetry then the deletion ping
+  // will be immediately triggered for sending, and we won't have a chance to fill it in later.
+  // This keeps the `deletion-ping` up-to-date when the user's account state changes.
+  onAccountInitOrChange() {
+    // We don't submit sync pings for self-hosters, so don't need to collect their device ids either.
+    if (!this.isProductionSyncUser()) {
+      return;
+    }
+    // Awkwardly async, but no need to await. If the user's account state changes while
+    // this promise is in flight, it will reject and we won't record any data in the ping.
+    // (And a new notification will trigger us to try again with the new state).
+    lazy.fxAccounts.device
+      .getLocalId()
+      .then(deviceId => {
+        let sanitizedDeviceId =
+          lazy.fxAccounts.telemetry.sanitizeDeviceId(deviceId);
+        // In the past we did not persist the FxA metrics identifiers to disk,
+        // so this might be missing until we can fetch it from the server for the
+        // first time. There will be a fresh notification tirggered when it's available.
+        if (sanitizedDeviceId) {
+          // Sanitized device ids are 64 characters long, but telemetry limits scalar strings to 50.
+          // The first 32 chars are sufficient to uniquely identify the device, so just send those.
+          // It's hard to change the sync ping itself to only send 32 chars, to b/w compat reasons.
+          sanitizedDeviceId = sanitizedDeviceId.substr(0, 32);
+          Services.telemetry.scalarSet(
+            "deletion.request.sync_device_id",
+            sanitizedDeviceId
+          );
+        }
+      })
+      .catch(err => {
+        log.warn(
+          `Failed to set sync identifiers in the deletion-request ping: ${err}`
+        );
+      });
+  }
+
+  // This keeps the `deletion-request` ping up-to-date when the user signs out,
+  // clearing the now-nonexistent sync device id.
+  onAccountLogout() {
+    Services.telemetry.scalarSet("deletion.request.sync_device_id", "");
+  }
+
+  _checkCurrent(topic) {
+    if (!this.current) {
+      // This is only `info` because it happens when we do a tabs "quick-write"
+      log.info(
+        `Observed notification ${topic} but no current sync is being recorded.`
+      );
+      return false;
+    }
+    return true;
+  }
+
+  _shouldSubmitForDataChange() {
+    let newID = lazy.fxAccounts.telemetry.getSanitizedUID() || EMPTY_UID;
+    let oldID = this.lastUID;
+    if (
+      newID != EMPTY_UID &&
+      oldID != EMPTY_UID &&
+      // Both are "real" uids, so we care if they've changed.
+      newID != oldID
+    ) {
+      log.trace(
+        `shouldSubmitForDataChange - uid from '${oldID}' -> '${newID}'`
+      );
+      return true;
+    }
+    // We've gone from knowing one of the ids to not knowing it (which we
+    // ignore) or we've gone from not knowing it to knowing it (which is fine),
+    // Now check the node type because a change there also means we should
+    // submit.
+    if (
+      this.lastSyncNodeType &&
+      this.currentSyncNodeType != this.lastSyncNodeType
+    ) {
+      log.trace(
+        `shouldSubmitForDataChange - nodeType from '${this.lastSyncNodeType}' -> '${this.currentSyncNodeType}'`
+      );
+      return true;
+    }
+    log.trace("shouldSubmitForDataChange - no need to submit");
+    return false;
+  }
+
+  maybeSubmitForDataChange() {
+    if (this._shouldSubmitForDataChange()) {
+      log.info(
+        "Early submission of sync telemetry due to changed IDs/NodeType"
+      );
+      this.finish("idchange"); // this actually submits.
+      this.lastSubmissionTime = Services.telemetry.msSinceProcessStart();
+    }
+
+    // Only update the last UIDs if we actually know them.
+    let current_uid = lazy.fxAccounts.telemetry.getSanitizedUID();
+    if (current_uid) {
+      this.lastUID = current_uid;
+    }
+    if (this.currentSyncNodeType) {
+      this.lastSyncNodeType = this.currentSyncNodeType;
+    }
+  }
+
+  maybeSubmitForInterval() {
+    // We want to submit the ping every `this.submissionInterval` but only when
+    // there's no current sync in progress, otherwise we may end up submitting
+    // the sync and the events caused by it in different pings.
+    if (
+      this.current == null &&
+      Services.telemetry.msSinceProcessStart() - this.lastSubmissionTime >
+        this.submissionInterval
+    ) {
+      this.finish("schedule");
+      this.lastSubmissionTime = Services.telemetry.msSinceProcessStart();
+    }
+  }
+
+  onSyncFinished(error) {
+    if (!this.current) {
+      log.warn("onSyncFinished but we aren't recording");
+      return;
+    }
+    this.current.finished(error);
+    this.currentSyncNodeType = this.current.syncNodeType;
+    let current = this.current;
+    this.current = null;
+    this.takeTelemetryRecord(current);
+  }
+
+  takeTelemetryRecord(record) {
+    // We check for "data change" before appending the current sync to payloads,
+    // as it is the current sync which has the data with the new data, and thus
+    // must go in the *next* submission.
+    this.maybeSubmitForDataChange();
+    if (this.payloads.length < this.maxPayloadCount) {
+      this.payloads.push(record.toJSON());
+    } else {
+      ++this.discarded;
+    }
+    // If we are submitting due to timing, it's desirable that the most recent
+    // sync is included, so we check after appending the record.
+    this.maybeSubmitForInterval();
+  }
+
+  _addHistogram(hist) {
+    let histogram = Services.telemetry.getHistogramById(hist);
+    let s = histogram.snapshot();
+    this.histograms[hist] = s;
+  }
+
+  _recordEvent(eventDetails) {
+    this.maybeSubmitForDataChange();
+
+    if (this.events.length >= this.maxEventsCount) {
+      log.warn("discarding event - already queued our maximum", eventDetails);
+      return;
+    }
+
+    let { object, method, value, extra } = eventDetails;
+    if (extra) {
+      extra = normalizeExtraTelemetryFields(extra);
+      eventDetails = { object, method, value, extra };
+    }
+
+    if (!validateTelemetryEvent(eventDetails)) {
+      // we've already logged what the problem is...
+      return;
+    }
+    log.debug("recording event", eventDetails);
+
+    if (extra && lazy.Resource.serverTime && !extra.serverTime) {
+      extra.serverTime = String(lazy.Resource.serverTime);
+    }
+    let category = "sync";
+    let ts = Math.floor(tryGetMonotonicTimestamp());
+
+    // An event record is a simple array with at least 4 items.
+    let event = [ts, category, method, object];
+    // It may have up to 6 elements if |extra| is defined
+    if (value) {
+      event.push(value);
+      if (extra) {
+        event.push(extra);
+      }
+    } else if (extra) {
+      event.push(null); // a null for the empty value.
+      event.push(extra);
+    }
+    this.events.push(event);
+    this.maybeSubmitForInterval();
+  }
+
+  observe(subject, topic, data) {
+    log.trace(`observed ${topic} ${data}`);
+
+    switch (topic) {
+      case "weave:service:ready":
+      case "weave:service:login:got-hashed-id":
+      case "fxaccounts:new_device_id":
+        this.onAccountInitOrChange();
+        break;
+
+      case "fxaccounts:onlogout":
+        this.onAccountLogout();
+        break;
+
+      /* sync itself state changes */
+      case "weave:service:sync:start":
+        this.onSyncStarted(data);
+        break;
+
+      case "weave:service:sync:finish":
+        if (this._checkCurrent(topic)) {
+          this.onSyncFinished(null);
+        }
+        break;
+
+      case "weave:service:sync:error":
+        // argument needs to be truthy (this should always be the case)
+        this.onSyncFinished(subject || "Unknown");
+        break;
+
+      /* engine sync state changes */
+      case "weave:engine:sync:start":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineStart(data);
+        }
+        break;
+      case "weave:engine:sync:finish":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineStop(data, null);
+        }
+        break;
+
+      case "weave:engine:sync:error":
+        if (this._checkCurrent(topic)) {
+          // argument needs to be truthy (this should always be the case)
+          this.current.onEngineStop(data, subject || "Unknown");
+        }
+        break;
+
+      /* engine counts */
+      case "weave:engine:sync:applied":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineApplied(data, subject);
+        }
+        break;
+
+      case "weave:engine:sync:step":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineStep(data, subject);
+        }
+        break;
+
+      case "weave:engine:sync:uploaded":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineUploaded(data, subject);
+        }
+        break;
+
+      case "weave:engine:validate:finish":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineValidated(data, subject);
+        }
+        break;
+
+      case "weave:engine:validate:error":
+        if (this._checkCurrent(topic)) {
+          this.current.onEngineValidateError(data, subject || "Unknown");
+        }
+        break;
+
+      case "weave:telemetry:event":
+      case "fxa:telemetry:event":
+        this._recordEvent(subject);
+        break;
+
+      case "weave:telemetry:histogram":
+        this._addHistogram(data);
+        break;
+
+      case "weave:telemetry:migration":
+        this._addMigrationRecord(data, subject);
+        break;
+
+      default:
+        log.warn(`unexpected observer topic ${topic}`);
+        break;
+    }
+  }
+
+  // Transform an exception into a standard description. Exposed here for when
+  // this module isn't directly responsible for knowing the transform should
+  // happen (for example, when including an error in the |extra| field of
+  // event telemetry)
+  transformError(error) {
+    // Certain parts of sync will use this pattern as a way to communicate to
+    // processIncoming to abort the processing. However, there's no guarantee
+    // this can only happen then.
+    if (typeof error == "object" && error.code && error.cause) {
+      error = error.cause;
+    }
+    if (lazy.Async.isShutdownException(error)) {
+      return { name: "shutdownerror" };
+    }
+
+    if (typeof error === "string") {
+      if (error.startsWith("error.")) {
+        // This is hacky, but I can't imagine that it's not also accurate.
+        return { name: "othererror", error };
+      }
+      error = ErrorSanitizer.cleanErrorMessage(error);
+      return { name: "unexpectederror", error };
+    }
+
+    if (error instanceof lazy.AuthenticationError) {
+      return { name: "autherror", from: error.source };
+    }
+
+    if (DOMException.isInstance(error)) {
+      return {
+        name: "unexpectederror",
+        error: ErrorSanitizer.cleanErrorMessage(error.message, error),
+      };
+    }
+
+    let httpCode =
+      error.status || (error.response && error.response.status) || error.code;
+
+    if (httpCode) {
+      return { name: "httperror", code: httpCode };
+    }
+
+    if (error.failureCode) {
+      return { name: "othererror", error: error.failureCode };
+    }
+
+    if (error.result) {
+      // many "nsresult" errors are actually network errors - if they are
+      // associated with the "network" module we assume that's true.
+      // We also assume NS_ERROR_ABORT is such an error - for almost everything
+      // we care about, it acually is (eg, if the connection fails early enough
+      // or if we have a captive portal etc) - we don't lose anything by this
+      // assumption, it's just that the error will no longer be in the "nserror"
+      // category, so our analysis can still find them.
+      if (
+        error.result == Cr.NS_ERROR_ABORT ||
+        NS_ERROR_GET_MODULE(error.result) == NS_ERROR_MODULE_NETWORK
+      ) {
+        return { name: "httperror", code: error.result };
+      }
+      return { name: "nserror", code: error.result };
+    }
+    // It's probably an Error object, but it also could be some
+    // other object that may or may not override toString to do
+    // something useful.
+    let msg = String(error);
+    if (msg.startsWith("[object")) {
+      // Nothing useful in the default, check for a string "message" property.
+      if (typeof error.message == "string") {
+        msg = String(error.message);
+      } else {
+        // Hopefully it won't come to this...
+        msg = JSON.stringify(error);
+      }
+    }
+    return {
+      name: "unexpectederror",
+      error: ErrorSanitizer.cleanErrorMessage(msg),
+    };
+  }
+}
+
+export var SyncTelemetry = new SyncTelemetryImpl(ENGINES);
diff --git a/services/sync/modules/util.sys.mjs b/services/sync/modules/util.sys.mjs
new file mode 100644
index 0000000000..e68ae0e19f
--- /dev/null
+++ b/services/sync/modules/util.sys.mjs
@@ -0,0 +1,780 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { Observers } from "resource://services-common/observers.sys.mjs";
+
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+import { CryptoUtils } from "resource://services-crypto/utils.sys.mjs";
+
+import {
+  DEVICE_TYPE_DESKTOP,
+  MAXIMUM_BACKOFF_INTERVAL,
+  PREFS_BRANCH,
+  SYNC_KEY_DECODED_LENGTH,
+  SYNC_KEY_ENCODED_LENGTH,
+  WEAVE_VERSION,
+} from "resource://services-sync/constants.sys.mjs";
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+import * as FxAccountsCommon from "resource://gre/modules/FxAccountsCommon.sys.mjs";
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "cryptoSDR",
+  "@mozilla.org/login-manager/crypto/SDR;1",
+  "nsILoginManagerCrypto"
+);
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "localDeviceType",
+  "services.sync.client.type",
+  DEVICE_TYPE_DESKTOP
+);
+
+/*
+ * Custom exception types.
+ */
+class LockException extends Error {
+  constructor(message) {
+    super(message);
+    this.name = "LockException";
+  }
+}
+
+class HMACMismatch extends Error {
+  constructor(message) {
+    super(message);
+    this.name = "HMACMismatch";
+  }
+}
+
+/*
+ * Utility functions
+ */
+export var Utils = {
+  // Aliases from CryptoUtils.
+  generateRandomBytesLegacy: CryptoUtils.generateRandomBytesLegacy,
+  computeHTTPMACSHA1: CryptoUtils.computeHTTPMACSHA1,
+  digestUTF8: CryptoUtils.digestUTF8,
+  digestBytes: CryptoUtils.digestBytes,
+  sha256: CryptoUtils.sha256,
+  hkdfExpand: CryptoUtils.hkdfExpand,
+  pbkdf2Generate: CryptoUtils.pbkdf2Generate,
+  getHTTPMACSHA1Header: CryptoUtils.getHTTPMACSHA1Header,
+
+  /**
+   * The string to use as the base User-Agent in Sync requests.
+   * This string will look something like
+   *
+   *   Firefox/49.0a1 (Windows NT 6.1; WOW64; rv:46.0) FxSync/1.51.0.20160516142357.desktop
+   */
+  _userAgent: null,
+  get userAgent() {
+    if (!this._userAgent) {
+      let hph = Cc["@mozilla.org/network/protocol;1?name=http"].getService(
+        Ci.nsIHttpProtocolHandler
+      );
+      /* eslint-disable no-multi-spaces */
+      this._userAgent =
+        Services.appinfo.name +
+        "/" +
+        Services.appinfo.version + // Product.
+        " (" +
+        hph.oscpu +
+        ")" + // (oscpu)
+        " FxSync/" +
+        WEAVE_VERSION +
+        "." + // Sync.
+        Services.appinfo.appBuildID +
+        "."; // Build.
+      /* eslint-enable no-multi-spaces */
+    }
+    return this._userAgent + lazy.localDeviceType;
+  },
+
+  /**
+   * Wrap a [promise-returning] function to catch all exceptions and log them.
+   *
+   * @usage MyObj._catch = Utils.catch;
+   *        MyObj.foo = function() { this._catch(func)(); }
+   *
+   * Optionally pass a function which will be called if an
+   * exception occurs.
+   */
+  catch(func, exceptionCallback) {
+    let thisArg = this;
+    return async function WrappedCatch() {
+      try {
+        return await func.call(thisArg);
+      } catch (ex) {
+        thisArg._log.debug(
+          "Exception calling " + (func.name || "anonymous function"),
+          ex
+        );
+        if (exceptionCallback) {
+          return exceptionCallback.call(thisArg, ex);
+        }
+        return null;
+      }
+    };
+  },
+
+  throwLockException(label) {
+    throw new LockException(`Could not acquire lock. Label: "${label}".`);
+  },
+
+  /**
+   * Wrap a [promise-returning] function to call lock before calling the function
+   * then unlock when it finishes executing or if it threw an error.
+   *
+   * @usage MyObj._lock = Utils.lock;
+   *        MyObj.foo = async function() { await this._lock(func)(); }
+   */
+  lock(label, func) {
+    let thisArg = this;
+    return async function WrappedLock() {
+      if (!thisArg.lock()) {
+        Utils.throwLockException(label);
+      }
+
+      try {
+        return await func.call(thisArg);
+      } finally {
+        thisArg.unlock();
+      }
+    };
+  },
+
+  isLockException: function isLockException(ex) {
+    return ex instanceof LockException;
+  },
+
+  /**
+   * Wrap [promise-returning] functions to notify when it starts and
+   * finishes executing or if it threw an error.
+   *
+   * The message is a combination of a provided prefix, the local name, and
+   * the event. Possible events are: "start", "finish", "error". The subject
+   * is the function's return value on "finish" or the caught exception on
+   * "error". The data argument is the predefined data value.
+   *
+   * Example:
+   *
+   * @usage function MyObj(name) {
+   *          this.name = name;
+   *          this._notify = Utils.notify("obj:");
+   *        }
+   *        MyObj.prototype = {
+   *          foo: function() this._notify("func", "data-arg", async function () {
+   *            //...
+   *          }(),
+   *        };
+   */
+  notify(prefix) {
+    return function NotifyMaker(name, data, func) {
+      let thisArg = this;
+      let notify = function (state, subject) {
+        let mesg = prefix + name + ":" + state;
+        thisArg._log.trace("Event: " + mesg);
+        Observers.notify(mesg, subject, data);
+      };
+
+      return async function WrappedNotify() {
+        notify("start", null);
+        try {
+          let ret = await func.call(thisArg);
+          notify("finish", ret);
+          return ret;
+        } catch (ex) {
+          notify("error", ex);
+          throw ex;
+        }
+      };
+    };
+  },
+
+  /**
+   * GUIDs are 9 random bytes encoded with base64url (RFC 4648).
+   * That makes them 12 characters long with 72 bits of entropy.
+   */
+  makeGUID: function makeGUID() {
+    return CommonUtils.encodeBase64URL(Utils.generateRandomBytesLegacy(9));
+  },
+
+  _base64url_regex: /^[-abcdefghijklmnopqrstuvwxyz0123456789_]{12}$/i,
+  checkGUID: function checkGUID(guid) {
+    return !!guid && this._base64url_regex.test(guid);
+  },
+
+  /**
+   * Add a simple getter/setter to an object that defers access of a property
+   * to an inner property.
+   *
+   * @param obj
+   *        Object to add properties to defer in its prototype
+   * @param defer
+   *        Property of obj to defer to
+   * @param prop
+   *        Property name to defer (or an array of property names)
+   */
+  deferGetSet: function Utils_deferGetSet(obj, defer, prop) {
+    if (Array.isArray(prop)) {
+      return prop.map(prop => Utils.deferGetSet(obj, defer, prop));
+    }
+
+    let prot = obj.prototype;
+
+    // Create a getter if it doesn't exist yet
+    if (!prot.__lookupGetter__(prop)) {
+      prot.__defineGetter__(prop, function () {
+        return this[defer][prop];
+      });
+    }
+
+    // Create a setter if it doesn't exist yet
+    if (!prot.__lookupSetter__(prop)) {
+      prot.__defineSetter__(prop, function (val) {
+        this[defer][prop] = val;
+      });
+    }
+  },
+
+  deepEquals: function eq(a, b) {
+    // If they're triple equals, then it must be equals!
+    if (a === b) {
+      return true;
+    }
+
+    // If they weren't equal, they must be objects to be different
+    if (typeof a != "object" || typeof b != "object") {
+      return false;
+    }
+
+    // But null objects won't have properties to compare
+    if (a === null || b === null) {
+      return false;
+    }
+
+    // Make sure all of a's keys have a matching value in b
+    for (let k in a) {
+      if (!eq(a[k], b[k])) {
+        return false;
+      }
+    }
+
+    // Do the same for b's keys but skip those that we already checked
+    for (let k in b) {
+      if (!(k in a) && !eq(a[k], b[k])) {
+        return false;
+      }
+    }
+
+    return true;
+  },
+
+  // Generator and discriminator for HMAC exceptions.
+  // Split these out in case we want to make them richer in future, and to
+  // avoid inevitable confusion if the message changes.
+  throwHMACMismatch: function throwHMACMismatch(shouldBe, is) {
+    throw new HMACMismatch(
+      `Record SHA256 HMAC mismatch: should be ${shouldBe}, is ${is}`
+    );
+  },
+
+  isHMACMismatch: function isHMACMismatch(ex) {
+    return ex instanceof HMACMismatch;
+  },
+
+  /**
+   * Turn RFC 4648 base32 into our own user-friendly version.
+   *   ABCDEFGHIJKLMNOPQRSTUVWXYZ234567
+   * becomes
+   *   abcdefghijk8mn9pqrstuvwxyz234567
+   */
+  base32ToFriendly: function base32ToFriendly(input) {
+    return input.toLowerCase().replace(/l/g, "8").replace(/o/g, "9");
+  },
+
+  base32FromFriendly: function base32FromFriendly(input) {
+    return input.toUpperCase().replace(/8/g, "L").replace(/9/g, "O");
+  },
+
+  /**
+   * Key manipulation.
+   */
+
+  // Return an octet string in friendly base32 *with no trailing =*.
+  encodeKeyBase32: function encodeKeyBase32(keyData) {
+    return Utils.base32ToFriendly(CommonUtils.encodeBase32(keyData)).slice(
+      0,
+      SYNC_KEY_ENCODED_LENGTH
+    );
+  },
+
+  decodeKeyBase32: function decodeKeyBase32(encoded) {
+    return CommonUtils.decodeBase32(
+      Utils.base32FromFriendly(Utils.normalizePassphrase(encoded))
+    ).slice(0, SYNC_KEY_DECODED_LENGTH);
+  },
+
+  jsonFilePath(...args) {
+    let [fileName] = args.splice(-1);
+
+    return PathUtils.join(
+      PathUtils.profileDir,
+      "weave",
+      ...args,
+      `${fileName}.json`
+    );
+  },
+
+  /**
+   * Load a JSON file from disk in the profile directory.
+   *
+   * @param filePath
+   *        JSON file path load from profile. Loaded file will be
+   *        extension.
+   * @param that
+   *        Object to use for logging.
+   *
+   * @return Promise<>
+   *        Promise resolved when the write has been performed.
+   */
+  async jsonLoad(filePath, that) {
+    let path;
+    if (Array.isArray(filePath)) {
+      path = Utils.jsonFilePath(...filePath);
+    } else {
+      path = Utils.jsonFilePath(filePath);
+    }
+
+    if (that._log && that._log.trace) {
+      that._log.trace("Loading json from disk: " + path);
+    }
+
+    try {
+      return await IOUtils.readJSON(path);
+    } catch (e) {
+      if (!DOMException.isInstance(e) || e.name !== "NotFoundError") {
+        if (that._log) {
+          that._log.debug("Failed to load json", e);
+        }
+      }
+      return null;
+    }
+  },
+
+  /**
+   * Save a json-able object to disk in the profile directory.
+   *
+   * @param filePath
+   *        JSON file path save to <filePath>.json
+   * @param that
+   *        Object to use for logging.
+   * @param obj
+   *        Function to provide json-able object to save. If this isn't a
+   *        function, it'll be used as the object to make a json string.*
+   *        Function called when the write has been performed. Optional.
+   *
+   * @return Promise<>
+   *        Promise resolved when the write has been performed.
+   */
+  async jsonSave(filePath, that, obj) {
+    let path = PathUtils.join(
+      PathUtils.profileDir,
+      "weave",
+      ...(filePath + ".json").split("/")
+    );
+    let dir = PathUtils.parent(path);
+
+    await IOUtils.makeDirectory(dir, { createAncestors: true });
+
+    if (that._log) {
+      that._log.trace("Saving json to disk: " + path);
+    }
+
+    let json = typeof obj == "function" ? obj.call(that) : obj;
+
+    return IOUtils.writeJSON(path, json);
+  },
+
+  /**
+   * Helper utility function to fit an array of records so that when serialized,
+   * they will be within payloadSizeMaxBytes. Returns a new array without the
+   * items.
+   *
+   * Note: This shouldn't be used for extremely large record sizes as
+   * it uses JSON.stringify, which could lead to a heavy performance hit.
+   * See Bug 1815151 for more details.
+   *
+   */
+  tryFitItems(records, payloadSizeMaxBytes) {
+    // Copy this so that callers don't have to do it in advance.
+    records = records.slice();
+    let encoder = Utils.utf8Encoder;
+    const computeSerializedSize = () =>
+      encoder.encode(JSON.stringify(records)).byteLength;
+    // Figure out how many records we can pack into a payload.
+    // We use byteLength here because the data is not encrypted in ascii yet.
+    let size = computeSerializedSize();
+    // See bug 535326 comment 8 for an explanation of the estimation
+    const maxSerializedSize = (payloadSizeMaxBytes / 4) * 3 - 1500;
+    if (maxSerializedSize < 0) {
+      // This is probably due to a test, but it causes very bad behavior if a
+      // test causes this accidentally. We could throw, but there's an obvious/
+      // natural way to handle it, so we do that instead (otherwise we'd have a
+      // weird lower bound of ~1125b on the max record payload size).
+      return [];
+    }
+    if (size > maxSerializedSize) {
+      // Estimate a little more than the direct fraction to maximize packing
+      let cutoff = Math.ceil((records.length * maxSerializedSize) / size);
+      records = records.slice(0, cutoff + 1);
+
+      // Keep dropping off the last entry until the data fits.
+      while (computeSerializedSize() > maxSerializedSize) {
+        records.pop();
+      }
+    }
+    return records;
+  },
+
+  /**
+   * Move a json file in the profile directory. Will fail if a file exists at the
+   * destination.
+   *
+   * @returns a promise that resolves to undefined on success, or rejects on failure
+   *
+   * @param aFrom
+   *        Current path to the JSON file saved on disk, relative to profileDir/weave
+   *        .json will be appended to the file name.
+   * @param aTo
+   *        New path to the JSON file saved on disk, relative to profileDir/weave
+   *        .json will be appended to the file name.
+   * @param that
+   *        Object to use for logging
+   */
+  jsonMove(aFrom, aTo, that) {
+    let pathFrom = PathUtils.join(
+      PathUtils.profileDir,
+      "weave",
+      ...(aFrom + ".json").split("/")
+    );
+    let pathTo = PathUtils.join(
+      PathUtils.profileDir,
+      "weave",
+      ...(aTo + ".json").split("/")
+    );
+    if (that._log) {
+      that._log.trace("Moving " + pathFrom + " to " + pathTo);
+    }
+    return IOUtils.move(pathFrom, pathTo, { noOverwrite: true });
+  },
+
+  /**
+   * Removes a json file in the profile directory.
+   *
+   * @returns a promise that resolves to undefined on success, or rejects on failure
+   *
+   * @param filePath
+   *        Current path to the JSON file saved on disk, relative to profileDir/weave
+   *        .json will be appended to the file name.
+   * @param that
+   *        Object to use for logging
+   */
+  jsonRemove(filePath, that) {
+    let path = PathUtils.join(
+      PathUtils.profileDir,
+      "weave",
+      ...(filePath + ".json").split("/")
+    );
+    if (that._log) {
+      that._log.trace("Deleting " + path);
+    }
+    return IOUtils.remove(path, { ignoreAbsent: true });
+  },
+
+  /**
+   * The following are the methods supported for UI use:
+   *
+   * * isPassphrase:
+   *     determines whether a string is either a normalized or presentable
+   *     passphrase.
+   * * normalizePassphrase:
+   *     take a presentable passphrase and reduce it to a normalized
+   *     representation for storage. normalizePassphrase can safely be called
+   *     on normalized input.
+   */
+
+  isPassphrase(s) {
+    if (s) {
+      return /^[abcdefghijkmnpqrstuvwxyz23456789]{26}$/.test(
+        Utils.normalizePassphrase(s)
+      );
+    }
+    return false;
+  },
+
+  normalizePassphrase: function normalizePassphrase(pp) {
+    // Short var name... have you seen the lines below?!
+    // Allow leading and trailing whitespace.
+    pp = pp.trim().toLowerCase();
+
+    // 20-char sync key.
+    if (pp.length == 23 && [5, 11, 17].every(i => pp[i] == "-")) {
+      return (
+        pp.slice(0, 5) + pp.slice(6, 11) + pp.slice(12, 17) + pp.slice(18, 23)
+      );
+    }
+
+    // "Modern" 26-char key.
+    if (pp.length == 31 && [1, 7, 13, 19, 25].every(i => pp[i] == "-")) {
+      return (
+        pp.slice(0, 1) +
+        pp.slice(2, 7) +
+        pp.slice(8, 13) +
+        pp.slice(14, 19) +
+        pp.slice(20, 25) +
+        pp.slice(26, 31)
+      );
+    }
+
+    // Something else -- just return.
+    return pp;
+  },
+
+  /**
+   * Create an array like the first but without elements of the second. Reuse
+   * arrays if possible.
+   */
+  arraySub: function arraySub(minuend, subtrahend) {
+    if (!minuend.length || !subtrahend.length) {
+      return minuend;
+    }
+    let setSubtrahend = new Set(subtrahend);
+    return minuend.filter(i => !setSubtrahend.has(i));
+  },
+
+  /**
+   * Build the union of two arrays. Reuse arrays if possible.
+   */
+  arrayUnion: function arrayUnion(foo, bar) {
+    if (!foo.length) {
+      return bar;
+    }
+    if (!bar.length) {
+      return foo;
+    }
+    return foo.concat(Utils.arraySub(bar, foo));
+  },
+
+  /**
+   * Add all the items in `items` to the provided Set in-place.
+   *
+   * @return The provided set.
+   */
+  setAddAll(set, items) {
+    for (let item of items) {
+      set.add(item);
+    }
+    return set;
+  },
+
+  /**
+   * Delete every items in `items` to the provided Set in-place.
+   *
+   * @return The provided set.
+   */
+  setDeleteAll(set, items) {
+    for (let item of items) {
+      set.delete(item);
+    }
+    return set;
+  },
+
+  /**
+   * Take the first `size` items from the Set `items`.
+   *
+   * @return A Set of size at most `size`
+   */
+  subsetOfSize(items, size) {
+    let result = new Set();
+    let count = 0;
+    for (let item of items) {
+      if (count++ == size) {
+        return result;
+      }
+      result.add(item);
+    }
+    return result;
+  },
+
+  bind2: function Async_bind2(object, method) {
+    return function innerBind() {
+      return method.apply(object, arguments);
+    };
+  },
+
+  /**
+   * Is there a master password configured and currently locked?
+   */
+  mpLocked() {
+    return !lazy.cryptoSDR.isLoggedIn;
+  },
+
+  // If Master Password is enabled and locked, present a dialog to unlock it.
+  // Return whether the system is unlocked.
+  ensureMPUnlocked() {
+    if (lazy.cryptoSDR.uiBusy) {
+      return false;
+    }
+    try {
+      lazy.cryptoSDR.encrypt("bacon");
+      return true;
+    } catch (e) {}
+    return false;
+  },
+
+  /**
+   * Return a value for a backoff interval.  Maximum is eight hours, unless
+   * Status.backoffInterval is higher.
+   *
+   */
+  calculateBackoff: function calculateBackoff(
+    attempts,
+    baseInterval,
+    statusInterval
+  ) {
+    let backoffInterval =
+      attempts * (Math.floor(Math.random() * baseInterval) + baseInterval);
+    return Math.max(
+      Math.min(backoffInterval, MAXIMUM_BACKOFF_INTERVAL),
+      statusInterval
+    );
+  },
+
+  /**
+   * Return a set of hostnames (including the protocol) which may have
+   * credentials for sync itself stored in the login manager.
+   *
+   * In general, these hosts will not have their passwords synced, will be
+   * reset when we drop sync credentials, etc.
+   */
+  getSyncCredentialsHosts() {
+    let result = new Set();
+    // the FxA host
+    result.add(FxAccountsCommon.FXA_PWDMGR_HOST);
+    // We used to include the FxA hosts (hence the Set() result) but we now
+    // don't give them special treatment (hence the Set() with exactly 1 item)
+    return result;
+  },
+
+  /**
+   * Helper to implement a more efficient version of fairly common pattern:
+   *
+   * Utils.defineLazyIDProperty(this, "syncID", "services.sync.client.syncID")
+   *
+   * is equivalent to (but more efficient than) the following:
+   *
+   * Foo.prototype = {
+   *   ...
+   *   get syncID() {
+   *     let syncID = Svc.PrefBranch.getStringPref("client.syncID", "");
+   *     return syncID == "" ? this.syncID = Utils.makeGUID() : syncID;
+   *   },
+   *   set syncID(value) {
+   *     Svc.PrefBranch.setStringPref("client.syncID", value);
+   *   },
+   *   ...
+   * };
+   */
+  defineLazyIDProperty(object, propName, prefName) {
+    // An object that exists to be the target of the lazy pref getter.
+    // We can't use `object` (at least, not using `propName`) since XPCOMUtils
+    // will stomp on any setter we define.
+    const storage = {};
+    XPCOMUtils.defineLazyPreferenceGetter(storage, "value", prefName, "");
+    Object.defineProperty(object, propName, {
+      configurable: true,
+      enumerable: true,
+      get() {
+        let value = storage.value;
+        if (!value) {
+          value = Utils.makeGUID();
+          Services.prefs.setStringPref(prefName, value);
+        }
+        return value;
+      },
+      set(value) {
+        Services.prefs.setStringPref(prefName, value);
+      },
+    });
+  },
+
+  getDeviceType() {
+    return lazy.localDeviceType;
+  },
+
+  formatTimestamp(date) {
+    // Format timestamp as: "%Y-%m-%d %H:%M:%S"
+    let year = String(date.getFullYear());
+    let month = String(date.getMonth() + 1).padStart(2, "0");
+    let day = String(date.getDate()).padStart(2, "0");
+    let hours = String(date.getHours()).padStart(2, "0");
+    let minutes = String(date.getMinutes()).padStart(2, "0");
+    let seconds = String(date.getSeconds()).padStart(2, "0");
+
+    return `${year}-${month}-${day} ${hours}:${minutes}:${seconds}`;
+  },
+
+  *walkTree(tree, parent = null) {
+    if (tree) {
+      // Skip root node
+      if (parent) {
+        yield [tree, parent];
+      }
+      if (tree.children) {
+        for (let child of tree.children) {
+          yield* Utils.walkTree(child, tree);
+        }
+      }
+    }
+  },
+};
+
+/**
+ * A subclass of Set that serializes as an Array when passed to JSON.stringify.
+ */
+export class SerializableSet extends Set {
+  toJSON() {
+    return Array.from(this);
+  }
+}
+
+ChromeUtils.defineLazyGetter(Utils, "_utf8Converter", function () {
+  let converter = Cc[
+    "@mozilla.org/intl/scriptableunicodeconverter"
+  ].createInstance(Ci.nsIScriptableUnicodeConverter);
+  converter.charset = "UTF-8";
+  return converter;
+});
+
+ChromeUtils.defineLazyGetter(Utils, "utf8Encoder", () => new TextEncoder());
+
+/*
+ * Commonly-used services
+ */
+export var Svc = {};
+
+Svc.PrefBranch = Services.prefs.getBranch(PREFS_BRANCH);
+Svc.Obs = Observers;
+
+Svc.Obs.add("xpcom-shutdown", function () {
+  for (let name in Svc) {
+    delete Svc[name];
+  }
+});
diff --git a/services/sync/moz.build b/services/sync/moz.build
new file mode 100644
index 0000000000..a00472056e
--- /dev/null
+++ b/services/sync/moz.build
@@ -0,0 +1,72 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Firefox", "Sync")
+
+XPCSHELL_TESTS_MANIFESTS += ["tests/unit/xpcshell.toml"]
+
+EXTRA_COMPONENTS += [
+    "SyncComponents.manifest",
+]
+
+EXTRA_JS_MODULES["services-sync"] += [
+    "modules/addonsreconciler.sys.mjs",
+    "modules/addonutils.sys.mjs",
+    "modules/bridged_engine.sys.mjs",
+    "modules/collection_validator.sys.mjs",
+    "modules/constants.sys.mjs",
+    "modules/doctor.sys.mjs",
+    "modules/engines.sys.mjs",
+    "modules/keys.sys.mjs",
+    "modules/main.sys.mjs",
+    "modules/policies.sys.mjs",
+    "modules/record.sys.mjs",
+    "modules/resource.sys.mjs",
+    "modules/service.sys.mjs",
+    "modules/status.sys.mjs",
+    "modules/sync_auth.sys.mjs",
+    "modules/SyncDisconnect.sys.mjs",
+    "modules/SyncedTabs.sys.mjs",
+    "modules/telemetry.sys.mjs",
+    "modules/UIState.sys.mjs",
+    "modules/util.sys.mjs",
+    "Weave.sys.mjs",
+]
+
+EXTRA_JS_MODULES["services-sync"].engines += [
+    "modules/engines/addons.sys.mjs",
+    "modules/engines/clients.sys.mjs",
+    "modules/engines/extension-storage.sys.mjs",
+    "modules/engines/passwords.sys.mjs",
+    "modules/engines/prefs.sys.mjs",
+]
+
+if not CONFIG["MOZ_THUNDERBIRD"]:
+    EXTRA_JS_MODULES["services-sync"].engines += [
+        "modules/engines/bookmarks.sys.mjs",
+        "modules/engines/forms.sys.mjs",
+        "modules/engines/history.sys.mjs",
+        "modules/engines/tabs.sys.mjs",
+    ]
+
+EXTRA_JS_MODULES["services-sync"].stages += [
+    "modules/stages/declined.sys.mjs",
+    "modules/stages/enginesync.sys.mjs",
+]
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+TESTING_JS_MODULES.services.sync += [
+    "modules-testing/fakeservices.sys.mjs",
+    "modules-testing/fxa_utils.sys.mjs",
+    "modules-testing/rotaryengine.sys.mjs",
+    "modules-testing/utils.sys.mjs",
+]
+
+SPHINX_TREES["/services/sync"] = "docs"
diff --git a/services/sync/tests/tps/.eslintrc.js b/services/sync/tests/tps/.eslintrc.js
new file mode 100644
index 0000000000..182e87933b
--- /dev/null
+++ b/services/sync/tests/tps/.eslintrc.js
@@ -0,0 +1,28 @@
+"use strict";
+
+module.exports = {
+  globals: {
+    // Injected into tests via tps.jsm
+    Addons: false,
+    Addresses: false,
+    Bookmarks: false,
+    CreditCards: false,
+    EnableEngines: false,
+    EnsureTracking: false,
+    ExtStorage: false,
+    Formdata: false,
+    History: false,
+    Login: false,
+    Passwords: false,
+    Phase: false,
+    Prefs: false,
+    STATE_DISABLED: false,
+    STATE_ENABLED: false,
+    Sync: false,
+    SYNC_WIPE_CLIENT: false,
+    SYNC_WIPE_REMOTE: false,
+    Tabs: false,
+    Windows: false,
+    WipeServer: false,
+  },
+};
diff --git a/services/sync/tests/tps/addons/api/restartless-xpi@tests.mozilla.org.json b/services/sync/tests/tps/addons/api/restartless-xpi@tests.mozilla.org.json
new file mode 100644
index 0000000000..8593bad089
--- /dev/null
+++ b/services/sync/tests/tps/addons/api/restartless-xpi@tests.mozilla.org.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Restartless Test XPI",
+      "type": "extension",
+      "guid": "restartless-xpi@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 485,
+            "url": "http://127.0.0.1:4567/addons/restartless.xpi"
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/tps/addons/api/test-webext@quality.mozilla.org.json b/services/sync/tests/tps/addons/api/test-webext@quality.mozilla.org.json
new file mode 100644
index 0000000000..298ecc2ead
--- /dev/null
+++ b/services/sync/tests/tps/addons/api/test-webext@quality.mozilla.org.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Tet Webext XPI",
+      "type": "extension",
+      "guid": "test-webext@quality.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 3412,
+            "url": "http://127.0.0.1:4567/addons/webextension.xpi"
+          }
+        ]
+      },
+      "last_updated": "2018-04-17T18:24:42Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/tps/addons/restartless.xpi b/services/sync/tests/tps/addons/restartless.xpi
new file mode 100644
index 0000000000..973bc00cb5
--- /dev/null
+++ b/services/sync/tests/tps/addons/restartless.xpi
diff --git a/services/sync/tests/tps/addons/webextension.xpi b/services/sync/tests/tps/addons/webextension.xpi
new file mode 100644
index 0000000000..0ed64f79ac
--- /dev/null
+++ b/services/sync/tests/tps/addons/webextension.xpi
diff --git a/services/sync/tests/tps/all_tests.json b/services/sync/tests/tps/all_tests.json
new file mode 100644
index 0000000000..ea92e04b8f
--- /dev/null
+++ b/services/sync/tests/tps/all_tests.json
@@ -0,0 +1,34 @@
+{
+  "tests": {
+    "test_bookmark_conflict.js": {},
+    "test_sync.js": {},
+    "test_prefs.js": {},
+    "test_tabs.js": {},
+    "test_passwords.js": {},
+    "test_history.js": {},
+    "test_formdata.js": {},
+    "test_bug530717.js": {},
+    "test_bug531489.js": {},
+    "test_bug538298.js": {},
+    "test_bug556509.js": {},
+    "test_bug562515.js": {},
+    "test_bug535326.js": {},
+    "test_bug501528.js": {},
+    "test_bug575423.js": {},
+    "test_bug546807.js": {},
+    "test_history_collision.js": {},
+    "test_privbrw_passwords.js": {},
+    "test_privbrw_tabs.js": {},
+    "test_bookmarks_in_same_named_folder.js": {},
+    "test_client_wipe.js": {},
+    "test_special_tabs.js": {},
+    "test_addon_restartless_xpi.js": { "disabled": "Bug 1498974" },
+    "test_addon_webext_xpi.js": { "disabled": "Bug 1498974" },
+    "test_addon_reconciling.js": { "disabled": "Bug 1498974" },
+    "test_addon_wipe.js": { "disabled": "Bug 1498974" },
+    "test_existing_bookmarks.js": {},
+    "test_addresses.js": {},
+    "test_creditcards.js": {},
+    "test_extstorage.js": {}
+  }
+}
diff --git a/services/sync/tests/tps/test_addon_reconciling.js b/services/sync/tests/tps/test_addon_reconciling.js
new file mode 100644
index 0000000000..9c928734c8
--- /dev/null
+++ b/services/sync/tests/tps/test_addon_reconciling.js
@@ -0,0 +1,45 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// This test verifies that record reconciling works as expected. It makes
+// similar changes to add-ons in separate profiles and does a sync to verify
+// the proper action is taken.
+EnableEngines(["addons"]);
+
+var phases = {
+  phase01: "profile1",
+  phase02: "profile2",
+  phase03: "profile1",
+  phase04: "profile2",
+  phase05: "profile1",
+  phase06: "profile2",
+};
+
+const id = "restartless-xpi@tests.mozilla.org";
+
+// Install the add-on in 2 profiles.
+Phase("phase01", [
+  [Addons.verifyNot, [id]],
+  [Addons.install, [id]],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+]);
+Phase("phase02", [
+  [Addons.verifyNot, [id]],
+  [Sync],
+  [Addons.verify, [id], STATE_ENABLED],
+]);
+
+// Now we disable in one and uninstall in the other.
+Phase("phase03", [
+  [Sync], // Get GUID updates, potentially.
+  [Addons.setEnabled, [id], STATE_DISABLED],
+  // We've changed the state, but don't want this profile to sync until phase5,
+  // so if we ran a validation now we'd be expecting to find errors.
+  [Addons.skipValidation],
+]);
+Phase("phase04", [[EnsureTracking], [Addons.uninstall, [id]], [Sync]]);
+
+// When we sync, the uninstall should take precedence because it was newer.
+Phase("phase05", [[Sync]]);
+Phase("phase06", [[Sync], [Addons.verifyNot, [id]]]);
diff --git a/services/sync/tests/tps/test_addon_restartless_xpi.js b/services/sync/tests/tps/test_addon_restartless_xpi.js
new file mode 100644
index 0000000000..2b14245a6e
--- /dev/null
+++ b/services/sync/tests/tps/test_addon_restartless_xpi.js
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// This test verifies that install of restartless extensions syncs to
+// other profiles.
+EnableEngines(["addons"]);
+
+var phases = {
+  phase01: "profile1",
+  phase02: "profile2",
+  phase03: "profile1",
+  phase04: "profile2",
+  phase05: "profile1",
+  phase06: "profile2",
+  phase07: "profile1",
+  phase08: "profile2",
+};
+
+const id = "restartless-xpi@tests.mozilla.org";
+
+// Verify install is synced
+Phase("phase01", [
+  [Addons.verifyNot, [id]],
+  [Addons.install, [id]],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+]);
+Phase("phase02", [
+  [Addons.verifyNot, [id]],
+  [Sync],
+  [Addons.verify, [id], STATE_ENABLED],
+]);
+
+// Now disable and see that is is synced.
+Phase("phase03", [
+  [EnsureTracking],
+  [Addons.setEnabled, [id], STATE_DISABLED],
+  [Addons.verify, [id], STATE_DISABLED],
+  [Sync],
+]);
+Phase("phase04", [[Sync], [Addons.verify, [id], STATE_DISABLED]]);
+
+// Enable and see it is synced.
+Phase("phase05", [
+  [EnsureTracking],
+  [Addons.setEnabled, [id], STATE_ENABLED],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+]);
+Phase("phase06", [[Sync], [Addons.verify, [id], STATE_ENABLED]]);
+
+// Uninstall and see it is synced.
+Phase("phase07", [
+  [EnsureTracking],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Addons.uninstall, [id]],
+  [Addons.verifyNot, [id]],
+  [Sync],
+]);
+Phase("phase08", [
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+  [Addons.verifyNot, [id]],
+]);
diff --git a/services/sync/tests/tps/test_addon_webext_xpi.js b/services/sync/tests/tps/test_addon_webext_xpi.js
new file mode 100644
index 0000000000..27065ee6af
--- /dev/null
+++ b/services/sync/tests/tps/test_addon_webext_xpi.js
@@ -0,0 +1,65 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// This test verifies that install of web extensions sync to other profiles.
+// It's more or less copied from test_addon_restartless_xpi with a different id.
+
+EnableEngines(["addons"]);
+
+var phases = {
+  phase01: "profile1",
+  phase02: "profile2",
+  phase03: "profile1",
+  phase04: "profile2",
+  phase05: "profile1",
+  phase06: "profile2",
+  phase07: "profile1",
+  phase08: "profile2",
+};
+
+const id = "test-webext@quality.mozilla.org";
+
+// Verify install is synced
+Phase("phase01", [
+  [Addons.verifyNot, [id]],
+  [Addons.install, [id]],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+]);
+Phase("phase02", [
+  [Addons.verifyNot, [id]],
+  [Sync],
+  [Addons.verify, [id], STATE_ENABLED],
+]);
+
+// Now disable and see that is is synced.
+Phase("phase03", [
+  [EnsureTracking],
+  [Addons.setEnabled, [id], STATE_DISABLED],
+  [Addons.verify, [id], STATE_DISABLED],
+  [Sync],
+]);
+Phase("phase04", [[Sync], [Addons.verify, [id], STATE_DISABLED]]);
+
+// Enable and see it is synced.
+Phase("phase05", [
+  [EnsureTracking],
+  [Addons.setEnabled, [id], STATE_ENABLED],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+]);
+Phase("phase06", [[Sync], [Addons.verify, [id], STATE_ENABLED]]);
+
+// Uninstall and see it is synced.
+Phase("phase07", [
+  [EnsureTracking],
+  [Addons.verify, [id], STATE_ENABLED],
+  [Addons.uninstall, [id]],
+  [Addons.verifyNot, [id]],
+  [Sync],
+]);
+Phase("phase08", [
+  [Addons.verify, [id], STATE_ENABLED],
+  [Sync],
+  [Addons.verifyNot, [id]],
+]);
diff --git a/services/sync/tests/tps/test_addon_wipe.js b/services/sync/tests/tps/test_addon_wipe.js
new file mode 100644
index 0000000000..038f01d014
--- /dev/null
+++ b/services/sync/tests/tps/test_addon_wipe.js
@@ -0,0 +1,31 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// This test ensures that a client wipe followed by an "initial" sync will
+// restore add-ons. This test should expose flaws in the reconciling logic,
+// specifically around AddonsReconciler. This test is in response to bug
+// 792990.
+
+EnableEngines(["addons"]);
+
+var phases = {
+  phase01: "profile1",
+  phase02: "profile1",
+  phase03: "profile1",
+};
+
+const id1 = "restartless-xpi@tests.mozilla.org";
+const id2 = "test-webext@quality.mozilla.org";
+
+Phase("phase01", [[Addons.install, [id1]], [Addons.install, [id2]], [Sync]]);
+Phase("phase02", [
+  [Addons.verify, [id1], STATE_ENABLED],
+  [Addons.verify, [id2], STATE_ENABLED],
+  [Sync, SYNC_WIPE_CLIENT],
+  [Sync],
+]);
+Phase("phase03", [
+  [Addons.verify, [id1], STATE_ENABLED],
+  [Addons.verify, [id2], STATE_ENABLED],
+  [Sync], // Sync to ensure that the addon validator can run without error
+]);
diff --git a/services/sync/tests/tps/test_addresses.js b/services/sync/tests/tps/test_addresses.js
new file mode 100644
index 0000000000..33af349dd5
--- /dev/null
+++ b/services/sync/tests/tps/test_addresses.js
@@ -0,0 +1,84 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* global Services */
+Services.prefs.setBoolPref("services.sync.engine.addresses", true);
+
+EnableEngines(["addresses"]);
+
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+};
+
+const address1 = [
+  {
+    "given-name": "Timothy",
+    "additional-name": "John",
+    "family-name": "Berners-Lee",
+    organization: "World Wide Web Consortium",
+    "street-address": "32 Vassar Street\nMIT Room 32-G524",
+    "address-level2": "Cambridge",
+    "address-level1": "MA",
+    "postal-code": "02139",
+    country: "US",
+    tel: "+16172535702",
+    email: "timbl@w3.org",
+    changes: {
+      organization: "W3C",
+    },
+    "unknown-1": "an unknown field from another client",
+  },
+];
+
+const address1_after = [
+  {
+    "given-name": "Timothy",
+    "additional-name": "John",
+    "family-name": "Berners-Lee",
+    organization: "W3C",
+    "street-address": "32 Vassar Street\nMIT Room 32-G524",
+    "address-level2": "Cambridge",
+    "address-level1": "MA",
+    "postal-code": "02139",
+    country: "US",
+    tel: "+16172535702",
+    email: "timbl@w3.org",
+    "unknown-1": "an unknown field from another client",
+  },
+];
+
+const address2 = [
+  {
+    "given-name": "John",
+    "additional-name": "R.",
+    "family-name": "Smith",
+    organization: "Mozilla",
+    "street-address":
+      "Geb\u00E4ude 3, 4. Obergeschoss\nSchlesische Stra\u00DFe 27",
+    "address-level2": "Berlin",
+    "address-level1": "BE",
+    "postal-code": "10997",
+    country: "DE",
+    tel: "+4930983333000",
+    email: "timbl@w3.org",
+  },
+];
+
+Phase("phase1", [[Addresses.add, address1], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [Addresses.verify, address1],
+  [Addresses.modify, address1],
+  [Addresses.add, address2],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  [Addresses.verify, address1_after],
+  [Addresses.verify, address2],
+  [Sync],
+]);
diff --git a/services/sync/tests/tps/test_bookmark_conflict.js b/services/sync/tests/tps/test_bookmark_conflict.js
new file mode 100644
index 0000000000..2832bded5d
--- /dev/null
+++ b/services/sync/tests/tps/test_bookmark_conflict.js
@@ -0,0 +1,138 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+// the initial list of bookmarks to add to the browser
+var bookmarksInitial = {
+  menu: [
+    { folder: "foldera" },
+    { folder: "folderb" },
+    { folder: "folderc" },
+    { folder: "folderd" },
+  ],
+
+  "menu/foldera": [{ uri: "http://www.cnn.com", title: "CNN" }],
+  "menu/folderb": [{ uri: "http://www.apple.com", title: "Apple", tags: [] }],
+  "menu/folderc": [{ uri: "http://www.yahoo.com", title: "Yahoo" }],
+
+  "menu/folderd": [],
+};
+
+// a list of bookmarks to delete during a 'delete' action on P2
+var bookmarksToDelete = {
+  menu: [{ folder: "foldera" }, { folder: "folderb" }],
+  "menu/folderc": [{ uri: "http://www.yahoo.com", title: "Yahoo" }],
+};
+
+// the modifications to make on P1, after P2 has synced, but before P1 has gotten
+// P2's changes
+var bookmarkMods = {
+  menu: [
+    { folder: "foldera" },
+    { folder: "folderb" },
+    { folder: "folderc" },
+    { folder: "folderd" },
+  ],
+
+  // we move this child out of its folder (p1), after deleting the folder (p2)
+  // and expect the child to come back to p2 after sync.
+  "menu/foldera": [
+    {
+      uri: "http://www.cnn.com",
+      title: "CNN",
+      changes: { location: "menu/folderd" },
+    },
+  ],
+
+  // we rename this child (p1) after deleting the folder (p2), and expect the child
+  // to be moved into great grandparent (menu)
+  "menu/folderb": [
+    {
+      uri: "http://www.apple.com",
+      title: "Apple",
+      tags: [],
+      changes: { title: "Mac" },
+    },
+  ],
+
+  // we move this child (p1) after deleting the child (p2) and expect it to survive
+  "menu/folderc": [
+    {
+      uri: "http://www.yahoo.com",
+      title: "Yahoo",
+      changes: { location: "menu/folderd" },
+    },
+  ],
+
+  "menu/folderd": [],
+};
+
+// a list of bookmarks to delete during a 'delete' action
+bookmarksToDelete = {
+  menu: [{ folder: "foldera" }, { folder: "folderb" }],
+  "menu/folderc": [{ uri: "http://www.yahoo.com", title: "Yahoo" }],
+};
+
+// expected bookmark state after conflict resolution
+var bookmarksExpected = {
+  menu: [
+    { folder: "folderc" },
+    { folder: "folderd" },
+    { uri: "http://www.apple.com", title: "Mac" },
+  ],
+
+  "menu/folderc": [],
+
+  "menu/folderd": [
+    { uri: "http://www.cnn.com", title: "CNN" },
+    { uri: "http://www.yahoo.com", title: "Yahoo" },
+  ],
+};
+
+// Add bookmarks to profile1 and sync.
+Phase("phase1", [
+  [Bookmarks.add, bookmarksInitial],
+  [Bookmarks.verify, bookmarksInitial],
+  [Sync],
+  [Bookmarks.verify, bookmarksInitial],
+]);
+
+// Sync to profile2 and verify that the bookmarks are present. Delete
+// bookmarks/folders, verify that it's not present, and sync
+Phase("phase2", [
+  [Sync],
+  [Bookmarks.verify, bookmarksInitial],
+  [Bookmarks.delete, bookmarksToDelete],
+  [Bookmarks.verifyNot, bookmarksToDelete],
+  [Sync],
+]);
+
+// Using profile1, modify the bookmarks, and sync *after* the modification,
+// and then sync again to propagate the reconciliation changes.
+Phase("phase3", [
+  [Bookmarks.verify, bookmarksInitial],
+  [Bookmarks.modify, bookmarkMods],
+  [Sync],
+  [Bookmarks.verify, bookmarksExpected],
+  [Bookmarks.verifyNot, bookmarksToDelete],
+]);
+
+// Back in profile2, do a sync and verify that we're in the expected state
+Phase("phase4", [
+  [Sync],
+  [Bookmarks.verify, bookmarksExpected],
+  [Bookmarks.verifyNot, bookmarksToDelete],
+]);
diff --git a/services/sync/tests/tps/test_bookmarks_in_same_named_folder.js b/services/sync/tests/tps/test_bookmarks_in_same_named_folder.js
new file mode 100644
index 0000000000..69dd8ba8a6
--- /dev/null
+++ b/services/sync/tests/tps/test_bookmarks_in_same_named_folder.js
@@ -0,0 +1,53 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// bug 558077
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2", phase3: "profile1" };
+
+var bookmarks_initial_1 = {
+  menu: [
+    { folder: "aaa", description: "foo" },
+    { uri: "http://www.mozilla.com" },
+  ],
+  "menu/aaa": [
+    { uri: "http://www.yahoo.com", title: "testing Yahoo" },
+    { uri: "http://www.google.com", title: "testing Google" },
+  ],
+};
+
+var bookmarks_initial_2 = {
+  menu: [
+    { folder: "aaa", description: "bar" },
+    { uri: "http://www.mozilla.com" },
+  ],
+  "menu/aaa": [
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+    },
+    { uri: "http://www.apple.com", tags: ["apple"] },
+  ],
+};
+
+Phase("phase1", [[Bookmarks.add, bookmarks_initial_1], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [Bookmarks.verify, bookmarks_initial_1],
+  [Bookmarks.add, bookmarks_initial_2],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  // XXX [Bookmarks.verify, bookmarks_initial_1],
+  [Bookmarks.verify, bookmarks_initial_2],
+]);
diff --git a/services/sync/tests/tps/test_bug501528.js b/services/sync/tests/tps/test_bug501528.js
new file mode 100644
index 0000000000..f86ba02403
--- /dev/null
+++ b/services/sync/tests/tps/test_bug501528.js
@@ -0,0 +1,75 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["passwords"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Password lists
+ */
+
+var passwords_initial = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "secret",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      password: "SeCrEt$$$",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+var passwords_after_first_update = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "SeCrEt$$$",
+    usernameField: "uname",
+    passwordField: "pword",
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[Passwords.add, passwords_initial], [Sync]]);
+
+Phase("phase2", [[Passwords.add, passwords_initial], [Sync]]);
+
+Phase("phase3", [
+  [Sync],
+  [Passwords.verify, passwords_initial],
+  [Passwords.modify, passwords_initial],
+  [Passwords.verify, passwords_after_first_update],
+  [Sync],
+]);
+
+Phase("phase4", [[Sync], [Passwords.verify, passwords_after_first_update]]);
diff --git a/services/sync/tests/tps/test_bug530717.js b/services/sync/tests/tps/test_bug530717.js
new file mode 100644
index 0000000000..37e6711cb5
--- /dev/null
+++ b/services/sync/tests/tps/test_bug530717.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["prefs"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2", phase3: "profile1" };
+
+/*
+ * Preference lists
+ */
+
+var prefs1 = [
+  { name: "browser.startup.homepage", value: "http://www.getfirefox.com" },
+  { name: "browser.urlbar.maxRichResults", value: 20 },
+  { name: "privacy.clearOnShutdown.siteSettings", value: true },
+];
+
+var prefs2 = [
+  { name: "browser.startup.homepage", value: "http://www.mozilla.com" },
+  { name: "browser.urlbar.maxRichResults", value: 18 },
+  { name: "privacy.clearOnShutdown.siteSettings", value: false },
+];
+
+/*
+ * Test phases
+ */
+
+// Add prefs to profile1 and sync.
+Phase("phase1", [[Prefs.modify, prefs1], [Prefs.verify, prefs1], [Sync]]);
+
+// Sync profile2 and verify same prefs are present.
+Phase("phase2", [[Sync], [Prefs.verify, prefs1]]);
+
+// Using profile1, change some prefs, then do another sync with wipe-client.
+// Verify that the cloud's prefs are restored, and the recent local changes
+// discarded.
+Phase("phase3", [
+  [Prefs.modify, prefs2],
+  [Prefs.verify, prefs2],
+  [Sync, SYNC_WIPE_CLIENT],
+  [Prefs.verify, prefs1],
+]);
diff --git a/services/sync/tests/tps/test_bug531489.js b/services/sync/tests/tps/test_bug531489.js
new file mode 100644
index 0000000000..3cc79c87ec
--- /dev/null
+++ b/services/sync/tests/tps/test_bug531489.js
@@ -0,0 +1,43 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2", phase3: "profile1" };
+
+/*
+ * Bookmark asset lists: these define bookmarks that are used during the test
+ */
+
+// the initial list of bookmarks to add to the browser
+var bookmarks_initial = {
+  menu: [
+    { folder: "foldera" },
+    { uri: "http://www.google.com", title: "Google" },
+  ],
+  "menu/foldera": [{ uri: "http://www.google.com", title: "Google" }],
+  toolbar: [{ uri: "http://www.google.com", title: "Google" }],
+};
+
+/*
+ * Test phases
+ */
+
+// Add three bookmarks with the same url to different locations and sync.
+Phase("phase1", [
+  [Bookmarks.add, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_initial],
+  [Sync],
+]);
+
+// Sync to profile2 and verify that all three bookmarks are present
+Phase("phase2", [[Sync], [Bookmarks.verify, bookmarks_initial]]);
+
+// Sync again to profile1 and verify that all three bookmarks are still
+// present.
+Phase("phase3", [[Sync], [Bookmarks.verify, bookmarks_initial]]);
diff --git a/services/sync/tests/tps/test_bug535326.js b/services/sync/tests/tps/test_bug535326.js
new file mode 100644
index 0000000000..ede8f45c10
--- /dev/null
+++ b/services/sync/tests/tps/test_bug535326.js
@@ -0,0 +1,148 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["tabs"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2" };
+
+var tabs1 = [
+  {
+    uri: "data:text/html,<html><head><title>Howdy</title></head><body>Howdy</body></html>",
+    title: "Howdy",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>America</title></head><body>America</body></html>",
+    title: "America",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Apple</title></head><body>Apple</body></html>",
+    title: "Apple",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>This</title></head><body>This</body></html>",
+    title: "This",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Bug</title></head><body>Bug</body></html>",
+    title: "Bug",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>IRC</title></head><body>IRC</body></html>",
+    title: "IRC",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Tinderbox</title></head><body>Tinderbox</body></html>",
+    title: "Tinderbox",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Fox</title></head><body>Fox</body></html>",
+    title: "Fox",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Hello</title></head><body>Hello</body></html>",
+    title: "Hello",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Eagle</title></head><body>Eagle</body></html>",
+    title: "Eagle",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Train</title></head><body>Train</body></html>",
+    title: "Train",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Macbook</title></head><body>Macbook</body></html>",
+    title: "Macbook",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Clock</title></head><body>Clock</body></html>",
+    title: "Clock",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Google</title></head><body>Google</body></html>",
+    title: "Google",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Human</title></head><body>Human</body></html>",
+    title: "Human",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Jetpack</title></head><body>Jetpack</body></html>",
+    title: "Jetpack",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Selenium</title></head><body>Selenium</body></html>",
+    title: "Selenium",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Mozilla</title></head><body>Mozilla</body></html>",
+    title: "Mozilla",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Firefox</title></head><body>Firefox</body></html>",
+    title: "Firefox",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Weave</title></head><body>Weave</body></html>",
+    title: "Weave",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Android</title></head><body>Android</body></html>",
+    title: "Android",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Bye</title></head><body>Bye</body></html>",
+    title: "Bye",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Hi</title></head><body>Hi</body></html>",
+    title: "Hi",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Final</title></head><body>Final</body></html>",
+    title: "Final",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Fennec</title></head><body>Fennec</body></html>",
+    title: "Fennec",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Mobile</title></head><body>Mobile</body></html>",
+    title: "Mobile",
+    profile: "profile1",
+  },
+];
+
+Phase("phase1", [[Tabs.add, tabs1], [Sync]]);
+
+Phase("phase2", [[Sync], [Tabs.verify, tabs1]]);
diff --git a/services/sync/tests/tps/test_bug538298.js b/services/sync/tests/tps/test_bug538298.js
new file mode 100644
index 0000000000..3f1abf2a96
--- /dev/null
+++ b/services/sync/tests/tps/test_bug538298.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Bookmark asset lists: these define bookmarks that are used during the test
+ */
+
+// the initial list of bookmarks to add to the browser
+var bookmarks_initial = {
+  toolbar: [
+    { uri: "http://www.google.com", title: "Google" },
+    {
+      uri: "http://www.cnn.com",
+      title: "CNN",
+      changes: {
+        position: "Google",
+      },
+    },
+    { uri: "http://www.mozilla.com", title: "Mozilla" },
+    {
+      uri: "http://www.firefox.com",
+      title: "Firefox",
+      changes: {
+        position: "Mozilla",
+      },
+    },
+  ],
+};
+
+var bookmarks_after_move = {
+  toolbar: [
+    { uri: "http://www.cnn.com", title: "CNN" },
+    { uri: "http://www.google.com", title: "Google" },
+    { uri: "http://www.firefox.com", title: "Firefox" },
+    { uri: "http://www.mozilla.com", title: "Mozilla" },
+  ],
+};
+
+/*
+ * Test phases
+ */
+
+// Add four bookmarks to the toolbar and sync.
+Phase("phase1", [
+  [Bookmarks.add, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_initial],
+  [Sync],
+]);
+
+// Sync to profile2 and verify that all four bookmarks are present.
+Phase("phase2", [[Sync], [Bookmarks.verify, bookmarks_initial]]);
+
+// Change the order of the toolbar bookmarks, and sync.
+Phase("phase3", [
+  [Sync],
+  [Bookmarks.verify, bookmarks_initial],
+  [Bookmarks.modify, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_after_move],
+  [Sync],
+]);
+
+// Go back to profile2, sync, and verify that the bookmarks are reordered
+// as expected.
+Phase("phase4", [[Sync], [Bookmarks.verify, bookmarks_after_move]]);
diff --git a/services/sync/tests/tps/test_bug546807.js b/services/sync/tests/tps/test_bug546807.js
new file mode 100644
index 0000000000..f02f632b41
--- /dev/null
+++ b/services/sync/tests/tps/test_bug546807.js
@@ -0,0 +1,38 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["tabs"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2" };
+
+/*
+ * Tabs data
+ */
+
+var tabs1 = [
+  { uri: "about:config", profile: "profile1" },
+  { uri: "about:credits", profile: "profile1" },
+  {
+    uri: "data:text/html,<html><head><title>Apple</title></head><body>Apple</body></html>",
+    title: "Apple",
+    profile: "profile1",
+  },
+];
+
+var tabs_absent = [
+  { uri: "about:config", profile: "profile1" },
+  { uri: "about:credits", profile: "profile1" },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[Tabs.add, tabs1], [Sync]]);
+
+Phase("phase2", [[Sync], [Tabs.verifyNot, tabs_absent]]);
diff --git a/services/sync/tests/tps/test_bug556509.js b/services/sync/tests/tps/test_bug556509.js
new file mode 100644
index 0000000000..3a46c3c23e
--- /dev/null
+++ b/services/sync/tests/tps/test_bug556509.js
@@ -0,0 +1,32 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2" };
+
+// the initial list of bookmarks to add to the browser
+var bookmarks_initial = {
+  menu: [{ folder: "testfolder", description: "it's just me, a test folder" }],
+  "menu/testfolder": [{ uri: "http://www.mozilla.com", title: "Mozilla" }],
+};
+
+/*
+ * Test phases
+ */
+
+// Add a bookmark folder which has a description, and sync.
+Phase("phase1", [
+  [Bookmarks.add, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_initial],
+  [Sync],
+]);
+
+// Sync to profile2 and verify that the bookmark folder is created, along
+// with its description.
+Phase("phase2", [[Sync], [Bookmarks.verify, bookmarks_initial]]);
diff --git a/services/sync/tests/tps/test_bug562515.js b/services/sync/tests/tps/test_bug562515.js
new file mode 100644
index 0000000000..3a1a6cbee2
--- /dev/null
+++ b/services/sync/tests/tps/test_bug562515.js
@@ -0,0 +1,90 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Bookmark lists
+ */
+
+// the initial list of bookmarks to add to the browser
+var bookmarks_initial = {
+  menu: [
+    {
+      uri: "http://www.google.com",
+      tags: ["google", "computers", "internet", "www"],
+    },
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+      keyword: "bz",
+    },
+    { folder: "foldera" },
+    { uri: "http://www.mozilla.com" },
+    { separator: true },
+    { folder: "folderb" },
+  ],
+  "menu/foldera": [
+    { uri: "http://www.yahoo.com", title: "testing Yahoo" },
+    {
+      uri: "http://www.cnn.com",
+      description: "This is a description of the site a at www.cnn.com",
+    },
+  ],
+  "menu/folderb": [{ uri: "http://www.apple.com", tags: ["apple", "mac"] }],
+  toolbar: [
+    {
+      uri: "place:queryType=0&sort=8&maxResults=10&beginTimeRef=1&beginTime=0",
+      title: "Visited Today",
+    },
+  ],
+};
+
+// a list of bookmarks to delete during a 'delete' action
+var bookmarks_to_delete = {
+  menu: [
+    {
+      uri: "http://www.google.com",
+      tags: ["google", "computers", "internet", "www"],
+    },
+  ],
+  "menu/foldera": [{ uri: "http://www.yahoo.com", title: "testing Yahoo" }],
+};
+
+/*
+ * Test phases
+ */
+
+// add bookmarks to profile1 and sync
+Phase("phase1", [
+  [Bookmarks.add, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_initial],
+  [Sync],
+]);
+
+// sync to profile2 and verify that the bookmarks are present
+Phase("phase2", [[Sync], [Bookmarks.verify, bookmarks_initial]]);
+
+// delete some bookmarks from profile1, then sync with "wipe-client"
+// set; finally, verify that the deleted bookmarks were restored.
+Phase("phase3", [
+  [Bookmarks.delete, bookmarks_to_delete],
+  [Bookmarks.verifyNot, bookmarks_to_delete],
+  [Sync, SYNC_WIPE_CLIENT],
+  [Bookmarks.verify, bookmarks_initial],
+]);
+
+// sync profile2 again, verify no bookmarks have been deleted
+Phase("phase4", [[Sync], [Bookmarks.verify, bookmarks_initial]]);
diff --git a/services/sync/tests/tps/test_bug575423.js b/services/sync/tests/tps/test_bug575423.js
new file mode 100644
index 0000000000..53f46db5b7
--- /dev/null
+++ b/services/sync/tests/tps/test_bug575423.js
@@ -0,0 +1,67 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["history"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2" };
+
+/*
+ * History data
+ */
+
+// the history data to add to the browser
+var history1 = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -1 },
+    ],
+  },
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+// Another history data to add to the browser
+var history2 = [
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -36 },
+    ],
+  },
+  {
+    uri: "http://www.google.com/language_tools?hl=en",
+    title: "Language Tools",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -40 },
+    ],
+  },
+];
+
+/*
+ * Test phases
+ */
+Phase("phase1", [
+  [History.add, history1],
+  [Sync],
+  [History.add, history2],
+  [Sync],
+]);
+
+Phase("phase2", [[Sync], [History.verify, history2]]);
diff --git a/services/sync/tests/tps/test_client_wipe.js b/services/sync/tests/tps/test_client_wipe.js
new file mode 100644
index 0000000000..e0fb7a97ac
--- /dev/null
+++ b/services/sync/tests/tps/test_client_wipe.js
@@ -0,0 +1,142 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2", phase3: "profile1" };
+
+/*
+ * Bookmark lists
+ */
+
+// the initial list of bookmarks to add to the browser
+var bookmarks_initial = {
+  toolbar: [
+    { uri: "http://www.google.com", title: "Google" },
+    {
+      uri: "http://www.cnn.com",
+      title: "CNN",
+      changes: {
+        position: "Google",
+      },
+    },
+    { uri: "http://www.mozilla.com", title: "Mozilla" },
+    {
+      uri: "http://www.firefox.com",
+      title: "Firefox",
+      changes: {
+        position: "Mozilla",
+      },
+    },
+  ],
+};
+
+var bookmarks_after_move = {
+  toolbar: [
+    { uri: "http://www.cnn.com", title: "CNN" },
+    { uri: "http://www.google.com", title: "Google" },
+    { uri: "http://www.firefox.com", title: "Firefox" },
+    { uri: "http://www.mozilla.com", title: "Mozilla" },
+  ],
+};
+
+/*
+ * Password data
+ */
+
+// Initial password data
+var passwords_initial = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "secret",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      password: "SeCrEt$$$",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+// Password after first modify action has been performed
+var passwords_after_change = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "SeCrEt$$$",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      username: "james",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+/*
+ * Prefs to use in the test
+ */
+var prefs1 = [
+  { name: "browser.startup.homepage", value: "http://www.getfirefox.com" },
+  { name: "browser.urlbar.maxRichResults", value: 20 },
+  { name: "privacy.clearOnShutdown.siteSettings", value: true },
+];
+
+var prefs2 = [
+  { name: "browser.startup.homepage", value: "http://www.mozilla.com" },
+  { name: "browser.urlbar.maxRichResults", value: 18 },
+  { name: "privacy.clearOnShutdown.siteSettings", value: false },
+];
+
+/*
+ * Test phases
+ */
+
+// Add prefs,passwords and bookmarks to profile1 and sync.
+Phase("phase1", [
+  [Passwords.add, passwords_initial],
+  [Bookmarks.add, bookmarks_initial],
+  [Prefs.modify, prefs1],
+  [Prefs.verify, prefs1],
+  [Sync],
+]);
+
+// Sync profile2 and verify same prefs,passwords and bookmarks are present.
+Phase("phase2", [
+  [Sync],
+  [Prefs.verify, prefs1],
+  [Passwords.verify, passwords_initial],
+  [Bookmarks.verify, bookmarks_initial],
+]);
+
+// Using profile1, change some prefs,bookmarks and pwds, then do another sync with wipe-client.
+// Verify that the cloud's  settings are restored, and the recent local changes
+// discarded.
+Phase("phase3", [
+  [Prefs.modify, prefs2],
+  [Passwords.modify, passwords_initial],
+  [Bookmarks.modify, bookmarks_initial],
+  [Prefs.verify, prefs2],
+  [Passwords.verify, passwords_after_change],
+  [Bookmarks.verify, bookmarks_after_move],
+  [Sync, SYNC_WIPE_CLIENT],
+  [Prefs.verify, prefs1],
+  [Passwords.verify, passwords_initial],
+  [Bookmarks.verify, bookmarks_initial],
+]);
diff --git a/services/sync/tests/tps/test_creditcards.js b/services/sync/tests/tps/test_creditcards.js
new file mode 100644
index 0000000000..fea2b8a541
--- /dev/null
+++ b/services/sync/tests/tps/test_creditcards.js
@@ -0,0 +1,62 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* global Services */
+Services.prefs.setBoolPref("services.sync.engine.creditcards", true);
+
+EnableEngines(["creditcards"]);
+
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+};
+
+const cc1 = [
+  {
+    "cc-name": "John Doe",
+    "cc-number": "4716179744040592",
+    "cc-exp-month": 4,
+    "cc-exp-year": 2050,
+    "unknown-1": "an unknown field from another client",
+    changes: {
+      "cc-exp-year": 2051,
+    },
+  },
+];
+
+const cc1_after = [
+  {
+    "cc-name": "John Doe",
+    "cc-number": "4716179744040592",
+    "cc-exp-month": 4,
+    "cc-exp-year": 2051,
+    "unknown-1": "an unknown field from another client",
+  },
+];
+
+const cc2 = [
+  {
+    "cc-name": "Timothy Berners-Lee",
+    "cc-number": "2221000374457678",
+    "cc-exp-month": 12,
+    "cc-exp-year": 2050,
+  },
+];
+
+Phase("phase1", [[CreditCards.add, cc1], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [CreditCards.verify, cc1],
+  [CreditCards.modify, cc1],
+  [CreditCards.add, cc2],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  [CreditCards.verifyNot, cc1],
+  [CreditCards.verify, cc1_after],
+  [CreditCards.verify, cc2],
+]);
diff --git a/services/sync/tests/tps/test_existing_bookmarks.js b/services/sync/tests/tps/test_existing_bookmarks.js
new file mode 100644
index 0000000000..cde41a96e6
--- /dev/null
+++ b/services/sync/tests/tps/test_existing_bookmarks.js
@@ -0,0 +1,80 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["bookmarks"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile2",
+  phase4: "profile1",
+};
+
+/*
+ * Bookmark lists
+ */
+var bookmarks_initial = {
+  menu: [
+    {
+      uri: "http://www.google.com",
+      title: "Google",
+      changes: {
+        title: "google",
+      },
+    },
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+    },
+    { uri: "http://www.mozilla.com" },
+    {
+      uri: "http://www.cnn.com",
+      description: "This is a description of the site a at www.cnn.com",
+      changes: {
+        description: "Global news",
+      },
+    },
+  ],
+};
+
+var bookmarks_after = {
+  menu: [
+    { uri: "http://www.google.com", title: "google" },
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+    },
+    { uri: "http://www.mozilla.com" },
+    { uri: "http://www.cnn.com", description: "Global news" },
+  ],
+};
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [
+  [Bookmarks.add, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_initial],
+  [Sync],
+]);
+
+Phase("phase2", [
+  [Bookmarks.add, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_initial],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Bookmarks.verify, bookmarks_initial],
+  [Bookmarks.modify, bookmarks_initial],
+  [Bookmarks.verify, bookmarks_after],
+  [Sync],
+]);
+
+Phase("phase4", [[Sync], [Bookmarks.verify, bookmarks_after]]);
diff --git a/services/sync/tests/tps/test_extstorage.js b/services/sync/tests/tps/test_extstorage.js
new file mode 100644
index 0000000000..cf25187bc3
--- /dev/null
+++ b/services/sync/tests/tps/test_extstorage.js
@@ -0,0 +1,154 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["addons"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+  phase5: "profile1",
+};
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [
+  [
+    ExtStorage.set,
+    "ext-1",
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": {
+        sk_1: "v1",
+        sk_2: "v2",
+      },
+    },
+  ],
+  [
+    ExtStorage.verify,
+    "ext-1",
+    null,
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": {
+        sk_1: "v1",
+        sk_2: "v2",
+      },
+    },
+  ],
+  [Sync],
+]);
+
+Phase("phase2", [
+  [Sync],
+  [
+    ExtStorage.set,
+    "ext-1",
+    {
+      "key-2": "value from profile 2",
+    },
+  ],
+  [
+    ExtStorage.verify,
+    "ext-1",
+    null,
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": "value from profile 2",
+    },
+  ],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  [
+    ExtStorage.verify,
+    "ext-1",
+    null,
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": "value from profile 2",
+    },
+  ],
+  [
+    ExtStorage.set,
+    "ext-1",
+    {
+      "key-2": "value from profile 1",
+    },
+  ],
+  // exit without syncing.
+]);
+
+Phase("phase4", [
+  [Sync],
+  [
+    ExtStorage.verify,
+    "ext-1",
+    null,
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": "value from profile 2",
+    },
+  ],
+  [
+    ExtStorage.set,
+    "ext-1",
+    {
+      "key-2": "second value from profile 2",
+    },
+  ],
+  [Sync],
+]);
+
+Phase("phase5", [
+  [
+    ExtStorage.verify,
+    "ext-1",
+    null,
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": "value from profile 1",
+    },
+  ],
+  [Sync],
+  [
+    ExtStorage.verify,
+    "ext-1",
+    null,
+    {
+      "key-1": {
+        sub_key_1: "value 1",
+        sub_key_2: "value 2",
+      },
+      "key-2": "second value from profile 2",
+    },
+  ],
+]);
diff --git a/services/sync/tests/tps/test_formdata.js b/services/sync/tests/tps/test_formdata.js
new file mode 100644
index 0000000000..764b12342f
--- /dev/null
+++ b/services/sync/tests/tps/test_formdata.js
@@ -0,0 +1,63 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["forms"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Form data asset lists: these define form values that are used in the tests.
+ */
+
+var formdata1 = [
+  { fieldname: "testing", value: "success", date: -1 },
+  { fieldname: "testing", value: "failure", date: -2 },
+  { fieldname: "username", value: "joe" },
+];
+
+var formdata2 = [
+  { fieldname: "testing", value: "success", date: -1 },
+  { fieldname: "username", value: "joe" },
+];
+
+var formdata_delete = [{ fieldname: "testing", value: "failure" }];
+
+var formdata_new = [{ fieldname: "new-field", value: "new-value" }];
+/*
+ * Test phases
+ */
+
+Phase("phase1", [
+  [Formdata.add, formdata1],
+  [Formdata.verify, formdata1],
+  [Sync],
+]);
+
+Phase("phase2", [[Sync], [Formdata.verify, formdata1]]);
+
+Phase("phase3", [
+  [Sync],
+  [Formdata.delete, formdata_delete],
+  [Formdata.verifyNot, formdata_delete],
+  [Formdata.verify, formdata2],
+  // add new data after the first Sync, ensuring the tracker works.
+  [Formdata.add, formdata_new],
+  [Sync],
+]);
+
+Phase("phase4", [
+  [Sync],
+  [Formdata.verify, formdata2],
+  [Formdata.verify, formdata_new],
+  [Formdata.verifyNot, formdata_delete],
+]);
diff --git a/services/sync/tests/tps/test_history.js b/services/sync/tests/tps/test_history.js
new file mode 100644
index 0000000000..e1df78c762
--- /dev/null
+++ b/services/sync/tests/tps/test_history.js
@@ -0,0 +1,129 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["history"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2" };
+
+/*
+ * History asset lists: these define history entries that are used during
+ * the test
+ */
+
+// the initial list of history items to add to the browser
+var history1 = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -1 },
+    ],
+  },
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+  {
+    uri: "http://www.google.com/language_tools?hl=en",
+    title: "Language Tools",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -40 },
+    ],
+  },
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 1, date: -1 },
+      { type: 1, date: -20 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+// a list of items to delete from the history
+var history_to_delete = [
+  { uri: "http://www.cnn.com/" },
+  { begin: -24, end: -1 },
+  { host: "www.google.com" },
+];
+
+// a list which reflects items that should be in the history after
+// the above items are deleted
+var history2 = [
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+// a list which includes history entries that should not be present
+// after deletion of the history_to_delete entries
+var history_not = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -1 },
+    ],
+  },
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+  {
+    uri: "http://www.google.com/language_tools?hl=en",
+    title: "Language Tools",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -40 },
+    ],
+  },
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 1, date: -20 },
+    ],
+  },
+];
+
+/*
+ * Test phases
+ * Note:  there is no test phase in which deleted history entries are
+ * synced to other clients.  This functionality is not supported by
+ * Sync, see bug 446517.
+ */
+
+Phase("phase1", [[History.add, history1], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [History.verify, history1],
+  [History.delete, history_to_delete],
+  [History.verify, history2],
+  [History.verifyNot, history_not],
+  [Sync],
+]);
diff --git a/services/sync/tests/tps/test_history_collision.js b/services/sync/tests/tps/test_history_collision.js
new file mode 100644
index 0000000000..ea428d1051
--- /dev/null
+++ b/services/sync/tests/tps/test_history_collision.js
@@ -0,0 +1,98 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["history"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * History lists
+ */
+
+// the initial list of history to add to the browser
+var history1 = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [{ type: 1, date: 0 }],
+  },
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+// the history to delete
+var history_to_delete = [
+  { uri: "http://www.cnn.com/", title: "CNN" },
+  { begin: -36, end: -1 },
+];
+
+var history_not = [
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+var history_after_delete = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [{ type: 1, date: 0 }],
+  },
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [{ type: 1, date: 0 }],
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[History.add, history1], [Sync]]);
+
+Phase("phase2", [[Sync], [History.add, history1], [Sync, SYNC_WIPE_REMOTE]]);
+
+Phase("phase3", [
+  [Sync],
+  [History.verify, history1],
+  [History.delete, history_to_delete],
+  [History.verify, history_after_delete],
+  [History.verifyNot, history_not],
+  [Sync],
+]);
+
+Phase("phase4", [
+  [Sync],
+  [History.verify, history_after_delete],
+  [History.verifyNot, history_not],
+]);
diff --git a/services/sync/tests/tps/test_passwords.js b/services/sync/tests/tps/test_passwords.js
new file mode 100644
index 0000000000..2dfd7de8dd
--- /dev/null
+++ b/services/sync/tests/tps/test_passwords.js
@@ -0,0 +1,119 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["passwords"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Password asset lists: these define password entries that are used during
+ * the test
+ */
+
+// initial password list to be loaded into the browser
+var passwords_initial = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "SeCrEt123",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      password: "zippity-do-dah",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+// expected state of passwords after the changes in the above list are applied
+var passwords_after_first_update = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "zippity-do-dah",
+    usernameField: "uname",
+    passwordField: "pword",
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+var passwords_to_delete = [
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+var passwords_absent = [
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+// expected state of passwords after the delete operation
+var passwords_after_second_update = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "zippity-do-dah",
+    usernameField: "uname",
+    passwordField: "pword",
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[Passwords.add, passwords_initial], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [Passwords.verify, passwords_initial],
+  [Passwords.modify, passwords_initial],
+  [Passwords.verify, passwords_after_first_update],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  [Passwords.verify, passwords_after_first_update],
+  [Passwords.delete, passwords_to_delete],
+  [Passwords.verify, passwords_after_second_update],
+  [Passwords.verifyNot, passwords_absent],
+  [Sync],
+]);
+
+Phase("phase4", [
+  [Sync],
+  [Passwords.verify, passwords_after_second_update],
+  [Passwords.verifyNot, passwords_absent],
+]);
diff --git a/services/sync/tests/tps/test_prefs.js b/services/sync/tests/tps/test_prefs.js
new file mode 100644
index 0000000000..3e26e947dd
--- /dev/null
+++ b/services/sync/tests/tps/test_prefs.js
@@ -0,0 +1,35 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["prefs"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2", phase3: "profile1" };
+
+var prefs1 = [
+  { name: "browser.startup.homepage", value: "http://www.getfirefox.com" },
+  { name: "browser.urlbar.maxRichResults", value: 20 },
+  { name: "privacy.clearOnShutdown.siteSettings", value: true },
+];
+
+var prefs2 = [
+  { name: "browser.startup.homepage", value: "http://www.mozilla.com" },
+  { name: "browser.urlbar.maxRichResults", value: 18 },
+  { name: "privacy.clearOnShutdown.siteSettings", value: false },
+];
+
+Phase("phase1", [[Prefs.modify, prefs1], [Prefs.verify, prefs1], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [Prefs.verify, prefs1],
+  [Prefs.modify, prefs2],
+  [Prefs.verify, prefs2],
+  [Sync],
+]);
+
+Phase("phase3", [[Sync], [Prefs.verify, prefs2]]);
diff --git a/services/sync/tests/tps/test_privbrw_passwords.js b/services/sync/tests/tps/test_privbrw_passwords.js
new file mode 100644
index 0000000000..dcde6c02b8
--- /dev/null
+++ b/services/sync/tests/tps/test_privbrw_passwords.js
@@ -0,0 +1,105 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in strict JSON format, as it will get parsed by the Python
+ * testrunner (no single quotes, extra comma's, etc).
+ */
+EnableEngines(["passwords"]);
+
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Password data
+ */
+
+// Initial password data
+var passwords_initial = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "secret",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      password: "SeCrEt$$$",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+// Password after first modify action has been performed
+var passwords_after_first_change = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "SeCrEt$$$",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      username: "james",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+// Password after second modify action has been performed
+var passwords_after_second_change = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "james",
+    password: "SeCrEt$$$",
+    usernameField: "uname",
+    passwordField: "pword",
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "jack",
+    password: "secretlogin",
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[Passwords.add, passwords_initial], [Sync]]);
+
+Phase("phase2", [
+  [Sync],
+  [Passwords.verify, passwords_initial],
+  [Passwords.modify, passwords_initial],
+  [Passwords.verify, passwords_after_first_change],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  [Windows.add, { private: true }],
+  [Passwords.verify, passwords_after_first_change],
+  [Passwords.modify, passwords_after_first_change],
+  [Passwords.verify, passwords_after_second_change],
+  [Sync],
+]);
+
+Phase("phase4", [[Sync], [Passwords.verify, passwords_after_second_change]]);
diff --git a/services/sync/tests/tps/test_privbrw_tabs.js b/services/sync/tests/tps/test_privbrw_tabs.js
new file mode 100644
index 0000000000..c7c877677f
--- /dev/null
+++ b/services/sync/tests/tps/test_privbrw_tabs.js
@@ -0,0 +1,86 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in strict JSON format, as it will get parsed by the Python
+ * testrunner (no single quotes, extra comma's, etc).
+ */
+EnableEngines(["tabs"]);
+
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Tabs data
+ */
+
+var tabs1 = [
+  {
+    uri: "data:text/html,<html><head><title>Firefox</title></head><body>Firefox</body></html>",
+    title: "Firefox",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Weave</title></head><body>Weave</body></html>",
+    title: "Weave",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Apple</title></head><body>Apple</body></html>",
+    title: "Apple",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>IRC</title></head><body>IRC</body></html>",
+    title: "IRC",
+    profile: "profile1",
+  },
+];
+
+var tabs2 = [
+  {
+    uri: "data:text/html,<html><head><title>Tinderbox</title></head><body>Tinderbox</body></html>",
+    title: "Tinderbox",
+    profile: "profile2",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Fox</title></head><body>Fox</body></html>",
+    title: "Fox",
+    profile: "profile2",
+  },
+];
+
+var tabs3 = [
+  {
+    uri: "data:text/html,<html><head><title>Jetpack</title></head><body>Jetpack</body></html>",
+    title: "Jetpack",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Selenium</title></head><body>Selenium</body></html>",
+    title: "Selenium",
+    profile: "profile1",
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[Tabs.add, tabs1], [Sync]]);
+
+Phase("phase2", [[Sync], [Tabs.verify, tabs1], [Tabs.add, tabs2], [Sync]]);
+
+Phase("phase3", [
+  [Sync],
+  [Windows.add, { private: true }],
+  [Tabs.add, tabs3],
+  [Sync],
+]);
+
+Phase("phase4", [[Sync], [Tabs.verifyNot, tabs3]]);
diff --git a/services/sync/tests/tps/test_special_tabs.js b/services/sync/tests/tps/test_special_tabs.js
new file mode 100644
index 0000000000..9e1eae88e3
--- /dev/null
+++ b/services/sync/tests/tps/test_special_tabs.js
@@ -0,0 +1,63 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Bug 532173 - Dont sync tabs like about:* , weave firstrun etc
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in strict JSON format, as it will get parsed by the Python
+ * testrunner (no single quotes, extra comma's, etc).
+ */
+EnableEngines(["tabs"]);
+
+var phases = { phase1: "profile1", phase2: "profile2" };
+
+var tabs1 = [
+  {
+    uri: "data:text/html,<html><head><title>Firefox</title></head><body>Firefox</body></html>",
+    title: "Firefox",
+    profile: "profile1",
+  },
+  { uri: "about:robots", title: "About", profile: "profile1" },
+  { uri: "about:credits", title: "Credits", profile: "profile1" },
+  {
+    uri: "data:text/html,<html><head><title>Mozilla</title></head><body>Mozilla</body></html>",
+    title: "Mozilla",
+    profile: "profile1",
+  },
+  {
+    uri: "http://www.mozilla.com/en-US/firefox/sync/firstrun.html",
+    title: "Firstrun",
+    profile: "profile1",
+  },
+];
+
+var tabs2 = [
+  {
+    uri: "data:text/html,<html><head><title>Firefox</title></head><body>Firefox</body></html>",
+    title: "Firefox",
+    profile: "profile1",
+  },
+  {
+    uri: "data:text/html,<html><head><title>Mozilla</title></head><body>Mozilla</body></html>",
+    title: "Mozilla",
+    profile: "profile1",
+  },
+];
+
+var tabs3 = [
+  {
+    uri: "http://www.mozilla.com/en-US/firefox/sync/firstrun.html",
+    title: "Firstrun",
+    profile: "profile1",
+  },
+  { uri: "about:robots", title: "About", profile: "profile1" },
+  { uri: "about:credits", title: "Credits", profile: "profile1" },
+];
+
+/*
+ * Test phases
+ */
+Phase("phase1", [[Tabs.add, tabs1], [Sync]]);
+
+Phase("phase2", [[Sync], [Tabs.verify, tabs2], [Tabs.verifyNot, tabs3]]);
diff --git a/services/sync/tests/tps/test_sync.js b/services/sync/tests/tps/test_sync.js
new file mode 100644
index 0000000000..3d14430c60
--- /dev/null
+++ b/services/sync/tests/tps/test_sync.js
@@ -0,0 +1,403 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in strict JSON format, as it will get parsed by the Python
+ * testrunner (no single quotes, extra comma's, etc).
+ */
+
+var phases = {
+  phase1: "profile1",
+  phase2: "profile2",
+  phase3: "profile1",
+  phase4: "profile2",
+};
+
+/*
+ * Bookmark asset lists: these define bookmarks that are used during the test
+ */
+
+// the initial list of bookmarks to be added to the browser
+var bookmarks_initial = {
+  menu: [
+    {
+      uri: "http://www.google.com",
+      tags: ["google", "computers", "internet", "www"],
+      changes: {
+        title: "Google",
+        tags: ["google", "computers", "misc"],
+      },
+    },
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+      keyword: "bz",
+      changes: {
+        keyword: "bugzilla",
+      },
+    },
+    { folder: "foldera" },
+    { uri: "http://www.mozilla.com" },
+    { separator: true },
+    { folder: "folderb" },
+  ],
+  "menu/foldera": [
+    {
+      uri: "http://www.yahoo.com",
+      title: "testing Yahoo",
+      changes: {
+        location: "menu/folderb",
+      },
+    },
+    {
+      uri: "http://www.cnn.com",
+      description: "This is a description of the site a at www.cnn.com",
+      changes: {
+        uri: "http://money.cnn.com",
+        description: "new description",
+      },
+    },
+  ],
+  "menu/folderb": [
+    {
+      uri: "http://www.apple.com",
+      tags: ["apple", "mac"],
+      changes: {
+        uri: "http://www.apple.com/iphone/",
+        title: "iPhone",
+        location: "menu",
+        position: "Google",
+        tags: [],
+      },
+    },
+  ],
+  toolbar: [
+    {
+      uri: "place:queryType=0&sort=8&maxResults=10&beginTimeRef=1&beginTime=0",
+      title: "Visited Today",
+    },
+  ],
+};
+
+// the state of bookmarks after the first 'modify' action has been performed
+// on them
+var bookmarks_after_first_modify = {
+  menu: [
+    {
+      uri: "http://www.apple.com/iphone/",
+      title: "iPhone",
+      before: "Google",
+      tags: [],
+    },
+    {
+      uri: "http://www.google.com",
+      title: "Google",
+      tags: ["google", "computers", "misc"],
+    },
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+      keyword: "bugzilla",
+    },
+    { folder: "foldera" },
+    { uri: "http://www.mozilla.com" },
+    { separator: true },
+    {
+      folder: "folderb",
+      changes: {
+        location: "menu/foldera",
+        folder: "Folder B",
+        description: "folder description",
+      },
+    },
+  ],
+  "menu/foldera": [
+    {
+      uri: "http://money.cnn.com",
+      title: "http://www.cnn.com",
+      description: "new description",
+    },
+  ],
+  "menu/folderb": [{ uri: "http://www.yahoo.com", title: "testing Yahoo" }],
+  toolbar: [
+    {
+      uri: "place:queryType=0&sort=8&maxResults=10&beginTimeRef=1&beginTime=0",
+      title: "Visited Today",
+    },
+  ],
+};
+
+// a list of bookmarks to delete during a 'delete' action
+var bookmarks_to_delete = {
+  menu: [
+    {
+      uri: "http://www.google.com",
+      title: "Google",
+      tags: ["google", "computers", "misc"],
+    },
+  ],
+};
+
+// the state of bookmarks after the second 'modify' action has been performed
+// on them
+var bookmarks_after_second_modify = {
+  menu: [
+    { uri: "http://www.apple.com/iphone/", title: "iPhone" },
+    {
+      uri: "http://bugzilla.mozilla.org/show_bug.cgi?id=%s",
+      title: "Bugzilla",
+      keyword: "bugzilla",
+    },
+    { folder: "foldera" },
+    { uri: "http://www.mozilla.com" },
+    { separator: true },
+  ],
+  "menu/foldera": [
+    {
+      uri: "http://money.cnn.com",
+      title: "http://www.cnn.com",
+      description: "new description",
+    },
+    { folder: "Folder B", description: "folder description" },
+  ],
+  "menu/foldera/Folder B": [
+    { uri: "http://www.yahoo.com", title: "testing Yahoo" },
+  ],
+};
+
+// a list of bookmarks which should not be present after the last
+// 'delete' and 'modify' actions
+var bookmarks_absent = {
+  menu: [
+    { uri: "http://www.google.com", title: "Google" },
+    { folder: "folderb" },
+    { folder: "Folder B" },
+  ],
+};
+
+/*
+ * History asset lists: these define history entries that are used during
+ * the test
+ */
+
+// the initial list of history items to add to the browser
+var history_initial = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -1 },
+    ],
+  },
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+  {
+    uri: "http://www.google.com/language_tools?hl=en",
+    title: "Language Tools",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -40 },
+    ],
+  },
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 1, date: -1 },
+      { type: 1, date: -20 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+// a list of history entries to delete during a 'delete' action
+var history_to_delete = [
+  { uri: "http://www.cnn.com/" },
+  { begin: -24, end: -1 },
+  { host: "www.google.com" },
+];
+
+// the expected history entries after the first 'delete' action
+var history_after_delete = [
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -36 },
+    ],
+  },
+];
+
+// history entries expected to not exist after a 'delete' action
+var history_absent = [
+  {
+    uri: "http://www.google.com/",
+    title: "Google",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -1 },
+    ],
+  },
+  {
+    uri: "http://www.cnn.com/",
+    title: "CNN",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 2, date: -36 },
+    ],
+  },
+  {
+    uri: "http://www.google.com/language_tools?hl=en",
+    title: "Language Tools",
+    visits: [
+      { type: 1, date: 0 },
+      { type: 2, date: -40 },
+    ],
+  },
+  {
+    uri: "http://www.mozilla.com/",
+    title: "Mozilla",
+    visits: [
+      { type: 1, date: -1 },
+      { type: 1, date: -20 },
+    ],
+  },
+];
+
+/*
+ * Password asset lists: these define password entries that are used during
+ * the test
+ */
+
+// the initial list of passwords to add to the browser
+var passwords_initial = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "SeCrEt123",
+    usernameField: "uname",
+    passwordField: "pword",
+    changes: {
+      password: "zippity-do-dah",
+    },
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+// the expected state of passwords after the first 'modify' action
+var passwords_after_first_modify = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "zippity-do-dah",
+    usernameField: "uname",
+    passwordField: "pword",
+  },
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+// a list of passwords to delete during a 'delete' action
+var passwords_to_delete = [
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+// a list of passwords expected to be absent after 'delete' and 'modify'
+// actions
+var passwords_absent = [
+  {
+    hostname: "http://www.example.com",
+    realm: "login",
+    username: "joe",
+    password: "secretlogin",
+  },
+];
+
+// the expected state of passwords after the seconds 'modify' action
+var passwords_after_second_modify = [
+  {
+    hostname: "http://www.example.com",
+    submitURL: "http://login.example.com",
+    username: "joe",
+    password: "zippity-do-dah",
+    usernameField: "uname",
+    passwordField: "pword",
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [
+  [Bookmarks.add, bookmarks_initial],
+  [Passwords.add, passwords_initial],
+  [History.add, history_initial],
+  [Sync],
+]);
+
+Phase("phase2", [
+  [Sync],
+  [Bookmarks.verify, bookmarks_initial],
+  [Passwords.verify, passwords_initial],
+  [History.verify, history_initial],
+  [Bookmarks.modify, bookmarks_initial],
+  [Passwords.modify, passwords_initial],
+  [History.delete, history_to_delete],
+  [Bookmarks.verify, bookmarks_after_first_modify],
+  [Passwords.verify, passwords_after_first_modify],
+  [History.verify, history_after_delete],
+  [History.verifyNot, history_absent],
+  [Sync],
+]);
+
+Phase("phase3", [
+  [Sync],
+  [Bookmarks.verify, bookmarks_after_first_modify],
+  [Passwords.verify, passwords_after_first_modify],
+  [History.verify, history_after_delete],
+  [Bookmarks.modify, bookmarks_after_first_modify],
+  [Passwords.modify, passwords_after_first_modify],
+  [Bookmarks.delete, bookmarks_to_delete],
+  [Passwords.delete, passwords_to_delete],
+  [Bookmarks.verify, bookmarks_after_second_modify],
+  [Passwords.verify, passwords_after_second_modify],
+  [Bookmarks.verifyNot, bookmarks_absent],
+  [Passwords.verifyNot, passwords_absent],
+  [Sync],
+]);
+
+Phase("phase4", [
+  [Sync],
+  [Bookmarks.verify, bookmarks_after_second_modify],
+  [Passwords.verify, passwords_after_second_modify],
+  [Bookmarks.verifyNot, bookmarks_absent],
+  [Passwords.verifyNot, passwords_absent],
+  [History.verifyNot, history_absent],
+]);
diff --git a/services/sync/tests/tps/test_tabs.js b/services/sync/tests/tps/test_tabs.js
new file mode 100644
index 0000000000..58e8d358cd
--- /dev/null
+++ b/services/sync/tests/tps/test_tabs.js
@@ -0,0 +1,42 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+EnableEngines(["tabs"]);
+
+/*
+ * The list of phases mapped to their corresponding profiles.  The object
+ * here must be in JSON format as it will get parsed by the Python
+ * testrunner. It is parsed by the YAML package, so it relatively flexible.
+ */
+var phases = { phase1: "profile1", phase2: "profile2", phase3: "profile1" };
+
+/*
+ * Tab lists.
+ */
+
+var tabs1 = [
+  { uri: "https://www.mozilla.org/en-US/firefox/", profile: "profile1" },
+  {
+    uri: "https://example.com/",
+    title: "Example Domain",
+    profile: "profile1",
+  },
+];
+
+var tabs2 = [
+  { uri: "https://www.mozilla.org/en-US/contribute/", profile: "profile2" },
+  {
+    uri: "https://example.com/",
+    profile: "profile2",
+  },
+];
+
+/*
+ * Test phases
+ */
+
+Phase("phase1", [[Tabs.add, tabs1], [Sync]]);
+
+Phase("phase2", [[Sync], [Tabs.verify, tabs1], [Tabs.add, tabs2], [Sync]]);
+
+Phase("phase3", [[Sync], [Tabs.verify, tabs2]]);
diff --git a/services/sync/tests/unit/addon1-search.json b/services/sync/tests/unit/addon1-search.json
new file mode 100644
index 0000000000..55f8af8857
--- /dev/null
+++ b/services/sync/tests/unit/addon1-search.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Non-Restartless Test Extension",
+      "type": "extension",
+      "guid": "addon1@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 485,
+            "url": "http://127.0.0.1:8888/addon1.xpi"
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/unit/bootstrap1-search.json b/services/sync/tests/unit/bootstrap1-search.json
new file mode 100644
index 0000000000..8cd1cf43ed
--- /dev/null
+++ b/services/sync/tests/unit/bootstrap1-search.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Restartless Test Extension",
+      "type": "extension",
+      "guid": "bootstrap1@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 485,
+            "url": "http://127.0.0.1:8888/bootstrap1.xpi"
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/unit/head_appinfo.js b/services/sync/tests/unit/head_appinfo.js
new file mode 100644
index 0000000000..79379ccea8
--- /dev/null
+++ b/services/sync/tests/unit/head_appinfo.js
@@ -0,0 +1,58 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* import-globals-from ../../../common/tests/unit/head_helpers.js */
+
+var { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+
+// Required to avoid failures.
+do_get_profile();
+
+// Init FormHistoryStartup and pretend we opened a profile.
+var fhs = Cc["@mozilla.org/satchel/form-history-startup;1"].getService(
+  Ci.nsIObserver
+);
+fhs.observe(null, "profile-after-change", null);
+
+// An app is going to have some prefs set which xpcshell tests don't.
+Services.prefs.setStringPref(
+  "identity.sync.tokenserver.uri",
+  "http://token-server"
+);
+
+// Make sure to provide the right OS so crypto loads the right binaries
+function getOS() {
+  switch (mozinfo.os) {
+    case "win":
+      return "WINNT";
+    case "mac":
+      return "Darwin";
+    default:
+      return "Linux";
+  }
+}
+
+const { updateAppInfo } = ChromeUtils.importESModule(
+  "resource://testing-common/AppInfo.sys.mjs"
+);
+updateAppInfo({
+  name: "XPCShell",
+  ID: "xpcshell@tests.mozilla.org",
+  version: "1",
+  platformVersion: "",
+  OS: getOS(),
+});
+
+// Register resource aliases. Normally done in SyncComponents.manifest.
+function addResourceAlias() {
+  const resProt = Services.io
+    .getProtocolHandler("resource")
+    .QueryInterface(Ci.nsIResProtocolHandler);
+  for (let s of ["common", "sync", "crypto"]) {
+    let uri = Services.io.newURI("resource://gre/modules/services-" + s + "/");
+    resProt.setSubstitution("services-" + s, uri);
+  }
+}
+addResourceAlias();
diff --git a/services/sync/tests/unit/head_errorhandler_common.js b/services/sync/tests/unit/head_errorhandler_common.js
new file mode 100644
index 0000000000..fcf44ec43b
--- /dev/null
+++ b/services/sync/tests/unit/head_errorhandler_common.js
@@ -0,0 +1,195 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* import-globals-from head_appinfo.js */
+/* import-globals-from ../../../common/tests/unit/head_helpers.js */
+/* import-globals-from head_helpers.js */
+/* import-globals-from head_http_server.js */
+
+// This file expects Service to be defined in the global scope when EHTestsCommon
+// is used (from service.js).
+/* global Service */
+
+var { Changeset, EngineManager, Store, SyncEngine, Tracker, LegacyTracker } =
+  ChromeUtils.importESModule("resource://services-sync/engines.sys.mjs");
+var {
+  ABORT_SYNC_COMMAND,
+  CLIENT_NOT_CONFIGURED,
+  CREDENTIALS_CHANGED,
+  DEFAULT_DOWNLOAD_BATCH_SIZE,
+  DEFAULT_GUID_FETCH_BATCH_SIZE,
+  DEFAULT_KEYBUNDLE_NAME,
+  DEVICE_TYPE_DESKTOP,
+  DEVICE_TYPE_MOBILE,
+  ENGINE_APPLY_FAIL,
+  ENGINE_BATCH_INTERRUPTED,
+  ENGINE_DOWNLOAD_FAIL,
+  ENGINE_SUCCEEDED,
+  ENGINE_UNKNOWN_FAIL,
+  ENGINE_UPLOAD_FAIL,
+  HMAC_EVENT_INTERVAL,
+  IDLE_OBSERVER_BACK_DELAY,
+  LOGIN_FAILED,
+  LOGIN_FAILED_INVALID_PASSPHRASE,
+  LOGIN_FAILED_LOGIN_REJECTED,
+  LOGIN_FAILED_NETWORK_ERROR,
+  LOGIN_FAILED_NO_PASSPHRASE,
+  LOGIN_FAILED_NO_USERNAME,
+  LOGIN_FAILED_SERVER_ERROR,
+  LOGIN_SUCCEEDED,
+  MASTER_PASSWORD_LOCKED,
+  MASTER_PASSWORD_LOCKED_RETRY_INTERVAL,
+  MAXIMUM_BACKOFF_INTERVAL,
+  MAX_ERROR_COUNT_BEFORE_BACKOFF,
+  MAX_HISTORY_DOWNLOAD,
+  MAX_HISTORY_UPLOAD,
+  METARECORD_DOWNLOAD_FAIL,
+  MINIMUM_BACKOFF_INTERVAL,
+  MULTI_DEVICE_THRESHOLD,
+  NO_SYNC_NODE_FOUND,
+  NO_SYNC_NODE_INTERVAL,
+  OVER_QUOTA,
+  PREFS_BRANCH,
+  RESPONSE_OVER_QUOTA,
+  SCORE_INCREMENT_MEDIUM,
+  SCORE_INCREMENT_SMALL,
+  SCORE_INCREMENT_XLARGE,
+  SCORE_UPDATE_DELAY,
+  SERVER_MAINTENANCE,
+  SINGLE_USER_THRESHOLD,
+  SQLITE_MAX_VARIABLE_NUMBER,
+  STATUS_DISABLED,
+  STATUS_OK,
+  STORAGE_VERSION,
+  SYNC_FAILED,
+  SYNC_FAILED_PARTIAL,
+  SYNC_KEY_DECODED_LENGTH,
+  SYNC_KEY_ENCODED_LENGTH,
+  SYNC_SUCCEEDED,
+  URI_LENGTH_MAX,
+  VERSION_OUT_OF_DATE,
+  WEAVE_VERSION,
+  kFirefoxShuttingDown,
+  kFirstSyncChoiceNotMade,
+  kSyncBackoffNotMet,
+  kSyncMasterPasswordLocked,
+  kSyncNetworkOffline,
+  kSyncNotConfigured,
+  kSyncWeaveDisabled,
+} = ChromeUtils.importESModule("resource://services-sync/constants.sys.mjs");
+var { BulkKeyBundle, SyncKeyBundle } = ChromeUtils.importESModule(
+  "resource://services-sync/keys.sys.mjs"
+);
+
+// Common code for test_errorhandler_{1,2}.js -- pulled out to make it less
+// monolithic and take less time to execute.
+const EHTestsCommon = {
+  service_unavailable(request, response) {
+    let body = "Service Unavailable";
+    response.setStatusLine(request.httpVersion, 503, "Service Unavailable");
+    response.setHeader("Retry-After", "42");
+    response.bodyOutputStream.write(body, body.length);
+  },
+
+  async sync_httpd_setup() {
+    let clientsEngine = Service.clientsEngine;
+    let clientsSyncID = await clientsEngine.resetLocalSyncID();
+    let catapultEngine = Service.engineManager.get("catapult");
+    let catapultSyncID = await catapultEngine.resetLocalSyncID();
+    let global = new ServerWBO("global", {
+      syncID: Service.syncID,
+      storageVersion: STORAGE_VERSION,
+      engines: {
+        clients: { version: clientsEngine.version, syncID: clientsSyncID },
+        catapult: { version: catapultEngine.version, syncID: catapultSyncID },
+      },
+    });
+    let clientsColl = new ServerCollection({}, true);
+
+    // Tracking info/collections.
+    let collectionsHelper = track_collections_helper();
+    let upd = collectionsHelper.with_updated_collection;
+
+    let handler_401 = httpd_handler(401, "Unauthorized");
+    return httpd_setup({
+      // Normal server behaviour.
+      "/1.1/johndoe/storage/meta/global": upd("meta", global.handler()),
+      "/1.1/johndoe/info/collections": collectionsHelper.handler,
+      "/1.1/johndoe/storage/crypto/keys": upd(
+        "crypto",
+        new ServerWBO("keys").handler()
+      ),
+      "/1.1/johndoe/storage/clients": upd("clients", clientsColl.handler()),
+
+      // Credentials are wrong or node reallocated.
+      "/1.1/janedoe/storage/meta/global": handler_401,
+      "/1.1/janedoe/info/collections": handler_401,
+
+      // Maintenance or overloaded (503 + Retry-After) at info/collections.
+      "/1.1/broken.info/info/collections": EHTestsCommon.service_unavailable,
+
+      // Maintenance or overloaded (503 + Retry-After) at meta/global.
+      "/1.1/broken.meta/storage/meta/global": EHTestsCommon.service_unavailable,
+      "/1.1/broken.meta/info/collections": collectionsHelper.handler,
+
+      // Maintenance or overloaded (503 + Retry-After) at crypto/keys.
+      "/1.1/broken.keys/storage/meta/global": upd("meta", global.handler()),
+      "/1.1/broken.keys/info/collections": collectionsHelper.handler,
+      "/1.1/broken.keys/storage/crypto/keys": EHTestsCommon.service_unavailable,
+
+      // Maintenance or overloaded (503 + Retry-After) at wiping collection.
+      "/1.1/broken.wipe/info/collections": collectionsHelper.handler,
+      "/1.1/broken.wipe/storage/meta/global": upd("meta", global.handler()),
+      "/1.1/broken.wipe/storage/crypto/keys": upd(
+        "crypto",
+        new ServerWBO("keys").handler()
+      ),
+      "/1.1/broken.wipe/storage": EHTestsCommon.service_unavailable,
+      "/1.1/broken.wipe/storage/clients": upd("clients", clientsColl.handler()),
+      "/1.1/broken.wipe/storage/catapult": EHTestsCommon.service_unavailable,
+    });
+  },
+
+  CatapultEngine: (function () {
+    function CatapultEngine() {
+      SyncEngine.call(this, "Catapult", Service);
+    }
+    CatapultEngine.prototype = {
+      exception: null, // tests fill this in
+      async _sync() {
+        if (this.exception) {
+          throw this.exception;
+        }
+      },
+    };
+    Object.setPrototypeOf(CatapultEngine.prototype, SyncEngine.prototype);
+
+    return CatapultEngine;
+  })(),
+
+  async generateCredentialsChangedFailure() {
+    // Make sync fail due to changed credentials. We simply re-encrypt
+    // the keys with a different Sync Key, without changing the local one.
+    let newSyncKeyBundle = new BulkKeyBundle("crypto");
+    await newSyncKeyBundle.generateRandom();
+    let keys = Service.collectionKeys.asWBO();
+    await keys.encrypt(newSyncKeyBundle);
+    return keys.upload(Service.resource(Service.cryptoKeysURL));
+  },
+
+  async setUp(server) {
+    syncTestLogging();
+    await configureIdentity({ username: "johndoe" }, server);
+    return EHTestsCommon.generateAndUploadKeys();
+  },
+
+  async generateAndUploadKeys() {
+    await generateNewKeys(Service.collectionKeys);
+    let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+    await serverKeys.encrypt(Service.identity.syncKeyBundle);
+    let response = await serverKeys.upload(
+      Service.resource(Service.cryptoKeysURL)
+    );
+    return response.success;
+  },
+};
diff --git a/services/sync/tests/unit/head_helpers.js b/services/sync/tests/unit/head_helpers.js
new file mode 100644
index 0000000000..e79e55e57f
--- /dev/null
+++ b/services/sync/tests/unit/head_helpers.js
@@ -0,0 +1,709 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* import-globals-from head_appinfo.js */
+/* import-globals-from ../../../common/tests/unit/head_helpers.js */
+/* import-globals-from head_errorhandler_common.js */
+/* import-globals-from head_http_server.js */
+
+// This file expects Service to be defined in the global scope when EHTestsCommon
+// is used (from service.js).
+/* global Service */
+
+var { AddonTestUtils, MockAsyncShutdown } = ChromeUtils.importESModule(
+  "resource://testing-common/AddonTestUtils.sys.mjs"
+);
+var { Async } = ChromeUtils.importESModule(
+  "resource://services-common/async.sys.mjs"
+);
+var { CommonUtils } = ChromeUtils.importESModule(
+  "resource://services-common/utils.sys.mjs"
+);
+var { PlacesTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/PlacesTestUtils.sys.mjs"
+);
+var { sinon } = ChromeUtils.importESModule(
+  "resource://testing-common/Sinon.sys.mjs"
+);
+var { SerializableSet, Svc, Utils, getChromeWindow } =
+  ChromeUtils.importESModule("resource://services-sync/util.sys.mjs");
+var { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+var { PlacesUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/PlacesUtils.sys.mjs"
+);
+var { PlacesSyncUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/PlacesSyncUtils.sys.mjs"
+);
+var { ObjectUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/ObjectUtils.sys.mjs"
+);
+var {
+  MockFxaStorageManager,
+  SyncTestingInfrastructure,
+  configureFxAccountIdentity,
+  configureIdentity,
+  encryptPayload,
+  getLoginTelemetryScalar,
+  makeFxAccountsInternalMock,
+  makeIdentityConfig,
+  promiseNamedTimer,
+  promiseZeroTimer,
+  sumHistogram,
+  syncTestLogging,
+  waitForZeroTimer,
+} = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/utils.sys.mjs"
+);
+ChromeUtils.defineESModuleGetters(this, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+});
+
+add_setup(async function head_setup() {
+  // Initialize logging. This will sometimes be reset by a pref reset,
+  // so it's also called as part of SyncTestingInfrastructure().
+  syncTestLogging();
+  // If a test imports Service, make sure it is initialized first.
+  if (typeof Service !== "undefined") {
+    await Service.promiseInitialized;
+  }
+});
+
+ChromeUtils.defineLazyGetter(this, "SyncPingSchema", function () {
+  let { FileUtils } = ChromeUtils.importESModule(
+    "resource://gre/modules/FileUtils.sys.mjs"
+  );
+  let { NetUtil } = ChromeUtils.importESModule(
+    "resource://gre/modules/NetUtil.sys.mjs"
+  );
+  let stream = Cc["@mozilla.org/network/file-input-stream;1"].createInstance(
+    Ci.nsIFileInputStream
+  );
+  let schema;
+  try {
+    let schemaFile = do_get_file("sync_ping_schema.json");
+    stream.init(schemaFile, FileUtils.MODE_RDONLY, FileUtils.PERMS_FILE, 0);
+
+    let bytes = NetUtil.readInputStream(stream, stream.available());
+    schema = JSON.parse(new TextDecoder().decode(bytes));
+  } finally {
+    stream.close();
+  }
+
+  // Allow tests to make whatever engines they want, this shouldn't cause
+  // validation failure.
+  schema.definitions.engine.properties.name = { type: "string" };
+  return schema;
+});
+
+ChromeUtils.defineLazyGetter(this, "SyncPingValidator", function () {
+  const { JsonSchema } = ChromeUtils.importESModule(
+    "resource://gre/modules/JsonSchema.sys.mjs"
+  );
+  return new JsonSchema.Validator(SyncPingSchema);
+});
+
+// This is needed for loadAddonTestFunctions().
+var gGlobalScope = this;
+
+function ExtensionsTestPath(path) {
+  if (path[0] != "/") {
+    throw Error("Path must begin with '/': " + path);
+  }
+
+  return "../../../../toolkit/mozapps/extensions/test/xpcshell" + path;
+}
+
+function webExtensionsTestPath(path) {
+  if (path[0] != "/") {
+    throw Error("Path must begin with '/': " + path);
+  }
+
+  return "../../../../toolkit/components/extensions/test/xpcshell" + path;
+}
+
+/**
+ * Loads the WebExtension test functions by importing its test file.
+ */
+function loadWebExtensionTestFunctions() {
+  /* import-globals-from ../../../../toolkit/components/extensions/test/xpcshell/head_sync.js */
+  const path = webExtensionsTestPath("/head_sync.js");
+  let file = do_get_file(path);
+  let uri = Services.io.newFileURI(file);
+  Services.scriptloader.loadSubScript(uri.spec, gGlobalScope);
+}
+
+/**
+ * Installs an add-on from an addonInstall
+ *
+ * @param  install addonInstall instance to install
+ */
+async function installAddonFromInstall(install) {
+  await install.install();
+
+  Assert.notEqual(null, install.addon);
+  Assert.notEqual(null, install.addon.syncGUID);
+
+  return install.addon;
+}
+
+/**
+ * Convenience function to install an add-on from the extensions unit tests.
+ *
+ * @param  file
+ *         Add-on file to install.
+ * @param  reconciler
+ *         addons reconciler, if passed we will wait on the events to be
+ *         processed before resolving
+ * @return addon object that was installed
+ */
+async function installAddon(file, reconciler = null) {
+  let install = await AddonManager.getInstallForFile(file);
+  Assert.notEqual(null, install);
+  const addon = await installAddonFromInstall(install);
+  if (reconciler) {
+    await reconciler.queueCaller.promiseCallsComplete();
+  }
+  return addon;
+}
+
+/**
+ * Convenience function to uninstall an add-on.
+ *
+ * @param addon
+ *        Addon instance to uninstall
+ * @param reconciler
+ *        addons reconciler, if passed we will wait on the events to be
+ *        processed before resolving
+ */
+async function uninstallAddon(addon, reconciler = null) {
+  const uninstallPromise = new Promise(res => {
+    let listener = {
+      onUninstalled(uninstalled) {
+        if (uninstalled.id == addon.id) {
+          AddonManager.removeAddonListener(listener);
+          res(uninstalled);
+        }
+      },
+    };
+    AddonManager.addAddonListener(listener);
+  });
+  addon.uninstall();
+  await uninstallPromise;
+  if (reconciler) {
+    await reconciler.queueCaller.promiseCallsComplete();
+  }
+}
+
+async function generateNewKeys(collectionKeys, collections = null) {
+  let wbo = await collectionKeys.generateNewKeysWBO(collections);
+  let modified = new_timestamp();
+  collectionKeys.setContents(wbo.cleartext, modified);
+}
+
+// Helpers for testing open tabs.
+// These reflect part of the internal structure of TabEngine,
+// and stub part of Service.wm.
+
+function mockShouldSkipWindow(win) {
+  return win.closed || win.mockIsPrivate;
+}
+
+function mockGetTabState(tab) {
+  return tab;
+}
+
+function mockGetWindowEnumerator(urls) {
+  let elements = [];
+
+  const numWindows = 1;
+  for (let w = 0; w < numWindows; ++w) {
+    let tabs = [];
+    let win = {
+      closed: false,
+      mockIsPrivate: false,
+      gBrowser: {
+        tabs,
+      },
+    };
+    elements.push(win);
+
+    let lastAccessed = 2000;
+    for (let url of urls) {
+      tabs.push({
+        linkedBrowser: {
+          currentURI: Services.io.newURI(url),
+          contentTitle: "title",
+        },
+        lastAccessed,
+      });
+      lastAccessed += 1000;
+    }
+  }
+
+  // Always include a closed window and a private window.
+  elements.push({
+    closed: true,
+    mockIsPrivate: false,
+    gBrowser: {
+      tabs: [],
+    },
+  });
+
+  elements.push({
+    closed: false,
+    mockIsPrivate: true,
+    gBrowser: {
+      tabs: [],
+    },
+  });
+
+  return elements.values();
+}
+
+// Helper function to get the sync telemetry and add the typically used test
+// engine names to its list of allowed engines.
+function get_sync_test_telemetry() {
+  let { SyncTelemetry } = ChromeUtils.importESModule(
+    "resource://services-sync/telemetry.sys.mjs"
+  );
+  SyncTelemetry.tryRefreshDevices = function () {};
+  let testEngines = ["rotary", "steam", "sterling", "catapult", "nineties"];
+  for (let engineName of testEngines) {
+    SyncTelemetry.allowedEngines.add(engineName);
+  }
+  SyncTelemetry.submissionInterval = -1;
+  return SyncTelemetry;
+}
+
+function assert_valid_ping(record) {
+  // Our JSON validator does not like `undefined` values, even though they will
+  // be skipped when we serialize to JSON.
+  record = JSON.parse(JSON.stringify(record));
+
+  // This is called as the test harness tears down due to shutdown. This
+  // will typically have no recorded syncs, and the validator complains about
+  // it. So ignore such records (but only ignore when *both* shutdown and
+  // no Syncs - either of them not being true might be an actual problem)
+  if (record && (record.why != "shutdown" || !!record.syncs.length)) {
+    const result = SyncPingValidator.validate(record);
+    if (!result.valid) {
+      if (result.errors.length) {
+        // validation failed - using a simple |deepEqual([], errors)| tends to
+        // truncate the validation errors in the output and doesn't show that
+        // the ping actually was - so be helpful.
+        info("telemetry ping validation failed");
+        info("the ping data is: " + JSON.stringify(record, undefined, 2));
+        info(
+          "the validation failures: " +
+            JSON.stringify(result.errors, undefined, 2)
+        );
+        ok(
+          false,
+          "Sync telemetry ping validation failed - see output above for details"
+        );
+      }
+    }
+    equal(record.version, 1);
+    record.syncs.forEach(p => {
+      lessOrEqual(p.when, Date.now());
+    });
+  }
+}
+
+// Asserts that `ping` is a ping that doesn't contain any failure information
+function assert_success_ping(ping) {
+  ok(!!ping);
+  assert_valid_ping(ping);
+  ping.syncs.forEach(record => {
+    ok(!record.failureReason, JSON.stringify(record.failureReason));
+    equal(undefined, record.status);
+    greater(record.engines.length, 0);
+    for (let e of record.engines) {
+      ok(!e.failureReason);
+      equal(undefined, e.status);
+      if (e.validation) {
+        equal(undefined, e.validation.problems);
+        equal(undefined, e.validation.failureReason);
+      }
+      if (e.outgoing) {
+        for (let o of e.outgoing) {
+          equal(undefined, o.failed);
+          notEqual(undefined, o.sent);
+        }
+      }
+      if (e.incoming) {
+        equal(undefined, e.incoming.failed);
+        equal(undefined, e.incoming.newFailed);
+        notEqual(undefined, e.incoming.applied || e.incoming.reconciled);
+      }
+    }
+  });
+}
+
+// Hooks into telemetry to validate all pings after calling.
+function validate_all_future_pings() {
+  let telem = get_sync_test_telemetry();
+  telem.submit = assert_valid_ping;
+}
+
+function wait_for_pings(expectedPings) {
+  return new Promise(resolve => {
+    let telem = get_sync_test_telemetry();
+    let oldSubmit = telem.submit;
+    let pings = [];
+    telem.submit = function (record) {
+      pings.push(record);
+      if (pings.length == expectedPings) {
+        telem.submit = oldSubmit;
+        resolve(pings);
+      }
+    };
+  });
+}
+
+async function wait_for_ping(callback, allowErrorPings, getFullPing = false) {
+  let pingsPromise = wait_for_pings(1);
+  await callback();
+  let [record] = await pingsPromise;
+  if (allowErrorPings) {
+    assert_valid_ping(record);
+  } else {
+    assert_success_ping(record);
+  }
+  if (getFullPing) {
+    return record;
+  }
+  equal(record.syncs.length, 1);
+  return record.syncs[0];
+}
+
+// Perform a sync and validate all telemetry caused by the sync. If fnValidate
+// is null, we just check the ping records success. If fnValidate is specified,
+// then the sync must have recorded just a single sync, and that sync will be
+// passed to the function to be checked.
+async function sync_and_validate_telem(
+  fnValidate = null,
+  wantFullPing = false
+) {
+  let numErrors = 0;
+  let telem = get_sync_test_telemetry();
+  let oldSubmit = telem.submit;
+  try {
+    telem.submit = function (record) {
+      // This is called via an observer, so failures here don't cause the test
+      // to fail :(
+      try {
+        // All pings must be valid.
+        assert_valid_ping(record);
+        if (fnValidate) {
+          // for historical reasons most of these callbacks expect a "sync"
+          // record, not the entire ping.
+          if (wantFullPing) {
+            fnValidate(record);
+          } else {
+            Assert.equal(record.syncs.length, 1);
+            fnValidate(record.syncs[0]);
+          }
+        } else {
+          // no validation function means it must be a "success" ping.
+          assert_success_ping(record);
+        }
+      } catch (ex) {
+        print("Failure in ping validation callback", ex, "\n", ex.stack);
+        numErrors += 1;
+      }
+    };
+    await Service.sync();
+    Assert.ok(numErrors == 0, "There were telemetry validation errors");
+  } finally {
+    telem.submit = oldSubmit;
+  }
+}
+
+// Used for the (many) cases where we do a 'partial' sync, where only a single
+// engine is actually synced, but we still want to ensure we're generating a
+// valid ping. Returns a promise that resolves to the ping, or rejects with the
+// thrown error after calling an optional callback.
+async function sync_engine_and_validate_telem(
+  engine,
+  allowErrorPings,
+  onError,
+  wantFullPing = false
+) {
+  let telem = get_sync_test_telemetry();
+  let caughtError = null;
+  // Clear out status, so failures from previous syncs won't show up in the
+  // telemetry ping.
+  let { Status } = ChromeUtils.importESModule(
+    "resource://services-sync/status.sys.mjs"
+  );
+  Status._engines = {};
+  Status.partial = false;
+  // Ideally we'd clear these out like we do with engines, (probably via
+  // Status.resetSync()), but this causes *numerous* tests to fail, so we just
+  // assume that if no failureReason or engine failures are set, and the
+  // status properties are the same as they were initially, that it's just
+  // a leftover.
+  // This is only an issue since we're triggering the sync of just one engine,
+  // without doing any other parts of the sync.
+  let initialServiceStatus = Status._service;
+  let initialSyncStatus = Status._sync;
+
+  let oldSubmit = telem.submit;
+  let submitPromise = new Promise((resolve, reject) => {
+    telem.submit = function (ping) {
+      telem.submit = oldSubmit;
+      ping.syncs.forEach(record => {
+        if (record && record.status) {
+          // did we see anything to lead us to believe that something bad actually happened
+          let realProblem =
+            record.failureReason ||
+            record.engines.some(e => {
+              if (e.failureReason || e.status) {
+                return true;
+              }
+              if (e.outgoing && e.outgoing.some(o => o.failed > 0)) {
+                return true;
+              }
+              return e.incoming && e.incoming.failed;
+            });
+          if (!realProblem) {
+            // no, so if the status is the same as it was initially, just assume
+            // that its leftover and that we can ignore it.
+            if (record.status.sync && record.status.sync == initialSyncStatus) {
+              delete record.status.sync;
+            }
+            if (
+              record.status.service &&
+              record.status.service == initialServiceStatus
+            ) {
+              delete record.status.service;
+            }
+            if (!record.status.sync && !record.status.service) {
+              delete record.status;
+            }
+          }
+        }
+      });
+      if (allowErrorPings) {
+        assert_valid_ping(ping);
+      } else {
+        assert_success_ping(ping);
+      }
+      equal(ping.syncs.length, 1);
+      if (caughtError) {
+        if (onError) {
+          onError(ping.syncs[0], ping);
+        }
+        reject(caughtError);
+      } else if (wantFullPing) {
+        resolve(ping);
+      } else {
+        resolve(ping.syncs[0]);
+      }
+    };
+  });
+  // neuter the scheduler as it interacts badly with some of the tests - the
+  // engine being synced usually isn't the registered engine, so we see
+  // scored incremented and not removed, which schedules unexpected syncs.
+  let oldObserve = Service.scheduler.observe;
+  Service.scheduler.observe = () => {};
+  try {
+    Svc.Obs.notify("weave:service:sync:start");
+    try {
+      await engine.sync();
+    } catch (e) {
+      caughtError = e;
+    }
+    if (caughtError) {
+      Svc.Obs.notify("weave:service:sync:error", caughtError);
+    } else {
+      Svc.Obs.notify("weave:service:sync:finish");
+    }
+  } finally {
+    Service.scheduler.observe = oldObserve;
+  }
+  return submitPromise;
+}
+
+// Returns a promise that resolves once the specified observer notification
+// has fired.
+function promiseOneObserver(topic, callback) {
+  return new Promise((resolve, reject) => {
+    let observer = function (subject, data) {
+      Svc.Obs.remove(topic, observer);
+      resolve({ subject, data });
+    };
+    Svc.Obs.add(topic, observer);
+  });
+}
+
+async function registerRotaryEngine() {
+  let { RotaryEngine } = ChromeUtils.importESModule(
+    "resource://testing-common/services/sync/rotaryengine.sys.mjs"
+  );
+  await Service.engineManager.clear();
+
+  await Service.engineManager.register(RotaryEngine);
+  let engine = Service.engineManager.get("rotary");
+  let syncID = await engine.resetLocalSyncID();
+  engine.enabled = true;
+
+  return { engine, syncID, tracker: engine._tracker };
+}
+
+// Set the validation prefs to attempt validation every time to avoid non-determinism.
+function enableValidationPrefs(engines = ["bookmarks"]) {
+  for (let engine of engines) {
+    Svc.PrefBranch.setIntPref(`engine.${engine}.validation.interval`, 0);
+    Svc.PrefBranch.setIntPref(
+      `engine.${engine}.validation.percentageChance`,
+      100
+    );
+    Svc.PrefBranch.setIntPref(`engine.${engine}.validation.maxRecords`, -1);
+    Svc.PrefBranch.setBoolPref(`engine.${engine}.validation.enabled`, true);
+  }
+}
+
+async function serverForEnginesWithKeys(users, engines, callback) {
+  // Generate and store a fake default key bundle to avoid resetting the client
+  // before the first sync.
+  let wbo = await Service.collectionKeys.generateNewKeysWBO();
+  let modified = new_timestamp();
+  Service.collectionKeys.setContents(wbo.cleartext, modified);
+
+  let allEngines = [Service.clientsEngine].concat(engines);
+
+  let globalEngines = {};
+  for (let engine of allEngines) {
+    let syncID = await engine.resetLocalSyncID();
+    globalEngines[engine.name] = { version: engine.version, syncID };
+  }
+
+  let contents = {
+    meta: {
+      global: {
+        syncID: Service.syncID,
+        storageVersion: STORAGE_VERSION,
+        engines: globalEngines,
+      },
+    },
+    crypto: {
+      keys: encryptPayload(wbo.cleartext),
+    },
+  };
+  for (let engine of allEngines) {
+    contents[engine.name] = {};
+  }
+
+  return serverForUsers(users, contents, callback);
+}
+
+async function serverForFoo(engine, callback) {
+  // The bookmarks engine *always* tracks changes, meaning we might try
+  // and sync due to the bookmarks we ourselves create! Worse, because we
+  // do an engine sync only, there's no locking - so we end up with multiple
+  // syncs running. Neuter that by making the threshold very large.
+  Service.scheduler.syncThreshold = 10000000;
+  return serverForEnginesWithKeys({ foo: "password" }, engine, callback);
+}
+
+// Places notifies history observers asynchronously, so `addVisits` might return
+// before the tracker receives the notification. This helper registers an
+// observer that resolves once the expected notification fires.
+async function promiseVisit(expectedType, expectedURI) {
+  return new Promise(resolve => {
+    function done(type, uri) {
+      if (uri == expectedURI.spec && type == expectedType) {
+        PlacesObservers.removeListener(
+          ["page-visited", "page-removed"],
+          observer.handlePlacesEvents
+        );
+        resolve();
+      }
+    }
+    let observer = {
+      handlePlacesEvents(events) {
+        Assert.equal(events.length, 1);
+
+        if (events[0].type === "page-visited") {
+          done("added", events[0].url);
+        } else if (events[0].type === "page-removed") {
+          Assert.ok(events[0].isRemovedFromStore);
+          done("removed", events[0].url);
+        }
+      },
+    };
+    PlacesObservers.addListener(
+      ["page-visited", "page-removed"],
+      observer.handlePlacesEvents
+    );
+  });
+}
+
+async function addVisit(
+  suffix,
+  referrer = null,
+  transition = PlacesUtils.history.TRANSITION_LINK
+) {
+  let uriString = "http://getfirefox.com/" + suffix;
+  let uri = CommonUtils.makeURI(uriString);
+  _("Adding visit for URI " + uriString);
+
+  let visitAddedPromise = promiseVisit("added", uri);
+  await PlacesTestUtils.addVisits({
+    uri,
+    visitDate: Date.now() * 1000,
+    transition,
+    referrer,
+  });
+  await visitAddedPromise;
+
+  return uri;
+}
+
+function bookmarkNodesToInfos(nodes) {
+  return nodes.map(node => {
+    let info = {
+      guid: node.guid,
+      index: node.index,
+    };
+    if (node.children) {
+      info.children = bookmarkNodesToInfos(node.children);
+    }
+    return info;
+  });
+}
+
+async function assertBookmarksTreeMatches(rootGuid, expected, message) {
+  let root = await PlacesUtils.promiseBookmarksTree(rootGuid, {
+    includeItemIds: true,
+  });
+  let actual = bookmarkNodesToInfos(root.children);
+
+  if (!ObjectUtils.deepEqual(actual, expected)) {
+    _(`Expected structure for ${rootGuid}`, JSON.stringify(expected));
+    _(`Actual structure for ${rootGuid}`, JSON.stringify(actual));
+    throw new Assert.constructor.AssertionError({ actual, expected, message });
+  }
+}
+
+function add_bookmark_test(task) {
+  const { BookmarksEngine } = ChromeUtils.importESModule(
+    "resource://services-sync/engines/bookmarks.sys.mjs"
+  );
+
+  add_task(async function () {
+    _(`Running bookmarks test ${task.name}`);
+    let engine = new BookmarksEngine(Service);
+    await engine.initialize();
+    await engine._resetClient();
+    try {
+      await task(engine);
+    } finally {
+      await engine.finalize();
+    }
+  });
+}
diff --git a/services/sync/tests/unit/head_http_server.js b/services/sync/tests/unit/head_http_server.js
new file mode 100644
index 0000000000..84dbb33951
--- /dev/null
+++ b/services/sync/tests/unit/head_http_server.js
@@ -0,0 +1,1265 @@
+/* import-globals-from head_appinfo.js */
+/* import-globals-from ../../../common/tests/unit/head_helpers.js */
+/* import-globals-from head_helpers.js */
+
+var Cm = Components.manager;
+
+// Shared logging for all HTTP server functions.
+var { Log } = ChromeUtils.importESModule("resource://gre/modules/Log.sys.mjs");
+var { CommonUtils } = ChromeUtils.importESModule(
+  "resource://services-common/utils.sys.mjs"
+);
+var { TestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/TestUtils.sys.mjs"
+);
+var {
+  MockFxaStorageManager,
+  SyncTestingInfrastructure,
+  configureFxAccountIdentity,
+  configureIdentity,
+  encryptPayload,
+  getLoginTelemetryScalar,
+  makeFxAccountsInternalMock,
+  makeIdentityConfig,
+  promiseNamedTimer,
+  promiseZeroTimer,
+  sumHistogram,
+  syncTestLogging,
+  waitForZeroTimer,
+} = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/utils.sys.mjs"
+);
+
+const SYNC_HTTP_LOGGER = "Sync.Test.Server";
+
+// While the sync code itself uses 1.5, the tests hard-code 1.1,
+// so we're sticking with 1.1 here.
+const SYNC_API_VERSION = "1.1";
+
+// Use the same method that record.js does, which mirrors the server.
+// The server returns timestamps with 1/100 sec granularity. Note that this is
+// subject to change: see Bug 650435.
+function new_timestamp() {
+  return round_timestamp(Date.now());
+}
+
+// Rounds a millisecond timestamp `t` to seconds, with centisecond precision.
+function round_timestamp(t) {
+  return Math.round(t / 10) / 100;
+}
+
+function return_timestamp(request, response, timestamp) {
+  if (!timestamp) {
+    timestamp = new_timestamp();
+  }
+  let body = "" + timestamp;
+  response.setHeader("X-Weave-Timestamp", body);
+  response.setStatusLine(request.httpVersion, 200, "OK");
+  writeBytesToOutputStream(response.bodyOutputStream, body);
+  return timestamp;
+}
+
+function has_hawk_header(req) {
+  return (
+    req.hasHeader("Authorization") &&
+    req.getHeader("Authorization").startsWith("Hawk")
+  );
+}
+
+function basic_auth_header(user, password) {
+  return "Basic " + btoa(user + ":" + CommonUtils.encodeUTF8(password));
+}
+
+function basic_auth_matches(req, user, password) {
+  if (!req.hasHeader("Authorization")) {
+    return false;
+  }
+
+  let expected = basic_auth_header(user, CommonUtils.encodeUTF8(password));
+  return req.getHeader("Authorization") == expected;
+}
+
+function httpd_basic_auth_handler(body, metadata, response) {
+  if (basic_auth_matches(metadata, "guest", "guest")) {
+    response.setStatusLine(metadata.httpVersion, 200, "OK, authorized");
+    response.setHeader("WWW-Authenticate", 'Basic realm="secret"', false);
+  } else {
+    body = "This path exists and is protected - failed";
+    response.setStatusLine(metadata.httpVersion, 401, "Unauthorized");
+    response.setHeader("WWW-Authenticate", 'Basic realm="secret"', false);
+  }
+  writeBytesToOutputStream(response.bodyOutputStream, body);
+}
+
+/*
+ * Represent a WBO on the server
+ */
+function ServerWBO(id, initialPayload, modified) {
+  if (!id) {
+    throw new Error("No ID for ServerWBO!");
+  }
+  this.id = id;
+  if (!initialPayload) {
+    return;
+  }
+
+  if (typeof initialPayload == "object") {
+    initialPayload = JSON.stringify(initialPayload);
+  }
+  this.payload = initialPayload;
+  this.modified = modified || new_timestamp();
+  this.sortindex = 0;
+}
+ServerWBO.prototype = {
+  get data() {
+    return JSON.parse(this.payload);
+  },
+
+  get() {
+    return { id: this.id, modified: this.modified, payload: this.payload };
+  },
+
+  put(input) {
+    input = JSON.parse(input);
+    this.payload = input.payload;
+    this.modified = new_timestamp();
+    this.sortindex = input.sortindex || 0;
+  },
+
+  delete() {
+    delete this.payload;
+    delete this.modified;
+    delete this.sortindex;
+  },
+
+  // This handler sets `newModified` on the response body if the collection
+  // timestamp has changed. This allows wrapper handlers to extract information
+  // that otherwise would exist only in the body stream.
+  handler() {
+    let self = this;
+
+    return function (request, response) {
+      var statusCode = 200;
+      var status = "OK";
+      var body;
+
+      switch (request.method) {
+        case "GET":
+          if (self.payload) {
+            body = JSON.stringify(self.get());
+          } else {
+            statusCode = 404;
+            status = "Not Found";
+            body = "Not Found";
+          }
+          break;
+
+        case "PUT":
+          self.put(readBytesFromInputStream(request.bodyInputStream));
+          body = JSON.stringify(self.modified);
+          response.setHeader("Content-Type", "application/json");
+          response.newModified = self.modified;
+          break;
+
+        case "DELETE":
+          self.delete();
+          let ts = new_timestamp();
+          body = JSON.stringify(ts);
+          response.setHeader("Content-Type", "application/json");
+          response.newModified = ts;
+          break;
+      }
+      response.setHeader("X-Weave-Timestamp", "" + new_timestamp(), false);
+      response.setStatusLine(request.httpVersion, statusCode, status);
+      writeBytesToOutputStream(response.bodyOutputStream, body);
+    };
+  },
+
+  /**
+   * Get the cleartext data stored in the payload.
+   *
+   * This isn't `get cleartext`, because `x.cleartext.blah = 3;` wouldn't work,
+   * which seems like a footgun.
+   */
+  getCleartext() {
+    return JSON.parse(JSON.parse(this.payload).ciphertext);
+  },
+
+  /**
+   * Setter for getCleartext(), but lets you adjust the modified timestamp too.
+   * Returns this ServerWBO object.
+   */
+  setCleartext(cleartext, modifiedTimestamp = this.modified) {
+    this.payload = JSON.stringify(encryptPayload(cleartext));
+    this.modified = modifiedTimestamp;
+    return this;
+  },
+};
+
+/**
+ * Represent a collection on the server. The '_wbos' attribute is a
+ * mapping of id -> ServerWBO objects.
+ *
+ * Note that if you want these records to be accessible individually,
+ * you need to register their handlers with the server separately, or use a
+ * containing HTTP server that will do so on your behalf.
+ *
+ * @param wbos
+ *        An object mapping WBO IDs to ServerWBOs.
+ * @param acceptNew
+ *        If true, POSTs to this collection URI will result in new WBOs being
+ *        created and wired in on the fly.
+ * @param timestamp
+ *        An optional timestamp value to initialize the modified time of the
+ *        collection. This should be in the format returned by new_timestamp().
+ *
+ * @return the new ServerCollection instance.
+ *
+ */
+function ServerCollection(wbos, acceptNew, timestamp) {
+  this._wbos = wbos || {};
+  this.acceptNew = acceptNew || false;
+
+  /*
+   * Track modified timestamp.
+   * We can't just use the timestamps of contained WBOs: an empty collection
+   * has a modified time.
+   */
+  this.timestamp = timestamp || new_timestamp();
+  this._log = Log.repository.getLogger(SYNC_HTTP_LOGGER);
+}
+ServerCollection.prototype = {
+  /**
+   * Convenience accessor for our WBO keys.
+   * Excludes deleted items, of course.
+   *
+   * @param filter
+   *        A predicate function (applied to the ID and WBO) which dictates
+   *        whether to include the WBO's ID in the output.
+   *
+   * @return an array of IDs.
+   */
+  keys: function keys(filter) {
+    let ids = [];
+    for (let [id, wbo] of Object.entries(this._wbos)) {
+      if (wbo.payload && (!filter || filter(id, wbo))) {
+        ids.push(id);
+      }
+    }
+    return ids;
+  },
+
+  /**
+   * Convenience method to get an array of WBOs.
+   * Optionally provide a filter function.
+   *
+   * @param filter
+   *        A predicate function, applied to the WBO, which dictates whether to
+   *        include the WBO in the output.
+   *
+   * @return an array of ServerWBOs.
+   */
+  wbos: function wbos(filter) {
+    let os = [];
+    for (let wbo of Object.values(this._wbos)) {
+      if (wbo.payload) {
+        os.push(wbo);
+      }
+    }
+
+    if (filter) {
+      return os.filter(filter);
+    }
+    return os;
+  },
+
+  /**
+   * Convenience method to get an array of parsed ciphertexts.
+   *
+   * @return an array of the payloads of each stored WBO.
+   */
+  payloads() {
+    return this.wbos().map(wbo => wbo.getCleartext());
+  },
+
+  // Just for syntactic elegance.
+  wbo: function wbo(id) {
+    return this._wbos[id];
+  },
+
+  payload: function payload(id) {
+    return this.wbo(id).payload;
+  },
+
+  cleartext(id) {
+    return this.wbo(id).getCleartext();
+  },
+
+  /**
+   * Insert the provided WBO under its ID.
+   *
+   * @return the provided WBO.
+   */
+  insertWBO: function insertWBO(wbo) {
+    this.timestamp = Math.max(this.timestamp, wbo.modified);
+    return (this._wbos[wbo.id] = wbo);
+  },
+
+  /**
+   * Update an existing WBO's cleartext using a callback function that modifies
+   * the record in place, or returns a new record.
+   */
+  updateRecord(id, updateCallback, optTimestamp) {
+    let wbo = this.wbo(id);
+    if (!wbo) {
+      throw new Error("No record with provided ID");
+    }
+    let curCleartext = wbo.getCleartext();
+    // Allow update callback to either return a new cleartext, or modify in place.
+    let newCleartext = updateCallback(curCleartext) || curCleartext;
+    wbo.setCleartext(newCleartext, optTimestamp);
+    // It is already inserted, but we might need to update our timestamp based
+    // on it's `modified` value, if `optTimestamp` was provided.
+    return this.insertWBO(wbo);
+  },
+
+  /**
+   * Insert a record, which may either an object with a cleartext property, or
+   * the cleartext property itself.
+   */
+  insertRecord(record, timestamp = Math.round(Date.now() / 10) / 100) {
+    if (typeof timestamp != "number") {
+      throw new TypeError("insertRecord: Timestamp is not a number.");
+    }
+    if (!record.id) {
+      throw new Error("Attempt to insert record with no id");
+    }
+    // Allow providing either the cleartext directly, or the CryptoWrapper-like.
+    let cleartext = record.cleartext || record;
+    return this.insert(record.id, encryptPayload(cleartext), timestamp);
+  },
+
+  /**
+   * Insert the provided payload as part of a new ServerWBO with the provided
+   * ID.
+   *
+   * @param id
+   *        The GUID for the WBO.
+   * @param payload
+   *        The payload, as provided to the ServerWBO constructor.
+   * @param modified
+   *        An optional modified time for the ServerWBO.
+   *
+   * @return the inserted WBO.
+   */
+  insert: function insert(id, payload, modified) {
+    return this.insertWBO(new ServerWBO(id, payload, modified));
+  },
+
+  /**
+   * Removes an object entirely from the collection.
+   *
+   * @param id
+   *        (string) ID to remove.
+   */
+  remove: function remove(id) {
+    delete this._wbos[id];
+  },
+
+  _inResultSet(wbo, options) {
+    return (
+      wbo.payload &&
+      (!options.ids || options.ids.includes(wbo.id)) &&
+      (!options.newer || wbo.modified > options.newer) &&
+      (!options.older || wbo.modified < options.older)
+    );
+  },
+
+  count(options) {
+    options = options || {};
+    let c = 0;
+    for (let wbo of Object.values(this._wbos)) {
+      if (wbo.modified && this._inResultSet(wbo, options)) {
+        c++;
+      }
+    }
+    return c;
+  },
+
+  get(options, request) {
+    let data = [];
+    for (let wbo of Object.values(this._wbos)) {
+      if (wbo.modified && this._inResultSet(wbo, options)) {
+        data.push(wbo);
+      }
+    }
+    switch (options.sort) {
+      case "newest":
+        data.sort((a, b) => b.modified - a.modified);
+        break;
+
+      case "oldest":
+        data.sort((a, b) => a.modified - b.modified);
+        break;
+
+      case "index":
+        data.sort((a, b) => b.sortindex - a.sortindex);
+        break;
+
+      default:
+        if (options.sort) {
+          this._log.error(
+            "Error: client requesting unknown sort order",
+            options.sort
+          );
+          throw new Error("Unknown sort order");
+        }
+        // If the client didn't request a sort order, shuffle the records
+        // to ensure that we don't accidentally depend on the default order.
+        TestUtils.shuffle(data);
+    }
+    if (options.full) {
+      data = data.map(wbo => wbo.get());
+      let start = options.offset || 0;
+      if (options.limit) {
+        let numItemsPastOffset = data.length - start;
+        data = data.slice(start, start + options.limit);
+        // use options as a backchannel to set x-weave-next-offset
+        if (numItemsPastOffset > options.limit) {
+          options.nextOffset = start + options.limit;
+        }
+      } else if (start) {
+        data = data.slice(start);
+      }
+
+      if (request && request.getHeader("accept") == "application/newlines") {
+        this._log.error(
+          "Error: client requesting application/newlines content"
+        );
+        throw new Error(
+          "This server should not serve application/newlines content"
+        );
+      }
+
+      // Use options as a backchannel to report count.
+      options.recordCount = data.length;
+    } else {
+      data = data.map(wbo => wbo.id);
+      let start = options.offset || 0;
+      if (options.limit) {
+        data = data.slice(start, start + options.limit);
+        options.nextOffset = start + options.limit;
+      } else if (start) {
+        data = data.slice(start);
+      }
+      options.recordCount = data.length;
+    }
+    return JSON.stringify(data);
+  },
+
+  post(input) {
+    input = JSON.parse(input);
+    let success = [];
+    let failed = {};
+
+    // This will count records where we have an existing ServerWBO
+    // registered with us as successful and all other records as failed.
+    for (let key in input) {
+      let record = input[key];
+      let wbo = this.wbo(record.id);
+      if (!wbo && this.acceptNew) {
+        this._log.debug(
+          "Creating WBO " + JSON.stringify(record.id) + " on the fly."
+        );
+        wbo = new ServerWBO(record.id);
+        this.insertWBO(wbo);
+      }
+      if (wbo) {
+        wbo.payload = record.payload;
+        wbo.modified = new_timestamp();
+        wbo.sortindex = record.sortindex || 0;
+        success.push(record.id);
+      } else {
+        failed[record.id] = "no wbo configured";
+      }
+    }
+    return { modified: new_timestamp(), success, failed };
+  },
+
+  delete(options) {
+    let deleted = [];
+    for (let wbo of Object.values(this._wbos)) {
+      if (this._inResultSet(wbo, options)) {
+        this._log.debug("Deleting " + JSON.stringify(wbo));
+        deleted.push(wbo.id);
+        wbo.delete();
+      }
+    }
+    return deleted;
+  },
+
+  // This handler sets `newModified` on the response body if the collection
+  // timestamp has changed.
+  handler() {
+    let self = this;
+
+    return function (request, response) {
+      var statusCode = 200;
+      var status = "OK";
+      var body;
+
+      // Parse queryString
+      let options = {};
+      for (let chunk of request.queryString.split("&")) {
+        if (!chunk) {
+          continue;
+        }
+        chunk = chunk.split("=");
+        if (chunk.length == 1) {
+          options[chunk[0]] = "";
+        } else {
+          options[chunk[0]] = chunk[1];
+        }
+      }
+      // The real servers return 400 if ids= is specified without a list of IDs.
+      if (options.hasOwnProperty("ids")) {
+        if (!options.ids) {
+          response.setStatusLine(request.httpVersion, "400", "Bad Request");
+          body = "Bad Request";
+          writeBytesToOutputStream(response.bodyOutputStream, body);
+          return;
+        }
+        options.ids = options.ids.split(",");
+      }
+      if (options.newer) {
+        options.newer = parseFloat(options.newer);
+      }
+      if (options.older) {
+        options.older = parseFloat(options.older);
+      }
+      if (options.limit) {
+        options.limit = parseInt(options.limit, 10);
+      }
+      if (options.offset) {
+        options.offset = parseInt(options.offset, 10);
+      }
+
+      switch (request.method) {
+        case "GET":
+          body = self.get(options, request);
+          // see http://moz-services-docs.readthedocs.io/en/latest/storage/apis-1.5.html
+          // for description of these headers.
+          let { recordCount: records, nextOffset } = options;
+
+          self._log.info("Records: " + records + ", nextOffset: " + nextOffset);
+          if (records != null) {
+            response.setHeader("X-Weave-Records", "" + records);
+          }
+          if (nextOffset) {
+            response.setHeader("X-Weave-Next-Offset", "" + nextOffset);
+          }
+          response.setHeader("X-Last-Modified", "" + self.timestamp);
+          break;
+
+        case "POST":
+          let res = self.post(
+            readBytesFromInputStream(request.bodyInputStream),
+            request
+          );
+          body = JSON.stringify(res);
+          response.newModified = res.modified;
+          break;
+
+        case "DELETE":
+          self._log.debug("Invoking ServerCollection.DELETE.");
+          let deleted = self.delete(options, request);
+          let ts = new_timestamp();
+          body = JSON.stringify(ts);
+          response.newModified = ts;
+          response.deleted = deleted;
+          break;
+      }
+      response.setHeader("X-Weave-Timestamp", "" + new_timestamp(), false);
+
+      // Update the collection timestamp to the appropriate modified time.
+      // This is either a value set by the handler, or the current time.
+      if (request.method != "GET") {
+        self.timestamp =
+          response.newModified >= 0 ? response.newModified : new_timestamp();
+      }
+      response.setHeader("X-Last-Modified", "" + self.timestamp, false);
+
+      response.setStatusLine(request.httpVersion, statusCode, status);
+      writeBytesToOutputStream(response.bodyOutputStream, body);
+    };
+  },
+};
+
+/*
+ * Test setup helpers.
+ */
+function sync_httpd_setup(handlers) {
+  handlers["/1.1/foo/storage/meta/global"] = new ServerWBO(
+    "global",
+    {}
+  ).handler();
+  return httpd_setup(handlers);
+}
+
+/*
+ * Track collection modified times. Return closures.
+ *
+ * XXX - DO NOT USE IN NEW TESTS
+ *
+ * This code has very limited and very hacky timestamp support - the test
+ * server now has more complete and correct support - using this helper
+ * may cause strangeness wrt timestamp headers and 412 responses.
+ */
+function track_collections_helper() {
+  /*
+   * Our tracking object.
+   */
+  let collections = {};
+
+  /*
+   * Update the timestamp of a collection.
+   */
+  function update_collection(coll, ts) {
+    _("Updating collection " + coll + " to " + ts);
+    let timestamp = ts || new_timestamp();
+    collections[coll] = timestamp;
+  }
+
+  /*
+   * Invoke a handler, updating the collection's modified timestamp unless
+   * it's a GET request.
+   */
+  function with_updated_collection(coll, f) {
+    return function (request, response) {
+      f.call(this, request, response);
+
+      // Update the collection timestamp to the appropriate modified time.
+      // This is either a value set by the handler, or the current time.
+      if (request.method != "GET") {
+        update_collection(coll, response.newModified);
+      }
+    };
+  }
+
+  /*
+   * Return the info/collections object.
+   */
+  function info_collections(request, response) {
+    let body = "Error.";
+    switch (request.method) {
+      case "GET":
+        body = JSON.stringify(collections);
+        break;
+      default:
+        throw new Error("Non-GET on info_collections.");
+    }
+
+    response.setHeader("Content-Type", "application/json");
+    response.setHeader("X-Weave-Timestamp", "" + new_timestamp(), false);
+    response.setStatusLine(request.httpVersion, 200, "OK");
+    writeBytesToOutputStream(response.bodyOutputStream, body);
+  }
+
+  return {
+    collections,
+    handler: info_collections,
+    with_updated_collection,
+    update_collection,
+  };
+}
+
+// ===========================================================================//
+// httpd.js-based Sync server.                                               //
+// ===========================================================================//
+
+/**
+ * In general, the preferred way of using SyncServer is to directly introspect
+ * it. Callbacks are available for operations which are hard to verify through
+ * introspection, such as deletions.
+ *
+ * One of the goals of this server is to provide enough hooks for test code to
+ * find out what it needs without monkeypatching. Use this object as your
+ * prototype, and override as appropriate.
+ */
+var SyncServerCallback = {
+  onCollectionDeleted: function onCollectionDeleted(user, collection) {},
+  onItemDeleted: function onItemDeleted(user, collection, wboID) {},
+
+  /**
+   * Called at the top of every request.
+   *
+   * Allows the test to inspect the request. Hooks should be careful not to
+   * modify or change state of the request or they may impact future processing.
+   * The response is also passed so the callback can set headers etc - but care
+   * must be taken to not screw with the response body or headers that may
+   * conflict with normal operation of this server.
+   */
+  onRequest: function onRequest(request, response) {},
+};
+
+/**
+ * Construct a new test Sync server. Takes a callback object (e.g.,
+ * SyncServerCallback) as input.
+ */
+function SyncServer(callback) {
+  this.callback = callback || Object.create(SyncServerCallback);
+  this.server = new HttpServer();
+  this.started = false;
+  this.users = {};
+  this._log = Log.repository.getLogger(SYNC_HTTP_LOGGER);
+
+  // Install our own default handler. This allows us to mess around with the
+  // whole URL space.
+  let handler = this.server._handler;
+  handler._handleDefault = this.handleDefault.bind(this, handler);
+}
+SyncServer.prototype = {
+  server: null, // HttpServer.
+  users: null, // Map of username => {collections, password}.
+
+  /**
+   * Start the SyncServer's underlying HTTP server.
+   *
+   * @param port
+   *        The numeric port on which to start. -1 implies the default, a
+   *        randomly chosen port.
+   * @param cb
+   *        A callback function (of no arguments) which is invoked after
+   *        startup.
+   */
+  start: function start(port = -1, cb) {
+    if (this.started) {
+      this._log.warn("Warning: server already started on " + this.port);
+      return;
+    }
+    try {
+      this.server.start(port);
+      let i = this.server.identity;
+      this.port = i.primaryPort;
+      this.baseURI =
+        i.primaryScheme + "://" + i.primaryHost + ":" + i.primaryPort + "/";
+      this.started = true;
+      if (cb) {
+        cb();
+      }
+    } catch (ex) {
+      _("==========================================");
+      _("Got exception starting Sync HTTP server.");
+      _("Error: " + Log.exceptionStr(ex));
+      _("Is there a process already listening on port " + port + "?");
+      _("==========================================");
+      do_throw(ex);
+    }
+  },
+
+  /**
+   * Stop the SyncServer's HTTP server.
+   *
+   * @param cb
+   *        A callback function. Invoked after the server has been stopped.
+   *
+   */
+  stop: function stop(cb) {
+    if (!this.started) {
+      this._log.warn(
+        "SyncServer: Warning: server not running. Can't stop me now!"
+      );
+      return;
+    }
+
+    this.server.stop(cb);
+    this.started = false;
+  },
+
+  /**
+   * Return a server timestamp for a record.
+   * The server returns timestamps with 1/100 sec granularity. Note that this is
+   * subject to change: see Bug 650435.
+   */
+  timestamp: function timestamp() {
+    return new_timestamp();
+  },
+
+  /**
+   * Create a new user, complete with an empty set of collections.
+   *
+   * @param username
+   *        The username to use. An Error will be thrown if a user by that name
+   *        already exists.
+   * @param password
+   *        A password string.
+   *
+   * @return a user object, as would be returned by server.user(username).
+   */
+  registerUser: function registerUser(username, password) {
+    if (username in this.users) {
+      throw new Error("User already exists.");
+    }
+    this.users[username] = {
+      password,
+      collections: {},
+    };
+    return this.user(username);
+  },
+
+  userExists: function userExists(username) {
+    return username in this.users;
+  },
+
+  getCollection: function getCollection(username, collection) {
+    return this.users[username].collections[collection];
+  },
+
+  _insertCollection: function _insertCollection(collections, collection, wbos) {
+    let coll = new ServerCollection(wbos, true);
+    coll.collectionHandler = coll.handler();
+    collections[collection] = coll;
+    return coll;
+  },
+
+  createCollection: function createCollection(username, collection, wbos) {
+    if (!(username in this.users)) {
+      throw new Error("Unknown user.");
+    }
+    let collections = this.users[username].collections;
+    if (collection in collections) {
+      throw new Error("Collection already exists.");
+    }
+    return this._insertCollection(collections, collection, wbos);
+  },
+
+  /**
+   * Accept a map like the following:
+   * {
+   *   meta: {global: {version: 1, ...}},
+   *   crypto: {"keys": {}, foo: {bar: 2}},
+   *   bookmarks: {}
+   * }
+   * to cause collections and WBOs to be created.
+   * If a collection already exists, no error is raised.
+   * If a WBO already exists, it will be updated to the new contents.
+   */
+  createContents: function createContents(username, collections) {
+    if (!(username in this.users)) {
+      throw new Error("Unknown user.");
+    }
+    let userCollections = this.users[username].collections;
+    for (let [id, contents] of Object.entries(collections)) {
+      let coll =
+        userCollections[id] || this._insertCollection(userCollections, id);
+      for (let [wboID, payload] of Object.entries(contents)) {
+        coll.insert(wboID, payload);
+      }
+    }
+  },
+
+  /**
+   * Insert a WBO in an existing collection.
+   */
+  insertWBO: function insertWBO(username, collection, wbo) {
+    if (!(username in this.users)) {
+      throw new Error("Unknown user.");
+    }
+    let userCollections = this.users[username].collections;
+    if (!(collection in userCollections)) {
+      throw new Error("Unknown collection.");
+    }
+    userCollections[collection].insertWBO(wbo);
+    return wbo;
+  },
+
+  /**
+   * Delete all of the collections for the named user.
+   *
+   * @param username
+   *        The name of the affected user.
+   *
+   * @return a timestamp.
+   */
+  deleteCollections: function deleteCollections(username) {
+    if (!(username in this.users)) {
+      throw new Error("Unknown user.");
+    }
+    let userCollections = this.users[username].collections;
+    for (let name in userCollections) {
+      let coll = userCollections[name];
+      this._log.trace("Bulk deleting " + name + " for " + username + "...");
+      coll.delete({});
+    }
+    this.users[username].collections = {};
+    return this.timestamp();
+  },
+
+  /**
+   * Simple accessor to allow collective binding and abbreviation of a bunch of
+   * methods. Yay!
+   * Use like this:
+   *
+   *   let u = server.user("john");
+   *   u.collection("bookmarks").wbo("abcdefg").payload;  // Etc.
+   *
+   * @return a proxy for the user data stored in this server.
+   */
+  user: function user(username) {
+    let collection = this.getCollection.bind(this, username);
+    let createCollection = this.createCollection.bind(this, username);
+    let createContents = this.createContents.bind(this, username);
+    let modified = function (collectionName) {
+      return collection(collectionName).timestamp;
+    };
+    let deleteCollections = this.deleteCollections.bind(this, username);
+    return {
+      collection,
+      createCollection,
+      createContents,
+      deleteCollections,
+      modified,
+    };
+  },
+
+  /*
+   * Regular expressions for splitting up Sync request paths.
+   * Sync URLs are of the form:
+   *   /$apipath/$version/$user/$further
+   * where $further is usually:
+   *   storage/$collection/$wbo
+   * or
+   *   storage/$collection
+   * or
+   *   info/$op
+   * We assume for the sake of simplicity that $apipath is empty.
+   *
+   * N.B., we don't follow any kind of username spec here, because as far as I
+   * can tell there isn't one. See Bug 689671. Instead we follow the Python
+   * server code.
+   *
+   * Path: [all, version, username, first, rest]
+   * Storage: [all, collection?, id?]
+   */
+  pathRE:
+    /^\/([0-9]+(?:\.[0-9]+)?)\/([-._a-zA-Z0-9]+)(?:\/([^\/]+)(?:\/(.+))?)?$/,
+  storageRE: /^([-_a-zA-Z0-9]+)(?:\/([-_a-zA-Z0-9]+)\/?)?$/,
+
+  defaultHeaders: {},
+
+  /**
+   * HTTP response utility.
+   */
+  respond: function respond(req, resp, code, status, body, headers) {
+    resp.setStatusLine(req.httpVersion, code, status);
+    if (!headers) {
+      headers = this.defaultHeaders;
+    }
+    for (let header in headers) {
+      let value = headers[header];
+      resp.setHeader(header, value);
+    }
+    resp.setHeader("X-Weave-Timestamp", "" + this.timestamp(), false);
+    writeBytesToOutputStream(resp.bodyOutputStream, body);
+  },
+
+  /**
+   * This is invoked by the HttpServer. `this` is bound to the SyncServer;
+   * `handler` is the HttpServer's handler.
+   *
+   * TODO: need to use the correct Sync API response codes and errors here.
+   * TODO: Basic Auth.
+   * TODO: check username in path against username in BasicAuth.
+   */
+  handleDefault: function handleDefault(handler, req, resp) {
+    try {
+      this._handleDefault(handler, req, resp);
+    } catch (e) {
+      if (e instanceof HttpError) {
+        this.respond(req, resp, e.code, e.description, "", {});
+      } else {
+        throw e;
+      }
+    }
+  },
+
+  _handleDefault: function _handleDefault(handler, req, resp) {
+    this._log.debug(
+      "SyncServer: Handling request: " + req.method + " " + req.path
+    );
+
+    if (this.callback.onRequest) {
+      this.callback.onRequest(req, resp);
+    }
+
+    let parts = this.pathRE.exec(req.path);
+    if (!parts) {
+      this._log.debug("SyncServer: Unexpected request: bad URL " + req.path);
+      throw HTTP_404;
+    }
+
+    let [, version, username, first, rest] = parts;
+    // Doing a float compare of the version allows for us to pretend there was
+    // a node-reassignment - eg, we could re-assign from "1.1/user/" to
+    // "1.10/user" - this server will then still accept requests with the new
+    // URL while any code in sync itself which compares URLs will see a
+    // different URL.
+    if (parseFloat(version) != parseFloat(SYNC_API_VERSION)) {
+      this._log.debug("SyncServer: Unknown version.");
+      throw HTTP_404;
+    }
+
+    if (!this.userExists(username)) {
+      this._log.debug("SyncServer: Unknown user.");
+      throw HTTP_401;
+    }
+
+    // Hand off to the appropriate handler for this path component.
+    if (first in this.toplevelHandlers) {
+      let newHandler = this.toplevelHandlers[first];
+      return newHandler.call(
+        this,
+        newHandler,
+        req,
+        resp,
+        version,
+        username,
+        rest
+      );
+    }
+    this._log.debug("SyncServer: Unknown top-level " + first);
+    throw HTTP_404;
+  },
+
+  /**
+   * Compute the object that is returned for an info/collections request.
+   */
+  infoCollections: function infoCollections(username) {
+    let responseObject = {};
+    let colls = this.users[username].collections;
+    for (let coll in colls) {
+      responseObject[coll] = colls[coll].timestamp;
+    }
+    this._log.trace(
+      "SyncServer: info/collections returning " + JSON.stringify(responseObject)
+    );
+    return responseObject;
+  },
+
+  /**
+   * Collection of the handler methods we use for top-level path components.
+   */
+  toplevelHandlers: {
+    storage: function handleStorage(
+      handler,
+      req,
+      resp,
+      version,
+      username,
+      rest
+    ) {
+      let respond = this.respond.bind(this, req, resp);
+      if (!rest || !rest.length) {
+        this._log.debug(
+          "SyncServer: top-level storage " + req.method + " request."
+        );
+
+        // TODO: verify if this is spec-compliant.
+        if (req.method != "DELETE") {
+          respond(405, "Method Not Allowed", "[]", { Allow: "DELETE" });
+          return undefined;
+        }
+
+        // Delete all collections and track the timestamp for the response.
+        let timestamp = this.user(username).deleteCollections();
+
+        // Return timestamp and OK for deletion.
+        respond(200, "OK", JSON.stringify(timestamp));
+        return undefined;
+      }
+
+      let match = this.storageRE.exec(rest);
+      if (!match) {
+        this._log.warn("SyncServer: Unknown storage operation " + rest);
+        throw HTTP_404;
+      }
+      let [, collection, wboID] = match;
+      let coll = this.getCollection(username, collection);
+
+      let checkXIUSFailure = () => {
+        if (req.hasHeader("x-if-unmodified-since")) {
+          let xius = parseFloat(req.getHeader("x-if-unmodified-since"));
+          // Sadly the way our tests are setup, we often end up with xius of
+          // zero (typically when syncing just one engine, so the date from
+          // info/collections isn't used) - so we allow that to work.
+          // Further, the Python server treats non-existing collections as
+          // having a timestamp of 0.
+          let collTimestamp = coll ? coll.timestamp : 0;
+          if (xius && xius < collTimestamp) {
+            this._log.info(
+              `x-if-unmodified-since mismatch - request wants ${xius} but our collection has ${collTimestamp}`
+            );
+            respond(412, "precondition failed", "precondition failed");
+            return true;
+          }
+        }
+        return false;
+      };
+
+      switch (req.method) {
+        case "GET": {
+          if (!coll) {
+            if (wboID) {
+              respond(404, "Not found", "Not found");
+              return undefined;
+            }
+            // *cries inside*: - apparently the real sync server returned 200
+            // here for some time, then returned 404 for some time (bug 687299),
+            // and now is back to 200 (bug 963332).
+            respond(200, "OK", "[]");
+            return undefined;
+          }
+          if (!wboID) {
+            return coll.collectionHandler(req, resp);
+          }
+          let wbo = coll.wbo(wboID);
+          if (!wbo) {
+            respond(404, "Not found", "Not found");
+            return undefined;
+          }
+          return wbo.handler()(req, resp);
+        }
+        case "DELETE": {
+          if (!coll) {
+            respond(200, "OK", "{}");
+            return undefined;
+          }
+          if (checkXIUSFailure()) {
+            return undefined;
+          }
+          if (wboID) {
+            let wbo = coll.wbo(wboID);
+            if (wbo) {
+              wbo.delete();
+              this.callback.onItemDeleted(username, collection, wboID);
+            }
+            respond(200, "OK", "{}");
+            return undefined;
+          }
+          coll.collectionHandler(req, resp);
+
+          // Spot if this is a DELETE for some IDs, and don't blow away the
+          // whole collection!
+          //
+          // We already handled deleting the WBOs by invoking the deleted
+          // collection's handler. However, in the case of
+          //
+          //   DELETE storage/foobar
+          //
+          // we also need to remove foobar from the collections map. This
+          // clause tries to differentiate the above request from
+          //
+          //  DELETE storage/foobar?ids=foo,baz
+          //
+          // and do the right thing.
+          // TODO: less hacky method.
+          if (-1 == req.queryString.indexOf("ids=")) {
+            // When you delete the entire collection, we drop it.
+            this._log.debug("Deleting entire collection.");
+            delete this.users[username].collections[collection];
+            this.callback.onCollectionDeleted(username, collection);
+          }
+
+          // Notify of item deletion.
+          let deleted = resp.deleted || [];
+          for (let i = 0; i < deleted.length; ++i) {
+            this.callback.onItemDeleted(username, collection, deleted[i]);
+          }
+          return undefined;
+        }
+        case "PUT":
+          // PUT and POST have slightly different XIUS semantics - for PUT,
+          // the check is against the item, whereas for POST it is against
+          // the collection. So first, a special-case for PUT.
+          if (req.hasHeader("x-if-unmodified-since")) {
+            let xius = parseFloat(req.getHeader("x-if-unmodified-since"));
+            // treat and xius of zero as if it wasn't specified - this happens
+            // in some of our tests for a new collection.
+            if (xius > 0) {
+              let wbo = coll.wbo(wboID);
+              if (xius < wbo.modified) {
+                this._log.info(
+                  `x-if-unmodified-since mismatch - request wants ${xius} but wbo has ${wbo.modified}`
+                );
+                respond(412, "precondition failed", "precondition failed");
+                return undefined;
+              }
+              wbo.handler()(req, resp);
+              coll.timestamp = resp.newModified;
+              return resp;
+            }
+          }
+        // fall through to post.
+        case "POST":
+          if (checkXIUSFailure()) {
+            return undefined;
+          }
+          if (!coll) {
+            coll = this.createCollection(username, collection);
+          }
+
+          if (wboID) {
+            let wbo = coll.wbo(wboID);
+            if (!wbo) {
+              this._log.trace(
+                "SyncServer: creating WBO " + collection + "/" + wboID
+              );
+              wbo = coll.insert(wboID);
+            }
+            // Rather than instantiate each WBO's handler function, do it once
+            // per request. They get hit far less often than do collections.
+            wbo.handler()(req, resp);
+            coll.timestamp = resp.newModified;
+            return resp;
+          }
+          return coll.collectionHandler(req, resp);
+        default:
+          throw new Error("Request method " + req.method + " not implemented.");
+      }
+    },
+
+    info: function handleInfo(handler, req, resp, version, username, rest) {
+      switch (rest) {
+        case "collections":
+          let body = JSON.stringify(this.infoCollections(username));
+          this.respond(req, resp, 200, "OK", body, {
+            "Content-Type": "application/json",
+          });
+          return;
+        case "collection_usage":
+        case "collection_counts":
+        case "quota":
+          // TODO: implement additional info methods.
+          this.respond(req, resp, 200, "OK", "TODO");
+          return;
+        default:
+          // TODO
+          this._log.warn("SyncServer: Unknown info operation " + rest);
+          throw HTTP_404;
+      }
+    },
+  },
+};
+
+/**
+ * Test helper.
+ */
+function serverForUsers(users, contents, callback) {
+  let server = new SyncServer(callback);
+  for (let [user, pass] of Object.entries(users)) {
+    server.registerUser(user, pass);
+    server.createContents(user, contents);
+  }
+  server.start();
+  return server;
+}
diff --git a/services/sync/tests/unit/missing-sourceuri.json b/services/sync/tests/unit/missing-sourceuri.json
new file mode 100644
index 0000000000..dcd487726a
--- /dev/null
+++ b/services/sync/tests/unit/missing-sourceuri.json
@@ -0,0 +1,20 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Restartless Test Extension",
+      "type": "extension",
+      "guid": "missing-sourceuri@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 485
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/unit/missing-xpi-search.json b/services/sync/tests/unit/missing-xpi-search.json
new file mode 100644
index 0000000000..55f6432b29
--- /dev/null
+++ b/services/sync/tests/unit/missing-xpi-search.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Non-Restartless Test Extension",
+      "type": "extension",
+      "guid": "missing-xpi@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 485,
+            "url": "http://127.0.0.1:8888/THIS_DOES_NOT_EXIST.xpi"
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/unit/prefs_test_prefs_store.js b/services/sync/tests/unit/prefs_test_prefs_store.js
new file mode 100644
index 0000000000..63851a6934
--- /dev/null
+++ b/services/sync/tests/unit/prefs_test_prefs_store.js
@@ -0,0 +1,47 @@
+// This is a "preferences" file used by test_prefs_store.js
+
+/* global pref, user_pref */
+
+// The prefs that control what should be synced.
+// Most of these are "default" prefs, so the value itself will not sync.
+pref("services.sync.prefs.sync.testing.int", true);
+pref("services.sync.prefs.sync.testing.string", true);
+pref("services.sync.prefs.sync.testing.bool", true);
+pref("services.sync.prefs.sync.testing.dont.change", true);
+// This is a default pref, but has the special "sync-seen" pref.
+pref("services.sync.prefs.sync.testing.seen", true);
+pref("services.sync.prefs.sync-seen.testing.seen", false);
+
+// this one is a user pref, so it *will* sync.
+user_pref("services.sync.prefs.sync.testing.turned.off", false);
+pref("services.sync.prefs.sync.testing.nonexistent", true);
+pref("services.sync.prefs.sync.testing.default", true);
+pref("services.sync.prefs.sync.testing.synced.url", true);
+// We shouldn't sync the URL, or the flag that says we should sync the pref
+// (otherwise some other client might overwrite our local value).
+user_pref("services.sync.prefs.sync.testing.unsynced.url", true);
+
+// The preference values - these are all user_prefs, otherwise their value
+// will not be synced.
+user_pref("testing.int", 123);
+user_pref("testing.string", "ohai");
+user_pref("testing.bool", true);
+user_pref("testing.dont.change", "Please don't change me.");
+user_pref("testing.turned.off", "I won't get synced.");
+user_pref("testing.not.turned.on", "I won't get synced either!");
+// Some url we don't want to sync
+user_pref(
+  "testing.unsynced.url",
+  "moz-extension://d5d31b00-b944-4afb-bd3d-d0326551a0ae"
+);
+user_pref("testing.synced.url", "https://www.example.com");
+
+// A pref that exists but still has the default value - will be synced with
+// null as the value.
+pref("testing.default", "I'm the default value");
+
+// A pref that has the default value - it will start syncing as soon as
+// we see a change, even if the change is to the default.
+pref("testing.seen", "the value");
+
+// A pref that shouldn't be synced
diff --git a/services/sync/tests/unit/rewrite-search.json b/services/sync/tests/unit/rewrite-search.json
new file mode 100644
index 0000000000..740b6f2c30
--- /dev/null
+++ b/services/sync/tests/unit/rewrite-search.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "Rewrite Test Extension",
+      "type": "extension",
+      "guid": "rewrite@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 485,
+            "url": "http://127.0.0.1:8888/require.xpi?src=api"
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/unit/sync_ping_schema.json b/services/sync/tests/unit/sync_ping_schema.json
new file mode 100644
index 0000000000..a9866e3550
--- /dev/null
+++ b/services/sync/tests/unit/sync_ping_schema.json
@@ -0,0 +1,262 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "schema for Sync pings, documentation avaliable in toolkit/components/telemetry/docs/sync-ping.rst",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["version", "syncs", "why", "uid"],
+  "properties": {
+    "version": { "type": "integer", "minimum": 0 },
+    "os": { "type": "object" },
+    "discarded": { "type": "integer", "minimum": 1 },
+    "why": { "enum": ["shutdown", "schedule", "idchange"] },
+    "uid": {
+      "type": "string",
+      "pattern": "^[0-9a-f]{32}$"
+    },
+    "deviceID": {
+      "type": "string",
+      "pattern": "^[0-9a-f]{64}$"
+    },
+    "devices": {
+      "type": "array",
+      "items": { "$ref": "#/definitions/device" }
+    },
+    "sessionStartDate": { "type": "string" },
+    "syncs": {
+      "type": "array",
+      "minItems": 0,
+      "items": { "$ref": "#/definitions/payload" }
+    },
+    "syncNodeType": {
+      "type": "string"
+    },
+    "events": {
+      "type": "array",
+      "minItems": 1,
+      "items": { "$ref": "#/definitions/event" }
+    },
+    "migrations": {
+      "type": "array",
+      "minItems": 1,
+      "items": { "$ref": "#/definitions/migration" }
+    },
+    "histograms": {
+      "type": "object",
+      "additionalProperties": {
+        "type": "object",
+        "properties": {
+          "min": { "type": "integer" },
+          "max": { "type": "integer" },
+          "histogram_type": { "type": "integer" },
+          "sum": { "type": "integer" },
+          "ranges": { "type": "array" },
+          "counts": { "type": "array" }
+        }
+      }
+    }
+  },
+  "definitions": {
+    "payload": {
+      "type": "object",
+      "additionalProperties": false,
+      "required": ["when", "took"],
+      "properties": {
+        "didLogin": { "type": "boolean" },
+        "when": { "type": "integer" },
+        "status": {
+          "type": "object",
+          "anyOf": [{ "required": ["sync"] }, { "required": ["service"] }],
+          "additionalProperties": false,
+          "properties": {
+            "sync": { "type": "string" },
+            "service": { "type": "string" }
+          }
+        },
+        "why": { "type": "string" },
+        "took": { "type": "integer", "minimum": -1 },
+        "failureReason": { "$ref": "#/definitions/error" },
+        "engines": {
+          "type": "array",
+          "minItems": 1,
+          "items": { "$ref": "#/definitions/engine" }
+        }
+      }
+    },
+    "device": {
+      "required": ["id"],
+      "additionalProperties": false,
+      "type": "object",
+      "properties": {
+        "id": { "type": "string", "pattern": "^[0-9a-f]{64}$" },
+        "os": { "type": "string" },
+        "version": { "type": "string" },
+        "type": { "type": "string" },
+        "syncID": { "type": "string", "pattern": "^[0-9a-f]{64}$" }
+      }
+    },
+    "engine": {
+      "required": ["name"],
+      "additionalProperties": false,
+      "properties": {
+        "failureReason": { "$ref": "#/definitions/error" },
+        "name": { "type": "string" },
+        "took": { "type": "integer", "minimum": 1 },
+        "status": { "type": "string" },
+        "incoming": {
+          "type": "object",
+          "additionalProperties": false,
+          "anyOf": [{ "required": ["applied"] }, { "required": ["failed"] }],
+          "properties": {
+            "applied": { "type": "integer", "minimum": 1 },
+            "failed": { "type": "integer", "minimum": 1 },
+            "failedReasons": {
+              "type": "array",
+              "minItems": 1,
+              "$ref": "#/definitions/namedCount"
+            }
+          }
+        },
+        "outgoing": {
+          "type": "array",
+          "minItems": 1,
+          "items": { "$ref": "#/definitions/outgoingBatch" }
+        },
+        "steps": {
+          "type": "array",
+          "minItems": 1,
+          "$ref": "#/definitions/step"
+        },
+        "validation": {
+          "type": "object",
+          "additionalProperties": false,
+          "anyOf": [
+            { "required": ["checked"] },
+            { "required": ["failureReason"] }
+          ],
+          "properties": {
+            "checked": { "type": "integer", "minimum": 0 },
+            "failureReason": { "$ref": "#/definitions/error" },
+            "took": { "type": "integer" },
+            "version": { "type": "integer" },
+            "problems": {
+              "type": "array",
+              "minItems": 1,
+              "$ref": "#/definitions/namedCount"
+            }
+          }
+        }
+      }
+    },
+    "outgoingBatch": {
+      "type": "object",
+      "additionalProperties": false,
+      "anyOf": [{ "required": ["sent"] }, { "required": ["failed"] }],
+      "properties": {
+        "sent": { "type": "integer", "minimum": 1 },
+        "failed": { "type": "integer", "minimum": 1 },
+        "failedReasons": {
+          "type": "array",
+          "minItems": 1,
+          "$ref": "#/definitions/namedCount"
+        }
+      }
+    },
+    "event": {
+      "type": "array",
+      "minItems": 4,
+      "maxItems": 6
+    },
+    "migration": {
+      "oneOf": [{ "$ref": "#/definitions/webextMigration" }]
+    },
+    "webextMigration": {
+      "required": ["type"],
+      "properties": {
+        "type": { "enum": ["webext-storage"] },
+        "entries": { "type": "integer" },
+        "entriesSuccessful": { "type": "integer" },
+        "extensions": { "type": "integer" },
+        "extensionsSuccessful": { "type": "integer" },
+        "openFailure": { "type": "boolean" }
+      }
+    },
+    "error": {
+      "oneOf": [
+        { "$ref": "#/definitions/httpError" },
+        { "$ref": "#/definitions/nsError" },
+        { "$ref": "#/definitions/shutdownError" },
+        { "$ref": "#/definitions/authError" },
+        { "$ref": "#/definitions/otherError" },
+        { "$ref": "#/definitions/unexpectedError" },
+        { "$ref": "#/definitions/sqlError" }
+      ]
+    },
+    "httpError": {
+      "required": ["name", "code"],
+      "properties": {
+        "name": { "enum": ["httperror"] },
+        "code": { "type": "integer" }
+      }
+    },
+    "nsError": {
+      "required": ["name", "code"],
+      "properties": {
+        "name": { "enum": ["nserror"] },
+        "code": { "type": "integer" }
+      }
+    },
+    "shutdownError": {
+      "required": ["name"],
+      "properties": {
+        "name": { "enum": ["shutdownerror"] }
+      }
+    },
+    "authError": {
+      "required": ["name"],
+      "properties": {
+        "name": { "enum": ["autherror"] },
+        "from": { "enum": ["tokenserver", "fxaccounts", "hawkclient"] }
+      }
+    },
+    "otherError": {
+      "required": ["name"],
+      "properties": {
+        "name": { "enum": ["othererror"] },
+        "error": { "type": "string" }
+      }
+    },
+    "unexpectedError": {
+      "required": ["name"],
+      "properties": {
+        "name": { "enum": ["unexpectederror"] },
+        "error": { "type": "string" }
+      }
+    },
+    "sqlError": {
+      "required": ["name"],
+      "properties": {
+        "name": { "enum": ["sqlerror"] },
+        "code": { "type": "integer" }
+      }
+    },
+    "step": {
+      "required": ["name"],
+      "properties": {
+        "name": { "type": "string" },
+        "took": { "type": "integer", "minimum": 1 },
+        "counts": {
+          "type": "array",
+          "minItems": 1,
+          "$ref": "#/definitions/namedCount"
+        }
+      }
+    },
+    "namedCount": {
+      "required": ["name", "count"],
+      "properties": {
+        "name": { "type": "string" },
+        "count": { "type": "integer" }
+      }
+    }
+  }
+}
diff --git a/services/sync/tests/unit/systemaddon-search.json b/services/sync/tests/unit/systemaddon-search.json
new file mode 100644
index 0000000000..a812714918
--- /dev/null
+++ b/services/sync/tests/unit/systemaddon-search.json
@@ -0,0 +1,21 @@
+{
+  "next": null,
+  "results": [
+    {
+      "name": "System Add-on Test",
+      "type": "extension",
+      "guid": "system1@tests.mozilla.org",
+      "current_version": {
+        "version": "1.0",
+        "files": [
+          {
+            "platform": "all",
+            "size": 999,
+            "url": "http://127.0.0.1:8888/system.xpi"
+          }
+        ]
+      },
+      "last_updated": "2011-09-05T20:42:09Z"
+    }
+  ]
+}
diff --git a/services/sync/tests/unit/test_412.js b/services/sync/tests/unit/test_412.js
new file mode 100644
index 0000000000..de0a2c087e
--- /dev/null
+++ b/services/sync/tests/unit/test_412.js
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { RotaryEngine } = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/rotaryengine.sys.mjs"
+);
+
+add_task(async function test_412_not_treated_as_failure() {
+  await Service.engineManager.register(RotaryEngine);
+  let engine = Service.engineManager.get("rotary");
+
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  // add an item to the server to the first sync advances lastModified.
+  let collection = server.getCollection("foo", "rotary");
+  let payload = encryptPayload({
+    id: "existing",
+    something: "existing record",
+  });
+  collection.insert("existing", payload);
+
+  let promiseObserved = promiseOneObserver("weave:engine:sync:finish");
+  try {
+    // Do sync.
+    _("initial sync to initialize the world");
+    await Service.sync();
+
+    // create a new record that should be uploaded and arrange for our lastSync
+    // timestamp to be wrong so we get a 412.
+    engine._store.items = { new: "new record" };
+    await engine._tracker.addChangedID("new", 0);
+
+    let saw412 = false;
+    let _uploadOutgoing = engine._uploadOutgoing;
+    engine._uploadOutgoing = async () => {
+      let lastSync = await engine.getLastSync();
+      await engine.setLastSync(lastSync - 2);
+      try {
+        await _uploadOutgoing.call(engine);
+      } catch (ex) {
+        saw412 = ex.status == 412;
+        throw ex;
+      }
+    };
+    _("Second sync - expecting a 412");
+    await Service.sync();
+    await promiseObserved;
+    ok(saw412, "did see a 412 error");
+    // But service status should be OK as the 412 shouldn't be treated as an error.
+    equal(Service.status.service, STATUS_OK);
+  } finally {
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_addon_utils.js b/services/sync/tests/unit/test_addon_utils.js
new file mode 100644
index 0000000000..c039bee16c
--- /dev/null
+++ b/services/sync/tests/unit/test_addon_utils.js
@@ -0,0 +1,156 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AddonUtils } = ChromeUtils.importESModule(
+  "resource://services-sync/addonutils.sys.mjs"
+);
+
+const HTTP_PORT = 8888;
+const SERVER_ADDRESS = "http://127.0.0.1:8888";
+
+Services.prefs.setStringPref(
+  "extensions.getAddons.get.url",
+  SERVER_ADDRESS + "/search/guid:%IDS%"
+);
+
+AddonTestUtils.init(this);
+AddonTestUtils.createAppInfo(
+  "xpcshell@tests.mozilla.org",
+  "XPCShell",
+  "1",
+  "1.9.2"
+);
+
+add_task(async function setup() {
+  await AddonTestUtils.promiseStartupManager();
+});
+
+function createAndStartHTTPServer(port = HTTP_PORT) {
+  try {
+    let server = new HttpServer();
+
+    server.registerFile(
+      "/search/guid:missing-sourceuri%40tests.mozilla.org",
+      do_get_file("missing-sourceuri.json")
+    );
+
+    server.registerFile(
+      "/search/guid:rewrite%40tests.mozilla.org",
+      do_get_file("rewrite-search.json")
+    );
+
+    server.start(port);
+
+    return server;
+  } catch (ex) {
+    _("Got exception starting HTTP server on port " + port);
+    _("Error: " + Log.exceptionStr(ex));
+    do_throw(ex);
+  }
+  return null; /* not hit, but keeps eslint happy! */
+}
+
+function run_test() {
+  syncTestLogging();
+
+  run_next_test();
+}
+
+add_task(async function test_handle_empty_source_uri() {
+  _("Ensure that search results without a sourceURI are properly ignored.");
+
+  let server = createAndStartHTTPServer();
+
+  const ID = "missing-sourceuri@tests.mozilla.org";
+
+  const result = await AddonUtils.installAddons([
+    { id: ID, requireSecureURI: false },
+  ]);
+
+  Assert.ok("installedIDs" in result);
+  Assert.equal(0, result.installedIDs.length);
+
+  Assert.ok("skipped" in result);
+  Assert.ok(result.skipped.includes(ID));
+
+  await promiseStopServer(server);
+});
+
+add_test(function test_ignore_untrusted_source_uris() {
+  _("Ensures that source URIs from insecure schemes are rejected.");
+
+  const bad = [
+    "http://example.com/foo.xpi",
+    "ftp://example.com/foo.xpi",
+    "silly://example.com/foo.xpi",
+  ];
+
+  const good = ["https://example.com/foo.xpi"];
+
+  for (let s of bad) {
+    let sourceURI = Services.io.newURI(s);
+    let addon = { sourceURI, name: "bad", id: "bad" };
+
+    let canInstall = AddonUtils.canInstallAddon(addon);
+    Assert.ok(!canInstall, "Correctly rejected a bad URL");
+  }
+
+  for (let s of good) {
+    let sourceURI = Services.io.newURI(s);
+    let addon = { sourceURI, name: "good", id: "good" };
+
+    let canInstall = AddonUtils.canInstallAddon(addon);
+    Assert.ok(canInstall, "Correctly accepted a good URL");
+  }
+  run_next_test();
+});
+
+add_task(async function test_source_uri_rewrite() {
+  _("Ensure that a 'src=api' query string is rewritten to 'src=sync'");
+
+  // This tests for conformance with bug 708134 so server-side metrics aren't
+  // skewed.
+
+  // We resort to monkeypatching because of the API design.
+  let oldFunction =
+    Object.getPrototypeOf(AddonUtils).installAddonFromSearchResult;
+
+  let installCalled = false;
+  Object.getPrototypeOf(AddonUtils).installAddonFromSearchResult =
+    async function testInstallAddon(addon, metadata) {
+      Assert.equal(
+        SERVER_ADDRESS + "/require.xpi?src=sync",
+        addon.sourceURI.spec
+      );
+
+      installCalled = true;
+
+      const install = await AddonUtils.getInstallFromSearchResult(addon);
+      Assert.equal(
+        SERVER_ADDRESS + "/require.xpi?src=sync",
+        install.sourceURI.spec
+      );
+      Assert.deepEqual(
+        install.installTelemetryInfo,
+        { source: "sync" },
+        "Got the expected installTelemetryInfo"
+      );
+
+      return { id: addon.id, addon, install };
+    };
+
+  let server = createAndStartHTTPServer();
+
+  let installOptions = {
+    id: "rewrite@tests.mozilla.org",
+    requireSecureURI: false,
+  };
+  await AddonUtils.installAddons([installOptions]);
+
+  Assert.ok(installCalled);
+  Object.getPrototypeOf(AddonUtils).installAddonFromSearchResult = oldFunction;
+
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_addons_engine.js b/services/sync/tests/unit/test_addons_engine.js
new file mode 100644
index 0000000000..d957ed8fd3
--- /dev/null
+++ b/services/sync/tests/unit/test_addons_engine.js
@@ -0,0 +1,277 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AddonManager } = ChromeUtils.importESModule(
+  "resource://gre/modules/AddonManager.sys.mjs"
+);
+const { CHANGE_INSTALLED } = ChromeUtils.importESModule(
+  "resource://services-sync/addonsreconciler.sys.mjs"
+);
+const { AddonsEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/addons.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+Services.prefs.setStringPref(
+  "extensions.getAddons.get.url",
+  "http://localhost:8888/search/guid:%IDS%"
+);
+Services.prefs.setBoolPref("extensions.install.requireSecureOrigin", false);
+
+let engine;
+let syncID;
+let reconciler;
+let tracker;
+
+AddonTestUtils.init(this);
+
+const ADDON_ID = "addon1@tests.mozilla.org";
+const XPI = AddonTestUtils.createTempWebExtensionFile({
+  manifest: {
+    name: "Test 1",
+    description: "Test Description",
+    browser_specific_settings: { gecko: { id: ADDON_ID } },
+  },
+});
+
+async function resetReconciler() {
+  reconciler._addons = {};
+  reconciler._changes = [];
+
+  await reconciler.saveState();
+
+  await tracker.clearChangedIDs();
+}
+
+add_task(async function setup() {
+  AddonTestUtils.createAppInfo(
+    "xpcshell@tests.mozilla.org",
+    "XPCShell",
+    "1",
+    "1.9.2"
+  );
+  AddonTestUtils.overrideCertDB();
+  await AddonTestUtils.promiseStartupManager();
+
+  await Service.engineManager.register(AddonsEngine);
+  engine = Service.engineManager.get("addons");
+  syncID = await engine.resetLocalSyncID();
+  reconciler = engine._reconciler;
+  tracker = engine._tracker;
+
+  reconciler.startListening();
+
+  // Don't flush to disk in the middle of an event listener!
+  // This causes test hangs on WinXP.
+  reconciler._shouldPersist = false;
+
+  await resetReconciler();
+});
+
+// This is a basic sanity test for the unit test itself. If this breaks, the
+// add-ons API likely changed upstream.
+add_task(async function test_addon_install() {
+  _("Ensure basic add-on APIs work as expected.");
+
+  let install = await AddonManager.getInstallForFile(XPI);
+  Assert.notEqual(install, null);
+  Assert.equal(install.type, "extension");
+  Assert.equal(install.name, "Test 1");
+
+  await resetReconciler();
+});
+
+add_task(async function test_find_dupe() {
+  _("Ensure the _findDupe() implementation is sane.");
+
+  // This gets invoked at the top of sync, which is bypassed by this
+  // test, so we do it manually.
+  await engine._refreshReconcilerState();
+
+  let addon = await installAddon(XPI, reconciler);
+
+  let record = {
+    id: Utils.makeGUID(),
+    addonID: ADDON_ID,
+    enabled: true,
+    applicationID: Services.appinfo.ID,
+    source: "amo",
+  };
+
+  let dupe = await engine._findDupe(record);
+  Assert.equal(addon.syncGUID, dupe);
+
+  record.id = addon.syncGUID;
+  dupe = await engine._findDupe(record);
+  Assert.equal(null, dupe);
+
+  await uninstallAddon(addon, reconciler);
+  await resetReconciler();
+});
+
+add_task(async function test_get_changed_ids() {
+  let timerPrecision = Services.prefs.getBoolPref(
+    "privacy.reduceTimerPrecision"
+  );
+  Services.prefs.setBoolPref("privacy.reduceTimerPrecision", false);
+
+  registerCleanupFunction(function () {
+    Services.prefs.setBoolPref("privacy.reduceTimerPrecision", timerPrecision);
+  });
+
+  _("Ensure getChangedIDs() has the appropriate behavior.");
+
+  _("Ensure getChangedIDs() returns an empty object by default.");
+  let changes = await engine.getChangedIDs();
+  Assert.equal("object", typeof changes);
+  Assert.equal(0, Object.keys(changes).length);
+
+  _("Ensure tracker changes are populated.");
+  let now = new Date();
+  let changeTime = now.getTime() / 1000;
+  let guid1 = Utils.makeGUID();
+  await tracker.addChangedID(guid1, changeTime);
+
+  changes = await engine.getChangedIDs();
+  Assert.equal("object", typeof changes);
+  Assert.equal(1, Object.keys(changes).length);
+  Assert.ok(guid1 in changes);
+  Assert.equal(changeTime, changes[guid1]);
+
+  await tracker.clearChangedIDs();
+
+  _("Ensure reconciler changes are populated.");
+  let addon = await installAddon(XPI, reconciler);
+  await tracker.clearChangedIDs(); // Just in case.
+  changes = await engine.getChangedIDs();
+  Assert.equal("object", typeof changes);
+  Assert.equal(1, Object.keys(changes).length);
+  Assert.ok(addon.syncGUID in changes);
+  _(
+    "Change time: " + changeTime + ", addon change: " + changes[addon.syncGUID]
+  );
+  Assert.ok(changes[addon.syncGUID] >= changeTime);
+
+  let oldTime = changes[addon.syncGUID];
+  let guid2 = addon.syncGUID;
+  await uninstallAddon(addon, reconciler);
+  changes = await engine.getChangedIDs();
+  Assert.equal(1, Object.keys(changes).length);
+  Assert.ok(guid2 in changes);
+  Assert.ok(changes[guid2] > oldTime);
+
+  _("Ensure non-syncable add-ons aren't picked up by reconciler changes.");
+  reconciler._addons = {};
+  reconciler._changes = [];
+  let record = {
+    id: "DUMMY",
+    guid: Utils.makeGUID(),
+    enabled: true,
+    installed: true,
+    modified: new Date(),
+    type: "UNSUPPORTED",
+    scope: 0,
+    foreignInstall: false,
+  };
+  reconciler.addons.DUMMY = record;
+  await reconciler._addChange(record.modified, CHANGE_INSTALLED, record);
+
+  changes = await engine.getChangedIDs();
+  _(JSON.stringify(changes));
+  Assert.equal(0, Object.keys(changes).length);
+
+  await resetReconciler();
+});
+
+add_task(async function test_disabled_install_semantics() {
+  _("Ensure that syncing a disabled add-on preserves proper state.");
+
+  // This is essentially a test for bug 712542, which snuck into the original
+  // add-on sync drop. It ensures that when an add-on is installed that the
+  // disabled state and incoming syncGUID is preserved, even on the next sync.
+  const USER = "foo";
+  const PASSWORD = "password";
+
+  let server = new SyncServer();
+  server.start();
+  await SyncTestingInfrastructure(server, USER, PASSWORD);
+
+  await generateNewKeys(Service.collectionKeys);
+
+  let contents = {
+    meta: {
+      global: { engines: { addons: { version: engine.version, syncID } } },
+    },
+    crypto: {},
+    addons: {},
+  };
+
+  server.registerUser(USER, "password");
+  server.createContents(USER, contents);
+
+  let amoServer = new HttpServer();
+  amoServer.registerFile(
+    "/search/guid:addon1%40tests.mozilla.org",
+    do_get_file("addon1-search.json")
+  );
+
+  amoServer.registerFile("/addon1.xpi", XPI);
+  amoServer.start(8888);
+
+  // Insert an existing record into the server.
+  let id = Utils.makeGUID();
+  let now = Date.now() / 1000;
+
+  let record = encryptPayload({
+    id,
+    applicationID: Services.appinfo.ID,
+    addonID: ADDON_ID,
+    enabled: false,
+    deleted: false,
+    source: "amo",
+  });
+  let wbo = new ServerWBO(id, record, now - 2);
+  server.insertWBO(USER, "addons", wbo);
+
+  _("Performing sync of add-ons engine.");
+  await engine._sync();
+
+  // At this point the non-restartless extension should be staged for install.
+
+  // Don't need this server any more.
+  await promiseStopServer(amoServer);
+
+  // We ensure the reconciler has recorded the proper ID and enabled state.
+  let addon = reconciler.getAddonStateFromSyncGUID(id);
+  Assert.notEqual(null, addon);
+  Assert.equal(false, addon.enabled);
+
+  // We fake an app restart and perform another sync, just to make sure things
+  // are sane.
+  await AddonTestUtils.promiseRestartManager();
+
+  let collection = server.getCollection(USER, "addons");
+  engine.lastModified = collection.timestamp;
+  await engine._sync();
+
+  // The client should not upload a new record. The old record should be
+  // retained and unmodified.
+  Assert.equal(1, collection.count());
+
+  let payload = collection.payloads()[0];
+  Assert.notEqual(null, collection.wbo(id));
+  Assert.equal(ADDON_ID, payload.addonID);
+  Assert.ok(!payload.enabled);
+
+  await promiseStopServer(server);
+});
+
+add_test(function cleanup() {
+  // There's an xpcom-shutdown hook for this, but let's give this a shot.
+  reconciler.stopListening();
+  run_next_test();
+});
diff --git a/services/sync/tests/unit/test_addons_reconciler.js b/services/sync/tests/unit/test_addons_reconciler.js
new file mode 100644
index 0000000000..c72b18b00e
--- /dev/null
+++ b/services/sync/tests/unit/test_addons_reconciler.js
@@ -0,0 +1,209 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AddonsReconciler, CHANGE_INSTALLED, CHANGE_UNINSTALLED } =
+  ChromeUtils.importESModule(
+    "resource://services-sync/addonsreconciler.sys.mjs"
+  );
+const { AddonsEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/addons.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+AddonTestUtils.init(this);
+AddonTestUtils.createAppInfo(
+  "xpcshell@tests.mozilla.org",
+  "XPCShell",
+  "1",
+  "1.9.2"
+);
+AddonTestUtils.overrideCertDB();
+
+const ADDON_ID = "addon1@tests.mozilla.org";
+const XPI = AddonTestUtils.createTempWebExtensionFile({
+  manifest: {
+    name: "Test 1",
+    description: "Test Description",
+    browser_specific_settings: { gecko: { id: ADDON_ID } },
+  },
+});
+
+function makeAddonsReconciler() {
+  const log = Service.engineManager.get("addons")._log;
+  const queueCaller = Async.asyncQueueCaller(log);
+  return new AddonsReconciler(queueCaller);
+}
+
+add_task(async function setup() {
+  await AddonTestUtils.promiseStartupManager();
+  Svc.PrefBranch.setBoolPref("engine.addons", true);
+  await Service.engineManager.register(AddonsEngine);
+});
+
+add_task(async function test_defaults() {
+  _("Ensure new objects have reasonable defaults.");
+
+  let reconciler = makeAddonsReconciler();
+  await reconciler.ensureStateLoaded();
+
+  Assert.ok(!reconciler._listening);
+  Assert.equal("object", typeof reconciler.addons);
+  Assert.equal(0, Object.keys(reconciler.addons).length);
+  Assert.equal(0, reconciler._changes.length);
+  Assert.equal(0, reconciler._listeners.length);
+});
+
+add_task(async function test_load_state_empty_file() {
+  _("Ensure loading from a missing file results in defaults being set.");
+
+  let reconciler = makeAddonsReconciler();
+  await reconciler.ensureStateLoaded();
+
+  let loaded = await reconciler.loadState();
+  Assert.ok(!loaded);
+
+  Assert.equal("object", typeof reconciler.addons);
+  Assert.equal(0, Object.keys(reconciler.addons).length);
+  Assert.equal(0, reconciler._changes.length);
+});
+
+add_task(async function test_install_detection() {
+  _("Ensure that add-on installation results in appropriate side-effects.");
+
+  let reconciler = makeAddonsReconciler();
+  await reconciler.ensureStateLoaded();
+  reconciler.startListening();
+
+  let before = new Date();
+  let addon = await installAddon(XPI);
+  let after = new Date();
+
+  Assert.equal(1, Object.keys(reconciler.addons).length);
+  Assert.ok(addon.id in reconciler.addons);
+  let record = reconciler.addons[ADDON_ID];
+
+  const KEYS = [
+    "id",
+    "guid",
+    "enabled",
+    "installed",
+    "modified",
+    "type",
+    "scope",
+    "foreignInstall",
+  ];
+  for (let key of KEYS) {
+    Assert.ok(key in record);
+    Assert.notEqual(null, record[key]);
+  }
+
+  Assert.equal(addon.id, record.id);
+  Assert.equal(addon.syncGUID, record.guid);
+  Assert.ok(record.enabled);
+  Assert.ok(record.installed);
+  Assert.ok(record.modified >= before && record.modified <= after);
+  Assert.equal("extension", record.type);
+  Assert.ok(!record.foreignInstall);
+
+  Assert.equal(1, reconciler._changes.length);
+  let change = reconciler._changes[0];
+  Assert.ok(change[0] >= before && change[1] <= after);
+  Assert.equal(CHANGE_INSTALLED, change[1]);
+  Assert.equal(addon.id, change[2]);
+
+  await uninstallAddon(addon);
+});
+
+add_task(async function test_uninstall_detection() {
+  _("Ensure that add-on uninstallation results in appropriate side-effects.");
+
+  let reconciler = makeAddonsReconciler();
+  await reconciler.ensureStateLoaded();
+  reconciler.startListening();
+
+  reconciler._addons = {};
+  reconciler._changes = [];
+
+  let addon = await installAddon(XPI);
+  let id = addon.id;
+
+  reconciler._changes = [];
+  await uninstallAddon(addon, reconciler);
+
+  Assert.equal(1, Object.keys(reconciler.addons).length);
+  Assert.ok(id in reconciler.addons);
+
+  let record = reconciler.addons[id];
+  Assert.ok(!record.installed);
+
+  Assert.equal(1, reconciler._changes.length);
+  let change = reconciler._changes[0];
+  Assert.equal(CHANGE_UNINSTALLED, change[1]);
+  Assert.equal(id, change[2]);
+});
+
+add_task(async function test_load_state_future_version() {
+  _("Ensure loading a file from a future version results in no data loaded.");
+
+  const FILENAME = "TEST_LOAD_STATE_FUTURE_VERSION";
+
+  let reconciler = makeAddonsReconciler();
+  await reconciler.ensureStateLoaded();
+
+  // First we populate our new file.
+  let state = { version: 100, addons: { foo: {} }, changes: [[1, 1, "foo"]] };
+
+  // jsonSave() expects an object with ._log, so we give it a reconciler
+  // instance.
+  await Utils.jsonSave(FILENAME, reconciler, state);
+
+  let loaded = await reconciler.loadState(FILENAME);
+  Assert.ok(!loaded);
+
+  Assert.equal("object", typeof reconciler.addons);
+  Assert.equal(0, Object.keys(reconciler.addons).length);
+  Assert.equal(0, reconciler._changes.length);
+});
+
+add_task(async function test_prune_changes_before_date() {
+  _("Ensure that old changes are pruned properly.");
+
+  let reconciler = makeAddonsReconciler();
+  await reconciler.ensureStateLoaded();
+  reconciler._changes = [];
+
+  let now = new Date();
+  const HOUR_MS = 1000 * 60 * 60;
+
+  _("Ensure pruning an empty changes array works.");
+  reconciler.pruneChangesBeforeDate(now);
+  Assert.equal(0, reconciler._changes.length);
+
+  let old = new Date(now.getTime() - HOUR_MS);
+  let young = new Date(now.getTime() - 1000);
+  reconciler._changes.push([old, CHANGE_INSTALLED, "foo"]);
+  reconciler._changes.push([young, CHANGE_INSTALLED, "bar"]);
+  Assert.equal(2, reconciler._changes.length);
+
+  _("Ensure pruning with an old time won't delete anything.");
+  let threshold = new Date(old.getTime() - 1);
+  reconciler.pruneChangesBeforeDate(threshold);
+  Assert.equal(2, reconciler._changes.length);
+
+  _("Ensure pruning a single item works.");
+  threshold = new Date(young.getTime() - 1000);
+  reconciler.pruneChangesBeforeDate(threshold);
+  Assert.equal(1, reconciler._changes.length);
+  Assert.notEqual(undefined, reconciler._changes[0]);
+  Assert.equal(young, reconciler._changes[0][0]);
+  Assert.equal("bar", reconciler._changes[0][2]);
+
+  _("Ensure pruning all changes works.");
+  reconciler._changes.push([old, CHANGE_INSTALLED, "foo"]);
+  reconciler.pruneChangesBeforeDate(now);
+  Assert.equal(0, reconciler._changes.length);
+});
diff --git a/services/sync/tests/unit/test_addons_store.js b/services/sync/tests/unit/test_addons_store.js
new file mode 100644
index 0000000000..a0a3ac6c69
--- /dev/null
+++ b/services/sync/tests/unit/test_addons_store.js
@@ -0,0 +1,750 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AddonsEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/addons.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+const { SyncedRecordsTelemetry } = ChromeUtils.importESModule(
+  "resource://services-sync/telemetry.sys.mjs"
+);
+
+const HTTP_PORT = 8888;
+
+Services.prefs.setStringPref(
+  "extensions.getAddons.get.url",
+  "http://localhost:8888/search/guid:%IDS%"
+);
+// Note that all compat-override URLs currently 404, but that's OK - the main
+// thing is to avoid us hitting the real AMO.
+Services.prefs.setStringPref(
+  "extensions.getAddons.compatOverides.url",
+  "http://localhost:8888/compat-override/guid:%IDS%"
+);
+Services.prefs.setBoolPref("extensions.install.requireSecureOrigin", false);
+Services.prefs.setBoolPref("extensions.checkUpdateSecurity", false);
+
+AddonTestUtils.init(this);
+AddonTestUtils.createAppInfo(
+  "xpcshell@tests.mozilla.org",
+  "XPCShell",
+  "1",
+  "1.9.2"
+);
+AddonTestUtils.overrideCertDB();
+
+Services.prefs.setBoolPref("extensions.experiments.enabled", true);
+
+const SYSTEM_ADDON_ID = "system1@tests.mozilla.org";
+add_task(async function setupSystemAddon() {
+  const distroDir = FileUtils.getDir("ProfD", ["sysfeatures", "app0"]);
+  distroDir.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+  AddonTestUtils.registerDirectory("XREAppFeat", distroDir);
+
+  let xpi = await AddonTestUtils.createTempWebExtensionFile({
+    manifest: {
+      browser_specific_settings: { gecko: { id: SYSTEM_ADDON_ID } },
+    },
+  });
+
+  xpi.copyTo(distroDir, `${SYSTEM_ADDON_ID}.xpi`);
+
+  await AddonTestUtils.overrideBuiltIns({ system: [SYSTEM_ADDON_ID] });
+  await AddonTestUtils.promiseStartupManager();
+});
+
+const ID1 = "addon1@tests.mozilla.org";
+const ID2 = "addon2@tests.mozilla.org";
+const ID3 = "addon3@tests.mozilla.org";
+
+const ADDONS = {
+  test_addon1: {
+    manifest: {
+      browser_specific_settings: {
+        gecko: {
+          id: ID1,
+          update_url: "http://example.com/data/test_install.json",
+        },
+      },
+    },
+  },
+
+  test_addon2: {
+    manifest: {
+      browser_specific_settings: { gecko: { id: ID2 } },
+    },
+  },
+
+  test_addon3: {
+    manifest: {
+      browser_specific_settings: {
+        gecko: {
+          id: ID3,
+          strict_max_version: "0",
+        },
+      },
+    },
+  },
+};
+
+const SEARCH_RESULT = {
+  next: null,
+  results: [
+    {
+      name: "Test Extension",
+      type: "extension",
+      guid: "addon1@tests.mozilla.org",
+      current_version: {
+        version: "1.0",
+        files: [
+          {
+            platform: "all",
+            size: 485,
+            url: "http://localhost:8888/addon1.xpi",
+          },
+        ],
+      },
+      last_updated: "2018-10-27T04:12:00.826Z",
+    },
+  ],
+};
+
+const MISSING_SEARCH_RESULT = {
+  next: null,
+  results: [
+    {
+      name: "Test",
+      type: "extension",
+      guid: "missing-xpi@tests.mozilla.org",
+      current_version: {
+        version: "1.0",
+        files: [
+          {
+            platform: "all",
+            size: 123,
+            url: "http://localhost:8888/THIS_DOES_NOT_EXIST.xpi",
+          },
+        ],
+      },
+    },
+  ],
+};
+
+const XPIS = {};
+for (let [name, files] of Object.entries(ADDONS)) {
+  XPIS[name] = AddonTestUtils.createTempWebExtensionFile(files);
+}
+
+let engine;
+let store;
+let reconciler;
+
+const proxyService = Cc[
+  "@mozilla.org/network/protocol-proxy-service;1"
+].getService(Ci.nsIProtocolProxyService);
+
+const proxyFilter = {
+  proxyInfo: proxyService.newProxyInfo(
+    "http",
+    "localhost",
+    HTTP_PORT,
+    "",
+    "",
+    0,
+    4096,
+    null
+  ),
+
+  applyFilter(channel, defaultProxyInfo, callback) {
+    if (channel.URI.host === "example.com") {
+      callback.onProxyFilterResult(this.proxyInfo);
+    } else {
+      callback.onProxyFilterResult(defaultProxyInfo);
+    }
+  },
+};
+
+proxyService.registerChannelFilter(proxyFilter, 0);
+registerCleanupFunction(() => {
+  proxyService.unregisterChannelFilter(proxyFilter);
+});
+
+/**
+ * Create a AddonsRec for this application with the fields specified.
+ *
+ * @param  id       Sync GUID of record
+ * @param  addonId  ID of add-on
+ * @param  enabled  Boolean whether record is enabled
+ * @param  deleted  Boolean whether record was deleted
+ */
+function createRecordForThisApp(id, addonId, enabled, deleted) {
+  return {
+    id,
+    addonID: addonId,
+    enabled,
+    deleted: !!deleted,
+    applicationID: Services.appinfo.ID,
+    source: "amo",
+  };
+}
+
+function createAndStartHTTPServer(port) {
+  try {
+    let server = new HttpServer();
+
+    server.registerPathHandler(
+      "/search/guid:addon1%40tests.mozilla.org",
+      (req, resp) => {
+        resp.setHeader("Content-type", "application/json", true);
+        resp.write(JSON.stringify(SEARCH_RESULT));
+      }
+    );
+    server.registerPathHandler(
+      "/search/guid:missing-xpi%40tests.mozilla.org",
+      (req, resp) => {
+        resp.setHeader("Content-type", "application/json", true);
+        resp.write(JSON.stringify(MISSING_SEARCH_RESULT));
+      }
+    );
+    server.registerFile("/addon1.xpi", XPIS.test_addon1);
+
+    server.start(port);
+
+    return server;
+  } catch (ex) {
+    _("Got exception starting HTTP server on port " + port);
+    _("Error: " + Log.exceptionStr(ex));
+    do_throw(ex);
+  }
+  return null; /* not hit, but keeps eslint happy! */
+}
+
+// A helper function to ensure that the reconciler's current view of the addon
+// is the same as the addon itself. If it's not, then the reconciler missed a
+// change, and is likely to re-upload the addon next sync because of the change
+// it missed.
+async function checkReconcilerUpToDate(addon) {
+  let stateBefore = Object.assign({}, store.reconciler.addons[addon.id]);
+  await store.reconciler.rectifyStateFromAddon(addon);
+  let stateAfter = store.reconciler.addons[addon.id];
+  deepEqual(stateBefore, stateAfter);
+}
+
+add_task(async function setup() {
+  await Service.engineManager.register(AddonsEngine);
+  engine = Service.engineManager.get("addons");
+  store = engine._store;
+  reconciler = engine._reconciler;
+
+  reconciler.startListening();
+
+  // Don't flush to disk in the middle of an event listener!
+  // This causes test hangs on WinXP.
+  reconciler._shouldPersist = false;
+});
+
+add_task(async function test_remove() {
+  _("Ensure removing add-ons from deleted records works.");
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  let record = createRecordForThisApp(addon.syncGUID, ID1, true, true);
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(0, failed.length);
+  Assert.equal(null, countTelemetry.failedReasons);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+
+  let newAddon = await AddonManager.getAddonByID(ID1);
+  Assert.equal(null, newAddon);
+});
+
+add_task(async function test_apply_enabled() {
+  let countTelemetry = new SyncedRecordsTelemetry();
+  _("Ensures that changes to the userEnabled flag apply.");
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  Assert.ok(addon.isActive);
+  Assert.ok(!addon.userDisabled);
+
+  _("Ensure application of a disable record works as expected.");
+  let records = [];
+  records.push(createRecordForThisApp(addon.syncGUID, ID1, false, false));
+
+  let [failed] = await Promise.all([
+    store.applyIncomingBatch(records, countTelemetry),
+    AddonTestUtils.promiseAddonEvent("onDisabled"),
+  ]);
+  Assert.equal(0, failed.length);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+  addon = await AddonManager.getAddonByID(ID1);
+  Assert.ok(addon.userDisabled);
+  await checkReconcilerUpToDate(addon);
+  records = [];
+
+  _("Ensure enable record works as expected.");
+  records.push(createRecordForThisApp(addon.syncGUID, ID1, true, false));
+  [failed] = await Promise.all([
+    store.applyIncomingBatch(records, countTelemetry),
+    AddonTestUtils.promiseWebExtensionStartup(ID1),
+  ]);
+  Assert.equal(0, failed.length);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+  addon = await AddonManager.getAddonByID(ID1);
+  Assert.ok(!addon.userDisabled);
+  await checkReconcilerUpToDate(addon);
+  records = [];
+
+  _("Ensure enabled state updates don't apply if the ignore pref is set.");
+  records.push(createRecordForThisApp(addon.syncGUID, ID1, false, false));
+  Svc.PrefBranch.setBoolPref("addons.ignoreUserEnabledChanges", true);
+  failed = await store.applyIncomingBatch(records, countTelemetry);
+  Assert.equal(0, failed.length);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+  addon = await AddonManager.getAddonByID(ID1);
+  Assert.ok(!addon.userDisabled);
+  records = [];
+
+  await uninstallAddon(addon, reconciler);
+  Svc.PrefBranch.clearUserPref("addons.ignoreUserEnabledChanges");
+});
+
+add_task(async function test_apply_enabled_appDisabled() {
+  _(
+    "Ensures that changes to the userEnabled flag apply when the addon is appDisabled."
+  );
+
+  // this addon is appDisabled by default.
+  let addon = await installAddon(XPIS.test_addon3);
+  Assert.ok(addon.appDisabled);
+  Assert.ok(!addon.isActive);
+  Assert.ok(!addon.userDisabled);
+
+  _("Ensure application of a disable record works as expected.");
+  store.reconciler.pruneChangesBeforeDate(Date.now() + 10);
+  store.reconciler._changes = [];
+  let records = [];
+  let countTelemetry = new SyncedRecordsTelemetry();
+  records.push(createRecordForThisApp(addon.syncGUID, ID3, false, false));
+  let failed = await store.applyIncomingBatch(records, countTelemetry);
+  Assert.equal(0, failed.length);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+  addon = await AddonManager.getAddonByID(ID3);
+  Assert.ok(addon.userDisabled);
+  await checkReconcilerUpToDate(addon);
+  records = [];
+
+  _("Ensure enable record works as expected.");
+  records.push(createRecordForThisApp(addon.syncGUID, ID3, true, false));
+  failed = await store.applyIncomingBatch(records, countTelemetry);
+  Assert.equal(0, failed.length);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+  addon = await AddonManager.getAddonByID(ID3);
+  Assert.ok(!addon.userDisabled);
+  await checkReconcilerUpToDate(addon);
+  records = [];
+
+  await uninstallAddon(addon, reconciler);
+});
+
+add_task(async function test_ignore_different_appid() {
+  _(
+    "Ensure that incoming records with a different application ID are ignored."
+  );
+
+  // We test by creating a record that should result in an update.
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  Assert.ok(!addon.userDisabled);
+
+  let record = createRecordForThisApp(addon.syncGUID, ID1, false, false);
+  record.applicationID = "FAKE_ID";
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(0, failed.length);
+
+  let newAddon = await AddonManager.getAddonByID(ID1);
+  Assert.ok(!newAddon.userDisabled);
+
+  await uninstallAddon(addon, reconciler);
+});
+
+add_task(async function test_ignore_unknown_source() {
+  _("Ensure incoming records with unknown source are ignored.");
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+
+  let record = createRecordForThisApp(addon.syncGUID, ID1, false, false);
+  record.source = "DUMMY_SOURCE";
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(0, failed.length);
+
+  let newAddon = await AddonManager.getAddonByID(ID1);
+  Assert.ok(!newAddon.userDisabled);
+
+  await uninstallAddon(addon, reconciler);
+});
+
+add_task(async function test_apply_uninstall() {
+  _("Ensures that uninstalling an add-on from a record works.");
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+
+  let records = [];
+  let countTelemetry = new SyncedRecordsTelemetry();
+  records.push(createRecordForThisApp(addon.syncGUID, ID1, true, true));
+  let failed = await store.applyIncomingBatch(records, countTelemetry);
+  Assert.equal(0, failed.length);
+  Assert.equal(0, countTelemetry.incomingCounts.failed);
+
+  addon = await AddonManager.getAddonByID(ID1);
+  Assert.equal(null, addon);
+});
+
+add_task(async function test_addon_syncability() {
+  _("Ensure isAddonSyncable functions properly.");
+
+  Svc.PrefBranch.setStringPref(
+    "addons.trustedSourceHostnames",
+    "addons.mozilla.org,other.example.com"
+  );
+
+  Assert.ok(!(await store.isAddonSyncable(null)));
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  Assert.ok(await store.isAddonSyncable(addon));
+
+  let dummy = {};
+  const KEYS = [
+    "id",
+    "syncGUID",
+    "type",
+    "scope",
+    "foreignInstall",
+    "isSyncable",
+  ];
+  for (let k of KEYS) {
+    dummy[k] = addon[k];
+  }
+
+  Assert.ok(await store.isAddonSyncable(dummy));
+
+  dummy.type = "UNSUPPORTED";
+  Assert.ok(!(await store.isAddonSyncable(dummy)));
+  dummy.type = addon.type;
+
+  dummy.scope = 0;
+  Assert.ok(!(await store.isAddonSyncable(dummy)));
+  dummy.scope = addon.scope;
+
+  dummy.isSyncable = false;
+  Assert.ok(!(await store.isAddonSyncable(dummy)));
+  dummy.isSyncable = addon.isSyncable;
+
+  dummy.foreignInstall = true;
+  Assert.ok(!(await store.isAddonSyncable(dummy)));
+  dummy.foreignInstall = false;
+
+  await uninstallAddon(addon, reconciler);
+
+  Assert.ok(!store.isSourceURITrusted(null));
+
+  let trusted = [
+    "https://addons.mozilla.org/foo",
+    "https://other.example.com/foo",
+  ];
+
+  let untrusted = [
+    "http://addons.mozilla.org/foo", // non-https
+    "ftps://addons.mozilla.org/foo", // non-https
+    "https://untrusted.example.com/foo", // non-trusted hostname`
+  ];
+
+  for (let uri of trusted) {
+    Assert.ok(store.isSourceURITrusted(Services.io.newURI(uri)));
+  }
+
+  for (let uri of untrusted) {
+    Assert.ok(!store.isSourceURITrusted(Services.io.newURI(uri)));
+  }
+
+  Svc.PrefBranch.setStringPref("addons.trustedSourceHostnames", "");
+  for (let uri of trusted) {
+    Assert.ok(!store.isSourceURITrusted(Services.io.newURI(uri)));
+  }
+
+  Svc.PrefBranch.setStringPref(
+    "addons.trustedSourceHostnames",
+    "addons.mozilla.org"
+  );
+  Assert.ok(
+    store.isSourceURITrusted(
+      Services.io.newURI("https://addons.mozilla.org/foo")
+    )
+  );
+
+  Svc.PrefBranch.clearUserPref("addons.trustedSourceHostnames");
+});
+
+add_task(async function test_get_all_ids() {
+  _("Ensures that getAllIDs() returns an appropriate set.");
+
+  _("Installing two addons.");
+  // XXX - this test seems broken - at this point, before we've installed the
+  // addons below, store.getAllIDs() returns all addons installed by previous
+  // tests, even though those tests uninstalled the addon.
+  // So if any tests above ever add a new addon ID, they are going to need to
+  // be added here too.
+  // Assert.equal(0, Object.keys(store.getAllIDs()).length);
+  let addon1 = await installAddon(XPIS.test_addon1, reconciler);
+  let addon2 = await installAddon(XPIS.test_addon2, reconciler);
+  let addon3 = await installAddon(XPIS.test_addon3, reconciler);
+
+  _("Ensure they're syncable.");
+  Assert.ok(await store.isAddonSyncable(addon1));
+  Assert.ok(await store.isAddonSyncable(addon2));
+  Assert.ok(await store.isAddonSyncable(addon3));
+
+  let ids = await store.getAllIDs();
+
+  Assert.equal("object", typeof ids);
+  Assert.equal(3, Object.keys(ids).length);
+  Assert.ok(addon1.syncGUID in ids);
+  Assert.ok(addon2.syncGUID in ids);
+  Assert.ok(addon3.syncGUID in ids);
+
+  await uninstallAddon(addon1, reconciler);
+  await uninstallAddon(addon2, reconciler);
+  await uninstallAddon(addon3, reconciler);
+});
+
+add_task(async function test_change_item_id() {
+  _("Ensures that changeItemID() works properly.");
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+
+  let oldID = addon.syncGUID;
+  let newID = Utils.makeGUID();
+
+  await store.changeItemID(oldID, newID);
+
+  let newAddon = await AddonManager.getAddonByID(ID1);
+  Assert.notEqual(null, newAddon);
+  Assert.equal(newID, newAddon.syncGUID);
+
+  await uninstallAddon(newAddon, reconciler);
+});
+
+add_task(async function test_create() {
+  _("Ensure creating/installing an add-on from a record works.");
+
+  let server = createAndStartHTTPServer(HTTP_PORT);
+
+  let guid = Utils.makeGUID();
+  let record = createRecordForThisApp(guid, ID1, true, false);
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(0, failed.length);
+
+  let newAddon = await AddonManager.getAddonByID(ID1);
+  Assert.notEqual(null, newAddon);
+  Assert.equal(guid, newAddon.syncGUID);
+  Assert.ok(!newAddon.userDisabled);
+
+  await uninstallAddon(newAddon, reconciler);
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_create_missing_search() {
+  _("Ensures that failed add-on searches are handled gracefully.");
+
+  let server = createAndStartHTTPServer(HTTP_PORT);
+
+  // The handler for this ID is not installed, so a search should 404.
+  const id = "missing@tests.mozilla.org";
+  let guid = Utils.makeGUID();
+  let record = createRecordForThisApp(guid, id, true, false);
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(1, failed.length);
+  Assert.equal(guid, failed[0]);
+  Assert.equal(
+    countTelemetry.incomingCounts.failedReasons[0].name,
+    "GET <URL> failed (status 404)"
+  );
+  Assert.equal(countTelemetry.incomingCounts.failedReasons[0].count, 1);
+
+  let addon = await AddonManager.getAddonByID(id);
+  Assert.equal(null, addon);
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_create_bad_install() {
+  _("Ensures that add-ons without a valid install are handled gracefully.");
+
+  let server = createAndStartHTTPServer(HTTP_PORT);
+
+  // The handler returns a search result but the XPI will 404.
+  const id = "missing-xpi@tests.mozilla.org";
+  let guid = Utils.makeGUID();
+  let record = createRecordForThisApp(guid, id, true, false);
+  let countTelemetry = new SyncedRecordsTelemetry();
+  /* let failed = */ await store.applyIncomingBatch([record], countTelemetry);
+  // This addon had no source URI so was skipped - but it's not treated as
+  // failure.
+  // XXX - this test isn't testing what we thought it was. Previously the addon
+  // was not being installed due to requireSecureURL checking *before* we'd
+  // attempted to get the XPI.
+  // With requireSecureURL disabled we do see a download failure, but the addon
+  // *does* get added to |failed|.
+  // FTR: onDownloadFailed() is called with ERROR_NETWORK_FAILURE, so it's going
+  // to be tricky to distinguish a 404 from other transient network errors
+  // where we do want the addon to end up in |failed|.
+  // This is being tracked in bug 1284778.
+  // Assert.equal(0, failed.length);
+
+  let addon = await AddonManager.getAddonByID(id);
+  Assert.equal(null, addon);
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_ignore_system() {
+  _("Ensure we ignore system addons");
+  // Our system addon should not appear in getAllIDs
+  await engine._refreshReconcilerState();
+  let num = 0;
+  let ids = await store.getAllIDs();
+  for (let guid in ids) {
+    num += 1;
+    let addon = reconciler.getAddonStateFromSyncGUID(guid);
+    Assert.notEqual(addon.id, SYSTEM_ADDON_ID);
+  }
+  Assert.greater(num, 1, "should have seen at least one.");
+});
+
+add_task(async function test_incoming_system() {
+  _("Ensure we handle incoming records that refer to a system addon");
+  // eg, loop initially had a normal addon but it was then "promoted" to be a
+  // system addon but wanted to keep the same ID. The server record exists due
+  // to this.
+
+  // before we start, ensure the system addon isn't disabled.
+  Assert.ok(!(await AddonManager.getAddonByID(SYSTEM_ADDON_ID).userDisabled));
+
+  // Now simulate an incoming record with the same ID as the system addon,
+  // but flagged as disabled - it should not be applied.
+  let server = createAndStartHTTPServer(HTTP_PORT);
+  // We make the incoming record flag the system addon as disabled - it should
+  // be ignored.
+  let guid = Utils.makeGUID();
+  let record = createRecordForThisApp(guid, SYSTEM_ADDON_ID, false, false);
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(0, failed.length);
+
+  // The system addon should still not be userDisabled.
+  Assert.ok(!(await AddonManager.getAddonByID(SYSTEM_ADDON_ID).userDisabled));
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_wipe() {
+  _("Ensures that wiping causes add-ons to be uninstalled.");
+
+  await installAddon(XPIS.test_addon1, reconciler);
+
+  await store.wipe();
+
+  let addon = await AddonManager.getAddonByID(ID1);
+  Assert.equal(null, addon);
+});
+
+add_task(async function test_wipe_and_install() {
+  _("Ensure wipe followed by install works.");
+
+  // This tests the reset sync flow where remote data is replaced by local. The
+  // receiving client will see a wipe followed by a record which should undo
+  // the wipe.
+  let installed = await installAddon(XPIS.test_addon1, reconciler);
+
+  let record = createRecordForThisApp(installed.syncGUID, ID1, true, false);
+
+  await store.wipe();
+
+  let deleted = await AddonManager.getAddonByID(ID1);
+  Assert.equal(null, deleted);
+
+  // Re-applying the record can require re-fetching the XPI.
+  let server = createAndStartHTTPServer(HTTP_PORT);
+
+  await store.applyIncoming(record);
+
+  let fetched = await AddonManager.getAddonByID(record.addonID);
+  Assert.ok(!!fetched);
+
+  // wipe again to we are left with a clean slate.
+  await store.wipe();
+
+  await promiseStopServer(server);
+});
+
+// STR for what this is testing:
+// * Either:
+//   * Install then remove an addon, then delete addons.json from the profile
+//     or corrupt it (in which case the addon manager will remove it)
+//   * Install then remove an addon while addon caching is disabled, then
+//     re-enable addon caching.
+// * Install the same addon in a different profile, sync it.
+// * Sync this profile
+// Before bug 1467904, the addon would fail to install because this profile
+// has a copy of the addon in our addonsreconciler.json, but the addon manager
+// does *not* have a copy in its cache, and repopulating that cache would not
+// re-add it as the addon is no longer installed locally.
+add_task(async function test_incoming_reconciled_but_not_cached() {
+  _(
+    "Ensure we handle incoming records our reconciler has but the addon cache does not"
+  );
+
+  // Make sure addon is not installed.
+  let addon = await AddonManager.getAddonByID(ID1);
+  Assert.equal(null, addon);
+
+  Services.prefs.setBoolPref("extensions.getAddons.cache.enabled", false);
+
+  addon = await installAddon(XPIS.test_addon1, reconciler);
+  Assert.notEqual(await AddonManager.getAddonByID(ID1), null);
+  await uninstallAddon(addon, reconciler);
+
+  Services.prefs.setBoolPref("extensions.getAddons.cache.enabled", true);
+
+  // now pretend it is incoming.
+  let server = createAndStartHTTPServer(HTTP_PORT);
+  let guid = Utils.makeGUID();
+  let record = createRecordForThisApp(guid, ID1, true, false);
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch([record], countTelemetry);
+  Assert.equal(0, failed.length);
+
+  Assert.notEqual(await AddonManager.getAddonByID(ID1), null);
+
+  await promiseStopServer(server);
+});
+
+// NOTE: The test above must be the last test run due to the addon cache
+// being trashed. It is probably possible to fix that by running, eg,
+// AddonRespository.backgroundUpdateCheck() to rebuild the cache, but that
+// requires implementing more AMO functionality in our test server
+
+add_task(async function cleanup() {
+  // There's an xpcom-shutdown hook for this, but let's give this a shot.
+  reconciler.stopListening();
+});
diff --git a/services/sync/tests/unit/test_addons_tracker.js b/services/sync/tests/unit/test_addons_tracker.js
new file mode 100644
index 0000000000..f8473e4cfa
--- /dev/null
+++ b/services/sync/tests/unit/test_addons_tracker.js
@@ -0,0 +1,174 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AddonsEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/addons.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+AddonTestUtils.init(this);
+AddonTestUtils.createAppInfo(
+  "xpcshell@tests.mozilla.org",
+  "XPCShell",
+  "1",
+  "1.9.2"
+);
+AddonTestUtils.overrideCertDB();
+
+Services.prefs.setBoolPref("extensions.experiments.enabled", true);
+
+Svc.PrefBranch.setBoolPref("engine.addons", true);
+
+let reconciler;
+let tracker;
+
+const addon1ID = "addon1@tests.mozilla.org";
+
+const ADDONS = {
+  test_addon1: {
+    manifest: {
+      browser_specific_settings: { gecko: { id: addon1ID } },
+    },
+  },
+};
+
+const XPIS = {};
+
+async function cleanup() {
+  tracker.stop();
+
+  tracker.resetScore();
+  await tracker.clearChangedIDs();
+
+  reconciler._addons = {};
+  reconciler._changes = [];
+  await reconciler.saveState();
+}
+
+add_task(async function setup() {
+  await AddonTestUtils.promiseStartupManager();
+  for (let [name, data] of Object.entries(ADDONS)) {
+    XPIS[name] = AddonTestUtils.createTempWebExtensionFile(data);
+  }
+  await Service.engineManager.register(AddonsEngine);
+  let engine = Service.engineManager.get("addons");
+  reconciler = engine._reconciler;
+  tracker = engine._tracker;
+
+  await cleanup();
+});
+
+add_task(async function test_empty() {
+  _("Verify the tracker is empty to start with.");
+
+  Assert.equal(0, Object.keys(await tracker.getChangedIDs()).length);
+  Assert.equal(0, tracker.score);
+
+  await cleanup();
+});
+
+add_task(async function test_not_tracking() {
+  _("Ensures the tracker doesn't do anything when it isn't tracking.");
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  await uninstallAddon(addon, reconciler);
+
+  Assert.equal(0, Object.keys(await tracker.getChangedIDs()).length);
+  Assert.equal(0, tracker.score);
+
+  await cleanup();
+});
+
+add_task(async function test_track_install() {
+  _("Ensure that installing an add-on notifies tracker.");
+
+  reconciler.startListening();
+
+  tracker.start();
+
+  Assert.equal(0, tracker.score);
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  let changed = await tracker.getChangedIDs();
+
+  Assert.equal(1, Object.keys(changed).length);
+  Assert.ok(addon.syncGUID in changed);
+  Assert.equal(SCORE_INCREMENT_XLARGE, tracker.score);
+
+  await uninstallAddon(addon, reconciler);
+  await cleanup();
+});
+
+add_task(async function test_track_uninstall() {
+  _("Ensure that uninstalling an add-on notifies tracker.");
+
+  reconciler.startListening();
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  let guid = addon.syncGUID;
+  Assert.equal(0, tracker.score);
+
+  tracker.start();
+
+  await uninstallAddon(addon, reconciler);
+  let changed = await tracker.getChangedIDs();
+  Assert.equal(1, Object.keys(changed).length);
+  Assert.ok(guid in changed);
+  Assert.equal(SCORE_INCREMENT_XLARGE, tracker.score);
+
+  await cleanup();
+});
+
+add_task(async function test_track_user_disable() {
+  _("Ensure that tracker sees disabling of add-on");
+
+  reconciler.startListening();
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  Assert.ok(!addon.userDisabled);
+  Assert.ok(!addon.appDisabled);
+  Assert.ok(addon.isActive);
+
+  tracker.start();
+  Assert.equal(0, tracker.score);
+
+  _("Disabling add-on");
+  await addon.disable();
+  await reconciler.queueCaller.promiseCallsComplete();
+
+  let changed = await tracker.getChangedIDs();
+  Assert.equal(1, Object.keys(changed).length);
+  Assert.ok(addon.syncGUID in changed);
+  Assert.equal(SCORE_INCREMENT_XLARGE, tracker.score);
+
+  await uninstallAddon(addon, reconciler);
+  await cleanup();
+});
+
+add_task(async function test_track_enable() {
+  _("Ensure that enabling a disabled add-on notifies tracker.");
+
+  reconciler.startListening();
+
+  let addon = await installAddon(XPIS.test_addon1, reconciler);
+  await addon.disable();
+  await Async.promiseYield();
+
+  Assert.equal(0, tracker.score);
+
+  tracker.start();
+  await addon.enable();
+  await Async.promiseYield();
+  await reconciler.queueCaller.promiseCallsComplete();
+
+  let changed = await tracker.getChangedIDs();
+  Assert.equal(1, Object.keys(changed).length);
+  Assert.ok(addon.syncGUID in changed);
+  Assert.equal(SCORE_INCREMENT_XLARGE, tracker.score);
+
+  await uninstallAddon(addon, reconciler);
+  await cleanup();
+});
diff --git a/services/sync/tests/unit/test_addons_validator.js b/services/sync/tests/unit/test_addons_validator.js
new file mode 100644
index 0000000000..60f2f8bf43
--- /dev/null
+++ b/services/sync/tests/unit/test_addons_validator.js
@@ -0,0 +1,65 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { AddonValidator } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/addons.sys.mjs"
+);
+
+function getDummyServerAndClient() {
+  return {
+    server: [
+      {
+        id: "1111",
+        applicationID: Services.appinfo.ID,
+        addonID: "synced-addon@example.com",
+        enabled: true,
+        source: "amo",
+        understood: true,
+      },
+    ],
+    client: [
+      {
+        syncGUID: "1111",
+        id: "synced-addon@example.com",
+        type: "extension",
+        isSystem: false,
+        isSyncable: true,
+      },
+      {
+        syncGUID: "2222",
+        id: "system-addon@example.com",
+        type: "extension",
+        isSystem: true,
+        isSyncable: false,
+      },
+      {
+        // Plugins don't have a `syncedGUID`, but we don't sync them, so we
+        // shouldn't report them as client duplicates.
+        id: "some-plugin",
+        type: "plugin",
+      },
+      {
+        id: "another-plugin",
+        type: "plugin",
+      },
+    ],
+  };
+}
+
+add_task(async function test_valid() {
+  let { server, client } = getDummyServerAndClient();
+  let validator = new AddonValidator({
+    _findDupe(item) {
+      return null;
+    },
+    isAddonSyncable(item) {
+      return item.type != "plugin";
+    },
+  });
+  let { problemData, clientRecords, records, deletedRecords } =
+    await validator.compareClientWithServer(client, server);
+  equal(clientRecords.length, 4);
+  equal(records.length, 1);
+  equal(deletedRecords.length, 0);
+  deepEqual(problemData, validator.emptyProblemData());
+});
diff --git a/services/sync/tests/unit/test_bookmark_batch_fail.js b/services/sync/tests/unit/test_bookmark_batch_fail.js
new file mode 100644
index 0000000000..9644d730e4
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_batch_fail.js
@@ -0,0 +1,25 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+_("Making sure a failing sync reports a useful error");
+// `Service` is used as a global in head_helpers.js.
+// eslint-disable-next-line no-unused-vars
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_bookmark_test(async function run_test(engine) {
+  await engine.initialize();
+  engine._syncStartup = async function () {
+    throw new Error("FAIL!");
+  };
+
+  try {
+    _("Try calling the sync that should throw right away");
+    await engine._sync();
+    do_throw("Should have failed sync!");
+  } catch (ex) {
+    _("Making sure what we threw ended up as the exception:", ex);
+    Assert.equal(ex.message, "FAIL!");
+  }
+});
diff --git a/services/sync/tests/unit/test_bookmark_decline_undecline.js b/services/sync/tests/unit/test_bookmark_decline_undecline.js
new file mode 100644
index 0000000000..12139dd163
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_decline_undecline.js
@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+// A stored reference to the collection won't be valid after disabling.
+function getBookmarkWBO(server, guid) {
+  let coll = server.user("foo").collection("bookmarks");
+  if (!coll) {
+    return null;
+  }
+  return coll.wbo(guid);
+}
+
+add_task(async function test_decline_undecline() {
+  let engine = Service.engineManager.get("bookmarks");
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    let { guid: bzGuid } = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "https://bugzilla.mozilla.org",
+      index: PlacesUtils.bookmarks.DEFAULT_INDEX,
+      title: "bugzilla",
+    });
+
+    ok(!getBookmarkWBO(server, bzGuid), "Shouldn't have been uploaded yet");
+    await Service.sync();
+    ok(getBookmarkWBO(server, bzGuid), "Should be present on server");
+
+    engine.enabled = false;
+    await Service.sync();
+    ok(
+      !getBookmarkWBO(server, bzGuid),
+      "Shouldn't be present on server anymore"
+    );
+
+    engine.enabled = true;
+    await Service.sync();
+    ok(getBookmarkWBO(server, bzGuid), "Should be present on server again");
+  } finally {
+    await PlacesSyncUtils.bookmarks.reset();
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_bookmark_engine.js b/services/sync/tests/unit/test_bookmark_engine.js
new file mode 100644
index 0000000000..6274a6b836
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_engine.js
@@ -0,0 +1,1555 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { BookmarkHTMLUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/BookmarkHTMLUtils.sys.mjs"
+);
+const { BookmarkJSONUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/BookmarkJSONUtils.sys.mjs"
+);
+const { Bookmark, BookmarkFolder, BookmarksEngine, Livemark } =
+  ChromeUtils.importESModule(
+    "resource://services-sync/engines/bookmarks.sys.mjs"
+  );
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { SyncedRecordsTelemetry } = ChromeUtils.importESModule(
+  "resource://services-sync/telemetry.sys.mjs"
+);
+
+var recordedEvents = [];
+
+function checkRecordedEvents(object, expected, message) {
+  // Ignore event telemetry from the merger.
+  let checkEvents = recordedEvents.filter(event => event.object == object);
+  deepEqual(checkEvents, expected, message);
+  // and clear the list so future checks are easier to write.
+  recordedEvents = [];
+}
+
+async function fetchAllRecordIds() {
+  let db = await PlacesUtils.promiseDBConnection();
+  let rows = await db.executeCached(`
+    WITH RECURSIVE
+    syncedItems(id, guid) AS (
+      SELECT b.id, b.guid FROM moz_bookmarks b
+      WHERE b.guid IN ('menu________', 'toolbar_____', 'unfiled_____',
+                       'mobile______')
+      UNION ALL
+      SELECT b.id, b.guid FROM moz_bookmarks b
+      JOIN syncedItems s ON b.parent = s.id
+    )
+    SELECT guid FROM syncedItems`);
+  let recordIds = new Set();
+  for (let row of rows) {
+    let recordId = PlacesSyncUtils.bookmarks.guidToRecordId(
+      row.getResultByName("guid")
+    );
+    recordIds.add(recordId);
+  }
+  return recordIds;
+}
+
+async function cleanupEngine(engine) {
+  await engine.resetClient();
+  await engine._store.wipe();
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  Service.recordManager.clearCache();
+  // Note we don't finalize the engine here as add_bookmark_test() does.
+}
+
+async function cleanup(engine, server) {
+  await promiseStopServer(server);
+  await cleanupEngine(engine);
+}
+
+add_task(async function setup() {
+  await generateNewKeys(Service.collectionKeys);
+  await Service.engineManager.unregister("bookmarks");
+
+  Service.recordTelemetryEvent = (object, method, value, extra = undefined) => {
+    recordedEvents.push({ object, method, value, extra });
+  };
+});
+
+add_task(async function test_buffer_timeout() {
+  await Service.recordManager.clearCache();
+  await PlacesSyncUtils.bookmarks.reset();
+  let engine = new BookmarksEngine(Service);
+  engine._newWatchdog = function () {
+    // Return an already-aborted watchdog, so that we can abort merges
+    // immediately.
+    let watchdog = Async.watchdog();
+    watchdog.controller.abort();
+    return watchdog;
+  };
+  await engine.initialize();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("bookmarks");
+
+  try {
+    info("Insert local bookmarks");
+    await PlacesUtils.bookmarks.insertTree({
+      guid: PlacesUtils.bookmarks.unfiledGuid,
+      children: [
+        {
+          guid: "bookmarkAAAA",
+          url: "http://example.com/a",
+          title: "A",
+        },
+        {
+          guid: "bookmarkBBBB",
+          url: "http://example.com/b",
+          title: "B",
+        },
+      ],
+    });
+
+    info("Insert remote bookmarks");
+    collection.insert(
+      "menu",
+      encryptPayload({
+        id: "menu",
+        type: "folder",
+        parentid: "places",
+        title: "menu",
+        children: ["bookmarkCCCC", "bookmarkDDDD"],
+      })
+    );
+    collection.insert(
+      "bookmarkCCCC",
+      encryptPayload({
+        id: "bookmarkCCCC",
+        type: "bookmark",
+        parentid: "menu",
+        bmkUri: "http://example.com/c",
+        title: "C",
+      })
+    );
+    collection.insert(
+      "bookmarkDDDD",
+      encryptPayload({
+        id: "bookmarkDDDD",
+        type: "bookmark",
+        parentid: "menu",
+        bmkUri: "http://example.com/d",
+        title: "D",
+      })
+    );
+
+    info("We expect this sync to fail");
+    await Assert.rejects(
+      sync_engine_and_validate_telem(engine, true),
+      ex => ex.name == "InterruptedError"
+    );
+  } finally {
+    await cleanup(engine, server);
+    await engine.finalize();
+  }
+});
+
+add_bookmark_test(async function test_maintenance_after_failure(engine) {
+  _("Ensure we try to run maintenance if the engine fails to sync");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    let syncStartup = engine._syncStartup;
+    let syncError = new Error("Something is rotten in the state of Places");
+    engine._syncStartup = function () {
+      throw syncError;
+    };
+
+    Services.prefs.clearUserPref("places.database.lastMaintenance");
+
+    _("Ensure the sync fails and we run maintenance");
+    await Assert.rejects(
+      sync_engine_and_validate_telem(engine, true),
+      ex => ex == syncError
+    );
+    checkRecordedEvents(
+      "maintenance",
+      [
+        {
+          object: "maintenance",
+          method: "run",
+          value: "bookmarks",
+          extra: undefined,
+        },
+      ],
+      "Should record event for first maintenance run"
+    );
+
+    _("Sync again, but ensure maintenance doesn't run");
+    await Assert.rejects(
+      sync_engine_and_validate_telem(engine, true),
+      ex => ex == syncError
+    );
+    checkRecordedEvents(
+      "maintenance",
+      [],
+      "Should not record event if maintenance didn't run"
+    );
+
+    _("Fast-forward last maintenance pref; ensure maintenance runs");
+    Services.prefs.setIntPref(
+      "places.database.lastMaintenance",
+      Date.now() / 1000 - 14400
+    );
+    await Assert.rejects(
+      sync_engine_and_validate_telem(engine, true),
+      ex => ex == syncError
+    );
+    checkRecordedEvents(
+      "maintenance",
+      [
+        {
+          object: "maintenance",
+          method: "run",
+          value: "bookmarks",
+          extra: undefined,
+        },
+      ],
+      "Should record event for second maintenance run"
+    );
+
+    _("Fix sync failure; ensure we report success after maintenance");
+    engine._syncStartup = syncStartup;
+    await sync_engine_and_validate_telem(engine, false);
+    checkRecordedEvents(
+      "maintenance",
+      [
+        {
+          object: "maintenance",
+          method: "fix",
+          value: "bookmarks",
+          extra: undefined,
+        },
+      ],
+      "Should record event for successful sync after second maintenance"
+    );
+
+    await sync_engine_and_validate_telem(engine, false);
+    checkRecordedEvents(
+      "maintenance",
+      [],
+      "Should not record maintenance events after successful sync"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_bookmark_test(async function test_delete_invalid_roots_from_server(engine) {
+  _("Ensure that we delete the Places and Reading List roots from the server.");
+
+  enableValidationPrefs();
+
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  engine._tracker.start();
+
+  try {
+    let placesRecord = await store.createRecord("places");
+    collection.insert("places", encryptPayload(placesRecord.cleartext));
+
+    let listBmk = new Bookmark("bookmarks", Utils.makeGUID());
+    listBmk.bmkUri = "https://example.com";
+    listBmk.title = "Example reading list entry";
+    listBmk.parentName = "Reading List";
+    listBmk.parentid = "readinglist";
+    collection.insert(listBmk.id, encryptPayload(listBmk.cleartext));
+
+    let readingList = new BookmarkFolder("bookmarks", "readinglist");
+    readingList.title = "Reading List";
+    readingList.children = [listBmk.id];
+    readingList.parentName = "";
+    readingList.parentid = "places";
+    collection.insert("readinglist", encryptPayload(readingList.cleartext));
+
+    // Note that we don't insert a record for the toolbar, so the  engine will
+    // report a parent-child disagreement, since Firefox's `parentid` is
+    // `toolbar`.
+    let newBmk = new Bookmark("bookmarks", Utils.makeGUID());
+    newBmk.bmkUri = "http://getfirefox.com";
+    newBmk.title = "Get Firefox!";
+    newBmk.parentName = "Bookmarks Toolbar";
+    newBmk.parentid = "toolbar";
+    collection.insert(newBmk.id, encryptPayload(newBmk.cleartext));
+
+    deepEqual(
+      collection.keys().sort(),
+      ["places", "readinglist", listBmk.id, newBmk.id].sort(),
+      "Should store Places root, reading list items, and new bookmark on server"
+    );
+
+    let ping = await sync_engine_and_validate_telem(engine, true);
+    // In a real sync, the engine is named `bookmarks-buffered`.
+    // However, `sync_engine_and_validate_telem` simulates a sync where
+    // the engine isn't registered with the engine manager, so the recorder
+    // doesn't see its `overrideTelemetryName`.
+    let engineData = ping.engines.find(e => e.name == "bookmarks");
+    ok(engineData.validation, "Bookmarks engine should always run validation");
+    equal(
+      engineData.validation.checked,
+      6,
+      "Bookmarks engine should validate all items"
+    );
+    deepEqual(
+      engineData.validation.problems,
+      [
+        {
+          name: "parentChildDisagreements",
+          count: 1,
+        },
+      ],
+      "Bookmarks engine should report parent-child disagreement"
+    );
+    deepEqual(
+      engineData.steps.map(step => step.name),
+      [
+        "fetchLocalTree",
+        "fetchRemoteTree",
+        "merge",
+        "apply",
+        "notifyObservers",
+        "fetchLocalChangeRecords",
+      ],
+      "Bookmarks engine should report all merge steps"
+    );
+
+    deepEqual(
+      collection.keys().sort(),
+      ["menu", "mobile", "toolbar", "unfiled", newBmk.id].sort(),
+      "Should remove Places root and reading list items from server; upload local roots"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_bookmark_test(async function test_processIncoming_error_orderChildren(
+  engine
+) {
+  _(
+    "Ensure that _orderChildren() is called even when _processIncoming() throws an error."
+  );
+
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  try {
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      title: "Folder 1",
+    });
+
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    let bmk2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    });
+
+    let toolbar_record = await store.createRecord("toolbar");
+    collection.insert("toolbar", encryptPayload(toolbar_record.cleartext));
+
+    let bmk1_record = await store.createRecord(bmk1.guid);
+    collection.insert(bmk1.guid, encryptPayload(bmk1_record.cleartext));
+
+    let bmk2_record = await store.createRecord(bmk2.guid);
+    collection.insert(bmk2.guid, encryptPayload(bmk2_record.cleartext));
+
+    // Create a server record for folder1 where we flip the order of
+    // the children.
+    let folder1_record = await store.createRecord(folder1.guid);
+    let folder1_payload = folder1_record.cleartext;
+    folder1_payload.children.reverse();
+    collection.insert(folder1.guid, encryptPayload(folder1_payload));
+
+    // Create a bogus record that when synced down will provoke a
+    // network error which in turn provokes an exception in _processIncoming.
+    const BOGUS_GUID = "zzzzzzzzzzzz";
+    let bogus_record = collection.insert(BOGUS_GUID, "I'm a bogus record!");
+    bogus_record.get = function get() {
+      throw new Error("Sync this!");
+    };
+
+    // Make the 10 minutes old so it will only be synced in the toFetch phase.
+    bogus_record.modified = new_timestamp() - 60 * 10;
+    await engine.setLastSync(new_timestamp() - 60);
+    engine.toFetch = new SerializableSet([BOGUS_GUID]);
+
+    let error;
+    try {
+      await sync_engine_and_validate_telem(engine, true);
+    } catch (ex) {
+      error = ex;
+    }
+    ok(!!error);
+
+    // Verify that the bookmark order has been applied.
+    folder1_record = await store.createRecord(folder1.guid);
+    let new_children = folder1_record.children;
+    Assert.deepEqual(
+      new_children.sort(),
+      [folder1_payload.children[0], folder1_payload.children[1]].sort()
+    );
+
+    let localChildIds = await PlacesSyncUtils.bookmarks.fetchChildRecordIds(
+      folder1.guid
+    );
+    Assert.deepEqual(localChildIds.sort(), [bmk2.guid, bmk1.guid].sort());
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_bookmark_test(async function test_restorePromptsReupload(engine) {
+  await test_restoreOrImport(engine, { replace: true });
+});
+
+add_bookmark_test(async function test_importPromptsReupload(engine) {
+  await test_restoreOrImport(engine, { replace: false });
+});
+
+// Test a JSON restore or HTML import. Use JSON if `replace` is `true`, or
+// HTML otherwise.
+async function test_restoreOrImport(engine, { replace }) {
+  let verb = replace ? "restore" : "import";
+  let verbing = replace ? "restoring" : "importing";
+  let bookmarkUtils = replace ? BookmarkJSONUtils : BookmarkHTMLUtils;
+
+  _(`Ensure that ${verbing} from a backup will reupload all records.`);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  engine._tracker.start(); // We skip usual startup...
+
+  try {
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      title: "Folder 1",
+    });
+
+    _("Create a single record.");
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    _(`Get Firefox!: ${bmk1.guid}`);
+
+    let backupFilePath = PathUtils.join(
+      PathUtils.tempDir,
+      `t_b_e_${Date.now()}.json`
+    );
+
+    _("Make a backup.");
+
+    await bookmarkUtils.exportToFile(backupFilePath);
+
+    _("Create a different record and sync.");
+    let bmk2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    });
+    _(`Get Thunderbird!: ${bmk2.guid}`);
+
+    await PlacesUtils.bookmarks.remove(bmk1.guid);
+
+    let error;
+    try {
+      await sync_engine_and_validate_telem(engine, false);
+    } catch (ex) {
+      error = ex;
+      _("Got error: " + Log.exceptionStr(ex));
+    }
+    Assert.ok(!error);
+
+    _(
+      "Verify that there's only one bookmark on the server, and it's Thunderbird."
+    );
+    // Of course, there's also the Bookmarks Toolbar and Bookmarks Menu...
+    let wbos = collection.keys(function (id) {
+      return !["menu", "toolbar", "mobile", "unfiled", folder1.guid].includes(
+        id
+      );
+    });
+    Assert.equal(wbos.length, 1);
+    Assert.equal(wbos[0], bmk2.guid);
+
+    _(`Now ${verb} from a backup.`);
+    await bookmarkUtils.importFromFile(backupFilePath, { replace });
+
+    // If `replace` is `true`, we'll wipe the server on the next sync.
+    let bookmarksCollection = server.user("foo").collection("bookmarks");
+    _("Verify that we didn't wipe the server.");
+    Assert.ok(!!bookmarksCollection);
+
+    _("Ensure we have the bookmarks we expect locally.");
+    let recordIds = await fetchAllRecordIds();
+    _("GUIDs: " + JSON.stringify([...recordIds]));
+
+    let bookmarkRecordIds = new Map();
+    let count = 0;
+    for (let recordId of recordIds) {
+      count++;
+      let info = await PlacesUtils.bookmarks.fetch(
+        PlacesSyncUtils.bookmarks.recordIdToGuid(recordId)
+      );
+      // Only one bookmark, so _all_ should be Firefox!
+      if (info.type == PlacesUtils.bookmarks.TYPE_BOOKMARK) {
+        _(`Found URI ${info.url.href} for record ID ${recordId}`);
+        bookmarkRecordIds.set(info.url.href, recordId);
+      }
+    }
+    Assert.ok(bookmarkRecordIds.has("http://getfirefox.com/"));
+    if (!replace) {
+      Assert.ok(bookmarkRecordIds.has("http://getthunderbird.com/"));
+    }
+
+    _("Have the correct number of IDs locally, too.");
+    let expectedResults = [
+      "menu",
+      "toolbar",
+      "mobile",
+      "unfiled",
+      folder1.guid,
+      bmk1.guid,
+    ];
+    if (!replace) {
+      expectedResults.push("toolbar", folder1.guid, bmk2.guid);
+    }
+    Assert.equal(count, expectedResults.length);
+
+    _("Sync again. This'll wipe bookmarks from the server.");
+    try {
+      await sync_engine_and_validate_telem(engine, false);
+    } catch (ex) {
+      error = ex;
+      _("Got error: " + Log.exceptionStr(ex));
+    }
+    Assert.ok(!error);
+
+    _("Verify that there's the right bookmarks on the server.");
+    // Of course, there's also the Bookmarks Toolbar and Bookmarks Menu...
+    let payloads = server.user("foo").collection("bookmarks").payloads();
+    let bookmarkWBOs = payloads.filter(function (wbo) {
+      return wbo.type == "bookmark";
+    });
+
+    let folderWBOs = payloads.filter(function (wbo) {
+      return (
+        wbo.type == "folder" &&
+        wbo.id != "menu" &&
+        wbo.id != "toolbar" &&
+        wbo.id != "unfiled" &&
+        wbo.id != "mobile" &&
+        wbo.parentid != "menu"
+      );
+    });
+
+    let expectedFX = {
+      id: bookmarkRecordIds.get("http://getfirefox.com/"),
+      bmkUri: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    };
+    let expectedTB = {
+      id: bookmarkRecordIds.get("http://getthunderbird.com/"),
+      bmkUri: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    };
+
+    let expectedBookmarks;
+    if (replace) {
+      expectedBookmarks = [expectedFX];
+    } else {
+      expectedBookmarks = [expectedTB, expectedFX];
+    }
+
+    doCheckWBOs(bookmarkWBOs, expectedBookmarks);
+
+    _("Our old friend Folder 1 is still in play.");
+    let expectedFolder1 = { title: "Folder 1" };
+
+    let expectedFolders;
+    if (replace) {
+      expectedFolders = [expectedFolder1];
+    } else {
+      expectedFolders = [expectedFolder1, expectedFolder1];
+    }
+
+    doCheckWBOs(folderWBOs, expectedFolders);
+  } finally {
+    await cleanup(engine, server);
+  }
+}
+
+function doCheckWBOs(WBOs, expected) {
+  Assert.equal(WBOs.length, expected.length);
+  for (let i = 0; i < expected.length; i++) {
+    let lhs = WBOs[i];
+    let rhs = expected[i];
+    if ("id" in rhs) {
+      Assert.equal(lhs.id, rhs.id);
+    }
+    if ("bmkUri" in rhs) {
+      Assert.equal(lhs.bmkUri, rhs.bmkUri);
+    }
+    if ("title" in rhs) {
+      Assert.equal(lhs.title, rhs.title);
+    }
+  }
+}
+
+function FakeRecord(constructor, r) {
+  this.defaultCleartext = constructor.prototype.defaultCleartext;
+  constructor.call(this, "bookmarks", r.id);
+  for (let x in r) {
+    this[x] = r[x];
+  }
+  // Borrow the constructor's conversion functions.
+  this.toSyncBookmark = constructor.prototype.toSyncBookmark;
+  this.cleartextToString = constructor.prototype.cleartextToString;
+}
+
+// Bug 632287.
+// (Note that `test_mismatched_folder_types()` in
+//  toolkit/components/places/tests/sync/test_bookmark_kinds.js is an exact
+// copy of this test, so it's fine to remove it as part of bug 1449730)
+add_task(async function test_mismatched_types() {
+  _(
+    "Ensure that handling a record that changes type causes deletion " +
+      "then re-adding."
+  );
+
+  let oldRecord = {
+    id: "l1nZZXfB8nC7",
+    type: "folder",
+    parentName: "Bookmarks Toolbar",
+    title: "Innerst i Sneglehode",
+    description: null,
+    parentid: "toolbar",
+  };
+
+  let newRecord = {
+    id: "l1nZZXfB8nC7",
+    type: "livemark",
+    siteUri: "http://sneglehode.wordpress.com/",
+    feedUri: "http://sneglehode.wordpress.com/feed/",
+    parentName: "Bookmarks Toolbar",
+    title: "Innerst i Sneglehode",
+    description: null,
+    children: [
+      "HCRq40Rnxhrd",
+      "YeyWCV1RVsYw",
+      "GCceVZMhvMbP",
+      "sYi2hevdArlF",
+      "vjbZlPlSyGY8",
+      "UtjUhVyrpeG6",
+      "rVq8WMG2wfZI",
+      "Lx0tcy43ZKhZ",
+      "oT74WwV8_j4P",
+      "IztsItWVSo3-",
+    ],
+    parentid: "toolbar",
+  };
+
+  let engine = new BookmarksEngine(Service);
+  await engine.initialize();
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    let oldR = new FakeRecord(BookmarkFolder, oldRecord);
+    let newR = new FakeRecord(Livemark, newRecord);
+    oldR.parentid = PlacesUtils.bookmarks.toolbarGuid;
+    newR.parentid = PlacesUtils.bookmarks.toolbarGuid;
+
+    await store.applyIncoming(oldR);
+    await engine._apply();
+    _("Applied old. It's a folder.");
+    let oldID = await PlacesTestUtils.promiseItemId(oldR.id);
+    _("Old ID: " + oldID);
+    let oldInfo = await PlacesUtils.bookmarks.fetch(oldR.id);
+    Assert.equal(oldInfo.type, PlacesUtils.bookmarks.TYPE_FOLDER);
+
+    await store.applyIncoming(newR);
+    await engine._apply();
+  } finally {
+    await cleanup(engine, server);
+    await engine.finalize();
+  }
+});
+
+add_bookmark_test(async function test_misreconciled_root(engine) {
+  _("Ensure that we don't reconcile an arbitrary record with a root.");
+
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  // Log real hard for this test.
+  store._log.trace = store._log.debug;
+  engine._log.trace = engine._log.debug;
+
+  await engine._syncStartup();
+
+  // Let's find out where the toolbar is right now.
+  let toolbarBefore = await store.createRecord("toolbar", "bookmarks");
+  let toolbarIDBefore = await PlacesTestUtils.promiseItemId(
+    PlacesUtils.bookmarks.toolbarGuid
+  );
+  Assert.notEqual(-1, toolbarIDBefore);
+
+  let parentRecordIDBefore = toolbarBefore.parentid;
+  let parentGUIDBefore =
+    PlacesSyncUtils.bookmarks.recordIdToGuid(parentRecordIDBefore);
+  let parentIDBefore = await PlacesTestUtils.promiseItemId(parentGUIDBefore);
+  Assert.equal("string", typeof parentGUIDBefore);
+
+  _("Current parent: " + parentGUIDBefore + " (" + parentIDBefore + ").");
+
+  let to_apply = {
+    id: "zzzzzzzzzzzz",
+    type: "folder",
+    title: "Bookmarks Toolbar",
+    description: "Now you're for it.",
+    parentName: "",
+    parentid: "mobile", // Why not?
+    children: [],
+  };
+
+  let rec = new FakeRecord(BookmarkFolder, to_apply);
+
+  _("Applying record.");
+  let countTelemetry = new SyncedRecordsTelemetry();
+  await store.applyIncomingBatch([rec], countTelemetry);
+
+  // Ensure that afterwards, toolbar is still there.
+  // As of 2012-12-05, this only passes because Places doesn't use "toolbar" as
+  // the real GUID, instead using a generated one. Sync does the translation.
+  let toolbarAfter = await store.createRecord("toolbar", "bookmarks");
+  let parentRecordIDAfter = toolbarAfter.parentid;
+  let parentGUIDAfter =
+    PlacesSyncUtils.bookmarks.recordIdToGuid(parentRecordIDAfter);
+  let parentIDAfter = await PlacesTestUtils.promiseItemId(parentGUIDAfter);
+  Assert.equal(
+    await PlacesTestUtils.promiseItemGuid(toolbarIDBefore),
+    PlacesUtils.bookmarks.toolbarGuid
+  );
+  Assert.equal(parentGUIDBefore, parentGUIDAfter);
+  Assert.equal(parentIDBefore, parentIDAfter);
+
+  await cleanup(engine, server);
+});
+
+add_bookmark_test(async function test_invalid_url(engine) {
+  _("Ensure an incoming invalid bookmark URL causes an outgoing tombstone.");
+
+  let server = await serverForFoo(engine);
+  let collection = server.user("foo").collection("bookmarks");
+
+  await SyncTestingInfrastructure(server);
+  await engine._syncStartup();
+
+  // check the URL really is invalid.
+  let url = "https://www.42registry.42/";
+  Assert.throws(() => Services.io.newURI(url), /invalid/);
+
+  let guid = "abcdefabcdef";
+
+  let toolbar = new BookmarkFolder("bookmarks", "toolbar");
+  toolbar.title = "toolbar";
+  toolbar.parentName = "";
+  toolbar.parentid = "places";
+  toolbar.children = [guid];
+  collection.insert("toolbar", encryptPayload(toolbar.cleartext));
+
+  let item1 = new Bookmark("bookmarks", guid);
+  item1.bmkUri = "https://www.42registry.42/";
+  item1.title = "invalid url";
+  item1.parentName = "Bookmarks Toolbar";
+  item1.parentid = "toolbar";
+  item1.dateAdded = 1234;
+  collection.insert(guid, encryptPayload(item1.cleartext));
+
+  _("syncing.");
+  await sync_engine_and_validate_telem(engine, false);
+
+  // We should find the record now exists on the server as a tombstone.
+  let updated = collection.cleartext(guid);
+  Assert.ok(updated.deleted, "record was deleted");
+
+  let local = await PlacesUtils.bookmarks.fetch(guid);
+  Assert.deepEqual(local, null, "no local bookmark exists");
+
+  await cleanup(engine, server);
+});
+
+add_bookmark_test(async function test_sync_dateAdded(engine) {
+  await Service.recordManager.clearCache();
+  await PlacesSyncUtils.bookmarks.reset();
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  // TODO: Avoid random orange (bug 1374599), this is only necessary
+  // intermittently - reset the last sync date so that we'll get all bookmarks.
+  await engine.setLastSync(1);
+
+  engine._tracker.start(); // We skip usual startup...
+
+  // Just matters that it's in the past, not how far.
+  let now = Date.now();
+  let oneYearMS = 365 * 24 * 60 * 60 * 1000;
+
+  try {
+    let toolbar = new BookmarkFolder("bookmarks", "toolbar");
+    toolbar.title = "toolbar";
+    toolbar.parentName = "";
+    toolbar.parentid = "places";
+    toolbar.children = [
+      "abcdefabcdef",
+      "aaaaaaaaaaaa",
+      "bbbbbbbbbbbb",
+      "cccccccccccc",
+      "dddddddddddd",
+      "eeeeeeeeeeee",
+    ];
+    collection.insert("toolbar", encryptPayload(toolbar.cleartext));
+
+    let item1GUID = "abcdefabcdef";
+    let item1 = new Bookmark("bookmarks", item1GUID);
+    item1.bmkUri = "https://example.com";
+    item1.title = "asdf";
+    item1.parentName = "Bookmarks Toolbar";
+    item1.parentid = "toolbar";
+    item1.dateAdded = now - oneYearMS;
+    collection.insert(item1GUID, encryptPayload(item1.cleartext));
+
+    let item2GUID = "aaaaaaaaaaaa";
+    let item2 = new Bookmark("bookmarks", item2GUID);
+    item2.bmkUri = "https://example.com/2";
+    item2.title = "asdf2";
+    item2.parentName = "Bookmarks Toolbar";
+    item2.parentid = "toolbar";
+    item2.dateAdded = now + oneYearMS;
+    const item2LastModified = now / 1000 - 100;
+    collection.insert(
+      item2GUID,
+      encryptPayload(item2.cleartext),
+      item2LastModified
+    );
+
+    let item3GUID = "bbbbbbbbbbbb";
+    let item3 = new Bookmark("bookmarks", item3GUID);
+    item3.bmkUri = "https://example.com/3";
+    item3.title = "asdf3";
+    item3.parentName = "Bookmarks Toolbar";
+    item3.parentid = "toolbar";
+    // no dateAdded
+    collection.insert(item3GUID, encryptPayload(item3.cleartext));
+
+    let item4GUID = "cccccccccccc";
+    let item4 = new Bookmark("bookmarks", item4GUID);
+    item4.bmkUri = "https://example.com/4";
+    item4.title = "asdf4";
+    item4.parentName = "Bookmarks Toolbar";
+    item4.parentid = "toolbar";
+    // no dateAdded, but lastModified in past
+    const item4LastModified = (now - oneYearMS) / 1000;
+    collection.insert(
+      item4GUID,
+      encryptPayload(item4.cleartext),
+      item4LastModified
+    );
+
+    let item5GUID = "dddddddddddd";
+    let item5 = new Bookmark("bookmarks", item5GUID);
+    item5.bmkUri = "https://example.com/5";
+    item5.title = "asdf5";
+    item5.parentName = "Bookmarks Toolbar";
+    item5.parentid = "toolbar";
+    // no dateAdded, lastModified in (near) future.
+    const item5LastModified = (now + 60000) / 1000;
+    collection.insert(
+      item5GUID,
+      encryptPayload(item5.cleartext),
+      item5LastModified
+    );
+
+    let item6GUID = "eeeeeeeeeeee";
+    let item6 = new Bookmark("bookmarks", item6GUID);
+    item6.bmkUri = "https://example.com/6";
+    item6.title = "asdf6";
+    item6.parentName = "Bookmarks Toolbar";
+    item6.parentid = "toolbar";
+    const item6LastModified = (now - oneYearMS) / 1000;
+    collection.insert(
+      item6GUID,
+      encryptPayload(item6.cleartext),
+      item6LastModified
+    );
+
+    await sync_engine_and_validate_telem(engine, false);
+
+    let record1 = await store.createRecord(item1GUID);
+    let record2 = await store.createRecord(item2GUID);
+
+    equal(
+      item1.dateAdded,
+      record1.dateAdded,
+      "dateAdded in past should be synced"
+    );
+    equal(
+      record2.dateAdded,
+      item2LastModified * 1000,
+      "dateAdded in future should be ignored in favor of last modified"
+    );
+
+    let record3 = await store.createRecord(item3GUID);
+
+    ok(record3.dateAdded);
+    // Make sure it's within 24 hours of the right timestamp... This is a little
+    // dodgey but we only really care that it's basically accurate and has the
+    // right day.
+    ok(Math.abs(Date.now() - record3.dateAdded) < 24 * 60 * 60 * 1000);
+
+    let record4 = await store.createRecord(item4GUID);
+    equal(
+      record4.dateAdded,
+      item4LastModified * 1000,
+      "If no dateAdded is provided, lastModified should be used"
+    );
+
+    let record5 = await store.createRecord(item5GUID);
+    equal(
+      record5.dateAdded,
+      item5LastModified * 1000,
+      "If no dateAdded is provided, lastModified should be used (even if it's in the future)"
+    );
+
+    // Update item2 and try resyncing it.
+    item2.dateAdded = now - 100000;
+    collection.insert(
+      item2GUID,
+      encryptPayload(item2.cleartext),
+      now / 1000 - 50
+    );
+
+    // Also, add a local bookmark and make sure its date added makes it up to the server
+    let bz = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "https://bugzilla.mozilla.org/",
+      title: "Bugzilla",
+    });
+
+    // last sync did a POST, which doesn't advance its lastModified value.
+    // Next sync of the engine doesn't hit info/collections, so lastModified
+    // remains stale. Setting it to null side-steps that.
+    engine.lastModified = null;
+    await sync_engine_and_validate_telem(engine, false);
+
+    let newRecord2 = await store.createRecord(item2GUID);
+    equal(
+      newRecord2.dateAdded,
+      item2.dateAdded,
+      "dateAdded update should work for earlier date"
+    );
+
+    let bzWBO = collection.cleartext(bz.guid);
+    ok(bzWBO.dateAdded, "Locally added dateAdded lost");
+
+    let localRecord = await store.createRecord(bz.guid);
+    equal(
+      bzWBO.dateAdded,
+      localRecord.dateAdded,
+      "dateAdded should not change during upload"
+    );
+
+    item2.dateAdded += 10000;
+    collection.insert(
+      item2GUID,
+      encryptPayload(item2.cleartext),
+      now / 1000 - 10
+    );
+
+    engine.lastModified = null;
+    await sync_engine_and_validate_telem(engine, false);
+
+    let newerRecord2 = await store.createRecord(item2GUID);
+    equal(
+      newerRecord2.dateAdded,
+      newRecord2.dateAdded,
+      "dateAdded update should be ignored for later date if we know an earlier one "
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_buffer_hasDupe() {
+  await Service.recordManager.clearCache();
+  await PlacesSyncUtils.bookmarks.reset();
+  let engine = new BookmarksEngine(Service);
+  await engine.initialize();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("bookmarks");
+  engine._tracker.start(); // We skip usual startup...
+  try {
+    let guid1 = Utils.makeGUID();
+    let guid2 = Utils.makeGUID();
+    await PlacesUtils.bookmarks.insert({
+      guid: guid1,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "https://www.example.com",
+      title: "example.com",
+    });
+    await PlacesUtils.bookmarks.insert({
+      guid: guid2,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "https://www.example.com",
+      title: "example.com",
+    });
+
+    await sync_engine_and_validate_telem(engine, false);
+    // Make sure we set hasDupe on outgoing records
+    Assert.ok(collection.payloads().every(payload => payload.hasDupe));
+
+    await PlacesUtils.bookmarks.remove(guid1);
+
+    await sync_engine_and_validate_telem(engine, false);
+
+    let tombstone = JSON.parse(
+      JSON.parse(collection.payload(guid1)).ciphertext
+    );
+    // We shouldn't set hasDupe on tombstones.
+    Assert.ok(tombstone.deleted);
+    Assert.ok(!tombstone.hasDupe);
+
+    let record = JSON.parse(JSON.parse(collection.payload(guid2)).ciphertext);
+    // We should set hasDupe on weakly uploaded records.
+    Assert.ok(!record.deleted);
+    Assert.ok(
+      record.hasDupe,
+      "Bookmarks bookmark engine should set hasDupe for weakly uploaded records."
+    );
+
+    await sync_engine_and_validate_telem(engine, false);
+  } finally {
+    await cleanup(engine, server);
+    await engine.finalize();
+  }
+});
+
+// Bug 890217.
+add_bookmark_test(async function test_sync_imap_URLs(engine) {
+  await Service.recordManager.clearCache();
+  await PlacesSyncUtils.bookmarks.reset();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  engine._tracker.start(); // We skip usual startup...
+
+  try {
+    collection.insert(
+      "menu",
+      encryptPayload({
+        id: "menu",
+        type: "folder",
+        parentid: "places",
+        title: "Bookmarks Menu",
+        children: ["bookmarkAAAA"],
+      })
+    );
+    collection.insert(
+      "bookmarkAAAA",
+      encryptPayload({
+        id: "bookmarkAAAA",
+        type: "bookmark",
+        parentid: "menu",
+        bmkUri:
+          "imap://vs@eleven.vs.solnicky.cz:993/fetch%3EUID%3E/" +
+          "INBOX%3E56291?part=1.2&type=image/jpeg&filename=" +
+          "invalidazPrahy.jpg",
+        title:
+          "invalidazPrahy.jpg (JPEG Image, 1280x1024 pixels) - Scaled (71%)",
+      })
+    );
+
+    await PlacesUtils.bookmarks.insert({
+      guid: "bookmarkBBBB",
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url:
+        "imap://eleven.vs.solnicky.cz:993/fetch%3EUID%3E/" +
+        "CURRENT%3E2433?part=1.2&type=text/html&filename=TomEdwards.html",
+      title: "TomEdwards.html",
+    });
+
+    await sync_engine_and_validate_telem(engine, false);
+
+    let aInfo = await PlacesUtils.bookmarks.fetch("bookmarkAAAA");
+    equal(
+      aInfo.url.href,
+      "imap://vs@eleven.vs.solnicky.cz:993/" +
+        "fetch%3EUID%3E/INBOX%3E56291?part=1.2&type=image/jpeg&filename=" +
+        "invalidazPrahy.jpg",
+      "Remote bookmark A with IMAP URL should exist locally"
+    );
+
+    let bPayload = collection.cleartext("bookmarkBBBB");
+    equal(
+      bPayload.bmkUri,
+      "imap://eleven.vs.solnicky.cz:993/" +
+        "fetch%3EUID%3E/CURRENT%3E2433?part=1.2&type=text/html&filename=" +
+        "TomEdwards.html",
+      "Local bookmark B with IMAP URL should exist remotely"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_resume_buffer() {
+  await Service.recordManager.clearCache();
+  let engine = new BookmarksEngine(Service);
+  await engine.initialize();
+  await engine._store.wipe();
+  await engine.resetClient();
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  engine._tracker.start(); // We skip usual startup...
+
+  const batchChunkSize = 50;
+
+  engine._store._batchChunkSize = batchChunkSize;
+  try {
+    let children = [];
+
+    let timestamp = round_timestamp(Date.now());
+    // Add two chunks worth of records to the server
+    for (let i = 0; i < batchChunkSize * 2; ++i) {
+      let cleartext = {
+        id: Utils.makeGUID(),
+        type: "bookmark",
+        parentid: "toolbar",
+        title: `Bookmark ${i}`,
+        parentName: "Bookmarks Toolbar",
+        bmkUri: `https://example.com/${i}`,
+      };
+      let wbo = collection.insert(
+        cleartext.id,
+        encryptPayload(cleartext),
+        timestamp + 10 * i
+      );
+      // Something that is effectively random, but deterministic.
+      // (This is just to ensure we don't accidentally start using the
+      // sortindex again).
+      wbo.sortindex = 1000 + Math.round(Math.sin(i / 5) * 100);
+      children.push(cleartext.id);
+    }
+
+    // Add the parent of those records, and ensure its timestamp is the most recent.
+    collection.insert(
+      "toolbar",
+      encryptPayload({
+        id: "toolbar",
+        type: "folder",
+        parentid: "places",
+        title: "Bookmarks Toolbar",
+        children,
+      }),
+      timestamp + 10 * children.length
+    );
+
+    // Replace applyIncomingBatch with a custom one that calls the original,
+    // but forces it to throw on the 2nd chunk.
+    let origApplyIncomingBatch = engine._store.applyIncomingBatch;
+    engine._store.applyIncomingBatch = function (records) {
+      if (records.length > batchChunkSize) {
+        // Hacky way to make reading from the batchChunkSize'th record throw.
+        delete records[batchChunkSize];
+        Object.defineProperty(records, batchChunkSize, {
+          get() {
+            throw new Error("D:");
+          },
+        });
+      }
+      return origApplyIncomingBatch.call(this, records);
+    };
+
+    let caughtError;
+    _("We expect this to fail");
+    try {
+      await sync_engine_and_validate_telem(engine, true);
+    } catch (e) {
+      caughtError = e;
+    }
+    Assert.ok(caughtError, "Expected engine.sync to throw");
+    Assert.equal(caughtError.message, "D:");
+
+    // The buffer subtracts one second from the actual timestamp.
+    let lastSync = (await engine.getLastSync()) + 1;
+    // We poisoned the batchChunkSize'th record, so the last successfully
+    // applied record will be batchChunkSize - 1.
+    let expectedLastSync = timestamp + 10 * (batchChunkSize - 1);
+    Assert.equal(expectedLastSync, lastSync);
+
+    engine._store.applyIncomingBatch = origApplyIncomingBatch;
+
+    await sync_engine_and_validate_telem(engine, false);
+
+    // Check that all the children made it onto the correct record.
+    let toolbarRecord = await engine._store.createRecord("toolbar");
+    Assert.deepEqual(toolbarRecord.children.sort(), children.sort());
+  } finally {
+    await cleanup(engine, server);
+    await engine.finalize();
+  }
+});
+
+add_bookmark_test(async function test_livemarks(engine) {
+  _("Ensure we replace new and existing livemarks with tombstones");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("bookmarks");
+  let now = Date.now();
+
+  try {
+    _("Insert existing livemark");
+    let modifiedForA = now - 5 * 60 * 1000;
+    await PlacesUtils.bookmarks.insert({
+      guid: "livemarkAAAA",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "A",
+      lastModified: new Date(modifiedForA),
+      dateAdded: new Date(modifiedForA),
+      source: PlacesUtils.bookmarks.SOURCE_SYNC,
+    });
+    collection.insert(
+      "menu",
+      encryptPayload({
+        id: "menu",
+        type: "folder",
+        parentName: "",
+        title: "menu",
+        children: ["livemarkAAAA"],
+        parentid: "places",
+      }),
+      round_timestamp(modifiedForA)
+    );
+    collection.insert(
+      "livemarkAAAA",
+      encryptPayload({
+        id: "livemarkAAAA",
+        type: "livemark",
+        feedUri: "http://example.com/a",
+        parentName: "menu",
+        title: "A",
+        parentid: "menu",
+      }),
+      round_timestamp(modifiedForA)
+    );
+
+    _("Insert remotely updated livemark");
+    await PlacesUtils.bookmarks.insert({
+      guid: "livemarkBBBB",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      title: "B",
+      lastModified: new Date(now),
+      dateAdded: new Date(now),
+    });
+    collection.insert(
+      "toolbar",
+      encryptPayload({
+        id: "toolbar",
+        type: "folder",
+        parentName: "",
+        title: "toolbar",
+        children: ["livemarkBBBB"],
+        parentid: "places",
+      }),
+      round_timestamp(now)
+    );
+    collection.insert(
+      "livemarkBBBB",
+      encryptPayload({
+        id: "livemarkBBBB",
+        type: "livemark",
+        feedUri: "http://example.com/b",
+        parentName: "toolbar",
+        title: "B",
+        parentid: "toolbar",
+      }),
+      round_timestamp(now)
+    );
+
+    _("Insert new remote livemark");
+    collection.insert(
+      "unfiled",
+      encryptPayload({
+        id: "unfiled",
+        type: "folder",
+        parentName: "",
+        title: "unfiled",
+        children: ["livemarkCCCC"],
+        parentid: "places",
+      }),
+      round_timestamp(now)
+    );
+    collection.insert(
+      "livemarkCCCC",
+      encryptPayload({
+        id: "livemarkCCCC",
+        type: "livemark",
+        feedUri: "http://example.com/c",
+        parentName: "unfiled",
+        title: "C",
+        parentid: "unfiled",
+      }),
+      round_timestamp(now)
+    );
+
+    _("Bump last sync time to ignore A");
+    await engine.setLastSync(round_timestamp(now) - 60);
+
+    _("Sync");
+    await sync_engine_and_validate_telem(engine, false);
+
+    deepEqual(
+      collection.keys().sort(),
+      [
+        "livemarkAAAA",
+        "livemarkBBBB",
+        "livemarkCCCC",
+        "menu",
+        "mobile",
+        "toolbar",
+        "unfiled",
+      ],
+      "Should store original livemark A and tombstones for B and C on server"
+    );
+
+    let payloads = collection.payloads();
+
+    deepEqual(
+      payloads.find(payload => payload.id == "menu").children,
+      ["livemarkAAAA"],
+      "Should keep A in menu"
+    );
+    ok(
+      !payloads.find(payload => payload.id == "livemarkAAAA").deleted,
+      "Should not upload tombstone for A"
+    );
+
+    deepEqual(
+      payloads.find(payload => payload.id == "toolbar").children,
+      [],
+      "Should remove B from toolbar"
+    );
+    ok(
+      payloads.find(payload => payload.id == "livemarkBBBB").deleted,
+      "Should upload tombstone for B"
+    );
+
+    deepEqual(
+      payloads.find(payload => payload.id == "unfiled").children,
+      [],
+      "Should remove C from unfiled"
+    );
+    ok(
+      payloads.find(payload => payload.id == "livemarkCCCC").deleted,
+      "Should replace C with tombstone"
+    );
+
+    await assertBookmarksTreeMatches(
+      "",
+      [
+        {
+          guid: PlacesUtils.bookmarks.menuGuid,
+          index: 0,
+          children: [
+            {
+              guid: "livemarkAAAA",
+              index: 0,
+            },
+          ],
+        },
+        {
+          guid: PlacesUtils.bookmarks.toolbarGuid,
+          index: 1,
+        },
+        {
+          guid: PlacesUtils.bookmarks.unfiledGuid,
+          index: 3,
+        },
+        {
+          guid: PlacesUtils.bookmarks.mobileGuid,
+          index: 4,
+        },
+      ],
+      "Should keep A and remove B locally"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_bookmark_test(async function test_unknown_fields(engine) {
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("bookmarks");
+  try {
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      title: "Folder 1",
+    });
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    let bmk2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    });
+    let toolbar_record = await store.createRecord("toolbar");
+    collection.insert("toolbar", encryptPayload(toolbar_record.cleartext));
+
+    let folder1_record_without_unknown_fields = await store.createRecord(
+      folder1.guid
+    );
+    collection.insert(
+      folder1.guid,
+      encryptPayload(folder1_record_without_unknown_fields.cleartext)
+    );
+
+    // First bookmark record has an unknown string field
+    let bmk1_record = await store.createRecord(bmk1.guid);
+    console.log("bmk1_record: ", bmk1_record);
+    bmk1_record.cleartext.unknownStrField =
+      "an unknown field from another client";
+    collection.insert(bmk1.guid, encryptPayload(bmk1_record.cleartext));
+
+    // Second bookmark record as an unknown object field
+    let bmk2_record = await store.createRecord(bmk2.guid);
+    bmk2_record.cleartext.unknownObjField = {
+      name: "an unknown object from another client",
+    };
+    collection.insert(bmk2.guid, encryptPayload(bmk2_record.cleartext));
+
+    // Sync the two bookmarks
+    await sync_engine_and_validate_telem(engine, true);
+
+    // Add a folder could also have an unknown field
+    let folder1_record = await store.createRecord(folder1.guid);
+    folder1_record.cleartext.unknownStrField =
+      "a folder could also have an unknown field!";
+    collection.insert(folder1.guid, encryptPayload(folder1_record.cleartext));
+
+    // sync the new updates
+    await engine.setLastSync(1);
+    await sync_engine_and_validate_telem(engine, true);
+
+    let payloads = collection.payloads();
+    // Validate the server has the unknown fields at the top level (and now unknownFields)
+    let server_bmk1 = payloads.find(payload => payload.id == bmk1.guid);
+    deepEqual(
+      server_bmk1.unknownStrField,
+      "an unknown field from another client",
+      "unknown fields correctly on the record"
+    );
+    Assert.equal(server_bmk1.unknownFields, null);
+
+    // Check that the mirror table has unknown fields
+    let db = await PlacesUtils.promiseDBConnection();
+    let rows = await db.executeCached(
+      `
+      SELECT guid, title, unknownFields from items WHERE guid IN 
+      (:bmk1, :bmk2, :folder1)`,
+      { bmk1: bmk1.guid, bmk2: bmk2.guid, folder1: folder1.guid }
+    );
+    // We should have 3 rows that came from the server
+    Assert.equal(rows.length, 3);
+
+    // Bookmark 1 - unknown string field
+    let remote_bmk1 = rows.find(
+      row => row.getResultByName("guid") == bmk1.guid
+    );
+    Assert.equal(remote_bmk1.getResultByName("title"), "Get Firefox!");
+    deepEqual(JSON.parse(remote_bmk1.getResultByName("unknownFields")), {
+      unknownStrField: "an unknown field from another client",
+    });
+
+    // Bookmark 2 - unknown object field
+    let remote_bmk2 = rows.find(
+      row => row.getResultByName("guid") == bmk2.guid
+    );
+    Assert.equal(remote_bmk2.getResultByName("title"), "Get Thunderbird!");
+    deepEqual(JSON.parse(remote_bmk2.getResultByName("unknownFields")), {
+      unknownObjField: {
+        name: "an unknown object from another client",
+      },
+    });
+
+    // Folder with unknown field
+
+    // check the server still has the unknown field
+    deepEqual(
+      payloads.find(payload => payload.id == folder1.guid).unknownStrField,
+      "a folder could also have an unknown field!",
+      "Server still has the unknown field"
+    );
+
+    let remote_folder = rows.find(
+      row => row.getResultByName("guid") == folder1.guid
+    );
+    Assert.equal(remote_folder.getResultByName("title"), "Folder 1");
+    deepEqual(JSON.parse(remote_folder.getResultByName("unknownFields")), {
+      unknownStrField: "a folder could also have an unknown field!",
+    });
+  } finally {
+    await cleanup(engine, server);
+  }
+});
diff --git a/services/sync/tests/unit/test_bookmark_order.js b/services/sync/tests/unit/test_bookmark_order.js
new file mode 100644
index 0000000000..fc182b81ef
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_order.js
@@ -0,0 +1,586 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+_(
+  "Making sure after processing incoming bookmarks, they show up in the right order"
+);
+const { Bookmark, BookmarkFolder } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/bookmarks.sys.mjs"
+);
+const { Weave } = ChromeUtils.importESModule(
+  "resource://services-sync/main.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+async function serverForFoo(engine) {
+  await generateNewKeys(Service.collectionKeys);
+
+  let clientsEngine = Service.clientsEngine;
+  let clientsSyncID = await clientsEngine.resetLocalSyncID();
+  let engineSyncID = await engine.resetLocalSyncID();
+  return serverForUsers(
+    { foo: "password" },
+    {
+      meta: {
+        global: {
+          syncID: Service.syncID,
+          storageVersion: STORAGE_VERSION,
+          engines: {
+            clients: {
+              version: clientsEngine.version,
+              syncID: clientsSyncID,
+            },
+            [engine.name]: {
+              version: engine.version,
+              syncID: engineSyncID,
+            },
+          },
+        },
+      },
+      crypto: {
+        keys: encryptPayload({
+          id: "keys",
+          // Generate a fake default key bundle to avoid resetting the client
+          // before the first sync.
+          default: [
+            await Weave.Crypto.generateRandomKey(),
+            await Weave.Crypto.generateRandomKey(),
+          ],
+        }),
+      },
+      [engine.name]: {},
+    }
+  );
+}
+
+async function resolveConflict(
+  engine,
+  collection,
+  timestamp,
+  buildTree,
+  message
+) {
+  let guids = {
+    // These items don't exist on the server.
+    fx: Utils.makeGUID(),
+    nightly: Utils.makeGUID(),
+    support: Utils.makeGUID(),
+    customize: Utils.makeGUID(),
+
+    // These exist on the server, but in a different order, and `res`
+    // has completely different children.
+    res: Utils.makeGUID(),
+    tb: Utils.makeGUID(),
+
+    // These don't exist locally.
+    bz: Utils.makeGUID(),
+    irc: Utils.makeGUID(),
+    mdn: Utils.makeGUID(),
+  };
+
+  await PlacesUtils.bookmarks.insertTree({
+    guid: PlacesUtils.bookmarks.menuGuid,
+    children: [
+      {
+        guid: guids.fx,
+        title: "Get Firefox!",
+        url: "http://getfirefox.com/",
+      },
+      {
+        guid: guids.res,
+        title: "Resources",
+        type: PlacesUtils.bookmarks.TYPE_FOLDER,
+        children: [
+          {
+            guid: guids.nightly,
+            title: "Nightly",
+            url: "https://nightly.mozilla.org/",
+          },
+          {
+            guid: guids.support,
+            title: "Support",
+            url: "https://support.mozilla.org/",
+          },
+          {
+            guid: guids.customize,
+            title: "Customize",
+            url: "https://mozilla.org/firefox/customize/",
+          },
+        ],
+      },
+      {
+        title: "Get Thunderbird!",
+        guid: guids.tb,
+        url: "http://getthunderbird.com/",
+      },
+    ],
+  });
+
+  let serverRecords = [
+    {
+      id: "menu",
+      type: "folder",
+      title: "Bookmarks Menu",
+      parentid: "places",
+      children: [guids.tb, guids.res],
+    },
+    {
+      id: guids.tb,
+      type: "bookmark",
+      parentid: "menu",
+      bmkUri: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    },
+    {
+      id: guids.res,
+      type: "folder",
+      parentid: "menu",
+      title: "Resources",
+      children: [guids.irc, guids.bz, guids.mdn],
+    },
+    {
+      id: guids.bz,
+      type: "bookmark",
+      parentid: guids.res,
+      bmkUri: "https://bugzilla.mozilla.org/",
+      title: "Bugzilla",
+    },
+    {
+      id: guids.mdn,
+      type: "bookmark",
+      parentid: guids.res,
+      bmkUri: "https://developer.mozilla.org/",
+      title: "MDN",
+    },
+    {
+      id: guids.irc,
+      type: "bookmark",
+      parentid: guids.res,
+      bmkUri: "ircs://irc.mozilla.org/nightly",
+      title: "IRC",
+    },
+  ];
+  for (let record of serverRecords) {
+    collection.insert(record.id, encryptPayload(record), timestamp);
+  }
+
+  engine.lastModified = collection.timestamp;
+  await sync_engine_and_validate_telem(engine, false);
+
+  let expectedTree = buildTree(guids);
+  await assertBookmarksTreeMatches(
+    PlacesUtils.bookmarks.menuGuid,
+    expectedTree,
+    message
+  );
+}
+
+async function get_engine() {
+  return Service.engineManager.get("bookmarks");
+}
+
+add_task(async function test_local_order_newer() {
+  let engine = await get_engine();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    let collection = server.user("foo").collection("bookmarks");
+    let serverModified = Date.now() / 1000 - 120;
+    await resolveConflict(
+      engine,
+      collection,
+      serverModified,
+      guids => [
+        {
+          guid: guids.fx,
+          index: 0,
+        },
+        {
+          guid: guids.res,
+          index: 1,
+          children: [
+            {
+              guid: guids.nightly,
+              index: 0,
+            },
+            {
+              guid: guids.support,
+              index: 1,
+            },
+            {
+              guid: guids.customize,
+              index: 2,
+            },
+            {
+              guid: guids.irc,
+              index: 3,
+            },
+            {
+              guid: guids.bz,
+              index: 4,
+            },
+            {
+              guid: guids.mdn,
+              index: 5,
+            },
+          ],
+        },
+        {
+          guid: guids.tb,
+          index: 2,
+        },
+      ],
+      "Should use local order as base if remote is older"
+    );
+  } finally {
+    await engine.wipeClient();
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_remote_order_newer() {
+  let engine = await get_engine();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    let collection = server.user("foo").collection("bookmarks");
+    let serverModified = Date.now() / 1000 + 120;
+    await resolveConflict(
+      engine,
+      collection,
+      serverModified,
+      guids => [
+        {
+          guid: guids.tb,
+          index: 0,
+        },
+        {
+          guid: guids.res,
+          index: 1,
+          children: [
+            {
+              guid: guids.irc,
+              index: 0,
+            },
+            {
+              guid: guids.bz,
+              index: 1,
+            },
+            {
+              guid: guids.mdn,
+              index: 2,
+            },
+            {
+              guid: guids.nightly,
+              index: 3,
+            },
+            {
+              guid: guids.support,
+              index: 4,
+            },
+            {
+              guid: guids.customize,
+              index: 5,
+            },
+          ],
+        },
+        {
+          guid: guids.fx,
+          index: 2,
+        },
+      ],
+      "Should use remote order as base if local is older"
+    );
+  } finally {
+    await engine.wipeClient();
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_bookmark_order() {
+  let engine = await get_engine();
+  let store = engine._store;
+  _("Starting with a clean slate of no bookmarks");
+  await store.wipe();
+  await assertBookmarksTreeMatches(
+    "",
+    [
+      {
+        guid: PlacesUtils.bookmarks.menuGuid,
+        index: 0,
+      },
+      {
+        guid: PlacesUtils.bookmarks.toolbarGuid,
+        index: 1,
+      },
+      {
+        // Index 2 is the tags root. (Root indices depend on the order of the
+        // `CreateRoot` calls in `Database::CreateBookmarkRoots`).
+        guid: PlacesUtils.bookmarks.unfiledGuid,
+        index: 3,
+      },
+      {
+        guid: PlacesUtils.bookmarks.mobileGuid,
+        index: 4,
+      },
+    ],
+    "clean slate"
+  );
+
+  function bookmark(name, parent) {
+    let bm = new Bookmark("http://weave.server/my-bookmark");
+    bm.id = name;
+    bm.title = name;
+    bm.bmkUri = "http://uri/";
+    bm.parentid = parent || "unfiled";
+    bm.tags = [];
+    return bm;
+  }
+
+  function folder(name, parent, children) {
+    let bmFolder = new BookmarkFolder("http://weave.server/my-bookmark-folder");
+    bmFolder.id = name;
+    bmFolder.title = name;
+    bmFolder.parentid = parent || "unfiled";
+    bmFolder.children = children;
+    return bmFolder;
+  }
+
+  async function apply(records) {
+    for (record of records) {
+      await store.applyIncoming(record);
+    }
+    await engine._apply();
+  }
+  let id10 = "10_aaaaaaaaa";
+  _("basic add first bookmark");
+  await apply([bookmark(id10, "")]);
+  await assertBookmarksTreeMatches(
+    "",
+    [
+      {
+        guid: PlacesUtils.bookmarks.menuGuid,
+        index: 0,
+      },
+      {
+        guid: PlacesUtils.bookmarks.toolbarGuid,
+        index: 1,
+      },
+      {
+        guid: PlacesUtils.bookmarks.unfiledGuid,
+        index: 3,
+        children: [
+          {
+            guid: id10,
+            index: 0,
+          },
+        ],
+      },
+      {
+        guid: PlacesUtils.bookmarks.mobileGuid,
+        index: 4,
+      },
+    ],
+    "basic add first bookmark"
+  );
+  let id20 = "20_aaaaaaaaa";
+  _("basic append behind 10");
+  await apply([bookmark(id20, "")]);
+  await assertBookmarksTreeMatches(
+    "",
+    [
+      {
+        guid: PlacesUtils.bookmarks.menuGuid,
+        index: 0,
+      },
+      {
+        guid: PlacesUtils.bookmarks.toolbarGuid,
+        index: 1,
+      },
+      {
+        guid: PlacesUtils.bookmarks.unfiledGuid,
+        index: 3,
+        children: [
+          {
+            guid: id10,
+            index: 0,
+          },
+          {
+            guid: id20,
+            index: 1,
+          },
+        ],
+      },
+      {
+        guid: PlacesUtils.bookmarks.mobileGuid,
+        index: 4,
+      },
+    ],
+    "basic append behind 10"
+  );
+
+  let id31 = "31_aaaaaaaaa";
+  let id30 = "f30_aaaaaaaa";
+  _("basic create in folder");
+  let b31 = bookmark(id31, id30);
+  let f30 = folder(id30, "", [id31]);
+  await apply([b31, f30]);
+  await assertBookmarksTreeMatches(
+    "",
+    [
+      {
+        guid: PlacesUtils.bookmarks.menuGuid,
+        index: 0,
+      },
+      {
+        guid: PlacesUtils.bookmarks.toolbarGuid,
+        index: 1,
+      },
+      {
+        guid: PlacesUtils.bookmarks.unfiledGuid,
+        index: 3,
+        children: [
+          {
+            guid: id10,
+            index: 0,
+          },
+          {
+            guid: id20,
+            index: 1,
+          },
+          {
+            guid: id30,
+            index: 2,
+            children: [
+              {
+                guid: id31,
+                index: 0,
+              },
+            ],
+          },
+        ],
+      },
+      {
+        guid: PlacesUtils.bookmarks.mobileGuid,
+        index: 4,
+      },
+    ],
+    "basic create in folder"
+  );
+
+  let id41 = "41_aaaaaaaaa";
+  let id40 = "f40_aaaaaaaa";
+  _("insert missing parent -> append to unfiled");
+  await apply([bookmark(id41, id40)]);
+  await assertBookmarksTreeMatches(
+    "",
+    [
+      {
+        guid: PlacesUtils.bookmarks.menuGuid,
+        index: 0,
+      },
+      {
+        guid: PlacesUtils.bookmarks.toolbarGuid,
+        index: 1,
+      },
+      {
+        guid: PlacesUtils.bookmarks.unfiledGuid,
+        index: 3,
+        children: [
+          {
+            guid: id10,
+            index: 0,
+          },
+          {
+            guid: id20,
+            index: 1,
+          },
+          {
+            guid: id30,
+            index: 2,
+            children: [
+              {
+                guid: id31,
+                index: 0,
+              },
+            ],
+          },
+          {
+            guid: id41,
+            index: 3,
+          },
+        ],
+      },
+      {
+        guid: PlacesUtils.bookmarks.mobileGuid,
+        index: 4,
+      },
+    ],
+    "insert missing parent -> append to unfiled"
+  );
+
+  let id42 = "42_aaaaaaaaa";
+
+  _("insert another missing parent -> append");
+  await apply([bookmark(id42, id40)]);
+  await assertBookmarksTreeMatches(
+    "",
+    [
+      {
+        guid: PlacesUtils.bookmarks.menuGuid,
+        index: 0,
+      },
+      {
+        guid: PlacesUtils.bookmarks.toolbarGuid,
+        index: 1,
+      },
+      {
+        guid: PlacesUtils.bookmarks.unfiledGuid,
+        index: 3,
+        children: [
+          {
+            guid: id10,
+            index: 0,
+          },
+          {
+            guid: id20,
+            index: 1,
+          },
+          {
+            guid: id30,
+            index: 2,
+            children: [
+              {
+                guid: id31,
+                index: 0,
+              },
+            ],
+          },
+          {
+            guid: id41,
+            index: 3,
+          },
+          {
+            guid: id42,
+            index: 4,
+          },
+        ],
+      },
+      {
+        guid: PlacesUtils.bookmarks.mobileGuid,
+        index: 4,
+      },
+    ],
+    "insert another missing parent -> append"
+  );
+
+  await engine.wipeClient();
+  await Service.startOver();
+  await engine.finalize();
+});
diff --git a/services/sync/tests/unit/test_bookmark_places_query_rewriting.js b/services/sync/tests/unit/test_bookmark_places_query_rewriting.js
new file mode 100644
index 0000000000..e8dbbb48b1
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_places_query_rewriting.js
@@ -0,0 +1,57 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+_("Rewrite place: URIs.");
+const { BookmarkQuery, BookmarkFolder } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/bookmarks.sys.mjs"
+);
+// `Service` is used as a global in head_helpers.js.
+// eslint-disable-next-line no-unused-vars
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function makeTagRecord(id, uri) {
+  let tagRecord = new BookmarkQuery("bookmarks", id);
+  tagRecord.queryId = "MagicTags";
+  tagRecord.parentName = "Bookmarks Toolbar";
+  tagRecord.bmkUri = uri;
+  tagRecord.title = "tagtag";
+  tagRecord.folderName = "bar";
+  tagRecord.parentid = PlacesUtils.bookmarks.toolbarGuid;
+  return tagRecord;
+}
+
+add_bookmark_test(async function run_test(engine) {
+  let store = engine._store;
+
+  let toolbar = new BookmarkFolder("bookmarks", "toolbar");
+  toolbar.parentid = "places";
+  toolbar.children = ["abcdefabcdef"];
+
+  let uri = "place:folder=499&type=7&queryType=1";
+  let tagRecord = makeTagRecord("abcdefabcdef", uri);
+
+  _("Type: " + tagRecord.type);
+  _("Folder name: " + tagRecord.folderName);
+  await store.applyIncoming(toolbar);
+  await store.applyIncoming(tagRecord);
+  await engine._apply();
+
+  let insertedRecord = await store.createRecord("abcdefabcdef", "bookmarks");
+  Assert.equal(insertedRecord.bmkUri, "place:tag=bar");
+
+  _("... but not if the type is wrong.");
+  let wrongTypeURI = "place:folder=499&type=2&queryType=1";
+  let wrongTypeRecord = makeTagRecord("fedcbafedcba", wrongTypeURI);
+  await store.applyIncoming(wrongTypeRecord);
+  toolbar.children = ["fedcbafedcba"];
+  await store.applyIncoming(toolbar);
+  let expected = wrongTypeURI;
+  await engine._apply();
+  // the mirror appends a special param to these.
+  expected += "&excludeItems=1";
+
+  insertedRecord = await store.createRecord("fedcbafedcba", "bookmarks");
+  Assert.equal(insertedRecord.bmkUri, expected);
+});
diff --git a/services/sync/tests/unit/test_bookmark_record.js b/services/sync/tests/unit/test_bookmark_record.js
new file mode 100644
index 0000000000..c261027ed9
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_record.js
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Bookmark, BookmarkQuery, PlacesItem } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/bookmarks.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function prepareBookmarkItem(collection, id) {
+  let b = new Bookmark(collection, id);
+  b.cleartext.stuff = "my payload here";
+  return b;
+}
+
+add_task(async function test_bookmark_record() {
+  await configureIdentity();
+
+  await generateNewKeys(Service.collectionKeys);
+  let keyBundle = Service.identity.syncKeyBundle;
+
+  _("Creating a record");
+
+  let placesItem = new PlacesItem("bookmarks", "foo", "bookmark");
+  let bookmarkItem = prepareBookmarkItem("bookmarks", "foo");
+
+  _("Checking getTypeObject");
+  Assert.equal(placesItem.getTypeObject(placesItem.type), Bookmark);
+  Assert.equal(bookmarkItem.getTypeObject(bookmarkItem.type), Bookmark);
+
+  await bookmarkItem.encrypt(keyBundle);
+  _("Ciphertext is " + bookmarkItem.ciphertext);
+  Assert.ok(bookmarkItem.ciphertext != null);
+
+  _("Decrypting the record");
+
+  let payload = await bookmarkItem.decrypt(keyBundle);
+  Assert.equal(payload.stuff, "my payload here");
+  Assert.equal(bookmarkItem.getTypeObject(bookmarkItem.type), Bookmark);
+  Assert.notEqual(payload, bookmarkItem.payload); // wrap.data.payload is the encrypted one
+});
+
+add_task(async function test_query_foldername() {
+  // Bug 1443388
+  let checks = [
+    ["foo", "foo"],
+    ["", undefined],
+  ];
+  for (let [inVal, outVal] of checks) {
+    let bmk1 = new BookmarkQuery("bookmarks", Utils.makeGUID());
+    bmk1.fromSyncBookmark({
+      url: Services.io.newURI("https://example.com"),
+      folder: inVal,
+    });
+    Assert.strictEqual(bmk1.folderName, outVal);
+
+    // other direction
+    let bmk2 = new BookmarkQuery("bookmarks", Utils.makeGUID());
+    bmk2.folderName = inVal;
+    let record = bmk2.toSyncBookmark();
+    Assert.strictEqual(record.folder, outVal);
+  }
+});
diff --git a/services/sync/tests/unit/test_bookmark_store.js b/services/sync/tests/unit/test_bookmark_store.js
new file mode 100644
index 0000000000..2f4330ed2e
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_store.js
@@ -0,0 +1,425 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Bookmark, BookmarkFolder, BookmarkQuery, PlacesItem } =
+  ChromeUtils.importESModule(
+    "resource://services-sync/engines/bookmarks.sys.mjs"
+  );
+// `Service` is used as a global in head_helpers.js.
+// eslint-disable-next-line no-unused-vars
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const BookmarksToolbarTitle = "toolbar";
+
+// apply some test records without going via a test server.
+async function apply_records(engine, records) {
+  for (record of records) {
+    await engine._store.applyIncoming(record);
+  }
+  await engine._apply();
+}
+
+add_bookmark_test(async function test_ignore_specials(engine) {
+  _("Ensure that we can't delete bookmark roots.");
+  let store = engine._store;
+
+  // Belt...
+  let record = new BookmarkFolder("bookmarks", "toolbar", "folder");
+  record.deleted = true;
+  Assert.notEqual(
+    null,
+    await PlacesTestUtils.promiseItemId(PlacesUtils.bookmarks.toolbarGuid)
+  );
+
+  await apply_records(engine, [record]);
+
+  // Ensure that the toolbar exists.
+  Assert.notEqual(
+    null,
+    await PlacesTestUtils.promiseItemId(PlacesUtils.bookmarks.toolbarGuid)
+  );
+
+  await apply_records(engine, [record]);
+
+  Assert.notEqual(
+    null,
+    await PlacesTestUtils.promiseItemId(PlacesUtils.bookmarks.toolbarGuid)
+  );
+  await store.wipe();
+});
+
+add_bookmark_test(async function test_bookmark_create(engine) {
+  let store = engine._store;
+
+  try {
+    _("Ensure the record isn't present yet.");
+    let item = await PlacesUtils.bookmarks.fetch({
+      url: "http://getfirefox.com/",
+    });
+    Assert.equal(null, item);
+
+    _("Let's create a new record.");
+    let fxrecord = new Bookmark("bookmarks", "get-firefox1");
+    fxrecord.bmkUri = "http://getfirefox.com/";
+    fxrecord.title = "Get Firefox!";
+    fxrecord.tags = ["firefox", "awesome", "browser"];
+    fxrecord.keyword = "awesome";
+    fxrecord.parentName = BookmarksToolbarTitle;
+    fxrecord.parentid = "toolbar";
+    await apply_records(engine, [fxrecord]);
+
+    _("Verify it has been created correctly.");
+    item = await PlacesUtils.bookmarks.fetch(fxrecord.id);
+    Assert.equal(item.type, PlacesUtils.bookmarks.TYPE_BOOKMARK);
+    Assert.equal(item.url.href, "http://getfirefox.com/");
+    Assert.equal(item.title, fxrecord.title);
+    Assert.equal(item.parentGuid, PlacesUtils.bookmarks.toolbarGuid);
+    let keyword = await PlacesUtils.keywords.fetch(fxrecord.keyword);
+    Assert.equal(keyword.url.href, "http://getfirefox.com/");
+
+    _(
+      "Have the store create a new record object. Verify that it has the same data."
+    );
+    let newrecord = await store.createRecord(fxrecord.id);
+    Assert.ok(newrecord instanceof Bookmark);
+    for (let property of [
+      "type",
+      "bmkUri",
+      "title",
+      "keyword",
+      "parentName",
+      "parentid",
+    ]) {
+      Assert.equal(newrecord[property], fxrecord[property]);
+    }
+    Assert.ok(Utils.deepEquals(newrecord.tags.sort(), fxrecord.tags.sort()));
+
+    _("The calculated sort index is based on frecency data.");
+    Assert.ok(newrecord.sortindex >= 150);
+
+    _("Create a record with some values missing.");
+    let tbrecord = new Bookmark("bookmarks", "thunderbird1");
+    tbrecord.bmkUri = "http://getthunderbird.com/";
+    tbrecord.parentName = BookmarksToolbarTitle;
+    tbrecord.parentid = "toolbar";
+    await apply_records(engine, [tbrecord]);
+
+    _("Verify it has been created correctly.");
+    item = await PlacesUtils.bookmarks.fetch(tbrecord.id);
+    Assert.equal(item.type, PlacesUtils.bookmarks.TYPE_BOOKMARK);
+    Assert.equal(item.url.href, "http://getthunderbird.com/");
+    Assert.equal(item.title, "");
+    Assert.equal(item.parentGuid, PlacesUtils.bookmarks.toolbarGuid);
+    keyword = await PlacesUtils.keywords.fetch({
+      url: "http://getthunderbird.com/",
+    });
+    Assert.equal(null, keyword);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_bookmark_update(engine) {
+  let store = engine._store;
+
+  try {
+    _("Create a bookmark whose values we'll change.");
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    await PlacesUtils.keywords.insert({
+      url: "http://getfirefox.com/",
+      keyword: "firefox",
+    });
+
+    _("Update the record with some null values.");
+    let record = await store.createRecord(bmk1.guid);
+    record.title = null;
+    record.keyword = null;
+    record.tags = null;
+    await apply_records(engine, [record]);
+
+    _("Verify that the values have been cleared.");
+    let item = await PlacesUtils.bookmarks.fetch(bmk1.guid);
+    Assert.equal(item.title, "");
+    let keyword = await PlacesUtils.keywords.fetch({
+      url: "http://getfirefox.com/",
+    });
+    Assert.equal(null, keyword);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_bookmark_createRecord(engine) {
+  let store = engine._store;
+
+  try {
+    _("Create a bookmark without a title.");
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com/",
+    });
+
+    _("Verify that the record is created accordingly.");
+    let record = await store.createRecord(bmk1.guid);
+    Assert.equal(record.title, "");
+    Assert.equal(record.keyword, null);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_folder_create(engine) {
+  let store = engine._store;
+
+  try {
+    _("Create a folder.");
+    let folder = new BookmarkFolder("bookmarks", "testfolder-1");
+    folder.parentName = BookmarksToolbarTitle;
+    folder.parentid = "toolbar";
+    folder.title = "Test Folder";
+    await apply_records(engine, [folder]);
+
+    _("Verify it has been created correctly.");
+    let item = await PlacesUtils.bookmarks.fetch(folder.id);
+    Assert.equal(item.type, PlacesUtils.bookmarks.TYPE_FOLDER);
+    Assert.equal(item.title, folder.title);
+    Assert.equal(item.parentGuid, PlacesUtils.bookmarks.toolbarGuid);
+
+    _(
+      "Have the store create a new record object. Verify that it has the same data."
+    );
+    let newrecord = await store.createRecord(folder.id);
+    Assert.ok(newrecord instanceof BookmarkFolder);
+    for (let property of ["title", "parentName", "parentid"]) {
+      Assert.equal(newrecord[property], folder[property]);
+    }
+
+    _("Folders have high sort index to ensure they're synced first.");
+    Assert.equal(newrecord.sortindex, 1000000);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_folder_createRecord(engine) {
+  let store = engine._store;
+
+  try {
+    _("Create a folder.");
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      title: "Folder1",
+    });
+
+    _("Create two bookmarks in that folder without assigning them GUIDs.");
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    let bmk2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    });
+
+    _("Create a record for the folder and verify basic properties.");
+    let record = await store.createRecord(folder1.guid);
+    Assert.ok(record instanceof BookmarkFolder);
+    Assert.equal(record.title, "Folder1");
+    Assert.equal(record.parentid, "toolbar");
+    Assert.equal(record.parentName, BookmarksToolbarTitle);
+
+    _(
+      "Verify the folder's children. Ensures that the bookmarks were given GUIDs."
+    );
+    Assert.deepEqual(record.children, [bmk1.guid, bmk2.guid]);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_deleted(engine) {
+  let store = engine._store;
+
+  try {
+    _("Create a bookmark that will be deleted.");
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    // The engine needs to think we've previously synced it.
+    await PlacesTestUtils.markBookmarksAsSynced();
+
+    _("Delete the bookmark through the store.");
+    let record = new PlacesItem("bookmarks", bmk1.guid);
+    record.deleted = true;
+    await apply_records(engine, [record]);
+    _("Ensure it has been deleted.");
+    let item = await PlacesUtils.bookmarks.fetch(bmk1.guid);
+    let newrec = await store.createRecord(bmk1.guid);
+    Assert.equal(null, item);
+    Assert.equal(newrec.deleted, true);
+    _("Verify that the keyword has been cleared.");
+    let keyword = await PlacesUtils.keywords.fetch({
+      url: "http://getfirefox.com/",
+    });
+    Assert.equal(null, keyword);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_move_folder(engine) {
+  let store = engine._store;
+  store._childrenToOrder = {}; // *sob* - only needed for legacy.
+
+  try {
+    _("Create two folders and a bookmark in one of them.");
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      title: "Folder1",
+    });
+    let folder2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      title: "Folder2",
+    });
+    let bmk = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    // add records to the store that represent the current state.
+    await apply_records(engine, [
+      await store.createRecord(folder1.guid),
+      await store.createRecord(folder2.guid),
+      await store.createRecord(bmk.guid),
+    ]);
+
+    _("Now simulate incoming records reparenting it.");
+    let bmkRecord = await store.createRecord(bmk.guid);
+    Assert.equal(bmkRecord.parentid, folder1.guid);
+    bmkRecord.parentid = folder2.guid;
+
+    let folder1Record = await store.createRecord(folder1.guid);
+    Assert.deepEqual(folder1Record.children, [bmk.guid]);
+    folder1Record.children = [];
+    let folder2Record = await store.createRecord(folder2.guid);
+    Assert.deepEqual(folder2Record.children, []);
+    folder2Record.children = [bmk.guid];
+
+    await apply_records(engine, [bmkRecord, folder1Record, folder2Record]);
+
+    _("Verify the new parent.");
+    let movedBmk = await PlacesUtils.bookmarks.fetch(bmk.guid);
+    Assert.equal(movedBmk.parentGuid, folder2.guid);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+add_bookmark_test(async function test_move_order(engine) {
+  let store = engine._store;
+  let tracker = engine._tracker;
+
+  // Make sure the tracker is turned on.
+  tracker.start();
+  try {
+    _("Create two bookmarks");
+    let bmk1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com/",
+      title: "Get Firefox!",
+    });
+    let bmk2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getthunderbird.com/",
+      title: "Get Thunderbird!",
+    });
+
+    _("Verify order.");
+    let childIds = await PlacesSyncUtils.bookmarks.fetchChildRecordIds(
+      "toolbar"
+    );
+    Assert.deepEqual(childIds, [bmk1.guid, bmk2.guid]);
+    let toolbar = await store.createRecord("toolbar");
+    Assert.deepEqual(toolbar.children, [bmk1.guid, bmk2.guid]);
+
+    _("Move bookmarks around.");
+    store._childrenToOrder = {};
+    toolbar.children = [bmk2.guid, bmk1.guid];
+    await apply_records(engine, [
+      toolbar,
+      await store.createRecord(bmk1.guid),
+      await store.createRecord(bmk2.guid),
+    ]);
+    delete store._childrenToOrder;
+
+    _("Verify new order.");
+    let newChildIds = await PlacesSyncUtils.bookmarks.fetchChildRecordIds(
+      "toolbar"
+    );
+    Assert.deepEqual(newChildIds, [bmk2.guid, bmk1.guid]);
+  } finally {
+    await tracker.stop();
+    _("Clean up.");
+    await store.wipe();
+  }
+});
+
+// Tests Bug 806460, in which query records arrive with empty folder
+// names and missing bookmark URIs.
+add_bookmark_test(async function test_empty_query_doesnt_die(engine) {
+  let record = new BookmarkQuery("bookmarks", "8xoDGqKrXf1P");
+  record.folderName = "";
+  record.queryId = "";
+  record.parentName = "Toolbar";
+  record.parentid = "toolbar";
+
+  // These should not throw.
+  await apply_records(engine, [record]);
+
+  delete record.folderName;
+  await apply_records(engine, [record]);
+});
+
+add_bookmark_test(async function test_calculateIndex_for_invalid_url(engine) {
+  let store = engine._store;
+
+  let folderIndex = await store._calculateIndex({
+    type: "folder",
+  });
+  equal(folderIndex, 1000000, "Should use high sort index for folders");
+
+  let toolbarIndex = await store._calculateIndex({
+    parentid: "toolbar",
+  });
+  equal(toolbarIndex, 150, "Should bump sort index for toolbar bookmarks");
+
+  let validURLIndex = await store._calculateIndex({
+    bmkUri: "http://example.com/a",
+  });
+  greaterOrEqual(validURLIndex, 0, "Should use frecency for index");
+
+  let invalidURLIndex = await store._calculateIndex({
+    bmkUri: "!@#$%",
+  });
+  equal(invalidURLIndex, 0, "Should not throw for invalid URLs");
+});
diff --git a/services/sync/tests/unit/test_bookmark_tracker.js b/services/sync/tests/unit/test_bookmark_tracker.js
new file mode 100644
index 0000000000..9cfbb4de78
--- /dev/null
+++ b/services/sync/tests/unit/test_bookmark_tracker.js
@@ -0,0 +1,1275 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { PlacesTransactions } = ChromeUtils.importESModule(
+  "resource://gre/modules/PlacesTransactions.sys.mjs"
+);
+
+let engine;
+let store;
+let tracker;
+
+const DAY_IN_MS = 24 * 60 * 60 * 1000;
+
+add_task(async function setup() {
+  await Service.engineManager.switchAlternatives();
+  engine = Service.engineManager.get("bookmarks");
+  store = engine._store;
+  tracker = engine._tracker;
+});
+
+// Test helpers.
+async function verifyTrackerEmpty() {
+  await PlacesTestUtils.promiseAsyncUpdates();
+  let changes = await tracker.getChangedIDs();
+  deepEqual(changes, {});
+  equal(tracker.score, 0);
+}
+
+async function resetTracker() {
+  await PlacesTestUtils.markBookmarksAsSynced();
+  tracker.resetScore();
+}
+
+async function cleanup() {
+  await engine.setLastSync(0);
+  await store.wipe();
+  await resetTracker();
+  await tracker.stop();
+}
+
+// startTracking is a signal that the test wants to notice things that happen
+// after this is called (ie, things already tracked should be discarded.)
+async function startTracking() {
+  engine._tracker.start();
+  await PlacesTestUtils.markBookmarksAsSynced();
+}
+
+async function verifyTrackedItems(tracked) {
+  await PlacesTestUtils.promiseAsyncUpdates();
+  let changedIDs = await tracker.getChangedIDs();
+  let trackedIDs = new Set(Object.keys(changedIDs));
+  for (let guid of tracked) {
+    ok(guid in changedIDs, `${guid} should be tracked`);
+    ok(changedIDs[guid].modified > 0, `${guid} should have a modified time`);
+    ok(changedIDs[guid].counter >= -1, `${guid} should have a change counter`);
+    trackedIDs.delete(guid);
+  }
+  equal(
+    trackedIDs.size,
+    0,
+    `Unhandled tracked IDs: ${JSON.stringify(Array.from(trackedIDs))}`
+  );
+}
+
+async function verifyTrackedCount(expected) {
+  await PlacesTestUtils.promiseAsyncUpdates();
+  let changedIDs = await tracker.getChangedIDs();
+  do_check_attribute_count(changedIDs, expected);
+}
+
+// A debugging helper that dumps the full bookmarks tree.
+// Currently unused, but might come in handy
+// eslint-disable-next-line no-unused-vars
+async function dumpBookmarks() {
+  let columns = [
+    "id",
+    "title",
+    "guid",
+    "syncStatus",
+    "syncChangeCounter",
+    "position",
+  ];
+  return PlacesUtils.promiseDBConnection().then(connection => {
+    let all = [];
+    return connection
+      .executeCached(
+        `SELECT ${columns.join(", ")} FROM moz_bookmarks;`,
+        {},
+        row => {
+          let repr = {};
+          for (let column of columns) {
+            repr[column] = row.getResultByName(column);
+          }
+          all.push(repr);
+        }
+      )
+      .then(() => {
+        dump("All bookmarks:\n");
+        dump(JSON.stringify(all, undefined, 2));
+      });
+  });
+}
+
+add_task(async function test_tracking() {
+  _("Test starting and stopping the tracker");
+
+  // Remove existing tracking information for roots.
+  await startTracking();
+
+  let folder = await PlacesUtils.bookmarks.insert({
+    parentGuid: PlacesUtils.bookmarks.menuGuid,
+    title: "Test Folder",
+    type: PlacesUtils.bookmarks.TYPE_FOLDER,
+  });
+
+  // creating the folder should have made 2 changes - the folder itself and
+  // the parent of the folder.
+  await verifyTrackedCount(2);
+  // Reset the changes as the rest of the test doesn't want to see these.
+  await resetTracker();
+
+  function createBmk() {
+    return PlacesUtils.bookmarks.insert({
+      parentGuid: folder.guid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+  }
+
+  try {
+    _("Tell the tracker to start tracking changes.");
+    await startTracking();
+    await createBmk();
+    // We expect two changed items because the containing folder
+    // changed as well (new child).
+    await verifyTrackedCount(2);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+
+    _("Notifying twice won't do any harm.");
+    await createBmk();
+    await verifyTrackedCount(3);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_tracker_sql_batching() {
+  _(
+    "Test tracker does the correct thing when it is forced to batch SQL queries"
+  );
+
+  const SQLITE_MAX_VARIABLE_NUMBER = 999;
+  let numItems = SQLITE_MAX_VARIABLE_NUMBER * 2 + 10;
+
+  await startTracking();
+
+  let children = [];
+  for (let i = 0; i < numItems; i++) {
+    children.push({
+      url: "https://example.org/" + i,
+      title: "Sync Bookmark " + i,
+    });
+  }
+  let inserted = await PlacesUtils.bookmarks.insertTree({
+    guid: PlacesUtils.bookmarks.unfiledGuid,
+    children: [
+      {
+        type: PlacesUtils.bookmarks.TYPE_FOLDER,
+        children,
+      },
+    ],
+  });
+
+  Assert.equal(children.length, numItems);
+  Assert.equal(inserted.length, numItems + 1);
+  await verifyTrackedCount(numItems + 2); // The parent and grandparent are also tracked.
+  await resetTracker();
+
+  await PlacesUtils.bookmarks.remove(inserted[0]);
+  await verifyTrackedCount(numItems + 2);
+
+  await cleanup();
+});
+
+add_task(async function test_bookmarkAdded() {
+  _("Items inserted via the synchronous bookmarks API should be tracked");
+
+  try {
+    await startTracking();
+
+    _("Insert a folder using the sync API");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let syncFolder = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "Sync Folder",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+    await verifyTrackedItems(["menu", syncFolder.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+
+    await resetTracker();
+    await startTracking();
+
+    _("Insert a bookmark using the sync API");
+    totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let syncBmk = await PlacesUtils.bookmarks.insert({
+      parentGuid: syncFolder.guid,
+      url: "https://example.org/sync",
+      title: "Sync Bookmark",
+    });
+    await verifyTrackedItems([syncFolder.guid, syncBmk.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_bookmarkAdded() {
+  _("Items inserted via the asynchronous bookmarks API should be tracked");
+
+  try {
+    await startTracking();
+
+    _("Insert a folder using the async API");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let asyncFolder = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "Async Folder",
+    });
+    await verifyTrackedItems(["menu", asyncFolder.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+
+    await resetTracker();
+    await startTracking();
+
+    _("Insert a bookmark using the async API");
+    totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let asyncBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: asyncFolder.guid,
+      url: "https://example.org/async",
+      title: "Async Bookmark",
+    });
+    await verifyTrackedItems([asyncFolder.guid, asyncBmk.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+
+    await resetTracker();
+    await startTracking();
+
+    _("Insert a separator using the async API");
+    totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let asyncSep = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_SEPARATOR,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      index: asyncFolder.index,
+    });
+    await verifyTrackedItems(["menu", asyncSep.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemChanged() {
+  _("Items updated using the asynchronous bookmarks API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert a bookmark");
+    let fxBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`Firefox GUID: ${fxBmk.guid}`);
+
+    await startTracking();
+
+    _("Update the bookmark using the async API");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.update({
+      guid: fxBmk.guid,
+      title: "Download Firefox",
+      url: "https://www.mozilla.org/firefox",
+      // PlacesUtils.bookmarks.update rejects last modified dates older than
+      // the added date.
+      lastModified: new Date(Date.now() + DAY_IN_MS),
+    });
+
+    await verifyTrackedItems([fxBmk.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 1);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemChanged_itemDates() {
+  _("Changes to item dates should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert a bookmark");
+    let fx_bm = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`Firefox GUID: ${fx_bm.guid}`);
+
+    await startTracking();
+
+    _("Reset the bookmark's added date, should not be tracked");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let dateAdded = new Date(Date.now() - DAY_IN_MS);
+    await PlacesUtils.bookmarks.update({
+      guid: fx_bm.guid,
+      dateAdded,
+    });
+    await verifyTrackedCount(0);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges);
+
+    await resetTracker();
+
+    _(
+      "Reset the bookmark's added date and another property, should be tracked"
+    );
+    totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    dateAdded = new Date();
+    await PlacesUtils.bookmarks.update({
+      guid: fx_bm.guid,
+      dateAdded,
+      title: "test",
+    });
+    await verifyTrackedItems([fx_bm.guid]);
+    Assert.equal(tracker.score, 2 * SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 1);
+
+    await resetTracker();
+
+    _("Set the bookmark's last modified date");
+    totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    let fx_id = await PlacesTestUtils.promiseItemId(fx_bm.guid);
+    let dateModified = Date.now() * 1000;
+    PlacesUtils.bookmarks.setItemLastModified(fx_id, dateModified);
+    await verifyTrackedItems([fx_bm.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 1);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemTagged() {
+  _("Items tagged using the synchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Create a folder");
+    let folder = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "Parent",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+    _("Folder ID: " + folder);
+    _("Folder GUID: " + folder.guid);
+
+    _("Track changes to tags");
+    let uri = CommonUtils.makeURI("http://getfirefox.com");
+    let b = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder.guid,
+      url: uri,
+      title: "Get Firefox!",
+    });
+    _("New item is " + b);
+    _("GUID: " + b.guid);
+
+    await startTracking();
+
+    _("Tag the item");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    PlacesUtils.tagging.tagURI(uri, ["foo"]);
+
+    // bookmark should be tracked, folder should not be.
+    await verifyTrackedItems([b.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 6);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemUntagged() {
+  _("Items untagged using the synchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert tagged bookmarks");
+    let uri = CommonUtils.makeURI("http://getfirefox.com");
+    let fx1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: uri,
+      title: "Get Firefox!",
+    });
+    // Different parent and title; same URL.
+    let fx2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: uri,
+      title: "Download Firefox",
+    });
+    PlacesUtils.tagging.tagURI(uri, ["foo"]);
+
+    await startTracking();
+
+    _("Remove the tag");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    PlacesUtils.tagging.untagURI(uri, ["foo"]);
+
+    await verifyTrackedItems([fx1.guid, fx2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 4);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 5);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemUntagged() {
+  _("Items untagged using the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert tagged bookmarks");
+    let fxBmk1 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let fxBmk2 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com",
+      title: "Download Firefox",
+    });
+    let tag = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.tagsGuid,
+      title: "some tag",
+    });
+    let fxTag = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: tag.guid,
+      url: "http://getfirefox.com",
+    });
+
+    await startTracking();
+
+    _("Remove the tag using the async bookmarks API");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.remove(fxTag.guid);
+
+    await verifyTrackedItems([fxBmk1.guid, fxBmk2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 4);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 5);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemTagged() {
+  _("Items tagged using the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert untagged bookmarks");
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "Folder 1",
+    });
+    let fxBmk1 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: folder1.guid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let folder2 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "Folder 2",
+    });
+    // Different parent and title; same URL.
+    let fxBmk2 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: folder2.guid,
+      url: "http://getfirefox.com",
+      title: "Download Firefox",
+    });
+
+    await startTracking();
+
+    // This will change once tags are moved into a separate table (bug 424160).
+    // We specifically test this case because Bookmarks.jsm updates tagged
+    // bookmarks and notifies observers.
+    _("Insert a tag using the async bookmarks API");
+    let tag = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.tagsGuid,
+      title: "some tag",
+    });
+
+    _("Tag an item using the async bookmarks API");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: tag.guid,
+      url: "http://getfirefox.com",
+    });
+
+    await verifyTrackedItems([fxBmk1.guid, fxBmk2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 4);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 5);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemKeywordChanged() {
+  _("Keyword changes via the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert two bookmarks with the same URL");
+    let fxBmk1 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let fxBmk2 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com",
+      title: "Download Firefox",
+    });
+
+    await startTracking();
+
+    _("Add a keyword for both items");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.keywords.insert({
+      keyword: "the_keyword",
+      url: "http://getfirefox.com",
+      postData: "postData",
+    });
+
+    await verifyTrackedItems([fxBmk1.guid, fxBmk2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemKeywordDeleted() {
+  _("Keyword deletions via the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert two bookmarks with the same URL and keywords");
+    let fxBmk1 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let fxBmk2 = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      url: "http://getfirefox.com",
+      title: "Download Firefox",
+    });
+    await PlacesUtils.keywords.insert({
+      keyword: "the_keyword",
+      url: "http://getfirefox.com",
+    });
+
+    await startTracking();
+
+    _("Remove the keyword");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.keywords.remove("the_keyword");
+
+    await verifyTrackedItems([fxBmk1.guid, fxBmk2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_bookmarkAdded_filtered_root() {
+  _("Items outside the change roots should not be tracked");
+
+  try {
+    await startTracking();
+
+    _("Create a new root");
+    let root = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.rootGuid,
+      title: "New root",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+    _(`New root GUID: ${root.guid}`);
+
+    _("Insert a bookmark underneath the new root");
+    let untrackedBmk = await PlacesUtils.bookmarks.insert({
+      parentGuid: root.guid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+    _(`New untracked bookmark GUID: ${untrackedBmk.guid}`);
+
+    _("Insert a bookmark underneath the Places root");
+    let rootBmk = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.rootGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`New Places root bookmark GUID: ${rootBmk.guid}`);
+
+    _("New root and bookmark should be ignored");
+    await verifyTrackedItems([]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemDeleted_filtered_root() {
+  _("Deleted items outside the change roots should not be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert a bookmark underneath the Places root");
+    let rootBmk = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.rootGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`New Places root bookmark GUID: ${rootBmk.guid}`);
+
+    await startTracking();
+
+    await PlacesUtils.bookmarks.remove(rootBmk);
+
+    await verifyTrackedItems([]);
+    // We'll still increment the counter for the removed item.
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onPageAnnoChanged() {
+  _("Page annotations should not be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert a bookmark without an annotation");
+    let pageURI = "http://getfirefox.com";
+    await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: pageURI,
+      title: "Get Firefox!",
+    });
+
+    await startTracking();
+
+    _("Add a page annotation");
+    await PlacesUtils.history.update({
+      url: pageURI,
+      annotations: new Map([[PlacesUtils.CHARSET_ANNO, "UTF-16"]]),
+    });
+    await verifyTrackedItems([]);
+    Assert.equal(tracker.score, 0);
+    await resetTracker();
+
+    _("Remove the page annotation");
+    await PlacesUtils.history.update({
+      url: pageURI,
+      annotations: new Map([[PlacesUtils.CHARSET_ANNO, null]]),
+    });
+    await verifyTrackedItems([]);
+    Assert.equal(tracker.score, 0);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onFaviconChanged() {
+  _("Favicon changes should not be tracked");
+
+  try {
+    await tracker.stop();
+
+    let pageURI = CommonUtils.makeURI("http://getfirefox.com");
+    let iconURI = CommonUtils.makeURI("http://getfirefox.com/icon");
+    await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: pageURI,
+      title: "Get Firefox!",
+    });
+
+    await PlacesTestUtils.addVisits(pageURI);
+
+    await startTracking();
+
+    _("Favicon annotations should be ignored");
+    let iconURL =
+      "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAA" +
+      "AAAA6fptVAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==";
+
+    PlacesUtils.favicons.replaceFaviconDataFromDataURL(
+      iconURI,
+      iconURL,
+      0,
+      Services.scriptSecurityManager.getSystemPrincipal()
+    );
+
+    await new Promise(resolve => {
+      PlacesUtils.favicons.setAndFetchFaviconForPage(
+        pageURI,
+        iconURI,
+        true,
+        PlacesUtils.favicons.FAVICON_LOAD_NON_PRIVATE,
+        (uri, dataLen, data, mimeType) => {
+          resolve();
+        },
+        Services.scriptSecurityManager.getSystemPrincipal()
+      );
+    });
+    await verifyTrackedItems([]);
+    Assert.equal(tracker.score, 0);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemMoved_moveToFolder() {
+  _("Items moved via `moveToFolder` should be tracked");
+
+  try {
+    await tracker.stop();
+
+    await PlacesUtils.bookmarks.insertTree({
+      guid: PlacesUtils.bookmarks.menuGuid,
+      children: [
+        {
+          guid: "bookmarkAAAA",
+          title: "A",
+          url: "http://example.com/a",
+        },
+        {
+          guid: "bookmarkBBBB",
+          title: "B",
+          url: "http://example.com/b",
+        },
+        {
+          guid: "bookmarkCCCC",
+          title: "C",
+          url: "http://example.com/c",
+        },
+        {
+          guid: "bookmarkDDDD",
+          title: "D",
+          url: "http://example.com/d",
+        },
+      ],
+    });
+    await PlacesUtils.bookmarks.insertTree({
+      guid: PlacesUtils.bookmarks.toolbarGuid,
+      children: [
+        {
+          guid: "bookmarkEEEE",
+          title: "E",
+          url: "http://example.com/e",
+        },
+      ],
+    });
+
+    await startTracking();
+
+    _("Move (A B D) to the toolbar");
+    await PlacesUtils.bookmarks.moveToFolder(
+      ["bookmarkAAAA", "bookmarkBBBB", "bookmarkDDDD"],
+      PlacesUtils.bookmarks.toolbarGuid,
+      PlacesUtils.bookmarks.DEFAULT_INDEX
+    );
+
+    // Moving multiple bookmarks between two folders should track the old
+    // folder, new folder, and moved bookmarks.
+    await verifyTrackedItems([
+      "menu",
+      "toolbar",
+      "bookmarkAAAA",
+      "bookmarkBBBB",
+      "bookmarkDDDD",
+    ]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    await resetTracker();
+
+    _("Reorder toolbar children: (D A B E)");
+    await PlacesUtils.bookmarks.moveToFolder(
+      ["bookmarkDDDD", "bookmarkAAAA", "bookmarkBBBB"],
+      PlacesUtils.bookmarks.toolbarGuid,
+      0
+    );
+
+    // Reordering bookmarks in a folder should only track the folder, not the
+    // bookmarks.
+    await verifyTrackedItems(["toolbar"]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemMoved_update() {
+  _("Items moved via the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let tbBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+
+    await startTracking();
+
+    _("Repositioning a bookmark should track the folder");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.update({
+      guid: tbBmk.guid,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      index: 0,
+    });
+    await verifyTrackedItems(["menu"]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 1);
+    await resetTracker();
+
+    _("Reparenting a bookmark should track both folders and the bookmark");
+    totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.update({
+      guid: tbBmk.guid,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      index: PlacesUtils.bookmarks.DEFAULT_INDEX,
+    });
+    await verifyTrackedItems(["menu", "toolbar", tbBmk.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 3);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemMoved_reorder() {
+  _("Items reordered via the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Insert out-of-order bookmarks");
+    let fxBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`Firefox GUID: ${fxBmk.guid}`);
+
+    let tbBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+    _(`Thunderbird GUID: ${tbBmk.guid}`);
+
+    let mozBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "https://mozilla.org",
+      title: "Mozilla",
+    });
+    _(`Mozilla GUID: ${mozBmk.guid}`);
+
+    await startTracking();
+
+    _("Reorder bookmarks");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.reorder(PlacesUtils.bookmarks.menuGuid, [
+      mozBmk.guid,
+      fxBmk.guid,
+      tbBmk.guid,
+    ]);
+
+    // We only track the folder if we reorder its children, but we should
+    // bump the score for every changed item.
+    await verifyTrackedItems(["menu"]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 1);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemDeleted_removeFolderTransaction() {
+  _("Folders removed in a transaction should be tracked");
+
+  try {
+    await tracker.stop();
+
+    _("Create a folder with two children");
+    let folder = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "Test folder",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+    _(`Folder GUID: ${folder.guid}`);
+    let fx = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder.guid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`Firefox GUID: ${fx.guid}`);
+    let tb = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder.guid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+    _(`Thunderbird GUID: ${tb.guid}`);
+
+    await startTracking();
+
+    let txn = PlacesTransactions.Remove({ guid: folder.guid });
+    // We haven't executed the transaction yet.
+    await verifyTrackerEmpty();
+
+    _("Execute the remove folder transaction");
+    await txn.transact();
+    await verifyTrackedItems(["menu", folder.guid, fx.guid, tb.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    await resetTracker();
+
+    _("Undo the remove folder transaction");
+    await PlacesTransactions.undo();
+
+    await verifyTrackedItems(["menu", folder.guid, fx.guid, tb.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    await resetTracker();
+
+    _("Redo the transaction");
+    await PlacesTransactions.redo();
+    await verifyTrackedItems(["menu", folder.guid, fx.guid, tb.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_treeMoved() {
+  _("Moving an entire tree of bookmarks should track the parents");
+
+  try {
+    // Create a couple of parent folders.
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      test: "First test folder",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+
+    // A second folder in the first.
+    let folder2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      title: "Second test folder",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+
+    // Create a couple of bookmarks in the second folder.
+    await PlacesUtils.bookmarks.insert({
+      parentGuid: folder2.guid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    await PlacesUtils.bookmarks.insert({
+      parentGuid: folder2.guid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+
+    await startTracking();
+
+    // Move folder 2 to be a sibling of folder1.
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.update({
+      guid: folder2.guid,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      index: 0,
+    });
+
+    // the menu and both folders should be tracked, the children should not be.
+    await verifyTrackedItems(["menu", folder1.guid, folder2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 3);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemDeleted() {
+  _("Bookmarks deleted via the synchronous API should be tracked");
+
+  try {
+    await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let tb = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+
+    await startTracking();
+
+    // Delete the last item - the item and parent should be tracked.
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.remove(tb);
+
+    await verifyTrackedItems(["menu", tb.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemDeleted() {
+  _("Bookmarks deleted via the asynchronous API should be tracked");
+
+  try {
+    await tracker.stop();
+
+    let fxBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+
+    await startTracking();
+
+    _("Delete the first item");
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.remove(fxBmk.guid);
+
+    await verifyTrackedItems(["menu", fxBmk.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 2);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_async_onItemDeleted_eraseEverything() {
+  _("Erasing everything should track all deleted items");
+
+  try {
+    await tracker.stop();
+
+    let fxBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.mobileGuid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    _(`Firefox GUID: ${fxBmk.guid}`);
+    let tbBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.mobileGuid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+    _(`Thunderbird GUID: ${tbBmk.guid}`);
+    let mozBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "https://mozilla.org",
+      title: "Mozilla",
+    });
+    _(`Mozilla GUID: ${mozBmk.guid}`);
+    let mdnBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      url: "https://developer.mozilla.org",
+      title: "MDN",
+    });
+    _(`MDN GUID: ${mdnBmk.guid}`);
+    let bugsFolder = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+      title: "Bugs",
+    });
+    _(`Bugs folder GUID: ${bugsFolder.guid}`);
+    let bzBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: bugsFolder.guid,
+      url: "https://bugzilla.mozilla.org",
+      title: "Bugzilla",
+    });
+    _(`Bugzilla GUID: ${bzBmk.guid}`);
+    let bugsChildFolder = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+      parentGuid: bugsFolder.guid,
+      title: "Bugs child",
+    });
+    _(`Bugs child GUID: ${bugsChildFolder.guid}`);
+    let bugsGrandChildBmk = await PlacesUtils.bookmarks.insert({
+      type: PlacesUtils.bookmarks.TYPE_BOOKMARK,
+      parentGuid: bugsChildFolder.guid,
+      url: "https://example.com",
+      title: "Bugs grandchild",
+    });
+    _(`Bugs grandchild GUID: ${bugsGrandChildBmk.guid}`);
+
+    await startTracking();
+    // Simulate moving a synced item into a new folder. Deleting the folder
+    // should write a tombstone for the item, but not the folder.
+    await PlacesTestUtils.setBookmarkSyncFields({
+      guid: bugsChildFolder.guid,
+      syncStatus: PlacesUtils.bookmarks.SYNC_STATUS.NEW,
+    });
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.eraseEverything();
+
+    // bugsChildFolder's sync status is still "NEW", so it shouldn't be
+    // tracked. bugsGrandChildBmk is "NORMAL", so we *should* write a
+    // tombstone and track it.
+    await verifyTrackedItems([
+      "menu",
+      mozBmk.guid,
+      mdnBmk.guid,
+      "toolbar",
+      bugsFolder.guid,
+      "mobile",
+      fxBmk.guid,
+      tbBmk.guid,
+      "unfiled",
+      bzBmk.guid,
+      bugsGrandChildBmk.guid,
+    ]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 8);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 11);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
+
+add_task(async function test_onItemDeleted_tree() {
+  _("Deleting a tree of bookmarks should track all items");
+
+  try {
+    // Create a couple of parent folders.
+    let folder1 = await PlacesUtils.bookmarks.insert({
+      parentGuid: PlacesUtils.bookmarks.menuGuid,
+      title: "First test folder",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+
+    // A second folder in the first.
+    let folder2 = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder1.guid,
+      title: "Second test folder",
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+
+    // Create a couple of bookmarks in the second folder.
+    let fx = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder2.guid,
+      url: "http://getfirefox.com",
+      title: "Get Firefox!",
+    });
+    let tb = await PlacesUtils.bookmarks.insert({
+      parentGuid: folder2.guid,
+      url: "http://getthunderbird.com",
+      title: "Get Thunderbird!",
+    });
+
+    await startTracking();
+
+    // Delete folder2 - everything we created should be tracked.
+    let totalSyncChanges = PlacesUtils.bookmarks.totalSyncChanges;
+    await PlacesUtils.bookmarks.remove(folder2);
+
+    await verifyTrackedItems([fx.guid, tb.guid, folder1.guid, folder2.guid]);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(PlacesUtils.bookmarks.totalSyncChanges, totalSyncChanges + 4);
+  } finally {
+    _("Clean up.");
+    await cleanup();
+  }
+});
diff --git a/services/sync/tests/unit/test_bridged_engine.js b/services/sync/tests/unit/test_bridged_engine.js
new file mode 100644
index 0000000000..25a81f8f69
--- /dev/null
+++ b/services/sync/tests/unit/test_bridged_engine.js
@@ -0,0 +1,248 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { BridgedEngine, BridgeWrapperXPCOM } = ChromeUtils.importESModule(
+  "resource://services-sync/bridged_engine.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+// Wraps an `object` in a proxy so that its methods are bound to it. This
+// simulates how XPCOM class instances have all their methods bound.
+function withBoundMethods(object) {
+  return new Proxy(object, {
+    get(target, key) {
+      let value = target[key];
+      return typeof value == "function" ? value.bind(target) : value;
+    },
+  });
+}
+
+add_task(async function test_interface() {
+  class TestBridge {
+    constructor() {
+      this.storageVersion = 2;
+      this.syncID = "syncID111111";
+      this.clear();
+    }
+
+    clear() {
+      this.lastSyncMillis = 0;
+      this.wasSyncStarted = false;
+      this.incomingEnvelopes = [];
+      this.uploadedIDs = [];
+      this.wasSyncFinished = false;
+      this.wasReset = false;
+      this.wasWiped = false;
+    }
+
+    // `mozIBridgedSyncEngine` methods.
+
+    getLastSync(callback) {
+      CommonUtils.nextTick(() => callback.handleSuccess(this.lastSyncMillis));
+    }
+
+    setLastSync(millis, callback) {
+      this.lastSyncMillis = millis;
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+
+    resetSyncId(callback) {
+      CommonUtils.nextTick(() => callback.handleSuccess(this.syncID));
+    }
+
+    ensureCurrentSyncId(newSyncId, callback) {
+      equal(newSyncId, this.syncID, "Local and new sync IDs should match");
+      CommonUtils.nextTick(() => callback.handleSuccess(this.syncID));
+    }
+
+    syncStarted(callback) {
+      this.wasSyncStarted = true;
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+
+    storeIncoming(envelopes, callback) {
+      this.incomingEnvelopes.push(...envelopes.map(r => JSON.parse(r)));
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+
+    apply(callback) {
+      let outgoingEnvelopes = [
+        {
+          id: "hanson",
+          data: {
+            plants: ["seed", "flower 💐", "rose"],
+            canYouTell: false,
+          },
+        },
+        {
+          id: "sheryl-crow",
+          data: {
+            today: "winding 🛣",
+            tomorrow: "winding 🛣",
+          },
+        },
+      ].map(cleartext =>
+        JSON.stringify({
+          id: cleartext.id,
+          payload: JSON.stringify(cleartext),
+        })
+      );
+      CommonUtils.nextTick(() => callback.handleSuccess(outgoingEnvelopes));
+    }
+
+    setUploaded(millis, ids, callback) {
+      this.uploadedIDs.push(...ids);
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+
+    syncFinished(callback) {
+      this.wasSyncFinished = true;
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+
+    reset(callback) {
+      this.clear();
+      this.wasReset = true;
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+
+    wipe(callback) {
+      this.clear();
+      this.wasWiped = true;
+      CommonUtils.nextTick(() => callback.handleSuccess());
+    }
+  }
+
+  let bridge = new TestBridge();
+  let engine = new BridgedEngine("Nineties", Service);
+  engine._bridge = new BridgeWrapperXPCOM(withBoundMethods(bridge));
+  engine.enabled = true;
+
+  let server = await serverForFoo(engine);
+  try {
+    await SyncTestingInfrastructure(server);
+
+    info("Add server records");
+    let foo = server.user("foo");
+    let collection = foo.collection("nineties");
+    let now = new_timestamp();
+    collection.insert(
+      "backstreet",
+      encryptPayload({
+        id: "backstreet",
+        data: {
+          say: "I want it that way",
+          when: "never",
+        },
+      }),
+      now
+    );
+    collection.insert(
+      "tlc",
+      encryptPayload({
+        id: "tlc",
+        data: {
+          forbidden: ["scrubs 🚫"],
+          numberAvailable: false,
+        },
+      }),
+      now + 5
+    );
+
+    info("Sync the engine");
+    // Advance the last sync time to skip the Backstreet Boys...
+    bridge.lastSyncMillis = 1000 * (now + 2);
+    await sync_engine_and_validate_telem(engine, false);
+
+    let metaGlobal = foo.collection("meta").wbo("global").get();
+    deepEqual(
+      JSON.parse(metaGlobal.payload).engines.nineties,
+      {
+        version: 2,
+        syncID: "syncID111111",
+      },
+      "Should write storage version and sync ID to m/g"
+    );
+
+    greater(bridge.lastSyncMillis, 0, "Should update last sync time");
+    ok(
+      bridge.wasSyncStarted,
+      "Should have started sync before storing incoming"
+    );
+    deepEqual(
+      bridge.incomingEnvelopes
+        .sort((a, b) => a.id.localeCompare(b.id))
+        .map(({ payload, ...envelope }) => ({
+          cleartextAsObject: JSON.parse(payload),
+          ...envelope,
+        })),
+      [
+        {
+          id: "tlc",
+          modified: now + 5,
+          cleartextAsObject: {
+            id: "tlc",
+            data: {
+              forbidden: ["scrubs 🚫"],
+              numberAvailable: false,
+            },
+          },
+        },
+      ],
+      "Should stage incoming records from server"
+    );
+    deepEqual(
+      bridge.uploadedIDs.sort(),
+      ["hanson", "sheryl-crow"],
+      "Should mark new local records as uploaded"
+    );
+    ok(bridge.wasSyncFinished, "Should have finished sync after uploading");
+
+    deepEqual(
+      collection.keys().sort(),
+      ["backstreet", "hanson", "sheryl-crow", "tlc"],
+      "Should have all records on server"
+    );
+    let expectedRecords = [
+      {
+        id: "sheryl-crow",
+        data: {
+          today: "winding 🛣",
+          tomorrow: "winding 🛣",
+        },
+      },
+      {
+        id: "hanson",
+        data: {
+          plants: ["seed", "flower 💐", "rose"],
+          canYouTell: false,
+        },
+      },
+    ];
+    for (let expected of expectedRecords) {
+      let actual = collection.cleartext(expected.id);
+      deepEqual(
+        actual,
+        expected,
+        `Should upload record ${expected.id} from bridged engine`
+      );
+    }
+
+    await engine.resetClient();
+    ok(bridge.wasReset, "Should reset local storage for bridge");
+
+    await engine.wipeClient();
+    ok(bridge.wasWiped, "Should wipe local storage for bridge");
+
+    await engine.resetSyncID();
+    ok(
+      !foo.collection("nineties"),
+      "Should delete server collection after resetting sync ID"
+    );
+  } finally {
+    await promiseStopServer(server);
+    await engine.finalize();
+  }
+});
diff --git a/services/sync/tests/unit/test_clients_engine.js b/services/sync/tests/unit/test_clients_engine.js
new file mode 100644
index 0000000000..d910a67503
--- /dev/null
+++ b/services/sync/tests/unit/test_clients_engine.js
@@ -0,0 +1,2108 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { ClientEngine, ClientsRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/clients.sys.mjs"
+);
+const { CryptoWrapper } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const MORE_THAN_CLIENTS_TTL_REFRESH = 691200; // 8 days
+const LESS_THAN_CLIENTS_TTL_REFRESH = 86400; // 1 day
+
+let engine;
+
+/**
+ * Unpack the record with this ID, and verify that it has the same version that
+ * we should be putting into records.
+ */
+async function check_record_version(user, id) {
+  let payload = user.collection("clients").wbo(id).data;
+
+  let rec = new CryptoWrapper();
+  rec.id = id;
+  rec.collection = "clients";
+  rec.ciphertext = payload.ciphertext;
+  rec.hmac = payload.hmac;
+  rec.IV = payload.IV;
+
+  let cleartext = await rec.decrypt(
+    Service.collectionKeys.keyForCollection("clients")
+  );
+
+  _("Payload is " + JSON.stringify(cleartext));
+  equal(Services.appinfo.version, cleartext.version);
+  equal(1, cleartext.protocols.length);
+  equal("1.5", cleartext.protocols[0]);
+}
+
+// compare 2 different command arrays, taking into account that a flowID
+// attribute must exist, be unique in the commands, but isn't specified in
+// "expected" as the value isn't known.
+function compareCommands(actual, expected, description) {
+  let tweakedActual = JSON.parse(JSON.stringify(actual));
+  tweakedActual.map(elt => delete elt.flowID);
+  deepEqual(tweakedActual, expected, description);
+  // each item must have a unique flowID.
+  let allIDs = new Set(actual.map(elt => elt.flowID).filter(fid => !!fid));
+  equal(allIDs.size, actual.length, "all items have unique IDs");
+}
+
+async function syncClientsEngine(server) {
+  engine._lastFxADevicesFetch = 0;
+  engine.lastModified = server.getCollection("foo", "clients").timestamp;
+  await engine._sync();
+}
+
+add_task(async function setup() {
+  engine = Service.clientsEngine;
+});
+
+async function cleanup() {
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  await engine._tracker.clearChangedIDs();
+  await engine._resetClient();
+  // un-cleanup the logs (the resetBranch will have reset their levels), since
+  // not all the tests use SyncTestingInfrastructure, and it's cheap.
+  syncTestLogging();
+  // We don't finalize storage at cleanup, since we use the same clients engine
+  // instance across all tests.
+}
+
+add_task(async function test_bad_hmac() {
+  _("Ensure that Clients engine deletes corrupt records.");
+  let deletedCollections = [];
+  let deletedItems = [];
+  let callback = {
+    onItemDeleted(username, coll, wboID) {
+      deletedItems.push(coll + "/" + wboID);
+    },
+    onCollectionDeleted(username, coll) {
+      deletedCollections.push(coll);
+    },
+  };
+  Object.setPrototypeOf(callback, SyncServerCallback);
+  let server = await serverForFoo(engine, callback);
+  let user = server.user("foo");
+
+  function check_clients_count(expectedCount) {
+    let coll = user.collection("clients");
+
+    // Treat a non-existent collection as empty.
+    equal(expectedCount, coll ? coll.count() : 0);
+  }
+
+  function check_client_deleted(id) {
+    let coll = user.collection("clients");
+    let wbo = coll.wbo(id);
+    return !wbo || !wbo.payload;
+  }
+
+  async function uploadNewKeys() {
+    await generateNewKeys(Service.collectionKeys);
+    let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+    await serverKeys.encrypt(Service.identity.syncKeyBundle);
+    ok(
+      (await serverKeys.upload(Service.resource(Service.cryptoKeysURL))).success
+    );
+  }
+
+  try {
+    await configureIdentity({ username: "foo" }, server);
+    await Service.login();
+
+    await generateNewKeys(Service.collectionKeys);
+
+    _("First sync, client record is uploaded");
+    equal(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    check_clients_count(0);
+    await syncClientsEngine(server);
+    check_clients_count(1);
+    ok(engine.lastRecordUpload > 0);
+    ok(!engine.isFirstSync);
+
+    // Our uploaded record has a version.
+    await check_record_version(user, engine.localID);
+
+    // Initial setup can wipe the server, so clean up.
+    deletedCollections = [];
+    deletedItems = [];
+
+    _("Change our keys and our client ID, reupload keys.");
+    let oldLocalID = engine.localID; // Preserve to test for deletion!
+    engine.localID = Utils.makeGUID();
+    await engine.resetClient();
+    await generateNewKeys(Service.collectionKeys);
+    let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+    await serverKeys.encrypt(Service.identity.syncKeyBundle);
+    ok(
+      (await serverKeys.upload(Service.resource(Service.cryptoKeysURL))).success
+    );
+
+    _("Sync.");
+    await syncClientsEngine(server);
+
+    _("Old record " + oldLocalID + " was deleted, new one uploaded.");
+    check_clients_count(1);
+    check_client_deleted(oldLocalID);
+
+    _(
+      "Now change our keys but don't upload them. " +
+        "That means we get an HMAC error but redownload keys."
+    );
+    Service.lastHMACEvent = 0;
+    engine.localID = Utils.makeGUID();
+    await engine.resetClient();
+    await generateNewKeys(Service.collectionKeys);
+    deletedCollections = [];
+    deletedItems = [];
+    check_clients_count(1);
+    await syncClientsEngine(server);
+
+    _("Old record was not deleted, new one uploaded.");
+    equal(deletedCollections.length, 0);
+    equal(deletedItems.length, 0);
+    check_clients_count(2);
+
+    _(
+      "Now try the scenario where our keys are wrong *and* there's a bad record."
+    );
+    // Clean up and start fresh.
+    user.collection("clients")._wbos = {};
+    Service.lastHMACEvent = 0;
+    engine.localID = Utils.makeGUID();
+    await engine.resetClient();
+    deletedCollections = [];
+    deletedItems = [];
+    check_clients_count(0);
+
+    await uploadNewKeys();
+
+    // Sync once to upload a record.
+    await syncClientsEngine(server);
+    check_clients_count(1);
+
+    // Generate and upload new keys, so the old client record is wrong.
+    await uploadNewKeys();
+
+    // Create a new client record and new keys. Now our keys are wrong, as well
+    // as the object on the server. We'll download the new keys and also delete
+    // the bad client record.
+    oldLocalID = engine.localID; // Preserve to test for deletion!
+    engine.localID = Utils.makeGUID();
+    await engine.resetClient();
+    await generateNewKeys(Service.collectionKeys);
+    let oldKey = Service.collectionKeys.keyForCollection();
+
+    equal(deletedCollections.length, 0);
+    equal(deletedItems.length, 0);
+    await syncClientsEngine(server);
+    equal(deletedItems.length, 1);
+    check_client_deleted(oldLocalID);
+    check_clients_count(1);
+    let newKey = Service.collectionKeys.keyForCollection();
+    ok(!oldKey.equals(newKey));
+  } finally {
+    await cleanup();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_properties() {
+  _("Test lastRecordUpload property");
+  try {
+    equal(
+      Svc.PrefBranch.getPrefType("clients.lastRecordUpload"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    equal(engine.lastRecordUpload, 0);
+
+    let now = Date.now();
+    engine.lastRecordUpload = now / 1000;
+    equal(engine.lastRecordUpload, Math.floor(now / 1000));
+  } finally {
+    await cleanup();
+  }
+});
+
+add_task(async function test_full_sync() {
+  _("Ensure that Clients engine fetches all records for each sync.");
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+  let user = server.user("foo");
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let activeID = Utils.makeGUID();
+  user.collection("clients").insertRecord(
+    {
+      id: activeID,
+      name: "Active client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  let deletedID = Utils.makeGUID();
+  user.collection("clients").insertRecord(
+    {
+      id: deletedID,
+      name: "Client to delete",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    let store = engine._store;
+
+    _("First sync. 2 records downloaded; our record uploaded.");
+    strictEqual(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    await syncClientsEngine(server);
+    ok(engine.lastRecordUpload > 0);
+    ok(!engine.isFirstSync);
+    deepEqual(
+      user.collection("clients").keys().sort(),
+      [activeID, deletedID, engine.localID].sort(),
+      "Our record should be uploaded on first sync"
+    );
+    let ids = await store.getAllIDs();
+    deepEqual(
+      Object.keys(ids).sort(),
+      [activeID, deletedID, engine.localID].sort(),
+      "Other clients should be downloaded on first sync"
+    );
+
+    _("Delete a record, then sync again");
+    let collection = server.getCollection("foo", "clients");
+    collection.remove(deletedID);
+    // Simulate a timestamp update in info/collections.
+    await syncClientsEngine(server);
+
+    _("Record should be updated");
+    ids = await store.getAllIDs();
+    deepEqual(
+      Object.keys(ids).sort(),
+      [activeID, engine.localID].sort(),
+      "Deleted client should be removed on next sync"
+    );
+  } finally {
+    await cleanup();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_sync() {
+  _("Ensure that Clients engine uploads a new client record once a week.");
+
+  let server = await serverForFoo(engine);
+  let user = server.user("foo");
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  function clientWBO() {
+    return user.collection("clients").wbo(engine.localID);
+  }
+
+  try {
+    _("First sync. Client record is uploaded.");
+    equal(clientWBO(), undefined);
+    equal(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    await syncClientsEngine(server);
+    ok(!!clientWBO().payload);
+    ok(engine.lastRecordUpload > 0);
+    ok(!engine.isFirstSync);
+
+    _(
+      "Let's time travel more than a week back, new record should've been uploaded."
+    );
+    engine.lastRecordUpload -= MORE_THAN_CLIENTS_TTL_REFRESH;
+    let lastweek = engine.lastRecordUpload;
+    clientWBO().payload = undefined;
+    await syncClientsEngine(server);
+    ok(!!clientWBO().payload);
+    ok(engine.lastRecordUpload > lastweek);
+    ok(!engine.isFirstSync);
+
+    _("Remove client record.");
+    await engine.removeClientData();
+    equal(clientWBO().payload, undefined);
+
+    _("Time travel one day back, no record uploaded.");
+    engine.lastRecordUpload -= LESS_THAN_CLIENTS_TTL_REFRESH;
+    let yesterday = engine.lastRecordUpload;
+    await syncClientsEngine(server);
+    equal(clientWBO().payload, undefined);
+    equal(engine.lastRecordUpload, yesterday);
+    ok(!engine.isFirstSync);
+  } finally {
+    await cleanup();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_client_name_change() {
+  _("Ensure client name change incurs a client record update.");
+
+  let tracker = engine._tracker;
+
+  engine.localID; // Needed to increase the tracker changedIDs count.
+  let initialName = engine.localName;
+
+  tracker.start();
+  _("initial name: " + initialName);
+
+  // Tracker already has data, so clear it.
+  await tracker.clearChangedIDs();
+
+  let initialScore = tracker.score;
+
+  let changedIDs = await tracker.getChangedIDs();
+  equal(Object.keys(changedIDs).length, 0);
+
+  Services.prefs.setStringPref(
+    "identity.fxaccounts.account.device.name",
+    "new name"
+  );
+  await tracker.asyncObserver.promiseObserversComplete();
+
+  _("new name: " + engine.localName);
+  notEqual(initialName, engine.localName);
+  changedIDs = await tracker.getChangedIDs();
+  equal(Object.keys(changedIDs).length, 1);
+  ok(engine.localID in changedIDs);
+  ok(tracker.score > initialScore);
+  ok(tracker.score >= SCORE_INCREMENT_XLARGE);
+
+  await tracker.stop();
+
+  await cleanup();
+});
+
+add_task(async function test_fxa_device_id_change() {
+  _("Ensure an FxA device ID change incurs a client record update.");
+
+  let tracker = engine._tracker;
+
+  engine.localID; // Needed to increase the tracker changedIDs count.
+
+  tracker.start();
+
+  // Tracker already has data, so clear it.
+  await tracker.clearChangedIDs();
+
+  let initialScore = tracker.score;
+
+  let changedIDs = await tracker.getChangedIDs();
+  equal(Object.keys(changedIDs).length, 0);
+
+  Services.obs.notifyObservers(null, "fxaccounts:new_device_id");
+  await tracker.asyncObserver.promiseObserversComplete();
+
+  changedIDs = await tracker.getChangedIDs();
+  equal(Object.keys(changedIDs).length, 1);
+  ok(engine.localID in changedIDs);
+  ok(tracker.score > initialScore);
+  ok(tracker.score >= SINGLE_USER_THRESHOLD);
+
+  await tracker.stop();
+
+  await cleanup();
+});
+
+add_task(async function test_last_modified() {
+  _("Ensure that remote records have a sane serverLastModified attribute.");
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+  let user = server.user("foo");
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let activeID = Utils.makeGUID();
+  user.collection("clients").insertRecord(
+    {
+      id: activeID,
+      name: "Active client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    let collection = user.collection("clients");
+
+    _("Sync to download the record");
+    await syncClientsEngine(server);
+
+    equal(
+      engine._store._remoteClients[activeID].serverLastModified,
+      now - 10,
+      "last modified in the local record is correctly the server last-modified"
+    );
+
+    _("Modify the record and re-upload it");
+    // set a new name to make sure we really did upload.
+    engine._store._remoteClients[activeID].name = "New name";
+    engine._modified.set(activeID, 0);
+    // The sync above also did a POST, so adjust our lastModified.
+    engine.lastModified = server.getCollection("foo", "clients").timestamp;
+    await engine._uploadOutgoing();
+
+    _("Local record should have updated timestamp");
+    ok(engine._store._remoteClients[activeID].serverLastModified >= now);
+
+    _("Record on the server should have new name but not serverLastModified");
+    let payload = collection.cleartext(activeID);
+    equal(payload.name, "New name");
+    equal(payload.serverLastModified, undefined);
+  } finally {
+    await cleanup();
+    server.deleteCollections("foo");
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_send_command() {
+  _("Verifies _sendCommandToClient puts commands in the outbound queue.");
+
+  let store = engine._store;
+  let tracker = engine._tracker;
+  let remoteId = Utils.makeGUID();
+  let rec = new ClientsRec("clients", remoteId);
+
+  await store.create(rec);
+  await store.createRecord(remoteId, "clients");
+
+  let action = "testCommand";
+  let args = ["foo", "bar"];
+  let extra = { flowID: "flowy" };
+
+  await engine._sendCommandToClient(action, args, remoteId, extra);
+
+  let newRecord = store._remoteClients[remoteId];
+  let clientCommands = (await engine._readCommands())[remoteId];
+  notEqual(newRecord, undefined);
+  equal(clientCommands.length, 1);
+
+  let command = clientCommands[0];
+  equal(command.command, action);
+  equal(command.args.length, 2);
+  deepEqual(command.args, args);
+  ok(command.flowID);
+
+  const changes = await tracker.getChangedIDs();
+  notEqual(changes[remoteId], undefined);
+
+  await cleanup();
+});
+
+// The browser UI might call _addClientCommand indirectly without awaiting on the returned promise.
+// We need to make sure this doesn't result on commands not being saved.
+add_task(async function test_add_client_command_race() {
+  let promises = [];
+  for (let i = 0; i < 100; i++) {
+    promises.push(
+      engine._addClientCommand(`client-${i}`, { command: "cmd", args: [] })
+    );
+  }
+  await Promise.all(promises);
+
+  let localCommands = await engine._readCommands();
+  for (let i = 0; i < 100; i++) {
+    equal(localCommands[`client-${i}`].length, 1);
+  }
+});
+
+add_task(async function test_command_validation() {
+  _("Verifies that command validation works properly.");
+
+  let store = engine._store;
+
+  let testCommands = [
+    ["resetAll", [], true],
+    ["resetAll", ["foo"], false],
+    ["resetEngine", ["tabs"], true],
+    ["resetEngine", [], false],
+    ["wipeEngine", ["tabs"], true],
+    ["wipeEngine", [], false],
+    ["logout", [], true],
+    ["logout", ["foo"], false],
+    ["__UNKNOWN__", [], false],
+  ];
+
+  for (let [action, args, expectedResult] of testCommands) {
+    let remoteId = Utils.makeGUID();
+    let rec = new ClientsRec("clients", remoteId);
+
+    await store.create(rec);
+    await store.createRecord(remoteId, "clients");
+
+    await engine.sendCommand(action, args, remoteId);
+
+    let newRecord = store._remoteClients[remoteId];
+    notEqual(newRecord, undefined);
+
+    let clientCommands = (await engine._readCommands())[remoteId];
+
+    if (expectedResult) {
+      _("Ensuring command is sent: " + action);
+      equal(clientCommands.length, 1);
+
+      let command = clientCommands[0];
+      equal(command.command, action);
+      deepEqual(command.args, args);
+
+      notEqual(engine._tracker, undefined);
+      const changes = await engine._tracker.getChangedIDs();
+      notEqual(changes[remoteId], undefined);
+    } else {
+      _("Ensuring command is scrubbed: " + action);
+      equal(clientCommands, undefined);
+
+      if (store._tracker) {
+        equal(engine._tracker[remoteId], undefined);
+      }
+    }
+  }
+  await cleanup();
+});
+
+add_task(async function test_command_duplication() {
+  _("Ensures duplicate commands are detected and not added");
+
+  let store = engine._store;
+  let remoteId = Utils.makeGUID();
+  let rec = new ClientsRec("clients", remoteId);
+  await store.create(rec);
+  await store.createRecord(remoteId, "clients");
+
+  let action = "resetAll";
+  let args = [];
+
+  await engine.sendCommand(action, args, remoteId);
+  await engine.sendCommand(action, args, remoteId);
+
+  let clientCommands = (await engine._readCommands())[remoteId];
+  equal(clientCommands.length, 1);
+
+  _("Check variant args length");
+  await engine._saveCommands({});
+
+  action = "resetEngine";
+  await engine.sendCommand(action, [{ x: "foo" }], remoteId);
+  await engine.sendCommand(action, [{ x: "bar" }], remoteId);
+
+  _("Make sure we spot a real dupe argument.");
+  await engine.sendCommand(action, [{ x: "bar" }], remoteId);
+
+  clientCommands = (await engine._readCommands())[remoteId];
+  equal(clientCommands.length, 2);
+
+  await cleanup();
+});
+
+add_task(async function test_command_invalid_client() {
+  _("Ensures invalid client IDs are caught");
+
+  let id = Utils.makeGUID();
+  let error;
+
+  try {
+    await engine.sendCommand("wipeEngine", ["tabs"], id);
+  } catch (ex) {
+    error = ex;
+  }
+
+  equal(error.message.indexOf("Unknown remote client ID: "), 0);
+
+  await cleanup();
+});
+
+add_task(async function test_process_incoming_commands() {
+  _("Ensures local commands are executed");
+
+  engine.localCommands = [{ command: "logout", args: [] }];
+
+  let ev = "weave:service:logout:finish";
+
+  let logoutPromise = new Promise(resolve => {
+    var handler = function () {
+      Svc.Obs.remove(ev, handler);
+
+      resolve();
+    };
+
+    Svc.Obs.add(ev, handler);
+  });
+
+  // logout command causes processIncomingCommands to return explicit false.
+  ok(!(await engine.processIncomingCommands()));
+
+  await logoutPromise;
+
+  await cleanup();
+});
+
+add_task(async function test_filter_duplicate_names() {
+  _(
+    "Ensure that we exclude clients with identical names that haven't synced in a week."
+  );
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+  let user = server.user("foo");
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  // Synced recently.
+  let recentID = Utils.makeGUID();
+  user.collection("clients").insertRecord(
+    {
+      id: recentID,
+      name: "My Phone",
+      type: "mobile",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  // Dupe of our client, synced more than 1 week ago.
+  let dupeID = Utils.makeGUID();
+  user.collection("clients").insertRecord(
+    {
+      id: dupeID,
+      name: engine.localName,
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 604820
+  );
+
+  // Synced more than 1 week ago, but not a dupe.
+  let oldID = Utils.makeGUID();
+  user.collection("clients").insertRecord(
+    {
+      id: oldID,
+      name: "My old desktop",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 604820
+  );
+
+  try {
+    let store = engine._store;
+
+    _("First sync");
+    strictEqual(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    await syncClientsEngine(server);
+    ok(engine.lastRecordUpload > 0);
+    ok(!engine.isFirstSync);
+    deepEqual(
+      user.collection("clients").keys().sort(),
+      [recentID, dupeID, oldID, engine.localID].sort(),
+      "Our record should be uploaded on first sync"
+    );
+
+    let ids = await store.getAllIDs();
+    deepEqual(
+      Object.keys(ids).sort(),
+      [recentID, dupeID, oldID, engine.localID].sort(),
+      "Duplicate ID should remain in getAllIDs"
+    );
+    ok(
+      await engine._store.itemExists(dupeID),
+      "Dupe ID should be considered as existing for Sync methods."
+    );
+    ok(
+      !engine.remoteClientExists(dupeID),
+      "Dupe ID should not be considered as existing for external methods."
+    );
+
+    // dupe desktop should not appear in .deviceTypes.
+    equal(engine.deviceTypes.get("desktop"), 2);
+    equal(engine.deviceTypes.get("mobile"), 1);
+
+    // dupe desktop should not appear in stats
+    deepEqual(engine.stats, {
+      hasMobile: 1,
+      names: [engine.localName, "My Phone", "My old desktop"],
+      numClients: 3,
+    });
+
+    ok(engine.remoteClientExists(oldID), "non-dupe ID should exist.");
+    ok(!engine.remoteClientExists(dupeID), "dupe ID should not exist");
+    equal(
+      engine.remoteClients.length,
+      2,
+      "dupe should not be in remoteClients"
+    );
+
+    // Check that a subsequent Sync doesn't report anything as being processed.
+    let counts;
+    Svc.Obs.add("weave:engine:sync:applied", function observe(subject, data) {
+      Svc.Obs.remove("weave:engine:sync:applied", observe);
+      counts = subject;
+    });
+
+    await syncClientsEngine(server);
+    equal(counts.applied, 0); // We didn't report applying any records.
+    equal(counts.reconciled, 4); // We reported reconcilliation for all records
+    equal(counts.succeeded, 0);
+    equal(counts.failed, 0);
+    equal(counts.newFailed, 0);
+
+    _("Broadcast logout to all clients");
+    await engine.sendCommand("logout", []);
+    await syncClientsEngine(server);
+
+    let collection = server.getCollection("foo", "clients");
+    let recentPayload = collection.cleartext(recentID);
+    compareCommands(
+      recentPayload.commands,
+      [{ command: "logout", args: [] }],
+      "Should send commands to the recent client"
+    );
+
+    let oldPayload = collection.cleartext(oldID);
+    compareCommands(
+      oldPayload.commands,
+      [{ command: "logout", args: [] }],
+      "Should send commands to the week-old client"
+    );
+
+    let dupePayload = collection.cleartext(dupeID);
+    deepEqual(
+      dupePayload.commands,
+      [],
+      "Should not send commands to the dupe client"
+    );
+
+    _("Update the dupe client's modified time");
+    collection.insertRecord(
+      {
+        id: dupeID,
+        name: engine.localName,
+        type: "desktop",
+        commands: [],
+        version: "48",
+        protocols: ["1.5"],
+      },
+      now - 10
+    );
+
+    _("Second sync.");
+    await syncClientsEngine(server);
+
+    ids = await store.getAllIDs();
+    deepEqual(
+      Object.keys(ids).sort(),
+      [recentID, oldID, dupeID, engine.localID].sort(),
+      "Stale client synced, so it should no longer be marked as a dupe"
+    );
+
+    ok(
+      engine.remoteClientExists(dupeID),
+      "Dupe ID should appear as it synced."
+    );
+
+    // Recently synced dupe desktop should appear in .deviceTypes.
+    equal(engine.deviceTypes.get("desktop"), 3);
+
+    // Recently synced dupe desktop should now appear in stats
+    deepEqual(engine.stats, {
+      hasMobile: 1,
+      names: [engine.localName, "My Phone", engine.localName, "My old desktop"],
+      numClients: 4,
+    });
+
+    ok(
+      engine.remoteClientExists(dupeID),
+      "recently synced dupe ID should now exist"
+    );
+    equal(
+      engine.remoteClients.length,
+      3,
+      "recently synced dupe should now be in remoteClients"
+    );
+  } finally {
+    await cleanup();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_command_sync() {
+  _("Ensure that commands are synced across clients.");
+
+  await engine._store.wipe();
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let user = server.user("foo");
+  let remoteId = Utils.makeGUID();
+
+  function clientWBO(id) {
+    return user.collection("clients").wbo(id);
+  }
+
+  _("Create remote client record");
+  user.collection("clients").insertRecord({
+    id: remoteId,
+    name: "Remote client",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    protocols: ["1.5"],
+  });
+
+  try {
+    _("Syncing.");
+    await syncClientsEngine(server);
+
+    _("Checking remote record was downloaded.");
+    let clientRecord = engine._store._remoteClients[remoteId];
+    notEqual(clientRecord, undefined);
+    equal(clientRecord.commands.length, 0);
+
+    _("Send a command to the remote client.");
+    await engine.sendCommand("wipeEngine", ["tabs"]);
+    let clientCommands = (await engine._readCommands())[remoteId];
+    equal(clientCommands.length, 1);
+    await syncClientsEngine(server);
+
+    _("Checking record was uploaded.");
+    notEqual(clientWBO(engine.localID).payload, undefined);
+    ok(engine.lastRecordUpload > 0);
+    ok(!engine.isFirstSync);
+
+    notEqual(clientWBO(remoteId).payload, undefined);
+
+    Svc.PrefBranch.setStringPref("client.GUID", remoteId);
+    await engine._resetClient();
+    equal(engine.localID, remoteId);
+    _("Performing sync on resetted client.");
+    await syncClientsEngine(server);
+    notEqual(engine.localCommands, undefined);
+    equal(engine.localCommands.length, 1);
+
+    let command = engine.localCommands[0];
+    equal(command.command, "wipeEngine");
+    equal(command.args.length, 1);
+    equal(command.args[0], "tabs");
+  } finally {
+    await cleanup();
+
+    try {
+      let collection = server.getCollection("foo", "clients");
+      collection.remove(remoteId);
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_clients_not_in_fxa_list() {
+  _("Ensure that clients not in the FxA devices list are marked as stale.");
+
+  await engine._store.wipe();
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let remoteId = Utils.makeGUID();
+  let remoteId2 = Utils.makeGUID();
+  let collection = server.getCollection("foo", "clients");
+
+  _("Create remote client records");
+  collection.insertRecord({
+    id: remoteId,
+    name: "Remote client",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    fxaDeviceId: remoteId,
+    protocols: ["1.5"],
+  });
+
+  collection.insertRecord({
+    id: remoteId2,
+    name: "Remote client 2",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    fxaDeviceId: remoteId2,
+    protocols: ["1.5"],
+  });
+
+  let fxAccounts = engine.fxAccounts;
+  engine.fxAccounts = {
+    notifyDevices() {
+      return Promise.resolve(true);
+    },
+    device: {
+      getLocalId() {
+        return fxAccounts.device.getLocalId();
+      },
+      getLocalName() {
+        return fxAccounts.device.getLocalName();
+      },
+      getLocalType() {
+        return fxAccounts.device.getLocalType();
+      },
+      recentDeviceList: [{ id: remoteId }],
+      refreshDeviceList() {
+        return Promise.resolve(true);
+      },
+    },
+    _internal: {
+      now() {
+        return Date.now();
+      },
+    },
+  };
+
+  try {
+    _("Syncing.");
+    await syncClientsEngine(server);
+
+    ok(!engine._store._remoteClients[remoteId].stale);
+    ok(engine._store._remoteClients[remoteId2].stale);
+  } finally {
+    engine.fxAccounts = fxAccounts;
+    await cleanup();
+
+    try {
+      collection.remove(remoteId);
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_dupe_device_ids() {
+  _(
+    "Ensure that we mark devices with duplicate fxaDeviceIds but older lastModified as stale."
+  );
+
+  await engine._store.wipe();
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let remoteId = Utils.makeGUID();
+  let remoteId2 = Utils.makeGUID();
+  let remoteDeviceId = Utils.makeGUID();
+
+  let collection = server.getCollection("foo", "clients");
+
+  _("Create remote client records");
+  collection.insertRecord(
+    {
+      id: remoteId,
+      name: "Remote client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      fxaDeviceId: remoteDeviceId,
+      protocols: ["1.5"],
+    },
+    new_timestamp() - 3
+  );
+  collection.insertRecord({
+    id: remoteId2,
+    name: "Remote client",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    fxaDeviceId: remoteDeviceId,
+    protocols: ["1.5"],
+  });
+
+  let fxAccounts = engine.fxAccounts;
+  engine.fxAccounts = {
+    notifyDevices() {
+      return Promise.resolve(true);
+    },
+    device: {
+      getLocalId() {
+        return fxAccounts.device.getLocalId();
+      },
+      getLocalName() {
+        return fxAccounts.device.getLocalName();
+      },
+      getLocalType() {
+        return fxAccounts.device.getLocalType();
+      },
+      recentDeviceList: [{ id: remoteDeviceId }],
+      refreshDeviceList() {
+        return Promise.resolve(true);
+      },
+    },
+    _internal: {
+      now() {
+        return Date.now();
+      },
+    },
+  };
+
+  try {
+    _("Syncing.");
+    await syncClientsEngine(server);
+
+    ok(engine._store._remoteClients[remoteId].stale);
+    ok(!engine._store._remoteClients[remoteId2].stale);
+  } finally {
+    engine.fxAccounts = fxAccounts;
+    await cleanup();
+
+    try {
+      collection.remove(remoteId);
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_refresh_fxa_device_list() {
+  _("Ensure we refresh the fxa device list when we expect to.");
+
+  await engine._store.wipe();
+  engine._lastFxaDeviceRefresh = 0;
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let numRefreshes = 0;
+  let now = Date.now();
+  let fxAccounts = engine.fxAccounts;
+  engine.fxAccounts = {
+    notifyDevices() {
+      return Promise.resolve(true);
+    },
+    device: {
+      getLocalId() {
+        return fxAccounts.device.getLocalId();
+      },
+      getLocalName() {
+        return fxAccounts.device.getLocalName();
+      },
+      getLocalType() {
+        return fxAccounts.device.getLocalType();
+      },
+      recentDeviceList: [],
+      refreshDeviceList() {
+        numRefreshes += 1;
+        return Promise.resolve(true);
+      },
+    },
+    _internal: {
+      now() {
+        return now;
+      },
+    },
+  };
+
+  try {
+    _("Syncing.");
+    await syncClientsEngine(server);
+    Assert.equal(numRefreshes, 1, "first sync should refresh");
+    now += 1000; // a second later.
+    await syncClientsEngine(server);
+    Assert.equal(numRefreshes, 1, "next sync should not refresh");
+    now += 60 * 60 * 2 * 1000; // 2 hours later
+    await syncClientsEngine(server);
+    Assert.equal(numRefreshes, 2, "2 hours later should refresh");
+    now += 1000; // a second later.
+    Assert.equal(numRefreshes, 2, "next sync should not refresh");
+  } finally {
+    await cleanup();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_optional_client_fields() {
+  _("Ensure that we produce records with the fields added in Bug 1097222.");
+
+  const SUPPORTED_PROTOCOL_VERSIONS = ["1.5"];
+  let local = await engine._store.createRecord(engine.localID, "clients");
+  equal(local.name, engine.localName);
+  equal(local.type, engine.localType);
+  equal(local.version, Services.appinfo.version);
+  deepEqual(local.protocols, SUPPORTED_PROTOCOL_VERSIONS);
+
+  // Optional fields.
+  // Make sure they're what they ought to be...
+  equal(local.os, Services.appinfo.OS);
+  equal(local.appPackage, Services.appinfo.ID);
+
+  // ... and also that they're non-empty.
+  ok(!!local.os);
+  ok(!!local.appPackage);
+  ok(!!local.application);
+
+  // We don't currently populate device or formfactor.
+  // See Bug 1100722, Bug 1100723.
+
+  await cleanup();
+});
+
+add_task(async function test_merge_commands() {
+  _("Verifies local commands for remote clients are merged with the server's");
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let collection = server.getCollection("foo", "clients");
+
+  let desktopID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: desktopID,
+      name: "Desktop client",
+      type: "desktop",
+      commands: [
+        {
+          command: "wipeEngine",
+          args: ["history"],
+          flowID: Utils.makeGUID(),
+        },
+      ],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  let mobileID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: mobileID,
+      name: "Mobile client",
+      type: "mobile",
+      commands: [
+        {
+          command: "logout",
+          args: [],
+          flowID: Utils.makeGUID(),
+        },
+      ],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    _("First sync. 2 records downloaded.");
+    strictEqual(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    await syncClientsEngine(server);
+
+    _("Broadcast logout to all clients");
+    await engine.sendCommand("logout", []);
+    await syncClientsEngine(server);
+
+    let desktopPayload = collection.cleartext(desktopID);
+    compareCommands(
+      desktopPayload.commands,
+      [
+        {
+          command: "wipeEngine",
+          args: ["history"],
+        },
+        {
+          command: "logout",
+          args: [],
+        },
+      ],
+      "Should send the logout command to the desktop client"
+    );
+
+    let mobilePayload = collection.cleartext(mobileID);
+    compareCommands(
+      mobilePayload.commands,
+      [{ command: "logout", args: [] }],
+      "Should not send a duplicate logout to the mobile client"
+    );
+  } finally {
+    await cleanup();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_duplicate_remote_commands() {
+  _(
+    "Verifies local commands for remote clients are sent only once (bug 1289287)"
+  );
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let collection = server.getCollection("foo", "clients");
+
+  let desktopID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: desktopID,
+      name: "Desktop client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    _("First sync. 1 record downloaded.");
+    strictEqual(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    await syncClientsEngine(server);
+
+    _("Send command to client to wipe history engine");
+    await engine.sendCommand("wipeEngine", ["history"]);
+    await syncClientsEngine(server);
+
+    _(
+      "Simulate the desktop client consuming the command and syncing to the server"
+    );
+    collection.insertRecord(
+      {
+        id: desktopID,
+        name: "Desktop client",
+        type: "desktop",
+        commands: [],
+        version: "48",
+        protocols: ["1.5"],
+      },
+      now - 10
+    );
+
+    _("Send another command to the desktop client to wipe tabs engine");
+    await engine.sendCommand("wipeEngine", ["tabs"], desktopID);
+    await syncClientsEngine(server);
+
+    let desktopPayload = collection.cleartext(desktopID);
+    compareCommands(
+      desktopPayload.commands,
+      [
+        {
+          command: "wipeEngine",
+          args: ["tabs"],
+        },
+      ],
+      "Should only send the second command to the desktop client"
+    );
+  } finally {
+    await cleanup();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_upload_after_reboot() {
+  _("Multiple downloads, reboot, then upload (bug 1289287)");
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let collection = server.getCollection("foo", "clients");
+
+  let deviceBID = Utils.makeGUID();
+  let deviceCID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: deviceBID,
+      name: "Device B",
+      type: "desktop",
+      commands: [
+        {
+          command: "wipeEngine",
+          args: ["history"],
+          flowID: Utils.makeGUID(),
+        },
+      ],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+  collection.insertRecord(
+    {
+      id: deviceCID,
+      name: "Device C",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    _("First sync. 2 records downloaded.");
+    strictEqual(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+    await syncClientsEngine(server);
+
+    _("Send command to client to wipe tab engine");
+    await engine.sendCommand("wipeEngine", ["tabs"], deviceBID);
+
+    const oldUploadOutgoing = SyncEngine.prototype._uploadOutgoing;
+    SyncEngine.prototype._uploadOutgoing = async () =>
+      engine._onRecordsWritten([], [deviceBID]);
+    await syncClientsEngine(server);
+
+    let deviceBPayload = collection.cleartext(deviceBID);
+    compareCommands(
+      deviceBPayload.commands,
+      [
+        {
+          command: "wipeEngine",
+          args: ["history"],
+        },
+      ],
+      "Should be the same because the upload failed"
+    );
+
+    _("Simulate the client B consuming the command and syncing to the server");
+    collection.insertRecord(
+      {
+        id: deviceBID,
+        name: "Device B",
+        type: "desktop",
+        commands: [],
+        version: "48",
+        protocols: ["1.5"],
+      },
+      now - 10
+    );
+
+    // Simulate reboot
+    SyncEngine.prototype._uploadOutgoing = oldUploadOutgoing;
+    engine = Service.clientsEngine = new ClientEngine(Service);
+    await engine.initialize();
+
+    await syncClientsEngine(server);
+
+    deviceBPayload = collection.cleartext(deviceBID);
+    compareCommands(
+      deviceBPayload.commands,
+      [
+        {
+          command: "wipeEngine",
+          args: ["tabs"],
+        },
+      ],
+      "Should only had written our outgoing command"
+    );
+  } finally {
+    await cleanup();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_keep_cleared_commands_after_reboot() {
+  _(
+    "Download commands, fail upload, reboot, then apply new commands (bug 1289287)"
+  );
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let collection = server.getCollection("foo", "clients");
+
+  let deviceBID = Utils.makeGUID();
+  let deviceCID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: engine.localID,
+      name: "Device A",
+      type: "desktop",
+      commands: [
+        {
+          command: "wipeEngine",
+          args: ["history"],
+          flowID: Utils.makeGUID(),
+        },
+        {
+          command: "wipeEngine",
+          args: ["tabs"],
+          flowID: Utils.makeGUID(),
+        },
+      ],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+  collection.insertRecord(
+    {
+      id: deviceBID,
+      name: "Device B",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+  collection.insertRecord(
+    {
+      id: deviceCID,
+      name: "Device C",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    _("First sync. Download remote and our record.");
+    strictEqual(engine.lastRecordUpload, 0);
+    ok(engine.isFirstSync);
+
+    const oldUploadOutgoing = SyncEngine.prototype._uploadOutgoing;
+    SyncEngine.prototype._uploadOutgoing = async () =>
+      engine._onRecordsWritten([], [deviceBID]);
+    let commandsProcessed = 0;
+    engine.service.wipeClient = _engine => {
+      commandsProcessed++;
+    };
+
+    await syncClientsEngine(server);
+    await engine.processIncomingCommands(); // Not called by the engine.sync(), gotta call it ourselves
+    equal(commandsProcessed, 2, "We processed 2 commands");
+
+    let localRemoteRecord = collection.cleartext(engine.localID);
+    compareCommands(
+      localRemoteRecord.commands,
+      [
+        {
+          command: "wipeEngine",
+          args: ["history"],
+        },
+        {
+          command: "wipeEngine",
+          args: ["tabs"],
+        },
+      ],
+      "Should be the same because the upload failed"
+    );
+
+    // Another client sends a wipe command
+    collection.insertRecord(
+      {
+        id: engine.localID,
+        name: "Device A",
+        type: "desktop",
+        commands: [
+          {
+            command: "wipeEngine",
+            args: ["history"],
+            flowID: Utils.makeGUID(),
+          },
+          {
+            command: "wipeEngine",
+            args: ["tabs"],
+            flowID: Utils.makeGUID(),
+          },
+          {
+            command: "wipeEngine",
+            args: ["bookmarks"],
+            flowID: Utils.makeGUID(),
+          },
+        ],
+        version: "48",
+        protocols: ["1.5"],
+      },
+      now - 5
+    );
+
+    // Simulate reboot
+    SyncEngine.prototype._uploadOutgoing = oldUploadOutgoing;
+    engine = Service.clientsEngine = new ClientEngine(Service);
+    await engine.initialize();
+
+    commandsProcessed = 0;
+    engine.service.wipeClient = _engine => {
+      commandsProcessed++;
+    };
+    await syncClientsEngine(server);
+    await engine.processIncomingCommands();
+    equal(
+      commandsProcessed,
+      1,
+      "We processed one command (the other were cleared)"
+    );
+
+    localRemoteRecord = collection.cleartext(deviceBID);
+    deepEqual(localRemoteRecord.commands, [], "Should be empty");
+  } finally {
+    await cleanup();
+
+    // Reset service (remove mocks)
+    engine = Service.clientsEngine = new ClientEngine(Service);
+    await engine.initialize();
+    await engine._resetClient();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_deleted_commands() {
+  _("Verifies commands for a deleted client are discarded");
+
+  let now = new_timestamp();
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let collection = server.getCollection("foo", "clients");
+
+  let activeID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: activeID,
+      name: "Active client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  let deletedID = Utils.makeGUID();
+  collection.insertRecord(
+    {
+      id: deletedID,
+      name: "Client to delete",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    },
+    now - 10
+  );
+
+  try {
+    _("First sync. 2 records downloaded.");
+    await syncClientsEngine(server);
+
+    _("Delete a record on the server.");
+    collection.remove(deletedID);
+
+    _("Broadcast a command to all clients");
+    await engine.sendCommand("logout", []);
+    await syncClientsEngine(server);
+
+    deepEqual(
+      collection.keys().sort(),
+      [activeID, engine.localID].sort(),
+      "Should not reupload deleted clients"
+    );
+
+    let activePayload = collection.cleartext(activeID);
+    compareCommands(
+      activePayload.commands,
+      [{ command: "logout", args: [] }],
+      "Should send the command to the active client"
+    );
+  } finally {
+    await cleanup();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_command_sync() {
+  _("Notify other clients when writing their record.");
+
+  await engine._store.wipe();
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.getCollection("foo", "clients");
+  let remoteId = Utils.makeGUID();
+  let remoteId2 = Utils.makeGUID();
+
+  _("Create remote client record 1");
+  collection.insertRecord({
+    id: remoteId,
+    name: "Remote client",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    protocols: ["1.5"],
+  });
+
+  _("Create remote client record 2");
+  collection.insertRecord({
+    id: remoteId2,
+    name: "Remote client 2",
+    type: "mobile",
+    commands: [],
+    version: "48",
+    protocols: ["1.5"],
+  });
+
+  try {
+    equal(collection.count(), 2, "2 remote records written");
+    await syncClientsEngine(server);
+    equal(
+      collection.count(),
+      3,
+      "3 remote records written (+1 for the synced local record)"
+    );
+
+    await engine.sendCommand("wipeEngine", ["tabs"]);
+    await engine._tracker.addChangedID(engine.localID);
+    const getClientFxaDeviceId = sinon
+      .stub(engine, "getClientFxaDeviceId")
+      .callsFake(id => "fxa-" + id);
+    const engineMock = sinon.mock(engine);
+    let _notifyCollectionChanged = engineMock
+      .expects("_notifyCollectionChanged")
+      .withArgs(["fxa-" + remoteId, "fxa-" + remoteId2]);
+    _("Syncing.");
+    await syncClientsEngine(server);
+    _notifyCollectionChanged.verify();
+
+    engineMock.restore();
+    getClientFxaDeviceId.restore();
+  } finally {
+    await cleanup();
+    await engine._tracker.clearChangedIDs();
+
+    try {
+      server.deleteCollections("foo");
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function ensureSameFlowIDs() {
+  let events = [];
+  let origRecordTelemetryEvent = Service.recordTelemetryEvent;
+  Service.recordTelemetryEvent = (object, method, value, extra) => {
+    events.push({ object, method, value, extra });
+  };
+
+  let server = await serverForFoo(engine);
+  try {
+    // Setup 2 clients, send them a command, and ensure we get to events
+    // written, both with the same flowID.
+    await SyncTestingInfrastructure(server);
+    let collection = server.getCollection("foo", "clients");
+
+    let remoteId = Utils.makeGUID();
+    let remoteId2 = Utils.makeGUID();
+
+    _("Create remote client record 1");
+    collection.insertRecord({
+      id: remoteId,
+      name: "Remote client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    });
+
+    _("Create remote client record 2");
+    collection.insertRecord({
+      id: remoteId2,
+      name: "Remote client 2",
+      type: "mobile",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    });
+
+    await syncClientsEngine(server);
+    await engine.sendCommand("wipeEngine", ["tabs"]);
+    await syncClientsEngine(server);
+    equal(events.length, 2);
+    // we don't know what the flowID is, but do know it should be the same.
+    equal(events[0].extra.flowID, events[1].extra.flowID);
+    // Wipe remote clients to ensure deduping doesn't prevent us from adding the command.
+    for (let client of Object.values(engine._store._remoteClients)) {
+      client.commands = [];
+    }
+    // check it's correctly used when we specify a flow ID
+    events.length = 0;
+    let flowID = Utils.makeGUID();
+    await engine.sendCommand("wipeEngine", ["tabs"], null, { flowID });
+    await syncClientsEngine(server);
+    equal(events.length, 2);
+    equal(events[0].extra.flowID, flowID);
+    equal(events[1].extra.flowID, flowID);
+
+    // Wipe remote clients to ensure deduping doesn't prevent us from adding the command.
+    for (let client of Object.values(engine._store._remoteClients)) {
+      client.commands = [];
+    }
+
+    // and that it works when something else is in "extra"
+    events.length = 0;
+    await engine.sendCommand("wipeEngine", ["tabs"], null, {
+      reason: "testing",
+    });
+    await syncClientsEngine(server);
+    equal(events.length, 2);
+    equal(events[0].extra.flowID, events[1].extra.flowID);
+    equal(events[0].extra.reason, "testing");
+    equal(events[1].extra.reason, "testing");
+    // Wipe remote clients to ensure deduping doesn't prevent us from adding the command.
+    for (let client of Object.values(engine._store._remoteClients)) {
+      client.commands = [];
+    }
+
+    // and when both are specified.
+    events.length = 0;
+    await engine.sendCommand("wipeEngine", ["tabs"], null, {
+      reason: "testing",
+      flowID,
+    });
+    await syncClientsEngine(server);
+    equal(events.length, 2);
+    equal(events[0].extra.flowID, flowID);
+    equal(events[1].extra.flowID, flowID);
+    equal(events[0].extra.reason, "testing");
+    equal(events[1].extra.reason, "testing");
+    // Wipe remote clients to ensure deduping doesn't prevent us from adding the command.
+    for (let client of Object.values(engine._store._remoteClients)) {
+      client.commands = [];
+    }
+  } finally {
+    Service.recordTelemetryEvent = origRecordTelemetryEvent;
+    cleanup();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_duplicate_commands_telemetry() {
+  let events = [];
+  let origRecordTelemetryEvent = Service.recordTelemetryEvent;
+  Service.recordTelemetryEvent = (object, method, value, extra) => {
+    events.push({ object, method, value, extra });
+  };
+
+  let server = await serverForFoo(engine);
+  try {
+    await SyncTestingInfrastructure(server);
+    let collection = server.getCollection("foo", "clients");
+
+    let remoteId = Utils.makeGUID();
+    let remoteId2 = Utils.makeGUID();
+
+    _("Create remote client record 1");
+    collection.insertRecord({
+      id: remoteId,
+      name: "Remote client",
+      type: "desktop",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    });
+
+    _("Create remote client record 2");
+    collection.insertRecord({
+      id: remoteId2,
+      name: "Remote client 2",
+      type: "mobile",
+      commands: [],
+      version: "48",
+      protocols: ["1.5"],
+    });
+
+    await syncClientsEngine(server);
+    // Make sure deduping works before syncing
+    await engine.sendCommand("wipeEngine", ["history"], remoteId);
+    await engine.sendCommand("wipeEngine", ["history"], remoteId);
+    equal(events.length, 1);
+    await syncClientsEngine(server);
+    // And after syncing.
+    await engine.sendCommand("wipeEngine", ["history"], remoteId);
+    equal(events.length, 1);
+    // Ensure we aren't deduping commands to different clients
+    await engine.sendCommand("wipeEngine", ["history"], remoteId2);
+    equal(events.length, 2);
+  } finally {
+    Service.recordTelemetryEvent = origRecordTelemetryEvent;
+    cleanup();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_other_clients_notified_on_first_sync() {
+  _(
+    "Ensure that other clients are notified when we upload our client record for the first time."
+  );
+
+  await engine.resetLastSync();
+  await engine._store.wipe();
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  const fxAccounts = engine.fxAccounts;
+  let calls = 0;
+  engine.fxAccounts = {
+    device: {
+      getLocalId() {
+        return fxAccounts.device.getLocalId();
+      },
+      getLocalName() {
+        return fxAccounts.device.getLocalName();
+      },
+      getLocalType() {
+        return fxAccounts.device.getLocalType();
+      },
+    },
+    notifyDevices() {
+      calls++;
+      return Promise.resolve(true);
+    },
+    _internal: {
+      now() {
+        return Date.now();
+      },
+    },
+  };
+
+  try {
+    engine.lastRecordUpload = 0;
+    _("First sync, should notify other clients");
+    await syncClientsEngine(server);
+    equal(calls, 1);
+
+    _("Second sync, should not notify other clients");
+    await syncClientsEngine(server);
+    equal(calls, 1);
+  } finally {
+    engine.fxAccounts = fxAccounts;
+    cleanup();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(
+  async function device_disconnected_notification_updates_known_stale_clients() {
+    const spyUpdate = sinon.spy(engine, "updateKnownStaleClients");
+
+    Services.obs.notifyObservers(
+      null,
+      "fxaccounts:device_disconnected",
+      JSON.stringify({ isLocalDevice: false })
+    );
+    ok(spyUpdate.calledOnce, "updateKnownStaleClients should be called");
+    spyUpdate.resetHistory();
+
+    Services.obs.notifyObservers(
+      null,
+      "fxaccounts:device_disconnected",
+      JSON.stringify({ isLocalDevice: true })
+    );
+    ok(spyUpdate.notCalled, "updateKnownStaleClients should not be called");
+
+    spyUpdate.restore();
+  }
+);
+
+add_task(async function update_known_stale_clients() {
+  const makeFakeClient = id => ({ id, fxaDeviceId: `fxa-${id}` });
+  const clients = [
+    makeFakeClient("one"),
+    makeFakeClient("two"),
+    makeFakeClient("three"),
+  ];
+  const stubRemoteClients = sinon
+    .stub(engine._store, "_remoteClients")
+    .get(() => {
+      return clients;
+    });
+  const stubFetchFxADevices = sinon
+    .stub(engine, "_fetchFxADevices")
+    .callsFake(() => {
+      engine._knownStaleFxADeviceIds = ["fxa-one", "fxa-two"];
+    });
+
+  engine._knownStaleFxADeviceIds = null;
+  await engine.updateKnownStaleClients();
+  ok(clients[0].stale);
+  ok(clients[1].stale);
+  ok(!clients[2].stale);
+
+  stubRemoteClients.restore();
+  stubFetchFxADevices.restore();
+});
+
+add_task(async function test_create_record_command_limit() {
+  await engine._store.wipe();
+  await generateNewKeys(Service.collectionKeys);
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  const fakeLimit = 4 * 1024;
+
+  let maxSizeStub = sinon
+    .stub(Service, "getMemcacheMaxRecordPayloadSize")
+    .callsFake(() => fakeLimit);
+
+  let user = server.user("foo");
+  let remoteId = Utils.makeGUID();
+
+  _("Create remote client record");
+  user.collection("clients").insertRecord({
+    id: remoteId,
+    name: "Remote client",
+    type: "desktop",
+    commands: [],
+    version: "57",
+    protocols: ["1.5"],
+  });
+
+  try {
+    _("Initial sync.");
+    await syncClientsEngine(server);
+
+    _("Send a fairly sane number of commands.");
+
+    for (let i = 0; i < 5; ++i) {
+      await engine.sendCommand("wipeEngine", [`history: ${i}`], remoteId);
+    }
+
+    await syncClientsEngine(server);
+
+    _("Make sure they all fit and weren't dropped.");
+    let parsedServerRecord = user.collection("clients").cleartext(remoteId);
+
+    equal(parsedServerRecord.commands.length, 5);
+
+    await engine.sendCommand("wipeEngine", ["history"], remoteId);
+
+    _("Send a not-sane number of commands.");
+    // Much higher than the maximum number of commands we could actually fit.
+    for (let i = 0; i < 500; ++i) {
+      await engine.sendCommand("wipeEngine", [`tabs: ${i}`], remoteId);
+    }
+
+    await syncClientsEngine(server);
+
+    _("Ensure we didn't overflow the server limit.");
+    let wbo = user.collection("clients").wbo(remoteId);
+    less(wbo.payload.length, fakeLimit);
+
+    _(
+      "And that the data we uploaded is both sane json and containing some commands."
+    );
+    let remoteCommands = wbo.getCleartext().commands;
+    greater(remoteCommands.length, 2);
+    let firstCommand = remoteCommands[0];
+    _(
+      "The first command should still be present, since it had a high priority"
+    );
+    equal(firstCommand.command, "wipeEngine");
+    _("And the last command in the list should be the last command we sent.");
+    let lastCommand = remoteCommands[remoteCommands.length - 1];
+    equal(lastCommand.command, "wipeEngine");
+    deepEqual(lastCommand.args, ["tabs: 499"]);
+  } finally {
+    maxSizeStub.restore();
+    await cleanup();
+    try {
+      let collection = server.getCollection("foo", "clients");
+      collection.remove(remoteId);
+    } finally {
+      await promiseStopServer(server);
+    }
+  }
+});
diff --git a/services/sync/tests/unit/test_clients_escape.js b/services/sync/tests/unit/test_clients_escape.js
new file mode 100644
index 0000000000..53ec7fd7a9
--- /dev/null
+++ b/services/sync/tests/unit/test_clients_escape.js
@@ -0,0 +1,57 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function test_clients_escape() {
+  _("Set up test fixtures.");
+
+  await configureIdentity();
+  let keyBundle = Service.identity.syncKeyBundle;
+
+  let engine = Service.clientsEngine;
+
+  try {
+    _("Test that serializing client records results in uploadable ascii");
+    engine.localID = "ascii";
+    engine.localName = "wéävê";
+
+    _("Make sure we have the expected record");
+    let record = await engine._createRecord("ascii");
+    Assert.equal(record.id, "ascii");
+    Assert.equal(record.name, "wéävê");
+
+    _("Encrypting record...");
+    await record.encrypt(keyBundle);
+    _("Encrypted.");
+
+    let serialized = JSON.stringify(record);
+    let checkCount = 0;
+    _("Checking for all ASCII:", serialized);
+    for (let ch of serialized) {
+      let code = ch.charCodeAt(0);
+      _("Checking asciiness of '", ch, "'=", code);
+      Assert.ok(code < 128);
+      checkCount++;
+    }
+
+    _("Processed", checkCount, "characters out of", serialized.length);
+    Assert.equal(checkCount, serialized.length);
+
+    _("Making sure the record still looks like it did before");
+    await record.decrypt(keyBundle);
+    Assert.equal(record.id, "ascii");
+    Assert.equal(record.name, "wéävê");
+
+    _("Sanity check that creating the record also gives the same");
+    record = await engine._createRecord("ascii");
+    Assert.equal(record.id, "ascii");
+    Assert.equal(record.name, "wéävê");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
diff --git a/services/sync/tests/unit/test_collection_getBatched.js b/services/sync/tests/unit/test_collection_getBatched.js
new file mode 100644
index 0000000000..f5425abe92
--- /dev/null
+++ b/services/sync/tests/unit/test_collection_getBatched.js
@@ -0,0 +1,187 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Collection, WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function recordRange(lim, offset, total) {
+  let res = [];
+  for (let i = offset; i < Math.min(lim + offset, total); ++i) {
+    res.push({ id: String(i), payload: "test:" + i });
+  }
+  return res;
+}
+
+function get_test_collection_info({
+  totalRecords,
+  batchSize,
+  lastModified,
+  throwAfter = Infinity,
+  interruptedAfter = Infinity,
+}) {
+  let coll = new Collection("http://example.com/test/", WBORecord, Service);
+  coll.full = true;
+  let requests = [];
+  let responses = [];
+  coll.get = async function () {
+    let limit = +this.limit;
+    let offset = 0;
+    if (this.offset) {
+      equal(this.offset.slice(0, 6), "foobar");
+      offset = +this.offset.slice(6);
+    }
+    requests.push({
+      limit,
+      offset,
+      spec: this.spec,
+      headers: Object.assign({}, this.headers),
+    });
+    if (--throwAfter === 0) {
+      throw new Error("Some Network Error");
+    }
+    let body = recordRange(limit, offset, totalRecords);
+    let response = {
+      obj: body,
+      success: true,
+      status: 200,
+      headers: {},
+    };
+    if (--interruptedAfter === 0) {
+      response.success = false;
+      response.status = 412;
+      response.body = "";
+    } else if (offset + limit < totalRecords) {
+      // Ensure we're treating this as an opaque string, since the docs say
+      // it might not be numeric.
+      response.headers["x-weave-next-offset"] = "foobar" + (offset + batchSize);
+    }
+    response.headers["x-last-modified"] = lastModified;
+    responses.push(response);
+    return response;
+  };
+  return { responses, requests, coll };
+}
+
+add_task(async function test_success() {
+  const totalRecords = 11;
+  const batchSize = 2;
+  const lastModified = "111111";
+  let { responses, requests, coll } = get_test_collection_info({
+    totalRecords,
+    batchSize,
+    lastModified,
+  });
+  let { response, records } = await coll.getBatched(batchSize);
+
+  equal(requests.length, Math.ceil(totalRecords / batchSize));
+
+  equal(records.length, totalRecords);
+  checkRecordsOrder(records);
+
+  // ensure we're returning the last response
+  equal(responses[responses.length - 1], response);
+
+  // check first separately since its a bit of a special case
+  ok(!requests[0].headers["x-if-unmodified-since"]);
+  ok(!requests[0].offset);
+  equal(requests[0].limit, batchSize);
+  let expectedOffset = 2;
+  for (let i = 1; i < requests.length; ++i) {
+    let req = requests[i];
+    equal(req.headers["x-if-unmodified-since"], lastModified);
+    equal(req.limit, batchSize);
+    if (i !== requests.length - 1) {
+      equal(req.offset, expectedOffset);
+    }
+
+    expectedOffset += batchSize;
+  }
+
+  // ensure we cleaned up anything that would break further
+  // use of this collection.
+  ok(!coll._headers["x-if-unmodified-since"]);
+  ok(!coll.offset);
+  ok(!coll.limit || coll.limit == Infinity);
+});
+
+add_task(async function test_total_limit() {
+  _("getBatched respects the (initial) value of the limit property");
+  const totalRecords = 100;
+  const recordLimit = 11;
+  const batchSize = 2;
+  const lastModified = "111111";
+  let { requests, coll } = get_test_collection_info({
+    totalRecords,
+    batchSize,
+    lastModified,
+  });
+  coll.limit = recordLimit;
+  let { records } = await coll.getBatched(batchSize);
+  checkRecordsOrder(records);
+
+  equal(requests.length, Math.ceil(recordLimit / batchSize));
+  equal(records.length, recordLimit);
+
+  for (let i = 0; i < requests.length; ++i) {
+    let req = requests[i];
+    if (i !== requests.length - 1) {
+      equal(req.limit, batchSize);
+    } else {
+      equal(req.limit, recordLimit % batchSize);
+    }
+  }
+
+  equal(coll._limit, recordLimit);
+});
+
+add_task(async function test_412() {
+  _("We shouldn't record records if we get a 412 in the middle of a batch");
+  const totalRecords = 11;
+  const batchSize = 2;
+  const lastModified = "111111";
+  let { responses, requests, coll } = get_test_collection_info({
+    totalRecords,
+    batchSize,
+    lastModified,
+    interruptedAfter: 3,
+  });
+  let { response, records } = await coll.getBatched(batchSize);
+
+  equal(requests.length, 3);
+  equal(records.length, 0); // we should not get any records
+
+  // ensure we're returning the last response
+  equal(responses[responses.length - 1], response);
+
+  ok(!response.success);
+  equal(response.status, 412);
+});
+
+add_task(async function test_get_throws() {
+  _("getBatched() should throw if a get() throws");
+  const totalRecords = 11;
+  const batchSize = 2;
+  const lastModified = "111111";
+  let { requests, coll } = get_test_collection_info({
+    totalRecords,
+    batchSize,
+    lastModified,
+    throwAfter: 3,
+  });
+
+  await Assert.rejects(coll.getBatched(batchSize), /Some Network Error/);
+
+  equal(requests.length, 3);
+});
+
+function checkRecordsOrder(records) {
+  ok(!!records.length);
+  for (let i = 0; i < records.length; i++) {
+    equal(records[i].id, String(i));
+    equal(records[i].payload, "test:" + i);
+  }
+}
diff --git a/services/sync/tests/unit/test_collections_recovery.js b/services/sync/tests/unit/test_collections_recovery.js
new file mode 100644
index 0000000000..fd923bc272
--- /dev/null
+++ b/services/sync/tests/unit/test_collections_recovery.js
@@ -0,0 +1,95 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Verify that we wipe the server if we have to regenerate keys.
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function test_missing_crypto_collection() {
+  enableValidationPrefs();
+
+  let johnHelper = track_collections_helper();
+  let johnU = johnHelper.with_updated_collection;
+  let johnColls = johnHelper.collections;
+
+  let empty = false;
+  function maybe_empty(handler) {
+    return function (request, response) {
+      if (empty) {
+        let body = "{}";
+        response.setStatusLine(request.httpVersion, 200, "OK");
+        response.bodyOutputStream.write(body, body.length);
+      } else {
+        handler(request, response);
+      }
+    };
+  }
+
+  let handlers = {
+    "/1.1/johndoe/info/collections": maybe_empty(johnHelper.handler),
+    "/1.1/johndoe/storage/crypto/keys": johnU(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe/storage/meta/global": johnU(
+      "meta",
+      new ServerWBO("global").handler()
+    ),
+  };
+  let collections = [
+    "clients",
+    "bookmarks",
+    "forms",
+    "history",
+    "passwords",
+    "prefs",
+    "tabs",
+  ];
+  // Disable addon sync because AddonManager won't be initialized here.
+  await Service.engineManager.unregister("addons");
+  await Service.engineManager.unregister("extension-storage");
+
+  for (let coll of collections) {
+    handlers["/1.1/johndoe/storage/" + coll] = johnU(
+      coll,
+      new ServerCollection({}, true).handler()
+    );
+  }
+  let server = httpd_setup(handlers);
+  await configureIdentity({ username: "johndoe" }, server);
+
+  try {
+    let fresh = 0;
+    let orig = Service._freshStart;
+    Service._freshStart = async function () {
+      _("Called _freshStart.");
+      await orig.call(Service);
+      fresh++;
+    };
+
+    _("Startup, no meta/global: freshStart called once.");
+    await sync_and_validate_telem();
+    Assert.equal(fresh, 1);
+    fresh = 0;
+
+    _("Regular sync: no need to freshStart.");
+    await Service.sync();
+    Assert.equal(fresh, 0);
+
+    _("Simulate a bad info/collections.");
+    delete johnColls.crypto;
+    await sync_and_validate_telem();
+    Assert.equal(fresh, 1);
+    fresh = 0;
+
+    _("Regular sync: no need to freshStart.");
+    await sync_and_validate_telem();
+    Assert.equal(fresh, 0);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_corrupt_keys.js b/services/sync/tests/unit/test_corrupt_keys.js
new file mode 100644
index 0000000000..06d7335985
--- /dev/null
+++ b/services/sync/tests/unit/test_corrupt_keys.js
@@ -0,0 +1,248 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Weave } = ChromeUtils.importESModule(
+  "resource://services-sync/main.sys.mjs"
+);
+const { HistoryEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/history.sys.mjs"
+);
+const { CryptoWrapper, WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function test_locally_changed_keys() {
+  enableValidationPrefs();
+
+  let hmacErrorCount = 0;
+  function counting(f) {
+    return async function () {
+      hmacErrorCount++;
+      return f.call(this);
+    };
+  }
+
+  Service.handleHMACEvent = counting(Service.handleHMACEvent);
+
+  let server = new SyncServer();
+  let johndoe = server.registerUser("johndoe", "password");
+  johndoe.createContents({
+    meta: {},
+    crypto: {},
+    clients: {},
+  });
+  server.start();
+
+  try {
+    Svc.PrefBranch.setStringPref("registerEngines", "Tab");
+
+    await configureIdentity({ username: "johndoe" }, server);
+    // We aren't doing a .login yet, so fudge the cluster URL.
+    Service.clusterURL = Service.identity._token.endpoint;
+
+    await Service.engineManager.register(HistoryEngine);
+    // Disable addon sync because AddonManager won't be initialized here.
+    await Service.engineManager.unregister("addons");
+    await Service.engineManager.unregister("extension-storage");
+
+    async function corrupt_local_keys() {
+      Service.collectionKeys._default.keyPair = [
+        await Weave.Crypto.generateRandomKey(),
+        await Weave.Crypto.generateRandomKey(),
+      ];
+    }
+
+    _("Setting meta.");
+
+    // Bump version on the server.
+    let m = new WBORecord("meta", "global");
+    m.payload = {
+      syncID: "foooooooooooooooooooooooooo",
+      storageVersion: STORAGE_VERSION,
+    };
+    await m.upload(Service.resource(Service.metaURL));
+
+    _(
+      "New meta/global: " +
+        JSON.stringify(johndoe.collection("meta").wbo("global"))
+    );
+
+    // Upload keys.
+    await generateNewKeys(Service.collectionKeys);
+    let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+    await serverKeys.encrypt(Service.identity.syncKeyBundle);
+    Assert.ok(
+      (await serverKeys.upload(Service.resource(Service.cryptoKeysURL))).success
+    );
+
+    // Check that login works.
+    Assert.ok(await Service.login());
+    Assert.ok(Service.isLoggedIn);
+
+    // Sync should upload records.
+    await sync_and_validate_telem();
+
+    // Tabs exist.
+    _("Tabs modified: " + johndoe.modified("tabs"));
+    Assert.ok(johndoe.modified("tabs") > 0);
+
+    // Let's create some server side history records.
+    let liveKeys = Service.collectionKeys.keyForCollection("history");
+    _("Keys now: " + liveKeys.keyPair);
+    let visitType = Ci.nsINavHistoryService.TRANSITION_LINK;
+    let history = johndoe.createCollection("history");
+    for (let i = 0; i < 5; i++) {
+      let id = "record-no--" + i;
+      let modified = Date.now() / 1000 - 60 * (i + 10);
+
+      let w = new CryptoWrapper("history", "id");
+      w.cleartext = {
+        id,
+        histUri: "http://foo/bar?" + id,
+        title: id,
+        sortindex: i,
+        visits: [{ date: (modified - 5) * 1000000, type: visitType }],
+        deleted: false,
+      };
+      await w.encrypt(liveKeys);
+
+      let payload = { ciphertext: w.ciphertext, IV: w.IV, hmac: w.hmac };
+      history.insert(id, payload, modified);
+    }
+
+    history.timestamp = Date.now() / 1000;
+    let old_key_time = johndoe.modified("crypto");
+    _("Old key time: " + old_key_time);
+
+    // Check that we can decrypt one.
+    let rec = new CryptoWrapper("history", "record-no--0");
+    await rec.fetch(
+      Service.resource(Service.storageURL + "history/record-no--0")
+    );
+    _(JSON.stringify(rec));
+    Assert.ok(!!(await rec.decrypt(liveKeys)));
+
+    Assert.equal(hmacErrorCount, 0);
+
+    // Fill local key cache with bad data.
+    await corrupt_local_keys();
+    _(
+      "Keys now: " + Service.collectionKeys.keyForCollection("history").keyPair
+    );
+
+    Assert.equal(hmacErrorCount, 0);
+
+    _("HMAC error count: " + hmacErrorCount);
+    // Now syncing should succeed, after one HMAC error.
+    await sync_and_validate_telem(ping => {
+      Assert.equal(
+        ping.engines.find(e => e.name == "history").incoming.applied,
+        5
+      );
+    });
+
+    Assert.equal(hmacErrorCount, 1);
+    _(
+      "Keys now: " + Service.collectionKeys.keyForCollection("history").keyPair
+    );
+
+    // And look! We downloaded history!
+    Assert.ok(
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--0")
+    );
+    Assert.ok(
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--1")
+    );
+    Assert.ok(
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--2")
+    );
+    Assert.ok(
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--3")
+    );
+    Assert.ok(
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--4")
+    );
+    Assert.equal(hmacErrorCount, 1);
+
+    _("Busting some new server values.");
+    // Now what happens if we corrupt the HMAC on the server?
+    for (let i = 5; i < 10; i++) {
+      let id = "record-no--" + i;
+      let modified = 1 + Date.now() / 1000;
+
+      let w = new CryptoWrapper("history", "id");
+      w.cleartext = {
+        id,
+        histUri: "http://foo/bar?" + id,
+        title: id,
+        sortindex: i,
+        visits: [{ date: (modified - 5) * 1000000, type: visitType }],
+        deleted: false,
+      };
+      await w.encrypt(Service.collectionKeys.keyForCollection("history"));
+      w.hmac = w.hmac.toUpperCase();
+
+      let payload = { ciphertext: w.ciphertext, IV: w.IV, hmac: w.hmac };
+      history.insert(id, payload, modified);
+    }
+    history.timestamp = Date.now() / 1000;
+
+    _("Server key time hasn't changed.");
+    Assert.equal(johndoe.modified("crypto"), old_key_time);
+
+    _("Resetting HMAC error timer.");
+    Service.lastHMACEvent = 0;
+
+    _("Syncing...");
+    await sync_and_validate_telem(ping => {
+      Assert.equal(
+        ping.engines.find(e => e.name == "history").incoming.failed,
+        5
+      );
+    });
+
+    _(
+      "Keys now: " + Service.collectionKeys.keyForCollection("history").keyPair
+    );
+    _(
+      "Server keys have been updated, and we skipped over 5 more HMAC errors without adjusting history."
+    );
+    Assert.ok(johndoe.modified("crypto") > old_key_time);
+    Assert.equal(hmacErrorCount, 6);
+    Assert.equal(
+      false,
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--5")
+    );
+    Assert.equal(
+      false,
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--6")
+    );
+    Assert.equal(
+      false,
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--7")
+    );
+    Assert.equal(
+      false,
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--8")
+    );
+    Assert.equal(
+      false,
+      await PlacesUtils.history.hasVisits("http://foo/bar?record-no--9")
+    );
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
+
+function run_test() {
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+  validate_all_future_pings();
+
+  run_next_test();
+}
diff --git a/services/sync/tests/unit/test_declined.js b/services/sync/tests/unit/test_declined.js
new file mode 100644
index 0000000000..af7f8eb8c5
--- /dev/null
+++ b/services/sync/tests/unit/test_declined.js
@@ -0,0 +1,194 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { DeclinedEngines } = ChromeUtils.importESModule(
+  "resource://services-sync/stages/declined.sys.mjs"
+);
+const { EngineSynchronizer } = ChromeUtils.importESModule(
+  "resource://services-sync/stages/enginesync.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Observers } = ChromeUtils.importESModule(
+  "resource://services-common/observers.sys.mjs"
+);
+
+function PetrolEngine() {}
+PetrolEngine.prototype.name = "petrol";
+
+function DieselEngine() {}
+DieselEngine.prototype.name = "diesel";
+
+function DummyEngine() {}
+DummyEngine.prototype.name = "dummy";
+
+function ActualEngine() {}
+ActualEngine.prototype.name = "actual";
+Object.setPrototypeOf(ActualEngine.prototype, SyncEngine.prototype);
+
+function getEngineManager() {
+  let manager = new EngineManager(Service);
+  Service.engineManager = manager;
+  manager._engines = {
+    petrol: new PetrolEngine(),
+    diesel: new DieselEngine(),
+    dummy: new DummyEngine(),
+    actual: new ActualEngine(),
+  };
+  return manager;
+}
+
+/**
+ * 'Fetch' a meta/global record that doesn't mention declined.
+ *
+ * Push it into the EngineSynchronizer to set enabled; verify that those are
+ * correct.
+ *
+ * Then push it into DeclinedEngines to set declined; verify that none are
+ * declined, and a notification is sent for our locally disabled-but-not-
+ * declined engines.
+ */
+add_task(async function testOldMeta() {
+  let meta = {
+    payload: {
+      engines: {
+        petrol: 1,
+        diesel: 2,
+        nonlocal: 3, // Enabled but not supported.
+      },
+    },
+  };
+
+  _("Record: " + JSON.stringify(meta));
+
+  let manager = getEngineManager();
+
+  // Update enabled from meta/global.
+  let engineSync = new EngineSynchronizer(Service);
+  await engineSync._updateEnabledFromMeta(meta, 3, manager);
+
+  Assert.ok(manager._engines.petrol.enabled, "'petrol' locally enabled.");
+  Assert.ok(manager._engines.diesel.enabled, "'diesel' locally enabled.");
+  Assert.ok(
+    !("nonlocal" in manager._engines),
+    "We don't know anything about the 'nonlocal' engine."
+  );
+  Assert.ok(!manager._engines.actual.enabled, "'actual' not locally enabled.");
+  Assert.ok(!manager.isDeclined("actual"), "'actual' not declined, though.");
+
+  let declinedEngines = new DeclinedEngines(Service);
+
+  function onNotDeclined(subject, topic, data) {
+    Observers.remove("weave:engines:notdeclined", onNotDeclined);
+    Assert.ok(
+      subject.undecided.has("actual"),
+      "EngineManager observed that 'actual' was undecided."
+    );
+
+    let declined = manager.getDeclined();
+    _("Declined: " + JSON.stringify(declined));
+
+    Assert.ok(!meta.changed, "No need to upload a new meta/global.");
+    run_next_test();
+  }
+
+  Observers.add("weave:engines:notdeclined", onNotDeclined);
+
+  declinedEngines.updateDeclined(meta, manager);
+});
+
+/**
+ * 'Fetch' a meta/global that declines an engine we don't
+ * recognize. Ensure that we track that declined engine along
+ * with any we locally declined, and that the meta/global
+ * record is marked as changed and includes all declined
+ * engines.
+ */
+add_task(async function testDeclinedMeta() {
+  let meta = {
+    payload: {
+      engines: {
+        petrol: 1,
+        diesel: 2,
+        nonlocal: 3, // Enabled but not supported.
+      },
+      declined: ["nonexistent"], // Declined and not supported.
+    },
+  };
+
+  _("Record: " + JSON.stringify(meta));
+
+  let manager = getEngineManager();
+  manager._engines.petrol.enabled = true;
+  manager._engines.diesel.enabled = true;
+  manager._engines.dummy.enabled = true;
+  manager._engines.actual.enabled = false; // Disabled but not declined.
+
+  manager.decline(["localdecline"]); // Declined and not supported.
+
+  let declinedEngines = new DeclinedEngines(Service);
+
+  function onNotDeclined(subject, topic, data) {
+    Observers.remove("weave:engines:notdeclined", onNotDeclined);
+    Assert.ok(
+      subject.undecided.has("actual"),
+      "EngineManager observed that 'actual' was undecided."
+    );
+
+    let declined = manager.getDeclined();
+    _("Declined: " + JSON.stringify(declined));
+
+    Assert.equal(
+      declined.indexOf("actual"),
+      -1,
+      "'actual' is locally disabled, but not marked as declined."
+    );
+
+    Assert.equal(
+      declined.indexOf("clients"),
+      -1,
+      "'clients' is enabled and not remotely declined."
+    );
+    Assert.equal(
+      declined.indexOf("petrol"),
+      -1,
+      "'petrol' is enabled and not remotely declined."
+    );
+    Assert.equal(
+      declined.indexOf("diesel"),
+      -1,
+      "'diesel' is enabled and not remotely declined."
+    );
+    Assert.equal(
+      declined.indexOf("dummy"),
+      -1,
+      "'dummy' is enabled and not remotely declined."
+    );
+
+    Assert.ok(
+      0 <= declined.indexOf("nonexistent"),
+      "'nonexistent' was declined on the server."
+    );
+
+    Assert.ok(
+      0 <= declined.indexOf("localdecline"),
+      "'localdecline' was declined locally."
+    );
+
+    // The meta/global is modified, too.
+    Assert.ok(
+      0 <= meta.payload.declined.indexOf("nonexistent"),
+      "meta/global's declined contains 'nonexistent'."
+    );
+    Assert.ok(
+      0 <= meta.payload.declined.indexOf("localdecline"),
+      "meta/global's declined contains 'localdecline'."
+    );
+    Assert.strictEqual(true, meta.changed, "meta/global was changed.");
+  }
+
+  Observers.add("weave:engines:notdeclined", onNotDeclined);
+
+  declinedEngines.updateDeclined(meta, manager);
+});
diff --git a/services/sync/tests/unit/test_disconnect_shutdown.js b/services/sync/tests/unit/test_disconnect_shutdown.js
new file mode 100644
index 0000000000..0606c93a7d
--- /dev/null
+++ b/services/sync/tests/unit/test_disconnect_shutdown.js
@@ -0,0 +1,101 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { SyncDisconnect, SyncDisconnectInternal } = ChromeUtils.importESModule(
+  "resource://services-sync/SyncDisconnect.sys.mjs"
+);
+const { AsyncShutdown } = ChromeUtils.importESModule(
+  "resource://gre/modules/AsyncShutdown.sys.mjs"
+);
+const { PREF_LAST_FXA_USER } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccountsCommon.sys.mjs"
+);
+
+add_task(async function test_shutdown_blocker() {
+  let spySignout = sinon.stub(
+    SyncDisconnectInternal,
+    "doSyncAndAccountDisconnect"
+  );
+
+  // We don't need to check for the lock regularly as we end up aborting the wait.
+  SyncDisconnectInternal.lockRetryInterval = 1000;
+  // Force the retry count to a very large value - this test should never
+  // abort due to the retry count and we want the test to fail (aka timeout)
+  // should our abort code not work.
+  SyncDisconnectInternal.lockRetryCount = 10000;
+  // mock the "browser" sanitize function - it should not be called by
+  // this test.
+  let spyBrowser = sinon.stub(SyncDisconnectInternal, "doSanitizeBrowserData");
+  // mock Sync
+  let mockEngine1 = {
+    enabled: true,
+    name: "Test Engine 1",
+    wipeClient: sinon.spy(),
+  };
+  let mockEngine2 = {
+    enabled: false,
+    name: "Test Engine 2",
+    wipeClient: sinon.spy(),
+  };
+
+  // This weave mock never gives up the lock.
+  let Weave = {
+    Service: {
+      enabled: true,
+      lock: () => false, // so we never get the lock.
+      unlock: sinon.spy(),
+
+      engineManager: {
+        getAll: sinon.stub().returns([mockEngine1, mockEngine2]),
+      },
+      errorHandler: {
+        resetFileLog: sinon.spy(),
+      },
+    },
+  };
+  let weaveStub = sinon.stub(SyncDisconnectInternal, "getWeave");
+  weaveStub.returns(Weave);
+
+  Services.prefs.setStringPref(PREF_LAST_FXA_USER, "dGVzdEBleGFtcGxlLmNvbQ==");
+
+  let promiseDisconnected = SyncDisconnect.disconnect(true);
+
+  // Pretend we hit the shutdown blocker.
+  info("simulating quitApplicationGranted");
+  Services.prefs.setBoolPref("toolkit.asyncshutdown.testing", true);
+  AsyncShutdown.quitApplicationGranted._trigger();
+  Services.prefs.clearUserPref("toolkit.asyncshutdown.testing");
+
+  info("waiting for disconnect to complete");
+  await promiseDisconnected;
+
+  Assert.ok(
+    !Services.prefs.prefHasUserValue(PREF_LAST_FXA_USER),
+    "Should have reset different user warning pref"
+  );
+  Assert.equal(
+    Weave.Service.unlock.callCount,
+    0,
+    "should not have unlocked at the end"
+  );
+  Assert.ok(!Weave.Service.enabled, "Weave should be and remain disabled");
+  Assert.equal(
+    Weave.Service.errorHandler.resetFileLog.callCount,
+    1,
+    "should have reset the log"
+  );
+  Assert.equal(
+    mockEngine1.wipeClient.callCount,
+    1,
+    "enabled engine should have been wiped"
+  );
+  Assert.equal(
+    mockEngine2.wipeClient.callCount,
+    0,
+    "disabled engine should not have been wiped"
+  );
+  Assert.equal(spyBrowser.callCount, 1, "should not sanitize the browser");
+  Assert.equal(spySignout.callCount, 1, "should have signed out of FxA");
+});
diff --git a/services/sync/tests/unit/test_engine.js b/services/sync/tests/unit/test_engine.js
new file mode 100644
index 0000000000..31a08d5bc9
--- /dev/null
+++ b/services/sync/tests/unit/test_engine.js
@@ -0,0 +1,246 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Observers } = ChromeUtils.importESModule(
+  "resource://services-common/observers.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function SteamStore(engine) {
+  Store.call(this, "Steam", engine);
+  this.wasWiped = false;
+}
+SteamStore.prototype = {
+  async wipe() {
+    this.wasWiped = true;
+  },
+};
+Object.setPrototypeOf(SteamStore.prototype, Store.prototype);
+
+function SteamTracker(name, engine) {
+  LegacyTracker.call(this, name || "Steam", engine);
+}
+Object.setPrototypeOf(SteamTracker.prototype, LegacyTracker.prototype);
+
+function SteamEngine(name, service) {
+  SyncEngine.call(this, name, service);
+  this.wasReset = false;
+  this.wasSynced = false;
+}
+SteamEngine.prototype = {
+  _storeObj: SteamStore,
+  _trackerObj: SteamTracker,
+
+  async _resetClient() {
+    this.wasReset = true;
+  },
+
+  async _sync() {
+    this.wasSynced = true;
+  },
+};
+Object.setPrototypeOf(SteamEngine.prototype, SyncEngine.prototype);
+
+var engineObserver = {
+  topics: [],
+
+  observe(subject, topic, data) {
+    Assert.equal(data, "steam");
+    this.topics.push(topic);
+  },
+
+  reset() {
+    this.topics = [];
+  },
+};
+Observers.add("weave:engine:reset-client:start", engineObserver);
+Observers.add("weave:engine:reset-client:finish", engineObserver);
+Observers.add("weave:engine:wipe-client:start", engineObserver);
+Observers.add("weave:engine:wipe-client:finish", engineObserver);
+Observers.add("weave:engine:sync:start", engineObserver);
+Observers.add("weave:engine:sync:finish", engineObserver);
+
+async function cleanup(engine) {
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  engine.wasReset = false;
+  engine.wasSynced = false;
+  engineObserver.reset();
+  await engine._tracker.clearChangedIDs();
+  await engine.finalize();
+}
+
+add_task(async function test_members() {
+  _("Engine object members");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  Assert.equal(engine.Name, "Steam");
+  Assert.equal(engine.prefName, "steam");
+  Assert.ok(engine._store instanceof SteamStore);
+  Assert.ok(engine._tracker instanceof SteamTracker);
+});
+
+add_task(async function test_score() {
+  _("Engine.score corresponds to tracker.score and is readonly");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  Assert.equal(engine.score, 0);
+  engine._tracker.score += 5;
+  Assert.equal(engine.score, 5);
+
+  try {
+    engine.score = 10;
+  } catch (ex) {
+    // Setting an attribute that has a getter produces an error in
+    // Firefox <= 3.6 and is ignored in later versions.  Either way,
+    // the attribute's value won't change.
+  }
+  Assert.equal(engine.score, 5);
+});
+
+add_task(async function test_resetClient() {
+  _("Engine.resetClient calls _resetClient");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  Assert.ok(!engine.wasReset);
+
+  await engine.resetClient();
+  Assert.ok(engine.wasReset);
+  Assert.equal(engineObserver.topics[0], "weave:engine:reset-client:start");
+  Assert.equal(engineObserver.topics[1], "weave:engine:reset-client:finish");
+
+  await cleanup(engine);
+});
+
+add_task(async function test_invalidChangedIDs() {
+  _("Test that invalid changed IDs on disk don't end up live.");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  let tracker = engine._tracker;
+
+  await tracker._beforeSave();
+  await IOUtils.writeUTF8(tracker._storage.path, "5", {
+    tmpPath: tracker._storage.path + ".tmp",
+  });
+
+  ok(!tracker._storage.dataReady);
+  const changes = await tracker.getChangedIDs();
+  changes.placeholder = true;
+  deepEqual(
+    changes,
+    { placeholder: true },
+    "Accessing changed IDs should load changes from disk as a side effect"
+  );
+  ok(tracker._storage.dataReady);
+
+  Assert.ok(changes.placeholder);
+  await cleanup(engine);
+});
+
+add_task(async function test_wipeClient() {
+  _("Engine.wipeClient calls resetClient, wipes store, clears changed IDs");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  Assert.ok(!engine.wasReset);
+  Assert.ok(!engine._store.wasWiped);
+  Assert.ok(await engine._tracker.addChangedID("a-changed-id"));
+  let changes = await engine._tracker.getChangedIDs();
+  Assert.ok("a-changed-id" in changes);
+
+  await engine.wipeClient();
+  Assert.ok(engine.wasReset);
+  Assert.ok(engine._store.wasWiped);
+  changes = await engine._tracker.getChangedIDs();
+  Assert.equal(JSON.stringify(changes), "{}");
+  Assert.equal(engineObserver.topics[0], "weave:engine:wipe-client:start");
+  Assert.equal(engineObserver.topics[1], "weave:engine:reset-client:start");
+  Assert.equal(engineObserver.topics[2], "weave:engine:reset-client:finish");
+  Assert.equal(engineObserver.topics[3], "weave:engine:wipe-client:finish");
+
+  await cleanup(engine);
+});
+
+add_task(async function test_enabled() {
+  _("Engine.enabled corresponds to preference");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  try {
+    Assert.ok(!engine.enabled);
+    Svc.PrefBranch.setBoolPref("engine.steam", true);
+    Assert.ok(engine.enabled);
+
+    engine.enabled = false;
+    Assert.ok(!Svc.PrefBranch.getBoolPref("engine.steam"));
+  } finally {
+    await cleanup(engine);
+  }
+});
+
+add_task(async function test_sync() {
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  try {
+    _("Engine.sync doesn't call _sync if it's not enabled");
+    Assert.ok(!engine.enabled);
+    Assert.ok(!engine.wasSynced);
+    await engine.sync();
+
+    Assert.ok(!engine.wasSynced);
+
+    _("Engine.sync calls _sync if it's enabled");
+    engine.enabled = true;
+
+    await engine.sync();
+    Assert.ok(engine.wasSynced);
+    Assert.equal(engineObserver.topics[0], "weave:engine:sync:start");
+    Assert.equal(engineObserver.topics[1], "weave:engine:sync:finish");
+  } finally {
+    await cleanup(engine);
+  }
+});
+
+add_task(async function test_disabled_no_track() {
+  _("When an engine is disabled, its tracker is not tracking.");
+  let engine = new SteamEngine("Steam", Service);
+  await engine.initialize();
+  let tracker = engine._tracker;
+  Assert.equal(engine, tracker.engine);
+
+  Assert.ok(!engine.enabled);
+  Assert.ok(!tracker._isTracking);
+  let changes = await tracker.getChangedIDs();
+  do_check_empty(changes);
+
+  Assert.ok(!tracker.engineIsEnabled());
+  Assert.ok(!tracker._isTracking);
+  changes = await tracker.getChangedIDs();
+  do_check_empty(changes);
+
+  let promisePrefChangeHandled = Promise.withResolvers();
+  const origMethod = tracker.onEngineEnabledChanged;
+  tracker.onEngineEnabledChanged = async (...args) => {
+    await origMethod.apply(tracker, args);
+    promisePrefChangeHandled.resolve();
+  };
+
+  engine.enabled = true; // Also enables the tracker automatically.
+  await promisePrefChangeHandled.promise;
+  Assert.ok(tracker._isTracking);
+  changes = await tracker.getChangedIDs();
+  do_check_empty(changes);
+
+  await tracker.addChangedID("abcdefghijkl");
+  changes = await tracker.getChangedIDs();
+  Assert.ok(0 < changes.abcdefghijkl);
+  promisePrefChangeHandled = Promise.withResolvers();
+  Svc.PrefBranch.setBoolPref("engine." + engine.prefName, false);
+  await promisePrefChangeHandled.promise;
+  Assert.ok(!tracker._isTracking);
+  changes = await tracker.getChangedIDs();
+  do_check_empty(changes);
+
+  await cleanup(engine);
+});
diff --git a/services/sync/tests/unit/test_engine_abort.js b/services/sync/tests/unit/test_engine_abort.js
new file mode 100644
index 0000000000..f9bbf9d338
--- /dev/null
+++ b/services/sync/tests/unit/test_engine_abort.js
@@ -0,0 +1,79 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { RotaryEngine } = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/rotaryengine.sys.mjs"
+);
+
+add_task(async function test_processIncoming_abort() {
+  _(
+    "An abort exception, raised in applyIncoming, will abort _processIncoming."
+  );
+  let engine = new RotaryEngine(Service);
+
+  let collection = new ServerCollection();
+  let id = Utils.makeGUID();
+  let payload = encryptPayload({ id, denomination: "Record No. " + id });
+  collection.insert(id, payload);
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  _("Create some server data.");
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+  _("Fake applyIncoming to abort.");
+  engine._store.applyIncoming = async function (record) {
+    let ex = {
+      code: SyncEngine.prototype.eEngineAbortApplyIncoming,
+      cause: "Nooo",
+    };
+    _("Throwing: " + JSON.stringify(ex));
+    throw ex;
+  };
+
+  _("Trying _processIncoming. It will throw after aborting.");
+  let err;
+  try {
+    await engine._syncStartup();
+    await engine._processIncoming();
+  } catch (ex) {
+    err = ex;
+  }
+
+  Assert.equal(err, "Nooo");
+  err = undefined;
+
+  _("Trying engine.sync(). It will abort without error.");
+  try {
+    // This will quietly fail.
+    await engine.sync();
+  } catch (ex) {
+    err = ex;
+  }
+
+  Assert.equal(err, undefined);
+
+  await promiseStopServer(server);
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  Service.recordManager.clearCache();
+
+  await engine._tracker.clearChangedIDs();
+  await engine.finalize();
+});
diff --git a/services/sync/tests/unit/test_engine_changes_during_sync.js b/services/sync/tests/unit/test_engine_changes_during_sync.js
new file mode 100644
index 0000000000..891bea41ec
--- /dev/null
+++ b/services/sync/tests/unit/test_engine_changes_during_sync.js
@@ -0,0 +1,611 @@
+const { FormHistory } = ChromeUtils.importESModule(
+  "resource://gre/modules/FormHistory.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Bookmark, BookmarkFolder, BookmarkQuery } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/bookmarks.sys.mjs"
+);
+const { HistoryRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/history.sys.mjs"
+);
+const { FormRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/forms.sys.mjs"
+);
+const { LoginRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/passwords.sys.mjs"
+);
+const { PrefRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/prefs.sys.mjs"
+);
+
+const LoginInfo = Components.Constructor(
+  "@mozilla.org/login-manager/loginInfo;1",
+  Ci.nsILoginInfo,
+  "init"
+);
+
+/**
+ * We don't test the clients or tabs engines because neither has conflict
+ * resolution logic. The clients engine syncs twice per global sync, and
+ * custom conflict resolution logic for commands that doesn't use
+ * timestamps. Tabs doesn't have conflict resolution at all, since it's
+ * read-only.
+ */
+
+async function assertChildGuids(folderGuid, expectedChildGuids, message) {
+  let tree = await PlacesUtils.promiseBookmarksTree(folderGuid);
+  let childGuids = tree.children.map(child => child.guid);
+  deepEqual(childGuids, expectedChildGuids, message);
+}
+
+async function cleanup(engine, server) {
+  await engine._tracker.stop();
+  await engine._store.wipe();
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  Service.recordManager.clearCache();
+  await promiseStopServer(server);
+}
+
+add_task(async function test_history_change_during_sync() {
+  _("Ensure that we don't bump the score when applying history records.");
+
+  enableValidationPrefs();
+
+  let engine = Service.engineManager.get("history");
+  let server = await serverForEnginesWithKeys({ foo: "password" }, [engine]);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("history");
+
+  // Override `uploadOutgoing` to insert a record while we're applying
+  // changes. The tracker should ignore this change.
+  let uploadOutgoing = engine._uploadOutgoing;
+  engine._uploadOutgoing = async function () {
+    engine._uploadOutgoing = uploadOutgoing;
+    try {
+      await uploadOutgoing.call(this);
+    } finally {
+      _("Inserting local history visit");
+      await addVisit("during_sync");
+      await engine._tracker.asyncObserver.promiseObserversComplete();
+    }
+  };
+
+  engine._tracker.start();
+
+  try {
+    let remoteRec = new HistoryRec("history", "UrOOuzE5QM-e");
+    remoteRec.histUri = "http://getfirefox.com/";
+    remoteRec.title = "Get Firefox!";
+    remoteRec.visits = [
+      {
+        date: PlacesUtils.toPRTime(Date.now()),
+        type: PlacesUtils.history.TRANSITION_TYPED,
+      },
+    ];
+    collection.insert(remoteRec.id, encryptPayload(remoteRec.cleartext));
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score for visits added during sync"
+    );
+
+    equal(
+      collection.count(),
+      1,
+      "New local visit should not exist on server after first sync"
+    );
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score during second history sync"
+    );
+
+    equal(
+      collection.count(),
+      2,
+      "New local visit should exist on server after second sync"
+    );
+  } finally {
+    engine._uploadOutgoing = uploadOutgoing;
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_passwords_change_during_sync() {
+  _("Ensure that we don't bump the score when applying passwords.");
+
+  enableValidationPrefs();
+
+  let engine = Service.engineManager.get("passwords");
+  let server = await serverForEnginesWithKeys({ foo: "password" }, [engine]);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("passwords");
+
+  let uploadOutgoing = engine._uploadOutgoing;
+  engine._uploadOutgoing = async function () {
+    engine._uploadOutgoing = uploadOutgoing;
+    try {
+      await uploadOutgoing.call(this);
+    } finally {
+      _("Inserting local password");
+      let login = new LoginInfo(
+        "https://example.com",
+        "",
+        null,
+        "username",
+        "password",
+        "",
+        ""
+      );
+      await Services.logins.addLoginAsync(login);
+      await engine._tracker.asyncObserver.promiseObserversComplete();
+    }
+  };
+
+  engine._tracker.start();
+
+  try {
+    let remoteRec = new LoginRec(
+      "passwords",
+      "{765e3d6e-071d-d640-a83d-81a7eb62d3ed}"
+    );
+    remoteRec.formSubmitURL = "";
+    remoteRec.httpRealm = "";
+    remoteRec.hostname = "https://mozilla.org";
+    remoteRec.username = "username";
+    remoteRec.password = "sekrit";
+    remoteRec.timeCreated = Date.now();
+    remoteRec.timePasswordChanged = Date.now();
+    collection.insert(remoteRec.id, encryptPayload(remoteRec.cleartext));
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score for passwords added during first sync"
+    );
+
+    equal(
+      collection.count(),
+      1,
+      "New local password should not exist on server after first sync"
+    );
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score during second passwords sync"
+    );
+
+    equal(
+      collection.count(),
+      2,
+      "New local password should exist on server after second sync"
+    );
+  } finally {
+    engine._uploadOutgoing = uploadOutgoing;
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_prefs_change_during_sync() {
+  _("Ensure that we don't bump the score when applying prefs.");
+
+  const TEST_PREF = "test.duringSync";
+  // create a "control pref" for the pref we sync.
+  Services.prefs.setBoolPref("services.sync.prefs.sync.test.duringSync", true);
+
+  enableValidationPrefs();
+
+  let engine = Service.engineManager.get("prefs");
+  let server = await serverForEnginesWithKeys({ foo: "password" }, [engine]);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("prefs");
+
+  let uploadOutgoing = engine._uploadOutgoing;
+  engine._uploadOutgoing = async function () {
+    engine._uploadOutgoing = uploadOutgoing;
+    try {
+      await uploadOutgoing.call(this);
+    } finally {
+      _("Updating local pref value");
+      // Change the value of a synced pref.
+      Services.prefs.setStringPref(TEST_PREF, "hello");
+      await engine._tracker.asyncObserver.promiseObserversComplete();
+    }
+  };
+
+  engine._tracker.start();
+
+  try {
+    // All synced prefs are stored in a single record, so we'll only ever
+    // have one record on the server. This test just checks that we don't
+    // track or upload prefs changed during the sync.
+    let guid = CommonUtils.encodeBase64URL(Services.appinfo.ID);
+    let remoteRec = new PrefRec("prefs", guid);
+    remoteRec.value = {
+      [TEST_PREF]: "world",
+    };
+    collection.insert(remoteRec.id, encryptPayload(remoteRec.cleartext));
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score for prefs added during first sync"
+    );
+    let payloads = collection.payloads();
+    equal(
+      payloads.length,
+      1,
+      "Should not upload multiple prefs records after first sync"
+    );
+    equal(
+      payloads[0].value[TEST_PREF],
+      "world",
+      "Should not upload pref value changed during first sync"
+    );
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score during second prefs sync"
+    );
+    payloads = collection.payloads();
+    equal(
+      payloads.length,
+      1,
+      "Should not upload multiple prefs records after second sync"
+    );
+    equal(
+      payloads[0].value[TEST_PREF],
+      "hello",
+      "Should upload changed pref value during second sync"
+    );
+  } finally {
+    engine._uploadOutgoing = uploadOutgoing;
+    await cleanup(engine, server);
+    Services.prefs.clearUserPref(TEST_PREF);
+  }
+});
+
+add_task(async function test_forms_change_during_sync() {
+  _("Ensure that we don't bump the score when applying form records.");
+
+  enableValidationPrefs();
+
+  let engine = Service.engineManager.get("forms");
+  let server = await serverForEnginesWithKeys({ foo: "password" }, [engine]);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("forms");
+
+  let uploadOutgoing = engine._uploadOutgoing;
+  engine._uploadOutgoing = async function () {
+    engine._uploadOutgoing = uploadOutgoing;
+    try {
+      await uploadOutgoing.call(this);
+    } finally {
+      _("Inserting local form history entry");
+      await FormHistory.update([
+        {
+          op: "add",
+          fieldname: "favoriteDrink",
+          value: "cocoa",
+        },
+      ]);
+      await engine._tracker.asyncObserver.promiseObserversComplete();
+    }
+  };
+
+  engine._tracker.start();
+
+  try {
+    // Add an existing remote form history entry. We shouldn't bump the score when
+    // we apply this record.
+    let remoteRec = new FormRec("forms", "Tl9dHgmJSR6FkyxS");
+    remoteRec.name = "name";
+    remoteRec.value = "alice";
+    collection.insert(remoteRec.id, encryptPayload(remoteRec.cleartext));
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score for forms added during first sync"
+    );
+
+    equal(
+      collection.count(),
+      1,
+      "New local form should not exist on server after first sync"
+    );
+
+    await sync_engine_and_validate_telem(engine, true);
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should not bump global score during second forms sync"
+    );
+
+    equal(
+      collection.count(),
+      2,
+      "New local form should exist on server after second sync"
+    );
+  } finally {
+    engine._uploadOutgoing = uploadOutgoing;
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_bookmark_change_during_sync() {
+  _("Ensure that we track bookmark changes made during a sync.");
+
+  enableValidationPrefs();
+  let schedulerProto = Object.getPrototypeOf(Service.scheduler);
+  let syncThresholdDescriptor = Object.getOwnPropertyDescriptor(
+    schedulerProto,
+    "syncThreshold"
+  );
+  Object.defineProperty(Service.scheduler, "syncThreshold", {
+    // Trigger resync if any changes exist, rather than deciding based on the
+    // normal sync threshold.
+    get: () => 0,
+  });
+
+  let engine = Service.engineManager.get("bookmarks");
+  let server = await serverForEnginesWithKeys({ foo: "password" }, [engine]);
+  await SyncTestingInfrastructure(server);
+
+  // Already-tracked bookmarks that shouldn't be uploaded during the first sync.
+  let bzBmk = await PlacesUtils.bookmarks.insert({
+    parentGuid: PlacesUtils.bookmarks.menuGuid,
+    url: "https://bugzilla.mozilla.org/",
+    title: "Bugzilla",
+  });
+  _(`Bugzilla GUID: ${bzBmk.guid}`);
+
+  await PlacesTestUtils.setBookmarkSyncFields({
+    guid: bzBmk.guid,
+    syncChangeCounter: 0,
+    syncStatus: PlacesUtils.bookmarks.SYNC_STATUS.NORMAL,
+  });
+
+  let collection = server.user("foo").collection("bookmarks");
+
+  let bmk3; // New child of Folder 1, created locally during sync.
+
+  let uploadOutgoing = engine._uploadOutgoing;
+  engine._uploadOutgoing = async function () {
+    engine._uploadOutgoing = uploadOutgoing;
+    try {
+      await uploadOutgoing.call(this);
+    } finally {
+      _("Inserting bookmark into local store");
+      bmk3 = await PlacesUtils.bookmarks.insert({
+        parentGuid: folder1.guid,
+        url: "https://mozilla.org/",
+        title: "Mozilla",
+      });
+      await engine._tracker.asyncObserver.promiseObserversComplete();
+    }
+  };
+
+  // New bookmarks that should be uploaded during the first sync.
+  let folder1 = await PlacesUtils.bookmarks.insert({
+    type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+    title: "Folder 1",
+  });
+  _(`Folder GUID: ${folder1.guid}`);
+
+  let tbBmk = await PlacesUtils.bookmarks.insert({
+    parentGuid: folder1.guid,
+    url: "http://getthunderbird.com/",
+    title: "Get Thunderbird!",
+  });
+  _(`Thunderbird GUID: ${tbBmk.guid}`);
+
+  engine._tracker.start();
+
+  try {
+    let bmk2_guid = "get-firefox1"; // New child of Folder 1, created remotely.
+    let folder2_guid = "folder2-1111"; // New folder, created remotely.
+    let tagQuery_guid = "tag-query111"; // New tag query child of Folder 2, created remotely.
+    let bmk4_guid = "example-org1"; // New tagged child of Folder 2, created remotely.
+    {
+      // An existing record changed on the server that should not trigger
+      // another sync when applied.
+      let remoteBzBmk = new Bookmark("bookmarks", bzBmk.guid);
+      remoteBzBmk.bmkUri = "https://bugzilla.mozilla.org/";
+      remoteBzBmk.description = "New description";
+      remoteBzBmk.title = "Bugzilla";
+      remoteBzBmk.tags = ["new", "tags"];
+      remoteBzBmk.parentName = "Bookmarks Menu";
+      remoteBzBmk.parentid = "menu";
+      collection.insert(bzBmk.guid, encryptPayload(remoteBzBmk.cleartext));
+
+      let remoteFolder = new BookmarkFolder("bookmarks", folder2_guid);
+      remoteFolder.title = "Folder 2";
+      remoteFolder.children = [bmk4_guid, tagQuery_guid];
+      remoteFolder.parentName = "Bookmarks Menu";
+      remoteFolder.parentid = "menu";
+      collection.insert(folder2_guid, encryptPayload(remoteFolder.cleartext));
+
+      let remoteFxBmk = new Bookmark("bookmarks", bmk2_guid);
+      remoteFxBmk.bmkUri = "http://getfirefox.com/";
+      remoteFxBmk.description = "Firefox is awesome.";
+      remoteFxBmk.title = "Get Firefox!";
+      remoteFxBmk.tags = ["firefox", "awesome", "browser"];
+      remoteFxBmk.keyword = "awesome";
+      remoteFxBmk.parentName = "Folder 1";
+      remoteFxBmk.parentid = folder1.guid;
+      collection.insert(bmk2_guid, encryptPayload(remoteFxBmk.cleartext));
+
+      // A tag query referencing a nonexistent tag folder, which we should
+      // create locally when applying the record.
+      let remoteTagQuery = new BookmarkQuery("bookmarks", tagQuery_guid);
+      remoteTagQuery.bmkUri = "place:type=7&folder=999";
+      remoteTagQuery.title = "Taggy tags";
+      remoteTagQuery.folderName = "taggy";
+      remoteTagQuery.parentName = "Folder 2";
+      remoteTagQuery.parentid = folder2_guid;
+      collection.insert(
+        tagQuery_guid,
+        encryptPayload(remoteTagQuery.cleartext)
+      );
+
+      // A bookmark that should appear in the results for the tag query.
+      let remoteTaggedBmk = new Bookmark("bookmarks", bmk4_guid);
+      remoteTaggedBmk.bmkUri = "https://example.org/";
+      remoteTaggedBmk.title = "Tagged bookmark";
+      remoteTaggedBmk.tags = ["taggy"];
+      remoteTaggedBmk.parentName = "Folder 2";
+      remoteTaggedBmk.parentid = folder2_guid;
+      collection.insert(bmk4_guid, encryptPayload(remoteTaggedBmk.cleartext));
+
+      collection.insert(
+        "toolbar",
+        encryptPayload({
+          id: "toolbar",
+          type: "folder",
+          title: "toolbar",
+          children: [folder1.guid],
+          parentName: "places",
+          parentid: "places",
+        })
+      );
+
+      collection.insert(
+        "menu",
+        encryptPayload({
+          id: "menu",
+          type: "folder",
+          title: "menu",
+          children: [bzBmk.guid, folder2_guid],
+          parentName: "places",
+          parentid: "places",
+        })
+      );
+
+      collection.insert(
+        folder1.guid,
+        encryptPayload({
+          id: folder1.guid,
+          type: "folder",
+          title: "Folder 1",
+          children: [bmk2_guid],
+          parentName: "toolbar",
+          parentid: "toolbar",
+        })
+      );
+    }
+
+    await assertChildGuids(
+      folder1.guid,
+      [tbBmk.guid],
+      "Folder should have 1 child before first sync"
+    );
+
+    let pingsPromise = wait_for_pings(2);
+
+    let changes = await PlacesSyncUtils.bookmarks.pullChanges();
+    deepEqual(
+      Object.keys(changes).sort(),
+      [folder1.guid, tbBmk.guid, "menu", "mobile", "toolbar", "unfiled"].sort(),
+      "Should track bookmark and folder created before first sync"
+    );
+
+    // Unlike the tests above, we can't use `sync_engine_and_validate_telem`
+    // because the bookmarks engine will automatically schedule a follow-up
+    // sync for us.
+    _("Perform first sync and immediate follow-up sync");
+    Service.sync({ engines: ["bookmarks"] });
+
+    let pings = await pingsPromise;
+    equal(pings.length, 2, "Should submit two pings");
+    ok(
+      pings.every(p => {
+        assert_success_ping(p);
+        return p.syncs.length == 1;
+      }),
+      "Should submit 1 sync per ping"
+    );
+
+    strictEqual(
+      Service.scheduler.globalScore,
+      0,
+      "Should reset global score after follow-up sync"
+    );
+    ok(bmk3, "Should insert bookmark during first sync to simulate change");
+    ok(
+      collection.wbo(bmk3.guid),
+      "Changed bookmark should be uploaded after follow-up sync"
+    );
+
+    let bmk2 = await PlacesUtils.bookmarks.fetch({
+      guid: bmk2_guid,
+    });
+    ok(bmk2, "Remote bookmark should be applied during first sync");
+    {
+      // We only check child GUIDs, and not their order, because the exact
+      // order is an implementation detail.
+      let folder1Children = await PlacesSyncUtils.bookmarks.fetchChildRecordIds(
+        folder1.guid
+      );
+      deepEqual(
+        folder1Children.sort(),
+        [bmk2_guid, tbBmk.guid, bmk3.guid].sort(),
+        "Folder 1 should have 3 children after first sync"
+      );
+    }
+    await assertChildGuids(
+      folder2_guid,
+      [bmk4_guid, tagQuery_guid],
+      "Folder 2 should have 2 children after first sync"
+    );
+    let taggedURIs = [];
+    await PlacesUtils.bookmarks.fetch({ tags: ["taggy"] }, b =>
+      taggedURIs.push(b.url)
+    );
+    equal(taggedURIs.length, 1, "Should have 1 tagged URI");
+    equal(
+      taggedURIs[0].href,
+      "https://example.org/",
+      "Synced tagged bookmark should appear in tagged URI list"
+    );
+
+    changes = await PlacesSyncUtils.bookmarks.pullChanges();
+    deepEqual(
+      changes,
+      {},
+      "Should have already uploaded changes in follow-up sync"
+    );
+
+    // First ping won't include validation data, since we've changed bookmarks
+    // and `canValidate` will indicate it can't proceed.
+    let engineData = pings.map(p => {
+      return p.syncs[0].engines.find(e => e.name == "bookmarks-buffered");
+    });
+    ok(engineData[0].validation, "Engine should validate after first sync");
+    ok(engineData[1].validation, "Engine should validate after second sync");
+  } finally {
+    Object.defineProperty(
+      schedulerProto,
+      "syncThreshold",
+      syncThresholdDescriptor
+    );
+    engine._uploadOutgoing = uploadOutgoing;
+    await cleanup(engine, server);
+  }
+});
diff --git a/services/sync/tests/unit/test_enginemanager.js b/services/sync/tests/unit/test_enginemanager.js
new file mode 100644
index 0000000000..3e366be54f
--- /dev/null
+++ b/services/sync/tests/unit/test_enginemanager.js
@@ -0,0 +1,232 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function PetrolEngine() {}
+PetrolEngine.prototype.name = "petrol";
+PetrolEngine.prototype.finalize = async function () {};
+
+function DieselEngine() {}
+DieselEngine.prototype.name = "diesel";
+DieselEngine.prototype.finalize = async function () {};
+
+function DummyEngine() {}
+DummyEngine.prototype.name = "dummy";
+DummyEngine.prototype.finalize = async function () {};
+
+class ActualEngine extends SyncEngine {
+  constructor(service) {
+    super("Actual", service);
+  }
+}
+
+add_task(async function test_basics() {
+  _("We start out with a clean slate");
+
+  let manager = new EngineManager(Service);
+
+  let engines = await manager.getAll();
+  Assert.equal(engines.length, 0);
+  Assert.equal(await manager.get("dummy"), undefined);
+
+  _("Register an engine");
+  await manager.register(DummyEngine);
+  let dummy = await manager.get("dummy");
+  Assert.ok(dummy instanceof DummyEngine);
+
+  engines = await manager.getAll();
+  Assert.equal(engines.length, 1);
+  Assert.equal(engines[0], dummy);
+
+  _("Register an already registered engine is ignored");
+  await manager.register(DummyEngine);
+  Assert.equal(await manager.get("dummy"), dummy);
+
+  _("Register multiple engines in one go");
+  await manager.register([PetrolEngine, DieselEngine]);
+  let petrol = await manager.get("petrol");
+  let diesel = await manager.get("diesel");
+  Assert.ok(petrol instanceof PetrolEngine);
+  Assert.ok(diesel instanceof DieselEngine);
+
+  engines = await manager.getAll();
+  Assert.equal(engines.length, 3);
+  Assert.notEqual(engines.indexOf(petrol), -1);
+  Assert.notEqual(engines.indexOf(diesel), -1);
+
+  _("Retrieve multiple engines in one go");
+  engines = await manager.get(["dummy", "diesel"]);
+  Assert.equal(engines.length, 2);
+  Assert.notEqual(engines.indexOf(dummy), -1);
+  Assert.notEqual(engines.indexOf(diesel), -1);
+
+  _("getEnabled() only returns enabled engines");
+  engines = await manager.getEnabled();
+  Assert.equal(engines.length, 0);
+
+  petrol.enabled = true;
+  engines = await manager.getEnabled();
+  Assert.equal(engines.length, 1);
+  Assert.equal(engines[0], petrol);
+
+  dummy.enabled = true;
+  diesel.enabled = true;
+  engines = await manager.getEnabled();
+  Assert.equal(engines.length, 3);
+
+  _("getEnabled() returns enabled engines in sorted order");
+  petrol.syncPriority = 1;
+  dummy.syncPriority = 2;
+  diesel.syncPriority = 3;
+
+  engines = await manager.getEnabled();
+
+  Assert.deepEqual(engines, [petrol, dummy, diesel]);
+
+  _("Changing the priorities should change the order in getEnabled()");
+
+  dummy.syncPriority = 4;
+
+  engines = await manager.getEnabled();
+
+  Assert.deepEqual(engines, [petrol, diesel, dummy]);
+
+  _("Unregister an engine by name");
+  await manager.unregister("dummy");
+  Assert.equal(await manager.get("dummy"), undefined);
+  engines = await manager.getAll();
+  Assert.equal(engines.length, 2);
+  Assert.equal(engines.indexOf(dummy), -1);
+
+  _("Unregister an engine by value");
+  // manager.unregister() checks for instanceof Engine, so let's make one:
+  await manager.register(ActualEngine);
+  let actual = await manager.get("actual");
+  Assert.ok(actual instanceof ActualEngine);
+  Assert.ok(actual instanceof SyncEngine);
+
+  await manager.unregister(actual);
+  Assert.equal(await manager.get("actual"), undefined);
+});
+
+class AutoEngine {
+  constructor(type) {
+    this.name = "automobile";
+    this.type = type;
+    this.initializeCalled = false;
+    this.finalizeCalled = false;
+    this.isActive = false;
+  }
+
+  async initialize() {
+    Assert.ok(!this.initializeCalled);
+    Assert.equal(AutoEngine.current, undefined);
+    this.initializeCalled = true;
+    this.isActive = true;
+    AutoEngine.current = this;
+  }
+
+  async finalize() {
+    Assert.equal(AutoEngine.current, this);
+    Assert.ok(!this.finalizeCalled);
+    Assert.ok(this.isActive);
+    this.finalizeCalled = true;
+    this.isActive = false;
+    AutoEngine.current = undefined;
+  }
+}
+
+class GasolineEngine extends AutoEngine {
+  constructor() {
+    super("gasoline");
+  }
+}
+
+class ElectricEngine extends AutoEngine {
+  constructor() {
+    super("electric");
+  }
+}
+
+add_task(async function test_alternates() {
+  let manager = new EngineManager(Service);
+  let engines = await manager.getAll();
+  Assert.equal(engines.length, 0);
+
+  const prefName = "services.sync.engines.automobile.electric";
+  Services.prefs.clearUserPref(prefName);
+
+  await manager.registerAlternatives(
+    "automobile",
+    prefName,
+    ElectricEngine,
+    GasolineEngine
+  );
+
+  let gasEngine = manager.get("automobile");
+  Assert.equal(gasEngine.type, "gasoline");
+
+  Assert.ok(gasEngine.isActive);
+  Assert.ok(gasEngine.initializeCalled);
+  Assert.ok(!gasEngine.finalizeCalled);
+  Assert.equal(AutoEngine.current, gasEngine);
+
+  _("Check that setting the controlling pref to false makes no difference");
+  Services.prefs.setBoolPref(prefName, false);
+  Assert.equal(manager.get("automobile"), gasEngine);
+  Assert.ok(gasEngine.isActive);
+  Assert.ok(gasEngine.initializeCalled);
+  Assert.ok(!gasEngine.finalizeCalled);
+
+  _("Even after the call to switchAlternatives");
+  await manager.switchAlternatives();
+  Assert.equal(manager.get("automobile"), gasEngine);
+  Assert.ok(gasEngine.isActive);
+  Assert.ok(gasEngine.initializeCalled);
+  Assert.ok(!gasEngine.finalizeCalled);
+
+  _("Set the pref to true, we still shouldn't switch yet");
+  Services.prefs.setBoolPref(prefName, true);
+  Assert.equal(manager.get("automobile"), gasEngine);
+  Assert.ok(gasEngine.isActive);
+  Assert.ok(gasEngine.initializeCalled);
+  Assert.ok(!gasEngine.finalizeCalled);
+
+  _("Now we expect to switch from gas to electric");
+  await manager.switchAlternatives();
+  let elecEngine = manager.get("automobile");
+  Assert.equal(elecEngine.type, "electric");
+  Assert.ok(elecEngine.isActive);
+  Assert.ok(elecEngine.initializeCalled);
+  Assert.ok(!elecEngine.finalizeCalled);
+  Assert.equal(AutoEngine.current, elecEngine);
+
+  Assert.ok(!gasEngine.isActive);
+  Assert.ok(gasEngine.finalizeCalled);
+
+  _("Switch back, and ensure we get a new instance that got initialized again");
+  Services.prefs.setBoolPref(prefName, false);
+  await manager.switchAlternatives();
+
+  // First make sure we deactivated the electric engine as we should
+  Assert.ok(!elecEngine.isActive);
+  Assert.ok(elecEngine.initializeCalled);
+  Assert.ok(elecEngine.finalizeCalled);
+
+  let newGasEngine = manager.get("automobile");
+  Assert.notEqual(newGasEngine, gasEngine);
+  Assert.equal(newGasEngine.type, "gasoline");
+
+  Assert.ok(newGasEngine.isActive);
+  Assert.ok(newGasEngine.initializeCalled);
+  Assert.ok(!newGasEngine.finalizeCalled);
+
+  _("Make sure unregister removes the alt info too");
+  await manager.unregister("automobile");
+  Assert.equal(manager.get("automobile"), null);
+  Assert.ok(newGasEngine.finalizeCalled);
+  Assert.deepEqual(Object.keys(manager._altEngineInfo), []);
+});
diff --git a/services/sync/tests/unit/test_errorhandler_1.js b/services/sync/tests/unit/test_errorhandler_1.js
new file mode 100644
index 0000000000..2d52b93a02
--- /dev/null
+++ b/services/sync/tests/unit/test_errorhandler_1.js
@@ -0,0 +1,341 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+
+const fakeServer = new SyncServer();
+fakeServer.start();
+const fakeServerUrl = "http://localhost:" + fakeServer.port;
+
+registerCleanupFunction(function () {
+  return promiseStopServer(fakeServer).finally(() => {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  });
+});
+
+let engine;
+add_task(async function setup() {
+  await Service.engineManager.clear();
+  await Service.engineManager.register(EHTestsCommon.CatapultEngine);
+  engine = Service.engineManager.get("catapult");
+});
+
+async function clean() {
+  let promiseLogReset = promiseOneObserver("weave:service:reset-file-log");
+  await Service.startOver();
+  await promiseLogReset;
+  Status.resetSync();
+  Status.resetBackoff();
+  // Move log levels back to trace (startOver will have reversed this), sicne
+  syncTestLogging();
+}
+
+add_task(async function test_401_logout() {
+  enableValidationPrefs();
+
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  // By calling sync, we ensure we're logged in.
+  await sync_and_validate_telem();
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  Assert.ok(Service.isLoggedIn);
+
+  let promiseErrors = new Promise(res => {
+    Svc.Obs.add("weave:service:sync:error", onSyncError);
+    function onSyncError() {
+      _("Got weave:service:sync:error in first sync.");
+      Svc.Obs.remove("weave:service:sync:error", onSyncError);
+
+      // Wait for the automatic next sync.
+      Svc.Obs.add("weave:service:login:error", onLoginError);
+      function onLoginError() {
+        _("Got weave:service:login:error in second sync.");
+        Svc.Obs.remove("weave:service:login:error", onLoginError);
+        res();
+      }
+    }
+  });
+
+  // Make sync fail due to login rejected.
+  await configureIdentity({ username: "janedoe" }, server);
+  Service._updateCachedURLs();
+
+  _("Starting first sync.");
+  await sync_and_validate_telem(ping => {
+    deepEqual(ping.failureReason, { name: "httperror", code: 401 });
+  });
+  _("First sync done.");
+
+  await promiseErrors;
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR);
+  Assert.ok(!Service.isLoggedIn);
+
+  // Clean up.
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_credentials_changed_logout() {
+  enableValidationPrefs();
+
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  // By calling sync, we ensure we're logged in.
+  await sync_and_validate_telem();
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  Assert.ok(Service.isLoggedIn);
+
+  await EHTestsCommon.generateCredentialsChangedFailure();
+
+  await sync_and_validate_telem(ping => {
+    equal(ping.status.sync, CREDENTIALS_CHANGED);
+    deepEqual(ping.failureReason, {
+      name: "unexpectederror",
+      error: "Error: Aborting sync, remote setup failed",
+    });
+  });
+
+  Assert.equal(Status.sync, CREDENTIALS_CHANGED);
+  Assert.ok(!Service.isLoggedIn);
+
+  // Clean up.
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_login_non_network_error() {
+  enableValidationPrefs();
+
+  // Test non-network errors are reported
+  // when calling sync
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+  Service.identity._syncKeyBundle = null;
+
+  await Service.sync();
+  Assert.equal(Status.login, LOGIN_FAILED_NO_PASSPHRASE);
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_sync_non_network_error() {
+  enableValidationPrefs();
+
+  // Test non-network errors are reported
+  // when calling sync
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  // By calling sync, we ensure we're logged in.
+  await Service.sync();
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  Assert.ok(Service.isLoggedIn);
+
+  await EHTestsCommon.generateCredentialsChangedFailure();
+
+  await sync_and_validate_telem(ping => {
+    equal(ping.status.sync, CREDENTIALS_CHANGED);
+    deepEqual(ping.failureReason, {
+      name: "unexpectederror",
+      error: "Error: Aborting sync, remote setup failed",
+    });
+  });
+
+  Assert.equal(Status.sync, CREDENTIALS_CHANGED);
+  // If we clean this tick, telemetry won't get the right error
+  await Async.promiseYield();
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_login_sync_network_error() {
+  enableValidationPrefs();
+
+  // Test network errors are reported when calling sync.
+  await configureIdentity({ username: "broken.wipe" });
+  Service.clusterURL = fakeServerUrl;
+
+  await Service.sync();
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR);
+
+  await clean();
+});
+
+add_task(async function test_sync_network_error() {
+  enableValidationPrefs();
+
+  // Test network errors are reported when calling sync.
+  Services.io.offline = true;
+
+  await Service.sync();
+  Assert.equal(Status.sync, LOGIN_FAILED_NETWORK_ERROR);
+
+  Services.io.offline = false;
+  await clean();
+});
+
+add_task(async function test_login_non_network_error() {
+  enableValidationPrefs();
+
+  // Test non-network errors are reported
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+  Service.identity._syncKeyBundle = null;
+
+  await Service.sync();
+  Assert.equal(Status.login, LOGIN_FAILED_NO_PASSPHRASE);
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_sync_non_network_error() {
+  enableValidationPrefs();
+
+  // Test non-network errors are reported
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  // By calling sync, we ensure we're logged in.
+  await Service.sync();
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  Assert.ok(Service.isLoggedIn);
+
+  await EHTestsCommon.generateCredentialsChangedFailure();
+
+  await Service.sync();
+  Assert.equal(Status.sync, CREDENTIALS_CHANGED);
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_login_network_error() {
+  enableValidationPrefs();
+
+  await configureIdentity({ username: "johndoe" });
+  Service.clusterURL = fakeServerUrl;
+
+  // Test network errors are not reported.
+
+  await Service.sync();
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR);
+
+  Services.io.offline = false;
+  await clean();
+});
+
+add_task(async function test_sync_network_error() {
+  enableValidationPrefs();
+
+  // Test network errors are not reported.
+  Services.io.offline = true;
+
+  await Service.sync();
+  Assert.equal(Status.sync, LOGIN_FAILED_NETWORK_ERROR);
+
+  Services.io.offline = false;
+  await clean();
+});
+
+add_task(async function test_sync_server_maintenance_error() {
+  enableValidationPrefs();
+
+  // Test server maintenance errors are not reported.
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  const BACKOFF = 42;
+  engine.enabled = true;
+  engine.exception = { status: 503, headers: { "retry-after": BACKOFF } };
+
+  Assert.equal(Status.service, STATUS_OK);
+
+  await sync_and_validate_telem(ping => {
+    equal(ping.status.sync, SERVER_MAINTENANCE);
+    deepEqual(ping.engines.find(e => e.failureReason).failureReason, {
+      name: "httperror",
+      code: 503,
+    });
+  });
+
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+  Assert.equal(Status.sync, SERVER_MAINTENANCE);
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_info_collections_login_server_maintenance_error() {
+  enableValidationPrefs();
+
+  // Test info/collections server maintenance errors are not reported.
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  await configureIdentity({ username: "broken.info" }, server);
+
+  let backoffInterval;
+  Svc.Obs.add(
+    "weave:service:backoff:interval",
+    function observe(subject, data) {
+      Svc.Obs.remove("weave:service:backoff:interval", observe);
+      backoffInterval = subject;
+    }
+  );
+
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(Status.service, STATUS_OK);
+
+  await Service.sync();
+
+  Assert.ok(Status.enforceBackoff);
+  Assert.equal(backoffInterval, 42);
+  Assert.equal(Status.service, LOGIN_FAILED);
+  Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_meta_global_login_server_maintenance_error() {
+  enableValidationPrefs();
+
+  // Test meta/global server maintenance errors are not reported.
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  await configureIdentity({ username: "broken.meta" }, server);
+
+  let backoffInterval;
+  Svc.Obs.add(
+    "weave:service:backoff:interval",
+    function observe(subject, data) {
+      Svc.Obs.remove("weave:service:backoff:interval", observe);
+      backoffInterval = subject;
+    }
+  );
+
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(Status.service, STATUS_OK);
+
+  await Service.sync();
+
+  Assert.ok(Status.enforceBackoff);
+  Assert.equal(backoffInterval, 42);
+  Assert.equal(Status.service, LOGIN_FAILED);
+  Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+  await clean();
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_errorhandler_2.js b/services/sync/tests/unit/test_errorhandler_2.js
new file mode 100644
index 0000000000..5cab4d832d
--- /dev/null
+++ b/services/sync/tests/unit/test_errorhandler_2.js
@@ -0,0 +1,550 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+const { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+
+const fakeServer = new SyncServer();
+fakeServer.start();
+
+registerCleanupFunction(function () {
+  return promiseStopServer(fakeServer).finally(() => {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  });
+});
+
+const logsdir = FileUtils.getDir("ProfD", ["weave", "logs"]);
+logsdir.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+
+function removeLogFiles() {
+  let entries = logsdir.directoryEntries;
+  while (entries.hasMoreElements()) {
+    let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+    logfile.remove(false);
+  }
+}
+
+function getLogFiles() {
+  let result = [];
+  let entries = logsdir.directoryEntries;
+  while (entries.hasMoreElements()) {
+    result.push(entries.getNext().QueryInterface(Ci.nsIFile));
+  }
+  return result;
+}
+
+let engine;
+add_task(async function setup() {
+  await Service.engineManager.clear();
+  await Service.engineManager.register(EHTestsCommon.CatapultEngine);
+  engine = Service.engineManager.get("catapult");
+});
+
+async function clean() {
+  let promiseLogReset = promiseOneObserver("weave:service:reset-file-log");
+  await Service.startOver();
+  await promiseLogReset;
+  Status.resetSync();
+  Status.resetBackoff();
+  removeLogFiles();
+  // Move log levels back to trace (startOver will have reversed this), sicne
+  syncTestLogging();
+}
+
+add_task(async function test_crypto_keys_login_server_maintenance_error() {
+  enableValidationPrefs();
+
+  Status.resetSync();
+  // Test crypto/keys server maintenance errors are not reported.
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  await configureIdentity({ username: "broken.keys" }, server);
+
+  // Force re-download of keys
+  Service.collectionKeys.clear();
+
+  let backoffInterval;
+  Svc.Obs.add(
+    "weave:service:backoff:interval",
+    function observe(subject, data) {
+      Svc.Obs.remove("weave:service:backoff:interval", observe);
+      backoffInterval = subject;
+    }
+  );
+
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(Status.service, STATUS_OK);
+
+  let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+  await Service.sync();
+  await promiseObserved;
+
+  Assert.ok(Status.enforceBackoff);
+  Assert.equal(backoffInterval, 42);
+  Assert.equal(Status.service, LOGIN_FAILED);
+  Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_lastSync_not_updated_on_complete_failure() {
+  enableValidationPrefs();
+
+  // Test info/collections prolonged server maintenance errors are reported.
+  let server = await EHTestsCommon.sync_httpd_setup();
+  await EHTestsCommon.setUp(server);
+
+  await configureIdentity({ username: "johndoe" }, server);
+
+  // Do an initial sync that we expect to be successful.
+  let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+  await sync_and_validate_telem();
+  await promiseObserved;
+
+  Assert.equal(Status.service, STATUS_OK);
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+  let lastSync = Svc.PrefBranch.getStringPref("lastSync");
+
+  Assert.ok(lastSync);
+
+  // Report server maintenance on info/collections requests
+  server.registerPathHandler(
+    "/1.1/johndoe/info/collections",
+    EHTestsCommon.service_unavailable
+  );
+
+  promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+  await sync_and_validate_telem(() => {});
+  await promiseObserved;
+
+  Assert.equal(Status.sync, SERVER_MAINTENANCE);
+  Assert.equal(Status.service, SYNC_FAILED);
+
+  // We shouldn't update lastSync on complete failure.
+  Assert.equal(lastSync, Svc.PrefBranch.getStringPref("lastSync"));
+
+  await clean();
+  await promiseStopServer(server);
+});
+
+add_task(
+  async function test_sync_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test server maintenance errors are reported
+    // when calling syncAndReportErrors.
+    let server = await EHTestsCommon.sync_httpd_setup();
+    await EHTestsCommon.setUp(server);
+
+    const BACKOFF = 42;
+    engine.enabled = true;
+    engine.exception = { status: 503, headers: { "retry-after": BACKOFF } };
+
+    Assert.equal(Status.service, STATUS_OK);
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+    Assert.equal(Status.sync, SERVER_MAINTENANCE);
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(
+  async function test_info_collections_login_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test info/collections server maintenance errors are reported
+    // when calling syncAndReportErrors.
+    let server = await EHTestsCommon.sync_httpd_setup();
+    await EHTestsCommon.setUp(server);
+
+    await configureIdentity({ username: "broken.info" }, server);
+
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+
+    Assert.ok(!Status.enforceBackoff);
+    Assert.equal(Status.service, STATUS_OK);
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Status.service, LOGIN_FAILED);
+    Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(
+  async function test_meta_global_login_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test meta/global server maintenance errors are reported
+    // when calling syncAndReportErrors.
+    let server = await EHTestsCommon.sync_httpd_setup();
+    await EHTestsCommon.setUp(server);
+
+    await configureIdentity({ username: "broken.meta" }, server);
+
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+
+    Assert.ok(!Status.enforceBackoff);
+    Assert.equal(Status.service, STATUS_OK);
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Status.service, LOGIN_FAILED);
+    Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(
+  async function test_download_crypto_keys_login_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test crypto/keys server maintenance errors are reported
+    // when calling syncAndReportErrors.
+    let server = await EHTestsCommon.sync_httpd_setup();
+    await EHTestsCommon.setUp(server);
+
+    await configureIdentity({ username: "broken.keys" }, server);
+    // Force re-download of keys
+    Service.collectionKeys.clear();
+
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+
+    Assert.ok(!Status.enforceBackoff);
+    Assert.equal(Status.service, STATUS_OK);
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Status.service, LOGIN_FAILED);
+    Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(
+  async function test_upload_crypto_keys_login_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test crypto/keys server maintenance errors are reported
+    // when calling syncAndReportErrors.
+    let server = await EHTestsCommon.sync_httpd_setup();
+
+    // Start off with an empty account, do not upload a key.
+    await configureIdentity({ username: "broken.keys" }, server);
+
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+
+    Assert.ok(!Status.enforceBackoff);
+    Assert.equal(Status.service, STATUS_OK);
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Status.service, LOGIN_FAILED);
+    Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(
+  async function test_wipeServer_login_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test crypto/keys server maintenance errors are reported
+    // when calling syncAndReportErrors.
+    let server = await EHTestsCommon.sync_httpd_setup();
+
+    // Start off with an empty account, do not upload a key.
+    await configureIdentity({ username: "broken.wipe" }, server);
+
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+
+    Assert.ok(!Status.enforceBackoff);
+    Assert.equal(Status.service, STATUS_OK);
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Status.service, LOGIN_FAILED);
+    Assert.equal(Status.login, SERVER_MAINTENANCE);
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(
+  async function test_wipeRemote_syncAndReportErrors_server_maintenance_error() {
+    enableValidationPrefs();
+
+    // Test that we report prolonged server maintenance errors that occur whilst
+    // wiping all remote devices.
+    let server = await EHTestsCommon.sync_httpd_setup();
+
+    await configureIdentity({ username: "broken.wipe" }, server);
+    await EHTestsCommon.generateAndUploadKeys();
+
+    engine.exception = null;
+    engine.enabled = true;
+
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+
+    Assert.ok(!Status.enforceBackoff);
+    Assert.equal(Status.service, STATUS_OK);
+
+    Svc.PrefBranch.setStringPref("firstSync", "wipeRemote");
+
+    let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+    await Service.sync();
+    await promiseObserved;
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Status.service, SYNC_FAILED);
+    Assert.equal(Status.sync, SERVER_MAINTENANCE);
+    Assert.equal(Svc.PrefBranch.getStringPref("firstSync"), "wipeRemote");
+
+    await clean();
+    await promiseStopServer(server);
+  }
+);
+
+add_task(async function test_sync_engine_generic_fail() {
+  enableValidationPrefs();
+
+  equal(getLogFiles().length, 0);
+
+  let server = await EHTestsCommon.sync_httpd_setup();
+  engine.enabled = true;
+  engine.sync = async function sync() {
+    Svc.Obs.notify("weave:engine:sync:error", ENGINE_UNKNOWN_FAIL, "catapult");
+  };
+  let lastSync = Svc.PrefBranch.getStringPref("lastSync", null);
+  let log = Log.repository.getLogger("Sync.ErrorHandler");
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+
+  Assert.equal(Status.engines.catapult, undefined);
+
+  let promiseObserved = new Promise(res => {
+    Svc.Obs.add("weave:engine:sync:finish", function onEngineFinish() {
+      Svc.Obs.remove("weave:engine:sync:finish", onEngineFinish);
+
+      log.info("Adding reset-file-log observer.");
+      Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+        Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+        res();
+      });
+    });
+  });
+
+  Assert.ok(await EHTestsCommon.setUp(server));
+  await sync_and_validate_telem(ping => {
+    deepEqual(ping.status.service, SYNC_FAILED_PARTIAL);
+    deepEqual(ping.engines.find(e => e.status).status, ENGINE_UNKNOWN_FAIL);
+  });
+
+  await promiseObserved;
+
+  _("Status.engines: " + JSON.stringify(Status.engines));
+  Assert.equal(Status.engines.catapult, ENGINE_UNKNOWN_FAIL);
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+
+  // lastSync should update on partial failure.
+  Assert.notEqual(lastSync, Svc.PrefBranch.getStringPref("lastSync"));
+
+  // Test Error log was written on SYNC_FAILED_PARTIAL.
+  let logFiles = getLogFiles();
+  equal(logFiles.length, 1);
+  Assert.ok(
+    logFiles[0].leafName.startsWith("error-sync-"),
+    logFiles[0].leafName
+  );
+
+  await clean();
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_logs_on_sync_error() {
+  enableValidationPrefs();
+
+  _(
+    "Ensure that an error is still logged when weave:service:sync:error " +
+      "is notified, despite shouldReportError returning false."
+  );
+
+  let log = Log.repository.getLogger("Sync.ErrorHandler");
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+  log.info("TESTING");
+
+  // Ensure that we report no error.
+  Status.login = MASTER_PASSWORD_LOCKED;
+
+  let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+  Svc.Obs.notify("weave:service:sync:error", {});
+  await promiseObserved;
+
+  // Test that error log was written.
+  let logFiles = getLogFiles();
+  equal(logFiles.length, 1);
+  Assert.ok(
+    logFiles[0].leafName.startsWith("error-sync-"),
+    logFiles[0].leafName
+  );
+
+  await clean();
+});
+
+add_task(async function test_logs_on_login_error() {
+  enableValidationPrefs();
+
+  _(
+    "Ensure that an error is still logged when weave:service:login:error " +
+      "is notified, despite shouldReportError returning false."
+  );
+
+  let log = Log.repository.getLogger("Sync.ErrorHandler");
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+  log.info("TESTING");
+
+  // Ensure that we report no error.
+  Status.login = MASTER_PASSWORD_LOCKED;
+
+  let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+  Svc.Obs.notify("weave:service:login:error", {});
+  await promiseObserved;
+
+  // Test that error log was written.
+  let logFiles = getLogFiles();
+  equal(logFiles.length, 1);
+  Assert.ok(
+    logFiles[0].leafName.startsWith("error-sync-"),
+    logFiles[0].leafName
+  );
+
+  await clean();
+});
+
+// This test should be the last one since it monkeypatches the engine object
+// and we should only have one engine object throughout the file (bug 629664).
+add_task(async function test_engine_applyFailed() {
+  enableValidationPrefs();
+
+  let server = await EHTestsCommon.sync_httpd_setup();
+
+  engine.enabled = true;
+  delete engine.exception;
+  engine.sync = async function sync() {
+    Svc.Obs.notify("weave:engine:sync:applied", { newFailed: 1 }, "catapult");
+  };
+
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+
+  let promiseObserved = promiseOneObserver("weave:service:reset-file-log");
+
+  Assert.equal(Status.engines.catapult, undefined);
+  Assert.ok(await EHTestsCommon.setUp(server));
+  await Service.sync();
+  await promiseObserved;
+
+  Assert.equal(Status.engines.catapult, ENGINE_APPLY_FAIL);
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+
+  // Test Error log was written on SYNC_FAILED_PARTIAL.
+  let logFiles = getLogFiles();
+  equal(logFiles.length, 1);
+  Assert.ok(
+    logFiles[0].leafName.startsWith("error-sync-"),
+    logFiles[0].leafName
+  );
+
+  await clean();
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_errorhandler_filelog.js b/services/sync/tests/unit/test_errorhandler_filelog.js
new file mode 100644
index 0000000000..66260b3f59
--- /dev/null
+++ b/services/sync/tests/unit/test_errorhandler_filelog.js
@@ -0,0 +1,473 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// `Service` is used as a global in head_helpers.js.
+// eslint-disable-next-line no-unused-vars
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { logManager } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccountsCommon.sys.mjs"
+);
+const { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+
+const logsdir = FileUtils.getDir("ProfD", ["weave", "logs"]);
+logsdir.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+
+// Delay to wait before cleanup, to allow files to age.
+// This is so large because the file timestamp granularity is per-second, and
+// so otherwise we can end up with all of our files -- the ones we want to
+// keep, and the ones we want to clean up -- having the same modified time.
+const CLEANUP_DELAY = 2000;
+const DELAY_BUFFER = 500; // Buffer for timers on different OS platforms.
+
+function run_test() {
+  validate_all_future_pings();
+  run_next_test();
+}
+
+add_test(function test_noOutput() {
+  // Ensure that the log appender won't print anything.
+  logManager._fileAppender.level = Log.Level.Fatal + 1;
+
+  // Clear log output from startup.
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnSuccess", false);
+  Svc.Obs.notify("weave:service:sync:finish");
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLogOuter() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLogOuter);
+    // Clear again without having issued any output.
+    Svc.PrefBranch.setBoolPref("log.appender.file.logOnSuccess", true);
+
+    Svc.Obs.add("weave:service:reset-file-log", function onResetFileLogInner() {
+      Svc.Obs.remove("weave:service:reset-file-log", onResetFileLogInner);
+
+      logManager._fileAppender.level = Log.Level.Trace;
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    });
+
+    // Fake a successful sync.
+    Svc.Obs.notify("weave:service:sync:finish");
+  });
+});
+
+add_test(function test_logOnSuccess_false() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnSuccess", false);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  log.info("this won't show up");
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+    // No log file was written.
+    Assert.ok(!logsdir.directoryEntries.hasMoreElements());
+
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    run_next_test();
+  });
+
+  // Fake a successful sync.
+  Svc.Obs.notify("weave:service:sync:finish");
+});
+
+function readFile(file, callback) {
+  NetUtil.asyncFetch(
+    {
+      uri: NetUtil.newURI(file),
+      loadUsingSystemPrincipal: true,
+    },
+    function (inputStream, statusCode, request) {
+      let data = NetUtil.readInputStreamToString(
+        inputStream,
+        inputStream.available()
+      );
+      callback(statusCode, data);
+    }
+  );
+}
+
+add_test(function test_logOnSuccess_true() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnSuccess", true);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  const MESSAGE = "this WILL show up";
+  log.info(MESSAGE);
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+
+    // Exactly one log file was written.
+    let entries = logsdir.directoryEntries;
+    Assert.ok(entries.hasMoreElements());
+    let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+    Assert.equal(logfile.leafName.slice(-4), ".txt");
+    Assert.ok(logfile.leafName.startsWith("success-sync-"), logfile.leafName);
+    Assert.ok(!entries.hasMoreElements());
+
+    // Ensure the log message was actually written to file.
+    readFile(logfile, function (error, data) {
+      Assert.ok(Components.isSuccessCode(error));
+      Assert.notEqual(data.indexOf(MESSAGE), -1);
+
+      // Clean up.
+      try {
+        logfile.remove(false);
+      } catch (ex) {
+        dump("Couldn't delete file: " + ex.message + "\n");
+        // Stupid Windows box.
+      }
+
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    });
+  });
+
+  // Fake a successful sync.
+  Svc.Obs.notify("weave:service:sync:finish");
+});
+
+add_test(function test_sync_error_logOnError_false() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", false);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  log.info("this won't show up");
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+    // No log file was written.
+    Assert.ok(!logsdir.directoryEntries.hasMoreElements());
+
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    run_next_test();
+  });
+
+  // Fake an unsuccessful sync.
+  Svc.Obs.notify("weave:service:sync:error");
+});
+
+add_test(function test_sync_error_logOnError_true() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  const MESSAGE = "this WILL show up";
+  log.info(MESSAGE);
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+
+    // Exactly one log file was written.
+    let entries = logsdir.directoryEntries;
+    Assert.ok(entries.hasMoreElements());
+    let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+    Assert.equal(logfile.leafName.slice(-4), ".txt");
+    Assert.ok(logfile.leafName.startsWith("error-sync-"), logfile.leafName);
+    Assert.ok(!entries.hasMoreElements());
+
+    // Ensure the log message was actually written to file.
+    readFile(logfile, function (error, data) {
+      Assert.ok(Components.isSuccessCode(error));
+      Assert.notEqual(data.indexOf(MESSAGE), -1);
+
+      // Clean up.
+      try {
+        logfile.remove(false);
+      } catch (ex) {
+        dump("Couldn't delete file: " + ex.message + "\n");
+        // Stupid Windows box.
+      }
+
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    });
+  });
+
+  // Fake an unsuccessful sync.
+  Svc.Obs.notify("weave:service:sync:error");
+});
+
+add_test(function test_login_error_logOnError_false() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", false);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  log.info("this won't show up");
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+    // No log file was written.
+    Assert.ok(!logsdir.directoryEntries.hasMoreElements());
+
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    run_next_test();
+  });
+
+  // Fake an unsuccessful login.
+  Svc.Obs.notify("weave:service:login:error");
+});
+
+add_test(function test_login_error_logOnError_true() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  const MESSAGE = "this WILL show up";
+  log.info(MESSAGE);
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+
+    // Exactly one log file was written.
+    let entries = logsdir.directoryEntries;
+    Assert.ok(entries.hasMoreElements());
+    let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+    Assert.equal(logfile.leafName.slice(-4), ".txt");
+    Assert.ok(logfile.leafName.startsWith("error-sync-"), logfile.leafName);
+    Assert.ok(!entries.hasMoreElements());
+
+    // Ensure the log message was actually written to file.
+    readFile(logfile, function (error, data) {
+      Assert.ok(Components.isSuccessCode(error));
+      Assert.notEqual(data.indexOf(MESSAGE), -1);
+
+      // Clean up.
+      try {
+        logfile.remove(false);
+      } catch (ex) {
+        dump("Couldn't delete file: " + ex.message + "\n");
+        // Stupid Windows box.
+      }
+
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    });
+  });
+
+  // Fake an unsuccessful login.
+  Svc.Obs.notify("weave:service:login:error");
+});
+
+add_test(function test_noNewFailed_noErrorLog() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnSuccess", false);
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+    // No log file was written.
+    Assert.ok(!logsdir.directoryEntries.hasMoreElements());
+
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    run_next_test();
+  });
+  // failed is nonzero and newFailed is zero -- shouldn't write a log.
+  let count = {
+    applied: 8,
+    succeeded: 4,
+    failed: 5,
+    newFailed: 0,
+    reconciled: 4,
+  };
+  Svc.Obs.notify("weave:engine:sync:applied", count, "foobar-engine");
+  Svc.Obs.notify("weave:service:sync:finish");
+});
+
+add_test(function test_newFailed_errorLog() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnSuccess", false);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  const MESSAGE = "this WILL show up 2";
+  log.info(MESSAGE);
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+
+    // Exactly one log file was written.
+    let entries = logsdir.directoryEntries;
+    Assert.ok(entries.hasMoreElements());
+    let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+    Assert.equal(logfile.leafName.slice(-4), ".txt");
+    Assert.ok(logfile.leafName.startsWith("error-sync-"), logfile.leafName);
+    Assert.ok(!entries.hasMoreElements());
+
+    // Ensure the log message was actually written to file.
+    readFile(logfile, function (error, data) {
+      Assert.ok(Components.isSuccessCode(error));
+      Assert.notEqual(data.indexOf(MESSAGE), -1);
+
+      // Clean up.
+      try {
+        logfile.remove(false);
+      } catch (ex) {
+        dump("Couldn't delete file: " + ex.message + "\n");
+        // Stupid Windows box.
+      }
+
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    });
+  });
+  // newFailed is nonzero -- should write a log.
+  let count = {
+    applied: 8,
+    succeeded: 4,
+    failed: 5,
+    newFailed: 4,
+    reconciled: 4,
+  };
+
+  Svc.Obs.notify("weave:engine:sync:applied", count, "foobar-engine");
+  Svc.Obs.notify("weave:service:sync:finish");
+});
+
+add_test(function test_errorLog_dumpAddons() {
+  Svc.PrefBranch.setStringPref("log.logger", "Trace");
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+
+  Svc.Obs.add("weave:service:reset-file-log", function onResetFileLog() {
+    Svc.Obs.remove("weave:service:reset-file-log", onResetFileLog);
+
+    let entries = logsdir.directoryEntries;
+    Assert.ok(entries.hasMoreElements());
+    let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+    Assert.equal(logfile.leafName.slice(-4), ".txt");
+    Assert.ok(logfile.leafName.startsWith("error-sync-"), logfile.leafName);
+    Assert.ok(!entries.hasMoreElements());
+
+    // Ensure we logged some addon list (which is probably empty)
+    readFile(logfile, function (error, data) {
+      Assert.ok(Components.isSuccessCode(error));
+      Assert.notEqual(data.indexOf("Addons installed"), -1);
+
+      // Clean up.
+      try {
+        logfile.remove(false);
+      } catch (ex) {
+        dump("Couldn't delete file: " + ex.message + "\n");
+        // Stupid Windows box.
+      }
+
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    });
+  });
+
+  // Fake an unsuccessful sync.
+  Svc.Obs.notify("weave:service:sync:error");
+});
+
+// Check that error log files are deleted above an age threshold.
+add_test(async function test_logErrorCleanup_age() {
+  _("Beginning test_logErrorCleanup_age.");
+  let maxAge = CLEANUP_DELAY / 1000;
+  let oldLogs = [];
+  let numLogs = 10;
+  let errString = "some error log\n";
+
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+  Svc.PrefBranch.setIntPref("log.appender.file.maxErrorAge", maxAge);
+
+  _("Making some files.");
+  const logsDir = PathUtils.join(PathUtils.profileDir, "weave", "logs");
+  await IOUtils.makeDirectory(logsDir);
+  for (let i = 0; i < numLogs; i++) {
+    let now = Date.now();
+    let filename = "error-sync-" + now + "" + i + ".txt";
+    let newLog = new FileUtils.File(PathUtils.join(logsDir, filename));
+    let foStream = FileUtils.openFileOutputStream(newLog);
+    foStream.write(errString, errString.length);
+    foStream.close();
+    _("  > Created " + filename);
+    oldLogs.push(newLog.leafName);
+  }
+
+  Svc.Obs.add(
+    "services-tests:common:log-manager:cleanup-logs",
+    function onCleanupLogs() {
+      Svc.Obs.remove(
+        "services-tests:common:log-manager:cleanup-logs",
+        onCleanupLogs
+      );
+
+      // Only the newest created log file remains.
+      let entries = logsdir.directoryEntries;
+      Assert.ok(entries.hasMoreElements());
+      let logfile = entries.getNext().QueryInterface(Ci.nsIFile);
+      Assert.ok(
+        oldLogs.every(function (e) {
+          return e != logfile.leafName;
+        })
+      );
+      Assert.ok(!entries.hasMoreElements());
+
+      // Clean up.
+      try {
+        logfile.remove(false);
+      } catch (ex) {
+        dump("Couldn't delete file: " + ex.message + "\n");
+        // Stupid Windows box.
+      }
+
+      for (const pref of Svc.PrefBranch.getChildList("")) {
+        Svc.PrefBranch.clearUserPref(pref);
+      }
+      run_next_test();
+    }
+  );
+
+  let delay = CLEANUP_DELAY + DELAY_BUFFER;
+
+  _("Cleaning up logs after " + delay + "msec.");
+  CommonUtils.namedTimer(
+    function onTimer() {
+      Svc.Obs.notify("weave:service:sync:error");
+    },
+    delay,
+    this,
+    "cleanup-timer"
+  );
+});
+
+add_task(async function test_remove_log_on_startOver() {
+  Svc.PrefBranch.setBoolPref("log.appender.file.logOnError", true);
+
+  let log = Log.repository.getLogger("Sync.Test.FileLog");
+  const MESSAGE = "this WILL show up";
+  log.info(MESSAGE);
+
+  let promiseLogWritten = promiseOneObserver("weave:service:reset-file-log");
+  // Fake an unsuccessful sync.
+  Svc.Obs.notify("weave:service:sync:error");
+
+  await promiseLogWritten;
+  // Should have at least 1 log file.
+  let entries = logsdir.directoryEntries;
+  Assert.ok(entries.hasMoreElements());
+
+  // Fake a reset.
+  let promiseRemoved = promiseOneObserver("weave:service:remove-file-log");
+  Svc.Obs.notify("weave:service:start-over:finish");
+  await promiseRemoved;
+
+  // should be no files left.
+  Assert.ok(!logsdir.directoryEntries.hasMoreElements());
+});
diff --git a/services/sync/tests/unit/test_errorhandler_sync_checkServerError.js b/services/sync/tests/unit/test_errorhandler_sync_checkServerError.js
new file mode 100644
index 0000000000..d73d548cc7
--- /dev/null
+++ b/services/sync/tests/unit/test_errorhandler_sync_checkServerError.js
@@ -0,0 +1,294 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+const { FakeCryptoService } = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/fakeservices.sys.mjs"
+);
+
+var engineManager = Service.engineManager;
+
+function CatapultEngine() {
+  SyncEngine.call(this, "Catapult", Service);
+}
+CatapultEngine.prototype = {
+  exception: null, // tests fill this in
+  async _sync() {
+    throw this.exception;
+  },
+};
+Object.setPrototypeOf(CatapultEngine.prototype, SyncEngine.prototype);
+
+async function sync_httpd_setup() {
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+
+  let catapultEngine = engineManager.get("catapult");
+  let syncID = await catapultEngine.resetLocalSyncID();
+  let engines = { catapult: { version: catapultEngine.version, syncID } };
+
+  // Track these using the collections helper, which keeps modified times
+  // up-to-date.
+  let clientsColl = new ServerCollection({}, true);
+  let keysWBO = new ServerWBO("keys");
+  let globalWBO = new ServerWBO("global", {
+    storageVersion: STORAGE_VERSION,
+    syncID: Utils.makeGUID(),
+    engines,
+  });
+
+  let handlers = {
+    "/1.1/johndoe/info/collections": collectionsHelper.handler,
+    "/1.1/johndoe/storage/meta/global": upd("meta", globalWBO.handler()),
+    "/1.1/johndoe/storage/clients": upd("clients", clientsColl.handler()),
+    "/1.1/johndoe/storage/crypto/keys": upd("crypto", keysWBO.handler()),
+  };
+  return httpd_setup(handlers);
+}
+
+async function setUp(server) {
+  await configureIdentity({ username: "johndoe" }, server);
+  new FakeCryptoService();
+  syncTestLogging();
+}
+
+async function generateAndUploadKeys(server) {
+  await generateNewKeys(Service.collectionKeys);
+  let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+  await serverKeys.encrypt(Service.identity.syncKeyBundle);
+  let res = Service.resource(
+    server.baseURI + "/1.1/johndoe/storage/crypto/keys"
+  );
+  return (await serverKeys.upload(res)).success;
+}
+
+add_task(async function setup() {
+  await engineManager.clear();
+  validate_all_future_pings();
+  await engineManager.register(CatapultEngine);
+});
+
+add_task(async function test_backoff500() {
+  enableValidationPrefs();
+
+  _("Test: HTTP 500 sets backoff status.");
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let engine = engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = { status: 500 };
+
+  try {
+    Assert.ok(!Status.enforceBackoff);
+
+    // Forcibly create and upload keys here -- otherwise we don't get to the 500!
+    Assert.ok(await generateAndUploadKeys(server));
+
+    await Service.login();
+    await Service.sync();
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(Status.sync, SYNC_SUCCEEDED);
+    Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+  } finally {
+    Status.resetBackoff();
+    await Service.startOver();
+  }
+  await promiseStopServer(server);
+});
+
+add_task(async function test_backoff503() {
+  enableValidationPrefs();
+
+  _(
+    "Test: HTTP 503 with Retry-After header leads to backoff notification and sets backoff status."
+  );
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  const BACKOFF = 42;
+  let engine = engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = { status: 503, headers: { "retry-after": BACKOFF } };
+
+  let backoffInterval;
+  Svc.Obs.add("weave:service:backoff:interval", function (subject) {
+    backoffInterval = subject;
+  });
+
+  try {
+    Assert.ok(!Status.enforceBackoff);
+
+    Assert.ok(await generateAndUploadKeys(server));
+
+    await Service.login();
+    await Service.sync();
+
+    Assert.ok(Status.enforceBackoff);
+    Assert.equal(backoffInterval, BACKOFF);
+    Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+    Assert.equal(Status.sync, SERVER_MAINTENANCE);
+  } finally {
+    Status.resetBackoff();
+    Status.resetSync();
+    await Service.startOver();
+  }
+  await promiseStopServer(server);
+});
+
+add_task(async function test_overQuota() {
+  enableValidationPrefs();
+
+  _("Test: HTTP 400 with body error code 14 means over quota.");
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let engine = engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = {
+    status: 400,
+    toString() {
+      return "14";
+    },
+  };
+
+  try {
+    Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+    Assert.ok(await generateAndUploadKeys(server));
+
+    await Service.login();
+    await Service.sync();
+
+    Assert.equal(Status.sync, OVER_QUOTA);
+    Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+  } finally {
+    Status.resetSync();
+    await Service.startOver();
+  }
+  await promiseStopServer(server);
+});
+
+add_task(async function test_service_networkError() {
+  enableValidationPrefs();
+
+  _(
+    "Test: Connection refused error from Service.sync() leads to the right status code."
+  );
+  let server = await sync_httpd_setup();
+  await setUp(server);
+  await promiseStopServer(server);
+  // Provoke connection refused.
+  Service.clusterURL = "http://localhost:12345/";
+
+  try {
+    Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+    Service._loggedIn = true;
+    await Service.sync();
+
+    Assert.equal(Status.sync, LOGIN_FAILED_NETWORK_ERROR);
+    Assert.equal(Status.service, SYNC_FAILED);
+  } finally {
+    Status.resetSync();
+    await Service.startOver();
+  }
+});
+
+add_task(async function test_service_offline() {
+  enableValidationPrefs();
+
+  _(
+    "Test: Wanting to sync in offline mode leads to the right status code but does not increment the ignorable error count."
+  );
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  await promiseStopServer(server);
+  Services.io.offline = true;
+  Services.prefs.setBoolPref("network.dns.offline-localhost", false);
+
+  try {
+    Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+    Service._loggedIn = true;
+    await Service.sync();
+
+    Assert.equal(Status.sync, LOGIN_FAILED_NETWORK_ERROR);
+    Assert.equal(Status.service, SYNC_FAILED);
+  } finally {
+    Status.resetSync();
+    await Service.startOver();
+  }
+  Services.io.offline = false;
+  Services.prefs.clearUserPref("network.dns.offline-localhost");
+});
+
+add_task(async function test_engine_networkError() {
+  enableValidationPrefs();
+
+  _(
+    "Test: Network related exceptions from engine.sync() lead to the right status code."
+  );
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let engine = engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = Components.Exception(
+    "NS_ERROR_UNKNOWN_HOST",
+    Cr.NS_ERROR_UNKNOWN_HOST
+  );
+
+  try {
+    Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+    Assert.ok(await generateAndUploadKeys(server));
+
+    await Service.login();
+    await Service.sync();
+
+    Assert.equal(Status.sync, LOGIN_FAILED_NETWORK_ERROR);
+    Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+  } finally {
+    Status.resetSync();
+    await Service.startOver();
+  }
+  await promiseStopServer(server);
+});
+
+add_task(async function test_resource_timeout() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let engine = engineManager.get("catapult");
+  engine.enabled = true;
+  // Resource throws this when it encounters a timeout.
+  engine.exception = Components.Exception(
+    "Aborting due to channel inactivity.",
+    Cr.NS_ERROR_NET_TIMEOUT
+  );
+
+  try {
+    Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+    Assert.ok(await generateAndUploadKeys(server));
+
+    await Service.login();
+    await Service.sync();
+
+    Assert.equal(Status.sync, LOGIN_FAILED_NETWORK_ERROR);
+    Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+  } finally {
+    Status.resetSync();
+    await Service.startOver();
+  }
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_extension_storage_engine.js b/services/sync/tests/unit/test_extension_storage_engine.js
new file mode 100644
index 0000000000..a061812aca
--- /dev/null
+++ b/services/sync/tests/unit/test_extension_storage_engine.js
@@ -0,0 +1,275 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  Service: "resource://services-sync/service.sys.mjs",
+  extensionStorageSync: "resource://gre/modules/ExtensionStorageSync.sys.mjs",
+});
+
+const { ExtensionStorageEngineBridge, ExtensionStorageEngineKinto } =
+  ChromeUtils.importESModule(
+    "resource://services-sync/engines/extension-storage.sys.mjs"
+  );
+
+const { BridgeWrapperXPCOM } = ChromeUtils.importESModule(
+  "resource://services-sync/bridged_engine.sys.mjs"
+);
+
+Services.prefs.setStringPref("webextensions.storage.sync.log.level", "debug");
+
+add_task(async function test_switching_between_kinto_and_bridged() {
+  function assertUsingKinto(message) {
+    let kintoEngine = Service.engineManager.get("extension-storage");
+    Assert.ok(kintoEngine instanceof ExtensionStorageEngineKinto, message);
+  }
+  function assertUsingBridged(message) {
+    let bridgedEngine = Service.engineManager.get("extension-storage");
+    Assert.ok(bridgedEngine instanceof ExtensionStorageEngineBridge, message);
+  }
+
+  let isUsingKinto = Services.prefs.getBoolPref(
+    "webextensions.storage.sync.kinto",
+    false
+  );
+  if (isUsingKinto) {
+    assertUsingKinto("Should use Kinto engine before flipping pref");
+  } else {
+    assertUsingBridged("Should use bridged engine before flipping pref");
+  }
+
+  _("Flip pref");
+  Services.prefs.setBoolPref("webextensions.storage.sync.kinto", !isUsingKinto);
+  await Service.engineManager.switchAlternatives();
+
+  if (isUsingKinto) {
+    assertUsingBridged("Should use bridged engine after flipping pref");
+  } else {
+    assertUsingKinto("Should use Kinto engine after flipping pref");
+  }
+
+  _("Clean up");
+  Services.prefs.clearUserPref("webextensions.storage.sync.kinto");
+  await Service.engineManager.switchAlternatives();
+});
+
+add_task(async function test_enable() {
+  const PREF = "services.sync.engine.extension-storage.force";
+
+  let addonsEngine = Service.engineManager.get("addons");
+  let extensionStorageEngine = Service.engineManager.get("extension-storage");
+
+  try {
+    Assert.ok(
+      addonsEngine.enabled,
+      "Add-ons engine should be enabled by default"
+    );
+    Assert.ok(
+      extensionStorageEngine.enabled,
+      "Extension storage engine should be enabled by default"
+    );
+
+    addonsEngine.enabled = false;
+    Assert.ok(
+      !extensionStorageEngine.enabled,
+      "Disabling add-ons should disable extension storage"
+    );
+
+    extensionStorageEngine.enabled = true;
+    Assert.ok(
+      !extensionStorageEngine.enabled,
+      "Enabling extension storage without override pref shouldn't work"
+    );
+
+    Services.prefs.setBoolPref(PREF, true);
+    Assert.ok(
+      extensionStorageEngine.enabled,
+      "Setting override pref should enable extension storage"
+    );
+
+    extensionStorageEngine.enabled = false;
+    Assert.ok(
+      !extensionStorageEngine.enabled,
+      "Disabling extension storage engine with override pref should work"
+    );
+
+    extensionStorageEngine.enabled = true;
+    Assert.ok(
+      extensionStorageEngine.enabled,
+      "Enabling extension storage with override pref should work"
+    );
+  } finally {
+    addonsEngine.enabled = true;
+    Services.prefs.clearUserPref(PREF);
+  }
+});
+
+add_task(async function test_notifyPendingChanges() {
+  let engine = new ExtensionStorageEngineBridge(Service);
+
+  let extension = { id: "ext-1" };
+  let expectedChange = {
+    a: "b",
+    c: "d",
+  };
+
+  let lastSync = 0;
+  let syncID = Utils.makeGUID();
+  let error = null;
+  engine.component = {
+    QueryInterface: ChromeUtils.generateQI([
+      "mozIBridgedSyncEngine",
+      "mozIExtensionStorageArea",
+      "mozISyncedExtensionStorageArea",
+    ]),
+    ensureCurrentSyncId(id, callback) {
+      if (syncID != id) {
+        syncID = id;
+        lastSync = 0;
+      }
+      callback.handleSuccess(id);
+    },
+    resetSyncId(callback) {
+      callback.handleSuccess(syncID);
+    },
+    syncStarted(callback) {
+      callback.handleSuccess();
+    },
+    getLastSync(callback) {
+      callback.handleSuccess(lastSync);
+    },
+    setLastSync(lastSyncMillis, callback) {
+      lastSync = lastSyncMillis;
+      callback.handleSuccess();
+    },
+    apply(callback) {
+      callback.handleSuccess([]);
+    },
+    fetchPendingSyncChanges(callback) {
+      if (error) {
+        callback.handleError(Cr.NS_ERROR_FAILURE, error.message);
+      } else {
+        callback.onChanged(extension.id, JSON.stringify(expectedChange));
+        callback.handleSuccess();
+      }
+    },
+    setUploaded(modified, ids, callback) {
+      callback.handleSuccess();
+    },
+    syncFinished(callback) {
+      callback.handleSuccess();
+    },
+    takeMigrationInfo(callback) {
+      callback.handleSuccess(null);
+    },
+  };
+
+  engine._bridge = new BridgeWrapperXPCOM(engine.component);
+
+  let server = await serverForFoo(engine);
+
+  let actualChanges = [];
+  let listener = changes => actualChanges.push(changes);
+  extensionStorageSync.addOnChangedListener(extension, listener);
+
+  try {
+    await SyncTestingInfrastructure(server);
+
+    info("Sync engine; notify about changes");
+    await sync_engine_and_validate_telem(engine, false);
+    deepEqual(
+      actualChanges,
+      [expectedChange],
+      "Should notify about changes during sync"
+    );
+
+    error = new Error("oops!");
+    actualChanges = [];
+    await sync_engine_and_validate_telem(engine, false);
+    deepEqual(
+      actualChanges,
+      [],
+      "Should finish syncing even if notifying about changes fails"
+    );
+  } finally {
+    extensionStorageSync.removeOnChangedListener(extension, listener);
+    await promiseStopServer(server);
+    await engine.finalize();
+  }
+});
+
+// It's difficult to know what to test - there's already tests for the bridged
+// engine etc - so we just try and check that this engine conforms to the
+// mozIBridgedSyncEngine interface guarantees.
+add_task(async function test_engine() {
+  // Forcibly set the bridged engine in the engine manager. the reason we do
+  // this, unlike the other tests where we just create the engine, is so that
+  // telemetry can get at the engine's `overrideTelemetryName`, which it gets
+  // through the engine manager.
+  await Service.engineManager.unregister("extension-storage");
+  await Service.engineManager.register(ExtensionStorageEngineBridge);
+  let engine = Service.engineManager.get("extension-storage");
+  Assert.equal(engine.version, 1);
+
+  Assert.deepEqual(await engine.getSyncID(), null);
+  await engine.resetLocalSyncID();
+  Assert.notEqual(await engine.getSyncID(), null);
+
+  Assert.equal(await engine.getLastSync(), 0);
+  // lastSync is seconds on this side of the world, but milli-seconds on the other.
+  await engine.setLastSync(1234.567);
+  // should have 2 digit precision.
+  Assert.equal(await engine.getLastSync(), 1234.57);
+  await engine.setLastSync(0);
+
+  // Set some data.
+  await extensionStorageSync.set({ id: "ext-2" }, { ext_2_key: "ext_2_value" });
+  // Now do a sync with out regular test server.
+  let server = await serverForFoo(engine);
+  try {
+    await SyncTestingInfrastructure(server);
+
+    info("Add server records");
+    let foo = server.user("foo");
+    let collection = foo.collection("extension-storage");
+    let now = new_timestamp();
+
+    collection.insert(
+      "fakeguid0000",
+      encryptPayload({
+        id: "fakeguid0000",
+        extId: "ext-1",
+        data: JSON.stringify({ foo: "bar" }),
+      }),
+      now
+    );
+
+    info("Sync the engine");
+
+    let ping = await sync_engine_and_validate_telem(engine, false);
+    Assert.ok(ping.engines.find(e => e.name == "rust-webext-storage"));
+    Assert.equal(
+      ping.engines.find(e => e.name == "extension-storage"),
+      null
+    );
+
+    // We should have applied the data from the existing collection record.
+    Assert.deepEqual(await extensionStorageSync.get({ id: "ext-1" }, null), {
+      foo: "bar",
+    });
+
+    // should now be 2 records on the server.
+    let payloads = collection.payloads();
+    Assert.equal(payloads.length, 2);
+    // find the new one we wrote.
+    let newPayload =
+      payloads[0].id == "fakeguid0000" ? payloads[1] : payloads[0];
+    Assert.equal(newPayload.data, `{"ext_2_key":"ext_2_value"}`);
+    // should have updated the timestamp.
+    greater(await engine.getLastSync(), 0, "Should update last sync time");
+  } finally {
+    await promiseStopServer(server);
+    await engine.finalize();
+  }
+});
diff --git a/services/sync/tests/unit/test_extension_storage_engine_kinto.js b/services/sync/tests/unit/test_extension_storage_engine_kinto.js
new file mode 100644
index 0000000000..b074fe376c
--- /dev/null
+++ b/services/sync/tests/unit/test_extension_storage_engine_kinto.js
@@ -0,0 +1,136 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Services.prefs.setBoolPref("webextensions.storage.sync.kinto", true);
+
+const { ExtensionStorageEngineKinto: ExtensionStorageEngine } =
+  ChromeUtils.importESModule(
+    "resource://services-sync/engines/extension-storage.sys.mjs"
+  );
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { extensionStorageSyncKinto: extensionStorageSync } =
+  ChromeUtils.importESModule(
+    "resource://gre/modules/ExtensionStorageSyncKinto.sys.mjs"
+  );
+
+let engine;
+
+function mock(options) {
+  let calls = [];
+  let ret = function () {
+    calls.push(arguments);
+    return options.returns;
+  };
+  let proto = {
+    get calls() {
+      return calls;
+    },
+  };
+  Object.setPrototypeOf(proto, Function.prototype);
+  Object.setPrototypeOf(ret, proto);
+  return ret;
+}
+
+function setSkipChance(v) {
+  Services.prefs.setIntPref(
+    "services.sync.extension-storage.skipPercentageChance",
+    v
+  );
+}
+
+add_task(async function setup() {
+  await Service.engineManager.register(ExtensionStorageEngine);
+  engine = Service.engineManager.get("extension-storage");
+  do_get_profile(); // so we can use FxAccounts
+  loadWebExtensionTestFunctions();
+  setSkipChance(0);
+});
+
+add_task(async function test_calling_sync_calls__sync() {
+  let oldSync = ExtensionStorageEngine.prototype._sync;
+  let syncMock = (ExtensionStorageEngine.prototype._sync = mock({
+    returns: true,
+  }));
+  try {
+    // I wanted to call the main sync entry point for the entire
+    // package, but that fails because it tries to sync ClientEngine
+    // first, which fails.
+    await engine.sync();
+  } finally {
+    ExtensionStorageEngine.prototype._sync = oldSync;
+  }
+  equal(syncMock.calls.length, 1);
+});
+
+add_task(async function test_sync_skip() {
+  try {
+    // Do a few times to ensure we aren't getting "lucky" WRT Math.random()
+    for (let i = 0; i < 10; ++i) {
+      setSkipChance(100);
+      engine._tracker._score = 0;
+      ok(
+        !engine.shouldSkipSync("user"),
+        "Should allow explicitly requested syncs"
+      );
+      ok(!engine.shouldSkipSync("startup"), "Should allow startup syncs");
+      ok(
+        engine.shouldSkipSync("schedule"),
+        "Should skip scheduled syncs if skipProbability is 100"
+      );
+      engine._tracker._score = MULTI_DEVICE_THRESHOLD;
+      ok(
+        !engine.shouldSkipSync("schedule"),
+        "should allow scheduled syncs if tracker score is high"
+      );
+      engine._tracker._score = 0;
+      setSkipChance(0);
+      ok(
+        !engine.shouldSkipSync("schedule"),
+        "Should allow scheduled syncs if probability is 0"
+      );
+    }
+  } finally {
+    engine._tracker._score = 0;
+    setSkipChance(0);
+  }
+});
+
+add_task(async function test_calling_wipeClient_calls_clearAll() {
+  let oldClearAll = extensionStorageSync.clearAll;
+  let clearMock = (extensionStorageSync.clearAll = mock({
+    returns: Promise.resolve(),
+  }));
+  try {
+    await engine.wipeClient();
+  } finally {
+    extensionStorageSync.clearAll = oldClearAll;
+  }
+  equal(clearMock.calls.length, 1);
+});
+
+add_task(async function test_calling_sync_calls_ext_storage_sync() {
+  const extension = { id: "my-extension" };
+  let oldSync = extensionStorageSync.syncAll;
+  let syncMock = (extensionStorageSync.syncAll = mock({
+    returns: Promise.resolve(),
+  }));
+  try {
+    await withSyncContext(async function (context) {
+      // Set something so that everyone knows that we're using storage.sync
+      await extensionStorageSync.set(extension, { a: "b" }, context);
+      let ping = await sync_engine_and_validate_telem(engine, false);
+      Assert.ok(ping.engines.find(e => e.name == "extension-storage"));
+      Assert.equal(
+        ping.engines.find(e => e.name == "rust-webext-storage"),
+        null
+      );
+    });
+  } finally {
+    extensionStorageSync.syncAll = oldSync;
+  }
+  Assert.ok(syncMock.calls.length >= 1);
+});
diff --git a/services/sync/tests/unit/test_extension_storage_migration_telem.js b/services/sync/tests/unit/test_extension_storage_migration_telem.js
new file mode 100644
index 0000000000..a4b4c95f55
--- /dev/null
+++ b/services/sync/tests/unit/test_extension_storage_migration_telem.js
@@ -0,0 +1,81 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Import the rust-based and kinto-based implementations. Not great to grab
+// these as they're somewhat private, but we want to run the pings through our
+// validation machinery which is here in the sync test code.
+const { extensionStorageSync: rustImpl } = ChromeUtils.importESModule(
+  "resource://gre/modules/ExtensionStorageSync.sys.mjs"
+);
+const { extensionStorageSyncKinto: kintoImpl } = ChromeUtils.importESModule(
+  "resource://gre/modules/ExtensionStorageSyncKinto.sys.mjs"
+);
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { ExtensionStorageEngineBridge } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/extension-storage.sys.mjs"
+);
+
+Services.prefs.setBoolPref("webextensions.storage.sync.kinto", false);
+Services.prefs.setStringPref("webextensions.storage.sync.log.level", "debug");
+
+// It's tricky to force error cases here (the databases are opened with
+// exclusive locks) and that part of the code has coverage in the vendored
+// application-services webext-storage crate. So this just tests that the
+// migration data ends up in the ping, and exactly once.
+add_task(async function test_sync_migration_telem() {
+  // Set some stuff using the kinto-based impl prior to fully setting up sync.
+  let e1 = { id: "test@mozilla.com" };
+  let c1 = { extension: e1, callOnClose() {} };
+
+  let e2 = { id: "test-2@mozilla.com" };
+  let c2 = { extension: e2, callOnClose() {} };
+  await kintoImpl.set(e1, { foo: "bar" }, c1);
+  await kintoImpl.set(e1, { baz: "quux" }, c1);
+  await kintoImpl.set(e2, { second: "2nd" }, c2);
+
+  Assert.deepEqual(await rustImpl.get(e1, "foo", c1), { foo: "bar" });
+  Assert.deepEqual(await rustImpl.get(e1, "baz", c1), { baz: "quux" });
+  Assert.deepEqual(await rustImpl.get(e2, null, c2), { second: "2nd" });
+
+  // Explicitly unregister first. It's very possible this isn't needed for this
+  // case, however it's fairly harmless, we hope to uplift this patch to beta,
+  // and earlier today we had beta-only problems caused by this (bug 1629116)
+  await Service.engineManager.unregister("extension-storage");
+  await Service.engineManager.register(ExtensionStorageEngineBridge);
+  let engine = Service.engineManager.get("extension-storage");
+  let server = await serverForFoo(engine, undefined);
+  try {
+    await SyncTestingInfrastructure(server);
+    await Service.engineManager.switchAlternatives();
+
+    _("First sync");
+    let ping = await sync_engine_and_validate_telem(engine, false, null, true);
+    Assert.deepEqual(ping.migrations, [
+      {
+        type: "webext-storage",
+        entries: 3,
+        entriesSuccessful: 3,
+        extensions: 2,
+        extensionsSuccessful: 2,
+        openFailure: false,
+      },
+    ]);
+
+    // force another sync
+    await engine.setLastSync(0);
+    _("Second sync");
+
+    ping = await sync_engine_and_validate_telem(engine, false, null, true);
+    Assert.deepEqual(ping.migrations, undefined);
+  } finally {
+    await kintoImpl.clear(e1, c1);
+    await kintoImpl.clear(e2, c2);
+    await rustImpl.clear(e1, c1);
+    await rustImpl.clear(e2, c2);
+    await promiseStopServer(server);
+    await engine.finalize();
+  }
+});
diff --git a/services/sync/tests/unit/test_extension_storage_tracker_kinto.js b/services/sync/tests/unit/test_extension_storage_tracker_kinto.js
new file mode 100644
index 0000000000..2de56ae400
--- /dev/null
+++ b/services/sync/tests/unit/test_extension_storage_tracker_kinto.js
@@ -0,0 +1,44 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Services.prefs.setBoolPref("webextensions.storage.sync.kinto", true);
+
+const { ExtensionStorageEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/extension-storage.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { extensionStorageSyncKinto: extensionStorageSync } =
+  ChromeUtils.importESModule(
+    "resource://gre/modules/ExtensionStorageSyncKinto.sys.mjs"
+  );
+
+let engine;
+
+add_task(async function setup() {
+  await Service.engineManager.register(ExtensionStorageEngine);
+  engine = Service.engineManager.get("extension-storage");
+  do_get_profile(); // so we can use FxAccounts
+  loadWebExtensionTestFunctions();
+});
+
+add_task(async function test_changing_extension_storage_changes_score() {
+  const tracker = engine._tracker;
+  const extension = { id: "my-extension-id" };
+  tracker.start();
+  await withSyncContext(async function (context) {
+    await extensionStorageSync.set(extension, { a: "b" }, context);
+  });
+  Assert.equal(tracker.score, SCORE_INCREMENT_MEDIUM);
+
+  tracker.resetScore();
+  await withSyncContext(async function (context) {
+    await extensionStorageSync.remove(extension, "a", context);
+  });
+  Assert.equal(tracker.score, SCORE_INCREMENT_MEDIUM);
+
+  await tracker.stop();
+});
diff --git a/services/sync/tests/unit/test_form_validator.js b/services/sync/tests/unit/test_form_validator.js
new file mode 100644
index 0000000000..58ea8b855b
--- /dev/null
+++ b/services/sync/tests/unit/test_form_validator.js
@@ -0,0 +1,86 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { FormValidator } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/forms.sys.mjs"
+);
+
+function getDummyServerAndClient() {
+  return {
+    server: [
+      {
+        id: "11111",
+        guid: "11111",
+        name: "foo",
+        fieldname: "foo",
+        value: "bar",
+      },
+      {
+        id: "22222",
+        guid: "22222",
+        name: "foo2",
+        fieldname: "foo2",
+        value: "bar2",
+      },
+      {
+        id: "33333",
+        guid: "33333",
+        name: "foo3",
+        fieldname: "foo3",
+        value: "bar3",
+      },
+    ],
+    client: [
+      {
+        id: "11111",
+        guid: "11111",
+        name: "foo",
+        fieldname: "foo",
+        value: "bar",
+      },
+      {
+        id: "22222",
+        guid: "22222",
+        name: "foo2",
+        fieldname: "foo2",
+        value: "bar2",
+      },
+      {
+        id: "33333",
+        guid: "33333",
+        name: "foo3",
+        fieldname: "foo3",
+        value: "bar3",
+      },
+    ],
+  };
+}
+
+add_task(async function test_valid() {
+  let { server, client } = getDummyServerAndClient();
+  let validator = new FormValidator();
+  let { problemData, clientRecords, records, deletedRecords } =
+    await validator.compareClientWithServer(client, server);
+  equal(clientRecords.length, 3);
+  equal(records.length, 3);
+  equal(deletedRecords.length, 0);
+  deepEqual(problemData, validator.emptyProblemData());
+});
+
+add_task(async function test_formValidatorIgnoresMissingClients() {
+  // Since history form records are not deleted from the server, the
+  // |FormValidator| shouldn't set the |missingClient| flag in |problemData|.
+  let { server, client } = getDummyServerAndClient();
+  client.pop();
+
+  let validator = new FormValidator();
+  let { problemData, clientRecords, records, deletedRecords } =
+    await validator.compareClientWithServer(client, server);
+
+  equal(clientRecords.length, 2);
+  equal(records.length, 3);
+  equal(deletedRecords.length, 0);
+
+  let expected = validator.emptyProblemData();
+  deepEqual(problemData, expected);
+});
diff --git a/services/sync/tests/unit/test_forms_store.js b/services/sync/tests/unit/test_forms_store.js
new file mode 100644
index 0000000000..716487865f
--- /dev/null
+++ b/services/sync/tests/unit/test_forms_store.js
@@ -0,0 +1,176 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+_(
+  "Make sure the form store follows the Store api and correctly accesses the backend form storage"
+);
+const { FormEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/forms.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { SyncedRecordsTelemetry } = ChromeUtils.importESModule(
+  "resource://services-sync/telemetry.sys.mjs"
+);
+
+add_task(async function run_test() {
+  let engine = new FormEngine(Service);
+  await engine.initialize();
+  let store = engine._store;
+
+  async function applyEnsureNoFailures(records) {
+    let countTelemetry = new SyncedRecordsTelemetry();
+    Assert.equal(
+      (await store.applyIncomingBatch(records, countTelemetry)).length,
+      0
+    );
+  }
+
+  _("Remove any existing entries");
+  await store.wipe();
+  if ((await store.getAllIDs()).length) {
+    do_throw("Shouldn't get any ids!");
+  }
+
+  _("Add a form entry");
+  await applyEnsureNoFailures([
+    {
+      id: Utils.makeGUID(),
+      name: "name!!",
+      value: "value??",
+    },
+  ]);
+
+  _("Should have 1 entry now");
+  let id = "";
+  for (let _id in await store.getAllIDs()) {
+    if (id == "") {
+      id = _id;
+    } else {
+      do_throw("Should have only gotten one!");
+    }
+  }
+  Assert.ok(store.itemExists(id));
+
+  _("Should be able to find this entry as a dupe");
+  Assert.equal(
+    await engine._findDupe({ name: "name!!", value: "value??" }),
+    id
+  );
+
+  let rec = await store.createRecord(id);
+  _("Got record for id", id, rec);
+  Assert.equal(rec.name, "name!!");
+  Assert.equal(rec.value, "value??");
+
+  _("Create a non-existent id for delete");
+  Assert.ok((await store.createRecord("deleted!!")).deleted);
+
+  _("Try updating.. doesn't do anything yet");
+  await store.update({});
+
+  _("Remove all entries");
+  await store.wipe();
+  if ((await store.getAllIDs()).length) {
+    do_throw("Shouldn't get any ids!");
+  }
+
+  _("Add another entry");
+  await applyEnsureNoFailures([
+    {
+      id: Utils.makeGUID(),
+      name: "another",
+      value: "entry",
+    },
+  ]);
+  id = "";
+  for (let _id in await store.getAllIDs()) {
+    if (id == "") {
+      id = _id;
+    } else {
+      do_throw("Should have only gotten one!");
+    }
+  }
+
+  _("Change the id of the new entry to something else");
+  await store.changeItemID(id, "newid");
+
+  _("Make sure it's there");
+  Assert.ok(store.itemExists("newid"));
+
+  _("Remove the entry");
+  await store.remove({
+    id: "newid",
+  });
+  if ((await store.getAllIDs()).length) {
+    do_throw("Shouldn't get any ids!");
+  }
+
+  _("Removing the entry again shouldn't matter");
+  await store.remove({
+    id: "newid",
+  });
+  if ((await store.getAllIDs()).length) {
+    do_throw("Shouldn't get any ids!");
+  }
+
+  _("Add another entry to delete using applyIncomingBatch");
+  let toDelete = {
+    id: Utils.makeGUID(),
+    name: "todelete",
+    value: "entry",
+  };
+  await applyEnsureNoFailures([toDelete]);
+  id = "";
+  for (let _id in await store.getAllIDs()) {
+    if (id == "") {
+      id = _id;
+    } else {
+      do_throw("Should have only gotten one!");
+    }
+  }
+  Assert.ok(store.itemExists(id));
+  // mark entry as deleted
+  toDelete.id = id;
+  toDelete.deleted = true;
+  await applyEnsureNoFailures([toDelete]);
+  if ((await store.getAllIDs()).length) {
+    do_throw("Shouldn't get any ids!");
+  }
+
+  _("Add an entry to wipe");
+  await applyEnsureNoFailures([
+    {
+      id: Utils.makeGUID(),
+      name: "towipe",
+      value: "entry",
+    },
+  ]);
+
+  await store.wipe();
+
+  if ((await store.getAllIDs()).length) {
+    do_throw("Shouldn't get any ids!");
+  }
+
+  _("Ensure we work if formfill is disabled.");
+  Services.prefs.setBoolPref("browser.formfill.enable", false);
+  try {
+    // a search
+    if ((await store.getAllIDs()).length) {
+      do_throw("Shouldn't get any ids!");
+    }
+    // an update.
+    await applyEnsureNoFailures([
+      {
+        id: Utils.makeGUID(),
+        name: "some",
+        value: "entry",
+      },
+    ]);
+  } finally {
+    Services.prefs.clearUserPref("browser.formfill.enable");
+    await store.wipe();
+  }
+});
diff --git a/services/sync/tests/unit/test_forms_tracker.js b/services/sync/tests/unit/test_forms_tracker.js
new file mode 100644
index 0000000000..aee74381ad
--- /dev/null
+++ b/services/sync/tests/unit/test_forms_tracker.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { FormEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/forms.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function run_test() {
+  _("Verify we've got an empty tracker to work with.");
+  let engine = new FormEngine(Service);
+  await engine.initialize();
+  let tracker = engine._tracker;
+
+  let changes = await tracker.getChangedIDs();
+  do_check_empty(changes);
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  async function addEntry(name, value) {
+    await engine._store.create({ name, value });
+    await engine._tracker.asyncObserver.promiseObserversComplete();
+  }
+  async function removeEntry(name, value) {
+    let guid = await engine._findDupe({ name, value });
+    await engine._store.remove({ id: guid });
+    await engine._tracker.asyncObserver.promiseObserversComplete();
+  }
+
+  try {
+    _("Create an entry. Won't show because we haven't started tracking yet");
+    await addEntry("name", "John Doe");
+    changes = await tracker.getChangedIDs();
+    do_check_empty(changes);
+
+    _("Tell the tracker to start tracking changes.");
+    tracker.start();
+    await removeEntry("name", "John Doe");
+    await addEntry("email", "john@doe.com");
+    changes = await tracker.getChangedIDs();
+    do_check_attribute_count(changes, 2);
+
+    _("Notifying twice won't do any harm.");
+    tracker.start();
+    await addEntry("address", "Memory Lane");
+    changes = await tracker.getChangedIDs();
+    do_check_attribute_count(changes, 3);
+
+    _("Check that ignoreAll is respected");
+    await tracker.clearChangedIDs();
+    tracker.score = 0;
+    tracker.ignoreAll = true;
+    await addEntry("username", "johndoe123");
+    await addEntry("favoritecolor", "green");
+    await removeEntry("name", "John Doe");
+    tracker.ignoreAll = false;
+    changes = await tracker.getChangedIDs();
+    do_check_empty(changes);
+    equal(tracker.score, 0);
+
+    _("Let's stop tracking again.");
+    await tracker.clearChangedIDs();
+    await tracker.stop();
+    await removeEntry("address", "Memory Lane");
+    changes = await tracker.getChangedIDs();
+    do_check_empty(changes);
+
+    _("Notifying twice won't do any harm.");
+    await tracker.stop();
+    await removeEntry("email", "john@doe.com");
+    changes = await tracker.getChangedIDs();
+    do_check_empty(changes);
+  } finally {
+    _("Clean up.");
+    await engine._store.wipe();
+  }
+});
diff --git a/services/sync/tests/unit/test_fxa_node_reassignment.js b/services/sync/tests/unit/test_fxa_node_reassignment.js
new file mode 100644
index 0000000000..0b25df0183
--- /dev/null
+++ b/services/sync/tests/unit/test_fxa_node_reassignment.js
@@ -0,0 +1,399 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+_("Test that node reassignment happens correctly using the FxA identity mgr.");
+// The node-reassignment logic is quite different for FxA than for the legacy
+// provider.  In particular, there's no special request necessary for
+// reassignment - it comes from the token server - so we need to ensure the
+// Fxa cluster manager grabs a new token.
+
+const { RESTRequest } = ChromeUtils.importESModule(
+  "resource://services-common/rest.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+const { SyncAuthManager } = ChromeUtils.importESModule(
+  "resource://services-sync/sync_auth.sys.mjs"
+);
+
+add_task(async function setup() {
+  // Disables all built-in engines. Important for avoiding errors thrown by the
+  // add-ons engine.
+  await Service.engineManager.clear();
+
+  // Setup the sync auth manager.
+  Status.__authManager = Service.identity = new SyncAuthManager();
+});
+
+// API-compatible with SyncServer handler. Bind `handler` to something to use
+// as a ServerCollection handler.
+function handleReassign(handler, req, resp) {
+  resp.setStatusLine(req.httpVersion, 401, "Node reassignment");
+  resp.setHeader("Content-Type", "application/json");
+  let reassignBody = JSON.stringify({ error: "401inator in place" });
+  resp.bodyOutputStream.write(reassignBody, reassignBody.length);
+}
+
+var numTokenRequests = 0;
+
+function prepareServer(cbAfterTokenFetch) {
+  syncTestLogging();
+  let config = makeIdentityConfig({ username: "johndoe" });
+  // A server callback to ensure we don't accidentally hit the wrong endpoint
+  // after a node reassignment.
+  let callback = {
+    onRequest(req, resp) {
+      let full = `${req.scheme}://${req.host}:${req.port}${req.path}`;
+      let expected = config.fxaccount.token.endpoint;
+      Assert.ok(
+        full.startsWith(expected),
+        `request made to ${full}, expected ${expected}`
+      );
+    },
+  };
+  Object.setPrototypeOf(callback, SyncServerCallback);
+  let server = new SyncServer(callback);
+  server.registerUser("johndoe");
+  server.start();
+
+  // Set the token endpoint for the initial token request that's done implicitly
+  // via configureIdentity.
+  config.fxaccount.token.endpoint = server.baseURI + "1.1/johndoe/";
+  // And future token fetches will do magic around numReassigns.
+  let numReassigns = 0;
+  return configureIdentity(config).then(() => {
+    Service.identity._tokenServerClient = {
+      getTokenUsingOAuth() {
+        return new Promise(res => {
+          // Build a new URL with trailing zeros for the SYNC_VERSION part - this
+          // will still be seen as equivalent by the test server, but different
+          // by sync itself.
+          numReassigns += 1;
+          let trailingZeros = new Array(numReassigns + 1).join("0");
+          let token = config.fxaccount.token;
+          token.endpoint = server.baseURI + "1.1" + trailingZeros + "/johndoe";
+          token.uid = config.username;
+          _(`test server saw token fetch - endpoint now ${token.endpoint}`);
+          numTokenRequests += 1;
+          res(token);
+          if (cbAfterTokenFetch) {
+            cbAfterTokenFetch();
+          }
+        });
+      },
+    };
+    return server;
+  });
+}
+
+function getReassigned() {
+  try {
+    return Services.prefs.getBoolPref("services.sync.lastSyncReassigned");
+  } catch (ex) {
+    if (ex.result != Cr.NS_ERROR_UNEXPECTED) {
+      do_throw(
+        "Got exception retrieving lastSyncReassigned: " + Log.exceptionStr(ex)
+      );
+    }
+  }
+  return false;
+}
+
+/**
+ * Make a test request to `url`, then watch the result of two syncs
+ * to ensure that a node request was made.
+ * Runs `between` between the two. This can be used to undo deliberate failure
+ * setup, detach observers, etc.
+ */
+async function syncAndExpectNodeReassignment(
+  server,
+  firstNotification,
+  between,
+  secondNotification,
+  url
+) {
+  _("Starting syncAndExpectNodeReassignment\n");
+  let deferred = Promise.withResolvers();
+  async function onwards() {
+    let numTokenRequestsBefore;
+    function onFirstSync() {
+      _("First sync completed.");
+      Svc.Obs.remove(firstNotification, onFirstSync);
+      Svc.Obs.add(secondNotification, onSecondSync);
+
+      Assert.equal(Service.clusterURL, "");
+
+      // Track whether we fetched a new token.
+      numTokenRequestsBefore = numTokenRequests;
+
+      // Allow for tests to clean up error conditions.
+      between();
+    }
+    function onSecondSync() {
+      _("Second sync completed.");
+      Svc.Obs.remove(secondNotification, onSecondSync);
+      Service.scheduler.clearSyncTriggers();
+
+      // Make absolutely sure that any event listeners are done with their work
+      // before we proceed.
+      waitForZeroTimer(function () {
+        _("Second sync nextTick.");
+        Assert.equal(
+          numTokenRequests,
+          numTokenRequestsBefore + 1,
+          "fetched a new token"
+        );
+        Service.startOver().then(() => {
+          server.stop(deferred.resolve);
+        });
+      });
+    }
+
+    Svc.Obs.add(firstNotification, onFirstSync);
+    await Service.sync();
+  }
+
+  // Make sure that we really do get a 401 (but we can only do that if we are
+  // already logged in, as the login process is what sets up the URLs)
+  if (Service.isLoggedIn) {
+    _("Making request to " + url + " which should 401");
+    let request = new RESTRequest(url);
+    await request.get();
+    Assert.equal(request.response.status, 401);
+    CommonUtils.nextTick(onwards);
+  } else {
+    _("Skipping preliminary validation check for a 401 as we aren't logged in");
+    CommonUtils.nextTick(onwards);
+  }
+  await deferred.promise;
+}
+
+// Check that when we sync we don't request a new token by default - our
+// test setup has configured the client with a valid token, and that token
+// should be used to form the cluster URL.
+add_task(async function test_single_token_fetch() {
+  enableValidationPrefs();
+
+  _("Test a normal sync only fetches 1 token");
+
+  let numTokenFetches = 0;
+
+  function afterTokenFetch() {
+    numTokenFetches++;
+  }
+
+  // Set the cluster URL to an "old" version - this is to ensure we don't
+  // use that old cached version for the first sync but prefer the value
+  // we got from the token (and as above, we are also checking we don't grab
+  // a new token). If the test actually attempts to connect to this URL
+  // it will crash.
+  Service.clusterURL = "http://example.com/";
+
+  let server = await prepareServer(afterTokenFetch);
+
+  Assert.ok(!Service.isLoggedIn, "not already logged in");
+  await Service.sync();
+  Assert.equal(Status.sync, SYNC_SUCCEEDED, "sync succeeded");
+  Assert.equal(numTokenFetches, 0, "didn't fetch a new token");
+  // A bit hacky, but given we know how prepareServer works we can deduce
+  // that clusterURL we expect.
+  let expectedClusterURL = server.baseURI + "1.1/johndoe/";
+  Assert.equal(Service.clusterURL, expectedClusterURL);
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_momentary_401_engine() {
+  enableValidationPrefs();
+
+  _("Test a failure for engine URLs that's resolved by reassignment.");
+  let server = await prepareServer();
+  let john = server.user("johndoe");
+
+  _("Enabling the Rotary engine.");
+  let { engine, syncID, tracker } = await registerRotaryEngine();
+
+  // We need the server to be correctly set up prior to experimenting. Do this
+  // through a sync.
+  let global = {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    rotary: { version: engine.version, syncID },
+  };
+  john.createCollection("meta").insert("global", global);
+
+  _("First sync to prepare server contents.");
+  await Service.sync();
+
+  _("Setting up Rotary collection to 401.");
+  let rotary = john.createCollection("rotary");
+  let oldHandler = rotary.collectionHandler;
+  rotary.collectionHandler = handleReassign.bind(this, undefined);
+
+  // We want to verify that the clusterURL pref has been cleared after a 401
+  // inside a sync. Flag the Rotary engine to need syncing.
+  john.collection("rotary").timestamp += 1000;
+
+  function between() {
+    _("Undoing test changes.");
+    rotary.collectionHandler = oldHandler;
+
+    function onLoginStart() {
+      // lastSyncReassigned shouldn't be cleared until a sync has succeeded.
+      _("Ensuring that lastSyncReassigned is still set at next sync start.");
+      Svc.Obs.remove("weave:service:login:start", onLoginStart);
+      Assert.ok(getReassigned());
+    }
+
+    _("Adding observer that lastSyncReassigned is still set on login.");
+    Svc.Obs.add("weave:service:login:start", onLoginStart);
+  }
+
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:sync:finish",
+    between,
+    "weave:service:sync:finish",
+    Service.storageURL + "rotary"
+  );
+
+  await tracker.clearChangedIDs();
+  await Service.engineManager.unregister(engine);
+});
+
+// This test ends up being a failing info fetch *after we're already logged in*.
+add_task(async function test_momentary_401_info_collections_loggedin() {
+  enableValidationPrefs();
+
+  _(
+    "Test a failure for info/collections after login that's resolved by reassignment."
+  );
+  let server = await prepareServer();
+
+  _("First sync to prepare server contents.");
+  await Service.sync();
+
+  _("Arrange for info/collections to return a 401.");
+  let oldHandler = server.toplevelHandlers.info;
+  server.toplevelHandlers.info = handleReassign;
+
+  function undo() {
+    _("Undoing test changes.");
+    server.toplevelHandlers.info = oldHandler;
+  }
+
+  Assert.ok(Service.isLoggedIn, "already logged in");
+
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:sync:error",
+    undo,
+    "weave:service:sync:finish",
+    Service.infoURL
+  );
+});
+
+// This test ends up being a failing info fetch *before we're logged in*.
+// In this case we expect to recover during the login phase - so the first
+// sync succeeds.
+add_task(async function test_momentary_401_info_collections_loggedout() {
+  enableValidationPrefs();
+
+  _(
+    "Test a failure for info/collections before login that's resolved by reassignment."
+  );
+
+  let oldHandler;
+  let sawTokenFetch = false;
+
+  function afterTokenFetch() {
+    // After a single token fetch, we undo our evil handleReassign hack, so
+    // the next /info request returns the collection instead of a 401
+    server.toplevelHandlers.info = oldHandler;
+    sawTokenFetch = true;
+  }
+
+  let server = await prepareServer(afterTokenFetch);
+
+  // Return a 401 for the next /info request - it will be reset immediately
+  // after a new token is fetched.
+  oldHandler = server.toplevelHandlers.info;
+  server.toplevelHandlers.info = handleReassign;
+
+  Assert.ok(!Service.isLoggedIn, "not already logged in");
+
+  await Service.sync();
+  Assert.equal(Status.sync, SYNC_SUCCEEDED, "sync succeeded");
+  // sync was successful - check we grabbed a new token.
+  Assert.ok(sawTokenFetch, "a new token was fetched by this test.");
+  // and we are done.
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+// This test ends up being a failing meta/global fetch *after we're already logged in*.
+add_task(async function test_momentary_401_storage_loggedin() {
+  enableValidationPrefs();
+
+  _(
+    "Test a failure for any storage URL after login that's resolved by" +
+      "reassignment."
+  );
+  let server = await prepareServer();
+
+  _("First sync to prepare server contents.");
+  await Service.sync();
+
+  _("Arrange for meta/global to return a 401.");
+  let oldHandler = server.toplevelHandlers.storage;
+  server.toplevelHandlers.storage = handleReassign;
+
+  function undo() {
+    _("Undoing test changes.");
+    server.toplevelHandlers.storage = oldHandler;
+  }
+
+  Assert.ok(Service.isLoggedIn, "already logged in");
+
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:sync:error",
+    undo,
+    "weave:service:sync:finish",
+    Service.storageURL + "meta/global"
+  );
+});
+
+// This test ends up being a failing meta/global fetch *before we've logged in*.
+add_task(async function test_momentary_401_storage_loggedout() {
+  enableValidationPrefs();
+
+  _(
+    "Test a failure for any storage URL before login, not just engine parts. " +
+      "Resolved by reassignment."
+  );
+  let server = await prepareServer();
+
+  // Return a 401 for all storage requests.
+  let oldHandler = server.toplevelHandlers.storage;
+  server.toplevelHandlers.storage = handleReassign;
+
+  function undo() {
+    _("Undoing test changes.");
+    server.toplevelHandlers.storage = oldHandler;
+  }
+
+  Assert.ok(!Service.isLoggedIn, "already logged in");
+
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:login:error",
+    undo,
+    "weave:service:sync:finish",
+    Service.storageURL + "meta/global"
+  );
+});
diff --git a/services/sync/tests/unit/test_fxa_service_cluster.js b/services/sync/tests/unit/test_fxa_service_cluster.js
new file mode 100644
index 0000000000..0203d01ef5
--- /dev/null
+++ b/services/sync/tests/unit/test_fxa_service_cluster.js
@@ -0,0 +1,58 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { initializeIdentityWithTokenServerResponse } =
+  ChromeUtils.importESModule(
+    "resource://testing-common/services/sync/fxa_utils.sys.mjs"
+  );
+
+add_task(async function test_findCluster() {
+  _("Test FxA _findCluster()");
+
+  _("_findCluster() throws on 500 errors.");
+  initializeIdentityWithTokenServerResponse({
+    status: 500,
+    headers: [],
+    body: "",
+  });
+
+  await Assert.rejects(
+    Service.identity._findCluster(),
+    /TokenServerClientServerError/
+  );
+
+  _("_findCluster() returns null on authentication errors.");
+  initializeIdentityWithTokenServerResponse({
+    status: 401,
+    headers: { "content-type": "application/json" },
+    body: "{}",
+  });
+
+  let cluster = await Service.identity._findCluster();
+  Assert.strictEqual(cluster, null);
+
+  _("_findCluster() works with correct tokenserver response.");
+  let endpoint = "http://example.com/something";
+  initializeIdentityWithTokenServerResponse({
+    status: 200,
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({
+      api_endpoint: endpoint,
+      duration: 300,
+      id: "id",
+      key: "key",
+      uid: "uid",
+    }),
+  });
+
+  cluster = await Service.identity._findCluster();
+  // The cluster manager ensures a trailing "/"
+  Assert.strictEqual(cluster, endpoint + "/");
+
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+});
diff --git a/services/sync/tests/unit/test_history_engine.js b/services/sync/tests/unit/test_history_engine.js
new file mode 100644
index 0000000000..9cca379b0b
--- /dev/null
+++ b/services/sync/tests/unit/test_history_engine.js
@@ -0,0 +1,429 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { HistoryEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/history.sys.mjs"
+);
+
+// Use only for rawAddVisit.
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "asyncHistory",
+  "@mozilla.org/browser/history;1",
+  "mozIAsyncHistory"
+);
+async function rawAddVisit(id, uri, visitPRTime, transitionType) {
+  return new Promise((resolve, reject) => {
+    let results = [];
+    let handler = {
+      handleResult(result) {
+        results.push(result);
+      },
+      handleError(resultCode, placeInfo) {
+        do_throw(`updatePlaces gave error ${resultCode}!`);
+      },
+      handleCompletion(count) {
+        resolve({ results, count });
+      },
+    };
+    asyncHistory.updatePlaces(
+      [
+        {
+          guid: id,
+          uri: typeof uri == "string" ? CommonUtils.makeURI(uri) : uri,
+          visits: [{ visitDate: visitPRTime, transitionType }],
+        },
+      ],
+      handler
+    );
+  });
+}
+
+add_task(async function test_history_download_limit() {
+  let engine = new HistoryEngine(Service);
+  await engine.initialize();
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let lastSync = new_timestamp();
+
+  let collection = server.user("foo").collection("history");
+  for (let i = 0; i < 15; i++) {
+    let id = "place" + i.toString(10).padStart(7, "0");
+    let wbo = new ServerWBO(
+      id,
+      encryptPayload({
+        id,
+        histUri: "http://example.com/" + i,
+        title: "Page " + i,
+        visits: [
+          {
+            date: Date.now() * 1000,
+            type: PlacesUtils.history.TRANSITIONS.TYPED,
+          },
+          {
+            date: Date.now() * 1000,
+            type: PlacesUtils.history.TRANSITIONS.LINK,
+          },
+        ],
+      }),
+      lastSync + 1 + i
+    );
+    wbo.sortindex = 15 - i;
+    collection.insertWBO(wbo);
+  }
+
+  // We have 15 records on the server since the last sync, but our download
+  // limit is 5 records at a time. We should eventually fetch all 15.
+  await engine.setLastSync(lastSync);
+  engine.downloadBatchSize = 4;
+  engine.downloadLimit = 5;
+
+  // Don't actually fetch any backlogged records, so that we can inspect
+  // the backlog between syncs.
+  engine.guidFetchBatchSize = 0;
+
+  let ping = await sync_engine_and_validate_telem(engine, false);
+  deepEqual(ping.engines[0].incoming, { applied: 5 });
+
+  let backlogAfterFirstSync = Array.from(engine.toFetch).sort();
+  deepEqual(backlogAfterFirstSync, [
+    "place0000000",
+    "place0000001",
+    "place0000002",
+    "place0000003",
+    "place0000004",
+    "place0000005",
+    "place0000006",
+    "place0000007",
+    "place0000008",
+    "place0000009",
+  ]);
+
+  // We should have fast-forwarded the last sync time.
+  equal(await engine.getLastSync(), lastSync + 15);
+
+  engine.lastModified = collection.modified;
+  ping = await sync_engine_and_validate_telem(engine, false);
+  ok(!ping.engines[0].incoming);
+
+  // After the second sync, our backlog still contains the same GUIDs: we
+  // weren't able to make progress on fetching them, since our
+  // `guidFetchBatchSize` is 0.
+  let backlogAfterSecondSync = Array.from(engine.toFetch).sort();
+  deepEqual(backlogAfterFirstSync, backlogAfterSecondSync);
+
+  // Now add a newer record to the server.
+  let newWBO = new ServerWBO(
+    "placeAAAAAAA",
+    encryptPayload({
+      id: "placeAAAAAAA",
+      histUri: "http://example.com/a",
+      title: "New Page A",
+      visits: [
+        {
+          date: Date.now() * 1000,
+          type: PlacesUtils.history.TRANSITIONS.TYPED,
+        },
+      ],
+    }),
+    lastSync + 20
+  );
+  newWBO.sortindex = -1;
+  collection.insertWBO(newWBO);
+
+  engine.lastModified = collection.modified;
+  ping = await sync_engine_and_validate_telem(engine, false);
+  deepEqual(ping.engines[0].incoming, { applied: 1 });
+
+  // Our backlog should remain the same.
+  let backlogAfterThirdSync = Array.from(engine.toFetch).sort();
+  deepEqual(backlogAfterSecondSync, backlogAfterThirdSync);
+
+  equal(await engine.getLastSync(), lastSync + 20);
+
+  // Bump the fetch batch size to let the backlog make progress. We should
+  // make 3 requests to fetch 5 backlogged GUIDs.
+  engine.guidFetchBatchSize = 2;
+
+  engine.lastModified = collection.modified;
+  ping = await sync_engine_and_validate_telem(engine, false);
+  deepEqual(ping.engines[0].incoming, { applied: 5 });
+
+  deepEqual(Array.from(engine.toFetch).sort(), [
+    "place0000005",
+    "place0000006",
+    "place0000007",
+    "place0000008",
+    "place0000009",
+  ]);
+
+  // Sync again to clear out the backlog.
+  engine.lastModified = collection.modified;
+  ping = await sync_engine_and_validate_telem(engine, false);
+  deepEqual(ping.engines[0].incoming, { applied: 5 });
+
+  deepEqual(Array.from(engine.toFetch), []);
+
+  await engine.wipeClient();
+  await engine.finalize();
+});
+
+add_task(async function test_history_visit_roundtrip() {
+  let engine = new HistoryEngine(Service);
+  await engine.initialize();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  engine._tracker.start();
+
+  let id = "aaaaaaaaaaaa";
+  let oneHourMS = 60 * 60 * 1000;
+  // Insert a visit with a non-round microsecond timestamp (e.g. it's not evenly
+  // divisible by 1000). This will typically be the case for visits that occur
+  // during normal navigation.
+  let time = (Date.now() - oneHourMS) * 1000 + 555;
+  // We use the low level history api since it lets us provide microseconds
+  let { count } = await rawAddVisit(
+    id,
+    "https://www.example.com",
+    time,
+    PlacesUtils.history.TRANSITIONS.TYPED
+  );
+  equal(count, 1);
+  // Check that it was inserted and that we didn't round on the insert.
+  let visits = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "https://www.example.com"
+  );
+  equal(visits.length, 1);
+  equal(visits[0].date, time);
+
+  let collection = server.user("foo").collection("history");
+
+  // Sync the visit up to the server.
+  await sync_engine_and_validate_telem(engine, false);
+
+  collection.updateRecord(
+    id,
+    cleartext => {
+      // Double-check that we didn't round the visit's timestamp to the nearest
+      // millisecond when uploading.
+      equal(cleartext.visits[0].date, time);
+      // Add a remote visit so that we get past the deepEquals check in reconcile
+      // (otherwise the history engine will skip applying this record). The
+      // contents of this visit don't matter, beyond the fact that it needs to
+      // exist.
+      cleartext.visits.push({
+        date: (Date.now() - oneHourMS / 2) * 1000,
+        type: PlacesUtils.history.TRANSITIONS.LINK,
+      });
+    },
+    new_timestamp() + 10
+  );
+
+  // Force a remote sync.
+  await engine.setLastSync(new_timestamp() - 30);
+  await sync_engine_and_validate_telem(engine, false);
+
+  // Make sure that we didn't duplicate the visit when inserting. (Prior to bug
+  // 1423395, we would insert a duplicate visit, where the timestamp was
+  // effectively `Math.round(microsecondTimestamp / 1000) * 1000`.)
+  visits = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "https://www.example.com"
+  );
+  equal(visits.length, 2);
+
+  await engine.wipeClient();
+  await engine.finalize();
+});
+
+add_task(async function test_history_visit_dedupe_old() {
+  let engine = new HistoryEngine(Service);
+  await engine.initialize();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let initialVisits = Array.from({ length: 25 }, (_, index) => ({
+    transition: PlacesUtils.history.TRANSITION_LINK,
+    date: new Date(Date.UTC(2017, 10, 1 + index)),
+  }));
+  initialVisits.push({
+    transition: PlacesUtils.history.TRANSITION_LINK,
+    date: new Date(),
+  });
+  await PlacesUtils.history.insert({
+    url: "https://www.example.com",
+    visits: initialVisits,
+  });
+
+  let recentVisits = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "https://www.example.com"
+  );
+  equal(recentVisits.length, 20);
+  let { visits: allVisits, guid } = await PlacesUtils.history.fetch(
+    "https://www.example.com",
+    {
+      includeVisits: true,
+    }
+  );
+  equal(allVisits.length, 26);
+
+  let collection = server.user("foo").collection("history");
+
+  await sync_engine_and_validate_telem(engine, false);
+
+  collection.updateRecord(
+    guid,
+    data => {
+      data.visits.push(
+        // Add a couple remote visit equivalent to some old visits we have already
+        {
+          date: Date.UTC(2017, 10, 1) * 1000, // Nov 1, 2017
+          type: PlacesUtils.history.TRANSITIONS.LINK,
+        },
+        {
+          date: Date.UTC(2017, 10, 2) * 1000, // Nov 2, 2017
+          type: PlacesUtils.history.TRANSITIONS.LINK,
+        },
+        // Add a couple new visits to make sure we are still applying them.
+        {
+          date: Date.UTC(2017, 11, 4) * 1000, // Dec 4, 2017
+          type: PlacesUtils.history.TRANSITIONS.LINK,
+        },
+        {
+          date: Date.UTC(2017, 11, 5) * 1000, // Dec 5, 2017
+          type: PlacesUtils.history.TRANSITIONS.LINK,
+        }
+      );
+    },
+    new_timestamp() + 10
+  );
+
+  await engine.setLastSync(new_timestamp() - 30);
+  await sync_engine_and_validate_telem(engine, false);
+
+  allVisits = (
+    await PlacesUtils.history.fetch("https://www.example.com", {
+      includeVisits: true,
+    })
+  ).visits;
+
+  equal(allVisits.length, 28);
+  ok(
+    allVisits.find(x => x.date.getTime() === Date.UTC(2017, 11, 4)),
+    "Should contain the Dec. 4th visit"
+  );
+  ok(
+    allVisits.find(x => x.date.getTime() === Date.UTC(2017, 11, 5)),
+    "Should contain the Dec. 5th visit"
+  );
+
+  await engine.wipeClient();
+  await engine.finalize();
+});
+
+add_task(async function test_history_unknown_fields() {
+  let engine = new HistoryEngine(Service);
+  await engine.initialize();
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  engine._tracker.start();
+
+  let id = "aaaaaaaaaaaa";
+  let oneHourMS = 60 * 60 * 1000;
+  // Insert a visit with a non-round microsecond timestamp (e.g. it's not evenly
+  // divisible by 1000). This will typically be the case for visits that occur
+  // during normal navigation.
+  let time = (Date.now() - oneHourMS) * 1000 + 555;
+  // We use the low level history api since it lets us provide microseconds
+  let { count } = await rawAddVisit(
+    id,
+    "https://www.example.com",
+    time,
+    PlacesUtils.history.TRANSITIONS.TYPED
+  );
+  equal(count, 1);
+
+  let collection = server.user("foo").collection("history");
+
+  // Sync the visit up to the server.
+  await sync_engine_and_validate_telem(engine, false);
+
+  collection.updateRecord(
+    id,
+    cleartext => {
+      equal(cleartext.visits[0].date, time);
+
+      // Add unknown fields to an instance of a visit
+      cleartext.visits.push({
+        date: (Date.now() - oneHourMS / 2) * 1000,
+        type: PlacesUtils.history.TRANSITIONS.LINK,
+        unknownVisitField: "an unknown field could show up in a visit!",
+      });
+      cleartext.title = "A page title";
+      // Add unknown fields to the payload for this URL
+      cleartext.unknownStrField = "an unknown str field";
+      cleartext.unknownObjField = { newField: "a field within an object" };
+    },
+    new_timestamp() + 10
+  );
+
+  // Force a remote sync.
+  await engine.setLastSync(new_timestamp() - 30);
+  await sync_engine_and_validate_telem(engine, false);
+
+  // Add a new visit to ensure we're actually putting things back on the server
+  let newTime = (Date.now() - oneHourMS) * 1000 + 555;
+  await rawAddVisit(
+    id,
+    "https://www.example.com",
+    newTime,
+    PlacesUtils.history.TRANSITIONS.LINK
+  );
+
+  // Sync again
+  await engine.setLastSync(new_timestamp() - 30);
+  await sync_engine_and_validate_telem(engine, false);
+
+  let placeInfo = await PlacesSyncUtils.history.fetchURLInfoForGuid(id);
+
+  // Found the place we're looking for
+  Assert.equal(placeInfo.title, "A page title");
+  Assert.equal(placeInfo.url, "https://www.example.com/");
+
+  // It correctly returns any unknownFields that might've been
+  // stored in the moz_places_extra table
+  deepEqual(JSON.parse(placeInfo.unknownFields), {
+    unknownStrField: "an unknown str field",
+    unknownObjField: { newField: "a field within an object" },
+  });
+
+  // Getting visits via SyncUtils also will return unknownFields
+  // via the moz_historyvisits_extra table
+  let visits = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "https://www.example.com"
+  );
+  equal(visits.length, 3);
+
+  // fetchVisitsForURL is a sync method that gets called during upload
+  // so unknown field should already be at the top-level
+  deepEqual(
+    visits[0].unknownVisitField,
+    "an unknown field could show up in a visit!"
+  );
+
+  // Remote history record should have the fields back at the top level
+  let remotePlace = collection.payloads().find(rec => rec.id === id);
+  deepEqual(remotePlace.unknownStrField, "an unknown str field");
+  deepEqual(remotePlace.unknownObjField, {
+    newField: "a field within an object",
+  });
+
+  await engine.wipeClient();
+  await engine.finalize();
+});
diff --git a/services/sync/tests/unit/test_history_store.js b/services/sync/tests/unit/test_history_store.js
new file mode 100644
index 0000000000..07aee0dd01
--- /dev/null
+++ b/services/sync/tests/unit/test_history_store.js
@@ -0,0 +1,570 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { HistoryEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/history.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { SyncedRecordsTelemetry } = ChromeUtils.importESModule(
+  "resource://services-sync/telemetry.sys.mjs"
+);
+
+const TIMESTAMP1 = (Date.now() - 103406528) * 1000;
+const TIMESTAMP2 = (Date.now() - 6592903) * 1000;
+const TIMESTAMP3 = (Date.now() - 123894) * 1000;
+
+function promiseOnVisitObserved() {
+  return new Promise(res => {
+    let listener = new PlacesWeakCallbackWrapper(events => {
+      PlacesObservers.removeListener(["page-visited"], listener);
+      res();
+    });
+    PlacesObservers.addListener(["page-visited"], listener);
+  });
+}
+
+function isDateApproximately(actual, expected, skewMillis = 1000) {
+  let lowerBound = expected - skewMillis;
+  let upperBound = expected + skewMillis;
+  return actual >= lowerBound && actual <= upperBound;
+}
+
+let engine, store, fxuri, fxguid, tburi, tbguid;
+
+async function applyEnsureNoFailures(records) {
+  let countTelemetry = new SyncedRecordsTelemetry();
+  Assert.equal(
+    (await store.applyIncomingBatch(records, countTelemetry)).length,
+    0
+  );
+}
+
+add_task(async function setup() {
+  engine = new HistoryEngine(Service);
+  await engine.initialize();
+  store = engine._store;
+});
+
+add_task(async function test_store() {
+  _("Verify that we've got an empty store to work with.");
+  do_check_empty(await store.getAllIDs());
+
+  _("Let's create an entry in the database.");
+  fxuri = CommonUtils.makeURI("http://getfirefox.com/");
+
+  await PlacesTestUtils.addVisits({
+    uri: fxuri,
+    title: "Get Firefox!",
+    visitDate: TIMESTAMP1,
+  });
+  _("Verify that the entry exists.");
+  let ids = Object.keys(await store.getAllIDs());
+  Assert.equal(ids.length, 1);
+  fxguid = ids[0];
+  Assert.ok(await store.itemExists(fxguid));
+
+  _("If we query a non-existent record, it's marked as deleted.");
+  let record = await store.createRecord("non-existent");
+  Assert.ok(record.deleted);
+
+  _("Verify createRecord() returns a complete record.");
+  record = await store.createRecord(fxguid);
+  Assert.equal(record.histUri, fxuri.spec);
+  Assert.equal(record.title, "Get Firefox!");
+  Assert.equal(record.visits.length, 1);
+  Assert.equal(record.visits[0].date, TIMESTAMP1);
+  Assert.equal(record.visits[0].type, Ci.nsINavHistoryService.TRANSITION_LINK);
+
+  _("Let's modify the record and have the store update the database.");
+  let secondvisit = {
+    date: TIMESTAMP2,
+    type: Ci.nsINavHistoryService.TRANSITION_TYPED,
+  };
+  let onVisitObserved = promiseOnVisitObserved();
+  let updatedRec = await store.createRecord(fxguid);
+  updatedRec.cleartext.title = "Hol Dir Firefox!";
+  updatedRec.cleartext.visits.push(secondvisit);
+  await applyEnsureNoFailures([updatedRec]);
+  await onVisitObserved;
+  let queryres = await PlacesUtils.history.fetch(fxuri.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(queryres.title, "Hol Dir Firefox!");
+  Assert.deepEqual(queryres.visits, [
+    {
+      date: new Date(TIMESTAMP2 / 1000),
+      transition: Ci.nsINavHistoryService.TRANSITION_TYPED,
+    },
+    {
+      date: new Date(TIMESTAMP1 / 1000),
+      transition: Ci.nsINavHistoryService.TRANSITION_LINK,
+    },
+  ]);
+  await PlacesUtils.history.clear();
+});
+
+add_task(async function test_store_create() {
+  _("Create a brand new record through the store.");
+  tbguid = Utils.makeGUID();
+  tburi = CommonUtils.makeURI("http://getthunderbird.com");
+  let onVisitObserved = promiseOnVisitObserved();
+  let record = await store.createRecord(tbguid);
+  record.cleartext = {
+    id: tbguid,
+    histUri: tburi.spec,
+    title: "The bird is the word!",
+    visits: [
+      { date: TIMESTAMP3, type: Ci.nsINavHistoryService.TRANSITION_TYPED },
+    ],
+  };
+  await applyEnsureNoFailures([record]);
+  await onVisitObserved;
+  Assert.ok(await store.itemExists(tbguid));
+  do_check_attribute_count(await store.getAllIDs(), 1);
+  let queryres = await PlacesUtils.history.fetch(tburi.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(queryres.title, "The bird is the word!");
+  Assert.deepEqual(queryres.visits, [
+    {
+      date: new Date(TIMESTAMP3 / 1000),
+      transition: Ci.nsINavHistoryService.TRANSITION_TYPED,
+    },
+  ]);
+  await PlacesUtils.history.clear();
+});
+
+add_task(async function test_null_title() {
+  _(
+    "Make sure we handle a null title gracefully (it can happen in some cases, e.g. for resource:// URLs)"
+  );
+  let resguid = Utils.makeGUID();
+  let resuri = CommonUtils.makeURI("unknown://title");
+  let record = await store.createRecord(resguid);
+  record.cleartext = {
+    id: resguid,
+    histUri: resuri.spec,
+    title: null,
+    visits: [
+      { date: TIMESTAMP3, type: Ci.nsINavHistoryService.TRANSITION_TYPED },
+    ],
+  };
+  await applyEnsureNoFailures([record]);
+  do_check_attribute_count(await store.getAllIDs(), 1);
+
+  let queryres = await PlacesUtils.history.fetch(resuri.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(queryres.title, "");
+  Assert.deepEqual(queryres.visits, [
+    {
+      date: new Date(TIMESTAMP3 / 1000),
+      transition: Ci.nsINavHistoryService.TRANSITION_TYPED,
+    },
+  ]);
+  await PlacesUtils.history.clear();
+});
+
+add_task(async function test_invalid_records() {
+  _("Make sure we handle invalid URLs in places databases gracefully.");
+  await PlacesUtils.withConnectionWrapper(
+    "test_invalid_record",
+    async function (db) {
+      await db.execute(
+        "INSERT INTO moz_places " +
+          "(url, url_hash, title, rev_host, visit_count, last_visit_date) " +
+          "VALUES ('invalid-uri', hash('invalid-uri'), 'Invalid URI', '.', 1, " +
+          TIMESTAMP3 +
+          ")"
+      );
+      // Add the corresponding visit to retain database coherence.
+      await db.execute(
+        "INSERT INTO moz_historyvisits " +
+          "(place_id, visit_date, visit_type, session) " +
+          "VALUES ((SELECT id FROM moz_places WHERE url_hash = hash('invalid-uri') AND url = 'invalid-uri'), " +
+          TIMESTAMP3 +
+          ", " +
+          Ci.nsINavHistoryService.TRANSITION_TYPED +
+          ", 1)"
+      );
+    }
+  );
+  do_check_attribute_count(await store.getAllIDs(), 1);
+
+  _("Make sure we report records with invalid URIs.");
+  let invalid_uri_guid = Utils.makeGUID();
+  let countTelemetry = new SyncedRecordsTelemetry();
+  let failed = await store.applyIncomingBatch(
+    [
+      {
+        id: invalid_uri_guid,
+        histUri: ":::::::::::::::",
+        title: "Doesn't have a valid URI",
+        visits: [
+          { date: TIMESTAMP3, type: Ci.nsINavHistoryService.TRANSITION_EMBED },
+        ],
+      },
+    ],
+    countTelemetry
+  );
+  Assert.equal(failed.length, 1);
+  Assert.equal(failed[0], invalid_uri_guid);
+  Assert.equal(
+    countTelemetry.incomingCounts.failedReasons[0].name,
+    "<URL> is not a valid URL."
+  );
+  Assert.equal(countTelemetry.incomingCounts.failedReasons[0].count, 1);
+
+  _("Make sure we handle records with invalid GUIDs gracefully (ignore).");
+  await applyEnsureNoFailures([
+    {
+      id: "invalid",
+      histUri: "http://invalid.guid/",
+      title: "Doesn't have a valid GUID",
+      visits: [
+        { date: TIMESTAMP3, type: Ci.nsINavHistoryService.TRANSITION_EMBED },
+      ],
+    },
+  ]);
+
+  _(
+    "Make sure we handle records with invalid visit codes or visit dates, gracefully ignoring those visits."
+  );
+  let no_date_visit_guid = Utils.makeGUID();
+  let no_type_visit_guid = Utils.makeGUID();
+  let invalid_type_visit_guid = Utils.makeGUID();
+  let non_integer_visit_guid = Utils.makeGUID();
+  countTelemetry = new SyncedRecordsTelemetry();
+  failed = await store.applyIncomingBatch(
+    [
+      {
+        id: no_date_visit_guid,
+        histUri: "http://no.date.visit/",
+        title: "Visit has no date",
+        visits: [{ type: Ci.nsINavHistoryService.TRANSITION_EMBED }],
+      },
+      {
+        id: no_type_visit_guid,
+        histUri: "http://no.type.visit/",
+        title: "Visit has no type",
+        visits: [{ date: TIMESTAMP3 }],
+      },
+      {
+        id: invalid_type_visit_guid,
+        histUri: "http://invalid.type.visit/",
+        title: "Visit has invalid type",
+        visits: [
+          {
+            date: TIMESTAMP3,
+            type: Ci.nsINavHistoryService.TRANSITION_LINK - 1,
+          },
+        ],
+      },
+      {
+        id: non_integer_visit_guid,
+        histUri: "http://non.integer.visit/",
+        title: "Visit has non-integer date",
+        visits: [
+          { date: 1234.567, type: Ci.nsINavHistoryService.TRANSITION_EMBED },
+        ],
+      },
+    ],
+    countTelemetry
+  );
+  Assert.equal(failed.length, 0);
+
+  // Make sure we can apply tombstones (both valid and invalid)
+  countTelemetry = new SyncedRecordsTelemetry();
+  failed = await store.applyIncomingBatch(
+    [
+      { id: no_date_visit_guid, deleted: true },
+      { id: "not-a-valid-guid", deleted: true },
+    ],
+    countTelemetry
+  );
+  Assert.deepEqual(failed, ["not-a-valid-guid"]);
+  Assert.equal(
+    countTelemetry.incomingCounts.failedReasons[0].name,
+    "<URL> is not a valid URL."
+  );
+
+  _("Make sure we handle records with javascript: URLs gracefully.");
+  await applyEnsureNoFailures(
+    [
+      {
+        id: Utils.makeGUID(),
+        histUri: "javascript:''",
+        title: "javascript:''",
+        visits: [
+          { date: TIMESTAMP3, type: Ci.nsINavHistoryService.TRANSITION_EMBED },
+        ],
+      },
+    ],
+    countTelemetry
+  );
+
+  _("Make sure we handle records without any visits gracefully.");
+  await applyEnsureNoFailures([
+    {
+      id: Utils.makeGUID(),
+      histUri: "http://getfirebug.com",
+      title: "Get Firebug!",
+      visits: [],
+    },
+  ]);
+});
+
+add_task(async function test_unknowingly_invalid_records() {
+  _("Make sure we handle rejection of records by places gracefully.");
+  let oldCAU = store._canAddURI;
+  store._canAddURI = () => true;
+  try {
+    _("Make sure that when places rejects this record we record it as failed");
+    let guid = Utils.makeGUID();
+    let countTelemetry = new SyncedRecordsTelemetry();
+    let invalidRecord = await store.createRecord(guid);
+    invalidRecord.cleartext = {
+      id: guid,
+      histUri: "javascript:''",
+      title: "javascript:''",
+      visits: [
+        {
+          date: TIMESTAMP3,
+          type: Ci.nsINavHistoryService.TRANSITION_EMBED,
+        },
+      ],
+    };
+    let result = await store.applyIncomingBatch(
+      [invalidRecord],
+      countTelemetry
+    );
+    deepEqual(result, [guid]);
+  } finally {
+    store._canAddURI = oldCAU;
+  }
+});
+
+add_task(async function test_clamp_visit_dates() {
+  let futureVisitTime = Date.now() + 5 * 60 * 1000;
+  let recentVisitTime = Date.now() - 5 * 60 * 1000;
+
+  let recordA = await store.createRecord("visitAAAAAAA");
+  recordA.cleartext = {
+    id: "visitAAAAAAA",
+    histUri: "http://example.com/a",
+    title: "A",
+    visits: [
+      {
+        date: "invalidDate",
+        type: Ci.nsINavHistoryService.TRANSITION_LINK,
+      },
+    ],
+  };
+  let recordB = await store.createRecord("visitBBBBBBB");
+  recordB.cleartext = {
+    id: "visitBBBBBBB",
+    histUri: "http://example.com/b",
+    title: "B",
+    visits: [
+      {
+        date: 100,
+        type: Ci.nsINavHistoryService.TRANSITION_TYPED,
+      },
+      {
+        date: 250,
+        type: Ci.nsINavHistoryService.TRANSITION_TYPED,
+      },
+      {
+        date: recentVisitTime * 1000,
+        type: Ci.nsINavHistoryService.TRANSITION_TYPED,
+      },
+    ],
+  };
+  let recordC = await store.createRecord("visitCCCCCCC");
+  recordC.cleartext = {
+    id: "visitCCCCCCC",
+    histUri: "http://example.com/c",
+    title: "D",
+    visits: [
+      {
+        date: futureVisitTime * 1000,
+        type: Ci.nsINavHistoryService.TRANSITION_BOOKMARK,
+      },
+    ],
+  };
+  let recordD = await store.createRecord("visitDDDDDDD");
+  recordD.cleartext = {
+    id: "visitDDDDDDD",
+    histUri: "http://example.com/d",
+    title: "D",
+    visits: [
+      {
+        date: recentVisitTime * 1000,
+        type: Ci.nsINavHistoryService.TRANSITION_DOWNLOAD,
+      },
+    ],
+  };
+  await applyEnsureNoFailures([recordA, recordB, recordC, recordD]);
+
+  let visitsForA = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "http://example.com/a"
+  );
+  deepEqual(visitsForA, [], "Should ignore visits with invalid dates");
+
+  let visitsForB = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "http://example.com/b"
+  );
+  deepEqual(
+    visitsForB,
+    [
+      {
+        date: recentVisitTime * 1000,
+        type: Ci.nsINavHistoryService.TRANSITION_TYPED,
+      },
+      {
+        // We should clamp visit dates older than original Mosaic release.
+        date: PlacesSyncUtils.bookmarks.EARLIEST_BOOKMARK_TIMESTAMP * 1000,
+        type: Ci.nsINavHistoryService.TRANSITION_TYPED,
+      },
+    ],
+    "Should record clamped visit and valid visit for B"
+  );
+
+  let visitsForC = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "http://example.com/c"
+  );
+  equal(visitsForC.length, 1, "Should record clamped future visit for C");
+  let visitDateForC = PlacesUtils.toDate(visitsForC[0].date);
+  ok(
+    isDateApproximately(visitDateForC, Date.now()),
+    "Should clamp future visit date for C to now"
+  );
+
+  let visitsForD = await PlacesSyncUtils.history.fetchVisitsForURL(
+    "http://example.com/d"
+  );
+  deepEqual(
+    visitsForD,
+    [
+      {
+        date: recentVisitTime * 1000,
+        type: Ci.nsINavHistoryService.TRANSITION_DOWNLOAD,
+      },
+    ],
+    "Should not clamp valid visit dates"
+  );
+});
+
+add_task(async function test_remove() {
+  _("Remove an existent record and a non-existent from the store.");
+  await applyEnsureNoFailures([
+    { id: fxguid, deleted: true },
+    { id: Utils.makeGUID(), deleted: true },
+  ]);
+  Assert.equal(false, await store.itemExists(fxguid));
+  let queryres = await PlacesUtils.history.fetch(fxuri.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(null, queryres);
+
+  _("Make sure wipe works.");
+  await store.wipe();
+  do_check_empty(await store.getAllIDs());
+  queryres = await PlacesUtils.history.fetch(fxuri.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(null, queryres);
+  queryres = await PlacesUtils.history.fetch(tburi.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(null, queryres);
+});
+
+add_task(async function test_chunking() {
+  let mvpi = store.MAX_VISITS_PER_INSERT;
+  store.MAX_VISITS_PER_INSERT = 3;
+  let checkChunks = function (input, expected) {
+    let chunks = Array.from(store._generateChunks(input));
+    deepEqual(chunks, expected);
+  };
+  try {
+    checkChunks([{ visits: ["x"] }], [[{ visits: ["x"] }]]);
+
+    // 3 should still be one chunk.
+    checkChunks([{ visits: ["x", "x", "x"] }], [[{ visits: ["x", "x", "x"] }]]);
+
+    // 4 should still be one chunk as we don't split individual records.
+    checkChunks(
+      [{ visits: ["x", "x", "x", "x"] }],
+      [[{ visits: ["x", "x", "x", "x"] }]]
+    );
+
+    // 4 in the first and 1 in the second should be 2 chunks.
+    checkChunks(
+      [{ visits: ["x", "x", "x", "x"] }, { visits: ["x"] }],
+      // expected
+      [[{ visits: ["x", "x", "x", "x"] }], [{ visits: ["x"] }]]
+    );
+
+    // we put multiple records into chunks
+    checkChunks(
+      [
+        { visits: ["x", "x"] },
+        { visits: ["x"] },
+        { visits: ["x"] },
+        { visits: ["x", "x"] },
+        { visits: ["x", "x", "x", "x"] },
+      ],
+      // expected
+      [
+        [{ visits: ["x", "x"] }, { visits: ["x"] }],
+        [{ visits: ["x"] }, { visits: ["x", "x"] }],
+        [{ visits: ["x", "x", "x", "x"] }],
+      ]
+    );
+  } finally {
+    store.MAX_VISITS_PER_INSERT = mvpi;
+  }
+});
+
+add_task(async function test_getAllIDs_filters_file_uris() {
+  let uri = CommonUtils.makeURI("file:///Users/eoger/tps/config.json");
+  let visitAddedPromise = promiseVisit("added", uri);
+  await PlacesTestUtils.addVisits({
+    uri,
+    visitDate: Date.now() * 1000,
+    transition: PlacesUtils.history.TRANSITION_LINK,
+  });
+  await visitAddedPromise;
+
+  do_check_attribute_count(await store.getAllIDs(), 0);
+
+  await PlacesUtils.history.clear();
+});
+
+add_task(async function test_applyIncomingBatch_filters_file_uris() {
+  const guid = Utils.makeGUID();
+  let uri = CommonUtils.makeURI("file:///Users/eoger/tps/config.json");
+  await applyEnsureNoFailures([
+    {
+      id: guid,
+      histUri: uri.spec,
+      title: "TPS CONFIG",
+      visits: [
+        { date: TIMESTAMP3, type: Ci.nsINavHistoryService.TRANSITION_TYPED },
+      ],
+    },
+  ]);
+  Assert.equal(false, await store.itemExists(guid));
+  let queryres = await PlacesUtils.history.fetch(uri.spec, {
+    includeVisits: true,
+  });
+  Assert.equal(null, queryres);
+});
+
+add_task(async function cleanup() {
+  _("Clean up.");
+  await PlacesUtils.history.clear();
+});
diff --git a/services/sync/tests/unit/test_history_tracker.js b/services/sync/tests/unit/test_history_tracker.js
new file mode 100644
index 0000000000..6f351d6984
--- /dev/null
+++ b/services/sync/tests/unit/test_history_tracker.js
@@ -0,0 +1,251 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { PlacesDBUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/PlacesDBUtils.sys.mjs"
+);
+const { HistoryEngine } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/history.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+let engine;
+let tracker;
+
+add_task(async function setup() {
+  await Service.engineManager.clear();
+  await Service.engineManager.register(HistoryEngine);
+  engine = Service.engineManager.get("history");
+  tracker = engine._tracker;
+});
+
+async function verifyTrackerEmpty() {
+  let changes = await engine.pullNewChanges();
+  do_check_empty(changes);
+  equal(tracker.score, 0);
+}
+
+async function verifyTrackedCount(expected) {
+  let changes = await engine.pullNewChanges();
+  do_check_attribute_count(changes, expected);
+}
+
+async function verifyTrackedItems(tracked) {
+  let changes = await engine.pullNewChanges();
+  let trackedIDs = new Set(Object.keys(changes));
+  for (let guid of tracked) {
+    ok(guid in changes, `${guid} should be tracked`);
+    ok(changes[guid] > 0, `${guid} should have a modified time`);
+    trackedIDs.delete(guid);
+  }
+  equal(
+    trackedIDs.size,
+    0,
+    `Unhandled tracked IDs: ${JSON.stringify(Array.from(trackedIDs))}`
+  );
+}
+
+async function resetTracker() {
+  await tracker.clearChangedIDs();
+  tracker.resetScore();
+}
+
+async function cleanup() {
+  await PlacesUtils.history.clear();
+  await resetTracker();
+  await tracker.stop();
+}
+
+add_task(async function test_empty() {
+  _("Verify we've got an empty, disabled tracker to work with.");
+  await verifyTrackerEmpty();
+  Assert.ok(!tracker._isTracking);
+
+  await cleanup();
+});
+
+add_task(async function test_not_tracking() {
+  _("Create history item. Won't show because we haven't started tracking yet");
+  await addVisit("not_tracking");
+  await verifyTrackerEmpty();
+
+  await cleanup();
+});
+
+add_task(async function test_start_tracking() {
+  _("Add hook for save completion.");
+  let savePromise = new Promise((resolve, reject) => {
+    let save = tracker._storage._save;
+    tracker._storage._save = async function () {
+      try {
+        await save.call(this);
+        resolve();
+      } catch (ex) {
+        reject(ex);
+      } finally {
+        tracker._storage._save = save;
+      }
+    };
+  });
+
+  _("Tell the tracker to start tracking changes.");
+  tracker.start();
+  let scorePromise = promiseOneObserver("weave:engine:score:updated");
+  await addVisit("start_tracking");
+  await scorePromise;
+
+  _("Score updated in test_start_tracking.");
+  await verifyTrackedCount(1);
+  Assert.equal(tracker.score, SCORE_INCREMENT_SMALL);
+
+  await savePromise;
+
+  _("changedIDs written to disk. Proceeding.");
+  await cleanup();
+});
+
+add_task(async function test_start_tracking_twice() {
+  _("Verifying preconditions.");
+  tracker.start();
+  await addVisit("start_tracking_twice1");
+  await verifyTrackedCount(1);
+  Assert.equal(tracker.score, SCORE_INCREMENT_SMALL);
+
+  _("Notifying twice won't do any harm.");
+  tracker.start();
+  let scorePromise = promiseOneObserver("weave:engine:score:updated");
+  await addVisit("start_tracking_twice2");
+  await scorePromise;
+
+  _("Score updated in test_start_tracking_twice.");
+  await verifyTrackedCount(2);
+  Assert.equal(tracker.score, 2 * SCORE_INCREMENT_SMALL);
+
+  await cleanup();
+});
+
+add_task(async function test_track_delete() {
+  _("Deletions are tracked.");
+
+  // This isn't present because we weren't tracking when it was visited.
+  await addVisit("track_delete");
+  let uri = CommonUtils.makeURI("http://getfirefox.com/track_delete");
+  let guid = await engine._store.GUIDForUri(uri.spec);
+  await verifyTrackerEmpty();
+
+  tracker.start();
+  let visitRemovedPromise = promiseVisit("removed", uri);
+  let scorePromise = promiseOneObserver("weave:engine:score:updated");
+  await PlacesUtils.history.remove(uri);
+  await Promise.all([scorePromise, visitRemovedPromise]);
+
+  await verifyTrackedItems([guid]);
+  Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+
+  await cleanup();
+});
+
+add_task(async function test_dont_track_expiration() {
+  _("Expirations are not tracked.");
+  let uriToRemove = await addVisit("to_remove");
+  let guidToRemove = await engine._store.GUIDForUri(uriToRemove.spec);
+
+  await resetTracker();
+  await verifyTrackerEmpty();
+
+  tracker.start();
+  let visitRemovedPromise = promiseVisit("removed", uriToRemove);
+  let scorePromise = promiseOneObserver("weave:engine:score:updated");
+
+  // Observe expiration.
+  Services.obs.addObserver(function onExpiration(aSubject, aTopic, aData) {
+    Services.obs.removeObserver(onExpiration, aTopic);
+    // Remove the remaining page to update its score.
+    PlacesUtils.history.remove(uriToRemove);
+  }, PlacesUtils.TOPIC_EXPIRATION_FINISHED);
+
+  // Force expiration of 1 entry.
+  Services.prefs.setIntPref("places.history.expiration.max_pages", 0);
+  Cc["@mozilla.org/places/expiration;1"]
+    .getService(Ci.nsIObserver)
+    .observe(null, "places-debug-start-expiration", 1);
+
+  await Promise.all([scorePromise, visitRemovedPromise]);
+  await verifyTrackedItems([guidToRemove]);
+
+  await cleanup();
+});
+
+add_task(async function test_stop_tracking() {
+  _("Let's stop tracking again.");
+  await tracker.stop();
+  await addVisit("stop_tracking");
+  await verifyTrackerEmpty();
+
+  await cleanup();
+});
+
+add_task(async function test_stop_tracking_twice() {
+  await tracker.stop();
+  await addVisit("stop_tracking_twice1");
+
+  _("Notifying twice won't do any harm.");
+  await tracker.stop();
+  await addVisit("stop_tracking_twice2");
+  await verifyTrackerEmpty();
+
+  await cleanup();
+});
+
+add_task(async function test_filter_file_uris() {
+  tracker.start();
+
+  let uri = CommonUtils.makeURI("file:///Users/eoger/tps/config.json");
+  let visitAddedPromise = promiseVisit("added", uri);
+  await PlacesTestUtils.addVisits({
+    uri,
+    visitDate: Date.now() * 1000,
+    transition: PlacesUtils.history.TRANSITION_LINK,
+  });
+  await visitAddedPromise;
+
+  await verifyTrackerEmpty();
+  await tracker.stop();
+  await cleanup();
+});
+
+add_task(async function test_filter_hidden() {
+  tracker.start();
+
+  _("Add visit; should be hidden by the redirect");
+  let hiddenURI = await addVisit("hidden");
+  let hiddenGUID = await engine._store.GUIDForUri(hiddenURI.spec);
+  _(`Hidden visit GUID: ${hiddenGUID}`);
+
+  _("Add redirect visit; should be tracked");
+  let trackedURI = await addVisit(
+    "redirect",
+    hiddenURI.spec,
+    PlacesUtils.history.TRANSITION_REDIRECT_PERMANENT
+  );
+  let trackedGUID = await engine._store.GUIDForUri(trackedURI.spec);
+  _(`Tracked visit GUID: ${trackedGUID}`);
+
+  _("Add visit for framed link; should be ignored");
+  let embedURI = await addVisit(
+    "framed_link",
+    null,
+    PlacesUtils.history.TRANSITION_FRAMED_LINK
+  );
+  let embedGUID = await engine._store.GUIDForUri(embedURI.spec);
+  _(`Framed link visit GUID: ${embedGUID}`);
+
+  _("Run Places maintenance to mark redirect visit as hidden");
+  await PlacesDBUtils.maintenanceOnIdle();
+
+  await verifyTrackedItems([trackedGUID]);
+
+  await cleanup();
+});
diff --git a/services/sync/tests/unit/test_hmac_error.js b/services/sync/tests/unit/test_hmac_error.js
new file mode 100644
index 0000000000..26dbc12dea
--- /dev/null
+++ b/services/sync/tests/unit/test_hmac_error.js
@@ -0,0 +1,250 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+// Track HMAC error counts.
+var hmacErrorCount = 0;
+(function () {
+  let hHE = Service.handleHMACEvent;
+  Service.handleHMACEvent = async function () {
+    hmacErrorCount++;
+    return hHE.call(Service);
+  };
+})();
+
+async function shared_setup() {
+  enableValidationPrefs();
+  syncTestLogging();
+
+  hmacErrorCount = 0;
+
+  let clientsEngine = Service.clientsEngine;
+  let clientsSyncID = await clientsEngine.resetLocalSyncID();
+
+  // Make sure RotaryEngine is the only one we sync.
+  let { engine, syncID, tracker } = await registerRotaryEngine();
+  await engine.setLastSync(123); // Needs to be non-zero so that tracker is queried.
+  engine._store.items = {
+    flying: "LNER Class A3 4472",
+    scotsman: "Flying Scotsman",
+  };
+  await tracker.addChangedID("scotsman", 0);
+  Assert.equal(1, Service.engineManager.getEnabled().length);
+
+  let engines = {
+    rotary: { version: engine.version, syncID },
+    clients: { version: clientsEngine.version, syncID: clientsSyncID },
+  };
+
+  // Common server objects.
+  let global = new ServerWBO("global", { engines });
+  let keysWBO = new ServerWBO("keys");
+  let rotaryColl = new ServerCollection({}, true);
+  let clientsColl = new ServerCollection({}, true);
+
+  return [engine, rotaryColl, clientsColl, keysWBO, global, tracker];
+}
+
+add_task(async function hmac_error_during_404() {
+  _("Attempt to replicate the HMAC error setup.");
+  let [engine, rotaryColl, clientsColl, keysWBO, global, tracker] =
+    await shared_setup();
+
+  // Hand out 404s for crypto/keys.
+  let keysHandler = keysWBO.handler();
+  let key404Counter = 0;
+  let keys404Handler = function (request, response) {
+    if (key404Counter > 0) {
+      let body = "Not Found";
+      response.setStatusLine(request.httpVersion, 404, body);
+      response.bodyOutputStream.write(body, body.length);
+      key404Counter--;
+      return;
+    }
+    keysHandler(request, response);
+  };
+
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+  let handlers = {
+    "/1.1/foo/info/collections": collectionsHelper.handler,
+    "/1.1/foo/storage/meta/global": upd("meta", global.handler()),
+    "/1.1/foo/storage/crypto/keys": upd("crypto", keys404Handler),
+    "/1.1/foo/storage/clients": upd("clients", clientsColl.handler()),
+    "/1.1/foo/storage/rotary": upd("rotary", rotaryColl.handler()),
+  };
+
+  let server = sync_httpd_setup(handlers);
+  // Do not instantiate SyncTestingInfrastructure; we need real crypto.
+  await configureIdentity({ username: "foo" }, server);
+  await Service.login();
+
+  try {
+    _("Syncing.");
+    await sync_and_validate_telem();
+
+    _(
+      "Partially resetting client, as if after a restart, and forcing redownload."
+    );
+    Service.collectionKeys.clear();
+    await engine.setLastSync(0); // So that we redownload records.
+    key404Counter = 1;
+    _("---------------------------");
+    await sync_and_validate_telem();
+    _("---------------------------");
+
+    // Two rotary items, one client record... no errors.
+    Assert.equal(hmacErrorCount, 0);
+  } finally {
+    await tracker.clearChangedIDs();
+    await Service.engineManager.unregister(engine);
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    Service.recordManager.clearCache();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function hmac_error_during_node_reassignment() {
+  _("Attempt to replicate an HMAC error during node reassignment.");
+  let [engine, rotaryColl, clientsColl, keysWBO, global, tracker] =
+    await shared_setup();
+
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+
+  // We'll provide a 401 mid-way through the sync. This function
+  // simulates shifting to a node which has no data.
+  function on401() {
+    _("Deleting server data...");
+    global.delete();
+    rotaryColl.delete();
+    keysWBO.delete();
+    clientsColl.delete();
+    delete collectionsHelper.collections.rotary;
+    delete collectionsHelper.collections.crypto;
+    delete collectionsHelper.collections.clients;
+    _("Deleted server data.");
+  }
+
+  let should401 = false;
+  function upd401(coll, handler) {
+    return function (request, response) {
+      if (should401 && request.method != "DELETE") {
+        on401();
+        should401 = false;
+        let body = '"reassigned!"';
+        response.setStatusLine(request.httpVersion, 401, "Node reassignment.");
+        response.bodyOutputStream.write(body, body.length);
+        return;
+      }
+      handler(request, response);
+    };
+  }
+
+  let handlers = {
+    "/1.1/foo/info/collections": collectionsHelper.handler,
+    "/1.1/foo/storage/meta/global": upd("meta", global.handler()),
+    "/1.1/foo/storage/crypto/keys": upd("crypto", keysWBO.handler()),
+    "/1.1/foo/storage/clients": upd401("clients", clientsColl.handler()),
+    "/1.1/foo/storage/rotary": upd("rotary", rotaryColl.handler()),
+  };
+
+  let server = sync_httpd_setup(handlers);
+  // Do not instantiate SyncTestingInfrastructure; we need real crypto.
+  await configureIdentity({ username: "foo" }, server);
+
+  _("Syncing.");
+  // First hit of clients will 401. This will happen after meta/global and
+  // keys -- i.e., in the middle of the sync, but before RotaryEngine.
+  should401 = true;
+
+  // Use observers to perform actions when our sync finishes.
+  // This allows us to observe the automatic next-tick sync that occurs after
+  // an abort.
+  function onSyncError() {
+    do_throw("Should not get a sync error!");
+  }
+  let onSyncFinished = function () {};
+  let obs = {
+    observe: function observe(subject, topic, data) {
+      switch (topic) {
+        case "weave:service:sync:error":
+          onSyncError();
+          break;
+        case "weave:service:sync:finish":
+          onSyncFinished();
+          break;
+      }
+    },
+  };
+
+  Svc.Obs.add("weave:service:sync:finish", obs);
+  Svc.Obs.add("weave:service:sync:error", obs);
+
+  // This kicks off the actual test. Split into a function here to allow this
+  // source file to broadly follow actual execution order.
+  async function onwards() {
+    _("== Invoking first sync.");
+    await Service.sync();
+    _("We should not simultaneously have data but no keys on the server.");
+    let hasData = rotaryColl.wbo("flying") || rotaryColl.wbo("scotsman");
+    let hasKeys = keysWBO.modified;
+
+    _("We correctly handle 401s by aborting the sync and starting again.");
+    Assert.ok(!hasData == !hasKeys);
+
+    _("Be prepared for the second (automatic) sync...");
+  }
+
+  _("Make sure that syncing again causes recovery.");
+  let callbacksPromise = new Promise(resolve => {
+    onSyncFinished = function () {
+      _("== First sync done.");
+      _("---------------------------");
+      onSyncFinished = function () {
+        _("== Second (automatic) sync done.");
+        let hasData = rotaryColl.wbo("flying") || rotaryColl.wbo("scotsman");
+        let hasKeys = keysWBO.modified;
+        Assert.ok(!hasData == !hasKeys);
+
+        // Kick off another sync. Can't just call it, because we're inside the
+        // lock...
+        (async () => {
+          await Async.promiseYield();
+          _("Now a fresh sync will get no HMAC errors.");
+          _(
+            "Partially resetting client, as if after a restart, and forcing redownload."
+          );
+          Service.collectionKeys.clear();
+          await engine.setLastSync(0);
+          hmacErrorCount = 0;
+
+          onSyncFinished = async function () {
+            // Two rotary items, one client record... no errors.
+            Assert.equal(hmacErrorCount, 0);
+
+            Svc.Obs.remove("weave:service:sync:finish", obs);
+            Svc.Obs.remove("weave:service:sync:error", obs);
+
+            await tracker.clearChangedIDs();
+            await Service.engineManager.unregister(engine);
+            for (const pref of Svc.PrefBranch.getChildList("")) {
+              Svc.PrefBranch.clearUserPref(pref);
+            }
+            Service.recordManager.clearCache();
+            server.stop(resolve);
+          };
+
+          Service.sync();
+        })().catch(console.error);
+      };
+    };
+  });
+  await onwards();
+  await callbacksPromise;
+});
diff --git a/services/sync/tests/unit/test_httpd_sync_server.js b/services/sync/tests/unit/test_httpd_sync_server.js
new file mode 100644
index 0000000000..6ac8ff5e04
--- /dev/null
+++ b/services/sync/tests/unit/test_httpd_sync_server.js
@@ -0,0 +1,250 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+add_test(function test_creation() {
+  // Explicit callback for this one.
+  let server = new SyncServer(Object.create(SyncServerCallback));
+  Assert.ok(!!server); // Just so we have a check.
+  server.start(null, function () {
+    _("Started on " + server.port);
+    server.stop(run_next_test);
+  });
+});
+
+add_test(function test_url_parsing() {
+  let server = new SyncServer();
+
+  // Check that we can parse a WBO URI.
+  let parts = server.pathRE.exec("/1.1/johnsmith/storage/crypto/keys");
+  let [all, version, username, first, rest] = parts;
+  Assert.equal(all, "/1.1/johnsmith/storage/crypto/keys");
+  Assert.equal(version, "1.1");
+  Assert.equal(username, "johnsmith");
+  Assert.equal(first, "storage");
+  Assert.equal(rest, "crypto/keys");
+  Assert.equal(null, server.pathRE.exec("/nothing/else"));
+
+  // Check that we can parse a collection URI.
+  parts = server.pathRE.exec("/1.1/johnsmith/storage/crypto");
+  [all, version, username, first, rest] = parts;
+  Assert.equal(all, "/1.1/johnsmith/storage/crypto");
+  Assert.equal(version, "1.1");
+  Assert.equal(username, "johnsmith");
+  Assert.equal(first, "storage");
+  Assert.equal(rest, "crypto");
+
+  // We don't allow trailing slash on storage URI.
+  parts = server.pathRE.exec("/1.1/johnsmith/storage/");
+  Assert.equal(parts, undefined);
+
+  // storage alone is a valid request.
+  parts = server.pathRE.exec("/1.1/johnsmith/storage");
+  [all, version, username, first, rest] = parts;
+  Assert.equal(all, "/1.1/johnsmith/storage");
+  Assert.equal(version, "1.1");
+  Assert.equal(username, "johnsmith");
+  Assert.equal(first, "storage");
+  Assert.equal(rest, undefined);
+
+  parts = server.storageRE.exec("storage");
+  let collection;
+  [all, , collection] = parts;
+  Assert.equal(all, "storage");
+  Assert.equal(collection, undefined);
+
+  run_next_test();
+});
+
+const { RESTRequest } = ChromeUtils.importESModule(
+  "resource://services-common/rest.sys.mjs"
+);
+function localRequest(server, path) {
+  _("localRequest: " + path);
+  let url = server.baseURI.substr(0, server.baseURI.length - 1) + path;
+  _("url: " + url);
+  return new RESTRequest(url);
+}
+
+add_task(async function test_basic_http() {
+  let server = new SyncServer();
+  server.registerUser("john", "password");
+  Assert.ok(server.userExists("john"));
+  server.start();
+  _("Started on " + server.port);
+
+  let req = localRequest(server, "/1.1/john/storage/crypto/keys");
+  _("req is " + req);
+  // Shouldn't reject, beyond that we don't care.
+  await req.get();
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_info_collections() {
+  let server = new SyncServer(Object.create(SyncServerCallback));
+  function responseHasCorrectHeaders(r) {
+    Assert.equal(r.status, 200);
+    Assert.equal(r.headers["content-type"], "application/json");
+    Assert.ok("x-weave-timestamp" in r.headers);
+  }
+
+  server.registerUser("john", "password");
+  server.start();
+
+  let req = localRequest(server, "/1.1/john/info/collections");
+  await req.get();
+  responseHasCorrectHeaders(req.response);
+  Assert.equal(req.response.body, "{}");
+
+  let putReq = localRequest(server, "/1.1/john/storage/crypto/keys");
+  let payload = JSON.stringify({ foo: "bar" });
+  let putResp = await putReq.put(payload);
+
+  responseHasCorrectHeaders(putResp);
+
+  let putResponseBody = putResp.body;
+  _("PUT response body: " + JSON.stringify(putResponseBody));
+
+  // When we PUT something to crypto/keys, "crypto" appears in the response.
+  req = localRequest(server, "/1.1/john/info/collections");
+
+  await req.get();
+  responseHasCorrectHeaders(req.response);
+  let expectedColl = server.getCollection("john", "crypto");
+  Assert.ok(!!expectedColl);
+  let modified = expectedColl.timestamp;
+  Assert.ok(modified > 0);
+  Assert.equal(putResponseBody, modified);
+  Assert.equal(JSON.parse(req.response.body).crypto, modified);
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_storage_request() {
+  let keysURL = "/1.1/john/storage/crypto/keys?foo=bar";
+  let foosURL = "/1.1/john/storage/crypto/foos";
+  let storageURL = "/1.1/john/storage";
+
+  let server = new SyncServer();
+  let creation = server.timestamp();
+  server.registerUser("john", "password");
+
+  server.createContents("john", {
+    crypto: { foos: { foo: "bar" } },
+  });
+  let coll = server.user("john").collection("crypto");
+  Assert.ok(!!coll);
+
+  _("We're tracking timestamps.");
+  Assert.ok(coll.timestamp >= creation);
+
+  async function retrieveWBONotExists() {
+    let req = localRequest(server, keysURL);
+    let response = await req.get();
+    _("Body is " + response.body);
+    _("Modified is " + response.newModified);
+    Assert.equal(response.status, 404);
+    Assert.equal(response.body, "Not found");
+  }
+
+  async function retrieveWBOExists() {
+    let req = localRequest(server, foosURL);
+    let response = await req.get();
+    _("Body is " + response.body);
+    _("Modified is " + response.newModified);
+    let parsedBody = JSON.parse(response.body);
+    Assert.equal(parsedBody.id, "foos");
+    Assert.equal(parsedBody.modified, coll.wbo("foos").modified);
+    Assert.equal(JSON.parse(parsedBody.payload).foo, "bar");
+  }
+
+  async function deleteWBONotExists() {
+    let req = localRequest(server, keysURL);
+    server.callback.onItemDeleted = function (username, collection, wboID) {
+      do_throw("onItemDeleted should not have been called.");
+    };
+
+    let response = await req.delete();
+
+    _("Body is " + response.body);
+    _("Modified is " + response.newModified);
+    Assert.equal(response.status, 200);
+    delete server.callback.onItemDeleted;
+  }
+
+  async function deleteWBOExists() {
+    let req = localRequest(server, foosURL);
+    server.callback.onItemDeleted = function (username, collection, wboID) {
+      _("onItemDeleted called for " + collection + "/" + wboID);
+      delete server.callback.onItemDeleted;
+      Assert.equal(username, "john");
+      Assert.equal(collection, "crypto");
+      Assert.equal(wboID, "foos");
+    };
+    await req.delete();
+    _("Body is " + req.response.body);
+    _("Modified is " + req.response.newModified);
+    Assert.equal(req.response.status, 200);
+  }
+
+  async function deleteStorage() {
+    _("Testing DELETE on /storage.");
+    let now = server.timestamp();
+    _("Timestamp: " + now);
+    let req = localRequest(server, storageURL);
+    await req.delete();
+
+    _("Body is " + req.response.body);
+    _("Modified is " + req.response.newModified);
+    let parsedBody = JSON.parse(req.response.body);
+    Assert.ok(parsedBody >= now);
+    do_check_empty(server.users.john.collections);
+  }
+
+  async function getStorageFails() {
+    _("Testing that GET on /storage fails.");
+    let req = localRequest(server, storageURL);
+    await req.get();
+    Assert.equal(req.response.status, 405);
+    Assert.equal(req.response.headers.allow, "DELETE");
+  }
+
+  async function getMissingCollectionWBO() {
+    _("Testing that fetching a WBO from an on-existent collection 404s.");
+    let req = localRequest(server, storageURL + "/foobar/baz");
+    await req.get();
+    Assert.equal(req.response.status, 404);
+  }
+
+  server.start(null);
+
+  await retrieveWBONotExists();
+  await retrieveWBOExists();
+  await deleteWBOExists();
+  await deleteWBONotExists();
+  await getStorageFails();
+  await getMissingCollectionWBO();
+  await deleteStorage();
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_x_weave_records() {
+  let server = new SyncServer();
+  server.registerUser("john", "password");
+
+  server.createContents("john", {
+    crypto: { foos: { foo: "bar" }, bars: { foo: "baz" } },
+  });
+  server.start();
+
+  let wbo = localRequest(server, "/1.1/john/storage/crypto/foos");
+  await wbo.get();
+  Assert.equal(false, "x-weave-records" in wbo.response.headers);
+  let col = localRequest(server, "/1.1/john/storage/crypto");
+  await col.get();
+  // Collection fetches do.
+  Assert.equal(col.response.headers["x-weave-records"], "2");
+
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_interval_triggers.js b/services/sync/tests/unit/test_interval_triggers.js
new file mode 100644
index 0000000000..6f2821ec45
--- /dev/null
+++ b/services/sync/tests/unit/test_interval_triggers.js
@@ -0,0 +1,472 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+Svc.PrefBranch.setStringPref("registerEngines", "");
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+let scheduler;
+let clientsEngine;
+
+async function sync_httpd_setup() {
+  let clientsSyncID = await clientsEngine.resetLocalSyncID();
+  let global = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {
+      clients: { version: clientsEngine.version, syncID: clientsSyncID },
+    },
+  });
+  let clientsColl = new ServerCollection({}, true);
+
+  // Tracking info/collections.
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+
+  return httpd_setup({
+    "/1.1/johndoe/storage/meta/global": upd("meta", global.handler()),
+    "/1.1/johndoe/info/collections": collectionsHelper.handler,
+    "/1.1/johndoe/storage/crypto/keys": upd(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe/storage/clients": upd("clients", clientsColl.handler()),
+  });
+}
+
+async function setUp(server) {
+  syncTestLogging();
+  await configureIdentity({ username: "johndoe" }, server);
+  await generateNewKeys(Service.collectionKeys);
+  let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+  await serverKeys.encrypt(Service.identity.syncKeyBundle);
+  await serverKeys.upload(Service.resource(Service.cryptoKeysURL));
+}
+
+add_task(async function setup() {
+  scheduler = Service.scheduler;
+  clientsEngine = Service.clientsEngine;
+
+  // Don't remove stale clients when syncing. This is a test-only workaround
+  // that lets us add clients directly to the store, without losing them on
+  // the next sync.
+  clientsEngine._removeRemoteClient = async id => {};
+});
+
+add_task(async function test_successful_sync_adjustSyncInterval() {
+  enableValidationPrefs();
+
+  _("Test successful sync calling adjustSyncInterval");
+  let syncSuccesses = 0;
+  function onSyncFinish() {
+    _("Sync success.");
+    syncSuccesses++;
+  }
+  Svc.Obs.add("weave:service:sync:finish", onSyncFinish);
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Confirm defaults
+  Assert.ok(!scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.ok(!scheduler.hasIncomingItems);
+
+  _("Test as long as numClients <= 1 our sync interval is SINGLE_USER.");
+  // idle == true && numClients <= 1 && hasIncomingItems == false
+  scheduler.idle = true;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 1);
+  Assert.ok(scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // idle == false && numClients <= 1 && hasIncomingItems == false
+  scheduler.idle = false;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 2);
+  Assert.ok(!scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // idle == false && numClients <= 1 && hasIncomingItems == true
+  scheduler.hasIncomingItems = true;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 3);
+  Assert.ok(!scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // idle == true && numClients <= 1 && hasIncomingItems == true
+  scheduler.idle = true;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 4);
+  Assert.ok(scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  _(
+    "Test as long as idle && numClients > 1 our sync interval is idleInterval."
+  );
+  // idle == true && numClients > 1 && hasIncomingItems == true
+  await Service.clientsEngine._store.create({
+    id: "foo",
+    cleartext: { name: "bar", type: "mobile" },
+  });
+  await Service.sync();
+  Assert.equal(syncSuccesses, 5);
+  Assert.ok(scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.idleInterval);
+
+  // idle == true && numClients > 1 && hasIncomingItems == false
+  scheduler.hasIncomingItems = false;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 6);
+  Assert.ok(scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.idleInterval);
+
+  _("Test non-idle, numClients > 1, no incoming items => activeInterval.");
+  // idle == false && numClients > 1 && hasIncomingItems == false
+  scheduler.idle = false;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 7);
+  Assert.ok(!scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+
+  _("Test non-idle, numClients > 1, incoming items => immediateInterval.");
+  // idle == false && numClients > 1 && hasIncomingItems == true
+  scheduler.hasIncomingItems = true;
+  await Service.sync();
+  Assert.equal(syncSuccesses, 8);
+  Assert.ok(!scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems); // gets reset to false
+  Assert.equal(scheduler.syncInterval, scheduler.immediateInterval);
+
+  Svc.Obs.remove("weave:service:sync:finish", onSyncFinish);
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_unsuccessful_sync_adjustSyncInterval() {
+  enableValidationPrefs();
+
+  _("Test unsuccessful sync calling adjustSyncInterval");
+
+  let syncFailures = 0;
+  function onSyncError() {
+    _("Sync error.");
+    syncFailures++;
+  }
+  Svc.Obs.add("weave:service:sync:error", onSyncError);
+
+  _("Test unsuccessful sync calls adjustSyncInterval");
+  // Force sync to fail.
+  Svc.PrefBranch.setStringPref("firstSync", "notReady");
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Confirm defaults
+  Assert.ok(!scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.ok(!scheduler.hasIncomingItems);
+
+  _("Test as long as numClients <= 1 our sync interval is SINGLE_USER.");
+  // idle == true && numClients <= 1 && hasIncomingItems == false
+  scheduler.idle = true;
+  await Service.sync();
+  Assert.equal(syncFailures, 1);
+  Assert.ok(scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // idle == false && numClients <= 1 && hasIncomingItems == false
+  scheduler.idle = false;
+  await Service.sync();
+  Assert.equal(syncFailures, 2);
+  Assert.ok(!scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // idle == false && numClients <= 1 && hasIncomingItems == true
+  scheduler.hasIncomingItems = true;
+  await Service.sync();
+  Assert.equal(syncFailures, 3);
+  Assert.ok(!scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // idle == true && numClients <= 1 && hasIncomingItems == true
+  scheduler.idle = true;
+  await Service.sync();
+  Assert.equal(syncFailures, 4);
+  Assert.ok(scheduler.idle);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  _(
+    "Test as long as idle && numClients > 1 our sync interval is idleInterval."
+  );
+  // idle == true && numClients > 1 && hasIncomingItems == true
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 2);
+  scheduler.updateClientMode();
+
+  await Service.sync();
+  Assert.equal(syncFailures, 5);
+  Assert.ok(scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.idleInterval);
+
+  // idle == true && numClients > 1 && hasIncomingItems == false
+  scheduler.hasIncomingItems = false;
+  await Service.sync();
+  Assert.equal(syncFailures, 6);
+  Assert.ok(scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.idleInterval);
+
+  _("Test non-idle, numClients > 1, no incoming items => activeInterval.");
+  // idle == false && numClients > 1 && hasIncomingItems == false
+  scheduler.idle = false;
+  await Service.sync();
+  Assert.equal(syncFailures, 7);
+  Assert.ok(!scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+
+  _("Test non-idle, numClients > 1, incoming items => immediateInterval.");
+  // idle == false && numClients > 1 && hasIncomingItems == true
+  scheduler.hasIncomingItems = true;
+  await Service.sync();
+  Assert.equal(syncFailures, 8);
+  Assert.ok(!scheduler.idle);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.hasIncomingItems); // gets reset to false
+  Assert.equal(scheduler.syncInterval, scheduler.immediateInterval);
+
+  await Service.startOver();
+  Svc.Obs.remove("weave:service:sync:error", onSyncError);
+  await promiseStopServer(server);
+});
+
+add_task(async function test_back_triggers_sync() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Single device: no sync triggered.
+  scheduler.idle = true;
+  scheduler.observe(
+    null,
+    "active",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.ok(!scheduler.idle);
+
+  // Multiple devices: sync is triggered.
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 2);
+  scheduler.updateClientMode();
+
+  let promiseDone = promiseOneObserver("weave:service:sync:finish");
+
+  scheduler.idle = true;
+  scheduler.observe(
+    null,
+    "active",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.ok(!scheduler.idle);
+  await promiseDone;
+
+  Service.recordManager.clearCache();
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  scheduler.setDefaults();
+  await clientsEngine.resetClient();
+
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_adjust_interval_on_sync_error() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let syncFailures = 0;
+  function onSyncError() {
+    _("Sync error.");
+    syncFailures++;
+  }
+  Svc.Obs.add("weave:service:sync:error", onSyncError);
+
+  _("Test unsuccessful sync updates client mode & sync intervals");
+  // Force a sync fail.
+  Svc.PrefBranch.setStringPref("firstSync", "notReady");
+
+  Assert.equal(syncFailures, 0);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 2);
+  await Service.sync();
+
+  Assert.equal(syncFailures, 1);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+
+  Svc.Obs.remove("weave:service:sync:error", onSyncError);
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_bug671378_scenario() {
+  enableValidationPrefs();
+
+  // Test scenario similar to bug 671378. This bug appeared when a score
+  // update occurred that wasn't large enough to trigger a sync so
+  // scheduleNextSync() was called without a time interval parameter,
+  // setting nextSync to a non-zero value and preventing the timer from
+  // being adjusted in the next call to scheduleNextSync().
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let syncSuccesses = 0;
+  function onSyncFinish() {
+    _("Sync success.");
+    syncSuccesses++;
+  }
+  Svc.Obs.add("weave:service:sync:finish", onSyncFinish);
+
+  // After first sync call, syncInterval & syncTimer are singleDeviceInterval.
+  await Service.sync();
+  Assert.equal(syncSuccesses, 1);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.equal(scheduler.syncTimer.delay, scheduler.singleDeviceInterval);
+
+  let promiseDone = new Promise(resolve => {
+    // Wrap scheduleNextSync so we are notified when it is finished.
+    scheduler._scheduleNextSync = scheduler.scheduleNextSync;
+    scheduler.scheduleNextSync = function () {
+      scheduler._scheduleNextSync();
+
+      // Check on sync:finish scheduleNextSync sets the appropriate
+      // syncInterval and syncTimer values.
+      if (syncSuccesses == 2) {
+        Assert.notEqual(scheduler.nextSync, 0);
+        Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+        Assert.ok(scheduler.syncTimer.delay <= scheduler.activeInterval);
+
+        scheduler.scheduleNextSync = scheduler._scheduleNextSync;
+        Svc.Obs.remove("weave:service:sync:finish", onSyncFinish);
+        Service.startOver().then(() => {
+          server.stop(resolve);
+        });
+      }
+    };
+  });
+
+  // Set nextSync != 0
+  // syncInterval still hasn't been set by call to updateClientMode.
+  // Explicitly trying to invoke scheduleNextSync during a sync
+  // (to immitate a score update that isn't big enough to trigger a sync).
+  Svc.Obs.add("weave:service:sync:start", function onSyncStart() {
+    // Wait for other sync:start observers to be called so that
+    // nextSync is set to 0.
+    CommonUtils.nextTick(function () {
+      Svc.Obs.remove("weave:service:sync:start", onSyncStart);
+
+      scheduler.scheduleNextSync();
+      Assert.notEqual(scheduler.nextSync, 0);
+      Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+      Assert.equal(scheduler.syncTimer.delay, scheduler.singleDeviceInterval);
+    });
+  });
+
+  await Service.clientsEngine._store.create({
+    id: "foo",
+    cleartext: { name: "bar", type: "mobile" },
+  });
+  await Service.sync();
+  await promiseDone;
+});
+
+add_task(async function test_adjust_timer_larger_syncInterval() {
+  _(
+    "Test syncInterval > current timout period && nextSync != 0, syncInterval is NOT used."
+  );
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 2);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+
+  scheduler.scheduleNextSync();
+
+  // Ensure we have a small interval.
+  Assert.notEqual(scheduler.nextSync, 0);
+  Assert.equal(scheduler.syncTimer.delay, scheduler.activeInterval);
+
+  // Make interval large again
+  await clientsEngine._wipeClient();
+  Svc.PrefBranch.clearUserPref("clients.devices.mobile");
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  scheduler.scheduleNextSync();
+
+  // Ensure timer delay remains as the small interval.
+  Assert.notEqual(scheduler.nextSync, 0);
+  Assert.ok(scheduler.syncTimer.delay <= scheduler.activeInterval);
+
+  // SyncSchedule.
+  await Service.startOver();
+});
+
+add_task(async function test_adjust_timer_smaller_syncInterval() {
+  _(
+    "Test current timout > syncInterval period && nextSync != 0, syncInterval is used."
+  );
+  scheduler.scheduleNextSync();
+
+  // Ensure we have a large interval.
+  Assert.notEqual(scheduler.nextSync, 0);
+  Assert.equal(scheduler.syncTimer.delay, scheduler.singleDeviceInterval);
+
+  // Make interval smaller
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 2);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+
+  scheduler.scheduleNextSync();
+
+  // Ensure smaller timer delay is used.
+  Assert.notEqual(scheduler.nextSync, 0);
+  Assert.ok(scheduler.syncTimer.delay <= scheduler.activeInterval);
+
+  // SyncSchedule.
+  await Service.startOver();
+});
diff --git a/services/sync/tests/unit/test_keys.js b/services/sync/tests/unit/test_keys.js
new file mode 100644
index 0000000000..8cc5d4055c
--- /dev/null
+++ b/services/sync/tests/unit/test_keys.js
@@ -0,0 +1,242 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Weave } = ChromeUtils.importESModule(
+  "resource://services-sync/main.sys.mjs"
+);
+const { CollectionKeyManager, CryptoWrapper } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+
+var collectionKeys = new CollectionKeyManager();
+
+function do_check_keypair_eq(a, b) {
+  Assert.equal(2, a.length);
+  Assert.equal(2, b.length);
+  Assert.equal(a[0], b[0]);
+  Assert.equal(a[1], b[1]);
+}
+
+add_test(function test_set_invalid_values() {
+  _("Ensure that setting invalid encryption and HMAC key values is caught.");
+
+  let bundle = new BulkKeyBundle("foo");
+
+  let thrown = false;
+  try {
+    bundle.encryptionKey = null;
+  } catch (ex) {
+    thrown = true;
+    Assert.equal(ex.message.indexOf("Encryption key can only be set to"), 0);
+  } finally {
+    Assert.ok(thrown);
+    thrown = false;
+  }
+
+  try {
+    bundle.encryptionKey = ["trollololol"];
+  } catch (ex) {
+    thrown = true;
+    Assert.equal(ex.message.indexOf("Encryption key can only be set to"), 0);
+  } finally {
+    Assert.ok(thrown);
+    thrown = false;
+  }
+
+  try {
+    bundle.hmacKey = Utils.generateRandomBytesLegacy(15);
+  } catch (ex) {
+    thrown = true;
+    Assert.equal(ex.message.indexOf("HMAC key must be at least 128"), 0);
+  } finally {
+    Assert.ok(thrown);
+    thrown = false;
+  }
+
+  try {
+    bundle.hmacKey = null;
+  } catch (ex) {
+    thrown = true;
+    Assert.equal(ex.message.indexOf("HMAC key can only be set to string"), 0);
+  } finally {
+    Assert.ok(thrown);
+    thrown = false;
+  }
+
+  try {
+    bundle.hmacKey = ["trollolol"];
+  } catch (ex) {
+    thrown = true;
+    Assert.equal(ex.message.indexOf("HMAC key can only be set to"), 0);
+  } finally {
+    Assert.ok(thrown);
+    thrown = false;
+  }
+
+  try {
+    bundle.hmacKey = Utils.generateRandomBytesLegacy(15);
+  } catch (ex) {
+    thrown = true;
+    Assert.equal(ex.message.indexOf("HMAC key must be at least 128"), 0);
+  } finally {
+    Assert.ok(thrown);
+    thrown = false;
+  }
+
+  run_next_test();
+});
+
+add_task(async function test_ensureLoggedIn() {
+  let log = Log.repository.getLogger("Test");
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  await configureIdentity();
+
+  let keyBundle = Weave.Service.identity.syncKeyBundle;
+
+  /*
+   * Build a test version of storage/crypto/keys.
+   * Encrypt it with the sync key.
+   * Pass it into the CollectionKeyManager.
+   */
+
+  log.info("Building storage keys...");
+  let storage_keys = new CryptoWrapper("crypto", "keys");
+  let default_key64 = await Weave.Crypto.generateRandomKey();
+  let default_hmac64 = await Weave.Crypto.generateRandomKey();
+  let bookmarks_key64 = await Weave.Crypto.generateRandomKey();
+  let bookmarks_hmac64 = await Weave.Crypto.generateRandomKey();
+
+  storage_keys.cleartext = {
+    default: [default_key64, default_hmac64],
+    collections: { bookmarks: [bookmarks_key64, bookmarks_hmac64] },
+  };
+  storage_keys.modified = Date.now() / 1000;
+  storage_keys.id = "keys";
+
+  log.info("Encrypting storage keys...");
+
+  // Use passphrase (sync key) itself to encrypt the key bundle.
+  await storage_keys.encrypt(keyBundle);
+
+  // Sanity checking.
+  Assert.ok(null == storage_keys.cleartext);
+  Assert.ok(null != storage_keys.ciphertext);
+
+  log.info("Updating collection keys.");
+
+  // updateContents decrypts the object, releasing the payload for us to use.
+  // Returns true, because the default key has changed.
+  Assert.ok(await collectionKeys.updateContents(keyBundle, storage_keys));
+  let payload = storage_keys.cleartext;
+
+  _("CK: " + JSON.stringify(collectionKeys._collections));
+
+  // Test that the CollectionKeyManager returns a similar WBO.
+  let wbo = collectionKeys.asWBO("crypto", "keys");
+
+  _("WBO: " + JSON.stringify(wbo));
+  _("WBO cleartext: " + JSON.stringify(wbo.cleartext));
+
+  // Check the individual contents.
+  Assert.equal(wbo.collection, "crypto");
+  Assert.equal(wbo.id, "keys");
+  Assert.equal(undefined, wbo.modified);
+  Assert.equal(collectionKeys.lastModified, storage_keys.modified);
+  Assert.ok(!!wbo.cleartext.default);
+  do_check_keypair_eq(payload.default, wbo.cleartext.default);
+  do_check_keypair_eq(
+    payload.collections.bookmarks,
+    wbo.cleartext.collections.bookmarks
+  );
+
+  Assert.ok("bookmarks" in collectionKeys._collections);
+  Assert.equal(false, "tabs" in collectionKeys._collections);
+
+  _("Updating contents twice with the same data doesn't proceed.");
+  await storage_keys.encrypt(keyBundle);
+  Assert.equal(
+    false,
+    await collectionKeys.updateContents(keyBundle, storage_keys)
+  );
+
+  /*
+   * Test that we get the right keys out when we ask for
+   * a collection's tokens.
+   */
+  let b1 = new BulkKeyBundle("bookmarks");
+  b1.keyPairB64 = [bookmarks_key64, bookmarks_hmac64];
+  let b2 = collectionKeys.keyForCollection("bookmarks");
+  do_check_keypair_eq(b1.keyPair, b2.keyPair);
+
+  // Check key equality.
+  Assert.ok(b1.equals(b2));
+  Assert.ok(b2.equals(b1));
+
+  b1 = new BulkKeyBundle("[default]");
+  b1.keyPairB64 = [default_key64, default_hmac64];
+
+  Assert.ok(!b1.equals(b2));
+  Assert.ok(!b2.equals(b1));
+
+  b2 = collectionKeys.keyForCollection(null);
+  do_check_keypair_eq(b1.keyPair, b2.keyPair);
+
+  /*
+   * Checking for update times.
+   */
+  let info_collections = {};
+  Assert.ok(collectionKeys.updateNeeded(info_collections));
+  info_collections.crypto = 5000;
+  Assert.ok(!collectionKeys.updateNeeded(info_collections));
+  info_collections.crypto = 1 + Date.now() / 1000; // Add one in case computers are fast!
+  Assert.ok(collectionKeys.updateNeeded(info_collections));
+
+  collectionKeys.lastModified = null;
+  Assert.ok(collectionKeys.updateNeeded({}));
+
+  /*
+   * Check _compareKeyBundleCollections.
+   */
+  async function newBundle(name) {
+    let r = new BulkKeyBundle(name);
+    await r.generateRandom();
+    return r;
+  }
+  let k1 = await newBundle("k1");
+  let k2 = await newBundle("k2");
+  let k3 = await newBundle("k3");
+  let k4 = await newBundle("k4");
+  let k5 = await newBundle("k5");
+  let coll1 = { foo: k1, bar: k2 };
+  let coll2 = { foo: k1, bar: k2 };
+  let coll3 = { foo: k1, bar: k3 };
+  let coll4 = { foo: k4 };
+  let coll5 = { baz: k5, bar: k2 };
+  let coll6 = {};
+
+  let d1 = collectionKeys._compareKeyBundleCollections(coll1, coll2); // []
+  let d2 = collectionKeys._compareKeyBundleCollections(coll1, coll3); // ["bar"]
+  let d3 = collectionKeys._compareKeyBundleCollections(coll3, coll2); // ["bar"]
+  let d4 = collectionKeys._compareKeyBundleCollections(coll1, coll4); // ["bar", "foo"]
+  let d5 = collectionKeys._compareKeyBundleCollections(coll5, coll2); // ["baz", "foo"]
+  let d6 = collectionKeys._compareKeyBundleCollections(coll6, coll1); // ["bar", "foo"]
+  let d7 = collectionKeys._compareKeyBundleCollections(coll5, coll5); // []
+  let d8 = collectionKeys._compareKeyBundleCollections(coll6, coll6); // []
+
+  Assert.ok(d1.same);
+  Assert.ok(!d2.same);
+  Assert.ok(!d3.same);
+  Assert.ok(!d4.same);
+  Assert.ok(!d5.same);
+  Assert.ok(!d6.same);
+  Assert.ok(d7.same);
+  Assert.ok(d8.same);
+
+  Assert.deepEqual(d1.changed, []);
+  Assert.deepEqual(d2.changed, ["bar"]);
+  Assert.deepEqual(d3.changed, ["bar"]);
+  Assert.deepEqual(d4.changed, ["bar", "foo"]);
+  Assert.deepEqual(d5.changed, ["baz", "foo"]);
+  Assert.deepEqual(d6.changed, ["bar", "foo"]);
+});
diff --git a/services/sync/tests/unit/test_load_modules.js b/services/sync/tests/unit/test_load_modules.js
new file mode 100644
index 0000000000..93ef883d4b
--- /dev/null
+++ b/services/sync/tests/unit/test_load_modules.js
@@ -0,0 +1,59 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+const modules = [
+  "addonutils.sys.mjs",
+  "addonsreconciler.sys.mjs",
+  "constants.sys.mjs",
+  "engines/addons.sys.mjs",
+  "engines/clients.sys.mjs",
+  "engines/extension-storage.sys.mjs",
+  "engines/passwords.sys.mjs",
+  "engines/prefs.sys.mjs",
+  "engines.sys.mjs",
+  "keys.sys.mjs",
+  "main.sys.mjs",
+  "policies.sys.mjs",
+  "record.sys.mjs",
+  "resource.sys.mjs",
+  "service.sys.mjs",
+  "stages/declined.sys.mjs",
+  "stages/enginesync.sys.mjs",
+  "status.sys.mjs",
+  "sync_auth.sys.mjs",
+  "util.sys.mjs",
+];
+
+if (AppConstants.MOZ_APP_NAME != "thunderbird") {
+  modules.push(
+    "engines/bookmarks.sys.mjs",
+    "engines/forms.sys.mjs",
+    "engines/history.sys.mjs",
+    "engines/tabs.sys.mjs"
+  );
+}
+
+const testingModules = [
+  "fakeservices.sys.mjs",
+  "rotaryengine.sys.mjs",
+  "utils.sys.mjs",
+  "fxa_utils.sys.mjs",
+];
+
+function run_test() {
+  for (let m of modules) {
+    let res = "resource://services-sync/" + m;
+    _("Attempting to load " + res);
+    ChromeUtils.importESModule(res);
+  }
+
+  for (let m of testingModules) {
+    let res = "resource://testing-common/services/sync/" + m;
+    _("Attempting to load " + res);
+    ChromeUtils.importESModule(res);
+  }
+}
diff --git a/services/sync/tests/unit/test_node_reassignment.js b/services/sync/tests/unit/test_node_reassignment.js
new file mode 100644
index 0000000000..e3352af318
--- /dev/null
+++ b/services/sync/tests/unit/test_node_reassignment.js
@@ -0,0 +1,523 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+_(
+  "Test that node reassignment responses are respected on all kinds of " +
+    "requests."
+);
+
+const { RESTRequest } = ChromeUtils.importESModule(
+  "resource://services-common/rest.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function setup() {
+  validate_all_future_pings();
+});
+
+/**
+ * Emulate the following Zeus config:
+ * $draining = data.get($prefix . $host . " draining");
+ * if ($draining == "drain.") {
+ *   log.warn($log_host_db_status . " migrating=1 (node-reassignment)" .
+ *            $log_suffix);
+ *   http.sendResponse("401 Node reassignment", $content_type,
+ *                     '"server request: node reassignment"', "");
+ * }
+ */
+const reassignBody = '"server request: node reassignment"';
+
+// API-compatible with SyncServer handler. Bind `handler` to something to use
+// as a ServerCollection handler.
+function handleReassign(handler, req, resp) {
+  resp.setStatusLine(req.httpVersion, 401, "Node reassignment");
+  resp.setHeader("Content-Type", "application/json");
+  resp.bodyOutputStream.write(reassignBody, reassignBody.length);
+}
+
+async function prepareServer() {
+  let server = new SyncServer();
+  server.registerUser("johndoe");
+  server.start();
+  syncTestLogging();
+  await configureIdentity({ username: "johndoe" }, server);
+  return server;
+}
+
+function getReassigned() {
+  try {
+    return Services.prefs.getBoolPref("services.sync.lastSyncReassigned");
+  } catch (ex) {
+    if (ex.result != Cr.NS_ERROR_UNEXPECTED) {
+      do_throw(
+        "Got exception retrieving lastSyncReassigned: " + Log.exceptionStr(ex)
+      );
+    }
+  }
+  return false;
+}
+
+/**
+ * Make a test request to `url`, then watch the result of two syncs
+ * to ensure that a node request was made.
+ * Runs `between` between the two. This can be used to undo deliberate failure
+ * setup, detach observers, etc.
+ */
+async function syncAndExpectNodeReassignment(
+  server,
+  firstNotification,
+  between,
+  secondNotification,
+  url
+) {
+  let deferred = Promise.withResolvers();
+
+  let getTokenCount = 0;
+  let mockTSC = {
+    // TokenServerClient
+    async getTokenUsingOAuth() {
+      getTokenCount++;
+      return { endpoint: server.baseURI + "1.1/johndoe/" };
+    },
+  };
+  Service.identity._tokenServerClient = mockTSC;
+
+  // Make sure that it works!
+  let request = new RESTRequest(url);
+  let response = await request.get();
+  Assert.equal(response.status, 401);
+
+  function onFirstSync() {
+    _("First sync completed.");
+    Svc.Obs.remove(firstNotification, onFirstSync);
+    Svc.Obs.add(secondNotification, onSecondSync);
+
+    Assert.equal(Service.clusterURL, "");
+
+    // Allow for tests to clean up error conditions.
+    between();
+  }
+  function onSecondSync() {
+    _("Second sync completed.");
+    Svc.Obs.remove(secondNotification, onSecondSync);
+    Service.scheduler.clearSyncTriggers();
+
+    // Make absolutely sure that any event listeners are done with their work
+    // before we proceed.
+    waitForZeroTimer(function () {
+      _("Second sync nextTick.");
+      Assert.equal(getTokenCount, 1);
+      Service.startOver().then(() => {
+        server.stop(deferred.resolve);
+      });
+    });
+  }
+
+  Svc.Obs.add(firstNotification, onFirstSync);
+  await Service.sync();
+
+  await deferred.promise;
+}
+
+add_task(async function test_momentary_401_engine() {
+  enableValidationPrefs();
+
+  _("Test a failure for engine URLs that's resolved by reassignment.");
+  let server = await prepareServer();
+  let john = server.user("johndoe");
+
+  _("Enabling the Rotary engine.");
+  let { engine, syncID, tracker } = await registerRotaryEngine();
+
+  // We need the server to be correctly set up prior to experimenting. Do this
+  // through a sync.
+  let global = {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    rotary: { version: engine.version, syncID },
+  };
+  john.createCollection("meta").insert("global", global);
+
+  _("First sync to prepare server contents.");
+  await Service.sync();
+
+  let numResets = 0;
+  let observeReset = (obs, topic) => {
+    if (topic == "rotary") {
+      numResets += 1;
+    }
+  };
+  _("Adding observer that we saw an engine reset.");
+  Svc.Obs.add("weave:engine:reset-client:finish", observeReset);
+
+  _("Setting up Rotary collection to 401.");
+  let rotary = john.createCollection("rotary");
+  let oldHandler = rotary.collectionHandler;
+  rotary.collectionHandler = handleReassign.bind(this, undefined);
+
+  // We want to verify that the clusterURL pref has been cleared after a 401
+  // inside a sync. Flag the Rotary engine to need syncing.
+  john.collection("rotary").timestamp += 1000;
+
+  function between() {
+    _("Undoing test changes.");
+    rotary.collectionHandler = oldHandler;
+
+    function onLoginStart() {
+      // lastSyncReassigned shouldn't be cleared until a sync has succeeded.
+      _("Ensuring that lastSyncReassigned is still set at next sync start.");
+      Svc.Obs.remove("weave:service:login:start", onLoginStart);
+      Assert.ok(getReassigned());
+    }
+
+    _("Adding observer that lastSyncReassigned is still set on login.");
+    Svc.Obs.add("weave:service:login:start", onLoginStart);
+  }
+
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:sync:finish",
+    between,
+    "weave:service:sync:finish",
+    Service.storageURL + "rotary"
+  );
+
+  Svc.Obs.remove("weave:engine:reset-client:finish", observeReset);
+  Assert.equal(numResets, 1);
+  await tracker.clearChangedIDs();
+  await Service.engineManager.unregister(engine);
+});
+
+// This test ends up being a failing fetch *after we're already logged in*.
+add_task(async function test_momentary_401_info_collections() {
+  enableValidationPrefs();
+
+  _("Test a failure for info/collections that's resolved by reassignment.");
+  let server = await prepareServer();
+
+  _("First sync to prepare server contents.");
+  await Service.sync();
+
+  // Return a 401 for info requests, particularly info/collections.
+  let oldHandler = server.toplevelHandlers.info;
+  server.toplevelHandlers.info = handleReassign;
+
+  function undo() {
+    _("Undoing test changes.");
+    server.toplevelHandlers.info = oldHandler;
+  }
+
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:sync:error",
+    undo,
+    "weave:service:sync:finish",
+    Service.infoURL
+  );
+});
+
+add_task(async function test_momentary_401_storage_loggedin() {
+  enableValidationPrefs();
+
+  _(
+    "Test a failure for any storage URL, not just engine parts. " +
+      "Resolved by reassignment."
+  );
+  let server = await prepareServer();
+
+  _("Performing initial sync to ensure we are logged in.");
+  await Service.sync();
+
+  // Return a 401 for all storage requests.
+  let oldHandler = server.toplevelHandlers.storage;
+  server.toplevelHandlers.storage = handleReassign;
+
+  function undo() {
+    _("Undoing test changes.");
+    server.toplevelHandlers.storage = oldHandler;
+  }
+
+  Assert.ok(Service.isLoggedIn, "already logged in");
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:sync:error",
+    undo,
+    "weave:service:sync:finish",
+    Service.storageURL + "meta/global"
+  );
+});
+
+add_task(async function test_momentary_401_storage_loggedout() {
+  enableValidationPrefs();
+
+  _(
+    "Test a failure for any storage URL, not just engine parts. " +
+      "Resolved by reassignment."
+  );
+  let server = await prepareServer();
+
+  // Return a 401 for all storage requests.
+  let oldHandler = server.toplevelHandlers.storage;
+  server.toplevelHandlers.storage = handleReassign;
+
+  function undo() {
+    _("Undoing test changes.");
+    server.toplevelHandlers.storage = oldHandler;
+  }
+
+  Assert.ok(!Service.isLoggedIn, "not already logged in");
+  await syncAndExpectNodeReassignment(
+    server,
+    "weave:service:login:error",
+    undo,
+    "weave:service:sync:finish",
+    Service.storageURL + "meta/global"
+  );
+});
+
+add_task(async function test_loop_avoidance_storage() {
+  enableValidationPrefs();
+
+  _(
+    "Test that a repeated failure doesn't result in a sync loop " +
+      "if node reassignment cannot resolve the failure."
+  );
+
+  let server = await prepareServer();
+
+  // Return a 401 for all storage requests.
+  let oldHandler = server.toplevelHandlers.storage;
+  server.toplevelHandlers.storage = handleReassign;
+
+  let firstNotification = "weave:service:login:error";
+  let secondNotification = "weave:service:login:error";
+  let thirdNotification = "weave:service:sync:finish";
+
+  let deferred = Promise.withResolvers();
+
+  let getTokenCount = 0;
+  let mockTSC = {
+    // TokenServerClient
+    async getTokenUsingOAuth() {
+      getTokenCount++;
+      return { endpoint: server.baseURI + "1.1/johndoe/" };
+    },
+  };
+  Service.identity._tokenServerClient = mockTSC;
+
+  // Track the time. We want to make sure the duration between the first and
+  // second sync is small, and then that the duration between second and third
+  // is set to be large.
+  let now;
+
+  function onFirstSync() {
+    _("First sync completed.");
+    Svc.Obs.remove(firstNotification, onFirstSync);
+    Svc.Obs.add(secondNotification, onSecondSync);
+
+    Assert.equal(Service.clusterURL, "");
+
+    // We got a 401 mid-sync, and set the pref accordingly.
+    Assert.ok(Services.prefs.getBoolPref("services.sync.lastSyncReassigned"));
+
+    // Update the timestamp.
+    now = Date.now();
+  }
+
+  function onSecondSync() {
+    _("Second sync completed.");
+    Svc.Obs.remove(secondNotification, onSecondSync);
+    Svc.Obs.add(thirdNotification, onThirdSync);
+
+    // This sync occurred within the backoff interval.
+    let elapsedTime = Date.now() - now;
+    Assert.ok(elapsedTime < MINIMUM_BACKOFF_INTERVAL);
+
+    // This pref will be true until a sync completes successfully.
+    Assert.ok(getReassigned());
+
+    // The timer will be set for some distant time.
+    // We store nextSync in prefs, which offers us only limited resolution.
+    // Include that logic here.
+    let expectedNextSync =
+      1000 * Math.floor((now + MINIMUM_BACKOFF_INTERVAL) / 1000);
+    _("Next sync scheduled for " + Service.scheduler.nextSync);
+    _("Expected to be slightly greater than " + expectedNextSync);
+
+    Assert.ok(Service.scheduler.nextSync >= expectedNextSync);
+    Assert.ok(!!Service.scheduler.syncTimer);
+
+    // Undo our evil scheme.
+    server.toplevelHandlers.storage = oldHandler;
+
+    // Bring the timer forward to kick off a successful sync, so we can watch
+    // the pref get cleared.
+    Service.scheduler.scheduleNextSync(0);
+  }
+  function onThirdSync() {
+    Svc.Obs.remove(thirdNotification, onThirdSync);
+
+    // That'll do for now; no more syncs.
+    Service.scheduler.clearSyncTriggers();
+
+    // Make absolutely sure that any event listeners are done with their work
+    // before we proceed.
+    waitForZeroTimer(function () {
+      _("Third sync nextTick.");
+      Assert.ok(!getReassigned());
+      Assert.equal(getTokenCount, 2);
+      Service.startOver().then(() => {
+        server.stop(deferred.resolve);
+      });
+    });
+  }
+
+  Svc.Obs.add(firstNotification, onFirstSync);
+
+  now = Date.now();
+  await Service.sync();
+  await deferred.promise;
+});
+
+add_task(async function test_loop_avoidance_engine() {
+  enableValidationPrefs();
+
+  _(
+    "Test that a repeated 401 in an engine doesn't result in a sync loop " +
+      "if node reassignment cannot resolve the failure."
+  );
+  let server = await prepareServer();
+  let john = server.user("johndoe");
+
+  _("Enabling the Rotary engine.");
+  let { engine, syncID, tracker } = await registerRotaryEngine();
+  let deferred = Promise.withResolvers();
+
+  let getTokenCount = 0;
+  let mockTSC = {
+    // TokenServerClient
+    async getTokenUsingOAuth() {
+      getTokenCount++;
+      return { endpoint: server.baseURI + "1.1/johndoe/" };
+    },
+  };
+  Service.identity._tokenServerClient = mockTSC;
+
+  // We need the server to be correctly set up prior to experimenting. Do this
+  // through a sync.
+  let global = {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    rotary: { version: engine.version, syncID },
+  };
+  john.createCollection("meta").insert("global", global);
+
+  _("First sync to prepare server contents.");
+  await Service.sync();
+
+  _("Setting up Rotary collection to 401.");
+  let rotary = john.createCollection("rotary");
+  let oldHandler = rotary.collectionHandler;
+  rotary.collectionHandler = handleReassign.bind(this, undefined);
+
+  // Flag the Rotary engine to need syncing.
+  john.collection("rotary").timestamp += 1000;
+
+  function onLoginStart() {
+    // lastSyncReassigned shouldn't be cleared until a sync has succeeded.
+    _("Ensuring that lastSyncReassigned is still set at next sync start.");
+    Assert.ok(getReassigned());
+  }
+
+  function beforeSuccessfulSync() {
+    _("Undoing test changes.");
+    rotary.collectionHandler = oldHandler;
+  }
+
+  let firstNotification = "weave:service:sync:finish";
+  let secondNotification = "weave:service:sync:finish";
+  let thirdNotification = "weave:service:sync:finish";
+
+  // Track the time. We want to make sure the duration between the first and
+  // second sync is small, and then that the duration between second and third
+  // is set to be large.
+  let now;
+
+  function onFirstSync() {
+    _("First sync completed.");
+    Svc.Obs.remove(firstNotification, onFirstSync);
+    Svc.Obs.add(secondNotification, onSecondSync);
+
+    Assert.equal(Service.clusterURL, "");
+
+    _("Adding observer that lastSyncReassigned is still set on login.");
+    Svc.Obs.add("weave:service:login:start", onLoginStart);
+
+    // We got a 401 mid-sync, and set the pref accordingly.
+    Assert.ok(Services.prefs.getBoolPref("services.sync.lastSyncReassigned"));
+
+    // Update the timestamp.
+    now = Date.now();
+  }
+
+  function onSecondSync() {
+    _("Second sync completed.");
+    Svc.Obs.remove(secondNotification, onSecondSync);
+    Svc.Obs.add(thirdNotification, onThirdSync);
+
+    // This sync occurred within the backoff interval.
+    let elapsedTime = Date.now() - now;
+    Assert.ok(elapsedTime < MINIMUM_BACKOFF_INTERVAL);
+
+    // This pref will be true until a sync completes successfully.
+    Assert.ok(getReassigned());
+
+    // The timer will be set for some distant time.
+    // We store nextSync in prefs, which offers us only limited resolution.
+    // Include that logic here.
+    let expectedNextSync =
+      1000 * Math.floor((now + MINIMUM_BACKOFF_INTERVAL) / 1000);
+    _("Next sync scheduled for " + Service.scheduler.nextSync);
+    _("Expected to be slightly greater than " + expectedNextSync);
+
+    Assert.ok(Service.scheduler.nextSync >= expectedNextSync);
+    Assert.ok(!!Service.scheduler.syncTimer);
+
+    // Undo our evil scheme.
+    beforeSuccessfulSync();
+
+    // Bring the timer forward to kick off a successful sync, so we can watch
+    // the pref get cleared.
+    Service.scheduler.scheduleNextSync(0);
+  }
+
+  function onThirdSync() {
+    Svc.Obs.remove(thirdNotification, onThirdSync);
+
+    // That'll do for now; no more syncs.
+    Service.scheduler.clearSyncTriggers();
+
+    // Make absolutely sure that any event listeners are done with their work
+    // before we proceed.
+    waitForZeroTimer(function () {
+      _("Third sync nextTick.");
+      Assert.ok(!getReassigned());
+      Assert.equal(getTokenCount, 2);
+      Svc.Obs.remove("weave:service:login:start", onLoginStart);
+      Service.startOver().then(() => {
+        server.stop(deferred.resolve);
+      });
+    });
+  }
+
+  Svc.Obs.add(firstNotification, onFirstSync);
+
+  now = Date.now();
+  await Service.sync();
+  await deferred.promise;
+
+  await tracker.clearChangedIDs();
+  await Service.engineManager.unregister(engine);
+});
diff --git a/services/sync/tests/unit/test_password_engine.js b/services/sync/tests/unit/test_password_engine.js
new file mode 100644
index 0000000000..081403f63d
--- /dev/null
+++ b/services/sync/tests/unit/test_password_engine.js
@@ -0,0 +1,1257 @@
+const { FXA_PWDMGR_HOST, FXA_PWDMGR_REALM } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccountsCommon.sys.mjs"
+);
+const { LoginRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/passwords.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const LoginInfo = Components.Constructor(
+  "@mozilla.org/login-manager/loginInfo;1",
+  Ci.nsILoginInfo,
+  "init"
+);
+
+const { LoginCSVImport } = ChromeUtils.importESModule(
+  "resource://gre/modules/LoginCSVImport.sys.mjs"
+);
+
+const { FileTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/FileTestUtils.sys.mjs"
+);
+
+const PropertyBag = Components.Constructor(
+  "@mozilla.org/hash-property-bag;1",
+  Ci.nsIWritablePropertyBag
+);
+
+async function cleanup(engine, server) {
+  await engine._tracker.stop();
+  await engine.wipeClient();
+  engine.lastModified = null;
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  Service.recordManager.clearCache();
+  if (server) {
+    await promiseStopServer(server);
+  }
+}
+
+add_task(async function setup() {
+  // Disable addon sync because AddonManager won't be initialized here.
+  await Service.engineManager.unregister("addons");
+  await Service.engineManager.unregister("extension-storage");
+});
+
+add_task(async function test_ignored_fields() {
+  _("Only changes to syncable fields should be tracked");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  enableValidationPrefs();
+
+  let loginInfo = new LoginInfo(
+    "https://example.com",
+    "",
+    null,
+    "username",
+    "password",
+    "",
+    ""
+  );
+
+  // Setting syncCounter to -1 so that it will be incremented to 0 when added.
+  loginInfo.syncCounter = -1;
+  let login = await Services.logins.addLoginAsync(loginInfo);
+  login.QueryInterface(Ci.nsILoginMetaInfo); // For `guid`.
+
+  engine._tracker.start();
+
+  try {
+    let nonSyncableProps = new PropertyBag();
+    nonSyncableProps.setProperty("timeLastUsed", Date.now());
+    nonSyncableProps.setProperty("timesUsed", 3);
+    Services.logins.modifyLogin(login, nonSyncableProps);
+
+    let noChanges = await engine.pullNewChanges();
+    deepEqual(noChanges, {}, "Should not track non-syncable fields");
+
+    let syncableProps = new PropertyBag();
+    syncableProps.setProperty("username", "newuser");
+    Services.logins.modifyLogin(login, syncableProps);
+
+    let changes = await engine.pullNewChanges();
+    deepEqual(
+      Object.keys(changes),
+      [login.guid],
+      "Should track syncable fields"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_ignored_sync_credentials() {
+  _("Sync credentials in login manager should be ignored");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  enableValidationPrefs();
+
+  engine._tracker.start();
+
+  try {
+    let login = await Services.logins.addLoginAsync(
+      new LoginInfo(
+        FXA_PWDMGR_HOST,
+        null,
+        FXA_PWDMGR_REALM,
+        "fxa-uid",
+        "creds",
+        "",
+        ""
+      )
+    );
+
+    let noChanges = await engine.pullNewChanges();
+    deepEqual(noChanges, {}, "Should not track new FxA credentials");
+
+    let props = new PropertyBag();
+    props.setProperty("password", "newcreds");
+    Services.logins.modifyLogin(login, props);
+
+    noChanges = await engine.pullNewChanges();
+    deepEqual(noChanges, {}, "Should not track changes to FxA credentials");
+
+    let foundLogins = await Services.logins.searchLoginsAsync({
+      origin: FXA_PWDMGR_HOST,
+    });
+    equal(foundLogins.length, 1);
+    equal(foundLogins[0].syncCounter, 0);
+    equal(foundLogins[0].everSynced, false);
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_password_engine() {
+  _("Basic password sync test");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("passwords");
+
+  enableValidationPrefs();
+
+  _("Add new login to upload during first sync");
+  let newLogin;
+  {
+    let login = new LoginInfo(
+      "https://example.com",
+      "",
+      null,
+      "username",
+      "password",
+      "",
+      ""
+    );
+    await Services.logins.addLoginAsync(login);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://example.com",
+    });
+    equal(logins.length, 1, "Should find new login in login manager");
+    newLogin = logins[0].QueryInterface(Ci.nsILoginMetaInfo);
+
+    // Insert a server record that's older, so that we prefer the local one.
+    let rec = new LoginRec("passwords", newLogin.guid);
+    rec.formSubmitURL = newLogin.formActionOrigin;
+    rec.httpRealm = newLogin.httpRealm;
+    rec.hostname = newLogin.origin;
+    rec.username = newLogin.username;
+    rec.password = "sekrit";
+    let remotePasswordChangeTime = Date.now() - 1 * 60 * 60 * 24 * 1000;
+    rec.timeCreated = remotePasswordChangeTime;
+    rec.timePasswordChanged = remotePasswordChangeTime;
+    collection.insert(
+      newLogin.guid,
+      encryptPayload(rec.cleartext),
+      remotePasswordChangeTime / 1000
+    );
+  }
+
+  _("Add login with older password change time to replace during first sync");
+  let oldLogin;
+  {
+    let login = new LoginInfo(
+      "https://mozilla.com",
+      "",
+      null,
+      "us3r",
+      "0ldpa55",
+      "",
+      ""
+    );
+    await Services.logins.addLoginAsync(login);
+
+    let props = new PropertyBag();
+    let localPasswordChangeTime = Date.now() - 1 * 60 * 60 * 24 * 1000;
+    props.setProperty("timePasswordChanged", localPasswordChangeTime);
+    Services.logins.modifyLogin(login, props);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://mozilla.com",
+    });
+    equal(logins.length, 1, "Should find old login in login manager");
+    oldLogin = logins[0].QueryInterface(Ci.nsILoginMetaInfo);
+    equal(oldLogin.timePasswordChanged, localPasswordChangeTime);
+
+    let rec = new LoginRec("passwords", oldLogin.guid);
+    rec.hostname = oldLogin.origin;
+    rec.formSubmitURL = oldLogin.formActionOrigin;
+    rec.httpRealm = oldLogin.httpRealm;
+    rec.username = oldLogin.username;
+    // Change the password and bump the password change time to ensure we prefer
+    // the remote one during reconciliation.
+    rec.password = "n3wpa55";
+    rec.usernameField = oldLogin.usernameField;
+    rec.passwordField = oldLogin.usernameField;
+    rec.timeCreated = oldLogin.timeCreated;
+    rec.timePasswordChanged = Date.now();
+    collection.insert(oldLogin.guid, encryptPayload(rec.cleartext));
+  }
+
+  await engine._tracker.stop();
+
+  try {
+    await sync_engine_and_validate_telem(engine, false);
+
+    let newRec = collection.cleartext(newLogin.guid);
+    equal(
+      newRec.password,
+      "password",
+      "Should update remote password for newer login"
+    );
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://mozilla.com",
+    });
+    equal(
+      logins[0].password,
+      "n3wpa55",
+      "Should update local password for older login"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_outgoing() {
+  _("Test syncing outgoing records");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  let loginInfo = new LoginInfo(
+    "http://mozilla.com",
+    "http://mozilla.com",
+    null,
+    "theuser",
+    "thepassword",
+    "username",
+    "password"
+  );
+  let login = await Services.logins.addLoginAsync(loginInfo);
+
+  engine._tracker.start();
+
+  try {
+    let foundLogins = await Services.logins.searchLoginsAsync({
+      origin: "http://mozilla.com",
+    });
+    equal(foundLogins.length, 1);
+    equal(foundLogins[0].syncCounter, 1);
+    equal(foundLogins[0].everSynced, false);
+    equal(collection.count(), 0);
+
+    let guid = foundLogins[0].QueryInterface(Ci.nsILoginMetaInfo).guid;
+
+    let changes = await engine.getChangedIDs();
+    let change = changes[guid];
+    equal(Object.keys(changes).length, 1);
+    equal(change.counter, 1);
+    ok(!change.deleted);
+
+    // This test modifies the password and then performs a sync and
+    // then ensures that the synced record is correct. This is done twice
+    // to ensure that syncing occurs correctly when the server record does not
+    // yet exist and when it does already exist.
+    for (let i = 1; i <= 2; i++) {
+      _("Modify the password iteration " + i);
+      foundLogins[0].password = "newpassword" + i;
+      Services.logins.modifyLogin(login, foundLogins[0]);
+      foundLogins = await Services.logins.searchLoginsAsync({
+        origin: "http://mozilla.com",
+      });
+      equal(foundLogins.length, 1);
+      // On the first pass, the counter should be 2, one for the add and one for the modify.
+      // No sync has occurred yet so everSynced should be false.
+      // On the second pass, the counter will only be 1 for the modify. The everSynced
+      // property should be true as the sync happened on the last iteration.
+      equal(foundLogins[0].syncCounter, i == 2 ? 1 : 2);
+      equal(foundLogins[0].everSynced, i == 2);
+
+      changes = await engine.getChangedIDs();
+      change = changes[guid];
+      equal(Object.keys(changes).length, 1);
+      equal(change.counter, i == 2 ? 1 : 2);
+      ok(!change.deleted);
+
+      _("Perform sync after modifying the password");
+      await sync_engine_and_validate_telem(engine, false);
+
+      equal(Object.keys(await engine.getChangedIDs()), 0);
+
+      // The remote login should have the updated password.
+      let newRec = collection.cleartext(guid);
+      equal(
+        newRec.password,
+        "newpassword" + i,
+        "Should update remote password for login"
+      );
+
+      foundLogins = await Services.logins.searchLoginsAsync({
+        origin: "http://mozilla.com",
+      });
+      equal(foundLogins.length, 1);
+      equal(foundLogins[0].syncCounter, 0);
+      equal(foundLogins[0].everSynced, true);
+
+      login.password = "newpassword" + i;
+    }
+
+    // Next, modify the username and sync.
+    _("Modify the username");
+    foundLogins[0].username = "newuser";
+    Services.logins.modifyLogin(login, foundLogins[0]);
+    foundLogins = await Services.logins.searchLoginsAsync({
+      origin: "http://mozilla.com",
+    });
+    equal(foundLogins.length, 1);
+    equal(foundLogins[0].syncCounter, 1);
+    equal(foundLogins[0].everSynced, true);
+
+    _("Perform sync after modifying the username");
+    await sync_engine_and_validate_telem(engine, false);
+
+    // The remote login should have the updated password.
+    let newRec = collection.cleartext(guid);
+    equal(
+      newRec.username,
+      "newuser",
+      "Should update remote username for login"
+    );
+
+    foundLogins = await Services.logins.searchLoginsAsync({
+      origin: "http://mozilla.com",
+    });
+    equal(foundLogins.length, 1);
+    equal(foundLogins[0].syncCounter, 0);
+    equal(foundLogins[0].everSynced, true);
+
+    // Finally, remove the login. The server record should be marked as deleted.
+    _("Remove the login");
+    equal(collection.count(), 1);
+    equal(Services.logins.countLogins("", "", ""), 2);
+    equal((await Services.logins.getAllLogins()).length, 2);
+    ok(await engine._store.itemExists(guid));
+
+    ok((await engine._store.getAllIDs())[guid]);
+
+    Services.logins.removeLogin(foundLogins[0]);
+    foundLogins = await Services.logins.searchLoginsAsync({
+      origin: "http://mozilla.com",
+    });
+    equal(foundLogins.length, 0);
+
+    changes = await engine.getChangedIDs();
+    change = changes[guid];
+    equal(Object.keys(changes).length, 1);
+    equal(change.counter, 1);
+    ok(change.deleted);
+
+    _("Perform sync after removing the login");
+    await sync_engine_and_validate_telem(engine, false);
+
+    equal(collection.count(), 1);
+    let payload = collection.payloads()[0];
+    ok(payload.deleted);
+
+    equal(Object.keys(await engine.getChangedIDs()), 0);
+
+    // All of these should not include the deleted login. Only the FxA password should exist.
+    equal(Services.logins.countLogins("", "", ""), 1);
+    equal((await Services.logins.getAllLogins()).length, 1);
+    ok(!(await engine._store.itemExists(guid)));
+
+    // getAllIDs includes deleted items but skips the FxA login.
+    ok((await engine._store.getAllIDs())[guid]);
+    let deletedLogin = await engine._store._getLoginFromGUID(guid);
+
+    equal(deletedLogin.hostname, null, "deleted login hostname");
+    equal(
+      deletedLogin.formActionOrigin,
+      null,
+      "deleted login formActionOrigin"
+    );
+    equal(deletedLogin.formSubmitURL, null, "deleted login formSubmitURL");
+    equal(deletedLogin.httpRealm, null, "deleted login httpRealm");
+    equal(deletedLogin.username, null, "deleted login username");
+    equal(deletedLogin.password, null, "deleted login password");
+    equal(deletedLogin.usernameField, "", "deleted login usernameField");
+    equal(deletedLogin.passwordField, "", "deleted login passwordField");
+    equal(deletedLogin.unknownFields, null, "deleted login unknownFields");
+    equal(deletedLogin.timeCreated, 0, "deleted login timeCreated");
+    equal(deletedLogin.timeLastUsed, 0, "deleted login timeLastUsed");
+    equal(deletedLogin.timesUsed, 0, "deleted login timesUsed");
+
+    // These fields are not reset when the login is removed.
+    equal(deletedLogin.guid, guid, "deleted login guid");
+    equal(deletedLogin.everSynced, true, "deleted login everSynced");
+    equal(deletedLogin.syncCounter, 0, "deleted login syncCounter");
+    ok(
+      deletedLogin.timePasswordChanged > 0,
+      "deleted login timePasswordChanged"
+    );
+  } finally {
+    await engine._tracker.stop();
+
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_incoming() {
+  _("Test syncing incoming records");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  const checkFields = [
+    "formSubmitURL",
+    "hostname",
+    "httpRealm",
+    "username",
+    "password",
+    "usernameField",
+    "passwordField",
+    "timeCreated",
+  ];
+
+  let guid1 = Utils.makeGUID();
+  let details = {
+    formSubmitURL: "https://www.example.com",
+    hostname: "https://www.example.com",
+    httpRealm: null,
+    username: "camel",
+    password: "llama",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+
+  try {
+    // This test creates a remote server record and then verifies that the login
+    // has been added locally after the sync occurs.
+    _("Create remote login");
+    collection.insertRecord(Object.assign({}, details, { id: guid1 }));
+
+    _("Perform sync when remote login has been added");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+    equal(logins.length, 1);
+
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid1);
+    checkFields.forEach(field => {
+      equal(logins[0][field], details[field]);
+    });
+    equal(logins[0].timePasswordChanged, details.timePasswordChanged);
+    equal(logins[0].syncCounter, 0);
+    equal(logins[0].everSynced, true);
+
+    // Modify the password within the remote record and then sync again.
+    _("Perform sync when remote login's password has been modified");
+    let newTime = Date.now();
+    collection.updateRecord(
+      guid1,
+      cleartext => {
+        cleartext.password = "alpaca";
+      },
+      newTime / 1000 + 10
+    );
+
+    await engine.setLastSync(newTime / 1000 - 30);
+    await sync_engine_and_validate_telem(engine, false);
+
+    logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+    equal(logins.length, 1);
+
+    details.password = "alpaca";
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid1);
+    checkFields.forEach(field => {
+      equal(logins[0][field], details[field]);
+    });
+    ok(logins[0].timePasswordChanged > details.timePasswordChanged);
+    equal(logins[0].syncCounter, 0);
+    equal(logins[0].everSynced, true);
+
+    // Modify the username within the remote record and then sync again.
+    _("Perform sync when remote login's username has been modified");
+    newTime = Date.now();
+    collection.updateRecord(
+      guid1,
+      cleartext => {
+        cleartext.username = "guanaco";
+      },
+      newTime / 1000 + 10
+    );
+
+    await engine.setLastSync(newTime / 1000 - 30);
+    await sync_engine_and_validate_telem(engine, false);
+
+    logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+    equal(logins.length, 1);
+
+    details.username = "guanaco";
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid1);
+    checkFields.forEach(field => {
+      equal(logins[0][field], details[field]);
+    });
+    ok(logins[0].timePasswordChanged > details.timePasswordChanged);
+    equal(logins[0].syncCounter, 0);
+    equal(logins[0].everSynced, true);
+
+    // Mark the remote record as deleted and then sync again.
+    _("Perform sync when remote login has been marked for deletion");
+    newTime = Date.now();
+    collection.updateRecord(
+      guid1,
+      cleartext => {
+        cleartext.deleted = true;
+      },
+      newTime / 1000 + 10
+    );
+
+    await engine.setLastSync(newTime / 1000 - 30);
+    await sync_engine_and_validate_telem(engine, false);
+
+    logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+    equal(logins.length, 0);
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_incoming_deleted() {
+  _("Test syncing incoming deleted records");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  let guid1 = Utils.makeGUID();
+  let details2 = {
+    formSubmitURL: "https://www.example.org",
+    hostname: "https://www.example.org",
+    httpRealm: null,
+    username: "capybara",
+    password: "beaver",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+    deleted: true,
+  };
+
+  try {
+    // This test creates a remote server record that has been deleted
+    // and then verifies that the login is not imported locally.
+    _("Create remote login");
+    collection.insertRecord(Object.assign({}, details2, { id: guid1 }));
+
+    _("Perform sync when remote login has been deleted");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+    equal(logins.length, 0);
+    ok(!(await engine._store.getAllIDs())[guid1]);
+    ok(!(await engine._store.itemExists(guid1)));
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_incoming_deleted_localchanged_remotenewer() {
+  _(
+    "Test syncing incoming deleted records where the local login has been changed but the remote record is newer"
+  );
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  let loginInfo = new LoginInfo(
+    "http://mozilla.com",
+    "http://mozilla.com",
+    null,
+    "kangaroo",
+    "kaola",
+    "username",
+    "password"
+  );
+  let login = await Services.logins.addLoginAsync(loginInfo);
+  let guid = login.QueryInterface(Ci.nsILoginMetaInfo).guid;
+
+  try {
+    _("Perform sync on new login");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let foundLogins = await Services.logins.searchLoginsAsync({
+      origin: "http://mozilla.com",
+    });
+    foundLogins[0].password = "wallaby";
+    Services.logins.modifyLogin(login, foundLogins[0]);
+
+    // Use a time in the future to ensure that the remote record is newer.
+    collection.updateRecord(
+      guid,
+      cleartext => {
+        cleartext.deleted = true;
+      },
+      Date.now() / 1000 + 1000
+    );
+
+    _(
+      "Perform sync when remote login has been deleted and local login has been changed"
+    );
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://mozilla.com",
+    });
+    equal(logins.length, 0);
+    ok(await engine._store.getAllIDs());
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_incoming_deleted_localchanged_localnewer() {
+  _(
+    "Test syncing incoming deleted records where the local login has been changed but the local record is newer"
+  );
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  let loginInfo = new LoginInfo(
+    "http://www.mozilla.com",
+    "http://www.mozilla.com",
+    null,
+    "lion",
+    "tiger",
+    "username",
+    "password"
+  );
+  let login = await Services.logins.addLoginAsync(loginInfo);
+  let guid = login.QueryInterface(Ci.nsILoginMetaInfo).guid;
+
+  try {
+    _("Perform sync on new login");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let foundLogins = await Services.logins.searchLoginsAsync({
+      origin: "http://www.mozilla.com",
+    });
+    foundLogins[0].password = "cheetah";
+    Services.logins.modifyLogin(login, foundLogins[0]);
+
+    // Use a time in the past to ensure that the local record is newer.
+    collection.updateRecord(
+      guid,
+      cleartext => {
+        cleartext.deleted = true;
+      },
+      Date.now() / 1000 - 1000
+    );
+
+    _(
+      "Perform sync when remote login has been deleted and local login has been changed"
+    );
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "http://www.mozilla.com",
+    });
+    equal(logins.length, 1);
+    equal(logins[0].password, "cheetah");
+    equal(logins[0].syncCounter, 0);
+    equal(logins[0].everSynced, true);
+    ok(await engine._store.getAllIDs());
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_incoming_no_formactionorigin() {
+  _("Test syncing incoming a record where there is no formActionOrigin");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  const checkFields = [
+    "formSubmitURL",
+    "hostname",
+    "httpRealm",
+    "username",
+    "password",
+    "usernameField",
+    "passwordField",
+    "timeCreated",
+  ];
+
+  let guid1 = Utils.makeGUID();
+  let details = {
+    formSubmitURL: "",
+    hostname: "https://www.example.com",
+    httpRealm: null,
+    username: "rabbit",
+    password: "squirrel",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+
+  try {
+    // This test creates a remote server record and then verifies that the login
+    // has been added locally after the sync occurs.
+    _("Create remote login");
+    collection.insertRecord(Object.assign({}, details, { id: guid1 }));
+
+    _("Perform sync when remote login has been added");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+      formActionOrigin: "",
+    });
+    equal(logins.length, 1);
+
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid1);
+    checkFields.forEach(field => {
+      equal(logins[0][field], details[field]);
+    });
+    equal(logins[0].timePasswordChanged, details.timePasswordChanged);
+    equal(logins[0].syncCounter, 0);
+    equal(logins[0].everSynced, true);
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_password_dupe() {
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("passwords");
+
+  let guid1 = Utils.makeGUID();
+  let rec1 = new LoginRec("passwords", guid1);
+  let guid2 = Utils.makeGUID();
+  let cleartext = {
+    formSubmitURL: "https://www.example.com",
+    hostname: "https://www.example.com",
+    httpRealm: null,
+    username: "foo",
+    password: "bar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Math.round(Date.now()),
+    timePasswordChanged: Math.round(Date.now()),
+  };
+  rec1.cleartext = cleartext;
+
+  _("Create remote record with same details and guid1");
+  collection.insert(guid1, encryptPayload(rec1.cleartext));
+
+  _("Create remote record with guid2");
+  collection.insert(guid2, encryptPayload(cleartext));
+
+  _("Create local record with same details and guid1");
+  await engine._store.create(rec1);
+
+  try {
+    _("Perform sync");
+    await sync_engine_and_validate_telem(engine, true);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+
+    equal(logins.length, 1);
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid2);
+    equal(null, collection.payload(guid1));
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_updated_null_password_sync() {
+  _("Ensure updated null login username is converted to a string");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("passwords");
+
+  let guid1 = Utils.makeGUID();
+  let guid2 = Utils.makeGUID();
+  let remoteDetails = {
+    formSubmitURL: "https://www.nullupdateexample.com",
+    hostname: "https://www.nullupdateexample.com",
+    httpRealm: null,
+    username: null,
+    password: "bar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+  let localDetails = {
+    formSubmitURL: "https://www.nullupdateexample.com",
+    hostname: "https://www.nullupdateexample.com",
+    httpRealm: null,
+    username: "foo",
+    password: "foobar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+
+  _("Create remote record with same details and guid1");
+  collection.insertRecord(Object.assign({}, remoteDetails, { id: guid1 }));
+
+  try {
+    _("Create local updated login with null password");
+    await engine._store.update(Object.assign({}, localDetails, { id: guid2 }));
+
+    _("Perform sync");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.nullupdateexample.com",
+    });
+
+    equal(logins.length, 1);
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid1);
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_updated_undefined_password_sync() {
+  _("Ensure updated undefined login username is converted to a string");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("passwords");
+
+  let guid1 = Utils.makeGUID();
+  let guid2 = Utils.makeGUID();
+  let remoteDetails = {
+    formSubmitURL: "https://www.undefinedupdateexample.com",
+    hostname: "https://www.undefinedupdateexample.com",
+    httpRealm: null,
+    username: undefined,
+    password: "bar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+  let localDetails = {
+    formSubmitURL: "https://www.undefinedupdateexample.com",
+    hostname: "https://www.undefinedupdateexample.com",
+    httpRealm: null,
+    username: "foo",
+    password: "foobar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+
+  _("Create remote record with same details and guid1");
+  collection.insertRecord(Object.assign({}, remoteDetails, { id: guid1 }));
+
+  try {
+    _("Create local updated login with undefined password");
+    await engine._store.update(Object.assign({}, localDetails, { id: guid2 }));
+
+    _("Perform sync");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.undefinedupdateexample.com",
+    });
+
+    equal(logins.length, 1);
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).guid, guid1);
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_new_null_password_sync() {
+  _("Ensure new null login username is converted to a string");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let guid1 = Utils.makeGUID();
+  let rec1 = new LoginRec("passwords", guid1);
+  rec1.cleartext = {
+    formSubmitURL: "https://www.example.com",
+    hostname: "https://www.example.com",
+    httpRealm: null,
+    username: null,
+    password: "bar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+
+  try {
+    _("Create local login with null password");
+    await engine._store.create(rec1);
+
+    _("Perform sync");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+
+    equal(logins.length, 1);
+    notEqual(logins[0].QueryInterface(Ci.nsILoginMetaInfo).username, null);
+    notEqual(logins[0].QueryInterface(Ci.nsILoginMetaInfo).username, undefined);
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).username, "");
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_new_undefined_password_sync() {
+  _("Ensure new undefined login username is converted to a string");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let guid1 = Utils.makeGUID();
+  let rec1 = new LoginRec("passwords", guid1);
+  rec1.cleartext = {
+    formSubmitURL: "https://www.example.com",
+    hostname: "https://www.example.com",
+    httpRealm: null,
+    username: undefined,
+    password: "bar",
+    usernameField: "username-field",
+    passwordField: "password-field",
+    timeCreated: Date.now(),
+    timePasswordChanged: Date.now(),
+  };
+
+  try {
+    _("Create local login with undefined password");
+    await engine._store.create(rec1);
+
+    _("Perform sync");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://www.example.com",
+    });
+
+    equal(logins.length, 1);
+    notEqual(logins[0].QueryInterface(Ci.nsILoginMetaInfo).username, null);
+    notEqual(logins[0].QueryInterface(Ci.nsILoginMetaInfo).username, undefined);
+    equal(logins[0].QueryInterface(Ci.nsILoginMetaInfo).username, "");
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_sync_password_validation() {
+  // This test isn't in test_password_validator to avoid duplicating cleanup.
+  _("Ensure that if a password validation happens, it ends up in the ping");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  Svc.PrefBranch.setIntPref("engine.passwords.validation.interval", 0);
+  Svc.PrefBranch.setIntPref(
+    "engine.passwords.validation.percentageChance",
+    100
+  );
+  Svc.PrefBranch.setIntPref("engine.passwords.validation.maxRecords", -1);
+  Svc.PrefBranch.setBoolPref("engine.passwords.validation.enabled", true);
+
+  try {
+    let ping = await wait_for_ping(() => Service.sync());
+
+    let engineInfo = ping.engines.find(e => e.name == "passwords");
+    ok(engineInfo, "Engine should be in ping");
+
+    let validation = engineInfo.validation;
+    ok(validation, "Engine should have validation info");
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_roundtrip_unknown_fields() {
+  _(
+    "Testing that unknown fields from other clients get roundtripped back to server"
+  );
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("passwords");
+
+  enableValidationPrefs();
+
+  _("Add login with older password change time to replace during first sync");
+  let oldLogin;
+  {
+    let login = new LoginInfo(
+      "https://mozilla.com",
+      "",
+      null,
+      "us3r",
+      "0ldpa55",
+      "",
+      ""
+    );
+    await Services.logins.addLoginAsync(login);
+
+    let props = new PropertyBag();
+    let localPasswordChangeTime = Math.round(
+      Date.now() - 1 * 60 * 60 * 24 * 1000
+    );
+    props.setProperty("timePasswordChanged", localPasswordChangeTime);
+    Services.logins.modifyLogin(login, props);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://mozilla.com",
+    });
+    equal(logins.length, 1, "Should find old login in login manager");
+    oldLogin = logins[0].QueryInterface(Ci.nsILoginMetaInfo);
+    equal(oldLogin.timePasswordChanged, localPasswordChangeTime);
+
+    let rec = new LoginRec("passwords", oldLogin.guid);
+    rec.hostname = oldLogin.origin;
+    rec.formSubmitURL = oldLogin.formActionOrigin;
+    rec.httpRealm = oldLogin.httpRealm;
+    rec.username = oldLogin.username;
+    // Change the password and bump the password change time to ensure we prefer
+    // the remote one during reconciliation.
+    rec.password = "n3wpa55";
+    rec.usernameField = oldLogin.usernameField;
+    rec.passwordField = oldLogin.usernameField;
+    rec.timeCreated = oldLogin.timeCreated;
+    rec.timePasswordChanged = Math.round(Date.now());
+
+    // pretend other clients have some snazzy new fields
+    // we don't quite understand yet
+    rec.cleartext.someStrField = "I am a str";
+    rec.cleartext.someObjField = { newField: "I am a new field" };
+    collection.insert(oldLogin.guid, encryptPayload(rec.cleartext));
+  }
+
+  await engine._tracker.stop();
+
+  try {
+    await sync_engine_and_validate_telem(engine, false);
+
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: "https://mozilla.com",
+    });
+    equal(
+      logins[0].password,
+      "n3wpa55",
+      "Should update local password for older login"
+    );
+    let expectedUnknowns = JSON.stringify({
+      someStrField: "I am a str",
+      someObjField: { newField: "I am a new field" },
+    });
+    // Check that the local record has all unknown fields properly
+    // stringified
+    equal(logins[0].unknownFields, expectedUnknowns);
+
+    // Check that the server has the unknown fields unfurled and on the
+    // top-level record
+    let serverRec = collection.cleartext(oldLogin.guid);
+    equal(serverRec.someStrField, "I am a str");
+    equal(serverRec.someObjField.newField, "I am a new field");
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_new_passwords_from_csv() {
+  _("Test syncing records imported from a csv file");
+
+  let engine = Service.engineManager.get("passwords");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let collection = server.user("foo").collection("passwords");
+
+  engine._tracker.start();
+
+  let data = [
+    {
+      hostname: "https://example.com",
+      url: "https://example.com/path",
+      username: "exampleuser",
+      password: "examplepassword",
+    },
+    {
+      hostname: "https://mozilla.org",
+      url: "https://mozilla.org",
+      username: "mozillauser",
+      password: "mozillapassword",
+    },
+    {
+      hostname: "https://www.example.org",
+      url: "https://www.example.org/example1/example2",
+      username: "person",
+      password: "mypassword",
+    },
+  ];
+
+  let csvData = ["url,username,login_password"];
+  for (let row of data) {
+    csvData.push(row.url + "," + row.username + "," + row.password);
+  }
+
+  let csvFile = FileTestUtils.getTempFile(`firefox_logins.csv`);
+  await IOUtils.writeUTF8(csvFile.path, csvData.join("\r\n"));
+
+  await LoginCSVImport.importFromCSV(csvFile.path);
+
+  equal(
+    engine._tracker.score,
+    SCORE_INCREMENT_XLARGE,
+    "Should only get one update notification for import"
+  );
+
+  _("Ensure that the csv import is correct");
+  for (let item of data) {
+    let foundLogins = await Services.logins.searchLoginsAsync({
+      origin: item.hostname,
+    });
+    equal(foundLogins.length, 1);
+    equal(foundLogins[0].syncCounter, 1);
+    equal(foundLogins[0].everSynced, false);
+    equal(foundLogins[0].username, item.username);
+    equal(foundLogins[0].password, item.password);
+  }
+
+  _("Perform sync after modifying the password");
+  await sync_engine_and_validate_telem(engine, false);
+
+  _("Verify that the sync counter and status are updated");
+  for (let item of data) {
+    let foundLogins = await Services.logins.searchLoginsAsync({
+      origin: item.hostname,
+    });
+    equal(foundLogins.length, 1);
+    equal(foundLogins[0].syncCounter, 0);
+    equal(foundLogins[0].everSynced, true);
+    equal(foundLogins[0].username, item.username);
+    equal(foundLogins[0].password, item.password);
+    item.guid = foundLogins[0].guid;
+  }
+
+  equal(Object.keys(await engine.getChangedIDs()), 0);
+  equal(collection.count(), 3);
+
+  for (let item of data) {
+    // The remote login should have the imported username and password.
+    let newRec = collection.cleartext(item.guid);
+    equal(newRec.username, item.username);
+    equal(newRec.password, item.password);
+  }
+});
diff --git a/services/sync/tests/unit/test_password_store.js b/services/sync/tests/unit/test_password_store.js
new file mode 100644
index 0000000000..ed393d6241
--- /dev/null
+++ b/services/sync/tests/unit/test_password_store.js
@@ -0,0 +1,398 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { LoginRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/passwords.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { SyncedRecordsTelemetry } = ChromeUtils.importESModule(
+  "resource://services-sync/telemetry.sys.mjs"
+);
+
+async function checkRecord(
+  name,
+  record,
+  expectedCount,
+  timeCreated,
+  expectedTimeCreated,
+  timePasswordChanged,
+  expectedTimePasswordChanged,
+  recordIsUpdated
+) {
+  let engine = Service.engineManager.get("passwords");
+  let store = engine._store;
+
+  let logins = await Services.logins.searchLoginsAsync({
+    origin: record.hostname,
+    formActionOrigin: record.formSubmitURL,
+  });
+
+  _("Record" + name + ":" + JSON.stringify(logins));
+  _("Count" + name + ":" + logins.length);
+
+  Assert.equal(logins.length, expectedCount);
+
+  if (expectedCount > 0) {
+    Assert.ok(!!(await store.getAllIDs())[record.id]);
+    let stored_record = logins[0].QueryInterface(Ci.nsILoginMetaInfo);
+
+    if (timeCreated !== undefined) {
+      Assert.equal(stored_record.timeCreated, expectedTimeCreated);
+    }
+
+    if (timePasswordChanged !== undefined) {
+      if (recordIsUpdated) {
+        Assert.ok(
+          stored_record.timePasswordChanged >= expectedTimePasswordChanged
+        );
+      } else {
+        Assert.equal(
+          stored_record.timePasswordChanged,
+          expectedTimePasswordChanged
+        );
+      }
+      return stored_record.timePasswordChanged;
+    }
+  } else {
+    Assert.ok(!(await store.getAllIDs())[record.id]);
+  }
+  return undefined;
+}
+
+async function changePassword(
+  name,
+  hostname,
+  password,
+  expectedCount,
+  timeCreated,
+  expectedTimeCreated,
+  timePasswordChanged,
+  expectedTimePasswordChanged,
+  insert,
+  recordIsUpdated
+) {
+  const BOGUS_GUID = "zzzzzz" + hostname;
+  let record = new LoginRec("passwords", BOGUS_GUID);
+  record.cleartext = {
+    id: BOGUS_GUID,
+    hostname,
+    formSubmitURL: hostname,
+    username: "john",
+    password,
+    usernameField: "username",
+    passwordField: "password",
+  };
+
+  if (timeCreated !== undefined) {
+    record.timeCreated = timeCreated;
+  }
+
+  if (timePasswordChanged !== undefined) {
+    record.timePasswordChanged = timePasswordChanged;
+  }
+
+  let engine = Service.engineManager.get("passwords");
+  let store = engine._store;
+
+  if (insert) {
+    let countTelemetry = new SyncedRecordsTelemetry();
+    Assert.equal(
+      (await store.applyIncomingBatch([record], countTelemetry)).length,
+      0
+    );
+  }
+
+  return checkRecord(
+    name,
+    record,
+    expectedCount,
+    timeCreated,
+    expectedTimeCreated,
+    timePasswordChanged,
+    expectedTimePasswordChanged,
+    recordIsUpdated
+  );
+}
+
+async function test_apply_records_with_times(
+  hostname,
+  timeCreated,
+  timePasswordChanged
+) {
+  // The following record is going to be inserted in the store and it needs
+  // to be found there. Then its timestamps are going to be compared to
+  // the expected values.
+  await changePassword(
+    " ",
+    hostname,
+    "password",
+    1,
+    timeCreated,
+    timeCreated,
+    timePasswordChanged,
+    timePasswordChanged,
+    true
+  );
+}
+
+async function test_apply_multiple_records_with_times() {
+  // The following records are going to be inserted in the store and they need
+  // to be found there. Then their timestamps are going to be compared to
+  // the expected values.
+  await changePassword(
+    "A",
+    "http://foo.a.com",
+    "password",
+    1,
+    undefined,
+    undefined,
+    undefined,
+    undefined,
+    true
+  );
+  await changePassword(
+    "B",
+    "http://foo.b.com",
+    "password",
+    1,
+    1000,
+    1000,
+    undefined,
+    undefined,
+    true
+  );
+  await changePassword(
+    "C",
+    "http://foo.c.com",
+    "password",
+    1,
+    undefined,
+    undefined,
+    1000,
+    1000,
+    true
+  );
+  await changePassword(
+    "D",
+    "http://foo.d.com",
+    "password",
+    1,
+    1000,
+    1000,
+    1000,
+    1000,
+    true
+  );
+
+  // The following records are not going to be inserted in the store and they
+  // are not going to be found there.
+  await changePassword(
+    "NotInStoreA",
+    "http://foo.aaaa.com",
+    "password",
+    0,
+    undefined,
+    undefined,
+    undefined,
+    undefined,
+    false
+  );
+  await changePassword(
+    "NotInStoreB",
+    "http://foo.bbbb.com",
+    "password",
+    0,
+    1000,
+    1000,
+    undefined,
+    undefined,
+    false
+  );
+  await changePassword(
+    "NotInStoreC",
+    "http://foo.cccc.com",
+    "password",
+    0,
+    undefined,
+    undefined,
+    1000,
+    1000,
+    false
+  );
+  await changePassword(
+    "NotInStoreD",
+    "http://foo.dddd.com",
+    "password",
+    0,
+    1000,
+    1000,
+    1000,
+    1000,
+    false
+  );
+}
+
+async function test_apply_same_record_with_different_times() {
+  // The following record is going to be inserted multiple times in the store
+  // and it needs to be found there. Then its timestamps are going to be
+  // compared to the expected values.
+
+  /* eslint-disable no-unused-vars */
+  /* The eslint linter thinks that timePasswordChanged is unused, even though
+     it is passed as an argument to changePassword. */
+  var timePasswordChanged = 100;
+  timePasswordChanged = await changePassword(
+    "A",
+    "http://a.tn",
+    "password",
+    1,
+    100,
+    100,
+    100,
+    timePasswordChanged,
+    true
+  );
+  timePasswordChanged = await changePassword(
+    "A",
+    "http://a.tn",
+    "password",
+    1,
+    100,
+    100,
+    800,
+    timePasswordChanged,
+    true,
+    true
+  );
+  timePasswordChanged = await changePassword(
+    "A",
+    "http://a.tn",
+    "password",
+    1,
+    500,
+    100,
+    800,
+    timePasswordChanged,
+    true,
+    true
+  );
+  timePasswordChanged = await changePassword(
+    "A",
+    "http://a.tn",
+    "password2",
+    1,
+    500,
+    100,
+    1536213005222,
+    timePasswordChanged,
+    true,
+    true
+  );
+  timePasswordChanged = await changePassword(
+    "A",
+    "http://a.tn",
+    "password2",
+    1,
+    500,
+    100,
+    800,
+    timePasswordChanged,
+    true,
+    true
+  );
+  /* eslint-enable no-unused-vars */
+}
+
+async function test_LoginRec_toString(store, recordData) {
+  let rec = await store.createRecord(recordData.id);
+  ok(rec);
+  ok(!rec.toString().includes(rec.password));
+}
+
+add_task(async function run_test() {
+  const BOGUS_GUID_A = "zzzzzzzzzzzz";
+  const BOGUS_GUID_B = "yyyyyyyyyyyy";
+  let recordA = new LoginRec("passwords", BOGUS_GUID_A);
+  let recordB = new LoginRec("passwords", BOGUS_GUID_B);
+  recordA.cleartext = {
+    id: BOGUS_GUID_A,
+    hostname: "http://foo.bar.com",
+    formSubmitURL: "http://foo.bar.com",
+    httpRealm: "secure",
+    username: "john",
+    password: "smith",
+    usernameField: "username",
+    passwordField: "password",
+  };
+  recordB.cleartext = {
+    id: BOGUS_GUID_B,
+    hostname: "http://foo.baz.com",
+    formSubmitURL: "http://foo.baz.com",
+    username: "john",
+    password: "smith",
+    usernameField: "username",
+    passwordField: "password",
+    unknownStr: "an unknown string from another field",
+  };
+
+  let engine = Service.engineManager.get("passwords");
+  let store = engine._store;
+
+  try {
+    let countTelemetry = new SyncedRecordsTelemetry();
+    Assert.equal(
+      (await store.applyIncomingBatch([recordA, recordB], countTelemetry))
+        .length,
+      0
+    );
+
+    // Only the good record makes it to Services.logins.
+    let badLogins = await Services.logins.searchLoginsAsync({
+      origin: recordA.hostname,
+      formActionOrigin: recordA.formSubmitURL,
+      httpRealm: recordA.httpRealm,
+    });
+    let goodLogins = await Services.logins.searchLoginsAsync({
+      origin: recordB.hostname,
+      formActionOrigin: recordB.formSubmitURL,
+    });
+
+    _("Bad: " + JSON.stringify(badLogins));
+    _("Good: " + JSON.stringify(goodLogins));
+    _("Count: " + badLogins.length + ", " + goodLogins.length);
+
+    Assert.equal(goodLogins.length, 1);
+    Assert.equal(badLogins.length, 0);
+
+    // applyIncoming should've put any unknown fields from the server
+    // into a catch-all unknownFields field
+    Assert.equal(
+      goodLogins[0].unknownFields,
+      JSON.stringify({
+        unknownStr: "an unknown string from another field",
+      })
+    );
+
+    Assert.ok(!!(await store.getAllIDs())[BOGUS_GUID_B]);
+    Assert.ok(!(await store.getAllIDs())[BOGUS_GUID_A]);
+
+    await test_LoginRec_toString(store, recordB);
+
+    await test_apply_records_with_times(
+      "http://afoo.baz.com",
+      undefined,
+      undefined
+    );
+    await test_apply_records_with_times("http://bfoo.baz.com", 1000, undefined);
+    await test_apply_records_with_times("http://cfoo.baz.com", undefined, 2000);
+    await test_apply_records_with_times("http://dfoo.baz.com", 1000, 2000);
+
+    await test_apply_multiple_records_with_times();
+
+    await test_apply_same_record_with_different_times();
+  } finally {
+    await store.wipe();
+  }
+});
diff --git a/services/sync/tests/unit/test_password_tracker.js b/services/sync/tests/unit/test_password_tracker.js
new file mode 100644
index 0000000000..77b46d1d2c
--- /dev/null
+++ b/services/sync/tests/unit/test_password_tracker.js
@@ -0,0 +1,248 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { PasswordEngine, LoginRec } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/passwords.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+let engine;
+let store;
+let tracker;
+
+add_task(async function setup() {
+  await Service.engineManager.register(PasswordEngine);
+  engine = Service.engineManager.get("passwords");
+  store = engine._store;
+  tracker = engine._tracker;
+});
+
+add_task(async function test_tracking() {
+  let recordNum = 0;
+
+  _("Verify we've got an empty tracker to work with.");
+  let changes = await engine.getChangedIDs();
+  do_check_empty(changes);
+
+  let exceptionHappened = false;
+  try {
+    await tracker.getChangedIDs();
+  } catch (ex) {
+    exceptionHappened = true;
+  }
+  ok(exceptionHappened, "tracker does not keep track of changes");
+
+  async function createPassword() {
+    _("RECORD NUM: " + recordNum);
+    let record = new LoginRec("passwords", "GUID" + recordNum);
+    record.cleartext = {
+      id: "GUID" + recordNum,
+      hostname: "http://foo.bar.com",
+      formSubmitURL: "http://foo.bar.com",
+      username: "john" + recordNum,
+      password: "smith",
+      usernameField: "username",
+      passwordField: "password",
+    };
+    recordNum++;
+    let login = store._nsLoginInfoFromRecord(record);
+    await Services.logins.addLoginAsync(login);
+    await tracker.asyncObserver.promiseObserversComplete();
+  }
+
+  try {
+    tracker.start();
+    await createPassword();
+    changes = await engine.getChangedIDs();
+    do_check_attribute_count(changes, 1);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(changes.GUID0.counter, 1);
+    Assert.ok(typeof changes.GUID0.modified, "number");
+
+    _("Starting twice won't do any harm.");
+    tracker.start();
+    await createPassword();
+    changes = await engine.getChangedIDs();
+    do_check_attribute_count(changes, 2);
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+    Assert.equal(changes.GUID0.counter, 1);
+    Assert.equal(changes.GUID1.counter, 1);
+
+    // The tracker doesn't keep track of changes, so 3 changes
+    // should still be returned, but the score is not updated.
+    _("Let's stop tracking again.");
+    tracker.resetScore();
+    await tracker.stop();
+    await createPassword();
+    changes = await engine.getChangedIDs();
+    do_check_attribute_count(changes, 3);
+    Assert.equal(tracker.score, 0);
+    Assert.equal(changes.GUID0.counter, 1);
+    Assert.equal(changes.GUID1.counter, 1);
+    Assert.equal(changes.GUID2.counter, 1);
+
+    _("Stopping twice won't do any harm.");
+    await tracker.stop();
+    await createPassword();
+    changes = await engine.getChangedIDs();
+    do_check_attribute_count(changes, 4);
+    Assert.equal(tracker.score, 0);
+  } finally {
+    _("Clean up.");
+    await store.wipe();
+    tracker.resetScore();
+    await tracker.stop();
+  }
+});
+
+add_task(async function test_onWipe() {
+  _("Verify we've got an empty tracker to work with.");
+  const changes = await engine.getChangedIDs();
+  do_check_empty(changes);
+  Assert.equal(tracker.score, 0);
+
+  try {
+    _("A store wipe should increment the score");
+    tracker.start();
+    await store.wipe();
+    await tracker.asyncObserver.promiseObserversComplete();
+
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+  } finally {
+    tracker.resetScore();
+    await tracker.stop();
+  }
+});
+
+add_task(async function test_removeAllLogins() {
+  let recordNum = 0;
+  _("Verify that all tracked logins are removed.");
+
+  // Perform this test twice, the first time where a sync is not performed
+  // between adding and removing items and the second time where a sync is
+  // performed. In the former case, the logins will just be deleted because
+  // they have never been synced, so they won't be detected as changes. In
+  // the latter case, the logins have been synced so they will be marked for
+  // deletion.
+  for (let syncBeforeRemove of [false, true]) {
+    async function createPassword() {
+      _("RECORD NUM: " + recordNum);
+      let record = new LoginRec("passwords", "GUID" + recordNum);
+      record.cleartext = {
+        id: "GUID" + recordNum,
+        hostname: "http://foo.bar.com",
+        formSubmitURL: "http://foo.bar.com",
+        username: "john" + recordNum,
+        password: "smith",
+        usernameField: "username",
+        passwordField: "password",
+      };
+      recordNum++;
+      let login = store._nsLoginInfoFromRecord(record);
+      await Services.logins.addLoginAsync(login);
+
+      await tracker.asyncObserver.promiseObserversComplete();
+    }
+    try {
+      _("Tell tracker to start tracking changes");
+      tracker.start();
+      await createPassword();
+      await createPassword();
+      let changes = await engine.getChangedIDs();
+      do_check_attribute_count(changes, 2);
+      Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+
+      if (syncBeforeRemove) {
+        let logins = await Services.logins.getAllLogins();
+        for (let login of logins) {
+          engine.markSynced(login.guid);
+        }
+      }
+
+      _("Tell sync to remove all logins");
+      Services.logins.removeAllUserFacingLogins();
+      await tracker.asyncObserver.promiseObserversComplete();
+      changes = await engine.getChangedIDs();
+      do_check_attribute_count(changes, syncBeforeRemove ? 2 : 0);
+      Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 5);
+
+      let deletedGuids = await engine._store.getAllIDs();
+      if (syncBeforeRemove) {
+        for (let guid in deletedGuids) {
+          let deletedLogin = await engine._store._getLoginFromGUID(guid);
+
+          Assert.equal(deletedLogin.hostname, null, "deleted login hostname");
+          Assert.equal(
+            deletedLogin.formActionOrigin,
+            null,
+            "deleted login formActionOrigin"
+          );
+          Assert.equal(
+            deletedLogin.formSubmitURL,
+            null,
+            "deleted login formSubmitURL"
+          );
+          Assert.equal(deletedLogin.httpRealm, null, "deleted login httpRealm");
+          Assert.equal(deletedLogin.username, null, "deleted login username");
+          Assert.equal(deletedLogin.password, null, "deleted login password");
+          Assert.equal(
+            deletedLogin.usernameField,
+            "",
+            "deleted login usernameField"
+          );
+          Assert.equal(
+            deletedLogin.passwordField,
+            "",
+            "deleted login passwordField"
+          );
+          Assert.equal(
+            deletedLogin.unknownFields,
+            null,
+            "deleted login unknownFields"
+          );
+          Assert.equal(
+            deletedLogin.timeCreated,
+            0,
+            "deleted login timeCreated"
+          );
+          Assert.equal(
+            deletedLogin.timeLastUsed,
+            0,
+            "deleted login timeLastUsed"
+          );
+          Assert.equal(deletedLogin.timesUsed, 0, "deleted login timesUsed");
+
+          // These fields are not reset when the login is removed.
+          Assert.ok(deletedLogin.guid.startsWith("GUID"), "deleted login guid");
+          Assert.equal(
+            deletedLogin.everSynced,
+            true,
+            "deleted login everSynced"
+          );
+          Assert.equal(
+            deletedLogin.syncCounter,
+            2,
+            "deleted login syncCounter"
+          );
+          Assert.ok(
+            deletedLogin.timePasswordChanged > 0,
+            "deleted login timePasswordChanged"
+          );
+        }
+      } else {
+        Assert.equal(
+          Object.keys(deletedGuids).length,
+          0,
+          "no logins remain after removeAllUserFacingLogins"
+        );
+      }
+    } finally {
+      _("Clean up.");
+      await store.wipe();
+      tracker.resetScore();
+      await tracker.stop();
+    }
+  }
+});
diff --git a/services/sync/tests/unit/test_password_validator.js b/services/sync/tests/unit/test_password_validator.js
new file mode 100644
index 0000000000..445b119e1d
--- /dev/null
+++ b/services/sync/tests/unit/test_password_validator.js
@@ -0,0 +1,176 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { PasswordValidator } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/passwords.sys.mjs"
+);
+
+function getDummyServerAndClient() {
+  return {
+    server: [
+      {
+        id: "11111",
+        guid: "11111",
+        hostname: "https://www.11111.com",
+        formSubmitURL: "https://www.11111.com",
+        password: "qwerty123",
+        passwordField: "pass",
+        username: "foobar",
+        usernameField: "user",
+        httpRealm: null,
+      },
+      {
+        id: "22222",
+        guid: "22222",
+        hostname: "https://www.22222.org",
+        formSubmitURL: "https://www.22222.org",
+        password: "hunter2",
+        passwordField: "passwd",
+        username: "baz12345",
+        usernameField: "user",
+        httpRealm: null,
+      },
+      {
+        id: "33333",
+        guid: "33333",
+        hostname: "https://www.33333.com",
+        formSubmitURL: "https://www.33333.com",
+        password: "p4ssw0rd",
+        passwordField: "passwad",
+        username: "quux",
+        usernameField: "user",
+        httpRealm: null,
+      },
+    ],
+    client: [
+      {
+        id: "11111",
+        guid: "11111",
+        hostname: "https://www.11111.com",
+        formSubmitURL: "https://www.11111.com",
+        password: "qwerty123",
+        passwordField: "pass",
+        username: "foobar",
+        usernameField: "user",
+        httpRealm: null,
+      },
+      {
+        id: "22222",
+        guid: "22222",
+        hostname: "https://www.22222.org",
+        formSubmitURL: "https://www.22222.org",
+        password: "hunter2",
+        passwordField: "passwd",
+        username: "baz12345",
+        usernameField: "user",
+        httpRealm: null,
+      },
+      {
+        id: "33333",
+        guid: "33333",
+        hostname: "https://www.33333.com",
+        formSubmitURL: "https://www.33333.com",
+        password: "p4ssw0rd",
+        passwordField: "passwad",
+        username: "quux",
+        usernameField: "user",
+        httpRealm: null,
+      },
+    ],
+  };
+}
+
+add_task(async function test_valid() {
+  let { server, client } = getDummyServerAndClient();
+  let validator = new PasswordValidator();
+  let { problemData, clientRecords, records, deletedRecords } =
+    await validator.compareClientWithServer(client, server);
+  equal(clientRecords.length, 3);
+  equal(records.length, 3);
+  equal(deletedRecords.length, 0);
+  deepEqual(problemData, validator.emptyProblemData());
+});
+
+add_task(async function test_missing() {
+  let validator = new PasswordValidator();
+  {
+    let { server, client } = getDummyServerAndClient();
+
+    client.pop();
+
+    let { problemData, clientRecords, records, deletedRecords } =
+      await validator.compareClientWithServer(client, server);
+
+    equal(clientRecords.length, 2);
+    equal(records.length, 3);
+    equal(deletedRecords.length, 0);
+
+    let expected = validator.emptyProblemData();
+    expected.clientMissing.push("33333");
+    deepEqual(problemData, expected);
+  }
+  {
+    let { server, client } = getDummyServerAndClient();
+
+    server.pop();
+
+    let { problemData, clientRecords, records, deletedRecords } =
+      await validator.compareClientWithServer(client, server);
+
+    equal(clientRecords.length, 3);
+    equal(records.length, 2);
+    equal(deletedRecords.length, 0);
+
+    let expected = validator.emptyProblemData();
+    expected.serverMissing.push("33333");
+    deepEqual(problemData, expected);
+  }
+});
+
+add_task(async function test_deleted() {
+  let { server, client } = getDummyServerAndClient();
+  let deletionRecord = { id: "444444", guid: "444444", deleted: true };
+
+  server.push(deletionRecord);
+  let validator = new PasswordValidator();
+
+  let { problemData, clientRecords, records, deletedRecords } =
+    await validator.compareClientWithServer(client, server);
+
+  equal(clientRecords.length, 3);
+  equal(records.length, 4);
+  deepEqual(deletedRecords, [deletionRecord]);
+
+  let expected = validator.emptyProblemData();
+  deepEqual(problemData, expected);
+});
+
+add_task(async function test_duplicates() {
+  let validator = new PasswordValidator();
+  {
+    let { server, client } = getDummyServerAndClient();
+    client.push(Cu.cloneInto(client[0], {}));
+
+    let { problemData } = await validator.compareClientWithServer(
+      client,
+      server
+    );
+
+    let expected = validator.emptyProblemData();
+    expected.clientDuplicates.push("11111");
+    deepEqual(problemData, expected);
+  }
+  {
+    let { server, client } = getDummyServerAndClient();
+    server.push(Cu.cloneInto(server[server.length - 1], {}));
+
+    let { problemData } = await validator.compareClientWithServer(
+      client,
+      server
+    );
+
+    let expected = validator.emptyProblemData();
+    expected.duplicates.push("33333");
+    deepEqual(problemData, expected);
+  }
+});
diff --git a/services/sync/tests/unit/test_postqueue.js b/services/sync/tests/unit/test_postqueue.js
new file mode 100644
index 0000000000..2e687bce11
--- /dev/null
+++ b/services/sync/tests/unit/test_postqueue.js
@@ -0,0 +1,985 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+let { PostQueue } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+
+function makeRecord(nbytes) {
+  return {
+    toJSON: () => ({ payload: "x".repeat(nbytes) }),
+  };
+}
+
+// Note: This is 14 bytes. Tests make assumptions about this (even if it's just
+// in setting config.max_request_bytes to a specific value).
+makeRecord.nonPayloadOverhead = JSON.stringify(makeRecord(0).toJSON()).length;
+
+// Gives how many encoded bytes a request with the given payload
+// sizes will be (assuming the records were created by makeRecord)
+// requestBytesFor([20]) => 22, requestBytesFor([20, 20]) => 43
+function requestBytesFor(recordPayloadByteCounts) {
+  let requestBytes = 1;
+  for (let size of recordPayloadByteCounts) {
+    requestBytes += size + 1 + makeRecord.nonPayloadOverhead;
+  }
+  return requestBytes;
+}
+
+function makePostQueue(config, lastModTime, responseGenerator) {
+  let stats = {
+    posts: [],
+    batches: [],
+  };
+  let poster = (data, headers, batch, commit) => {
+    let payloadBytes = 0;
+    let numRecords = 0;
+    for (let record of JSON.parse(data)) {
+      if (config.max_record_payload_bytes) {
+        less(
+          record.payload.length,
+          config.max_record_payload_bytes,
+          "PostQueue should respect max_record_payload_bytes"
+        );
+      }
+      payloadBytes += record.payload.length;
+      ++numRecords;
+    }
+
+    let thisPost = {
+      nbytes: data.length,
+      batch,
+      commit,
+      payloadBytes,
+      numRecords,
+    };
+
+    if (headers.length) {
+      thisPost.headers = headers;
+    }
+
+    // check that we respected the provided limits for the post
+    if (config.max_post_records) {
+      lessOrEqual(
+        numRecords,
+        config.max_post_records,
+        "PostQueue should respect max_post_records"
+      );
+    }
+
+    if (config.max_post_bytes) {
+      less(
+        payloadBytes,
+        config.max_post_bytes,
+        "PostQueue should respect max_post_bytes"
+      );
+    }
+
+    if (config.max_request_bytes) {
+      less(
+        thisPost.nbytes,
+        config.max_request_bytes,
+        "PostQueue should respect max_request_bytes"
+      );
+    }
+
+    stats.posts.push(thisPost);
+
+    // Call this now so we can check if there's a batch id in it.
+    // Kind of cludgey, but allows us to have the correct batch id even
+    // before the next post is made.
+    let nextResponse = responseGenerator.next().value;
+
+    // Record info for the batch.
+
+    let curBatch = stats.batches[stats.batches.length - 1];
+    // If there's no batch, it committed, or we requested a new one,
+    // then we need to start a new one.
+    if (!curBatch || batch == "true" || curBatch.didCommit) {
+      curBatch = {
+        posts: 0,
+        payloadBytes: 0,
+        numRecords: 0,
+        didCommit: false,
+        batch,
+        serverBatch: false,
+      };
+      if (nextResponse.obj && nextResponse.obj.batch) {
+        curBatch.batch = nextResponse.obj.batch;
+        curBatch.serverBatch = true;
+      }
+      stats.batches.push(curBatch);
+    }
+
+    // If we provided a batch id, it must be the same as the current batch
+    if (batch && batch != "true") {
+      equal(curBatch.batch, batch);
+    }
+
+    curBatch.posts += 1;
+    curBatch.payloadBytes += payloadBytes;
+    curBatch.numRecords += numRecords;
+    curBatch.didCommit = commit;
+
+    // if this is an actual server batch (or it's a one-shot batch), check that
+    // we respected the provided total limits
+    if (commit && (batch == "true" || curBatch.serverBatch)) {
+      if (config.max_total_records) {
+        lessOrEqual(
+          curBatch.numRecords,
+          config.max_total_records,
+          "PostQueue should respect max_total_records"
+        );
+      }
+
+      if (config.max_total_bytes) {
+        less(
+          curBatch.payloadBytes,
+          config.max_total_bytes,
+          "PostQueue should respect max_total_bytes"
+        );
+      }
+    }
+
+    return Promise.resolve(nextResponse);
+  };
+
+  let done = () => {};
+  let pq = new PostQueue(poster, lastModTime, config, getTestLogger(), done);
+  return { pq, stats };
+}
+
+add_task(async function test_simple() {
+  let config = {
+    max_request_bytes: 1000,
+    max_record_payload_bytes: 1000,
+  };
+
+  const time = 11111111;
+
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 200,
+      headers: {
+        "x-weave-timestamp": time + 100,
+        "x-last-modified": time + 100,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  await pq.enqueue(makeRecord(10));
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([10]),
+      payloadBytes: 10,
+      numRecords: 1,
+      commit: true, // we don't know if we have batch semantics, so committed.
+      headers: [["x-if-unmodified-since", time]],
+      batch: "true",
+    },
+  ]);
+  deepEqual(stats.batches, [
+    {
+      posts: 1,
+      payloadBytes: 10,
+      numRecords: 1,
+      didCommit: true,
+      batch: "true",
+      serverBatch: false,
+    },
+  ]);
+});
+
+// Test we do the right thing when we need to make multiple posts when there
+// are no batch semantics
+add_task(async function test_max_request_bytes_no_batch() {
+  let config = {
+    max_request_bytes: 50,
+    max_record_payload_bytes: 50,
+  };
+
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 200,
+      headers: {
+        "x-weave-timestamp": time + 100,
+        "x-last-modified": time + 100,
+      },
+    };
+    yield {
+      success: true,
+      status: 200,
+      headers: {
+        "x-weave-timestamp": time + 200,
+        "x-last-modified": time + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  let payloadSize = 20 - makeRecord.nonPayloadOverhead;
+  await pq.enqueue(makeRecord(payloadSize)); // total size now 22 bytes - "[" + record + "]"
+  await pq.enqueue(makeRecord(payloadSize)); // total size now 43 bytes - "[" + record + "," + record + "]"
+  await pq.enqueue(makeRecord(payloadSize)); // this will exceed our byte limit, so will be in the 2nd POST.
+  await pq.flush(true);
+  deepEqual(stats.posts, [
+    {
+      nbytes: 43, // 43 for the first part
+      payloadBytes: payloadSize * 2,
+      numRecords: 2,
+      commit: false,
+      headers: [["x-if-unmodified-since", time]],
+      batch: "true",
+    },
+    {
+      nbytes: 22,
+      payloadBytes: payloadSize,
+      numRecords: 1,
+      commit: false, // we know we aren't in a batch, so never commit.
+      headers: [["x-if-unmodified-since", time + 100]],
+      batch: null,
+    },
+  ]);
+  equal(stats.batches.filter(x => x.didCommit).length, 0);
+  equal(pq.lastModified, time + 200);
+});
+
+add_task(async function test_max_record_payload_bytes_no_batch() {
+  let config = {
+    max_request_bytes: 100,
+    max_record_payload_bytes: 50,
+  };
+
+  const time = 11111111;
+
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 200,
+      headers: {
+        "x-weave-timestamp": time + 100,
+        "x-last-modified": time + 100,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  // Should trigger when the record really is too large to fit
+  let { enqueued } = await pq.enqueue(makeRecord(51));
+  ok(!enqueued);
+  // Shouldn't trigger when the encoded record is too big
+  ok(
+    (await pq.enqueue(makeRecord(50 - makeRecord.nonPayloadOverhead))).enqueued
+  ); // total size now 52 bytes - "[" + record + "]"
+  ok(
+    (await pq.enqueue(makeRecord(46 - makeRecord.nonPayloadOverhead))).enqueued
+  ); // total size now 99 bytes - "[" + record0 + "," + record1 + "]"
+
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: 99,
+      payloadBytes: 50 + 46 - makeRecord.nonPayloadOverhead * 2,
+      numRecords: 2,
+      commit: true, // we know we aren't in a batch, so never commit.
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 1,
+      payloadBytes: 50 + 46 - makeRecord.nonPayloadOverhead * 2,
+      numRecords: 2,
+      didCommit: true,
+      batch: "true",
+      serverBatch: false,
+    },
+  ]);
+
+  equal(pq.lastModified, time + 100);
+});
+
+// Batch tests.
+
+// Test making a single post when batch semantics are in place.
+
+add_task(async function test_single_batch() {
+  let config = {
+    max_post_bytes: 1000,
+    max_post_records: 100,
+    max_total_records: 200,
+    max_record_payload_bytes: 1000,
+  };
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time, "x-weave-timestamp": time + 100 },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([10]),
+      numRecords: 1,
+      payloadBytes: 10,
+      commit: true, // we don't know if we have batch semantics, so committed.
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 1,
+      payloadBytes: 10,
+      numRecords: 1,
+      didCommit: true,
+      batch: 1234,
+      serverBatch: true,
+    },
+  ]);
+});
+
+// Test we do the right thing when we need to make multiple posts due to
+// max_post_bytes when there are batch semantics in place.
+add_task(async function test_max_post_bytes_batch() {
+  let config = {
+    max_post_bytes: 50,
+    max_post_records: 4,
+    max_total_bytes: 5000,
+    max_total_records: 100,
+    max_record_payload_bytes: 50,
+    max_request_bytes: 4000,
+  };
+
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time, "x-weave-timestamp": time + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: {
+        "x-last-modified": time + 200,
+        "x-weave-timestamp": time + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // 20
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // 40
+  // 60 would overflow, so post
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // 20
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([20, 20]),
+      payloadBytes: 40,
+      numRecords: 2,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+    {
+      nbytes: requestBytesFor([20]),
+      payloadBytes: 20,
+      numRecords: 1,
+      commit: true,
+      batch: 1234,
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 2,
+      payloadBytes: 60,
+      numRecords: 3,
+      didCommit: true,
+      batch: 1234,
+      serverBatch: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time + 200);
+});
+
+// Test we do the right thing when we need to make multiple posts due to
+// max_request_bytes when there are batch semantics in place.
+add_task(async function test_max_request_bytes_batch() {
+  let config = {
+    max_post_bytes: 60,
+    max_post_records: 40,
+    max_total_bytes: 5000,
+    max_total_records: 100,
+    max_record_payload_bytes: 500,
+    max_request_bytes: 100,
+  };
+
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time, "x-weave-timestamp": time + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: {
+        "x-last-modified": time + 200,
+        "x-weave-timestamp": time + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  ok((await pq.enqueue(makeRecord(10))).enqueued); // post: 10, request: 26 (10 + 14 + 2)
+  ok((await pq.enqueue(makeRecord(10))).enqueued); // post: 20, request: 51 (10 + 14 + 1) * 2 + 1
+  ok((await pq.enqueue(makeRecord(10))).enqueued); // post: 30, request: 76 (10 + 14 + 1) * 3 + 1
+  // 1 more would be post: 40 (fine), request: 101, So we should post.
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([10, 10, 10]),
+      payloadBytes: 30,
+      numRecords: 3,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+    {
+      nbytes: requestBytesFor([10]),
+      payloadBytes: 10,
+      numRecords: 1,
+      commit: true,
+      batch: 1234,
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 2,
+      payloadBytes: 40,
+      numRecords: 4,
+      didCommit: true,
+      batch: 1234,
+      serverBatch: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time + 200);
+});
+
+// Test we do the right thing when the batch bytes limit is exceeded.
+add_task(async function test_max_total_bytes_batch() {
+  let config = {
+    max_post_bytes: 50,
+    max_post_records: 20,
+    max_total_bytes: 70,
+    max_total_records: 100,
+    max_record_payload_bytes: 50,
+    max_request_bytes: 500,
+  };
+
+  const time0 = 11111111;
+  const time1 = 22222222;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time0, "x-weave-timestamp": time0 + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time1, "x-weave-timestamp": time1 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 5678 },
+      headers: { "x-last-modified": time1, "x-weave-timestamp": time1 + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 5678 },
+      headers: {
+        "x-last-modified": time1 + 200,
+        "x-weave-timestamp": time1 + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time0, responseGenerator());
+
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // payloads = post: 20, batch: 20
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // payloads = post: 40, batch: 40
+
+  // this will exceed our POST byte limit, so will be in the 2nd POST - but still in the first batch.
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // payloads = post: 20, batch: 60
+
+  // this will exceed our batch byte limit, so will be in a new batch.
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // payloads = post: 20, batch: 20
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // payloads = post: 40, batch: 40
+  // This will exceed POST byte limit, so will be in the 4th post, part of the 2nd batch.
+  ok((await pq.enqueue(makeRecord(20))).enqueued); // payloads = post: 20, batch: 60
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([20, 20]),
+      payloadBytes: 40,
+      numRecords: 2,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time0]],
+    },
+    {
+      nbytes: requestBytesFor([20]),
+      payloadBytes: 20,
+      numRecords: 1,
+      commit: true,
+      batch: 1234,
+      headers: [["x-if-unmodified-since", time0]],
+    },
+    {
+      nbytes: requestBytesFor([20, 20]),
+      payloadBytes: 40,
+      numRecords: 2,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time1]],
+    },
+    {
+      nbytes: requestBytesFor([20]),
+      payloadBytes: 20,
+      numRecords: 1,
+      commit: true,
+      batch: 5678,
+      headers: [["x-if-unmodified-since", time1]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 2,
+      payloadBytes: 60,
+      numRecords: 3,
+      didCommit: true,
+      batch: 1234,
+      serverBatch: true,
+    },
+    {
+      posts: 2,
+      payloadBytes: 60,
+      numRecords: 3,
+      didCommit: true,
+      batch: 5678,
+      serverBatch: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time1 + 200);
+});
+
+// Test we split up the posts when we exceed the record limit when batch semantics
+// are in place.
+add_task(async function test_max_post_records_batch() {
+  let config = {
+    max_post_bytes: 1000,
+    max_post_records: 2,
+    max_total_bytes: 5000,
+    max_total_records: 100,
+    max_record_payload_bytes: 1000,
+    max_request_bytes: 1000,
+  };
+
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time, "x-weave-timestamp": time + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: {
+        "x-last-modified": time + 200,
+        "x-weave-timestamp": time + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+
+  // will exceed record limit of 2, so will be in 2nd post.
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([20, 20]),
+      numRecords: 2,
+      payloadBytes: 40,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+    {
+      nbytes: requestBytesFor([20]),
+      numRecords: 1,
+      payloadBytes: 20,
+      commit: true,
+      batch: 1234,
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 2,
+      payloadBytes: 60,
+      numRecords: 3,
+      batch: 1234,
+      serverBatch: true,
+      didCommit: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time + 200);
+});
+
+// Test we do the right thing when the batch record limit is exceeded.
+add_task(async function test_max_records_batch() {
+  let config = {
+    max_post_bytes: 1000,
+    max_post_records: 3,
+    max_total_bytes: 10000,
+    max_total_records: 5,
+    max_record_payload_bytes: 1000,
+    max_request_bytes: 10000,
+  };
+
+  const time0 = 11111111;
+  const time1 = 22222222;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time0, "x-weave-timestamp": time0 + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time1, "x-weave-timestamp": time1 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 5678 },
+      headers: { "x-last-modified": time1, "x-weave-timestamp": time1 + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 5678 },
+      headers: {
+        "x-last-modified": time1 + 200,
+        "x-weave-timestamp": time1 + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time0, responseGenerator());
+
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+
+  ok((await pq.enqueue(makeRecord(20))).enqueued);
+
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      // 3 records
+      nbytes: requestBytesFor([20, 20, 20]),
+      payloadBytes: 60,
+      numRecords: 3,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time0]],
+    },
+    {
+      // 2 records -- end batch1
+      nbytes: requestBytesFor([20, 20]),
+      payloadBytes: 40,
+      numRecords: 2,
+      commit: true,
+      batch: 1234,
+      headers: [["x-if-unmodified-since", time0]],
+    },
+    {
+      // 3 records
+      nbytes: requestBytesFor([20, 20, 20]),
+      payloadBytes: 60,
+      numRecords: 3,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time1]],
+    },
+    {
+      // 1 record -- end batch2
+      nbytes: requestBytesFor([20]),
+      payloadBytes: 20,
+      numRecords: 1,
+      commit: true,
+      batch: 5678,
+      headers: [["x-if-unmodified-since", time1]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 2,
+      payloadBytes: 100,
+      numRecords: 5,
+      batch: 1234,
+      serverBatch: true,
+      didCommit: true,
+    },
+    {
+      posts: 2,
+      payloadBytes: 80,
+      numRecords: 4,
+      batch: 5678,
+      serverBatch: true,
+      didCommit: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time1 + 200);
+});
+
+// Test we do the right thing when the limits are met but not exceeded.
+add_task(async function test_packed_batch() {
+  let config = {
+    max_post_bytes: 41,
+    max_post_records: 4,
+
+    max_total_bytes: 81,
+    max_total_records: 8,
+
+    max_record_payload_bytes: 20 + makeRecord.nonPayloadOverhead + 1,
+    max_request_bytes: requestBytesFor([10, 10, 10, 10]) + 1,
+  };
+
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: { "x-last-modified": time, "x-weave-timestamp": time + 100 },
+    };
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: {
+        "x-last-modified": time + 200,
+        "x-weave-timestamp": time + 200,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+  ok((await pq.enqueue(makeRecord(10))).enqueued);
+
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([10, 10, 10, 10]),
+      numRecords: 4,
+      payloadBytes: 40,
+      commit: false,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+    {
+      nbytes: requestBytesFor([10, 10, 10, 10]),
+      numRecords: 4,
+      payloadBytes: 40,
+      commit: true,
+      batch: 1234,
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 2,
+      payloadBytes: 80,
+      numRecords: 8,
+      batch: 1234,
+      serverBatch: true,
+      didCommit: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time + 200);
+});
+
+// Tests that check that a single record fails to enqueue for the provided config
+async function test_enqueue_failure_case(failureLimit, config) {
+  const time = 11111111;
+  function* responseGenerator() {
+    yield {
+      success: true,
+      status: 202,
+      obj: { batch: 1234 },
+      headers: {
+        "x-last-modified": time + 100,
+        "x-weave-timestamp": time + 100,
+      },
+    };
+  }
+
+  let { pq, stats } = makePostQueue(config, time, responseGenerator());
+  // Check on empty postqueue
+  let result = await pq.enqueue(makeRecord(failureLimit + 1));
+  ok(!result.enqueued);
+  notEqual(result.error, undefined);
+
+  ok((await pq.enqueue(makeRecord(5))).enqueued);
+
+  // check on nonempty postqueue
+  result = await pq.enqueue(makeRecord(failureLimit + 1));
+  ok(!result.enqueued);
+  notEqual(result.error, undefined);
+
+  // make sure that we keep working, skipping the bad record entirely
+  // (handling the error the queue reported is left up to caller)
+  ok((await pq.enqueue(makeRecord(5))).enqueued);
+
+  await pq.flush(true);
+
+  deepEqual(stats.posts, [
+    {
+      nbytes: requestBytesFor([5, 5]),
+      numRecords: 2,
+      payloadBytes: 10,
+      commit: true,
+      batch: "true",
+      headers: [["x-if-unmodified-since", time]],
+    },
+  ]);
+
+  deepEqual(stats.batches, [
+    {
+      posts: 1,
+      payloadBytes: 10,
+      numRecords: 2,
+      batch: 1234,
+      serverBatch: true,
+      didCommit: true,
+    },
+  ]);
+
+  equal(pq.lastModified, time + 100);
+}
+
+add_task(async function test_max_post_bytes_enqueue_failure() {
+  await test_enqueue_failure_case(50, {
+    max_post_bytes: 50,
+    max_post_records: 100,
+
+    max_total_bytes: 5000,
+    max_total_records: 100,
+
+    max_record_payload_bytes: 500,
+    max_request_bytes: 500,
+  });
+});
+
+add_task(async function test_max_request_bytes_enqueue_failure() {
+  await test_enqueue_failure_case(50, {
+    max_post_bytes: 500,
+    max_post_records: 100,
+
+    max_total_bytes: 5000,
+    max_total_records: 100,
+
+    max_record_payload_bytes: 500,
+    max_request_bytes: 50,
+  });
+});
+
+add_task(async function test_max_record_payload_bytes_enqueue_failure() {
+  await test_enqueue_failure_case(50, {
+    max_post_bytes: 500,
+    max_post_records: 100,
+
+    max_total_bytes: 5000,
+    max_total_records: 100,
+
+    max_record_payload_bytes: 50,
+    max_request_bytes: 500,
+  });
+});
diff --git a/services/sync/tests/unit/test_prefs_engine.js b/services/sync/tests/unit/test_prefs_engine.js
new file mode 100644
index 0000000000..77a4474cb4
--- /dev/null
+++ b/services/sync/tests/unit/test_prefs_engine.js
@@ -0,0 +1,134 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { getPrefsGUIDForTest } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/prefs.sys.mjs"
+);
+const PREFS_GUID = getPrefsGUIDForTest();
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+async function cleanup(engine, server) {
+  await engine._tracker.stop();
+  await engine.wipeClient();
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  Service.recordManager.clearCache();
+  await promiseStopServer(server);
+}
+
+add_task(async function test_modified_after_fail() {
+  let engine = Service.engineManager.get("prefs");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    // The homepage pref is synced by default.
+    _("Set homepage before first sync");
+    Services.prefs.setStringPref("browser.startup.homepage", "about:welcome");
+
+    _("First sync; create collection and pref record on server");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let collection = server.user("foo").collection("prefs");
+    equal(
+      collection.cleartext(PREFS_GUID).value["browser.startup.homepage"],
+      "about:welcome",
+      "Should upload homepage in pref record"
+    );
+    ok(
+      !engine._tracker.modified,
+      "Tracker shouldn't be modified after first sync"
+    );
+
+    // Our tracker only has a `modified` flag that's reset after a
+    // successful upload. Force it to remain set by failing the
+    // upload.
+    _("Second sync; flag tracker as modified and throw on upload");
+    Services.prefs.setStringPref("browser.startup.homepage", "about:robots");
+    engine._tracker.modified = true;
+    let oldPost = collection.post;
+    collection.post = () => {
+      throw new Error("Sync this!");
+    };
+    await Assert.rejects(
+      sync_engine_and_validate_telem(engine, true),
+      ex => ex.success === false
+    );
+    ok(
+      engine._tracker.modified,
+      "Tracker should remain modified after failed sync"
+    );
+
+    _("Third sync");
+    collection.post = oldPost;
+    await sync_engine_and_validate_telem(engine, false);
+    equal(
+      collection.cleartext(PREFS_GUID).value["browser.startup.homepage"],
+      "about:robots",
+      "Should upload new homepage on third sync"
+    );
+    ok(
+      !engine._tracker.modified,
+      "Tracker shouldn't be modified again after third sync"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
+
+add_task(async function test_allow_arbitrary() {
+  let engine = Service.engineManager.get("prefs");
+
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  try {
+    _("Create collection and pref record on server");
+    await sync_engine_and_validate_telem(engine, false);
+
+    let collection = server.user("foo").collection("prefs");
+
+    _("Insert arbitrary pref into remote record");
+    let cleartext1 = collection.cleartext(PREFS_GUID);
+    cleartext1.value.let_viruses_take_over = true;
+    collection.insert(
+      PREFS_GUID,
+      encryptPayload(cleartext1),
+      new_timestamp() + 5
+    );
+
+    _("Sync again; client shouldn't allow pref");
+    await sync_engine_and_validate_telem(engine, false);
+    ok(
+      !Services.prefs.getBoolPref("let_viruses_take_over", false),
+      "Shouldn't allow arbitrary remote prefs without control pref"
+    );
+
+    _("Sync with control pref set; client should set new pref");
+    Services.prefs.setBoolPref(
+      "services.sync.prefs.sync.let_viruses_take_over_take_two",
+      true
+    );
+
+    let cleartext2 = collection.cleartext(PREFS_GUID);
+    cleartext2.value.let_viruses_take_over_take_two = true;
+    collection.insert(
+      PREFS_GUID,
+      encryptPayload(cleartext2),
+      new_timestamp() + 5
+    );
+    // Reset the last sync time so that the engine fetches the record again.
+    await engine.setLastSync(0);
+    await sync_engine_and_validate_telem(engine, false);
+    ok(
+      Services.prefs.getBoolPref("let_viruses_take_over_take_two"),
+      "Should set arbitrary remote pref with control pref"
+    );
+  } finally {
+    await cleanup(engine, server);
+  }
+});
diff --git a/services/sync/tests/unit/test_prefs_store.js b/services/sync/tests/unit/test_prefs_store.js
new file mode 100644
index 0000000000..53ee68fc95
--- /dev/null
+++ b/services/sync/tests/unit/test_prefs_store.js
@@ -0,0 +1,391 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { PromiseTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/PromiseTestUtils.sys.mjs"
+);
+PromiseTestUtils.allowMatchingRejectionsGlobally(
+  /Unable to arm timer, the object has been finalized\./
+);
+PromiseTestUtils.allowMatchingRejectionsGlobally(
+  /IOUtils\.profileBeforeChange getter: IOUtils: profileBeforeChange phase has already finished/
+);
+
+const { PrefRec, getPrefsGUIDForTest } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/prefs.sys.mjs"
+);
+const PREFS_GUID = getPrefsGUIDForTest();
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const DEFAULT_THEME_ID = "default-theme@mozilla.org";
+const COMPACT_THEME_ID = "firefox-compact-light@mozilla.org";
+
+AddonTestUtils.init(this);
+AddonTestUtils.createAppInfo(
+  "xpcshell@tests.mozilla.org",
+  "XPCShell",
+  "1",
+  "1.9.2"
+);
+AddonTestUtils.overrideCertDB();
+
+add_task(async function run_test() {
+  _("Test fixtures.");
+  // Part of this test ensures the default theme, via the preference
+  // extensions.activeThemeID, is synced correctly - so we do a little
+  // addons initialization to allow this to work.
+
+  // Enable application scopes to ensure the builtin theme is going to
+  // be installed as part of the the addon manager startup.
+  Services.prefs.setIntPref(
+    "extensions.enabledScopes",
+    AddonManager.SCOPE_APPLICATION
+  );
+  await AddonTestUtils.promiseStartupManager();
+
+  // Install another built-in theme.
+  await AddonManager.installBuiltinAddon("resource://builtin-themes/light/");
+
+  const defaultThemeAddon = await AddonManager.getAddonByID(DEFAULT_THEME_ID);
+  ok(defaultThemeAddon, "Got an addon wrapper for the default theme");
+
+  const otherThemeAddon = await AddonManager.getAddonByID(COMPACT_THEME_ID);
+  ok(otherThemeAddon, "Got an addon wrapper for the compact theme");
+
+  await otherThemeAddon.enable();
+
+  // read our custom prefs file before doing anything.
+  Services.prefs.readDefaultPrefsFromFile(
+    do_get_file("prefs_test_prefs_store.js")
+  );
+
+  let engine = Service.engineManager.get("prefs");
+  let store = engine._store;
+  try {
+    _("Expect the compact light theme to be active");
+    Assert.strictEqual(
+      Services.prefs.getStringPref("extensions.activeThemeID"),
+      COMPACT_THEME_ID
+    );
+
+    _("The GUID corresponds to XUL App ID.");
+    let allIDs = await store.getAllIDs();
+    let ids = Object.keys(allIDs);
+    Assert.equal(ids.length, 1);
+    Assert.equal(ids[0], PREFS_GUID);
+    Assert.ok(allIDs[PREFS_GUID]);
+
+    Assert.ok(await store.itemExists(PREFS_GUID));
+    Assert.equal(false, await store.itemExists("random-gibberish"));
+
+    _("Unknown prefs record is created as deleted.");
+    let record = await store.createRecord("random-gibberish", "prefs");
+    Assert.ok(record.deleted);
+
+    _("Prefs record contains only prefs that should be synced.");
+    record = await store.createRecord(PREFS_GUID, "prefs");
+    Assert.strictEqual(record.value["testing.int"], 123);
+    Assert.strictEqual(record.value["testing.string"], "ohai");
+    Assert.strictEqual(record.value["testing.bool"], true);
+    // non-existing prefs get null as the value
+    Assert.strictEqual(record.value["testing.nonexistent"], null);
+    // as do prefs that have a default value.
+    Assert.strictEqual(record.value["testing.default"], null);
+    Assert.strictEqual(record.value["testing.turned.off"], undefined);
+    Assert.strictEqual(record.value["testing.not.turned.on"], undefined);
+
+    _("Prefs record contains the correct control prefs.");
+    // All control prefs which have the default value and where the pref
+    // itself is synced should appear, but with null as the value.
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.int"],
+      null
+    );
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.string"],
+      null
+    );
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.bool"],
+      null
+    );
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.dont.change"],
+      null
+    );
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.nonexistent"],
+      null
+    );
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.default"],
+      null
+    );
+
+    // but this control pref has a non-default value so that value is synced.
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.turned.off"],
+      false
+    );
+
+    _("Unsyncable prefs are treated correctly.");
+    // Prefs we consider unsyncable (since they are URLs that won't be stable on
+    // another firefox) shouldn't be included - neither the value nor the
+    // control pref should appear.
+    Assert.strictEqual(record.value["testing.unsynced.url"], undefined);
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.unsynced.url"],
+      undefined
+    );
+    // Other URLs with user prefs should be synced, though.
+    Assert.strictEqual(
+      record.value["testing.synced.url"],
+      "https://www.example.com"
+    );
+    Assert.strictEqual(
+      record.value["services.sync.prefs.sync.testing.synced.url"],
+      null
+    );
+
+    _("Update some prefs, including one that's to be reset/deleted.");
+    // This pref is not going to be reset or deleted as there's no "control pref"
+    // in either the incoming record or locally.
+    Services.prefs.setStringPref(
+      "testing.deleted-without-control-pref",
+      "I'm deleted-without-control-pref"
+    );
+    // Another pref with only a local control pref.
+    Services.prefs.setStringPref(
+      "testing.deleted-with-local-control-pref",
+      "I'm deleted-with-local-control-pref"
+    );
+    Services.prefs.setBoolPref(
+      "services.sync.prefs.sync.testing.deleted-with-local-control-pref",
+      true
+    );
+    // And a pref without a local control pref but one that's incoming.
+    Services.prefs.setStringPref(
+      "testing.deleted-with-incoming-control-pref",
+      "I'm deleted-with-incoming-control-pref"
+    );
+    record = new PrefRec("prefs", PREFS_GUID);
+    record.value = {
+      "extensions.activeThemeID": DEFAULT_THEME_ID,
+      "testing.int": 42,
+      "testing.string": "im in ur prefs",
+      "testing.bool": false,
+      "testing.deleted-without-control-pref": null,
+      "testing.deleted-with-local-control-pref": null,
+      "testing.deleted-with-incoming-control-pref": null,
+      "services.sync.prefs.sync.testing.deleted-with-incoming-control-pref": true,
+      "testing.somepref": "im a new pref from other device",
+      "services.sync.prefs.sync.testing.somepref": true,
+      // Pretend some a stale remote client is overwriting it with a value
+      // we consider unsyncable.
+      "testing.synced.url": "blob:ebeb707a-502e-40c6-97a5-dd4bda901463",
+      // Make sure we can replace the unsynced URL with a valid URL.
+      "testing.unsynced.url": "https://www.example.com/2",
+      // Make sure our "master control pref" is ignored.
+      "services.sync.prefs.dangerously_allow_arbitrary": true,
+      "services.sync.prefs.sync.services.sync.prefs.dangerously_allow_arbitrary": true,
+    };
+
+    const onceAddonEnabled = AddonTestUtils.promiseAddonEvent("onEnabled");
+
+    await store.update(record);
+    Assert.strictEqual(Services.prefs.getIntPref("testing.int"), 42);
+    Assert.strictEqual(
+      Services.prefs.getStringPref("testing.string"),
+      "im in ur prefs"
+    );
+    Assert.strictEqual(Services.prefs.getBoolPref("testing.bool"), false);
+    Assert.strictEqual(
+      Services.prefs.getStringPref("testing.deleted-without-control-pref"),
+      "I'm deleted-without-control-pref"
+    );
+    Assert.strictEqual(
+      Services.prefs.getPrefType("testing.deleted-with-local-control-pref"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.strictEqual(
+      Services.prefs.getStringPref(
+        "testing.deleted-with-incoming-control-pref"
+      ),
+      "I'm deleted-with-incoming-control-pref"
+    );
+    Assert.strictEqual(
+      Services.prefs.getStringPref("testing.dont.change"),
+      "Please don't change me."
+    );
+    Assert.strictEqual(
+      Services.prefs.getPrefType("testing.somepref"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.strictEqual(
+      Services.prefs.getStringPref("testing.synced.url"),
+      "https://www.example.com"
+    );
+    Assert.strictEqual(
+      Services.prefs.getStringPref("testing.unsynced.url"),
+      "https://www.example.com/2"
+    );
+    Assert.strictEqual(
+      Svc.PrefBranch.getPrefType("prefs.sync.testing.somepref"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.strictEqual(
+      Services.prefs.getBoolPref(
+        "services.sync.prefs.dangerously_allow_arbitrary"
+      ),
+      false
+    );
+    Assert.strictEqual(
+      Services.prefs.getPrefType(
+        "services.sync.prefs.sync.services.sync.prefs.dangerously_allow_arbitrary"
+      ),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+
+    await onceAddonEnabled;
+    ok(
+      !defaultThemeAddon.userDisabled,
+      "the default theme should have been enabled"
+    );
+    ok(
+      otherThemeAddon.userDisabled,
+      "the compact theme should have been disabled"
+    );
+
+    _("Only the current app's preferences are applied.");
+    record = new PrefRec("prefs", "some-fake-app");
+    record.value = {
+      "testing.int": 98,
+    };
+    await store.update(record);
+    Assert.equal(Services.prefs.getIntPref("testing.int"), 42);
+  } finally {
+    for (const pref of Services.prefs.getChildList("")) {
+      Services.prefs.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_dangerously_allow() {
+  _("services.sync.prefs.dangerously_allow_arbitrary");
+  // Bug 1538015 added a capability to "dangerously allow" arbitrary prefs.
+  // Bug 1854698 removed that capability but did keep the fact we never
+  // sync the pref which enabled the "dangerous" behaviour, just incase someone
+  // tries to sync it back to a profile which *does* support that pref.
+  Services.prefs.readDefaultPrefsFromFile(
+    do_get_file("prefs_test_prefs_store.js")
+  );
+
+  let engine = Service.engineManager.get("prefs");
+  let store = engine._store;
+  try {
+    // an incoming record with our old "dangerous" pref.
+    let record = new PrefRec("prefs", PREFS_GUID);
+    record.value = {
+      "services.sync.prefs.dangerously_allow_arbitrary": true,
+      "services.sync.prefs.sync.services.sync.prefs.dangerously_allow_arbitrary": true,
+    };
+    await store.update(record);
+    Assert.strictEqual(
+      Services.prefs.getBoolPref(
+        "services.sync.prefs.dangerously_allow_arbitrary"
+      ),
+      false
+    );
+    Assert.strictEqual(
+      Services.prefs.getPrefType(
+        "services.sync.prefs.sync.services.sync.prefs.dangerously_allow_arbitrary"
+      ),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+  } finally {
+    for (const pref of Services.prefs.getChildList("")) {
+      Services.prefs.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_incoming_sets_seen() {
+  _("Test the sync-seen allow-list");
+
+  let engine = Service.engineManager.get("prefs");
+  let store = engine._store;
+
+  Services.prefs.readDefaultPrefsFromFile(
+    do_get_file("prefs_test_prefs_store.js")
+  );
+  const defaultValue = "the value";
+  Assert.equal(Services.prefs.getStringPref("testing.seen"), defaultValue);
+
+  let record = await store.createRecord(PREFS_GUID, "prefs");
+  // Haven't seen a non-default value before, so remains null.
+  Assert.strictEqual(record.value["testing.seen"], null);
+
+  // pretend an incoming record with the default value - it might not be
+  // the default everywhere, so we treat it specially.
+  record = new PrefRec("prefs", PREFS_GUID);
+  record.value = {
+    "testing.seen": defaultValue,
+  };
+  await store.update(record);
+  // Our special control value should now be set.
+  Assert.strictEqual(
+    Services.prefs.getBoolPref("services.sync.prefs.sync-seen.testing.seen"),
+    true
+  );
+  // It's still the default value, so the value is not considered changed
+  Assert.equal(Services.prefs.prefHasUserValue("testing.seen"), false);
+
+  // But now that special control value is set, the record always contains the value.
+  record = await store.createRecord(PREFS_GUID, "prefs");
+  Assert.strictEqual(record.value["testing.seen"], defaultValue);
+});
+
+add_task(async function test_outgoing_when_changed() {
+  _("Test the 'seen' pref is set first sync of non-default value");
+
+  let engine = Service.engineManager.get("prefs");
+  let store = engine._store;
+  for (const pref of Services.prefs.getChildList("")) {
+    Services.prefs.clearUserPref(pref);
+  }
+
+  Services.prefs.readDefaultPrefsFromFile(
+    do_get_file("prefs_test_prefs_store.js")
+  );
+  const defaultValue = "the value";
+  Assert.equal(Services.prefs.getStringPref("testing.seen"), defaultValue);
+
+  let record = await store.createRecord(PREFS_GUID, "prefs");
+  // Haven't seen a non-default value before, so remains null.
+  Assert.strictEqual(record.value["testing.seen"], null);
+
+  // Change the value.
+  Services.prefs.setStringPref("testing.seen", "new value");
+  record = await store.createRecord(PREFS_GUID, "prefs");
+  // creating the record toggled that "seen" pref.
+  Assert.strictEqual(
+    Services.prefs.getBoolPref("services.sync.prefs.sync-seen.testing.seen"),
+    true
+  );
+  Assert.strictEqual(Services.prefs.getStringPref("testing.seen"), "new value");
+
+  // Resetting the pref does not change that seen value.
+  Services.prefs.clearUserPref("testing.seen");
+  Assert.strictEqual(
+    Services.prefs.getStringPref("testing.seen"),
+    defaultValue
+  );
+
+  record = await store.createRecord(PREFS_GUID, "prefs");
+  Assert.strictEqual(
+    Services.prefs.getBoolPref("services.sync.prefs.sync-seen.testing.seen"),
+    true
+  );
+});
diff --git a/services/sync/tests/unit/test_prefs_tracker.js b/services/sync/tests/unit/test_prefs_tracker.js
new file mode 100644
index 0000000000..ecf3f18420
--- /dev/null
+++ b/services/sync/tests/unit/test_prefs_tracker.js
@@ -0,0 +1,93 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function run_test() {
+  let engine = Service.engineManager.get("prefs");
+  let tracker = engine._tracker;
+
+  try {
+    _("tracker.modified corresponds to preference.");
+    // Assert preference is not defined.
+    Assert.equal(
+      Svc.PrefBranch.getPrefType("engine.prefs.modified"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.ok(!tracker.modified);
+
+    tracker.modified = true;
+    Assert.equal(Svc.PrefBranch.getBoolPref("engine.prefs.modified"), true);
+    Assert.ok(tracker.modified);
+
+    _("Engine's getChangedID() just returns the one GUID we have.");
+    let changedIDs = await engine.getChangedIDs();
+    let ids = Object.keys(changedIDs);
+    Assert.equal(ids.length, 1);
+    Assert.equal(ids[0], CommonUtils.encodeBase64URL(Services.appinfo.ID));
+
+    Svc.PrefBranch.setBoolPref("engine.prefs.modified", false);
+    Assert.ok(!tracker.modified);
+
+    _("No modified state, so no changed IDs.");
+    do_check_empty(await engine.getChangedIDs());
+
+    _("Initial score is 0");
+    Assert.equal(tracker.score, 0);
+
+    _("Test fixtures.");
+    Svc.PrefBranch.setBoolPref("prefs.sync.testing.int", true);
+
+    _(
+      "Test fixtures haven't upped the tracker score yet because it hasn't started tracking yet."
+    );
+    Assert.equal(tracker.score, 0);
+
+    _("Tell the tracker to start tracking changes.");
+    tracker.start();
+    Services.prefs.setIntPref("testing.int", 23);
+    await tracker.asyncObserver.promiseObserversComplete();
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE);
+    Assert.equal(tracker.modified, true);
+
+    _("Clearing changed IDs reset modified status.");
+    await tracker.clearChangedIDs();
+    Assert.equal(tracker.modified, false);
+
+    _("Resetting a pref ups the score, too.");
+    Services.prefs.clearUserPref("testing.int");
+    await tracker.asyncObserver.promiseObserversComplete();
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 2);
+    Assert.equal(tracker.modified, true);
+    await tracker.clearChangedIDs();
+
+    _("So does changing a pref sync pref.");
+    Svc.PrefBranch.setBoolPref("prefs.sync.testing.int", false);
+    await tracker.asyncObserver.promiseObserversComplete();
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(tracker.modified, true);
+    await tracker.clearChangedIDs();
+
+    _(
+      "Now that the pref sync pref has been flipped, changes to it won't be picked up."
+    );
+    Services.prefs.setIntPref("testing.int", 42);
+    await tracker.asyncObserver.promiseObserversComplete();
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(tracker.modified, false);
+    await tracker.clearChangedIDs();
+
+    _("Changing some other random pref won't do anything.");
+    Services.prefs.setStringPref("testing.other", "blergh");
+    await tracker.asyncObserver.promiseObserversComplete();
+    Assert.equal(tracker.score, SCORE_INCREMENT_XLARGE * 3);
+    Assert.equal(tracker.modified, false);
+  } finally {
+    await tracker.stop();
+    for (const pref of Services.prefs.getChildList("")) {
+      Services.prefs.clearUserPref(pref);
+    }
+  }
+});
diff --git a/services/sync/tests/unit/test_records_crypto.js b/services/sync/tests/unit/test_records_crypto.js
new file mode 100644
index 0000000000..8b841653cd
--- /dev/null
+++ b/services/sync/tests/unit/test_records_crypto.js
@@ -0,0 +1,189 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { CollectionKeyManager, CryptoWrapper } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+var cryptoWrap;
+
+function crypted_resource_handler(metadata, response) {
+  let obj = {
+    id: "resource",
+    modified: cryptoWrap.modified,
+    payload: JSON.stringify(cryptoWrap.payload),
+  };
+  return httpd_basic_auth_handler(JSON.stringify(obj), metadata, response);
+}
+
+function prepareCryptoWrap(collection, id) {
+  let w = new CryptoWrapper();
+  w.cleartext.stuff = "my payload here";
+  w.collection = collection;
+  w.id = id;
+  return w;
+}
+
+add_task(async function test_records_crypto() {
+  let server;
+
+  await configureIdentity({ username: "john@example.com" });
+  let keyBundle = Service.identity.syncKeyBundle;
+
+  try {
+    let log = Log.repository.getLogger("Test");
+    Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+    log.info("Setting up server and authenticator");
+
+    server = httpd_setup({ "/steam/resource": crypted_resource_handler });
+
+    log.info("Creating a record");
+
+    cryptoWrap = prepareCryptoWrap("steam", "resource");
+
+    log.info("cryptoWrap: " + cryptoWrap.toString());
+
+    log.info("Encrypting a record");
+
+    await cryptoWrap.encrypt(keyBundle);
+    log.info("Ciphertext is " + cryptoWrap.ciphertext);
+    Assert.ok(cryptoWrap.ciphertext != null);
+
+    let firstIV = cryptoWrap.IV;
+
+    log.info("Decrypting the record");
+
+    let payload = await cryptoWrap.decrypt(keyBundle);
+    Assert.equal(payload.stuff, "my payload here");
+    Assert.notEqual(payload, cryptoWrap.payload); // wrap.data.payload is the encrypted one
+
+    log.info("Make sure multiple decrypts cause failures");
+    let error = "";
+    try {
+      payload = await cryptoWrap.decrypt(keyBundle);
+    } catch (ex) {
+      error = ex;
+    }
+    Assert.equal(error.message, "No ciphertext: nothing to decrypt?");
+
+    log.info("Re-encrypting the record with alternate payload");
+
+    cryptoWrap.cleartext.stuff = "another payload";
+    await cryptoWrap.encrypt(keyBundle);
+    let secondIV = cryptoWrap.IV;
+    payload = await cryptoWrap.decrypt(keyBundle);
+    Assert.equal(payload.stuff, "another payload");
+
+    log.info("Make sure multiple encrypts use different IVs");
+    Assert.notEqual(firstIV, secondIV);
+
+    log.info(await "Make sure differing ids cause failures");
+    await cryptoWrap.encrypt(keyBundle);
+    cryptoWrap.data.id = "other";
+    error = "";
+    try {
+      await cryptoWrap.decrypt(keyBundle);
+    } catch (ex) {
+      error = ex;
+    }
+    Assert.equal(error.message, "Record id mismatch: resource != other");
+
+    log.info("Make sure wrong hmacs cause failures");
+    await cryptoWrap.encrypt(keyBundle);
+    cryptoWrap.hmac = "foo";
+    error = "";
+    try {
+      await cryptoWrap.decrypt(keyBundle);
+    } catch (ex) {
+      error = ex;
+    }
+    Assert.equal(
+      error.message.substr(0, 42),
+      "Record SHA256 HMAC mismatch: should be foo"
+    );
+
+    // Checking per-collection keys and default key handling.
+
+    await generateNewKeys(Service.collectionKeys);
+    let bookmarkItem = prepareCryptoWrap("bookmarks", "foo");
+    await bookmarkItem.encrypt(
+      Service.collectionKeys.keyForCollection("bookmarks")
+    );
+    log.info("Ciphertext is " + bookmarkItem.ciphertext);
+    Assert.ok(bookmarkItem.ciphertext != null);
+    log.info("Decrypting the record explicitly with the default key.");
+    Assert.equal(
+      (await bookmarkItem.decrypt(Service.collectionKeys._default)).stuff,
+      "my payload here"
+    );
+
+    // Per-collection keys.
+    // Generate a key for "bookmarks".
+    await generateNewKeys(Service.collectionKeys, ["bookmarks"]);
+    bookmarkItem = prepareCryptoWrap("bookmarks", "foo");
+    Assert.equal(bookmarkItem.collection, "bookmarks");
+
+    // Encrypt. This'll use the "bookmarks" encryption key, because we have a
+    // special key for it. The same key will need to be used for decryption.
+    await bookmarkItem.encrypt(
+      Service.collectionKeys.keyForCollection("bookmarks")
+    );
+    Assert.ok(bookmarkItem.ciphertext != null);
+
+    // Attempt to use the default key, because this is a collision that could
+    // conceivably occur in the real world. Decryption will error, because
+    // it's not the bookmarks key.
+    let err;
+    try {
+      await bookmarkItem.decrypt(Service.collectionKeys._default);
+    } catch (ex) {
+      err = ex;
+    }
+    Assert.equal("Record SHA256 HMAC mismatch", err.message.substr(0, 27));
+
+    // Explicitly check that it's using the bookmarks key.
+    // This should succeed.
+    Assert.equal(
+      (
+        await bookmarkItem.decrypt(
+          Service.collectionKeys.keyForCollection("bookmarks")
+        )
+      ).stuff,
+      "my payload here"
+    );
+
+    Assert.ok(Service.collectionKeys.hasKeysFor(["bookmarks"]));
+
+    // Add a key for some new collection and verify that it isn't the
+    // default key.
+    Assert.ok(!Service.collectionKeys.hasKeysFor(["forms"]));
+    Assert.ok(!Service.collectionKeys.hasKeysFor(["bookmarks", "forms"]));
+    let oldFormsKey = Service.collectionKeys.keyForCollection("forms");
+    Assert.equal(oldFormsKey, Service.collectionKeys._default);
+    let newKeys = await Service.collectionKeys.ensureKeysFor(["forms"]);
+    Assert.ok(newKeys.hasKeysFor(["forms"]));
+    Assert.ok(newKeys.hasKeysFor(["bookmarks", "forms"]));
+    let newFormsKey = newKeys.keyForCollection("forms");
+    Assert.notEqual(newFormsKey, oldFormsKey);
+
+    // Verify that this doesn't overwrite keys
+    let regetKeys = await newKeys.ensureKeysFor(["forms"]);
+    Assert.equal(regetKeys.keyForCollection("forms"), newFormsKey);
+
+    const emptyKeys = new CollectionKeyManager();
+    payload = {
+      default: Service.collectionKeys._default.keyPairB64,
+      collections: {},
+    };
+    // Verify that not passing `modified` doesn't throw
+    emptyKeys.setContents(payload, null);
+
+    log.info("Done!");
+  } finally {
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_records_wbo.js b/services/sync/tests/unit/test_records_wbo.js
new file mode 100644
index 0000000000..61ba33d749
--- /dev/null
+++ b/services/sync/tests/unit/test_records_wbo.js
@@ -0,0 +1,85 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_test(function test_toJSON() {
+  _("Create a record, for now without a TTL.");
+  let wbo = new WBORecord("coll", "a_record");
+  wbo.modified = 12345;
+  wbo.sortindex = 42;
+  wbo.payload = {};
+
+  _(
+    "Verify that the JSON representation contains the WBO properties, but not TTL."
+  );
+  let json = JSON.parse(JSON.stringify(wbo));
+  Assert.equal(json.modified, 12345);
+  Assert.equal(json.sortindex, 42);
+  Assert.equal(json.payload, "{}");
+  Assert.equal(false, "ttl" in json);
+
+  _("Set a TTL, make sure it's present in the JSON representation.");
+  wbo.ttl = 30 * 60;
+  json = JSON.parse(JSON.stringify(wbo));
+  Assert.equal(json.ttl, 30 * 60);
+  run_next_test();
+});
+
+add_task(async function test_fetch() {
+  let record = {
+    id: "asdf-1234-asdf-1234",
+    modified: 2454725.98283,
+    payload: JSON.stringify({ cheese: "roquefort" }),
+  };
+  let record2 = {
+    id: "record2",
+    modified: 2454725.98284,
+    payload: JSON.stringify({ cheese: "gruyere" }),
+  };
+  let coll = [
+    {
+      id: "record2",
+      modified: 2454725.98284,
+      payload: JSON.stringify({ cheese: "gruyere" }),
+    },
+  ];
+
+  _("Setting up server.");
+  let server = httpd_setup({
+    "/record": httpd_handler(200, "OK", JSON.stringify(record)),
+    "/record2": httpd_handler(200, "OK", JSON.stringify(record2)),
+    "/coll": httpd_handler(200, "OK", JSON.stringify(coll)),
+  });
+
+  try {
+    _("Fetching a WBO record");
+    let rec = new WBORecord("coll", "record");
+    await rec.fetch(Service.resource(server.baseURI + "/record"));
+    Assert.equal(rec.id, "asdf-1234-asdf-1234"); // NOT "record"!
+
+    Assert.equal(rec.modified, 2454725.98283);
+    Assert.equal(typeof rec.payload, "object");
+    Assert.equal(rec.payload.cheese, "roquefort");
+
+    _("Fetching a WBO record using the record manager");
+    let rec2 = await Service.recordManager.get(server.baseURI + "/record2");
+    Assert.equal(rec2.id, "record2");
+    Assert.equal(rec2.modified, 2454725.98284);
+    Assert.equal(typeof rec2.payload, "object");
+    Assert.equal(rec2.payload.cheese, "gruyere");
+    Assert.equal(Service.recordManager.response.status, 200);
+
+    // Testing collection extraction.
+    _("Extracting collection.");
+    let rec3 = new WBORecord("tabs", "foo"); // Create through constructor.
+    Assert.equal(rec3.collection, "tabs");
+  } finally {
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_resource.js b/services/sync/tests/unit/test_resource.js
new file mode 100644
index 0000000000..5182784639
--- /dev/null
+++ b/services/sync/tests/unit/test_resource.js
@@ -0,0 +1,554 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Observers } = ChromeUtils.importESModule(
+  "resource://services-common/observers.sys.mjs"
+);
+const { Resource } = ChromeUtils.importESModule(
+  "resource://services-sync/resource.sys.mjs"
+);
+const { SyncAuthManager } = ChromeUtils.importESModule(
+  "resource://services-sync/sync_auth.sys.mjs"
+);
+
+var fetched = false;
+function server_open(metadata, response) {
+  let body;
+  if (metadata.method == "GET") {
+    fetched = true;
+    body = "This path exists";
+    response.setStatusLine(metadata.httpVersion, 200, "OK");
+  } else {
+    body = "Wrong request method";
+    response.setStatusLine(metadata.httpVersion, 405, "Method Not Allowed");
+  }
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_protected(metadata, response) {
+  let body;
+
+  if (has_hawk_header(metadata)) {
+    body = "This path exists and is protected";
+    response.setStatusLine(metadata.httpVersion, 200, "OK, authorized");
+    response.setHeader("WWW-Authenticate", 'Basic realm="secret"', false);
+  } else {
+    body = "This path exists and is protected - failed";
+    response.setStatusLine(metadata.httpVersion, 401, "Unauthorized");
+    response.setHeader("WWW-Authenticate", 'Basic realm="secret"', false);
+  }
+
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_404(metadata, response) {
+  let body = "File not found";
+  response.setStatusLine(metadata.httpVersion, 404, "Not Found");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+var pacFetched = false;
+function server_pac(metadata, response) {
+  _("Invoked PAC handler.");
+  pacFetched = true;
+  let body = 'function FindProxyForURL(url, host) { return "DIRECT"; }';
+  response.setStatusLine(metadata.httpVersion, 200, "OK");
+  response.setHeader(
+    "Content-Type",
+    "application/x-ns-proxy-autoconfig",
+    false
+  );
+  response.bodyOutputStream.write(body, body.length);
+}
+
+var sample_data = {
+  some: "sample_data",
+  injson: "format",
+  number: 42,
+};
+
+function server_upload(metadata, response) {
+  let body;
+
+  let input = readBytesFromInputStream(metadata.bodyInputStream);
+  if (input == JSON.stringify(sample_data)) {
+    body = "Valid data upload via " + metadata.method;
+    response.setStatusLine(metadata.httpVersion, 200, "OK");
+  } else {
+    body = "Invalid data upload via " + metadata.method + ": " + input;
+    response.setStatusLine(metadata.httpVersion, 500, "Internal Server Error");
+  }
+
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_delete(metadata, response) {
+  let body;
+  if (metadata.method == "DELETE") {
+    body = "This resource has been deleted";
+    response.setStatusLine(metadata.httpVersion, 200, "OK");
+  } else {
+    body = "Wrong request method";
+    response.setStatusLine(metadata.httpVersion, 405, "Method Not Allowed");
+  }
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_json(metadata, response) {
+  let body = JSON.stringify(sample_data);
+  response.setStatusLine(metadata.httpVersion, 200, "OK");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+const TIMESTAMP = 1274380461;
+
+function server_timestamp(metadata, response) {
+  let body = "Thank you for your request";
+  response.setHeader("X-Weave-Timestamp", "" + TIMESTAMP, false);
+  response.setStatusLine(metadata.httpVersion, 200, "OK");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_backoff(metadata, response) {
+  let body = "Hey, back off!";
+  response.setHeader("X-Weave-Backoff", "600", false);
+  response.setStatusLine(metadata.httpVersion, 200, "OK");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_quota_notice(request, response) {
+  let body = "You're approaching quota.";
+  response.setHeader("X-Weave-Quota-Remaining", "1048576", false);
+  response.setStatusLine(request.httpVersion, 200, "OK");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_quota_error(request, response) {
+  let body = "14";
+  response.setHeader("X-Weave-Quota-Remaining", "-1024", false);
+  response.setStatusLine(request.httpVersion, 400, "OK");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function server_headers(metadata, response) {
+  let ignore_headers = [
+    "host",
+    "user-agent",
+    "accept-language",
+    "accept-encoding",
+    "accept-charset",
+    "keep-alive",
+    "connection",
+    "pragma",
+    "origin",
+    "cache-control",
+    "content-length",
+  ];
+  let headers = metadata.headers;
+  let header_names = [];
+  while (headers.hasMoreElements()) {
+    let header = headers.getNext().toString();
+    if (!ignore_headers.includes(header)) {
+      header_names.push(header);
+    }
+  }
+  header_names = header_names.sort();
+
+  headers = {};
+  for (let header of header_names) {
+    headers[header] = metadata.getHeader(header);
+  }
+  let body = JSON.stringify(headers);
+  response.setStatusLine(metadata.httpVersion, 200, "OK");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+var quotaValue;
+Observers.add("weave:service:quota:remaining", function (subject) {
+  quotaValue = subject;
+});
+
+function run_test() {
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  Svc.PrefBranch.setIntPref("network.numRetries", 1); // speed up test
+  run_next_test();
+}
+
+// This apparently has to come first in order for our PAC URL to be hit.
+// Don't put any other HTTP requests earlier in the file!
+add_task(async function test_proxy_auth_redirect() {
+  _(
+    "Ensure that a proxy auth redirect (which switches out our channel) " +
+      "doesn't break Resource."
+  );
+  let server = httpd_setup({
+    "/open": server_open,
+    "/pac2": server_pac,
+  });
+
+  PACSystemSettings.PACURI = server.baseURI + "/pac2";
+  installFakePAC();
+  let res = new Resource(server.baseURI + "/open");
+  let result = await res.get();
+  Assert.ok(pacFetched);
+  Assert.ok(fetched);
+  Assert.equal("This path exists", result.data);
+  pacFetched = fetched = false;
+  uninstallFakePAC();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_new_channel() {
+  _("Ensure a redirect to a new channel is handled properly.");
+
+  let resourceRequested = false;
+  function resourceHandler(metadata, response) {
+    resourceRequested = true;
+
+    let body = "Test";
+    response.setHeader("Content-Type", "text/plain");
+    response.bodyOutputStream.write(body, body.length);
+  }
+
+  let locationURL;
+  function redirectHandler(metadata, response) {
+    let body = "Redirecting";
+    response.setStatusLine(metadata.httpVersion, 307, "TEMPORARY REDIRECT");
+    response.setHeader("Location", locationURL);
+    response.bodyOutputStream.write(body, body.length);
+  }
+
+  let server = httpd_setup({
+    "/resource": resourceHandler,
+    "/redirect": redirectHandler,
+  });
+  locationURL = server.baseURI + "/resource";
+
+  let request = new Resource(server.baseURI + "/redirect");
+  let content = await request.get();
+  Assert.ok(resourceRequested);
+  Assert.equal(200, content.status);
+  Assert.ok("content-type" in content.headers);
+  Assert.equal("text/plain", content.headers["content-type"]);
+
+  await promiseStopServer(server);
+});
+
+var server;
+
+add_test(function setup() {
+  server = httpd_setup({
+    "/open": server_open,
+    "/protected": server_protected,
+    "/404": server_404,
+    "/upload": server_upload,
+    "/delete": server_delete,
+    "/json": server_json,
+    "/timestamp": server_timestamp,
+    "/headers": server_headers,
+    "/backoff": server_backoff,
+    "/pac2": server_pac,
+    "/quota-notice": server_quota_notice,
+    "/quota-error": server_quota_error,
+  });
+
+  run_next_test();
+});
+
+add_test(function test_members() {
+  _("Resource object members");
+  let uri = server.baseURI + "/open";
+  let res = new Resource(uri);
+  Assert.ok(res.uri instanceof Ci.nsIURI);
+  Assert.equal(res.uri.spec, uri);
+  Assert.equal(res.spec, uri);
+  Assert.equal(typeof res.headers, "object");
+  Assert.equal(typeof res.authenticator, "object");
+
+  run_next_test();
+});
+
+add_task(async function test_get() {
+  _("GET a non-password-protected resource");
+  let res = new Resource(server.baseURI + "/open");
+  let content = await res.get();
+  Assert.equal(content.data, "This path exists");
+  Assert.equal(content.status, 200);
+  Assert.ok(content.success);
+
+  // Observe logging messages.
+  let resLogger = res._log;
+  let dbg = resLogger.debug;
+  let debugMessages = [];
+  resLogger.debug = function (msg, extra) {
+    debugMessages.push(`${msg}: ${JSON.stringify(extra)}`);
+    dbg.call(this, msg);
+  };
+
+  // Since we didn't receive proper JSON data, accessing content.obj
+  // will result in a SyntaxError from JSON.parse
+  let didThrow = false;
+  try {
+    content.obj;
+  } catch (ex) {
+    didThrow = true;
+  }
+  Assert.ok(didThrow);
+  Assert.equal(debugMessages.length, 1);
+  Assert.equal(
+    debugMessages[0],
+    'Parse fail: Response body starts: "This path exists"'
+  );
+  resLogger.debug = dbg;
+});
+
+add_test(function test_basicauth() {
+  _("Test that the BasicAuthenticator doesn't screw up header case.");
+  let res1 = new Resource(server.baseURI + "/foo");
+  res1.setHeader("Authorization", "Basic foobar");
+  Assert.equal(res1._headers.authorization, "Basic foobar");
+  Assert.equal(res1.headers.authorization, "Basic foobar");
+
+  run_next_test();
+});
+
+add_task(async function test_get_protected_fail() {
+  _(
+    "GET a password protected resource (test that it'll fail w/o pass, no throw)"
+  );
+  let res2 = new Resource(server.baseURI + "/protected");
+  let content = await res2.get();
+  Assert.equal(content.data, "This path exists and is protected - failed");
+  Assert.equal(content.status, 401);
+  Assert.ok(!content.success);
+});
+
+add_task(async function test_get_protected_success() {
+  _("GET a password protected resource");
+  let identityConfig = makeIdentityConfig();
+  let syncAuthManager = new SyncAuthManager();
+  configureFxAccountIdentity(syncAuthManager, identityConfig);
+  let auth = syncAuthManager.getResourceAuthenticator();
+  let res3 = new Resource(server.baseURI + "/protected");
+  res3.authenticator = auth;
+  Assert.equal(res3.authenticator, auth);
+  let content = await res3.get();
+  Assert.equal(content.data, "This path exists and is protected");
+  Assert.equal(content.status, 200);
+  Assert.ok(content.success);
+});
+
+add_task(async function test_get_404() {
+  _("GET a non-existent resource (test that it'll fail, but not throw)");
+  let res4 = new Resource(server.baseURI + "/404");
+  let content = await res4.get();
+  Assert.equal(content.data, "File not found");
+  Assert.equal(content.status, 404);
+  Assert.ok(!content.success);
+
+  // Check some headers of the 404 response
+  Assert.equal(content.headers.connection, "close");
+  Assert.equal(content.headers.server, "httpd.js");
+  Assert.equal(content.headers["content-length"], 14);
+});
+
+add_task(async function test_put_string() {
+  _("PUT to a resource (string)");
+  let res_upload = new Resource(server.baseURI + "/upload");
+  let content = await res_upload.put(JSON.stringify(sample_data));
+  Assert.equal(content.data, "Valid data upload via PUT");
+  Assert.equal(content.status, 200);
+});
+
+add_task(async function test_put_object() {
+  _("PUT to a resource (object)");
+  let res_upload = new Resource(server.baseURI + "/upload");
+  let content = await res_upload.put(sample_data);
+  Assert.equal(content.data, "Valid data upload via PUT");
+  Assert.equal(content.status, 200);
+});
+
+add_task(async function test_post_string() {
+  _("POST to a resource (string)");
+  let res_upload = new Resource(server.baseURI + "/upload");
+  let content = await res_upload.post(JSON.stringify(sample_data));
+  Assert.equal(content.data, "Valid data upload via POST");
+  Assert.equal(content.status, 200);
+});
+
+add_task(async function test_post_object() {
+  _("POST to a resource (object)");
+  let res_upload = new Resource(server.baseURI + "/upload");
+  let content = await res_upload.post(sample_data);
+  Assert.equal(content.data, "Valid data upload via POST");
+  Assert.equal(content.status, 200);
+});
+
+add_task(async function test_delete() {
+  _("DELETE a resource");
+  let res6 = new Resource(server.baseURI + "/delete");
+  let content = await res6.delete();
+  Assert.equal(content.data, "This resource has been deleted");
+  Assert.equal(content.status, 200);
+});
+
+add_task(async function test_json_body() {
+  _("JSON conversion of response body");
+  let res7 = new Resource(server.baseURI + "/json");
+  let content = await res7.get();
+  Assert.equal(content.data, JSON.stringify(sample_data));
+  Assert.equal(content.status, 200);
+  Assert.equal(JSON.stringify(content.obj), JSON.stringify(sample_data));
+});
+
+add_task(async function test_weave_timestamp() {
+  _("X-Weave-Timestamp header updates Resource.serverTime");
+  // Before having received any response containing the
+  // X-Weave-Timestamp header, Resource.serverTime is null.
+  Assert.equal(Resource.serverTime, null);
+  let res8 = new Resource(server.baseURI + "/timestamp");
+  await res8.get();
+  Assert.equal(Resource.serverTime, TIMESTAMP);
+});
+
+add_task(async function test_get_default_headers() {
+  _("GET: Accept defaults to application/json");
+  let res_headers = new Resource(server.baseURI + "/headers");
+  let content = JSON.parse((await res_headers.get()).data);
+  Assert.equal(content.accept, "application/json;q=0.9,*/*;q=0.2");
+});
+
+add_task(async function test_put_default_headers() {
+  _(
+    "PUT: Accept defaults to application/json, Content-Type defaults to text/plain"
+  );
+  let res_headers = new Resource(server.baseURI + "/headers");
+  let content = JSON.parse((await res_headers.put("data")).data);
+  Assert.equal(content.accept, "application/json;q=0.9,*/*;q=0.2");
+  Assert.equal(content["content-type"], "text/plain");
+});
+
+add_task(async function test_post_default_headers() {
+  _(
+    "POST: Accept defaults to application/json, Content-Type defaults to text/plain"
+  );
+  let res_headers = new Resource(server.baseURI + "/headers");
+  let content = JSON.parse((await res_headers.post("data")).data);
+  Assert.equal(content.accept, "application/json;q=0.9,*/*;q=0.2");
+  Assert.equal(content["content-type"], "text/plain");
+});
+
+add_task(async function test_setHeader() {
+  _("setHeader(): setting simple header");
+  let res_headers = new Resource(server.baseURI + "/headers");
+  res_headers.setHeader("X-What-Is-Weave", "awesome");
+  Assert.equal(res_headers.headers["x-what-is-weave"], "awesome");
+  let content = JSON.parse((await res_headers.get()).data);
+  Assert.equal(content["x-what-is-weave"], "awesome");
+});
+
+add_task(async function test_setHeader_overwrite() {
+  _("setHeader(): setting multiple headers, overwriting existing header");
+  let res_headers = new Resource(server.baseURI + "/headers");
+  res_headers.setHeader("X-WHAT-is-Weave", "more awesomer");
+  res_headers.setHeader("X-Another-Header", "hello world");
+  Assert.equal(res_headers.headers["x-what-is-weave"], "more awesomer");
+  Assert.equal(res_headers.headers["x-another-header"], "hello world");
+  let content = JSON.parse((await res_headers.get()).data);
+  Assert.equal(content["x-what-is-weave"], "more awesomer");
+  Assert.equal(content["x-another-header"], "hello world");
+});
+
+add_task(async function test_put_override_content_type() {
+  _("PUT: override default Content-Type");
+  let res_headers = new Resource(server.baseURI + "/headers");
+  res_headers.setHeader("Content-Type", "application/foobar");
+  Assert.equal(res_headers.headers["content-type"], "application/foobar");
+  let content = JSON.parse((await res_headers.put("data")).data);
+  Assert.equal(content["content-type"], "application/foobar");
+});
+
+add_task(async function test_post_override_content_type() {
+  _("POST: override default Content-Type");
+  let res_headers = new Resource(server.baseURI + "/headers");
+  res_headers.setHeader("Content-Type", "application/foobar");
+  let content = JSON.parse((await res_headers.post("data")).data);
+  Assert.equal(content["content-type"], "application/foobar");
+});
+
+add_task(async function test_weave_backoff() {
+  _("X-Weave-Backoff header notifies observer");
+  let backoffInterval;
+  function onBackoff(subject, data) {
+    backoffInterval = subject;
+  }
+  Observers.add("weave:service:backoff:interval", onBackoff);
+
+  let res10 = new Resource(server.baseURI + "/backoff");
+  await res10.get();
+  Assert.equal(backoffInterval, 600);
+});
+
+add_task(async function test_quota_error() {
+  _("X-Weave-Quota-Remaining header notifies observer on successful requests.");
+  let res10 = new Resource(server.baseURI + "/quota-error");
+  let content = await res10.get();
+  Assert.equal(content.status, 400);
+  Assert.equal(quotaValue, undefined); // HTTP 400, so no observer notification.
+});
+
+add_task(async function test_quota_notice() {
+  let res10 = new Resource(server.baseURI + "/quota-notice");
+  let content = await res10.get();
+  Assert.equal(content.status, 200);
+  Assert.equal(quotaValue, 1048576);
+});
+
+add_task(async function test_preserve_exceptions() {
+  _("Error handling preserves exception information");
+  let res11 = new Resource("http://localhost:12345/does/not/exist");
+  await Assert.rejects(res11.get(), error => {
+    Assert.notEqual(error, null);
+    Assert.equal(error.result, Cr.NS_ERROR_CONNECTION_REFUSED);
+    Assert.equal(error.name, "NS_ERROR_CONNECTION_REFUSED");
+    return true;
+  });
+});
+
+add_task(async function test_timeout() {
+  _("Ensure channel timeouts are thrown appropriately.");
+  let res19 = new Resource(server.baseURI + "/json");
+  res19.ABORT_TIMEOUT = 0;
+  await Assert.rejects(res19.get(), error => {
+    Assert.equal(error.result, Cr.NS_ERROR_NET_TIMEOUT);
+    return true;
+  });
+});
+
+add_test(function test_uri_construction() {
+  _("Testing URI construction.");
+  let args = [];
+  args.push("newer=" + 1234);
+  args.push("limit=" + 1234);
+  args.push("sort=" + 1234);
+
+  let query = "?" + args.join("&");
+
+  let uri1 = CommonUtils.makeURI("http://foo/" + query).QueryInterface(
+    Ci.nsIURL
+  );
+  let uri2 = CommonUtils.makeURI("http://foo/").QueryInterface(Ci.nsIURL);
+  uri2 = uri2.mutate().setQuery(query).finalize().QueryInterface(Ci.nsIURL);
+  Assert.equal(uri1.query, uri2.query);
+
+  run_next_test();
+});
+
+/**
+ * End of tests that rely on a single HTTP server.
+ * All tests after this point must begin and end their own.
+ */
+add_test(function eliminate_server() {
+  server.stop(run_next_test);
+});
diff --git a/services/sync/tests/unit/test_resource_header.js b/services/sync/tests/unit/test_resource_header.js
new file mode 100644
index 0000000000..e45b4a9864
--- /dev/null
+++ b/services/sync/tests/unit/test_resource_header.js
@@ -0,0 +1,63 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { Resource } = ChromeUtils.importESModule(
+  "resource://services-sync/resource.sys.mjs"
+);
+
+var httpServer = new HttpServer();
+httpServer.registerPathHandler("/content", contentHandler);
+httpServer.start(-1);
+
+const HTTP_PORT = httpServer.identity.primaryPort;
+const TEST_URL = "http://localhost:" + HTTP_PORT + "/content";
+const BODY = "response body";
+
+// Keep headers for later inspection.
+var auth = null;
+var foo = null;
+function contentHandler(metadata, response) {
+  _("Handling request.");
+  auth = metadata.getHeader("Authorization");
+  foo = metadata.getHeader("X-Foo");
+
+  _("Extracted headers. " + auth + ", " + foo);
+
+  response.setHeader("Content-Type", "text/plain");
+  response.bodyOutputStream.write(BODY, BODY.length);
+}
+
+// Set a proxy function to cause an internal redirect.
+function triggerRedirect() {
+  const PROXY_FUNCTION =
+    "function FindProxyForURL(url, host) {" +
+    "  return 'PROXY a_non_existent_domain_x7x6c572v:80; " +
+    "PROXY localhost:" +
+    HTTP_PORT +
+    "';" +
+    "}";
+
+  let prefs = Services.prefs.getBranch("network.proxy.");
+  prefs.setIntPref("type", 2);
+  prefs.setStringPref("autoconfig_url", "data:text/plain," + PROXY_FUNCTION);
+}
+
+add_task(async function test_headers_copied() {
+  triggerRedirect();
+
+  _("Issuing request.");
+  let resource = new Resource(TEST_URL);
+  resource.setHeader("Authorization", "Basic foobar");
+  resource.setHeader("X-Foo", "foofoo");
+
+  let result = await resource.get(TEST_URL);
+  _("Result: " + result.data);
+
+  Assert.equal(result.data, BODY);
+  Assert.equal(auth, "Basic foobar");
+  Assert.equal(foo, "foofoo");
+
+  await promiseStopServer(httpServer);
+});
diff --git a/services/sync/tests/unit/test_resource_ua.js b/services/sync/tests/unit/test_resource_ua.js
new file mode 100644
index 0000000000..115fa85b84
--- /dev/null
+++ b/services/sync/tests/unit/test_resource_ua.js
@@ -0,0 +1,96 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Resource } = ChromeUtils.importESModule(
+  "resource://services-sync/resource.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+var httpProtocolHandler = Cc[
+  "@mozilla.org/network/protocol;1?name=http"
+].getService(Ci.nsIHttpProtocolHandler);
+
+// Tracking info/collections.
+var collectionsHelper = track_collections_helper();
+
+var meta_global;
+var server;
+
+var expectedUA;
+var ua;
+function uaHandler(f) {
+  return function (request, response) {
+    ua = request.getHeader("User-Agent");
+    return f(request, response);
+  };
+}
+
+add_task(async function setup() {
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+  meta_global = new ServerWBO("global");
+  server = httpd_setup({
+    "/1.1/johndoe/info/collections": uaHandler(collectionsHelper.handler),
+    "/1.1/johndoe/storage/meta/global": uaHandler(meta_global.handler()),
+  });
+
+  await configureIdentity({ username: "johndoe" }, server);
+  _("Server URL: " + server.baseURI);
+
+  // Note this string is missing the trailing ".destkop" as the test
+  // adjusts the "client.type" pref where that portion comes from.
+  expectedUA =
+    Services.appinfo.name +
+    "/" +
+    Services.appinfo.version +
+    " (" +
+    httpProtocolHandler.oscpu +
+    ")" +
+    " FxSync/" +
+    WEAVE_VERSION +
+    "." +
+    Services.appinfo.appBuildID;
+});
+
+add_task(async function test_fetchInfo() {
+  _("Testing _fetchInfo.");
+  await Service.login();
+  await Service._fetchInfo();
+  _("User-Agent: " + ua);
+  Assert.equal(ua, expectedUA + ".desktop");
+  ua = "";
+});
+
+add_task(async function test_desktop_post() {
+  _("Testing direct Resource POST.");
+  let r = new Resource(server.baseURI + "/1.1/johndoe/storage/meta/global");
+  await r.post("foo=bar");
+  _("User-Agent: " + ua);
+  Assert.equal(ua, expectedUA + ".desktop");
+  ua = "";
+});
+
+add_task(async function test_desktop_get() {
+  _("Testing async.");
+  Svc.PrefBranch.setStringPref("client.type", "desktop");
+  let r = new Resource(server.baseURI + "/1.1/johndoe/storage/meta/global");
+  await r.get();
+  _("User-Agent: " + ua);
+  Assert.equal(ua, expectedUA + ".desktop");
+  ua = "";
+});
+
+add_task(async function test_mobile_get() {
+  _("Testing mobile.");
+  Svc.PrefBranch.setStringPref("client.type", "mobile");
+  let r = new Resource(server.baseURI + "/1.1/johndoe/storage/meta/global");
+  await r.get();
+  _("User-Agent: " + ua);
+  Assert.equal(ua, expectedUA + ".mobile");
+  ua = "";
+});
+
+add_test(function tear_down() {
+  server.stop(run_next_test);
+});
diff --git a/services/sync/tests/unit/test_score_triggers.js b/services/sync/tests/unit/test_score_triggers.js
new file mode 100644
index 0000000000..c6afa06407
--- /dev/null
+++ b/services/sync/tests/unit/test_score_triggers.js
@@ -0,0 +1,151 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+
+// Tracking info/collections.
+var collectionsHelper = track_collections_helper();
+var upd = collectionsHelper.with_updated_collection;
+
+function sync_httpd_setup() {
+  let handlers = {};
+
+  handlers["/1.1/johndoe/storage/meta/global"] = new ServerWBO(
+    "global",
+    {}
+  ).handler();
+  handlers["/1.1/johndoe/storage/steam"] = new ServerWBO("steam", {}).handler();
+
+  handlers["/1.1/johndoe/info/collections"] = collectionsHelper.handler;
+  delete collectionsHelper.collections.crypto;
+  delete collectionsHelper.collections.meta;
+
+  let cr = new ServerWBO("keys");
+  handlers["/1.1/johndoe/storage/crypto/keys"] = upd("crypto", cr.handler());
+
+  let cl = new ServerCollection();
+  handlers["/1.1/johndoe/storage/clients"] = upd("clients", cl.handler());
+
+  return httpd_setup(handlers);
+}
+
+async function setUp(server) {
+  let engineInfo = await registerRotaryEngine();
+  await SyncTestingInfrastructure(server, "johndoe", "ilovejane");
+  return engineInfo;
+}
+
+add_task(async function test_tracker_score_updated() {
+  enableValidationPrefs();
+  let { engine, tracker } = await registerRotaryEngine();
+
+  let scoreUpdated = 0;
+
+  function onScoreUpdated() {
+    scoreUpdated++;
+  }
+
+  Svc.Obs.add("weave:engine:score:updated", onScoreUpdated);
+
+  try {
+    Assert.equal(engine.score, 0);
+
+    tracker.score += SCORE_INCREMENT_SMALL;
+    Assert.equal(engine.score, SCORE_INCREMENT_SMALL);
+
+    Assert.equal(scoreUpdated, 1);
+  } finally {
+    Svc.Obs.remove("weave:engine:score:updated", onScoreUpdated);
+    tracker.resetScore();
+    await tracker.clearChangedIDs();
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_sync_triggered() {
+  let server = sync_httpd_setup();
+  let { engine, tracker } = await setUp(server);
+
+  await Service.login();
+
+  Service.scheduler.syncThreshold = MULTI_DEVICE_THRESHOLD;
+
+  Assert.equal(Status.login, LOGIN_SUCCEEDED);
+  tracker.score += SCORE_INCREMENT_XLARGE;
+
+  await promiseOneObserver("weave:service:sync:finish");
+
+  await Service.startOver();
+  await promiseStopServer(server);
+
+  await tracker.clearChangedIDs();
+  await Service.engineManager.unregister(engine);
+});
+
+add_task(async function test_clients_engine_sync_triggered() {
+  enableValidationPrefs();
+
+  _("Ensure that client engine score changes trigger a sync.");
+
+  // The clients engine is not registered like other engines. Therefore,
+  // it needs special treatment throughout the code. Here, we verify the
+  // global score tracker gives it that treatment. See bug 676042 for more.
+
+  let server = sync_httpd_setup();
+  let { engine, tracker } = await setUp(server);
+  await Service.login();
+
+  Service.scheduler.syncThreshold = MULTI_DEVICE_THRESHOLD;
+  Assert.equal(Status.login, LOGIN_SUCCEEDED);
+  Service.clientsEngine._tracker.score += SCORE_INCREMENT_XLARGE;
+
+  await promiseOneObserver("weave:service:sync:finish");
+  _("Sync due to clients engine change completed.");
+
+  await Service.startOver();
+  await promiseStopServer(server);
+
+  await tracker.clearChangedIDs();
+  await Service.engineManager.unregister(engine);
+});
+
+add_task(async function test_incorrect_credentials_sync_not_triggered() {
+  enableValidationPrefs();
+
+  _(
+    "Ensure that score changes don't trigger a sync if Status.login != LOGIN_SUCCEEDED."
+  );
+  let server = sync_httpd_setup();
+  let { engine, tracker } = await setUp(server);
+
+  // Ensure we don't actually try to sync.
+  function onSyncStart() {
+    do_throw("Should not get here!");
+  }
+  Svc.Obs.add("weave:service:sync:start", onSyncStart);
+
+  // Faking incorrect credentials to prevent score update.
+  Status.login = LOGIN_FAILED_LOGIN_REJECTED;
+  tracker.score += SCORE_INCREMENT_XLARGE;
+
+  // First wait >100ms (nsITimers can take up to that much time to fire, so
+  // we can account for the timer in delayedAutoconnect) and then one event
+  // loop tick (to account for a possible call to weave:service:sync:start).
+  await promiseNamedTimer(150, {}, "timer");
+  await Async.promiseYield();
+
+  Svc.Obs.remove("weave:service:sync:start", onSyncStart);
+
+  Assert.equal(Status.login, LOGIN_FAILED_LOGIN_REJECTED);
+
+  await Service.startOver();
+  await promiseStopServer(server);
+
+  await tracker.clearChangedIDs();
+  await Service.engineManager.unregister(engine);
+});
diff --git a/services/sync/tests/unit/test_service_attributes.js b/services/sync/tests/unit/test_service_attributes.js
new file mode 100644
index 0000000000..86ca3c30c0
--- /dev/null
+++ b/services/sync/tests/unit/test_service_attributes.js
@@ -0,0 +1,92 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { FakeGUIDService } = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/fakeservices.sys.mjs"
+);
+
+add_task(async function test_urls() {
+  _("URL related Service properties correspond to preference settings.");
+  try {
+    Assert.equal(Service.clusterURL, "");
+    Assert.ok(!Service.userBaseURL);
+    Assert.equal(Service.infoURL, undefined);
+    Assert.equal(Service.storageURL, undefined);
+    Assert.equal(Service.metaURL, undefined);
+
+    _("The 'clusterURL' attribute updates preferences and cached URLs.");
+
+    // Since we don't have a cluster URL yet, these will still not be defined.
+    Assert.equal(Service.infoURL, undefined);
+    Assert.ok(!Service.userBaseURL);
+    Assert.equal(Service.storageURL, undefined);
+    Assert.equal(Service.metaURL, undefined);
+
+    Service.clusterURL = "http://weave.cluster/1.1/johndoe/";
+
+    Assert.equal(Service.userBaseURL, "http://weave.cluster/1.1/johndoe/");
+    Assert.equal(
+      Service.infoURL,
+      "http://weave.cluster/1.1/johndoe/info/collections"
+    );
+    Assert.equal(
+      Service.storageURL,
+      "http://weave.cluster/1.1/johndoe/storage/"
+    );
+    Assert.equal(
+      Service.metaURL,
+      "http://weave.cluster/1.1/johndoe/storage/meta/global"
+    );
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_test(function test_syncID() {
+  _("Service.syncID is auto-generated, corresponds to preference.");
+  new FakeGUIDService();
+
+  try {
+    // Ensure pristine environment
+    Assert.equal(
+      Svc.PrefBranch.getPrefType("client.syncID"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+
+    // Performing the first get on the attribute will generate a new GUID.
+    Assert.equal(Service.syncID, "fake-guid-00");
+    Assert.equal(Svc.PrefBranch.getStringPref("client.syncID"), "fake-guid-00");
+
+    Svc.PrefBranch.setStringPref("client.syncID", Utils.makeGUID());
+    Assert.equal(Svc.PrefBranch.getStringPref("client.syncID"), "fake-guid-01");
+    Assert.equal(Service.syncID, "fake-guid-01");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    new FakeGUIDService();
+    run_next_test();
+  }
+});
+
+add_test(function test_locked() {
+  _("The 'locked' attribute can be toggled with lock() and unlock()");
+
+  // Defaults to false
+  Assert.equal(Service.locked, false);
+
+  Assert.equal(Service.lock(), true);
+  Assert.equal(Service.locked, true);
+
+  // Locking again will return false
+  Assert.equal(Service.lock(), false);
+
+  Service.unlock();
+  Assert.equal(Service.locked, false);
+  run_next_test();
+});
diff --git a/services/sync/tests/unit/test_service_cluster.js b/services/sync/tests/unit/test_service_cluster.js
new file mode 100644
index 0000000000..b4c14f910d
--- /dev/null
+++ b/services/sync/tests/unit/test_service_cluster.js
@@ -0,0 +1,61 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function test_findCluster() {
+  syncTestLogging();
+  _("Test Service._findCluster()");
+  try {
+    let whenReadyToAuthenticate = Promise.withResolvers();
+    Service.identity.whenReadyToAuthenticate = whenReadyToAuthenticate;
+    whenReadyToAuthenticate.resolve(true);
+
+    Service.identity._ensureValidToken = () =>
+      Promise.reject(new Error("Connection refused"));
+
+    _("_findCluster() throws on network errors (e.g. connection refused).");
+    await Assert.rejects(Service.identity._findCluster(), /Connection refused/);
+
+    Service.identity._ensureValidToken = () =>
+      Promise.resolve({ endpoint: "http://weave.user.node" });
+
+    _("_findCluster() returns the user's cluster node");
+    let cluster = await Service.identity._findCluster();
+    Assert.equal(cluster, "http://weave.user.node/");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_setCluster() {
+  syncTestLogging();
+  _("Test Service._setCluster()");
+  try {
+    _("Check initial state.");
+    Assert.equal(Service.clusterURL, "");
+
+    Service.identity._findCluster = () => "http://weave.user.node/";
+
+    _("Set the cluster URL.");
+    Assert.ok(await Service.identity.setCluster());
+    Assert.equal(Service.clusterURL, "http://weave.user.node/");
+
+    _("Setting it again won't make a difference if it's the same one.");
+    Assert.ok(!(await Service.identity.setCluster()));
+    Assert.equal(Service.clusterURL, "http://weave.user.node/");
+
+    _("A 'null' response won't make a difference either.");
+    Service.identity._findCluster = () => null;
+    Assert.ok(!(await Service.identity.setCluster()));
+    Assert.equal(Service.clusterURL, "http://weave.user.node/");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
diff --git a/services/sync/tests/unit/test_service_detect_upgrade.js b/services/sync/tests/unit/test_service_detect_upgrade.js
new file mode 100644
index 0000000000..d0db19af93
--- /dev/null
+++ b/services/sync/tests/unit/test_service_detect_upgrade.js
@@ -0,0 +1,274 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { CryptoWrapper, WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function v4_upgrade() {
+  enableValidationPrefs();
+
+  let clients = new ServerCollection();
+  let meta_global = new ServerWBO("global");
+
+  // Tracking info/collections.
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+  let collections = collectionsHelper.collections;
+
+  let keysWBO = new ServerWBO("keys");
+  let server = httpd_setup({
+    // Special.
+    "/1.1/johndoe/info/collections": collectionsHelper.handler,
+    "/1.1/johndoe/storage/crypto/keys": upd("crypto", keysWBO.handler()),
+    "/1.1/johndoe/storage/meta/global": upd("meta", meta_global.handler()),
+
+    // Track modified times.
+    "/1.1/johndoe/storage/clients": upd("clients", clients.handler()),
+    "/1.1/johndoe/storage/tabs": upd("tabs", new ServerCollection().handler()),
+
+    // Just so we don't get 404s in the logs.
+    "/1.1/johndoe/storage/bookmarks": new ServerCollection().handler(),
+    "/1.1/johndoe/storage/forms": new ServerCollection().handler(),
+    "/1.1/johndoe/storage/history": new ServerCollection().handler(),
+    "/1.1/johndoe/storage/passwords": new ServerCollection().handler(),
+    "/1.1/johndoe/storage/prefs": new ServerCollection().handler(),
+  });
+
+  try {
+    Service.status.resetSync();
+
+    _("Logging in.");
+
+    await configureIdentity({ username: "johndoe" }, server);
+
+    await Service.login();
+    Assert.ok(Service.isLoggedIn);
+    await Service.verifyAndFetchSymmetricKeys();
+    Assert.ok(await Service._remoteSetup());
+
+    async function test_out_of_date() {
+      _("Old meta/global: " + JSON.stringify(meta_global));
+      meta_global.payload = JSON.stringify({
+        syncID: "foooooooooooooooooooooooooo",
+        storageVersion: STORAGE_VERSION + 1,
+      });
+      collections.meta = Date.now() / 1000;
+      _("New meta/global: " + JSON.stringify(meta_global));
+      Service.recordManager.set(Service.metaURL, meta_global);
+      try {
+        await Service.sync();
+      } catch (ex) {}
+      Assert.equal(Service.status.sync, VERSION_OUT_OF_DATE);
+    }
+
+    // See what happens when we bump the storage version.
+    _("Syncing after server has been upgraded.");
+    await test_out_of_date();
+
+    // Same should happen after a wipe.
+    _("Syncing after server has been upgraded and wiped.");
+    await Service.wipeServer();
+    await test_out_of_date();
+
+    // Now's a great time to test what happens when keys get replaced.
+    _("Syncing afresh...");
+    Service.logout();
+    Service.collectionKeys.clear();
+    meta_global.payload = JSON.stringify({
+      syncID: "foooooooooooooobbbbbbbbbbbb",
+      storageVersion: STORAGE_VERSION,
+    });
+    collections.meta = Date.now() / 1000;
+    Service.recordManager.set(Service.metaURL, meta_global);
+    await Service.login();
+    Assert.ok(Service.isLoggedIn);
+    await Service.sync();
+    Assert.ok(Service.isLoggedIn);
+
+    let serverDecrypted;
+    let serverKeys;
+    let serverResp;
+
+    async function retrieve_server_default() {
+      serverKeys = serverResp = serverDecrypted = null;
+
+      serverKeys = new CryptoWrapper("crypto", "keys");
+      serverResp = (
+        await serverKeys.fetch(Service.resource(Service.cryptoKeysURL))
+      ).response;
+      Assert.ok(serverResp.success);
+
+      serverDecrypted = await serverKeys.decrypt(
+        Service.identity.syncKeyBundle
+      );
+      _("Retrieved WBO:       " + JSON.stringify(serverDecrypted));
+      _("serverKeys:          " + JSON.stringify(serverKeys));
+
+      return serverDecrypted.default;
+    }
+
+    async function retrieve_and_compare_default(should_succeed) {
+      let serverDefault = await retrieve_server_default();
+      let localDefault = Service.collectionKeys.keyForCollection().keyPairB64;
+
+      _("Retrieved keyBundle: " + JSON.stringify(serverDefault));
+      _("Local keyBundle:     " + JSON.stringify(localDefault));
+
+      if (should_succeed) {
+        Assert.equal(
+          JSON.stringify(serverDefault),
+          JSON.stringify(localDefault)
+        );
+      } else {
+        Assert.notEqual(
+          JSON.stringify(serverDefault),
+          JSON.stringify(localDefault)
+        );
+      }
+    }
+
+    // Uses the objects set above.
+    async function set_server_keys(pair) {
+      serverDecrypted.default = pair;
+      serverKeys.cleartext = serverDecrypted;
+      await serverKeys.encrypt(Service.identity.syncKeyBundle);
+      await serverKeys.upload(Service.resource(Service.cryptoKeysURL));
+    }
+
+    _("Checking we have the latest keys.");
+    await retrieve_and_compare_default(true);
+
+    _("Update keys on server.");
+    await set_server_keys([
+      "KaaaaaaaaaaaHAtfmuRY0XEJ7LXfFuqvF7opFdBD/MY=",
+      "aaaaaaaaaaaapxMO6TEWtLIOv9dj6kBAJdzhWDkkkis=",
+    ]);
+
+    _("Checking that we no longer have the latest keys.");
+    await retrieve_and_compare_default(false);
+
+    _("Indeed, they're what we set them to...");
+    Assert.equal(
+      "KaaaaaaaaaaaHAtfmuRY0XEJ7LXfFuqvF7opFdBD/MY=",
+      (await retrieve_server_default())[0]
+    );
+
+    _("Sync. Should download changed keys automatically.");
+    let oldClientsModified = collections.clients;
+    let oldTabsModified = collections.tabs;
+
+    await Service.login();
+    await Service.sync();
+    _("New key should have forced upload of data.");
+    _("Tabs: " + oldTabsModified + " < " + collections.tabs);
+    _("Clients: " + oldClientsModified + " < " + collections.clients);
+    Assert.ok(collections.clients > oldClientsModified);
+    Assert.ok(collections.tabs > oldTabsModified);
+
+    _("... and keys will now match.");
+    await retrieve_and_compare_default(true);
+
+    // Clean up.
+    await Service.startOver();
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function v5_upgrade() {
+  enableValidationPrefs();
+
+  // Tracking info/collections.
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+
+  let keysWBO = new ServerWBO("keys");
+  let bulkWBO = new ServerWBO("bulk");
+  let clients = new ServerCollection();
+  let meta_global = new ServerWBO("global");
+
+  let server = httpd_setup({
+    // Special.
+    "/1.1/johndoe/storage/meta/global": upd("meta", meta_global.handler()),
+    "/1.1/johndoe/info/collections": collectionsHelper.handler,
+    "/1.1/johndoe/storage/crypto/keys": upd("crypto", keysWBO.handler()),
+    "/1.1/johndoe/storage/crypto/bulk": upd("crypto", bulkWBO.handler()),
+
+    // Track modified times.
+    "/1.1/johndoe/storage/clients": upd("clients", clients.handler()),
+    "/1.1/johndoe/storage/tabs": upd("tabs", new ServerCollection().handler()),
+  });
+
+  try {
+    Service.status.resetSync();
+
+    Service.clusterURL = server.baseURI + "/";
+
+    await configureIdentity({ username: "johndoe" }, server);
+
+    // Test an upgrade where the contents of the server would cause us to error
+    // -- keys decrypted with a different sync key, for example.
+    _("Testing v4 -> v5 (or similar) upgrade.");
+    async function update_server_keys(syncKeyBundle, wboName, collWBO) {
+      await generateNewKeys(Service.collectionKeys);
+      let serverKeys = Service.collectionKeys.asWBO("crypto", wboName);
+      await serverKeys.encrypt(syncKeyBundle);
+      let res = Service.resource(Service.storageURL + collWBO);
+      Assert.ok((await serverKeys.upload(res)).success);
+    }
+
+    _("Bumping version.");
+    // Bump version on the server.
+    let m = new WBORecord("meta", "global");
+    m.payload = {
+      syncID: "foooooooooooooooooooooooooo",
+      storageVersion: STORAGE_VERSION + 1,
+    };
+    await m.upload(Service.resource(Service.metaURL));
+
+    _("New meta/global: " + JSON.stringify(meta_global));
+
+    // Fill the keys with bad data.
+    let badKeys = new BulkKeyBundle("crypto");
+    await badKeys.generateRandom();
+    await update_server_keys(badKeys, "keys", "crypto/keys"); // v4
+    await update_server_keys(badKeys, "bulk", "crypto/bulk"); // v5
+
+    _("Generating new keys.");
+    await generateNewKeys(Service.collectionKeys);
+
+    // Now sync and see what happens. It should be a version fail, not a crypto
+    // fail.
+
+    _("Logging in.");
+    try {
+      await Service.login();
+    } catch (e) {
+      _("Exception: " + e);
+    }
+    _("Status: " + Service.status);
+    Assert.ok(!Service.isLoggedIn);
+    Assert.equal(VERSION_OUT_OF_DATE, Service.status.sync);
+
+    // Clean up.
+    await Service.startOver();
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
+
+function run_test() {
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  run_next_test();
+}
diff --git a/services/sync/tests/unit/test_service_login.js b/services/sync/tests/unit/test_service_login.js
new file mode 100644
index 0000000000..c75799d38e
--- /dev/null
+++ b/services/sync/tests/unit/test_service_login.js
@@ -0,0 +1,224 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+function login_handling(handler) {
+  return function (request, response) {
+    if (has_hawk_header(request)) {
+      handler(request, response);
+    } else {
+      let body = "Unauthorized";
+      response.setStatusLine(request.httpVersion, 401, "Unauthorized");
+      response.setHeader("Content-Type", "text/plain");
+      response.bodyOutputStream.write(body, body.length);
+    }
+  };
+}
+
+add_task(async function test_offline() {
+  try {
+    _("The right bits are set when we're offline.");
+    Services.io.offline = true;
+    Assert.ok(!(await Service.login()));
+    Assert.equal(Service.status.login, LOGIN_FAILED_NETWORK_ERROR);
+    Services.io.offline = false;
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+function setup() {
+  let janeHelper = track_collections_helper();
+  let janeU = janeHelper.with_updated_collection;
+  let johnHelper = track_collections_helper();
+  let johnU = johnHelper.with_updated_collection;
+
+  let server = httpd_setup({
+    "/1.1/johndoe/info/collections": login_handling(johnHelper.handler),
+    "/1.1/janedoe/info/collections": login_handling(janeHelper.handler),
+
+    // We need these handlers because we test login, and login
+    // is where keys are generated or fetched.
+    // TODO: have Jane fetch her keys, not generate them...
+    "/1.1/johndoe/storage/crypto/keys": johnU(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe/storage/meta/global": johnU(
+      "meta",
+      new ServerWBO("global").handler()
+    ),
+    "/1.1/janedoe/storage/crypto/keys": janeU(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/janedoe/storage/meta/global": janeU(
+      "meta",
+      new ServerWBO("global").handler()
+    ),
+  });
+
+  return server;
+}
+
+add_task(async function test_not_logged_in() {
+  let server = setup();
+  try {
+    await Service.login();
+    Assert.ok(!Service.isLoggedIn, "no user configured, so can't be logged in");
+    Assert.equal(Service._checkSync(), kSyncNotConfigured);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_login_logout() {
+  enableValidationPrefs();
+
+  let server = setup();
+
+  try {
+    _("Force the initial state.");
+    Service.status.service = STATUS_OK;
+    Assert.equal(Service.status.service, STATUS_OK);
+
+    _("Try logging in. It won't work because we're not configured yet.");
+    await Service.login();
+    Assert.equal(Service.status.service, CLIENT_NOT_CONFIGURED);
+    Assert.equal(Service.status.login, LOGIN_FAILED_NO_USERNAME);
+    Assert.ok(!Service.isLoggedIn);
+
+    _("Try again with a configured account");
+    await configureIdentity({ username: "johndoe" }, server);
+    await Service.login();
+    Assert.equal(Service.status.service, STATUS_OK);
+    Assert.equal(Service.status.login, LOGIN_SUCCEEDED);
+    Assert.ok(Service.isLoggedIn);
+
+    _("Logout.");
+    Service.logout();
+    Assert.ok(!Service.isLoggedIn);
+
+    _("Logging out again won't do any harm.");
+    Service.logout();
+    Assert.ok(!Service.isLoggedIn);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_login_on_sync() {
+  enableValidationPrefs();
+
+  let server = setup();
+  await configureIdentity({ username: "johndoe" }, server);
+
+  try {
+    _("Sync calls login.");
+    let oldLogin = Service.login;
+    let loginCalled = false;
+    Service.login = async function () {
+      loginCalled = true;
+      Service.status.login = LOGIN_SUCCEEDED;
+      this._loggedIn = false; // So that sync aborts.
+      return true;
+    };
+
+    await Service.sync();
+
+    Assert.ok(loginCalled);
+    Service.login = oldLogin;
+
+    // Stub mpLocked.
+    let mpLocked = true;
+    Utils.mpLocked = () => mpLocked;
+
+    // Stub scheduleNextSync. This gets called within checkSyncStatus if we're
+    // ready to sync, so use it as an indicator.
+    let scheduleNextSyncF = Service.scheduler.scheduleNextSync;
+    let scheduleCalled = false;
+    Service.scheduler.scheduleNextSync = function (wait) {
+      scheduleCalled = true;
+      scheduleNextSyncF.call(this, wait);
+    };
+
+    // Autoconnect still tries to connect in the background (useful behavior:
+    // for non-MP users and unlocked MPs, this will detect version expiry
+    // earlier).
+    //
+    // Consequently, non-MP users will be logged in as in the pre-Bug 543784 world,
+    // and checkSyncStatus reflects that by waiting for login.
+    //
+    // This process doesn't apply if your MP is still locked, so we make
+    // checkSyncStatus accept a locked MP in place of being logged in.
+    //
+    // This test exercises these two branches.
+
+    _("We're ready to sync if locked.");
+    Service.enabled = true;
+    Services.io.offline = false;
+    Service.scheduler.checkSyncStatus();
+    Assert.ok(scheduleCalled);
+
+    _("... and also if we're not locked.");
+    scheduleCalled = false;
+    mpLocked = false;
+    Service.scheduler.checkSyncStatus();
+    Assert.ok(scheduleCalled);
+    Service.scheduler.scheduleNextSync = scheduleNextSyncF;
+
+    // TODO: need better tests around master password prompting. See Bug 620583.
+
+    mpLocked = true;
+
+    // Testing exception handling if master password dialog is canceled.
+    // Do this by monkeypatching.
+    Service.identity.unlockAndVerifyAuthState = () =>
+      Promise.resolve(MASTER_PASSWORD_LOCKED);
+
+    let cSTCalled = false;
+    let lockedSyncCalled = false;
+
+    Service.scheduler.clearSyncTriggers = function () {
+      cSTCalled = true;
+    };
+    Service._lockedSync = async function () {
+      lockedSyncCalled = true;
+    };
+
+    _("If master password is canceled, login fails and we report lockage.");
+    Assert.ok(!(await Service.login()));
+    Assert.equal(Service.status.login, MASTER_PASSWORD_LOCKED);
+    Assert.equal(Service.status.service, LOGIN_FAILED);
+    _("Locked? " + Utils.mpLocked());
+    _("checkSync reports the correct term.");
+    Assert.equal(Service._checkSync(), kSyncMasterPasswordLocked);
+
+    _("Sync doesn't proceed and clears triggers if MP is still locked.");
+    await Service.sync();
+
+    Assert.ok(cSTCalled);
+    Assert.ok(!lockedSyncCalled);
+
+    // N.B., a bunch of methods are stubbed at this point. Be careful putting
+    // new tests after this point!
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_service_startOver.js b/services/sync/tests/unit/test_service_startOver.js
new file mode 100644
index 0000000000..22d92c76ef
--- /dev/null
+++ b/services/sync/tests/unit/test_service_startOver.js
@@ -0,0 +1,91 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function BlaEngine() {
+  SyncEngine.call(this, "Bla", Service);
+}
+BlaEngine.prototype = {
+  removed: false,
+  async removeClientData() {
+    this.removed = true;
+  },
+};
+Object.setPrototypeOf(BlaEngine.prototype, SyncEngine.prototype);
+
+add_task(async function setup() {
+  await Service.engineManager.register(BlaEngine);
+});
+
+add_task(async function test_resetLocalData() {
+  await configureIdentity();
+  Service.status.enforceBackoff = true;
+  Service.status.backoffInterval = 42;
+  Service.status.minimumNextSync = 23;
+
+  // Verify set up.
+  Assert.equal(Service.status.checkSetup(), STATUS_OK);
+
+  // Verify state that the observer sees.
+  let observerCalled = false;
+  Svc.Obs.add("weave:service:start-over", function onStartOver() {
+    Svc.Obs.remove("weave:service:start-over", onStartOver);
+    observerCalled = true;
+
+    Assert.equal(Service.status.service, CLIENT_NOT_CONFIGURED);
+  });
+
+  await Service.startOver();
+  Assert.ok(observerCalled);
+
+  // Verify the site was nuked from orbit.
+  Assert.equal(
+    Svc.PrefBranch.getPrefType("username"),
+    Ci.nsIPrefBranch.PREF_INVALID
+  );
+
+  Assert.equal(Service.status.service, CLIENT_NOT_CONFIGURED);
+  Assert.ok(!Service.status.enforceBackoff);
+  Assert.equal(Service.status.backoffInterval, 0);
+  Assert.equal(Service.status.minimumNextSync, 0);
+});
+
+add_task(async function test_removeClientData() {
+  let engine = Service.engineManager.get("bla");
+
+  // No cluster URL = no removal.
+  Assert.ok(!engine.removed);
+  await Service.startOver();
+  Assert.ok(!engine.removed);
+
+  Service.clusterURL = "https://localhost/";
+
+  Assert.ok(!engine.removed);
+  await Service.startOver();
+  Assert.ok(engine.removed);
+});
+
+add_task(async function test_reset_SyncScheduler() {
+  // Some non-default values for SyncScheduler's attributes.
+  Service.scheduler.idle = true;
+  Service.scheduler.hasIncomingItems = true;
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 42);
+  Service.scheduler.nextSync = Date.now();
+  Service.scheduler.syncThreshold = MULTI_DEVICE_THRESHOLD;
+  Service.scheduler.syncInterval = Service.scheduler.activeInterval;
+
+  await Service.startOver();
+
+  Assert.ok(!Service.scheduler.idle);
+  Assert.ok(!Service.scheduler.hasIncomingItems);
+  Assert.equal(Service.scheduler.numClients, 0);
+  Assert.equal(Service.scheduler.nextSync, 0);
+  Assert.equal(Service.scheduler.syncThreshold, SINGLE_USER_THRESHOLD);
+  Assert.equal(
+    Service.scheduler.syncInterval,
+    Service.scheduler.singleDeviceInterval
+  );
+});
diff --git a/services/sync/tests/unit/test_service_startup.js b/services/sync/tests/unit/test_service_startup.js
new file mode 100644
index 0000000000..66623a951a
--- /dev/null
+++ b/services/sync/tests/unit/test_service_startup.js
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+// Svc.PrefBranch.setStringPref("services.sync.log.appender.dump", "All");
+Svc.PrefBranch.setStringPref("registerEngines", "Tab,Bookmarks,Form,History");
+
+add_task(async function run_test() {
+  validate_all_future_pings();
+  _("When imported, Service.onStartup is called");
+
+  let xps = Cc["@mozilla.org/weave/service;1"].getService(
+    Ci.nsISupports
+  ).wrappedJSObject;
+  Assert.ok(!xps.enabled);
+
+  // Test fixtures
+  let { Service } = ChromeUtils.importESModule(
+    "resource://services-sync/service.sys.mjs"
+  );
+  Services.prefs.setStringPref("services.sync.username", "johndoe");
+  Assert.ok(xps.enabled);
+
+  _("Service is enabled.");
+  Assert.equal(Service.enabled, true);
+
+  _("Observers are notified of startup");
+  Assert.ok(!Service.status.ready);
+  Assert.ok(!xps.ready);
+
+  await promiseOneObserver("weave:service:ready");
+
+  Assert.ok(Service.status.ready);
+  Assert.ok(xps.ready);
+
+  _("Engines are registered.");
+  let engines = Service.engineManager.getAll();
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    // Thunderbird's engines are registered later, so they're not here yet.
+    Assert.deepEqual(
+      engines.map(engine => engine.name),
+      []
+    );
+  } else {
+    Assert.deepEqual(
+      engines.map(engine => engine.name),
+      ["tabs", "bookmarks", "forms", "history"]
+    );
+  }
+
+  // Clean up.
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+
+  do_test_finished();
+});
diff --git a/services/sync/tests/unit/test_service_sync_401.js b/services/sync/tests/unit/test_service_sync_401.js
new file mode 100644
index 0000000000..a0bde0b0ab
--- /dev/null
+++ b/services/sync/tests/unit/test_service_sync_401.js
@@ -0,0 +1,90 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function login_handling(handler) {
+  return function (request, response) {
+    if (
+      request.hasHeader("Authorization") &&
+      request.getHeader("Authorization").includes('Hawk id="id"')
+    ) {
+      handler(request, response);
+    } else {
+      let body = "Unauthorized";
+      response.setStatusLine(request.httpVersion, 401, "Unauthorized");
+      response.bodyOutputStream.write(body, body.length);
+    }
+  };
+}
+
+add_task(async function run_test() {
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+
+  let server = httpd_setup({
+    "/1.1/johndoe/storage/crypto/keys": upd(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe/storage/meta/global": upd(
+      "meta",
+      new ServerWBO("global").handler()
+    ),
+    "/1.1/johndoe/info/collections": login_handling(collectionsHelper.handler),
+  });
+
+  const GLOBAL_SCORE = 42;
+
+  try {
+    _("Set up test fixtures.");
+    await SyncTestingInfrastructure(server, "johndoe", "ilovejane");
+    Service.scheduler.globalScore = GLOBAL_SCORE;
+    // Avoid daily ping
+    Svc.PrefBranch.setIntPref("lastPing", Math.floor(Date.now() / 1000));
+
+    let threw = false;
+    Svc.Obs.add("weave:service:sync:error", function (subject, data) {
+      threw = true;
+    });
+
+    _("Initial state: We're successfully logged in.");
+    await Service.login();
+    Assert.ok(Service.isLoggedIn);
+    Assert.equal(Service.status.login, LOGIN_SUCCEEDED);
+
+    _("Simulate having changed the password somewhere else.");
+    Service.identity._token.id = "somethingelse";
+    Service.identity.unlockAndVerifyAuthState = () =>
+      Promise.resolve(LOGIN_FAILED_LOGIN_REJECTED);
+
+    _("Let's try to sync.");
+    await Service.sync();
+
+    _("Verify that sync() threw an exception.");
+    Assert.ok(threw);
+
+    _("We're no longer logged in.");
+    Assert.ok(!Service.isLoggedIn);
+
+    _("Sync status won't have changed yet, because we haven't tried again.");
+
+    _("globalScore is reset upon starting a sync.");
+    Assert.equal(Service.scheduler.globalScore, 0);
+
+    _("Our next sync will fail appropriately.");
+    try {
+      await Service.sync();
+    } catch (ex) {}
+    Assert.equal(Service.status.login, LOGIN_FAILED_LOGIN_REJECTED);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_service_sync_locked.js b/services/sync/tests/unit/test_service_sync_locked.js
new file mode 100644
index 0000000000..5a872e2708
--- /dev/null
+++ b/services/sync/tests/unit/test_service_sync_locked.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function run_test() {
+  validate_all_future_pings();
+  let debug = [];
+  let info = [];
+
+  function augmentLogger(old) {
+    let d = old.debug;
+    let i = old.info;
+    // For the purposes of this test we don't need to do full formatting
+    // of the 2nd param, as the ones we care about are always strings.
+    old.debug = function (m, p) {
+      debug.push(p ? m + ": " + (p.message || p) : m);
+      d.call(old, m, p);
+    };
+    old.info = function (m, p) {
+      info.push(p ? m + ": " + (p.message || p) : m);
+      i.call(old, m, p);
+    };
+    return old;
+  }
+
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  augmentLogger(Service._log);
+
+  // Avoid daily ping
+  Svc.PrefBranch.setIntPref("lastPing", Math.floor(Date.now() / 1000));
+
+  _("Check that sync will log appropriately if already in 'progress'.");
+  Service._locked = true;
+  await Service.sync();
+  Service._locked = false;
+
+  Assert.ok(
+    debug[debug.length - 2].startsWith(
+      'Exception calling WrappedLock: Could not acquire lock. Label: "service.js: login".'
+    )
+  );
+  Assert.equal(info[info.length - 1], "Cannot start sync: already syncing?");
+});
diff --git a/services/sync/tests/unit/test_service_sync_remoteSetup.js b/services/sync/tests/unit/test_service_sync_remoteSetup.js
new file mode 100644
index 0000000000..ec95e69c78
--- /dev/null
+++ b/services/sync/tests/unit/test_service_sync_remoteSetup.js
@@ -0,0 +1,241 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+// This sucks, but this test fails if this engine is enabled, due to dumb
+// things that aren't related to this engine. In short:
+// * Because the addon manager isn't initialized, the addons engine fails to
+//   initialize. So we end up writing a meta/global with `extension-storage`
+//   but not addons.
+// * After we sync, we discover 'addons' is locally enabled, but because it's
+//   not in m/g, we decide it's been remotely declined (and it decides this
+//   without even considering `declined`). So we disable 'addons'.
+// * Disabling 'addons' means 'extension-storage' is disabled - but because
+//   that *is* in meta/global we re-update meta/global to remove it.
+// * This test fails due to the extra, unexpected update of m/g.
+//
+// Another option would be to ensure the addons manager is initialized, but
+// that's a larger patch and still isn't strictly relevant to what's being
+// tested here, so...
+Services.prefs.setBoolPref(
+  "services.sync.engine.extension-storage.force",
+  false
+);
+
+add_task(async function run_test() {
+  enableValidationPrefs();
+
+  validate_all_future_pings();
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  let clients = new ServerCollection();
+  let meta_global = new ServerWBO("global");
+
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+  let collections = collectionsHelper.collections;
+
+  function wasCalledHandler(wbo) {
+    let handler = wbo.handler();
+    return function () {
+      wbo.wasCalled = true;
+      handler.apply(this, arguments);
+    };
+  }
+
+  let keysWBO = new ServerWBO("keys");
+  let cryptoColl = new ServerCollection({ keys: keysWBO });
+  let metaColl = new ServerCollection({ global: meta_global });
+  do_test_pending();
+
+  /**
+   * Handle the bulk DELETE request sent by wipeServer.
+   */
+  function storageHandler(request, response) {
+    Assert.equal("DELETE", request.method);
+    Assert.ok(request.hasHeader("X-Confirm-Delete"));
+
+    _("Wiping out all collections.");
+    cryptoColl.delete({});
+    clients.delete({});
+    metaColl.delete({});
+
+    let ts = new_timestamp();
+    collectionsHelper.update_collection("crypto", ts);
+    collectionsHelper.update_collection("clients", ts);
+    collectionsHelper.update_collection("meta", ts);
+    return_timestamp(request, response, ts);
+  }
+
+  const GLOBAL_PATH = "/1.1/johndoe/storage/meta/global";
+
+  let handlers = {
+    "/1.1/johndoe/storage": storageHandler,
+    "/1.1/johndoe/storage/crypto/keys": upd("crypto", keysWBO.handler()),
+    "/1.1/johndoe/storage/crypto": upd("crypto", cryptoColl.handler()),
+    "/1.1/johndoe/storage/clients": upd("clients", clients.handler()),
+    "/1.1/johndoe/storage/meta": upd("meta", wasCalledHandler(metaColl)),
+    "/1.1/johndoe/storage/meta/global": upd(
+      "meta",
+      wasCalledHandler(meta_global)
+    ),
+    "/1.1/johndoe/info/collections": collectionsHelper.handler,
+  };
+
+  function mockHandler(path, mock) {
+    server.registerPathHandler(path, mock(handlers[path]));
+    return {
+      restore() {
+        server.registerPathHandler(path, handlers[path]);
+      },
+    };
+  }
+
+  let server = httpd_setup(handlers);
+
+  try {
+    _("Checking Status.sync with no credentials.");
+    await Service.verifyAndFetchSymmetricKeys();
+    Assert.equal(Service.status.sync, CREDENTIALS_CHANGED);
+    Assert.equal(Service.status.login, LOGIN_FAILED_NO_PASSPHRASE);
+
+    await configureIdentity({ username: "johndoe" }, server);
+
+    await Service.login();
+    _("Checking that remoteSetup returns true when credentials have changed.");
+    (await Service.recordManager.get(Service.metaURL)).payload.syncID =
+      "foobar";
+    Assert.ok(await Service._remoteSetup());
+
+    let returnStatusCode = (method, code) => oldMethod => (req, res) => {
+      if (req.method === method) {
+        res.setStatusLine(req.httpVersion, code, "");
+      } else {
+        oldMethod(req, res);
+      }
+    };
+
+    let mock = mockHandler(GLOBAL_PATH, returnStatusCode("GET", 401));
+    Service.recordManager.del(Service.metaURL);
+    _(
+      "Checking that remoteSetup returns false on 401 on first get /meta/global."
+    );
+    Assert.equal(false, await Service._remoteSetup());
+    mock.restore();
+
+    await Service.login();
+    mock = mockHandler(GLOBAL_PATH, returnStatusCode("GET", 503));
+    Service.recordManager.del(Service.metaURL);
+    _(
+      "Checking that remoteSetup returns false on 503 on first get /meta/global."
+    );
+    Assert.equal(false, await Service._remoteSetup());
+    Assert.equal(Service.status.sync, METARECORD_DOWNLOAD_FAIL);
+    mock.restore();
+
+    await Service.login();
+    mock = mockHandler(GLOBAL_PATH, returnStatusCode("GET", 404));
+    Service.recordManager.del(Service.metaURL);
+    _("Checking that remoteSetup recovers on 404 on first get /meta/global.");
+    Assert.ok(await Service._remoteSetup());
+    mock.restore();
+
+    let makeOutdatedMeta = async () => {
+      Service.metaModified = 0;
+      let infoResponse = await Service._fetchInfo();
+      return {
+        status: infoResponse.status,
+        obj: {
+          crypto: infoResponse.obj.crypto,
+          clients: infoResponse.obj.clients,
+          meta: 1,
+        },
+      };
+    };
+
+    _(
+      "Checking that remoteSetup recovers on 404 on get /meta/global after clear cached one."
+    );
+    mock = mockHandler(GLOBAL_PATH, returnStatusCode("GET", 404));
+    Service.recordManager.set(Service.metaURL, { isNew: false });
+    Assert.ok(await Service._remoteSetup(await makeOutdatedMeta()));
+    mock.restore();
+
+    _(
+      "Checking that remoteSetup returns false on 503 on get /meta/global after clear cached one."
+    );
+    mock = mockHandler(GLOBAL_PATH, returnStatusCode("GET", 503));
+    Service.status.sync = "";
+    Service.recordManager.set(Service.metaURL, { isNew: false });
+    Assert.equal(false, await Service._remoteSetup(await makeOutdatedMeta()));
+    Assert.equal(Service.status.sync, "");
+    mock.restore();
+
+    metaColl.delete({});
+
+    _("Do an initial sync.");
+    await Service.sync();
+
+    _("Checking that remoteSetup returns true.");
+    Assert.ok(await Service._remoteSetup());
+
+    _("Verify that the meta record was uploaded.");
+    Assert.equal(meta_global.data.syncID, Service.syncID);
+    Assert.equal(meta_global.data.storageVersion, STORAGE_VERSION);
+    Assert.equal(
+      meta_global.data.engines.clients.version,
+      Service.clientsEngine.version
+    );
+    Assert.equal(
+      meta_global.data.engines.clients.syncID,
+      await Service.clientsEngine.getSyncID()
+    );
+
+    _(
+      "Set the collection info hash so that sync() will remember the modified times for future runs."
+    );
+    let lastSync = await Service.clientsEngine.getLastSync();
+    collections.meta = lastSync;
+    collections.clients = lastSync;
+    await Service.sync();
+
+    _("Sync again and verify that meta/global wasn't downloaded again");
+    meta_global.wasCalled = false;
+    await Service.sync();
+    Assert.ok(!meta_global.wasCalled);
+
+    _(
+      "Fake modified records. This will cause a redownload, but not reupload since it hasn't changed."
+    );
+    collections.meta += 42;
+    meta_global.wasCalled = false;
+
+    let metaModified = meta_global.modified;
+
+    await Service.sync();
+    Assert.ok(meta_global.wasCalled);
+    Assert.equal(metaModified, meta_global.modified);
+
+    // Try to screw up HMAC calculation.
+    // Re-encrypt keys with a new random keybundle, and upload them to the
+    // server, just as might happen with a second client.
+    _("Attempting to screw up HMAC by re-encrypting keys.");
+    let keys = Service.collectionKeys.asWBO();
+    let b = new BulkKeyBundle("hmacerror");
+    await b.generateRandom();
+    collections.crypto = keys.modified = 100 + Date.now() / 1000; // Future modification time.
+    await keys.encrypt(b);
+    await keys.upload(Service.resource(Service.cryptoKeysURL));
+
+    Assert.equal(false, await Service.verifyAndFetchSymmetricKeys());
+    Assert.equal(Service.status.login, LOGIN_FAILED_INVALID_PASSPHRASE);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    server.stop(do_test_finished);
+  }
+});
diff --git a/services/sync/tests/unit/test_service_sync_specified.js b/services/sync/tests/unit/test_service_sync_specified.js
new file mode 100644
index 0000000000..845cdb3669
--- /dev/null
+++ b/services/sync/tests/unit/test_service_sync_specified.js
@@ -0,0 +1,150 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+let syncedEngines = [];
+
+function SteamEngine() {
+  SyncEngine.call(this, "Steam", Service);
+}
+SteamEngine.prototype = {
+  async _sync() {
+    syncedEngines.push(this.name);
+  },
+};
+Object.setPrototypeOf(SteamEngine.prototype, SyncEngine.prototype);
+
+function StirlingEngine() {
+  SyncEngine.call(this, "Stirling", Service);
+}
+StirlingEngine.prototype = {
+  async _sync() {
+    syncedEngines.push(this.name);
+  },
+};
+Object.setPrototypeOf(StirlingEngine.prototype, SteamEngine.prototype);
+
+// Tracking info/collections.
+var collectionsHelper = track_collections_helper();
+var upd = collectionsHelper.with_updated_collection;
+
+function sync_httpd_setup(handlers) {
+  handlers["/1.1/johndoe/info/collections"] = collectionsHelper.handler;
+  delete collectionsHelper.collections.crypto;
+  delete collectionsHelper.collections.meta;
+
+  let cr = new ServerWBO("keys");
+  handlers["/1.1/johndoe/storage/crypto/keys"] = upd("crypto", cr.handler());
+
+  let cl = new ServerCollection();
+  handlers["/1.1/johndoe/storage/clients"] = upd("clients", cl.handler());
+
+  return httpd_setup(handlers);
+}
+
+async function setUp() {
+  syncedEngines = [];
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  engine.syncPriority = 1;
+
+  engine = Service.engineManager.get("stirling");
+  engine.enabled = true;
+  engine.syncPriority = 2;
+
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": new ServerWBO("global", {}).handler(),
+  });
+  await SyncTestingInfrastructure(server, "johndoe", "ilovejane");
+  return server;
+}
+
+add_task(async function setup() {
+  await Service.engineManager.clear();
+  validate_all_future_pings();
+
+  await Service.engineManager.register(SteamEngine);
+  await Service.engineManager.register(StirlingEngine);
+});
+
+add_task(async function test_noEngines() {
+  enableValidationPrefs();
+
+  _("Test: An empty array of engines to sync does nothing.");
+  let server = await setUp();
+
+  try {
+    _("Sync with no engines specified.");
+    await Service.sync({ engines: [] });
+    deepEqual(syncedEngines, [], "no engines were synced");
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_oneEngine() {
+  enableValidationPrefs();
+
+  _("Test: Only one engine is synced.");
+  let server = await setUp();
+
+  try {
+    _("Sync with 1 engine specified.");
+    await Service.sync({ engines: ["steam"] });
+    deepEqual(syncedEngines, ["steam"]);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_bothEnginesSpecified() {
+  enableValidationPrefs();
+
+  _("Test: All engines are synced when specified in the correct order (1).");
+  let server = await setUp();
+
+  try {
+    _("Sync with both engines specified.");
+    await Service.sync({ engines: ["steam", "stirling"] });
+    deepEqual(syncedEngines, ["steam", "stirling"]);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_bothEnginesSpecified() {
+  enableValidationPrefs();
+
+  _("Test: All engines are synced when specified in the correct order (2).");
+  let server = await setUp();
+
+  try {
+    _("Sync with both engines specified.");
+    await Service.sync({ engines: ["stirling", "steam"] });
+    deepEqual(syncedEngines, ["stirling", "steam"]);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_bothEnginesDefault() {
+  enableValidationPrefs();
+
+  _("Test: All engines are synced when nothing is specified.");
+  let server = await setUp();
+
+  try {
+    await Service.sync();
+    deepEqual(syncedEngines, ["steam", "stirling"]);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
diff --git a/services/sync/tests/unit/test_service_sync_updateEnabledEngines.js b/services/sync/tests/unit/test_service_sync_updateEnabledEngines.js
new file mode 100644
index 0000000000..fd8b3f71bc
--- /dev/null
+++ b/services/sync/tests/unit/test_service_sync_updateEnabledEngines.js
@@ -0,0 +1,587 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const { EngineSynchronizer } = ChromeUtils.importESModule(
+  "resource://services-sync/stages/enginesync.sys.mjs"
+);
+
+function QuietStore() {
+  Store.call("Quiet");
+}
+QuietStore.prototype = {
+  async getAllIDs() {
+    return [];
+  },
+};
+
+function SteamEngine() {
+  SyncEngine.call(this, "Steam", Service);
+}
+SteamEngine.prototype = {
+  // We're not interested in engine sync but what the service does.
+  _storeObj: QuietStore,
+
+  _sync: async function _sync() {
+    await this._syncStartup();
+  },
+};
+Object.setPrototypeOf(SteamEngine.prototype, SyncEngine.prototype);
+
+function StirlingEngine() {
+  SyncEngine.call(this, "Stirling", Service);
+}
+StirlingEngine.prototype = {
+  // This engine's enabled state is the same as the SteamEngine's.
+  get prefName() {
+    return "steam";
+  },
+};
+Object.setPrototypeOf(StirlingEngine.prototype, SteamEngine.prototype);
+
+// Tracking info/collections.
+var collectionsHelper = track_collections_helper();
+var upd = collectionsHelper.with_updated_collection;
+
+function sync_httpd_setup(handlers) {
+  handlers["/1.1/johndoe/info/collections"] = collectionsHelper.handler;
+  delete collectionsHelper.collections.crypto;
+  delete collectionsHelper.collections.meta;
+
+  let cr = new ServerWBO("keys");
+  handlers["/1.1/johndoe/storage/crypto/keys"] = upd("crypto", cr.handler());
+
+  let cl = new ServerCollection();
+  handlers["/1.1/johndoe/storage/clients"] = upd("clients", cl.handler());
+
+  return httpd_setup(handlers);
+}
+
+async function setUp(server) {
+  await SyncTestingInfrastructure(server, "johndoe", "ilovejane");
+  // Ensure that the server has valid keys so that logging in will work and not
+  // result in a server wipe, rendering many of these tests useless.
+  await generateNewKeys(Service.collectionKeys);
+  let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+  await serverKeys.encrypt(Service.identity.syncKeyBundle);
+  let { success } = await serverKeys.upload(
+    Service.resource(Service.cryptoKeysURL)
+  );
+  ok(success);
+}
+
+const PAYLOAD = 42;
+
+add_task(async function setup() {
+  await Service.engineManager.clear();
+  validate_all_future_pings();
+
+  await Service.engineManager.register(SteamEngine);
+  await Service.engineManager.register(StirlingEngine);
+});
+
+add_task(async function test_newAccount() {
+  enableValidationPrefs();
+
+  _("Test: New account does not disable locally enabled engines.");
+  let engine = Service.engineManager.get("steam");
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": new ServerWBO("global", {}).handler(),
+    "/1.1/johndoe/storage/steam": new ServerWBO("steam", {}).handler(),
+  });
+  await setUp(server);
+
+  try {
+    _("Engine is enabled from the beginning.");
+    Service._ignorePrefObserver = true;
+    engine.enabled = true;
+    Service._ignorePrefObserver = false;
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Engine continues to be enabled.");
+    Assert.ok(engine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_enabledLocally() {
+  enableValidationPrefs();
+
+  _("Test: Engine is disabled on remote clients and enabled locally");
+  Service.syncID = "abcdefghij";
+  let engine = Service.engineManager.get("steam");
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {},
+  });
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+    "/1.1/johndoe/storage/steam": new ServerWBO("steam", {}).handler(),
+  });
+  await setUp(server);
+
+  try {
+    _("Enable engine locally.");
+    engine.enabled = true;
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Meta record now contains the new engine.");
+    Assert.ok(!!metaWBO.data.engines.steam);
+
+    _("Engine continues to be enabled.");
+    Assert.ok(engine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_disabledLocally() {
+  enableValidationPrefs();
+
+  _("Test: Engine is enabled on remote clients and disabled locally");
+  Service.syncID = "abcdefghij";
+  let engine = Service.engineManager.get("steam");
+  let syncID = await engine.resetLocalSyncID();
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: { steam: { syncID, version: engine.version } },
+  });
+  let steamCollection = new ServerWBO("steam", PAYLOAD);
+
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+    "/1.1/johndoe/storage/steam": steamCollection.handler(),
+  });
+  await setUp(server);
+
+  try {
+    _("Disable engine locally.");
+    Service._ignorePrefObserver = true;
+    engine.enabled = true;
+    Service._ignorePrefObserver = false;
+    engine.enabled = false;
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Meta record no longer contains engine.");
+    Assert.ok(!metaWBO.data.engines.steam);
+
+    _("Server records are wiped.");
+    Assert.equal(steamCollection.payload, undefined);
+
+    _("Engine continues to be disabled.");
+    Assert.ok(!engine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_disabledLocally_wipe503() {
+  enableValidationPrefs();
+
+  _("Test: Engine is enabled on remote clients and disabled locally");
+  Service.syncID = "abcdefghij";
+  let engine = Service.engineManager.get("steam");
+  let syncID = await engine.resetLocalSyncID();
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: { steam: { syncID, version: engine.version } },
+  });
+
+  function service_unavailable(request, response) {
+    let body = "Service Unavailable";
+    response.setStatusLine(request.httpVersion, 503, "Service Unavailable");
+    response.setHeader("Retry-After", "23");
+    response.bodyOutputStream.write(body, body.length);
+  }
+
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+    "/1.1/johndoe/storage/steam": service_unavailable,
+  });
+  await setUp(server);
+
+  _("Disable engine locally.");
+  Service._ignorePrefObserver = true;
+  engine.enabled = true;
+  Service._ignorePrefObserver = false;
+  engine.enabled = false;
+
+  _("Sync.");
+  await Service.sync();
+  Assert.equal(Service.status.sync, SERVER_MAINTENANCE);
+
+  await Service.startOver();
+  await promiseStopServer(server);
+});
+
+add_task(async function test_enabledRemotely() {
+  enableValidationPrefs();
+
+  _("Test: Engine is disabled locally and enabled on a remote client");
+  Service.syncID = "abcdefghij";
+  let engine = Service.engineManager.get("steam");
+  let syncID = await engine.resetLocalSyncID();
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: { steam: { syncID, version: engine.version } },
+  });
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": upd("meta", metaWBO.handler()),
+
+    "/1.1/johndoe/storage/steam": upd(
+      "steam",
+      new ServerWBO("steam", {}).handler()
+    ),
+  });
+  await setUp(server);
+
+  // We need to be very careful how we do this, so that we don't trigger a
+  // fresh start!
+  try {
+    _("Upload some keys to avoid a fresh start.");
+    let wbo = await Service.collectionKeys.generateNewKeysWBO();
+    await wbo.encrypt(Service.identity.syncKeyBundle);
+    Assert.equal(
+      200,
+      (await wbo.upload(Service.resource(Service.cryptoKeysURL))).status
+    );
+
+    _("Engine is disabled.");
+    Assert.ok(!engine.enabled);
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Engine is enabled.");
+    Assert.ok(engine.enabled);
+
+    _("Meta record still present.");
+    Assert.equal(metaWBO.data.engines.steam.syncID, await engine.getSyncID());
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_disabledRemotelyTwoClients() {
+  enableValidationPrefs();
+
+  _(
+    "Test: Engine is enabled locally and disabled on a remote client... with two clients."
+  );
+  Service.syncID = "abcdefghij";
+  let engine = Service.engineManager.get("steam");
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {},
+  });
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": upd("meta", metaWBO.handler()),
+
+    "/1.1/johndoe/storage/steam": upd(
+      "steam",
+      new ServerWBO("steam", {}).handler()
+    ),
+  });
+  await setUp(server);
+
+  try {
+    _("Enable engine locally.");
+    Service._ignorePrefObserver = true;
+    engine.enabled = true;
+    Service._ignorePrefObserver = false;
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Disable engine by deleting from meta/global.");
+    let d = metaWBO.data;
+    delete d.engines.steam;
+    metaWBO.payload = JSON.stringify(d);
+    metaWBO.modified = Date.now() / 1000;
+
+    _("Add a second client and verify that the local pref is changed.");
+    Service.clientsEngine._store._remoteClients.foobar = {
+      name: "foobar",
+      type: "desktop",
+    };
+    await Service.sync();
+
+    _("Engine is disabled.");
+    Assert.ok(!engine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_disabledRemotely() {
+  enableValidationPrefs();
+
+  _("Test: Engine is enabled locally and disabled on a remote client");
+  Service.syncID = "abcdefghij";
+  let engine = Service.engineManager.get("steam");
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {},
+  });
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+    "/1.1/johndoe/storage/steam": new ServerWBO("steam", {}).handler(),
+  });
+  await setUp(server);
+
+  try {
+    _("Enable engine locally.");
+    Service._ignorePrefObserver = true;
+    engine.enabled = true;
+    Service._ignorePrefObserver = false;
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Engine is not disabled: only one client.");
+    Assert.ok(engine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_dependentEnginesEnabledLocally() {
+  enableValidationPrefs();
+
+  _("Test: Engine is disabled on remote clients and enabled locally");
+  Service.syncID = "abcdefghij";
+  let steamEngine = Service.engineManager.get("steam");
+  let stirlingEngine = Service.engineManager.get("stirling");
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {},
+  });
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+    "/1.1/johndoe/storage/steam": new ServerWBO("steam", {}).handler(),
+    "/1.1/johndoe/storage/stirling": new ServerWBO("stirling", {}).handler(),
+  });
+  await setUp(server);
+
+  try {
+    _("Enable engine locally. Doing it on one is enough.");
+    steamEngine.enabled = true;
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Meta record now contains the new engines.");
+    Assert.ok(!!metaWBO.data.engines.steam);
+    Assert.ok(!!metaWBO.data.engines.stirling);
+
+    _("Engines continue to be enabled.");
+    Assert.ok(steamEngine.enabled);
+    Assert.ok(stirlingEngine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_dependentEnginesDisabledLocally() {
+  enableValidationPrefs();
+
+  _(
+    "Test: Two dependent engines are enabled on remote clients and disabled locally"
+  );
+  Service.syncID = "abcdefghij";
+  let steamEngine = Service.engineManager.get("steam");
+  let steamSyncID = await steamEngine.resetLocalSyncID();
+  let stirlingEngine = Service.engineManager.get("stirling");
+  let stirlingSyncID = await stirlingEngine.resetLocalSyncID();
+  let metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {
+      steam: { syncID: steamSyncID, version: steamEngine.version },
+      stirling: { syncID: stirlingSyncID, version: stirlingEngine.version },
+    },
+  });
+
+  let steamCollection = new ServerWBO("steam", PAYLOAD);
+  let stirlingCollection = new ServerWBO("stirling", PAYLOAD);
+
+  let server = sync_httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+    "/1.1/johndoe/storage/steam": steamCollection.handler(),
+    "/1.1/johndoe/storage/stirling": stirlingCollection.handler(),
+  });
+  await setUp(server);
+
+  try {
+    _("Disable engines locally. Doing it on one is enough.");
+    Service._ignorePrefObserver = true;
+    steamEngine.enabled = true;
+    Assert.ok(stirlingEngine.enabled);
+    Service._ignorePrefObserver = false;
+    steamEngine.enabled = false;
+    Assert.ok(!stirlingEngine.enabled);
+
+    _("Sync.");
+    await Service.sync();
+
+    _("Meta record no longer contains engines.");
+    Assert.ok(!metaWBO.data.engines.steam);
+    Assert.ok(!metaWBO.data.engines.stirling);
+
+    _("Server records are wiped.");
+    Assert.equal(steamCollection.payload, undefined);
+    Assert.equal(stirlingCollection.payload, undefined);
+
+    _("Engines continue to be disabled.");
+    Assert.ok(!steamEngine.enabled);
+    Assert.ok(!stirlingEngine.enabled);
+  } finally {
+    await Service.startOver();
+    await promiseStopServer(server);
+  }
+});
+
+add_task(async function test_service_updateLocalEnginesState() {
+  Service.syncID = "abcdefghij";
+  const engine = Service.engineManager.get("steam");
+  const metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    declined: ["steam"],
+    engines: {},
+  });
+  const server = httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+  });
+  await SyncTestingInfrastructure(server, "johndoe");
+
+  // Disconnect sync.
+  await Service.startOver();
+  Service._ignorePrefObserver = true;
+  // Steam engine is enabled on our machine.
+  engine.enabled = true;
+  Service._ignorePrefObserver = false;
+  Service.identity._findCluster = () => server.baseURI + "/1.1/johndoe/";
+
+  // Update engine state from the server.
+  await Service.updateLocalEnginesState();
+  // Now disabled.
+  Assert.ok(!engine.enabled);
+});
+
+add_task(async function test_service_enableAfterUpdateState() {
+  Service.syncID = "abcdefghij";
+  const engine = Service.engineManager.get("steam");
+  const metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    declined: ["steam"],
+    engines: { someengine: {} },
+  });
+  const server = httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+  });
+  await SyncTestingInfrastructure(server, "johndoe");
+
+  // Disconnect sync.
+  await Service.startOver();
+  Service.identity._findCluster = () => server.baseURI + "/1.1/johndoe/";
+
+  // Update engine state from the server.
+  await Service.updateLocalEnginesState();
+  // Now disabled, reflecting what's on the server.
+  Assert.ok(!engine.enabled);
+  // Enable the engine, as though the user selected it via CWTS.
+  engine.enabled = true;
+
+  // Do the "reconcile local and remote states" dance.
+  let engineSync = new EngineSynchronizer(Service);
+  await engineSync._updateEnabledEngines();
+  await Service._maybeUpdateDeclined();
+  // engine should remain enabled.
+  Assert.ok(engine.enabled);
+  // engine should no longer appear in declined on the server.
+  Assert.deepEqual(metaWBO.data.declined, []);
+});
+
+add_task(async function test_service_disableAfterUpdateState() {
+  Service.syncID = "abcdefghij";
+  const engine = Service.engineManager.get("steam");
+  const metaWBO = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    declined: [],
+    engines: { steam: {} },
+  });
+  const server = httpd_setup({
+    "/1.1/johndoe/storage/meta/global": metaWBO.handler(),
+  });
+  await SyncTestingInfrastructure(server, "johndoe");
+
+  // Disconnect sync.
+  await Service.startOver();
+  Service.identity._findCluster = () => server.baseURI + "/1.1/johndoe/";
+
+  // Update engine state from the server.
+  await Service.updateLocalEnginesState();
+  // Now enabled, reflecting what's on the server.
+  Assert.ok(engine.enabled);
+  // Disable the engine, as though via CWTS.
+  engine.enabled = false;
+
+  // Do the "reconcile local and remote states" dance.
+  let engineSync = new EngineSynchronizer(Service);
+  await engineSync._updateEnabledEngines();
+  await Service._maybeUpdateDeclined();
+  // engine should remain disabled.
+  Assert.ok(!engine.enabled);
+  // engine should now appear in declined on the server.
+  Assert.deepEqual(metaWBO.data.declined, ["steam"]);
+  // and should have been removed from engines.
+  Assert.deepEqual(metaWBO.data.engines, {});
+});
+
+add_task(async function test_service_updateLocalEnginesState_no_meta_global() {
+  Service.syncID = "abcdefghij";
+  const engine = Service.engineManager.get("steam");
+  // The server doesn't contain /meta/global (sync was never enabled).
+  const server = httpd_setup({});
+  await SyncTestingInfrastructure(server, "johndoe");
+
+  // Disconnect sync.
+  await Service.startOver();
+  Service._ignorePrefObserver = true;
+  // Steam engine is enabled on our machine.
+  engine.enabled = true;
+  Service._ignorePrefObserver = false;
+  Service.identity._findCluster = () => server.baseURI + "/1.1/johndoe/";
+
+  // Update engine state from the server.
+  await Service.updateLocalEnginesState();
+  // Still enabled.
+  Assert.ok(engine.enabled);
+});
diff --git a/services/sync/tests/unit/test_service_verifyLogin.js b/services/sync/tests/unit/test_service_verifyLogin.js
new file mode 100644
index 0000000000..b99b5c692c
--- /dev/null
+++ b/services/sync/tests/unit/test_service_verifyLogin.js
@@ -0,0 +1,118 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function login_handling(handler) {
+  return function (request, response) {
+    if (has_hawk_header(request)) {
+      handler(request, response);
+    } else {
+      let body = "Unauthorized";
+      response.setStatusLine(request.httpVersion, 401, "Unauthorized");
+      response.bodyOutputStream.write(body, body.length);
+    }
+  };
+}
+
+function service_unavailable(request, response) {
+  let body = "Service Unavailable";
+  response.setStatusLine(request.httpVersion, 503, "Service Unavailable");
+  response.setHeader("Retry-After", "42");
+  response.bodyOutputStream.write(body, body.length);
+}
+
+function run_test() {
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  run_next_test();
+}
+
+add_task(async function test_verifyLogin() {
+  // This test expects a clean slate -- no saved passphrase.
+  Services.logins.removeAllUserFacingLogins();
+  let johnHelper = track_collections_helper();
+  let johnU = johnHelper.with_updated_collection;
+
+  do_test_pending();
+
+  let server = httpd_setup({
+    "/1.1/johndoe/info/collections": login_handling(johnHelper.handler),
+    "/1.1/janedoe/info/collections": service_unavailable,
+
+    "/1.1/johndoe/storage/crypto/keys": johnU(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe/storage/meta/global": johnU(
+      "meta",
+      new ServerWBO("global").handler()
+    ),
+  });
+
+  try {
+    _("Force the initial state.");
+    Service.status.service = STATUS_OK;
+    Assert.equal(Service.status.service, STATUS_OK);
+
+    _("Credentials won't check out because we're not configured yet.");
+    Service.status.resetSync();
+    Assert.equal(false, await Service.verifyLogin());
+    Assert.equal(Service.status.service, CLIENT_NOT_CONFIGURED);
+    Assert.equal(Service.status.login, LOGIN_FAILED_NO_USERNAME);
+
+    _("Success if syncBundleKey is set.");
+    Service.status.resetSync();
+    await configureIdentity({ username: "johndoe" }, server);
+    Assert.ok(await Service.verifyLogin());
+    Assert.equal(Service.status.service, STATUS_OK);
+    Assert.equal(Service.status.login, LOGIN_SUCCEEDED);
+
+    _(
+      "If verifyLogin() encounters a server error, it flips on the backoff flag and notifies observers on a 503 with Retry-After."
+    );
+    Service.status.resetSync();
+    await configureIdentity({ username: "janedoe" }, server);
+    Service._updateCachedURLs();
+    Assert.ok(!Service.status.enforceBackoff);
+    let backoffInterval;
+    Svc.Obs.add(
+      "weave:service:backoff:interval",
+      function observe(subject, data) {
+        Svc.Obs.remove("weave:service:backoff:interval", observe);
+        backoffInterval = subject;
+      }
+    );
+    Assert.equal(false, await Service.verifyLogin());
+    Assert.ok(Service.status.enforceBackoff);
+    Assert.equal(backoffInterval, 42);
+    Assert.equal(Service.status.service, LOGIN_FAILED);
+    Assert.equal(Service.status.login, SERVER_MAINTENANCE);
+
+    _(
+      "Ensure a network error when finding the cluster sets the right Status bits."
+    );
+    Service.status.resetSync();
+    Service.clusterURL = "";
+    Service.identity._findCluster = () => "http://localhost:12345/";
+    Assert.equal(false, await Service.verifyLogin());
+    Assert.equal(Service.status.service, LOGIN_FAILED);
+    Assert.equal(Service.status.login, LOGIN_FAILED_NETWORK_ERROR);
+
+    _(
+      "Ensure a network error when getting the collection info sets the right Status bits."
+    );
+    Service.status.resetSync();
+    Service.clusterURL = "http://localhost:12345/";
+    Assert.equal(false, await Service.verifyLogin());
+    Assert.equal(Service.status.service, LOGIN_FAILED);
+    Assert.equal(Service.status.login, LOGIN_FAILED_NETWORK_ERROR);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+    server.stop(do_test_finished);
+  }
+});
diff --git a/services/sync/tests/unit/test_service_wipeClient.js b/services/sync/tests/unit/test_service_wipeClient.js
new file mode 100644
index 0000000000..aa48868ca0
--- /dev/null
+++ b/services/sync/tests/unit/test_service_wipeClient.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+function CanDecryptEngine() {
+  SyncEngine.call(this, "CanDecrypt", Service);
+}
+CanDecryptEngine.prototype = {
+  // Override these methods with mocks for the test
+  async canDecrypt() {
+    return true;
+  },
+
+  wasWiped: false,
+  async wipeClient() {
+    this.wasWiped = true;
+  },
+};
+Object.setPrototypeOf(CanDecryptEngine.prototype, SyncEngine.prototype);
+
+function CannotDecryptEngine() {
+  SyncEngine.call(this, "CannotDecrypt", Service);
+}
+CannotDecryptEngine.prototype = {
+  // Override these methods with mocks for the test
+  async canDecrypt() {
+    return false;
+  },
+
+  wasWiped: false,
+  async wipeClient() {
+    this.wasWiped = true;
+  },
+};
+Object.setPrototypeOf(CannotDecryptEngine.prototype, SyncEngine.prototype);
+
+let canDecryptEngine;
+let cannotDecryptEngine;
+
+add_task(async function setup() {
+  await Service.engineManager.clear();
+
+  await Service.engineManager.register(CanDecryptEngine);
+  await Service.engineManager.register(CannotDecryptEngine);
+  canDecryptEngine = Service.engineManager.get("candecrypt");
+  cannotDecryptEngine = Service.engineManager.get("cannotdecrypt");
+});
+
+add_task(async function test_withEngineList() {
+  try {
+    _("Ensure initial scenario.");
+    Assert.ok(!canDecryptEngine.wasWiped);
+    Assert.ok(!cannotDecryptEngine.wasWiped);
+
+    _("Wipe local engine data.");
+    await Service.wipeClient(["candecrypt", "cannotdecrypt"]);
+
+    _("Ensure only the engine that can decrypt was wiped.");
+    Assert.ok(canDecryptEngine.wasWiped);
+    Assert.ok(!cannotDecryptEngine.wasWiped);
+  } finally {
+    canDecryptEngine.wasWiped = false;
+    cannotDecryptEngine.wasWiped = false;
+    await Service.startOver();
+  }
+});
+
+add_task(async function test_startOver_clears_keys() {
+  syncTestLogging();
+  await generateNewKeys(Service.collectionKeys);
+  Assert.ok(!!Service.collectionKeys.keyForCollection());
+  await Service.startOver();
+  syncTestLogging();
+  Assert.ok(!Service.collectionKeys.keyForCollection());
+});
diff --git a/services/sync/tests/unit/test_service_wipeServer.js b/services/sync/tests/unit/test_service_wipeServer.js
new file mode 100644
index 0000000000..9fc2592aa8
--- /dev/null
+++ b/services/sync/tests/unit/test_service_wipeServer.js
@@ -0,0 +1,240 @@
+Svc.PrefBranch.setStringPref("registerEngines", "");
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+// configure the identity we use for this test.
+const identityConfig = makeIdentityConfig({ username: "johndoe" });
+
+function FakeCollection() {
+  this.deleted = false;
+}
+FakeCollection.prototype = {
+  handler() {
+    let self = this;
+    return function (request, response) {
+      let body = "";
+      self.timestamp = new_timestamp();
+      let timestamp = "" + self.timestamp;
+      if (request.method == "DELETE") {
+        body = timestamp;
+        self.deleted = true;
+      }
+      response.setHeader("X-Weave-Timestamp", timestamp);
+      response.setStatusLine(request.httpVersion, 200, "OK");
+      response.bodyOutputStream.write(body, body.length);
+    };
+  },
+};
+
+async function setUpTestFixtures(server) {
+  Service.clusterURL = server.baseURI + "/";
+
+  await configureIdentity(identityConfig);
+}
+
+add_task(async function test_wipeServer_list_success() {
+  _("Service.wipeServer() deletes collections given as argument.");
+
+  let steam_coll = new FakeCollection();
+  let diesel_coll = new FakeCollection();
+
+  let server = httpd_setup({
+    "/1.1/johndoe/storage/steam": steam_coll.handler(),
+    "/1.1/johndoe/storage/diesel": diesel_coll.handler(),
+    "/1.1/johndoe/storage/petrol": httpd_handler(404, "Not Found"),
+  });
+
+  try {
+    await setUpTestFixtures(server);
+    await SyncTestingInfrastructure(server, "johndoe", "irrelevant");
+
+    _("Confirm initial environment.");
+    Assert.ok(!steam_coll.deleted);
+    Assert.ok(!diesel_coll.deleted);
+
+    _(
+      "wipeServer() will happily ignore the non-existent collection and use the timestamp of the last DELETE that was successful."
+    );
+    let timestamp = await Service.wipeServer(["steam", "diesel", "petrol"]);
+    Assert.equal(timestamp, diesel_coll.timestamp);
+
+    _(
+      "wipeServer stopped deleting after encountering an error with the 'petrol' collection, thus only 'steam' has been deleted."
+    );
+    Assert.ok(steam_coll.deleted);
+    Assert.ok(diesel_coll.deleted);
+  } finally {
+    await promiseStopServer(server);
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_wipeServer_list_503() {
+  _("Service.wipeServer() deletes collections given as argument.");
+
+  let steam_coll = new FakeCollection();
+  let diesel_coll = new FakeCollection();
+
+  let server = httpd_setup({
+    "/1.1/johndoe/storage/steam": steam_coll.handler(),
+    "/1.1/johndoe/storage/petrol": httpd_handler(503, "Service Unavailable"),
+    "/1.1/johndoe/storage/diesel": diesel_coll.handler(),
+  });
+
+  try {
+    await setUpTestFixtures(server);
+    await SyncTestingInfrastructure(server, "johndoe", "irrelevant");
+
+    _("Confirm initial environment.");
+    Assert.ok(!steam_coll.deleted);
+    Assert.ok(!diesel_coll.deleted);
+
+    _(
+      "wipeServer() will happily ignore the non-existent collection, delete the 'steam' collection and abort after an receiving an error on the 'petrol' collection."
+    );
+    let error;
+    try {
+      await Service.wipeServer(["non-existent", "steam", "petrol", "diesel"]);
+      do_throw("Should have thrown!");
+    } catch (ex) {
+      error = ex;
+    }
+    _("wipeServer() threw this exception: " + error);
+    Assert.equal(error.status, 503);
+
+    _(
+      "wipeServer stopped deleting after encountering an error with the 'petrol' collection, thus only 'steam' has been deleted."
+    );
+    Assert.ok(steam_coll.deleted);
+    Assert.ok(!diesel_coll.deleted);
+  } finally {
+    await promiseStopServer(server);
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_wipeServer_all_success() {
+  _("Service.wipeServer() deletes all the things.");
+
+  /**
+   * Handle the bulk DELETE request sent by wipeServer.
+   */
+  let deleted = false;
+  let serverTimestamp;
+  function storageHandler(request, response) {
+    Assert.equal("DELETE", request.method);
+    Assert.ok(request.hasHeader("X-Confirm-Delete"));
+    deleted = true;
+    serverTimestamp = return_timestamp(request, response);
+  }
+
+  let server = httpd_setup({
+    "/1.1/johndoe/storage": storageHandler,
+  });
+  await setUpTestFixtures(server);
+
+  _("Try deletion.");
+  await SyncTestingInfrastructure(server, "johndoe", "irrelevant");
+  let returnedTimestamp = await Service.wipeServer();
+  Assert.ok(deleted);
+  Assert.equal(returnedTimestamp, serverTimestamp);
+
+  await promiseStopServer(server);
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+});
+
+add_task(async function test_wipeServer_all_404() {
+  _("Service.wipeServer() accepts a 404.");
+
+  /**
+   * Handle the bulk DELETE request sent by wipeServer. Returns a 404.
+   */
+  let deleted = false;
+  let serverTimestamp;
+  function storageHandler(request, response) {
+    Assert.equal("DELETE", request.method);
+    Assert.ok(request.hasHeader("X-Confirm-Delete"));
+    deleted = true;
+    serverTimestamp = new_timestamp();
+    response.setHeader("X-Weave-Timestamp", "" + serverTimestamp);
+    response.setStatusLine(request.httpVersion, 404, "Not Found");
+  }
+
+  let server = httpd_setup({
+    "/1.1/johndoe/storage": storageHandler,
+  });
+  await setUpTestFixtures(server);
+
+  _("Try deletion.");
+  await SyncTestingInfrastructure(server, "johndoe", "irrelevant");
+  let returnedTimestamp = await Service.wipeServer();
+  Assert.ok(deleted);
+  Assert.equal(returnedTimestamp, serverTimestamp);
+
+  await promiseStopServer(server);
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+});
+
+add_task(async function test_wipeServer_all_503() {
+  _("Service.wipeServer() throws if it encounters a non-200/404 response.");
+
+  /**
+   * Handle the bulk DELETE request sent by wipeServer. Returns a 503.
+   */
+  function storageHandler(request, response) {
+    Assert.equal("DELETE", request.method);
+    Assert.ok(request.hasHeader("X-Confirm-Delete"));
+    response.setStatusLine(request.httpVersion, 503, "Service Unavailable");
+  }
+
+  let server = httpd_setup({
+    "/1.1/johndoe/storage": storageHandler,
+  });
+  await setUpTestFixtures(server);
+
+  _("Try deletion.");
+  let error;
+  try {
+    await SyncTestingInfrastructure(server, "johndoe", "irrelevant");
+    await Service.wipeServer();
+    do_throw("Should have thrown!");
+  } catch (ex) {
+    error = ex;
+  }
+  Assert.equal(error.status, 503);
+
+  await promiseStopServer(server);
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+});
+
+add_task(async function test_wipeServer_all_connectionRefused() {
+  _("Service.wipeServer() throws if it encounters a network problem.");
+  let server = httpd_setup({});
+  await setUpTestFixtures(server);
+
+  Service.clusterURL = "http://localhost:4352/";
+
+  _("Try deletion.");
+  try {
+    await Service.wipeServer();
+    do_throw("Should have thrown!");
+  } catch (ex) {
+    Assert.equal(ex.result, Cr.NS_ERROR_CONNECTION_REFUSED);
+  }
+
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_status.js b/services/sync/tests/unit/test_status.js
new file mode 100644
index 0000000000..5bcfa182c3
--- /dev/null
+++ b/services/sync/tests/unit/test_status.js
@@ -0,0 +1,83 @@
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+
+function run_test() {
+  // Check initial states
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.equal(Status.minimumNextSync, 0);
+
+  Assert.equal(Status.service, STATUS_OK);
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  Assert.equal(Status.login, LOGIN_SUCCEEDED);
+  if (Status.engines.length) {
+    do_throw("Status.engines should be empty.");
+  }
+  Assert.equal(Status.partial, false);
+
+  // Check login status
+  for (let code of [LOGIN_FAILED_NO_USERNAME, LOGIN_FAILED_NO_PASSPHRASE]) {
+    Status.login = code;
+    Assert.equal(Status.login, code);
+    Assert.equal(Status.service, CLIENT_NOT_CONFIGURED);
+    Status.resetSync();
+  }
+
+  Status.login = LOGIN_FAILED;
+  Assert.equal(Status.login, LOGIN_FAILED);
+  Assert.equal(Status.service, LOGIN_FAILED);
+  Status.resetSync();
+
+  Status.login = LOGIN_SUCCEEDED;
+  Assert.equal(Status.login, LOGIN_SUCCEEDED);
+  Assert.equal(Status.service, STATUS_OK);
+  Status.resetSync();
+
+  // Check sync status
+  Status.sync = SYNC_FAILED;
+  Assert.equal(Status.sync, SYNC_FAILED);
+  Assert.equal(Status.service, SYNC_FAILED);
+
+  Status.sync = SYNC_SUCCEEDED;
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  Assert.equal(Status.service, STATUS_OK);
+
+  Status.resetSync();
+
+  // Check engine status
+  Status.engines = ["testEng1", ENGINE_SUCCEEDED];
+  Assert.equal(Status.engines.testEng1, ENGINE_SUCCEEDED);
+  Assert.equal(Status.service, STATUS_OK);
+
+  Status.engines = ["testEng2", ENGINE_DOWNLOAD_FAIL];
+  Assert.equal(Status.engines.testEng1, ENGINE_SUCCEEDED);
+  Assert.equal(Status.engines.testEng2, ENGINE_DOWNLOAD_FAIL);
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+
+  Status.engines = ["testEng3", ENGINE_SUCCEEDED];
+  Assert.equal(Status.engines.testEng1, ENGINE_SUCCEEDED);
+  Assert.equal(Status.engines.testEng2, ENGINE_DOWNLOAD_FAIL);
+  Assert.equal(Status.engines.testEng3, ENGINE_SUCCEEDED);
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+
+  // Check resetSync
+  Status.sync = SYNC_FAILED;
+  Status.resetSync();
+
+  Assert.equal(Status.service, STATUS_OK);
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+  if (Status.engines.length) {
+    do_throw("Status.engines should be empty.");
+  }
+
+  // Check resetBackoff
+  Status.enforceBackoff = true;
+  Status.backOffInterval = 4815162342;
+  Status.backOffInterval = 42;
+  Status.resetBackoff();
+
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.equal(Status.minimumNextSync, 0);
+}
diff --git a/services/sync/tests/unit/test_status_checkSetup.js b/services/sync/tests/unit/test_status_checkSetup.js
new file mode 100644
index 0000000000..fe3e7cad8d
--- /dev/null
+++ b/services/sync/tests/unit/test_status_checkSetup.js
@@ -0,0 +1,26 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+
+add_task(async function test_status_checkSetup() {
+  try {
+    _("Fresh setup, we're not configured.");
+    Assert.equal(Status.checkSetup(), CLIENT_NOT_CONFIGURED);
+    Assert.equal(Status.login, LOGIN_FAILED_NO_USERNAME);
+    Status.resetSync();
+
+    _("Let's provide the syncKeyBundle");
+    await configureIdentity();
+
+    _("checkSetup()");
+    Assert.equal(Status.checkSetup(), STATUS_OK);
+    Status.resetSync();
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
diff --git a/services/sync/tests/unit/test_sync_auth_manager.js b/services/sync/tests/unit/test_sync_auth_manager.js
new file mode 100644
index 0000000000..9af40d26c6
--- /dev/null
+++ b/services/sync/tests/unit/test_sync_auth_manager.js
@@ -0,0 +1,1027 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { AuthenticationError, SyncAuthManager } = ChromeUtils.importESModule(
+  "resource://services-sync/sync_auth.sys.mjs"
+);
+const { Resource } = ChromeUtils.importESModule(
+  "resource://services-sync/resource.sys.mjs"
+);
+const { initializeIdentityWithTokenServerResponse } =
+  ChromeUtils.importESModule(
+    "resource://testing-common/services/sync/fxa_utils.sys.mjs"
+  );
+const { HawkClient } = ChromeUtils.importESModule(
+  "resource://services-common/hawkclient.sys.mjs"
+);
+const { FxAccounts } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccounts.sys.mjs"
+);
+const { FxAccountsClient } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccountsClient.sys.mjs"
+);
+const {
+  ERRNO_INVALID_AUTH_TOKEN,
+  ONLOGIN_NOTIFICATION,
+  ONVERIFIED_NOTIFICATION,
+} = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccountsCommon.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+const { TokenServerClient, TokenServerClientServerError } =
+  ChromeUtils.importESModule(
+    "resource://services-common/tokenserverclient.sys.mjs"
+  );
+const { AccountState } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccounts.sys.mjs"
+);
+
+const SECOND_MS = 1000;
+const MINUTE_MS = SECOND_MS * 60;
+const HOUR_MS = MINUTE_MS * 60;
+
+const MOCK_ACCESS_TOKEN =
+  "e3c5caf17f27a0d9e351926a928938b3737df43e91d4992a5a5fca9a7bdef8ba";
+
+var globalIdentityConfig = makeIdentityConfig();
+var globalSyncAuthManager = new SyncAuthManager();
+configureFxAccountIdentity(globalSyncAuthManager, globalIdentityConfig);
+
+/**
+ * Mock client clock and skew vs server in FxAccounts signed-in user module and
+ * API client.  sync_auth.js queries these values to construct HAWK
+ * headers.  We will use this to test clock skew compensation in these headers
+ * below.
+ */
+var MockFxAccountsClient = function () {
+  FxAccountsClient.apply(this);
+};
+MockFxAccountsClient.prototype = {
+  accountStatus() {
+    return Promise.resolve(true);
+  },
+  getScopedKeyData() {
+    return Promise.resolve({
+      "https://identity.mozilla.com/apps/oldsync": {
+        identifier: "https://identity.mozilla.com/apps/oldsync",
+        keyRotationSecret:
+          "0000000000000000000000000000000000000000000000000000000000000000",
+        keyRotationTimestamp: 1234567890123,
+      },
+    });
+  },
+};
+Object.setPrototypeOf(
+  MockFxAccountsClient.prototype,
+  FxAccountsClient.prototype
+);
+
+add_test(function test_initial_state() {
+  _("Verify initial state");
+  Assert.ok(!globalSyncAuthManager._token);
+  Assert.ok(!globalSyncAuthManager._hasValidToken());
+  run_next_test();
+});
+
+add_task(async function test_initialialize() {
+  _("Verify start after fetching token");
+  await globalSyncAuthManager._ensureValidToken();
+  Assert.ok(!!globalSyncAuthManager._token);
+  Assert.ok(globalSyncAuthManager._hasValidToken());
+});
+
+add_task(async function test_refreshOAuthTokenOn401() {
+  _("Refreshes the FXA OAuth token after a 401.");
+  let getTokenCount = 0;
+  let syncAuthManager = new SyncAuthManager();
+  let identityConfig = makeIdentityConfig();
+  let fxaInternal = makeFxAccountsInternalMock(identityConfig);
+  configureFxAccountIdentity(syncAuthManager, identityConfig, fxaInternal);
+  syncAuthManager._fxaService._internal.initialize();
+  syncAuthManager._fxaService.getOAuthToken = () => {
+    ++getTokenCount;
+    return Promise.resolve(MOCK_ACCESS_TOKEN);
+  };
+
+  let didReturn401 = false;
+  let didReturn200 = false;
+  let mockTSC = mockTokenServer(() => {
+    if (getTokenCount <= 1) {
+      didReturn401 = true;
+      return {
+        status: 401,
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({}),
+      };
+    }
+    didReturn200 = true;
+    return {
+      status: 200,
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({
+        id: "id",
+        key: "key",
+        api_endpoint: "http://example.com/",
+        uid: "uid",
+        duration: 300,
+      }),
+    };
+  });
+
+  syncAuthManager._tokenServerClient = mockTSC;
+
+  await syncAuthManager._ensureValidToken();
+
+  Assert.equal(getTokenCount, 2);
+  Assert.ok(didReturn401);
+  Assert.ok(didReturn200);
+  Assert.ok(syncAuthManager._token);
+  Assert.ok(syncAuthManager._hasValidToken());
+});
+
+add_task(async function test_initialializeWithAuthErrorAndDeletedAccount() {
+  _("Verify sync state with auth error + account deleted");
+
+  var identityConfig = makeIdentityConfig();
+  var syncAuthManager = new SyncAuthManager();
+
+  // Use the real `getOAuthToken` method that calls
+  // `mockFxAClient.accessTokenWithSessionToken`.
+  let fxaInternal = makeFxAccountsInternalMock(identityConfig);
+  delete fxaInternal.getOAuthToken;
+
+  configureFxAccountIdentity(syncAuthManager, identityConfig, fxaInternal);
+  syncAuthManager._fxaService._internal.initialize();
+
+  let accessTokenWithSessionTokenCalled = false;
+  let accountStatusCalled = false;
+  let sessionStatusCalled = false;
+
+  let AuthErrorMockFxAClient = function () {
+    FxAccountsClient.apply(this);
+  };
+  AuthErrorMockFxAClient.prototype = {
+    accessTokenWithSessionToken() {
+      accessTokenWithSessionTokenCalled = true;
+      return Promise.reject({
+        code: 401,
+        errno: ERRNO_INVALID_AUTH_TOKEN,
+      });
+    },
+    accountStatus() {
+      accountStatusCalled = true;
+      return Promise.resolve(false);
+    },
+    sessionStatus() {
+      sessionStatusCalled = true;
+      return Promise.resolve(false);
+    },
+  };
+  Object.setPrototypeOf(
+    AuthErrorMockFxAClient.prototype,
+    FxAccountsClient.prototype
+  );
+
+  let mockFxAClient = new AuthErrorMockFxAClient();
+  syncAuthManager._fxaService._internal._fxAccountsClient = mockFxAClient;
+
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    AuthenticationError,
+    "should reject due to an auth error"
+  );
+
+  Assert.ok(accessTokenWithSessionTokenCalled);
+  Assert.ok(sessionStatusCalled);
+  Assert.ok(accountStatusCalled);
+  Assert.ok(!syncAuthManager._token);
+  Assert.ok(!syncAuthManager._hasValidToken());
+});
+
+add_task(async function test_getResourceAuthenticator() {
+  _(
+    "SyncAuthManager supplies a Resource Authenticator callback which returns a Hawk header."
+  );
+  configureFxAccountIdentity(globalSyncAuthManager);
+  let authenticator = globalSyncAuthManager.getResourceAuthenticator();
+  Assert.ok(!!authenticator);
+  let req = {
+    uri: CommonUtils.makeURI("https://example.net/somewhere/over/the/rainbow"),
+    method: "GET",
+  };
+  let output = await authenticator(req, "GET");
+  Assert.ok("headers" in output);
+  Assert.ok("authorization" in output.headers);
+  Assert.ok(output.headers.authorization.startsWith("Hawk"));
+  _("Expected internal state after successful call.");
+  Assert.equal(
+    globalSyncAuthManager._token.uid,
+    globalIdentityConfig.fxaccount.token.uid
+  );
+});
+
+add_task(async function test_resourceAuthenticatorSkew() {
+  _(
+    "SyncAuthManager Resource Authenticator compensates for clock skew in Hawk header."
+  );
+
+  // Clock is skewed 12 hours into the future
+  // We pick a date in the past so we don't risk concealing bugs in code that
+  // uses new Date() instead of our given date.
+  let now =
+    new Date("Fri Apr 09 2004 00:00:00 GMT-0700").valueOf() + 12 * HOUR_MS;
+  let syncAuthManager = new SyncAuthManager();
+  let hawkClient = new HawkClient("https://example.net/v1", "/foo");
+
+  // mock fxa hawk client skew
+  hawkClient.now = function () {
+    dump("mocked client now: " + now + "\n");
+    return now;
+  };
+  // Imagine there's already been one fxa request and the hawk client has
+  // already detected skew vs the fxa auth server.
+  let localtimeOffsetMsec = -1 * 12 * HOUR_MS;
+  hawkClient._localtimeOffsetMsec = localtimeOffsetMsec;
+
+  let fxaClient = new MockFxAccountsClient();
+  fxaClient.hawk = hawkClient;
+
+  // Sanity check
+  Assert.equal(hawkClient.now(), now);
+  Assert.equal(hawkClient.localtimeOffsetMsec, localtimeOffsetMsec);
+
+  // Properly picked up by the client
+  Assert.equal(fxaClient.now(), now);
+  Assert.equal(fxaClient.localtimeOffsetMsec, localtimeOffsetMsec);
+
+  let identityConfig = makeIdentityConfig();
+  let fxaInternal = makeFxAccountsInternalMock(identityConfig);
+  fxaInternal._now_is = now;
+  fxaInternal.fxAccountsClient = fxaClient;
+
+  // Mocks within mocks...
+  configureFxAccountIdentity(
+    syncAuthManager,
+    globalIdentityConfig,
+    fxaInternal
+  );
+
+  Assert.equal(syncAuthManager._fxaService._internal.now(), now);
+  Assert.equal(
+    syncAuthManager._fxaService._internal.localtimeOffsetMsec,
+    localtimeOffsetMsec
+  );
+
+  Assert.equal(syncAuthManager._fxaService._internal.now(), now);
+  Assert.equal(
+    syncAuthManager._fxaService._internal.localtimeOffsetMsec,
+    localtimeOffsetMsec
+  );
+
+  let request = new Resource("https://example.net/i/like/pie/");
+  let authenticator = syncAuthManager.getResourceAuthenticator();
+  let output = await authenticator(request, "GET");
+  dump("output" + JSON.stringify(output));
+  let authHeader = output.headers.authorization;
+  Assert.ok(authHeader.startsWith("Hawk"));
+
+  // Skew correction is applied in the header and we're within the two-minute
+  // window.
+  Assert.equal(getTimestamp(authHeader), now - 12 * HOUR_MS);
+  Assert.ok(getTimestampDelta(authHeader, now) - 12 * HOUR_MS < 2 * MINUTE_MS);
+});
+
+add_task(async function test_RESTResourceAuthenticatorSkew() {
+  _(
+    "SyncAuthManager REST Resource Authenticator compensates for clock skew in Hawk header."
+  );
+
+  // Clock is skewed 12 hours into the future from our arbitary date
+  let now =
+    new Date("Fri Apr 09 2004 00:00:00 GMT-0700").valueOf() + 12 * HOUR_MS;
+  let syncAuthManager = new SyncAuthManager();
+  let hawkClient = new HawkClient("https://example.net/v1", "/foo");
+
+  // mock fxa hawk client skew
+  hawkClient.now = function () {
+    return now;
+  };
+  // Imagine there's already been one fxa request and the hawk client has
+  // already detected skew vs the fxa auth server.
+  hawkClient._localtimeOffsetMsec = -1 * 12 * HOUR_MS;
+
+  let fxaClient = new MockFxAccountsClient();
+  fxaClient.hawk = hawkClient;
+
+  let identityConfig = makeIdentityConfig();
+  let fxaInternal = makeFxAccountsInternalMock(identityConfig);
+  fxaInternal._now_is = now;
+  fxaInternal.fxAccountsClient = fxaClient;
+
+  configureFxAccountIdentity(
+    syncAuthManager,
+    globalIdentityConfig,
+    fxaInternal
+  );
+
+  Assert.equal(syncAuthManager._fxaService._internal.now(), now);
+
+  let request = new Resource("https://example.net/i/like/pie/");
+  let authenticator = syncAuthManager.getResourceAuthenticator();
+  let output = await authenticator(request, "GET");
+  dump("output" + JSON.stringify(output));
+  let authHeader = output.headers.authorization;
+  Assert.ok(authHeader.startsWith("Hawk"));
+
+  // Skew correction is applied in the header and we're within the two-minute
+  // window.
+  Assert.equal(getTimestamp(authHeader), now - 12 * HOUR_MS);
+  Assert.ok(getTimestampDelta(authHeader, now) - 12 * HOUR_MS < 2 * MINUTE_MS);
+});
+
+add_task(async function test_ensureLoggedIn() {
+  configureFxAccountIdentity(globalSyncAuthManager);
+  await globalSyncAuthManager._ensureValidToken();
+  Assert.equal(Status.login, LOGIN_SUCCEEDED, "original initialize worked");
+  Assert.ok(globalSyncAuthManager._token);
+
+  // arrange for no logged in user.
+  let fxa = globalSyncAuthManager._fxaService;
+  let signedInUser =
+    fxa._internal.currentAccountState.storageManager.accountData;
+  fxa._internal.currentAccountState.storageManager.accountData = null;
+  await Assert.rejects(
+    globalSyncAuthManager._ensureValidToken(true),
+    /no user is logged in/,
+    "expecting rejection due to no user"
+  );
+  // Restore the logged in user to what it was.
+  fxa._internal.currentAccountState.storageManager.accountData = signedInUser;
+  Status.login = LOGIN_FAILED_LOGIN_REJECTED;
+  await globalSyncAuthManager._ensureValidToken(true);
+  Assert.equal(Status.login, LOGIN_SUCCEEDED, "final ensureLoggedIn worked");
+});
+
+add_task(async function test_syncState() {
+  // Avoid polling for an unverified user.
+  let identityConfig = makeIdentityConfig();
+  let fxaInternal = makeFxAccountsInternalMock(identityConfig);
+  fxaInternal.startVerifiedCheck = () => {};
+  configureFxAccountIdentity(
+    globalSyncAuthManager,
+    globalIdentityConfig,
+    fxaInternal
+  );
+
+  // arrange for no logged in user.
+  let fxa = globalSyncAuthManager._fxaService;
+  let signedInUser =
+    fxa._internal.currentAccountState.storageManager.accountData;
+  fxa._internal.currentAccountState.storageManager.accountData = null;
+  await Assert.rejects(
+    globalSyncAuthManager._ensureValidToken(true),
+    /no user is logged in/,
+    "expecting rejection due to no user"
+  );
+  // Restore to an unverified user.
+  Services.prefs.setStringPref("services.sync.username", signedInUser.email);
+  signedInUser.verified = false;
+  fxa._internal.currentAccountState.storageManager.accountData = signedInUser;
+  Status.login = LOGIN_FAILED_LOGIN_REJECTED;
+  // The sync_auth observers are async, so call them directly.
+  await globalSyncAuthManager.observe(null, ONLOGIN_NOTIFICATION, "");
+  Assert.equal(
+    Status.login,
+    LOGIN_FAILED_LOGIN_REJECTED,
+    "should not have changed the login state for an unverified user"
+  );
+
+  // now pretend the user because verified.
+  signedInUser.verified = true;
+  await globalSyncAuthManager.observe(null, ONVERIFIED_NOTIFICATION, "");
+  Assert.equal(
+    Status.login,
+    LOGIN_SUCCEEDED,
+    "should have changed the login state to success"
+  );
+});
+
+add_task(async function test_tokenExpiration() {
+  _("SyncAuthManager notices token expiration:");
+  let bimExp = new SyncAuthManager();
+  configureFxAccountIdentity(bimExp, globalIdentityConfig);
+
+  let authenticator = bimExp.getResourceAuthenticator();
+  Assert.ok(!!authenticator);
+  let req = {
+    uri: CommonUtils.makeURI("https://example.net/somewhere/over/the/rainbow"),
+    method: "GET",
+  };
+  await authenticator(req, "GET");
+
+  // Mock the clock.
+  _("Forcing the token to expire ...");
+  Object.defineProperty(bimExp, "_now", {
+    value: function customNow() {
+      return Date.now() + 3000001;
+    },
+    writable: true,
+  });
+  Assert.ok(bimExp._token.expiration < bimExp._now());
+  _("... means SyncAuthManager knows to re-fetch it on the next call.");
+  Assert.ok(!bimExp._hasValidToken());
+});
+
+add_task(async function test_getTokenErrors() {
+  _("SyncAuthManager correctly handles various failures to get a token.");
+
+  _("Arrange for a 401 - Sync should reflect an auth error.");
+  initializeIdentityWithTokenServerResponse({
+    status: 401,
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({}),
+  });
+  let syncAuthManager = Service.identity;
+
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    AuthenticationError,
+    "should reject due to 401"
+  );
+  Assert.equal(Status.login, LOGIN_FAILED_LOGIN_REJECTED, "login was rejected");
+
+  // XXX - other interesting responses to return?
+
+  // And for good measure, some totally "unexpected" errors - we generally
+  // assume these problems are going to magically go away at some point.
+  _(
+    "Arrange for an empty body with a 200 response - should reflect a network error."
+  );
+  initializeIdentityWithTokenServerResponse({
+    status: 200,
+    headers: [],
+    body: "",
+  });
+  syncAuthManager = Service.identity;
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    TokenServerClientServerError,
+    "should reject due to non-JSON response"
+  );
+  Assert.equal(
+    Status.login,
+    LOGIN_FAILED_NETWORK_ERROR,
+    "login state is LOGIN_FAILED_NETWORK_ERROR"
+  );
+});
+
+add_task(async function test_refreshAccessTokenOn401() {
+  _("SyncAuthManager refreshes the FXA OAuth access token after a 401.");
+  var identityConfig = makeIdentityConfig();
+  var syncAuthManager = new SyncAuthManager();
+  // Use the real `getOAuthToken` method that calls
+  // `mockFxAClient.accessTokenWithSessionToken`.
+  let fxaInternal = makeFxAccountsInternalMock(identityConfig);
+  delete fxaInternal.getOAuthToken;
+  configureFxAccountIdentity(syncAuthManager, identityConfig, fxaInternal);
+  syncAuthManager._fxaService._internal.initialize();
+
+  let getTokenCount = 0;
+
+  let CheckSignMockFxAClient = function () {
+    FxAccountsClient.apply(this);
+  };
+  CheckSignMockFxAClient.prototype = {
+    accessTokenWithSessionToken() {
+      ++getTokenCount;
+      return Promise.resolve({ access_token: "token" });
+    },
+  };
+  Object.setPrototypeOf(
+    CheckSignMockFxAClient.prototype,
+    FxAccountsClient.prototype
+  );
+
+  let mockFxAClient = new CheckSignMockFxAClient();
+  syncAuthManager._fxaService._internal._fxAccountsClient = mockFxAClient;
+
+  let didReturn401 = false;
+  let didReturn200 = false;
+  let mockTSC = mockTokenServer(() => {
+    if (getTokenCount <= 1) {
+      didReturn401 = true;
+      return {
+        status: 401,
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({}),
+      };
+    }
+    didReturn200 = true;
+    return {
+      status: 200,
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({
+        id: "id",
+        key: "key",
+        api_endpoint: "http://example.com/",
+        uid: "uid",
+        duration: 300,
+      }),
+    };
+  });
+
+  syncAuthManager._tokenServerClient = mockTSC;
+
+  await syncAuthManager._ensureValidToken();
+
+  Assert.equal(getTokenCount, 2);
+  Assert.ok(didReturn401);
+  Assert.ok(didReturn200);
+  Assert.ok(syncAuthManager._token);
+  Assert.ok(syncAuthManager._hasValidToken());
+});
+
+add_task(async function test_getTokenErrorWithRetry() {
+  _("tokenserver sends an observer notification on various backoff headers.");
+
+  // Set Sync's backoffInterval to zero - after we simulated the backoff header
+  // it should reflect the value we sent.
+  Status.backoffInterval = 0;
+  _("Arrange for a 503 with a Retry-After header.");
+  initializeIdentityWithTokenServerResponse({
+    status: 503,
+    headers: { "content-type": "application/json", "retry-after": "100" },
+    body: JSON.stringify({}),
+  });
+  let syncAuthManager = Service.identity;
+
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    TokenServerClientServerError,
+    "should reject due to 503"
+  );
+
+  // The observer should have fired - check it got the value in the response.
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR, "login was rejected");
+  // Sync will have the value in ms with some slop - so check it is at least that.
+  Assert.ok(Status.backoffInterval >= 100000);
+
+  _("Arrange for a 200 with an X-Backoff header.");
+  Status.backoffInterval = 0;
+  initializeIdentityWithTokenServerResponse({
+    status: 503,
+    headers: { "content-type": "application/json", "x-backoff": "200" },
+    body: JSON.stringify({}),
+  });
+  syncAuthManager = Service.identity;
+
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    TokenServerClientServerError,
+    "should reject due to no token in response"
+  );
+
+  // The observer should have fired - check it got the value in the response.
+  Assert.ok(Status.backoffInterval >= 200000);
+});
+
+add_task(async function test_getKeysErrorWithBackoff() {
+  _(
+    "Auth server (via hawk) sends an observer notification on backoff headers."
+  );
+
+  // Set Sync's backoffInterval to zero - after we simulated the backoff header
+  // it should reflect the value we sent.
+  Status.backoffInterval = 0;
+  _("Arrange for a 503 with a X-Backoff header.");
+
+  let config = makeIdentityConfig();
+  // We want no scopedKeys so we attempt to fetch them.
+  delete config.fxaccount.user.scopedKeys;
+  config.fxaccount.user.keyFetchToken = "keyfetchtoken";
+  await initializeIdentityWithHAWKResponseFactory(
+    config,
+    function (method, data, uri) {
+      Assert.equal(method, "get");
+      Assert.equal(uri, "http://mockedserver:9999/account/keys");
+      return {
+        status: 503,
+        headers: { "content-type": "application/json", "x-backoff": "100" },
+        body: "{}",
+      };
+    }
+  );
+
+  let syncAuthManager = Service.identity;
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    TokenServerClientServerError,
+    "should reject due to 503"
+  );
+
+  // The observer should have fired - check it got the value in the response.
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR, "login was rejected");
+  // Sync will have the value in ms with some slop - so check it is at least that.
+  Assert.ok(Status.backoffInterval >= 100000);
+});
+
+add_task(async function test_getKeysErrorWithRetry() {
+  _("Auth server (via hawk) sends an observer notification on retry headers.");
+
+  // Set Sync's backoffInterval to zero - after we simulated the backoff header
+  // it should reflect the value we sent.
+  Status.backoffInterval = 0;
+  _("Arrange for a 503 with a Retry-After header.");
+
+  let config = makeIdentityConfig();
+  // We want no scopedKeys so we attempt to fetch them.
+  delete config.fxaccount.user.scopedKeys;
+  config.fxaccount.user.keyFetchToken = "keyfetchtoken";
+  await initializeIdentityWithHAWKResponseFactory(
+    config,
+    function (method, data, uri) {
+      Assert.equal(method, "get");
+      Assert.equal(uri, "http://mockedserver:9999/account/keys");
+      return {
+        status: 503,
+        headers: { "content-type": "application/json", "retry-after": "100" },
+        body: "{}",
+      };
+    }
+  );
+
+  let syncAuthManager = Service.identity;
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    TokenServerClientServerError,
+    "should reject due to 503"
+  );
+
+  // The observer should have fired - check it got the value in the response.
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR, "login was rejected");
+  // Sync will have the value in ms with some slop - so check it is at least that.
+  Assert.ok(Status.backoffInterval >= 100000);
+});
+
+add_task(async function test_getHAWKErrors() {
+  _("SyncAuthManager correctly handles various HAWK failures.");
+
+  _("Arrange for a 401 - Sync should reflect an auth error.");
+  let config = makeIdentityConfig();
+  await initializeIdentityWithHAWKResponseFactory(
+    config,
+    function (method, data, uri) {
+      if (uri == "http://mockedserver:9999/oauth/token") {
+        Assert.equal(method, "post");
+        return {
+          status: 401,
+          headers: { "content-type": "application/json" },
+          body: JSON.stringify({
+            code: 401,
+            errno: 110,
+            error: "invalid token",
+          }),
+        };
+      }
+      // For any follow-up requests that check account status.
+      return {
+        status: 200,
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({}),
+      };
+    }
+  );
+  Assert.equal(Status.login, LOGIN_FAILED_LOGIN_REJECTED, "login was rejected");
+
+  // XXX - other interesting responses to return?
+
+  // And for good measure, some totally "unexpected" errors - we generally
+  // assume these problems are going to magically go away at some point.
+  _(
+    "Arrange for an empty body with a 200 response - should reflect a network error."
+  );
+  await initializeIdentityWithHAWKResponseFactory(
+    config,
+    function (method, data, uri) {
+      Assert.equal(method, "post");
+      Assert.equal(uri, "http://mockedserver:9999/oauth/token");
+      return {
+        status: 200,
+        headers: [],
+        body: "",
+      };
+    }
+  );
+  Assert.equal(
+    Status.login,
+    LOGIN_FAILED_NETWORK_ERROR,
+    "login state is LOGIN_FAILED_NETWORK_ERROR"
+  );
+});
+
+add_task(async function test_getGetKeysFailing401() {
+  _("SyncAuthManager correctly handles 401 responses fetching keys.");
+
+  _("Arrange for a 401 - Sync should reflect an auth error.");
+  let config = makeIdentityConfig();
+  // We want no scopedKeys so we attempt to fetch them.
+  delete config.fxaccount.user.scopedKeys;
+  config.fxaccount.user.keyFetchToken = "keyfetchtoken";
+  await initializeIdentityWithHAWKResponseFactory(
+    config,
+    function (method, data, uri) {
+      Assert.equal(method, "get");
+      Assert.equal(uri, "http://mockedserver:9999/account/keys");
+      return {
+        status: 401,
+        headers: { "content-type": "application/json" },
+        body: "{}",
+      };
+    }
+  );
+  Assert.equal(Status.login, LOGIN_FAILED_LOGIN_REJECTED, "login was rejected");
+});
+
+add_task(async function test_getGetKeysFailing503() {
+  _("SyncAuthManager correctly handles 5XX responses fetching keys.");
+
+  _("Arrange for a 503 - Sync should reflect a network error.");
+  let config = makeIdentityConfig();
+  // We want no scopedKeys so we attempt to fetch them.
+  delete config.fxaccount.user.scopedKeys;
+  config.fxaccount.user.keyFetchToken = "keyfetchtoken";
+  await initializeIdentityWithHAWKResponseFactory(
+    config,
+    function (method, data, uri) {
+      Assert.equal(method, "get");
+      Assert.equal(uri, "http://mockedserver:9999/account/keys");
+      return {
+        status: 503,
+        headers: { "content-type": "application/json" },
+        body: "{}",
+      };
+    }
+  );
+  Assert.equal(
+    Status.login,
+    LOGIN_FAILED_NETWORK_ERROR,
+    "state reflects network error"
+  );
+});
+
+add_task(async function test_getKeysMissing() {
+  _(
+    "SyncAuthManager correctly handles getKeyForScope succeeding but not returning the key."
+  );
+
+  let syncAuthManager = new SyncAuthManager();
+  let identityConfig = makeIdentityConfig();
+  // our mock identity config already has scopedKeys remove them or we never
+  // try and fetch them.
+  delete identityConfig.fxaccount.user.scopedKeys;
+  identityConfig.fxaccount.user.keyFetchToken = "keyFetchToken";
+
+  configureFxAccountIdentity(syncAuthManager, identityConfig);
+
+  // Mock a fxAccounts object
+  let fxa = new FxAccounts({
+    fxAccountsClient: new MockFxAccountsClient(),
+    newAccountState(credentials) {
+      // We only expect this to be called with null indicating the (mock)
+      // storage should be read.
+      if (credentials) {
+        throw new Error("Not expecting to have credentials passed");
+      }
+      let storageManager = new MockFxaStorageManager();
+      storageManager.initialize(identityConfig.fxaccount.user);
+      return new AccountState(storageManager);
+    },
+    // And the keys object with a mock that returns no keys.
+    keys: {
+      getKeyForScope() {
+        return Promise.resolve(null);
+      },
+    },
+  });
+
+  syncAuthManager._fxaService = fxa;
+
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    /browser does not have the sync key, cannot sync/
+  );
+});
+
+add_task(async function test_getKeysUnexpecedError() {
+  _(
+    "SyncAuthManager correctly handles getKeyForScope throwing an unexpected error."
+  );
+
+  let syncAuthManager = new SyncAuthManager();
+  let identityConfig = makeIdentityConfig();
+  // our mock identity config already has scopedKeys - remove them or we never
+  // try and fetch them.
+  delete identityConfig.fxaccount.user.scopedKeys;
+  identityConfig.fxaccount.user.keyFetchToken = "keyFetchToken";
+
+  configureFxAccountIdentity(syncAuthManager, identityConfig);
+
+  // Mock a fxAccounts object
+  let fxa = new FxAccounts({
+    fxAccountsClient: new MockFxAccountsClient(),
+    newAccountState(credentials) {
+      // We only expect this to be called with null indicating the (mock)
+      // storage should be read.
+      if (credentials) {
+        throw new Error("Not expecting to have credentials passed");
+      }
+      let storageManager = new MockFxaStorageManager();
+      storageManager.initialize(identityConfig.fxaccount.user);
+      return new AccountState(storageManager);
+    },
+    // And the keys object with a mock that returns no keys.
+    keys: {
+      async getKeyForScope() {
+        throw new Error("well that was unexpected");
+      },
+    },
+  });
+
+  syncAuthManager._fxaService = fxa;
+
+  await Assert.rejects(
+    syncAuthManager._ensureValidToken(),
+    /well that was unexpected/
+  );
+});
+
+add_task(async function test_signedInUserMissing() {
+  _(
+    "SyncAuthManager detects getSignedInUser returning incomplete account data"
+  );
+
+  let syncAuthManager = new SyncAuthManager();
+  // Delete stored keys and the key fetch token.
+  delete globalIdentityConfig.fxaccount.user.scopedKeys;
+  delete globalIdentityConfig.fxaccount.user.keyFetchToken;
+
+  configureFxAccountIdentity(syncAuthManager, globalIdentityConfig);
+
+  let fxa = new FxAccounts({
+    fetchAndUnwrapKeys() {
+      return Promise.resolve({});
+    },
+    fxAccountsClient: new MockFxAccountsClient(),
+    newAccountState(credentials) {
+      // We only expect this to be called with null indicating the (mock)
+      // storage should be read.
+      if (credentials) {
+        throw new Error("Not expecting to have credentials passed");
+      }
+      let storageManager = new MockFxaStorageManager();
+      storageManager.initialize(globalIdentityConfig.fxaccount.user);
+      return new AccountState(storageManager);
+    },
+  });
+
+  syncAuthManager._fxaService = fxa;
+
+  let status = await syncAuthManager.unlockAndVerifyAuthState();
+  Assert.equal(status, LOGIN_FAILED_LOGIN_REJECTED);
+});
+
+// End of tests
+// Utility functions follow
+
+// Create a new sync_auth object and initialize it with a
+// hawk mock that simulates HTTP responses.
+// The callback function will be called each time the mocked hawk server wants
+// to make a request.  The result of the callback should be the mock response
+// object that will be returned to hawk.
+// A token server mock will be used that doesn't hit a server, so we move
+// directly to a hawk request.
+async function initializeIdentityWithHAWKResponseFactory(
+  config,
+  cbGetResponse
+) {
+  // A mock request object.
+  function MockRESTRequest(uri, credentials, extra) {
+    this._uri = uri;
+    this._credentials = credentials;
+    this._extra = extra;
+  }
+  MockRESTRequest.prototype = {
+    setHeader() {},
+    async post(data) {
+      this.response = cbGetResponse(
+        "post",
+        data,
+        this._uri,
+        this._credentials,
+        this._extra
+      );
+      return this.response;
+    },
+    async get() {
+      // Skip /status requests (sync_auth checks if the account still
+      // exists after an auth error)
+      if (this._uri.startsWith("http://mockedserver:9999/account/status")) {
+        this.response = {
+          status: 200,
+          headers: { "content-type": "application/json" },
+          body: JSON.stringify({ exists: true }),
+        };
+      } else {
+        this.response = cbGetResponse(
+          "get",
+          null,
+          this._uri,
+          this._credentials,
+          this._extra
+        );
+      }
+      return this.response;
+    },
+  };
+
+  // The hawk client.
+  function MockedHawkClient() {}
+  MockedHawkClient.prototype = new HawkClient("http://mockedserver:9999");
+  MockedHawkClient.prototype.constructor = MockedHawkClient;
+  MockedHawkClient.prototype.newHAWKAuthenticatedRESTRequest = function (
+    uri,
+    credentials,
+    extra
+  ) {
+    return new MockRESTRequest(uri, credentials, extra);
+  };
+  // Arrange for the same observerPrefix as FxAccountsClient uses
+  MockedHawkClient.prototype.observerPrefix = "FxA:hawk";
+
+  // tie it all together - configureFxAccountIdentity isn't useful here :(
+  let fxaClient = new MockFxAccountsClient();
+  fxaClient.hawk = new MockedHawkClient();
+  let internal = {
+    fxAccountsClient: fxaClient,
+    newAccountState(credentials) {
+      // We only expect this to be called with null indicating the (mock)
+      // storage should be read.
+      if (credentials) {
+        throw new Error("Not expecting to have credentials passed");
+      }
+      let storageManager = new MockFxaStorageManager();
+      storageManager.initialize(config.fxaccount.user);
+      return new AccountState(storageManager);
+    },
+  };
+  let fxa = new FxAccounts(internal);
+
+  globalSyncAuthManager._fxaService = fxa;
+  await Assert.rejects(
+    globalSyncAuthManager._ensureValidToken(true),
+    // TODO: Ideally this should have a specific check for an error.
+    () => true,
+    "expecting rejection due to hawk error"
+  );
+}
+
+function getTimestamp(hawkAuthHeader) {
+  return parseInt(/ts="(\d+)"/.exec(hawkAuthHeader)[1], 10) * SECOND_MS;
+}
+
+function getTimestampDelta(hawkAuthHeader, now = Date.now()) {
+  return Math.abs(getTimestamp(hawkAuthHeader) - now);
+}
+
+function mockTokenServer(func) {
+  let requestLog = Log.repository.getLogger("testing.mock-rest");
+  if (!requestLog.appenders.length) {
+    // might as well see what it says :)
+    requestLog.addAppender(new Log.DumpAppender());
+    requestLog.level = Log.Level.Trace;
+  }
+  function MockRESTRequest(url) {}
+  MockRESTRequest.prototype = {
+    _log: requestLog,
+    setHeader() {},
+    async get() {
+      this.response = func();
+      return this.response;
+    },
+  };
+  // The mocked TokenServer client which will get the response.
+  function MockTSC() {}
+  MockTSC.prototype = new TokenServerClient();
+  MockTSC.prototype.constructor = MockTSC;
+  MockTSC.prototype.newRESTRequest = function (url) {
+    return new MockRESTRequest(url);
+  };
+  // Arrange for the same observerPrefix as sync_auth uses.
+  MockTSC.prototype.observerPrefix = "weave:service";
+  return new MockTSC();
+}
diff --git a/services/sync/tests/unit/test_syncedtabs.js b/services/sync/tests/unit/test_syncedtabs.js
new file mode 100644
index 0000000000..79ab3e0686
--- /dev/null
+++ b/services/sync/tests/unit/test_syncedtabs.js
@@ -0,0 +1,342 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
+ * vim:set ts=2 sw=2 sts=2 et:
+ */
+"use strict";
+
+const { Weave } = ChromeUtils.importESModule(
+  "resource://services-sync/main.sys.mjs"
+);
+const { SyncedTabs } = ChromeUtils.importESModule(
+  "resource://services-sync/SyncedTabs.sys.mjs"
+);
+
+Log.repository.getLogger("Sync.RemoteTabs").addAppender(new Log.DumpAppender());
+
+// A mock "Tabs" engine which the SyncedTabs module will use instead of the real
+// engine. We pass a constructor that Sync creates.
+function MockTabsEngine() {
+  this.clients = {}; // We'll set this dynamically
+  // Mock fxAccounts + recentDeviceList as if we hit the FxA server
+  this.fxAccounts = {
+    device: {
+      recentDeviceList: [
+        {
+          id: 1,
+          name: "updated desktop name",
+          availableCommands: {
+            "https://identity.mozilla.com/cmd/open-uri": "baz",
+          },
+        },
+        {
+          id: 2,
+          name: "updated mobile name",
+          availableCommands: {
+            "https://identity.mozilla.com/cmd/open-uri": "boo",
+          },
+        },
+      ],
+    },
+  };
+}
+
+MockTabsEngine.prototype = {
+  name: "tabs",
+  enabled: true,
+
+  getAllClients() {
+    return Object.values(this.clients);
+  },
+
+  getOpenURLs() {
+    return new Set();
+  },
+};
+
+let tabsEngine;
+
+// A clients engine that doesn't need to be a constructor.
+let MockClientsEngine = {
+  clientSettings: null, // Set in `configureClients`.
+
+  isMobile(guid) {
+    if (!guid.endsWith("desktop") && !guid.endsWith("mobile")) {
+      throw new Error(
+        "this module expected guids to end with 'desktop' or 'mobile'"
+      );
+    }
+    return guid.endsWith("mobile");
+  },
+  remoteClientExists(id) {
+    return this.clientSettings[id] !== false;
+  },
+  getClientName(id) {
+    if (this.clientSettings[id]) {
+      return this.clientSettings[id];
+    }
+    let client = tabsEngine.clients[id];
+    let fxaDevice = tabsEngine.fxAccounts.device.recentDeviceList.find(
+      device => device.id === client.fxaDeviceId
+    );
+    return fxaDevice ? fxaDevice.name : client.clientName;
+  },
+
+  getClientFxaDeviceId(id) {
+    if (this.clientSettings[id]) {
+      return this.clientSettings[id];
+    }
+    return tabsEngine.clients[id].fxaDeviceId;
+  },
+
+  getClientType(id) {
+    return "desktop";
+  },
+};
+
+function configureClients(clients, clientSettings = {}) {
+  // each client record is expected to have an id.
+  for (let [guid, client] of Object.entries(clients)) {
+    client.id = guid;
+  }
+  tabsEngine.clients = clients;
+  // Apply clients collection overrides.
+  MockClientsEngine.clientSettings = clientSettings;
+  // Send an observer that pretends the engine just finished a sync.
+  Services.obs.notifyObservers(null, "weave:engine:sync:finish", "tabs");
+}
+
+add_task(async function setup() {
+  await Weave.Service.promiseInitialized;
+  // Configure Sync with our mock tabs engine and force it to become initialized.
+  await Weave.Service.engineManager.unregister("tabs");
+  await Weave.Service.engineManager.register(MockTabsEngine);
+  Weave.Service.clientsEngine = MockClientsEngine;
+  tabsEngine = Weave.Service.engineManager.get("tabs");
+
+  // Tell the Sync XPCOM service it is initialized.
+  let weaveXPCService = Cc["@mozilla.org/weave/service;1"].getService(
+    Ci.nsISupports
+  ).wrappedJSObject;
+  weaveXPCService.ready = true;
+});
+
+// The tests.
+add_task(async function test_noClients() {
+  // no clients, can't be tabs.
+  await configureClients({});
+
+  let tabs = await SyncedTabs.getTabClients();
+  equal(Object.keys(tabs).length, 0);
+});
+
+add_task(async function test_clientWithTabs() {
+  await configureClients({
+    guid_desktop: {
+      clientName: "My Desktop",
+      tabs: [
+        {
+          urlHistory: ["http://foo.com/"],
+          icon: "http://foo.com/favicon",
+          lastUsed: 1655745700, // Mon, 20 Jun 2022 17:21:40 GMT
+        },
+      ],
+    },
+    guid_mobile: {
+      clientName: "My Phone",
+      tabs: [],
+    },
+  });
+
+  let clients = await SyncedTabs.getTabClients();
+  equal(clients.length, 2);
+  clients.sort((a, b) => {
+    return a.name.localeCompare(b.name);
+  });
+  equal(clients[0].tabs.length, 1);
+  equal(clients[0].tabs[0].url, "http://foo.com/");
+  equal(clients[0].tabs[0].icon, "http://foo.com/favicon");
+  equal(clients[0].tabs[0].lastUsed, 1655745700);
+  // second client has no tabs.
+  equal(clients[1].tabs.length, 0);
+});
+
+add_task(async function test_staleClientWithTabs() {
+  await configureClients(
+    {
+      guid_desktop: {
+        clientName: "My Desktop",
+        tabs: [
+          {
+            urlHistory: ["http://foo.com/"],
+            icon: "http://foo.com/favicon",
+            lastUsed: 1655745750,
+          },
+        ],
+      },
+      guid_mobile: {
+        clientName: "My Phone",
+        tabs: [],
+      },
+      guid_stale_mobile: {
+        clientName: "My Deleted Phone",
+        tabs: [],
+      },
+      guid_stale_desktop: {
+        clientName: "My Deleted Laptop",
+        tabs: [
+          {
+            urlHistory: ["https://bar.com/"],
+            icon: "https://bar.com/favicon",
+            lastUsed: 1655745700,
+          },
+        ],
+      },
+      guid_stale_name_desktop: {
+        clientName: "My Generic Device",
+        tabs: [
+          {
+            urlHistory: ["https://example.edu/"],
+            icon: "https://example.edu/favicon",
+            lastUsed: 1655745800,
+          },
+        ],
+      },
+    },
+    {
+      guid_stale_mobile: false,
+      guid_stale_desktop: false,
+      // We should always use the device name from the clients collection, instead
+      // of the possibly stale tabs collection.
+      guid_stale_name_desktop: "My Laptop",
+    }
+  );
+  let clients = await SyncedTabs.getTabClients();
+  clients.sort((a, b) => {
+    return a.name.localeCompare(b.name);
+  });
+  equal(clients.length, 3);
+  equal(clients[0].name, "My Desktop");
+  equal(clients[0].tabs.length, 1);
+  equal(clients[0].tabs[0].url, "http://foo.com/");
+  equal(clients[0].tabs[0].lastUsed, 1655745750);
+  equal(clients[1].name, "My Laptop");
+  equal(clients[1].tabs.length, 1);
+  equal(clients[1].tabs[0].url, "https://example.edu/");
+  equal(clients[1].tabs[0].lastUsed, 1655745800);
+  equal(clients[2].name, "My Phone");
+  equal(clients[2].tabs.length, 0);
+});
+
+add_task(async function test_clientWithTabsIconsDisabled() {
+  Services.prefs.setBoolPref("services.sync.syncedTabs.showRemoteIcons", false);
+  await configureClients({
+    guid_desktop: {
+      clientName: "My Desktop",
+      tabs: [
+        {
+          urlHistory: ["http://foo.com/"],
+          icon: "http://foo.com/favicon",
+        },
+      ],
+    },
+  });
+
+  let clients = await SyncedTabs.getTabClients();
+  equal(clients.length, 1);
+  clients.sort((a, b) => {
+    return a.name.localeCompare(b.name);
+  });
+  equal(clients[0].tabs.length, 1);
+  equal(clients[0].tabs[0].url, "http://foo.com/");
+  // Expect the default favicon due to the pref being false.
+  equal(clients[0].tabs[0].icon, "page-icon:http://foo.com/");
+  Services.prefs.clearUserPref("services.sync.syncedTabs.showRemoteIcons");
+});
+
+add_task(async function test_filter() {
+  // Nothing matches.
+  await configureClients({
+    guid_desktop: {
+      clientName: "My Desktop",
+      tabs: [
+        {
+          urlHistory: ["http://foo.com/"],
+          title: "A test page.",
+        },
+        {
+          urlHistory: ["http://bar.com/"],
+          title: "Another page.",
+        },
+      ],
+    },
+  });
+
+  let clients = await SyncedTabs.getTabClients("foo");
+  equal(clients.length, 1);
+  equal(clients[0].tabs.length, 1);
+  equal(clients[0].tabs[0].url, "http://foo.com/");
+  // check it matches the title.
+  clients = await SyncedTabs.getTabClients("test");
+  equal(clients.length, 1);
+  equal(clients[0].tabs.length, 1);
+  equal(clients[0].tabs[0].url, "http://foo.com/");
+});
+
+add_task(async function test_duplicatesTabsAcrossClients() {
+  await configureClients({
+    guid_desktop: {
+      clientName: "My Desktop",
+      tabs: [
+        {
+          urlHistory: ["http://foo.com/"],
+          title: "A test page.",
+        },
+      ],
+    },
+    guid_mobile: {
+      clientName: "My Phone",
+      tabs: [
+        {
+          urlHistory: ["http://foo.com/"],
+          title: "A test page.",
+        },
+      ],
+    },
+  });
+
+  let clients = await SyncedTabs.getTabClients();
+  equal(clients.length, 2);
+  equal(clients[0].tabs.length, 1);
+  equal(clients[1].tabs.length, 1);
+  equal(clients[0].tabs[0].url, "http://foo.com/");
+  equal(clients[1].tabs[0].url, "http://foo.com/");
+});
+
+add_task(async function test_clientsTabUpdatedName() {
+  // See the "fxAccounts" object in the MockEngine above for the device list
+  await configureClients({
+    guid_desktop: {
+      clientName: "My Desktop",
+      tabs: [
+        {
+          urlHistory: ["http://foo.com/"],
+          icon: "http://foo.com/favicon",
+        },
+      ],
+      fxaDeviceId: 1,
+    },
+    guid_mobile: {
+      clientName: "My Phone",
+      tabs: [
+        {
+          urlHistory: ["http://bar.com/"],
+          icon: "http://bar.com/favicon",
+        },
+      ],
+      fxaDeviceId: 2,
+    },
+  });
+  let clients = await SyncedTabs.getTabClients();
+  equal(clients.length, 2);
+  equal(clients[0].name, "updated desktop name");
+  equal(clients[1].name, "updated mobile name");
+});
diff --git a/services/sync/tests/unit/test_syncengine.js b/services/sync/tests/unit/test_syncengine.js
new file mode 100644
index 0000000000..50995a4e40
--- /dev/null
+++ b/services/sync/tests/unit/test_syncengine.js
@@ -0,0 +1,302 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+async function makeSteamEngine() {
+  let engine = new SyncEngine("Steam", Service);
+  await engine.initialize();
+  return engine;
+}
+
+function guidSetOfSize(length) {
+  return new SerializableSet(Array.from({ length }, () => Utils.makeGUID()));
+}
+
+function assertSetsEqual(a, b) {
+  // Assert.deepEqual doesn't understand Set.
+  Assert.deepEqual(Array.from(a).sort(), Array.from(b).sort());
+}
+
+async function testSteamEngineStorage(test) {
+  try {
+    let setupEngine = await makeSteamEngine();
+
+    if (test.setup) {
+      await test.setup(setupEngine);
+    }
+
+    // Finalize the engine to flush the backlog and previous failed to disk.
+    await setupEngine.finalize();
+
+    if (test.beforeCheck) {
+      await test.beforeCheck();
+    }
+
+    let checkEngine = await makeSteamEngine();
+    await test.check(checkEngine);
+
+    await checkEngine.resetClient();
+    await checkEngine.finalize();
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+}
+
+let server;
+
+add_task(async function setup() {
+  server = httpd_setup({});
+});
+
+add_task(async function test_url_attributes() {
+  _("SyncEngine url attributes");
+  await SyncTestingInfrastructure(server);
+  Service.clusterURL = "https://cluster/1.1/foo/";
+  let engine = await makeSteamEngine();
+  try {
+    Assert.equal(engine.storageURL, "https://cluster/1.1/foo/storage/");
+    Assert.equal(engine.engineURL, "https://cluster/1.1/foo/storage/steam");
+    Assert.equal(engine.metaURL, "https://cluster/1.1/foo/storage/meta/global");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_syncID() {
+  _("SyncEngine.syncID corresponds to preference");
+  await SyncTestingInfrastructure(server);
+  let engine = await makeSteamEngine();
+  try {
+    // Ensure pristine environment
+    Assert.equal(
+      Svc.PrefBranch.getPrefType("steam.syncID"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.equal(await engine.getSyncID(), "");
+
+    // Performing the first get on the attribute will generate a new GUID.
+    Assert.equal(await engine.resetLocalSyncID(), "fake-guid-00");
+    Assert.equal(Svc.PrefBranch.getStringPref("steam.syncID"), "fake-guid-00");
+
+    Svc.PrefBranch.setStringPref("steam.syncID", Utils.makeGUID());
+    Assert.equal(Svc.PrefBranch.getStringPref("steam.syncID"), "fake-guid-01");
+    Assert.equal(await engine.getSyncID(), "fake-guid-01");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_lastSync() {
+  _("SyncEngine.lastSync corresponds to preferences");
+  await SyncTestingInfrastructure(server);
+  let engine = await makeSteamEngine();
+  try {
+    // Ensure pristine environment
+    Assert.equal(
+      Svc.PrefBranch.getPrefType("steam.lastSync"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.equal(await engine.getLastSync(), 0);
+
+    // Floats are properly stored as floats and synced with the preference
+    await engine.setLastSync(123.45);
+    Assert.equal(await engine.getLastSync(), 123.45);
+    Assert.equal(Svc.PrefBranch.getStringPref("steam.lastSync"), "123.45");
+
+    // Integer is properly stored
+    await engine.setLastSync(67890);
+    Assert.equal(await engine.getLastSync(), 67890);
+    Assert.equal(Svc.PrefBranch.getStringPref("steam.lastSync"), "67890");
+
+    // resetLastSync() resets the value (and preference) to 0
+    await engine.resetLastSync();
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(Svc.PrefBranch.getStringPref("steam.lastSync"), "0");
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_toFetch() {
+  _("SyncEngine.toFetch corresponds to file on disk");
+  await SyncTestingInfrastructure(server);
+
+  await testSteamEngineStorage({
+    toFetch: guidSetOfSize(3),
+    setup(engine) {
+      // Ensure pristine environment
+      Assert.equal(engine.toFetch.size, 0);
+
+      // Write file to disk
+      engine.toFetch = this.toFetch;
+      Assert.equal(engine.toFetch, this.toFetch);
+    },
+    check(engine) {
+      // toFetch is written asynchronously
+      assertSetsEqual(engine.toFetch, this.toFetch);
+    },
+  });
+
+  await testSteamEngineStorage({
+    toFetch: guidSetOfSize(4),
+    toFetch2: guidSetOfSize(5),
+    setup(engine) {
+      // Make sure it work for consecutive writes before the callback is executed.
+      engine.toFetch = this.toFetch;
+      Assert.equal(engine.toFetch, this.toFetch);
+
+      engine.toFetch = this.toFetch2;
+      Assert.equal(engine.toFetch, this.toFetch2);
+    },
+    check(engine) {
+      assertSetsEqual(engine.toFetch, this.toFetch2);
+    },
+  });
+
+  await testSteamEngineStorage({
+    toFetch: guidSetOfSize(2),
+    async beforeCheck() {
+      let toFetchPath = PathUtils.join(
+        PathUtils.profileDir,
+        "weave",
+        "toFetch",
+        "steam.json"
+      );
+      await IOUtils.writeJSON(toFetchPath, this.toFetch, {
+        tmpPath: toFetchPath + ".tmp",
+      });
+    },
+    check(engine) {
+      // Read file from disk
+      assertSetsEqual(engine.toFetch, this.toFetch);
+    },
+  });
+});
+
+add_task(async function test_previousFailed() {
+  _("SyncEngine.previousFailed corresponds to file on disk");
+  await SyncTestingInfrastructure(server);
+
+  await testSteamEngineStorage({
+    previousFailed: guidSetOfSize(3),
+    setup(engine) {
+      // Ensure pristine environment
+      Assert.equal(engine.previousFailed.size, 0);
+
+      // Write file to disk
+      engine.previousFailed = this.previousFailed;
+      Assert.equal(engine.previousFailed, this.previousFailed);
+    },
+    check(engine) {
+      // previousFailed is written asynchronously
+      assertSetsEqual(engine.previousFailed, this.previousFailed);
+    },
+  });
+
+  await testSteamEngineStorage({
+    previousFailed: guidSetOfSize(4),
+    previousFailed2: guidSetOfSize(5),
+    setup(engine) {
+      // Make sure it work for consecutive writes before the callback is executed.
+      engine.previousFailed = this.previousFailed;
+      Assert.equal(engine.previousFailed, this.previousFailed);
+
+      engine.previousFailed = this.previousFailed2;
+      Assert.equal(engine.previousFailed, this.previousFailed2);
+    },
+    check(engine) {
+      assertSetsEqual(engine.previousFailed, this.previousFailed2);
+    },
+  });
+
+  await testSteamEngineStorage({
+    previousFailed: guidSetOfSize(2),
+    async beforeCheck() {
+      let previousFailedPath = PathUtils.join(
+        PathUtils.profileDir,
+        "weave",
+        "failed",
+        "steam.json"
+      );
+      await IOUtils.writeJSON(previousFailedPath, this.previousFailed, {
+        tmpPath: previousFailedPath + ".tmp",
+      });
+    },
+    check(engine) {
+      // Read file from disk
+      assertSetsEqual(engine.previousFailed, this.previousFailed);
+    },
+  });
+});
+
+add_task(async function test_resetClient() {
+  _("SyncEngine.resetClient resets lastSync and toFetch");
+  await SyncTestingInfrastructure(server);
+  let engine = await makeSteamEngine();
+  try {
+    // Ensure pristine environment
+    Assert.equal(
+      Svc.PrefBranch.getPrefType("steam.lastSync"),
+      Ci.nsIPrefBranch.PREF_INVALID
+    );
+    Assert.equal(engine.toFetch.size, 0);
+
+    await engine.setLastSync(123.45);
+    engine.toFetch = guidSetOfSize(4);
+    engine.previousFailed = guidSetOfSize(3);
+
+    await engine.resetClient();
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(engine.toFetch.size, 0);
+    Assert.equal(engine.previousFailed.size, 0);
+  } finally {
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function test_wipeServer() {
+  _("SyncEngine.wipeServer deletes server data and resets the client.");
+  let engine = await makeSteamEngine();
+
+  const PAYLOAD = 42;
+  let steamCollection = new ServerWBO("steam", PAYLOAD);
+  let steamServer = httpd_setup({
+    "/1.1/foo/storage/steam": steamCollection.handler(),
+  });
+  await SyncTestingInfrastructure(steamServer);
+  do_test_pending();
+
+  try {
+    // Some data to reset.
+    await engine.setLastSync(123.45);
+    engine.toFetch = guidSetOfSize(3);
+
+    _("Wipe server data and reset client.");
+    await engine.wipeServer();
+    Assert.equal(steamCollection.payload, undefined);
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(engine.toFetch.size, 0);
+  } finally {
+    steamServer.stop(do_test_finished);
+    for (const pref of Svc.PrefBranch.getChildList("")) {
+      Svc.PrefBranch.clearUserPref(pref);
+    }
+  }
+});
+
+add_task(async function finish() {
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_syncengine_sync.js b/services/sync/tests/unit/test_syncengine_sync.js
new file mode 100644
index 0000000000..efd061d5bc
--- /dev/null
+++ b/services/sync/tests/unit/test_syncengine_sync.js
@@ -0,0 +1,1781 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Weave } = ChromeUtils.importESModule(
+  "resource://services-sync/main.sys.mjs"
+);
+const { WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { RotaryEngine } = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/rotaryengine.sys.mjs"
+);
+
+function makeRotaryEngine() {
+  return new RotaryEngine(Service);
+}
+
+async function clean(engine) {
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  Svc.PrefBranch.setStringPref("log.logger.engine.rotary", "Trace");
+  Service.recordManager.clearCache();
+  await engine._tracker.clearChangedIDs();
+  await engine.finalize();
+}
+
+async function cleanAndGo(engine, server) {
+  await clean(engine);
+  await promiseStopServer(server);
+}
+
+async function promiseClean(engine, server) {
+  await clean(engine);
+  await promiseStopServer(server);
+}
+
+async function createServerAndConfigureClient() {
+  let engine = new RotaryEngine(Service);
+  let syncID = await engine.resetLocalSyncID();
+
+  let contents = {
+    meta: {
+      global: { engines: { rotary: { version: engine.version, syncID } } },
+    },
+    crypto: {},
+    rotary: {},
+  };
+
+  const USER = "foo";
+  let server = new SyncServer();
+  server.registerUser(USER, "password");
+  server.createContents(USER, contents);
+  server.start();
+
+  await SyncTestingInfrastructure(server, USER);
+  Service._updateCachedURLs();
+
+  return [engine, server, USER];
+}
+
+/*
+ * Tests
+ *
+ * SyncEngine._sync() is divided into four rather independent steps:
+ *
+ * - _syncStartup()
+ * - _processIncoming()
+ * - _uploadOutgoing()
+ * - _syncFinish()
+ *
+ * In the spirit of unit testing, these are tested individually for
+ * different scenarios below.
+ */
+
+add_task(async function setup() {
+  await generateNewKeys(Service.collectionKeys);
+  Svc.PrefBranch.setStringPref("log.logger.engine.rotary", "Trace");
+});
+
+add_task(async function test_syncStartup_emptyOrOutdatedGlobalsResetsSync() {
+  _(
+    "SyncEngine._syncStartup resets sync and wipes server data if there's no or an outdated global record"
+  );
+
+  // Some server side data that's going to be wiped
+  let collection = new ServerCollection();
+  collection.insert(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+  collection.insert(
+    "scotsman",
+    encryptPayload({ id: "scotsman", denomination: "Flying Scotsman" })
+  );
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  engine._store.items = { rekolok: "Rekonstruktionslokomotive" };
+  try {
+    // Confirm initial environment
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.rekolok, undefined);
+    let metaGlobal = await Service.recordManager.get(engine.metaURL);
+    Assert.equal(metaGlobal.payload.engines, undefined);
+    Assert.ok(!!collection.payload("flying"));
+    Assert.ok(!!collection.payload("scotsman"));
+
+    await engine.setLastSync(Date.now() / 1000);
+
+    // Trying to prompt a wipe -- we no longer track CryptoMeta per engine,
+    // so it has nothing to check.
+    await engine._syncStartup();
+
+    // The meta/global WBO has been filled with data about the engine
+    let engineData = metaGlobal.payload.engines.rotary;
+    Assert.equal(engineData.version, engine.version);
+    Assert.equal(engineData.syncID, await engine.getSyncID());
+
+    // Sync was reset and server data was wiped
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(collection.payload("flying"), undefined);
+    Assert.equal(collection.payload("scotsman"), undefined);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_syncStartup_serverHasNewerVersion() {
+  _("SyncEngine._syncStartup ");
+
+  let global = new ServerWBO("global", {
+    engines: { rotary: { version: 23456 } },
+  });
+  let server = httpd_setup({
+    "/1.1/foo/storage/meta/global": global.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  try {
+    // The server has a newer version of the data and our engine can
+    // handle.  That should give us an exception.
+    let error;
+    try {
+      await engine._syncStartup();
+    } catch (ex) {
+      error = ex;
+    }
+    Assert.equal(error.failureCode, VERSION_OUT_OF_DATE);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_syncStartup_syncIDMismatchResetsClient() {
+  _("SyncEngine._syncStartup resets sync if syncIDs don't match");
+
+  let server = sync_httpd_setup({});
+
+  await SyncTestingInfrastructure(server);
+
+  // global record with a different syncID than our engine has
+  let engine = makeRotaryEngine();
+  let global = new ServerWBO("global", {
+    engines: { rotary: { version: engine.version, syncID: "foobar" } },
+  });
+  server.registerPathHandler("/1.1/foo/storage/meta/global", global.handler());
+
+  try {
+    // Confirm initial environment
+    Assert.equal(await engine.getSyncID(), "");
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.rekolok, undefined);
+
+    await engine.setLastSync(Date.now() / 1000);
+    await engine._syncStartup();
+
+    // The engine has assumed the server's syncID
+    Assert.equal(await engine.getSyncID(), "foobar");
+
+    // Sync was reset
+    Assert.equal(await engine.getLastSync(), 0);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_emptyServer() {
+  _("SyncEngine._processIncoming working with an empty server backend");
+
+  let collection = new ServerCollection();
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  try {
+    // Merely ensure that this code path is run without any errors
+    await engine._processIncoming();
+    Assert.equal(await engine.getLastSync(), 0);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_createFromServer() {
+  _("SyncEngine._processIncoming creates new records from server data");
+
+  // Some server records that will be downloaded
+  let collection = new ServerCollection();
+  collection.insert(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+  collection.insert(
+    "scotsman",
+    encryptPayload({ id: "scotsman", denomination: "Flying Scotsman" })
+  );
+
+  // Two pathological cases involving relative URIs gone wrong.
+  let pathologicalPayload = encryptPayload({
+    id: "../pathological",
+    denomination: "Pathological Case",
+  });
+  collection.insert("../pathological", pathologicalPayload);
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+    "/1.1/foo/storage/rotary/flying": collection.wbo("flying").handler(),
+    "/1.1/foo/storage/rotary/scotsman": collection.wbo("scotsman").handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  await generateNewKeys(Service.collectionKeys);
+
+  let engine = makeRotaryEngine();
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    // Confirm initial environment
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(engine.lastModified, null);
+    Assert.equal(engine._store.items.flying, undefined);
+    Assert.equal(engine._store.items.scotsman, undefined);
+    Assert.equal(engine._store.items["../pathological"], undefined);
+
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    // Timestamps of last sync and last server modification are set.
+    Assert.ok((await engine.getLastSync()) > 0);
+    Assert.ok(engine.lastModified > 0);
+
+    // Local records have been created from the server data.
+    Assert.equal(engine._store.items.flying, "LNER Class A3 4472");
+    Assert.equal(engine._store.items.scotsman, "Flying Scotsman");
+    Assert.equal(engine._store.items["../pathological"], "Pathological Case");
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_reconcile() {
+  _("SyncEngine._processIncoming updates local records");
+
+  let collection = new ServerCollection();
+
+  // This server record is newer than the corresponding client one,
+  // so it'll update its data.
+  collection.insert(
+    "newrecord",
+    encryptPayload({ id: "newrecord", denomination: "New stuff..." })
+  );
+
+  // This server record is newer than the corresponding client one,
+  // so it'll update its data.
+  collection.insert(
+    "newerserver",
+    encryptPayload({ id: "newerserver", denomination: "New data!" })
+  );
+
+  // This server record is 2 mins older than the client counterpart
+  // but identical to it, so we're expecting the client record's
+  // changedID to be reset.
+  collection.insert(
+    "olderidentical",
+    encryptPayload({
+      id: "olderidentical",
+      denomination: "Older but identical",
+    })
+  );
+  collection._wbos.olderidentical.modified -= 120;
+
+  // This item simply has different data than the corresponding client
+  // record (which is unmodified), so it will update the client as well
+  collection.insert(
+    "updateclient",
+    encryptPayload({ id: "updateclient", denomination: "Get this!" })
+  );
+
+  // This is a dupe of 'original'.
+  collection.insert(
+    "duplication",
+    encryptPayload({ id: "duplication", denomination: "Original Entry" })
+  );
+
+  // This record is marked as deleted, so we're expecting the client
+  // record to be removed.
+  collection.insert(
+    "nukeme",
+    encryptPayload({ id: "nukeme", denomination: "Nuke me!", deleted: true })
+  );
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  engine._store.items = {
+    newerserver: "New data, but not as new as server!",
+    olderidentical: "Older but identical",
+    updateclient: "Got data?",
+    original: "Original Entry",
+    long_original: "Long Original Entry",
+    nukeme: "Nuke me!",
+  };
+  // Make this record 1 min old, thus older than the one on the server
+  await engine._tracker.addChangedID("newerserver", Date.now() / 1000 - 60);
+  // This record has been changed 2 mins later than the one on the server
+  await engine._tracker.addChangedID("olderidentical", Date.now() / 1000);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    // Confirm initial environment
+    Assert.equal(engine._store.items.newrecord, undefined);
+    Assert.equal(
+      engine._store.items.newerserver,
+      "New data, but not as new as server!"
+    );
+    Assert.equal(engine._store.items.olderidentical, "Older but identical");
+    Assert.equal(engine._store.items.updateclient, "Got data?");
+    Assert.equal(engine._store.items.nukeme, "Nuke me!");
+    let changes = await engine._tracker.getChangedIDs();
+    Assert.ok(changes.olderidentical > 0);
+
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    // Timestamps of last sync and last server modification are set.
+    Assert.ok((await engine.getLastSync()) > 0);
+    Assert.ok(engine.lastModified > 0);
+
+    // The new record is created.
+    Assert.equal(engine._store.items.newrecord, "New stuff...");
+
+    // The 'newerserver' record is updated since the server data is newer.
+    Assert.equal(engine._store.items.newerserver, "New data!");
+
+    // The data for 'olderidentical' is identical on the server, so
+    // it's no longer marked as changed anymore.
+    Assert.equal(engine._store.items.olderidentical, "Older but identical");
+    changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.olderidentical, undefined);
+
+    // Updated with server data.
+    Assert.equal(engine._store.items.updateclient, "Get this!");
+
+    // The incoming ID is preferred.
+    Assert.equal(engine._store.items.original, undefined);
+    Assert.equal(engine._store.items.duplication, "Original Entry");
+    Assert.notEqual(engine._delete.ids.indexOf("original"), -1);
+
+    // The 'nukeme' record marked as deleted is removed.
+    Assert.equal(engine._store.items.nukeme, undefined);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_reconcile_local_deleted() {
+  _("Ensure local, duplicate ID is deleted on server.");
+
+  // When a duplicate is resolved, the local ID (which is never taken) should
+  // be deleted on the server.
+  let [engine, server, user] = await createServerAndConfigureClient();
+
+  let now = Date.now() / 1000 - 10;
+  await engine.setLastSync(now);
+  engine.lastModified = now + 1;
+
+  let record = encryptPayload({
+    id: "DUPE_INCOMING",
+    denomination: "incoming",
+  });
+  let wbo = new ServerWBO("DUPE_INCOMING", record, now + 2);
+  server.insertWBO(user, "rotary", wbo);
+
+  record = encryptPayload({ id: "DUPE_LOCAL", denomination: "local" });
+  wbo = new ServerWBO("DUPE_LOCAL", record, now - 1);
+  server.insertWBO(user, "rotary", wbo);
+
+  await engine._store.create({ id: "DUPE_LOCAL", denomination: "local" });
+  Assert.ok(await engine._store.itemExists("DUPE_LOCAL"));
+  Assert.equal("DUPE_LOCAL", await engine._findDupe({ id: "DUPE_INCOMING" }));
+
+  await engine._sync();
+
+  do_check_attribute_count(engine._store.items, 1);
+  Assert.ok("DUPE_INCOMING" in engine._store.items);
+
+  let collection = server.getCollection(user, "rotary");
+  Assert.equal(1, collection.count());
+  Assert.notEqual(undefined, collection.wbo("DUPE_INCOMING"));
+
+  await cleanAndGo(engine, server);
+});
+
+add_task(async function test_processIncoming_reconcile_equivalent() {
+  _("Ensure proper handling of incoming records that match local.");
+
+  let [engine, server, user] = await createServerAndConfigureClient();
+
+  let now = Date.now() / 1000 - 10;
+  await engine.setLastSync(now);
+  engine.lastModified = now + 1;
+
+  let record = encryptPayload({ id: "entry", denomination: "denomination" });
+  let wbo = new ServerWBO("entry", record, now + 2);
+  server.insertWBO(user, "rotary", wbo);
+
+  engine._store.items = { entry: "denomination" };
+  Assert.ok(await engine._store.itemExists("entry"));
+
+  await engine._sync();
+
+  do_check_attribute_count(engine._store.items, 1);
+
+  await cleanAndGo(engine, server);
+});
+
+add_task(
+  async function test_processIncoming_reconcile_locally_deleted_dupe_new() {
+    _(
+      "Ensure locally deleted duplicate record newer than incoming is handled."
+    );
+
+    // This is a somewhat complicated test. It ensures that if a client receives
+    // a modified record for an item that is deleted locally but with a different
+    // ID that the incoming record is ignored. This is a corner case for record
+    // handling, but it needs to be supported.
+    let [engine, server, user] = await createServerAndConfigureClient();
+
+    let now = Date.now() / 1000 - 10;
+    await engine.setLastSync(now);
+    engine.lastModified = now + 1;
+
+    let record = encryptPayload({
+      id: "DUPE_INCOMING",
+      denomination: "incoming",
+    });
+    let wbo = new ServerWBO("DUPE_INCOMING", record, now + 2);
+    server.insertWBO(user, "rotary", wbo);
+
+    // Simulate a locally-deleted item.
+    engine._store.items = {};
+    await engine._tracker.addChangedID("DUPE_LOCAL", now + 3);
+    Assert.equal(false, await engine._store.itemExists("DUPE_LOCAL"));
+    Assert.equal(false, await engine._store.itemExists("DUPE_INCOMING"));
+    Assert.equal("DUPE_LOCAL", await engine._findDupe({ id: "DUPE_INCOMING" }));
+
+    engine.lastModified = server.getCollection(user, engine.name).timestamp;
+    await engine._sync();
+
+    // After the sync, the server's payload for the original ID should be marked
+    // as deleted.
+    do_check_empty(engine._store.items);
+    let collection = server.getCollection(user, "rotary");
+    Assert.equal(1, collection.count());
+    wbo = collection.wbo("DUPE_INCOMING");
+    Assert.notEqual(null, wbo);
+    let payload = wbo.getCleartext();
+    Assert.ok(payload.deleted);
+
+    await cleanAndGo(engine, server);
+  }
+);
+
+add_task(
+  async function test_processIncoming_reconcile_locally_deleted_dupe_old() {
+    _(
+      "Ensure locally deleted duplicate record older than incoming is restored."
+    );
+
+    // This is similar to the above test except it tests the condition where the
+    // incoming record is newer than the local deletion, therefore overriding it.
+
+    let [engine, server, user] = await createServerAndConfigureClient();
+
+    let now = Date.now() / 1000 - 10;
+    await engine.setLastSync(now);
+    engine.lastModified = now + 1;
+
+    let record = encryptPayload({
+      id: "DUPE_INCOMING",
+      denomination: "incoming",
+    });
+    let wbo = new ServerWBO("DUPE_INCOMING", record, now + 2);
+    server.insertWBO(user, "rotary", wbo);
+
+    // Simulate a locally-deleted item.
+    engine._store.items = {};
+    await engine._tracker.addChangedID("DUPE_LOCAL", now + 1);
+    Assert.equal(false, await engine._store.itemExists("DUPE_LOCAL"));
+    Assert.equal(false, await engine._store.itemExists("DUPE_INCOMING"));
+    Assert.equal("DUPE_LOCAL", await engine._findDupe({ id: "DUPE_INCOMING" }));
+
+    await engine._sync();
+
+    // Since the remote change is newer, the incoming item should exist locally.
+    do_check_attribute_count(engine._store.items, 1);
+    Assert.ok("DUPE_INCOMING" in engine._store.items);
+    Assert.equal("incoming", engine._store.items.DUPE_INCOMING);
+
+    let collection = server.getCollection(user, "rotary");
+    Assert.equal(1, collection.count());
+    wbo = collection.wbo("DUPE_INCOMING");
+    let payload = wbo.getCleartext();
+    Assert.equal("incoming", payload.denomination);
+
+    await cleanAndGo(engine, server);
+  }
+);
+
+add_task(async function test_processIncoming_reconcile_changed_dupe() {
+  _("Ensure that locally changed duplicate record is handled properly.");
+
+  let [engine, server, user] = await createServerAndConfigureClient();
+
+  let now = Date.now() / 1000 - 10;
+  await engine.setLastSync(now);
+  engine.lastModified = now + 1;
+
+  // The local record is newer than the incoming one, so it should be retained.
+  let record = encryptPayload({
+    id: "DUPE_INCOMING",
+    denomination: "incoming",
+  });
+  let wbo = new ServerWBO("DUPE_INCOMING", record, now + 2);
+  server.insertWBO(user, "rotary", wbo);
+
+  await engine._store.create({ id: "DUPE_LOCAL", denomination: "local" });
+  await engine._tracker.addChangedID("DUPE_LOCAL", now + 3);
+  Assert.ok(await engine._store.itemExists("DUPE_LOCAL"));
+  Assert.equal("DUPE_LOCAL", await engine._findDupe({ id: "DUPE_INCOMING" }));
+
+  engine.lastModified = server.getCollection(user, engine.name).timestamp;
+  await engine._sync();
+
+  // The ID should have been changed to incoming.
+  do_check_attribute_count(engine._store.items, 1);
+  Assert.ok("DUPE_INCOMING" in engine._store.items);
+
+  // On the server, the local ID should be deleted and the incoming ID should
+  // have its payload set to what was in the local record.
+  let collection = server.getCollection(user, "rotary");
+  Assert.equal(1, collection.count());
+  wbo = collection.wbo("DUPE_INCOMING");
+  Assert.notEqual(undefined, wbo);
+  let payload = wbo.getCleartext();
+  Assert.equal("local", payload.denomination);
+
+  await cleanAndGo(engine, server);
+});
+
+add_task(async function test_processIncoming_reconcile_changed_dupe_new() {
+  _("Ensure locally changed duplicate record older than incoming is ignored.");
+
+  // This test is similar to the above except the incoming record is younger
+  // than the local record. The incoming record should be authoritative.
+  let [engine, server, user] = await createServerAndConfigureClient();
+
+  let now = Date.now() / 1000 - 10;
+  await engine.setLastSync(now);
+  engine.lastModified = now + 1;
+
+  let record = encryptPayload({
+    id: "DUPE_INCOMING",
+    denomination: "incoming",
+  });
+  let wbo = new ServerWBO("DUPE_INCOMING", record, now + 2);
+  server.insertWBO(user, "rotary", wbo);
+
+  await engine._store.create({ id: "DUPE_LOCAL", denomination: "local" });
+  await engine._tracker.addChangedID("DUPE_LOCAL", now + 1);
+  Assert.ok(await engine._store.itemExists("DUPE_LOCAL"));
+  Assert.equal("DUPE_LOCAL", await engine._findDupe({ id: "DUPE_INCOMING" }));
+
+  engine.lastModified = server.getCollection(user, engine.name).timestamp;
+  await engine._sync();
+
+  // The ID should have been changed to incoming.
+  do_check_attribute_count(engine._store.items, 1);
+  Assert.ok("DUPE_INCOMING" in engine._store.items);
+
+  // On the server, the local ID should be deleted and the incoming ID should
+  // have its payload retained.
+  let collection = server.getCollection(user, "rotary");
+  Assert.equal(1, collection.count());
+  wbo = collection.wbo("DUPE_INCOMING");
+  Assert.notEqual(undefined, wbo);
+  let payload = wbo.getCleartext();
+  Assert.equal("incoming", payload.denomination);
+  await cleanAndGo(engine, server);
+});
+
+add_task(async function test_processIncoming_resume_toFetch() {
+  _(
+    "toFetch and previousFailed items left over from previous syncs are fetched on the next sync, along with new items."
+  );
+
+  const LASTSYNC = Date.now() / 1000;
+
+  // Server records that will be downloaded
+  let collection = new ServerCollection();
+  collection.insert(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+  collection.insert(
+    "scotsman",
+    encryptPayload({ id: "scotsman", denomination: "Flying Scotsman" })
+  );
+  collection.insert(
+    "rekolok",
+    encryptPayload({ id: "rekolok", denomination: "Rekonstruktionslokomotive" })
+  );
+  for (let i = 0; i < 3; i++) {
+    let id = "failed" + i;
+    let payload = encryptPayload({ id, denomination: "Record No. " + i });
+    let wbo = new ServerWBO(id, payload);
+    wbo.modified = LASTSYNC - 10;
+    collection.insertWBO(wbo);
+  }
+
+  collection.wbo("flying").modified = collection.wbo("scotsman").modified =
+    LASTSYNC - 10;
+  collection._wbos.rekolok.modified = LASTSYNC + 10;
+
+  // Time travel 10 seconds into the future but still download the above WBOs.
+  let engine = makeRotaryEngine();
+  await engine.setLastSync(LASTSYNC);
+  engine.toFetch = new SerializableSet(["flying", "scotsman"]);
+  engine.previousFailed = new SerializableSet([
+    "failed0",
+    "failed1",
+    "failed2",
+  ]);
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+  try {
+    // Confirm initial environment
+    Assert.equal(engine._store.items.flying, undefined);
+    Assert.equal(engine._store.items.scotsman, undefined);
+    Assert.equal(engine._store.items.rekolok, undefined);
+
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    // Local records have been created from the server data.
+    Assert.equal(engine._store.items.flying, "LNER Class A3 4472");
+    Assert.equal(engine._store.items.scotsman, "Flying Scotsman");
+    Assert.equal(engine._store.items.rekolok, "Rekonstruktionslokomotive");
+    Assert.equal(engine._store.items.failed0, "Record No. 0");
+    Assert.equal(engine._store.items.failed1, "Record No. 1");
+    Assert.equal(engine._store.items.failed2, "Record No. 2");
+    Assert.equal(engine.previousFailed.size, 0);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_notify_count() {
+  _("Ensure that failed records are reported only once.");
+
+  const NUMBER_OF_RECORDS = 15;
+
+  // Engine that fails every 5 records.
+  let engine = makeRotaryEngine();
+  engine._store._applyIncomingBatch = engine._store.applyIncomingBatch;
+  engine._store.applyIncomingBatch = async function (records, countTelemetry) {
+    let sortedRecords = records.sort((a, b) => (a.id > b.id ? 1 : -1));
+    let recordsToApply = [],
+      recordsToFail = [];
+    for (let i = 0; i < sortedRecords.length; i++) {
+      (i % 5 === 0 ? recordsToFail : recordsToApply).push(sortedRecords[i]);
+    }
+    recordsToFail.forEach(() => {
+      countTelemetry.addIncomingFailedReason("failed message");
+    });
+    await engine._store._applyIncomingBatch(recordsToApply, countTelemetry);
+
+    return recordsToFail.map(record => record.id);
+  };
+
+  // Create a batch of server side records.
+  let collection = new ServerCollection();
+  for (var i = 0; i < NUMBER_OF_RECORDS; i++) {
+    let id = "record-no-" + i.toString(10).padStart(2, "0");
+    let payload = encryptPayload({ id, denomination: "Record No. " + id });
+    collection.insert(id, payload);
+  }
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+  try {
+    // Confirm initial environment.
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(engine.toFetch.size, 0);
+    Assert.equal(engine.previousFailed.size, 0);
+    do_check_empty(engine._store.items);
+
+    let called = 0;
+    let counts;
+    function onApplied(count) {
+      _("Called with " + JSON.stringify(counts));
+      counts = count;
+      called++;
+    }
+    Svc.Obs.add("weave:engine:sync:applied", onApplied);
+
+    // Do sync.
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    // Confirm failures.
+    do_check_attribute_count(engine._store.items, 12);
+    Assert.deepEqual(
+      Array.from(engine.previousFailed).sort(),
+      ["record-no-00", "record-no-05", "record-no-10"].sort()
+    );
+
+    // There are newly failed records and they are reported.
+    Assert.equal(called, 1);
+    Assert.equal(counts.failed, 3);
+    Assert.equal(counts.failedReasons[0].count, 3);
+    Assert.equal(counts.failedReasons[0].name, "failed message");
+    Assert.equal(counts.applied, 15);
+    Assert.equal(counts.newFailed, 3);
+    Assert.equal(counts.succeeded, 12);
+
+    // Sync again, 1 of the failed items are the same, the rest didn't fail.
+    await engine._processIncoming();
+
+    // Confirming removed failures.
+    do_check_attribute_count(engine._store.items, 14);
+    // After failing twice the record that failed again [record-no-00]
+    // should NOT be stored to try again
+    Assert.deepEqual(Array.from(engine.previousFailed), []);
+
+    Assert.equal(called, 2);
+    Assert.equal(counts.failed, 1);
+    Assert.equal(counts.failedReasons[0].count, 1);
+    Assert.equal(counts.failedReasons[0].name, "failed message");
+    Assert.equal(counts.applied, 3);
+    Assert.equal(counts.newFailed, 0);
+    Assert.equal(counts.succeeded, 2);
+
+    Svc.Obs.remove("weave:engine:sync:applied", onApplied);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_previousFailed() {
+  _("Ensure that failed records are retried.");
+
+  const NUMBER_OF_RECORDS = 14;
+
+  // Engine that alternates between failing and applying every 2 records.
+  let engine = makeRotaryEngine();
+  engine._store._applyIncomingBatch = engine._store.applyIncomingBatch;
+  engine._store.applyIncomingBatch = async function (records, countTelemetry) {
+    let sortedRecords = records.sort((a, b) => (a.id > b.id ? 1 : -1));
+    let recordsToApply = [],
+      recordsToFail = [];
+    let chunks = Array.from(PlacesUtils.chunkArray(sortedRecords, 2));
+    for (let i = 0; i < chunks.length; i++) {
+      (i % 2 === 0 ? recordsToFail : recordsToApply).push(...chunks[i]);
+    }
+    await engine._store._applyIncomingBatch(recordsToApply, countTelemetry);
+    return recordsToFail.map(record => record.id);
+  };
+
+  // Create a batch of server side records.
+  let collection = new ServerCollection();
+  for (var i = 0; i < NUMBER_OF_RECORDS; i++) {
+    let id = "record-no-" + i.toString(10).padStart(2, "0");
+    let payload = encryptPayload({ id, denomination: "Record No. " + i });
+    collection.insert(id, payload);
+  }
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+  try {
+    // Confirm initial environment.
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(engine.toFetch.size, 0);
+    Assert.equal(engine.previousFailed.size, 0);
+    do_check_empty(engine._store.items);
+
+    // Initial failed items in previousFailed to be reset.
+    let previousFailed = new SerializableSet([
+      Utils.makeGUID(),
+      Utils.makeGUID(),
+      Utils.makeGUID(),
+    ]);
+    engine.previousFailed = previousFailed;
+    Assert.equal(engine.previousFailed, previousFailed);
+
+    // Do sync.
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    // Expected result: 4 sync batches with 2 failures each => 8 failures
+    do_check_attribute_count(engine._store.items, 6);
+    Assert.deepEqual(
+      Array.from(engine.previousFailed).sort(),
+      [
+        "record-no-00",
+        "record-no-01",
+        "record-no-04",
+        "record-no-05",
+        "record-no-08",
+        "record-no-09",
+        "record-no-12",
+        "record-no-13",
+      ].sort()
+    );
+
+    // Sync again with the same failed items (records 0, 1, 8, 9).
+    await engine._processIncoming();
+
+    do_check_attribute_count(engine._store.items, 10);
+    // A second sync with the same failed items should NOT add the same items again.
+    // Items that did not fail a second time should no longer be in previousFailed.
+    Assert.deepEqual(Array.from(engine.previousFailed).sort(), []);
+
+    // Refetched items that didn't fail the second time are in engine._store.items.
+    Assert.equal(engine._store.items["record-no-04"], "Record No. 4");
+    Assert.equal(engine._store.items["record-no-05"], "Record No. 5");
+    Assert.equal(engine._store.items["record-no-12"], "Record No. 12");
+    Assert.equal(engine._store.items["record-no-13"], "Record No. 13");
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_failed_records() {
+  _(
+    "Ensure that failed records from _reconcile and applyIncomingBatch are refetched."
+  );
+
+  // Let's create three and a bit batches worth of server side records.
+  let APPLY_BATCH_SIZE = 50;
+  let collection = new ServerCollection();
+  const NUMBER_OF_RECORDS = APPLY_BATCH_SIZE * 3 + 5;
+  for (let i = 0; i < NUMBER_OF_RECORDS; i++) {
+    let id = "record-no-" + i;
+    let payload = encryptPayload({ id, denomination: "Record No. " + id });
+    let wbo = new ServerWBO(id, payload);
+    wbo.modified = Date.now() / 1000 + 60 * (i - APPLY_BATCH_SIZE * 3);
+    collection.insertWBO(wbo);
+  }
+
+  // Engine that batches but likes to throw on a couple of records,
+  // two in each batch: the even ones fail in reconcile, the odd ones
+  // in applyIncoming.
+  const BOGUS_RECORDS = [
+    "record-no-" + 42,
+    "record-no-" + 23,
+    "record-no-" + (42 + APPLY_BATCH_SIZE),
+    "record-no-" + (23 + APPLY_BATCH_SIZE),
+    "record-no-" + (42 + APPLY_BATCH_SIZE * 2),
+    "record-no-" + (23 + APPLY_BATCH_SIZE * 2),
+    "record-no-" + (2 + APPLY_BATCH_SIZE * 3),
+    "record-no-" + (1 + APPLY_BATCH_SIZE * 3),
+  ];
+  let engine = makeRotaryEngine();
+
+  engine.__reconcile = engine._reconcile;
+  engine._reconcile = async function _reconcile(record) {
+    if (BOGUS_RECORDS.indexOf(record.id) % 2 == 0) {
+      throw new Error("I don't like this record! Baaaaaah!");
+    }
+    return this.__reconcile.apply(this, arguments);
+  };
+  engine._store._applyIncoming = engine._store.applyIncoming;
+  engine._store.applyIncoming = async function (record) {
+    if (BOGUS_RECORDS.indexOf(record.id) % 2 == 1) {
+      throw new Error("I don't like this record! Baaaaaah!");
+    }
+    return this._applyIncoming.apply(this, arguments);
+  };
+
+  // Keep track of requests made of a collection.
+  let count = 0;
+  let uris = [];
+  function recording_handler(recordedCollection) {
+    let h = recordedCollection.handler();
+    return function (req, res) {
+      ++count;
+      uris.push(req.path + "?" + req.queryString);
+      return h(req, res);
+    };
+  }
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": recording_handler(collection),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    // Confirm initial environment
+    Assert.equal(await engine.getLastSync(), 0);
+    Assert.equal(engine.toFetch.size, 0);
+    Assert.equal(engine.previousFailed.size, 0);
+    do_check_empty(engine._store.items);
+
+    let observerSubject;
+    let observerData;
+    Svc.Obs.add("weave:engine:sync:applied", function onApplied(subject, data) {
+      Svc.Obs.remove("weave:engine:sync:applied", onApplied);
+      observerSubject = subject;
+      observerData = data;
+    });
+
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    // Ensure that all records but the bogus 4 have been applied.
+    do_check_attribute_count(
+      engine._store.items,
+      NUMBER_OF_RECORDS - BOGUS_RECORDS.length
+    );
+
+    // Ensure that the bogus records will be fetched again on the next sync.
+    Assert.equal(engine.previousFailed.size, BOGUS_RECORDS.length);
+    Assert.deepEqual(
+      Array.from(engine.previousFailed).sort(),
+      BOGUS_RECORDS.sort()
+    );
+
+    // Ensure the observer was notified
+    Assert.equal(observerData, engine.name);
+    Assert.equal(observerSubject.failed, BOGUS_RECORDS.length);
+    Assert.equal(observerSubject.newFailed, BOGUS_RECORDS.length);
+
+    // Testing batching of failed item fetches.
+    // Try to sync again. Ensure that we split the request into chunks to avoid
+    // URI length limitations.
+    async function batchDownload(batchSize) {
+      count = 0;
+      uris = [];
+      engine.guidFetchBatchSize = batchSize;
+      await engine._processIncoming();
+      _("Tried again. Requests: " + count + "; URIs: " + JSON.stringify(uris));
+      return count;
+    }
+
+    // There are 8 bad records, so this needs 3 fetches.
+    _("Test batching with ID batch size 3, normal mobile batch size.");
+    Assert.equal(await batchDownload(3), 3);
+
+    // Since there the previous batch failed again, there should be
+    // no more records to fetch
+    _("Test that the second time a record failed to sync, gets ignored");
+    Assert.equal(await batchDownload(BOGUS_RECORDS.length), 0);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_processIncoming_decrypt_failed() {
+  _("Ensure that records failing to decrypt are either replaced or refetched.");
+
+  // Some good and some bogus records. One doesn't contain valid JSON,
+  // the other will throw during decrypt.
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+  collection._wbos.nojson = new ServerWBO("nojson", "This is invalid JSON");
+  collection._wbos.nojson2 = new ServerWBO("nojson2", "This is invalid JSON");
+  collection._wbos.scotsman = new ServerWBO(
+    "scotsman",
+    encryptPayload({ id: "scotsman", denomination: "Flying Scotsman" })
+  );
+  collection._wbos.nodecrypt = new ServerWBO("nodecrypt", "Decrypt this!");
+  collection._wbos.nodecrypt2 = new ServerWBO("nodecrypt2", "Decrypt this!");
+
+  // Patch the fake crypto service to throw on the record above.
+  Weave.Crypto._decrypt = Weave.Crypto.decrypt;
+  Weave.Crypto.decrypt = function (ciphertext) {
+    if (ciphertext == "Decrypt this!") {
+      throw new Error(
+        "Derp! Cipher finalized failed. Im ur crypto destroyin ur recordz."
+      );
+    }
+    return this._decrypt.apply(this, arguments);
+  };
+
+  // Some broken records also exist locally.
+  let engine = makeRotaryEngine();
+  engine.enabled = true;
+  engine._store.items = { nojson: "Valid JSON", nodecrypt: "Valid ciphertext" };
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+  try {
+    // Confirm initial state
+    Assert.equal(engine.toFetch.size, 0);
+    Assert.equal(engine.previousFailed.size, 0);
+
+    let observerSubject;
+    let observerData;
+    Svc.Obs.add("weave:engine:sync:applied", function onApplied(subject, data) {
+      Svc.Obs.remove("weave:engine:sync:applied", onApplied);
+      observerSubject = subject;
+      observerData = data;
+    });
+
+    await engine.setLastSync(collection.wbo("nojson").modified - 1);
+    let ping = await sync_engine_and_validate_telem(engine, true);
+    Assert.equal(ping.engines[0].incoming.applied, 2);
+    Assert.equal(ping.engines[0].incoming.failed, 4);
+    console.log("incoming telem: ", ping.engines[0].incoming);
+    Assert.equal(
+      ping.engines[0].incoming.failedReasons[0].name,
+      "No ciphertext: nothing to decrypt?"
+    );
+    // There should be 4 of the same error
+    Assert.equal(ping.engines[0].incoming.failedReasons[0].count, 4);
+
+    Assert.equal(engine.previousFailed.size, 4);
+    Assert.ok(engine.previousFailed.has("nojson"));
+    Assert.ok(engine.previousFailed.has("nojson2"));
+    Assert.ok(engine.previousFailed.has("nodecrypt"));
+    Assert.ok(engine.previousFailed.has("nodecrypt2"));
+
+    // Ensure the observer was notified
+    Assert.equal(observerData, engine.name);
+    Assert.equal(observerSubject.applied, 2);
+    Assert.equal(observerSubject.failed, 4);
+    Assert.equal(observerSubject.failedReasons[0].count, 4);
+  } finally {
+    await promiseClean(engine, server);
+  }
+});
+
+add_task(async function test_uploadOutgoing_toEmptyServer() {
+  _("SyncEngine._uploadOutgoing uploads new records to server");
+
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO("flying");
+  collection._wbos.scotsman = new ServerWBO("scotsman");
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+    "/1.1/foo/storage/rotary/flying": collection.wbo("flying").handler(),
+    "/1.1/foo/storage/rotary/scotsman": collection.wbo("scotsman").handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let engine = makeRotaryEngine();
+  engine._store.items = {
+    flying: "LNER Class A3 4472",
+    scotsman: "Flying Scotsman",
+  };
+  // Mark one of these records as changed
+  await engine._tracker.addChangedID("scotsman", 0);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    await engine.setLastSync(123); // needs to be non-zero so that tracker is queried
+
+    // Confirm initial environment
+    Assert.equal(collection.payload("flying"), undefined);
+    Assert.equal(collection.payload("scotsman"), undefined);
+
+    await engine._syncStartup();
+    await engine._uploadOutgoing();
+
+    // Ensure the marked record ('scotsman') has been uploaded and is
+    // no longer marked.
+    Assert.equal(collection.payload("flying"), undefined);
+    Assert.ok(!!collection.payload("scotsman"));
+    Assert.equal(collection.cleartext("scotsman").id, "scotsman");
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.scotsman, undefined);
+
+    // The 'flying' record wasn't marked so it wasn't uploaded
+    Assert.equal(collection.payload("flying"), undefined);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+async function test_uploadOutgoing_max_record_payload_bytes(
+  allowSkippedRecord
+) {
+  _(
+    "SyncEngine._uploadOutgoing throws when payload is bigger than max_record_payload_bytes"
+  );
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO("flying");
+  collection._wbos.scotsman = new ServerWBO("scotsman");
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+    "/1.1/foo/storage/rotary/flying": collection.wbo("flying").handler(),
+    "/1.1/foo/storage/rotary/scotsman": collection.wbo("scotsman").handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let engine = makeRotaryEngine();
+  engine.allowSkippedRecord = allowSkippedRecord;
+  engine._store.items = { flying: "a".repeat(1024 * 1024), scotsman: "abcd" };
+
+  await engine._tracker.addChangedID("flying", 1000);
+  await engine._tracker.addChangedID("scotsman", 1000);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    await engine.setLastSync(1); // needs to be non-zero so that tracker is queried
+
+    // Confirm initial environment
+    Assert.equal(collection.payload("flying"), undefined);
+    Assert.equal(collection.payload("scotsman"), undefined);
+
+    await engine._syncStartup();
+    await engine._uploadOutgoing();
+
+    if (!allowSkippedRecord) {
+      do_throw("should not get here");
+    }
+
+    await engine.trackRemainingChanges();
+
+    // Check we uploaded the other record to the server
+    Assert.ok(collection.payload("scotsman"));
+    // And that we won't try to upload the huge record next time.
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.flying, undefined);
+  } catch (e) {
+    if (allowSkippedRecord) {
+      do_throw("should not get here");
+    }
+
+    await engine.trackRemainingChanges();
+
+    // Check that we will try to upload the huge record next time
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.flying, 1000);
+  } finally {
+    // Check we didn't upload the oversized record to the server
+    Assert.equal(collection.payload("flying"), undefined);
+    await cleanAndGo(engine, server);
+  }
+}
+
+add_task(
+  async function test_uploadOutgoing_max_record_payload_bytes_disallowSkippedRecords() {
+    return test_uploadOutgoing_max_record_payload_bytes(false);
+  }
+);
+
+add_task(
+  async function test_uploadOutgoing_max_record_payload_bytes_allowSkippedRecords() {
+    return test_uploadOutgoing_max_record_payload_bytes(true);
+  }
+);
+
+add_task(async function test_uploadOutgoing_failed() {
+  _(
+    "SyncEngine._uploadOutgoing doesn't clear the tracker of objects that failed to upload."
+  );
+
+  let collection = new ServerCollection();
+  // We only define the "flying" WBO on the server, not the "scotsman"
+  // and "peppercorn" ones.
+  collection._wbos.flying = new ServerWBO("flying");
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  engine._store.items = {
+    flying: "LNER Class A3 4472",
+    scotsman: "Flying Scotsman",
+    peppercorn: "Peppercorn Class",
+  };
+  // Mark these records as changed
+  const FLYING_CHANGED = 12345;
+  const SCOTSMAN_CHANGED = 23456;
+  const PEPPERCORN_CHANGED = 34567;
+  await engine._tracker.addChangedID("flying", FLYING_CHANGED);
+  await engine._tracker.addChangedID("scotsman", SCOTSMAN_CHANGED);
+  await engine._tracker.addChangedID("peppercorn", PEPPERCORN_CHANGED);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    await engine.setLastSync(123); // needs to be non-zero so that tracker is queried
+
+    // Confirm initial environment
+    Assert.equal(collection.payload("flying"), undefined);
+    let changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.flying, FLYING_CHANGED);
+    Assert.equal(changes.scotsman, SCOTSMAN_CHANGED);
+    Assert.equal(changes.peppercorn, PEPPERCORN_CHANGED);
+
+    engine.enabled = true;
+    await sync_engine_and_validate_telem(engine, true);
+
+    // Ensure the 'flying' record has been uploaded and is no longer marked.
+    Assert.ok(!!collection.payload("flying"));
+    changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.flying, undefined);
+
+    // The 'scotsman' and 'peppercorn' records couldn't be uploaded so
+    // they weren't cleared from the tracker.
+    Assert.equal(changes.scotsman, SCOTSMAN_CHANGED);
+    Assert.equal(changes.peppercorn, PEPPERCORN_CHANGED);
+  } finally {
+    await promiseClean(engine, server);
+  }
+});
+
+async function createRecordFailTelemetry(allowSkippedRecord) {
+  Services.prefs.setStringPref("services.sync.username", "foo");
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO("flying");
+  collection._wbos.scotsman = new ServerWBO("scotsman");
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  engine.allowSkippedRecord = allowSkippedRecord;
+  let oldCreateRecord = engine._store.createRecord;
+  engine._store.createRecord = async (id, col) => {
+    if (id != "flying") {
+      throw new Error("oops");
+    }
+    return oldCreateRecord.call(engine._store, id, col);
+  };
+  engine._store.items = {
+    flying: "LNER Class A3 4472",
+    scotsman: "Flying Scotsman",
+  };
+  // Mark these records as changed
+  const FLYING_CHANGED = 12345;
+  const SCOTSMAN_CHANGED = 23456;
+  await engine._tracker.addChangedID("flying", FLYING_CHANGED);
+  await engine._tracker.addChangedID("scotsman", SCOTSMAN_CHANGED);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  let ping;
+  try {
+    await engine.setLastSync(123); // needs to be non-zero so that tracker is queried
+
+    // Confirm initial environment
+    Assert.equal(collection.payload("flying"), undefined);
+    let changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.flying, FLYING_CHANGED);
+    Assert.equal(changes.scotsman, SCOTSMAN_CHANGED);
+
+    engine.enabled = true;
+    ping = await sync_engine_and_validate_telem(engine, true, onErrorPing => {
+      ping = onErrorPing;
+    });
+
+    if (!allowSkippedRecord) {
+      do_throw("should not get here");
+    }
+
+    // Ensure the 'flying' record has been uploaded and is no longer marked.
+    Assert.ok(!!collection.payload("flying"));
+    changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.flying, undefined);
+  } catch (err) {
+    if (allowSkippedRecord) {
+      do_throw("should not get here");
+    }
+
+    // Ensure the 'flying' record has not been uploaded and is still marked
+    Assert.ok(!collection.payload("flying"));
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.ok(changes.flying);
+  } finally {
+    // We reported in telemetry that we failed a record
+    Assert.equal(ping.engines[0].outgoing[0].failed, 1);
+    Assert.equal(ping.engines[0].outgoing[0].failedReasons[0].name, "oops");
+
+    // In any case, the 'scotsman' record couldn't be created so it wasn't
+    // uploaded nor it was not cleared from the tracker.
+    Assert.ok(!collection.payload("scotsman"));
+    const changes = await engine._tracker.getChangedIDs();
+    Assert.equal(changes.scotsman, SCOTSMAN_CHANGED);
+
+    engine._store.createRecord = oldCreateRecord;
+    await promiseClean(engine, server);
+  }
+}
+
+add_task(
+  async function test_uploadOutgoing_createRecord_throws_reported_telemetry() {
+    _(
+      "SyncEngine._uploadOutgoing reports a failed record to telemetry if createRecord throws"
+    );
+    await createRecordFailTelemetry(true);
+  }
+);
+
+add_task(
+  async function test_uploadOutgoing_createRecord_throws_dontAllowSkipRecord() {
+    _(
+      "SyncEngine._uploadOutgoing will throw if createRecord throws and allowSkipRecord is set to false"
+    );
+    await createRecordFailTelemetry(false);
+  }
+);
+
+add_task(async function test_uploadOutgoing_largeRecords() {
+  _(
+    "SyncEngine._uploadOutgoing throws on records larger than the max record payload size"
+  );
+
+  let collection = new ServerCollection();
+
+  let engine = makeRotaryEngine();
+  engine.allowSkippedRecord = false;
+  engine._store.items["large-item"] = "Y".repeat(
+    Service.getMaxRecordPayloadSize() * 2
+  );
+  await engine._tracker.addChangedID("large-item", 0);
+  collection.insert("large-item");
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  try {
+    await engine._syncStartup();
+    let error = null;
+    try {
+      await engine._uploadOutgoing();
+    } catch (e) {
+      error = e;
+    }
+    ok(!!error);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_syncFinish_deleteByIds() {
+  _(
+    "SyncEngine._syncFinish deletes server records slated for deletion (list of record IDs)."
+  );
+
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+  collection._wbos.scotsman = new ServerWBO(
+    "scotsman",
+    encryptPayload({ id: "scotsman", denomination: "Flying Scotsman" })
+  );
+  collection._wbos.rekolok = new ServerWBO(
+    "rekolok",
+    encryptPayload({ id: "rekolok", denomination: "Rekonstruktionslokomotive" })
+  );
+
+  let server = httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  try {
+    engine._delete = { ids: ["flying", "rekolok"] };
+    await engine._syncFinish();
+
+    // The 'flying' and 'rekolok' records were deleted while the
+    // 'scotsman' one wasn't.
+    Assert.equal(collection.payload("flying"), undefined);
+    Assert.ok(!!collection.payload("scotsman"));
+    Assert.equal(collection.payload("rekolok"), undefined);
+
+    // The deletion todo list has been reset.
+    Assert.equal(engine._delete.ids, undefined);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_syncFinish_deleteLotsInBatches() {
+  _(
+    "SyncEngine._syncFinish deletes server records in batches of 100 (list of record IDs)."
+  );
+
+  let collection = new ServerCollection();
+
+  // Let's count how many times the client does a DELETE request to the server
+  var noOfUploads = 0;
+  collection.delete = (function (orig) {
+    return function () {
+      noOfUploads++;
+      return orig.apply(this, arguments);
+    };
+  })(collection.delete);
+
+  // Create a bunch of records on the server
+  let now = Date.now();
+  for (var i = 0; i < 234; i++) {
+    let id = "record-no-" + i;
+    let payload = encryptPayload({ id, denomination: "Record No. " + i });
+    let wbo = new ServerWBO(id, payload);
+    wbo.modified = now / 1000 - 60 * (i + 110);
+    collection.insertWBO(wbo);
+  }
+
+  let server = httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let engine = makeRotaryEngine();
+  try {
+    // Confirm initial environment
+    Assert.equal(noOfUploads, 0);
+
+    // Declare what we want to have deleted: all records no. 100 and
+    // up and all records that are less than 200 mins old (which are
+    // records 0 thru 90).
+    engine._delete = { ids: [], newer: now / 1000 - 60 * 200.5 };
+    for (i = 100; i < 234; i++) {
+      engine._delete.ids.push("record-no-" + i);
+    }
+
+    await engine._syncFinish();
+
+    // Ensure that the appropriate server data has been wiped while
+    // preserving records 90 thru 200.
+    for (i = 0; i < 234; i++) {
+      let id = "record-no-" + i;
+      if (i <= 90 || i >= 100) {
+        Assert.equal(collection.payload(id), undefined);
+      } else {
+        Assert.ok(!!collection.payload(id));
+      }
+    }
+
+    // The deletion was done in batches
+    Assert.equal(noOfUploads, 2 + 1);
+
+    // The deletion todo list has been reset.
+    Assert.equal(engine._delete.ids, undefined);
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_sync_partialUpload() {
+  _("SyncEngine.sync() keeps changedIDs that couldn't be uploaded.");
+
+  let collection = new ServerCollection();
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+  let oldServerConfiguration = Service.serverConfiguration;
+  Service.serverConfiguration = {
+    max_post_records: 100,
+  };
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let engine = makeRotaryEngine();
+
+  // Let the third upload fail completely
+  var noOfUploads = 0;
+  collection.post = (function (orig) {
+    return function () {
+      if (noOfUploads == 2) {
+        throw new Error("FAIL!");
+      }
+      noOfUploads++;
+      return orig.apply(this, arguments);
+    };
+  })(collection.post);
+
+  // Create a bunch of records (and server side handlers)
+  for (let i = 0; i < 234; i++) {
+    let id = "record-no-" + i;
+    engine._store.items[id] = "Record No. " + i;
+    await engine._tracker.addChangedID(id, i);
+    // Let two items in the first upload batch fail.
+    if (i != 23 && i != 42) {
+      collection.insert(id);
+    }
+  }
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    await engine.setLastSync(123); // needs to be non-zero so that tracker is queried
+
+    engine.enabled = true;
+    let error;
+    try {
+      await sync_engine_and_validate_telem(engine, true);
+    } catch (ex) {
+      error = ex;
+    }
+
+    ok(!!error);
+
+    const changes = await engine._tracker.getChangedIDs();
+    for (let i = 0; i < 234; i++) {
+      let id = "record-no-" + i;
+      // Ensure failed records are back in the tracker:
+      // * records no. 23 and 42 were rejected by the server,
+      // * records after the third batch and higher couldn't be uploaded because
+      //   we failed hard on the 3rd upload.
+      if (i == 23 || i == 42 || i >= 200) {
+        Assert.equal(changes[id], i);
+      } else {
+        Assert.equal(false, id in changes);
+      }
+    }
+  } finally {
+    Service.serverConfiguration = oldServerConfiguration;
+    await promiseClean(engine, server);
+  }
+});
+
+add_task(async function test_canDecrypt_noCryptoKeys() {
+  _(
+    "SyncEngine.canDecrypt returns false if the engine fails to decrypt items on the server, e.g. due to a missing crypto key collection."
+  );
+
+  // Wipe collection keys so we can test the desired scenario.
+  Service.collectionKeys.clear();
+
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+  let engine = makeRotaryEngine();
+  try {
+    Assert.equal(false, await engine.canDecrypt());
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_canDecrypt_true() {
+  _(
+    "SyncEngine.canDecrypt returns true if the engine can decrypt the items on the server."
+  );
+
+  await generateNewKeys(Service.collectionKeys);
+
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO(
+    "flying",
+    encryptPayload({ id: "flying", denomination: "LNER Class A3 4472" })
+  );
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+  let engine = makeRotaryEngine();
+  try {
+    Assert.ok(await engine.canDecrypt());
+  } finally {
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_syncapplied_observer() {
+  const NUMBER_OF_RECORDS = 10;
+
+  let engine = makeRotaryEngine();
+
+  // Create a batch of server side records.
+  let collection = new ServerCollection();
+  for (var i = 0; i < NUMBER_OF_RECORDS; i++) {
+    let id = "record-no-" + i;
+    let payload = encryptPayload({ id, denomination: "Record No. " + id });
+    collection.insert(id, payload);
+  }
+
+  let server = httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  let numApplyCalls = 0;
+  let engine_name;
+  let count;
+  function onApplied(subject, data) {
+    numApplyCalls++;
+    engine_name = data;
+    count = subject;
+  }
+
+  Svc.Obs.add("weave:engine:sync:applied", onApplied);
+
+  try {
+    Service.scheduler.hasIncomingItems = false;
+
+    // Do sync.
+    await engine._syncStartup();
+    await engine._processIncoming();
+
+    do_check_attribute_count(engine._store.items, 10);
+
+    Assert.equal(numApplyCalls, 1);
+    Assert.equal(engine_name, "rotary");
+    Assert.equal(count.applied, 10);
+
+    Assert.ok(Service.scheduler.hasIncomingItems);
+  } finally {
+    await cleanAndGo(engine, server);
+    Service.scheduler.hasIncomingItems = false;
+    Svc.Obs.remove("weave:engine:sync:applied", onApplied);
+  }
+});
diff --git a/services/sync/tests/unit/test_syncscheduler.js b/services/sync/tests/unit/test_syncscheduler.js
new file mode 100644
index 0000000000..98b7937da3
--- /dev/null
+++ b/services/sync/tests/unit/test_syncscheduler.js
@@ -0,0 +1,1195 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { FxAccounts } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccounts.sys.mjs"
+);
+const { SyncAuthManager } = ChromeUtils.importESModule(
+  "resource://services-sync/sync_auth.sys.mjs"
+);
+const { SyncScheduler } = ChromeUtils.importESModule(
+  "resource://services-sync/policies.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { Status } = ChromeUtils.importESModule(
+  "resource://services-sync/status.sys.mjs"
+);
+
+function CatapultEngine() {
+  SyncEngine.call(this, "Catapult", Service);
+}
+CatapultEngine.prototype = {
+  exception: null, // tests fill this in
+  async _sync() {
+    throw this.exception;
+  },
+};
+Object.setPrototypeOf(CatapultEngine.prototype, SyncEngine.prototype);
+
+var scheduler = new SyncScheduler(Service);
+let clientsEngine;
+
+async function sync_httpd_setup() {
+  let clientsSyncID = await clientsEngine.resetLocalSyncID();
+  let global = new ServerWBO("global", {
+    syncID: Service.syncID,
+    storageVersion: STORAGE_VERSION,
+    engines: {
+      clients: { version: clientsEngine.version, syncID: clientsSyncID },
+    },
+  });
+  let clientsColl = new ServerCollection({}, true);
+
+  // Tracking info/collections.
+  let collectionsHelper = track_collections_helper();
+  let upd = collectionsHelper.with_updated_collection;
+
+  return httpd_setup({
+    "/1.1/johndoe@mozilla.com/storage/meta/global": upd(
+      "meta",
+      global.handler()
+    ),
+    "/1.1/johndoe@mozilla.com/info/collections": collectionsHelper.handler,
+    "/1.1/johndoe@mozilla.com/storage/crypto/keys": upd(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe@mozilla.com/storage/clients": upd(
+      "clients",
+      clientsColl.handler()
+    ),
+  });
+}
+
+async function setUp(server) {
+  await configureIdentity({ username: "johndoe@mozilla.com" }, server);
+
+  await generateNewKeys(Service.collectionKeys);
+  let serverKeys = Service.collectionKeys.asWBO("crypto", "keys");
+  await serverKeys.encrypt(Service.identity.syncKeyBundle);
+  let result = (
+    await serverKeys.upload(Service.resource(Service.cryptoKeysURL))
+  ).success;
+  return result;
+}
+
+async function cleanUpAndGo(server) {
+  await Async.promiseYield();
+  await clientsEngine._store.wipe();
+  await Service.startOver();
+  // Re-enable logging, which we just disabled.
+  syncTestLogging();
+  if (server) {
+    await promiseStopServer(server);
+  }
+}
+
+add_task(async function setup() {
+  await Service.promiseInitialized;
+  clientsEngine = Service.clientsEngine;
+  // Don't remove stale clients when syncing. This is a test-only workaround
+  // that lets us add clients directly to the store, without losing them on
+  // the next sync.
+  clientsEngine._removeRemoteClient = async id => {};
+  await Service.engineManager.clear();
+
+  validate_all_future_pings();
+
+  scheduler.setDefaults();
+
+  await Service.engineManager.register(CatapultEngine);
+});
+
+add_test(function test_prefAttributes() {
+  _("Test various attributes corresponding to preferences.");
+
+  const INTERVAL = 42 * 60 * 1000; // 42 minutes
+  const THRESHOLD = 3142;
+  const SCORE = 2718;
+  const TIMESTAMP1 = 1275493471649;
+
+  _(
+    "The 'nextSync' attribute stores a millisecond timestamp rounded down to the nearest second."
+  );
+  Assert.equal(scheduler.nextSync, 0);
+  scheduler.nextSync = TIMESTAMP1;
+  Assert.equal(scheduler.nextSync, Math.floor(TIMESTAMP1 / 1000) * 1000);
+
+  _("'syncInterval' defaults to singleDeviceInterval.");
+  Assert.equal(
+    Svc.PrefBranch.getPrefType("syncInterval"),
+    Ci.nsIPrefBranch.PREF_INVALID
+  );
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  _("'syncInterval' corresponds to a preference setting.");
+  scheduler.syncInterval = INTERVAL;
+  Assert.equal(scheduler.syncInterval, INTERVAL);
+  Assert.equal(Svc.PrefBranch.getIntPref("syncInterval"), INTERVAL);
+
+  _(
+    "'syncThreshold' corresponds to preference, defaults to SINGLE_USER_THRESHOLD"
+  );
+  Assert.equal(
+    Svc.PrefBranch.getPrefType("syncThreshold"),
+    Ci.nsIPrefBranch.PREF_INVALID
+  );
+  Assert.equal(scheduler.syncThreshold, SINGLE_USER_THRESHOLD);
+  scheduler.syncThreshold = THRESHOLD;
+  Assert.equal(scheduler.syncThreshold, THRESHOLD);
+
+  _("'globalScore' corresponds to preference, defaults to zero.");
+  Assert.equal(Svc.PrefBranch.getIntPref("globalScore"), 0);
+  Assert.equal(scheduler.globalScore, 0);
+  scheduler.globalScore = SCORE;
+  Assert.equal(scheduler.globalScore, SCORE);
+  Assert.equal(Svc.PrefBranch.getIntPref("globalScore"), SCORE);
+
+  _("Intervals correspond to default preferences.");
+  Assert.equal(
+    scheduler.singleDeviceInterval,
+    Svc.PrefBranch.getIntPref("scheduler.fxa.singleDeviceInterval") * 1000
+  );
+  Assert.equal(
+    scheduler.idleInterval,
+    Svc.PrefBranch.getIntPref("scheduler.idleInterval") * 1000
+  );
+  Assert.equal(
+    scheduler.activeInterval,
+    Svc.PrefBranch.getIntPref("scheduler.activeInterval") * 1000
+  );
+  Assert.equal(
+    scheduler.immediateInterval,
+    Svc.PrefBranch.getIntPref("scheduler.immediateInterval") * 1000
+  );
+
+  _("Custom values for prefs will take effect after a restart.");
+  Svc.PrefBranch.setIntPref("scheduler.fxa.singleDeviceInterval", 420);
+  Svc.PrefBranch.setIntPref("scheduler.idleInterval", 230);
+  Svc.PrefBranch.setIntPref("scheduler.activeInterval", 180);
+  Svc.PrefBranch.setIntPref("scheduler.immediateInterval", 31415);
+  scheduler.setDefaults();
+  Assert.equal(scheduler.idleInterval, 230000);
+  Assert.equal(scheduler.singleDeviceInterval, 420000);
+  Assert.equal(scheduler.activeInterval, 180000);
+  Assert.equal(scheduler.immediateInterval, 31415000);
+
+  _("Custom values for interval prefs can't be less than 60 seconds.");
+  Svc.PrefBranch.setIntPref("scheduler.fxa.singleDeviceInterval", 42);
+  Svc.PrefBranch.setIntPref("scheduler.idleInterval", 50);
+  Svc.PrefBranch.setIntPref("scheduler.activeInterval", 50);
+  Svc.PrefBranch.setIntPref("scheduler.immediateInterval", 10);
+  scheduler.setDefaults();
+  Assert.equal(scheduler.idleInterval, 60000);
+  Assert.equal(scheduler.singleDeviceInterval, 60000);
+  Assert.equal(scheduler.activeInterval, 60000);
+  Assert.equal(scheduler.immediateInterval, 60000);
+
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  scheduler.setDefaults();
+  run_next_test();
+});
+
+add_task(async function test_sync_skipped_low_score_no_resync() {
+  enableValidationPrefs();
+  let server = await sync_httpd_setup();
+
+  function SkipEngine() {
+    SyncEngine.call(this, "Skip", Service);
+    this.syncs = 0;
+  }
+
+  SkipEngine.prototype = {
+    _sync() {
+      do_throw("Should have been skipped");
+    },
+    shouldSkipSync() {
+      return true;
+    },
+  };
+  Object.setPrototypeOf(SkipEngine.prototype, SyncEngine.prototype);
+  await Service.engineManager.register(SkipEngine);
+
+  let engine = Service.engineManager.get("skip");
+  engine.enabled = true;
+  engine._tracker._score = 30;
+
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+  Assert.ok(await setUp(server));
+
+  let resyncDoneObserver = promiseOneObserver("weave:service:resyncs-finished");
+
+  let synced = false;
+  function onSyncStarted() {
+    Assert.ok(!synced, "Only should sync once");
+    synced = true;
+  }
+
+  await Service.sync();
+
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+  Svc.Obs.add("weave:service:sync:start", onSyncStarted);
+  await resyncDoneObserver;
+
+  Svc.Obs.remove("weave:service:sync:start", onSyncStarted);
+  engine._tracker._store = 0;
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_updateClientMode() {
+  _(
+    "Test updateClientMode adjusts scheduling attributes based on # of clients appropriately"
+  );
+  Assert.equal(scheduler.syncThreshold, SINGLE_USER_THRESHOLD);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.idle);
+
+  // Trigger a change in interval & threshold by noting there are multiple clients.
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.updateClientMode();
+
+  Assert.equal(scheduler.syncThreshold, MULTI_DEVICE_THRESHOLD);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.idle);
+
+  // Resets the number of clients to 0.
+  await clientsEngine.resetClient();
+  Svc.PrefBranch.clearUserPref("clients.devices.mobile");
+  scheduler.updateClientMode();
+
+  // Goes back to single user if # clients is 1.
+  Assert.equal(scheduler.numClients, 1);
+  Assert.equal(scheduler.syncThreshold, SINGLE_USER_THRESHOLD);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.idle);
+
+  await cleanUpAndGo();
+});
+
+add_task(async function test_masterpassword_locked_retry_interval() {
+  enableValidationPrefs();
+
+  _(
+    "Test Status.login = MASTER_PASSWORD_LOCKED results in reschedule at MASTER_PASSWORD interval"
+  );
+  let loginFailed = false;
+  Svc.Obs.add("weave:service:login:error", function onLoginError() {
+    Svc.Obs.remove("weave:service:login:error", onLoginError);
+    loginFailed = true;
+  });
+
+  let rescheduleInterval = false;
+
+  let oldScheduleAtInterval = SyncScheduler.prototype.scheduleAtInterval;
+  SyncScheduler.prototype.scheduleAtInterval = function (interval) {
+    rescheduleInterval = true;
+    Assert.equal(interval, MASTER_PASSWORD_LOCKED_RETRY_INTERVAL);
+  };
+
+  let oldVerifyLogin = Service.verifyLogin;
+  Service.verifyLogin = async function () {
+    Status.login = MASTER_PASSWORD_LOCKED;
+    return false;
+  };
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  await Service.sync();
+
+  Assert.ok(loginFailed);
+  Assert.equal(Status.login, MASTER_PASSWORD_LOCKED);
+  Assert.ok(rescheduleInterval);
+
+  Service.verifyLogin = oldVerifyLogin;
+  SyncScheduler.prototype.scheduleAtInterval = oldScheduleAtInterval;
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_calculateBackoff() {
+  Assert.equal(Status.backoffInterval, 0);
+
+  // Test no interval larger than the maximum backoff is used if
+  // Status.backoffInterval is smaller.
+  Status.backoffInterval = 5;
+  let backoffInterval = Utils.calculateBackoff(
+    50,
+    MAXIMUM_BACKOFF_INTERVAL,
+    Status.backoffInterval
+  );
+
+  Assert.equal(backoffInterval, MAXIMUM_BACKOFF_INTERVAL);
+
+  // Test Status.backoffInterval is used if it is
+  // larger than MAXIMUM_BACKOFF_INTERVAL.
+  Status.backoffInterval = MAXIMUM_BACKOFF_INTERVAL + 10;
+  backoffInterval = Utils.calculateBackoff(
+    50,
+    MAXIMUM_BACKOFF_INTERVAL,
+    Status.backoffInterval
+  );
+
+  Assert.equal(backoffInterval, MAXIMUM_BACKOFF_INTERVAL + 10);
+
+  await cleanUpAndGo();
+});
+
+add_task(async function test_scheduleNextSync_nowOrPast() {
+  enableValidationPrefs();
+
+  let promiseObserved = promiseOneObserver("weave:service:sync:finish");
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // We're late for a sync...
+  scheduler.scheduleNextSync(-1);
+  await promiseObserved;
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_scheduleNextSync_future_noBackoff() {
+  enableValidationPrefs();
+
+  _(
+    "scheduleNextSync() uses the current syncInterval if no interval is provided."
+  );
+  // Test backoffInterval is 0 as expected.
+  Assert.equal(Status.backoffInterval, 0);
+
+  _("Test setting sync interval when nextSync == 0");
+  scheduler.nextSync = 0;
+  scheduler.scheduleNextSync();
+
+  // nextSync - Date.now() might be smaller than expectedInterval
+  // since some time has passed since we called scheduleNextSync().
+  Assert.ok(scheduler.nextSync - Date.now() <= scheduler.syncInterval);
+  Assert.equal(scheduler.syncTimer.delay, scheduler.syncInterval);
+
+  _("Test setting sync interval when nextSync != 0");
+  scheduler.nextSync = Date.now() + scheduler.singleDeviceInterval;
+  scheduler.scheduleNextSync();
+
+  // nextSync - Date.now() might be smaller than expectedInterval
+  // since some time has passed since we called scheduleNextSync().
+  Assert.ok(scheduler.nextSync - Date.now() <= scheduler.syncInterval);
+  Assert.ok(scheduler.syncTimer.delay <= scheduler.syncInterval);
+
+  _(
+    "Scheduling requests for intervals larger than the current one will be ignored."
+  );
+  // Request a sync at a longer interval. The sync that's already scheduled
+  // for sooner takes precedence.
+  let nextSync = scheduler.nextSync;
+  let timerDelay = scheduler.syncTimer.delay;
+  let requestedInterval = scheduler.syncInterval * 10;
+  scheduler.scheduleNextSync(requestedInterval);
+  Assert.equal(scheduler.nextSync, nextSync);
+  Assert.equal(scheduler.syncTimer.delay, timerDelay);
+
+  // We can schedule anything we want if there isn't a sync scheduled.
+  scheduler.nextSync = 0;
+  scheduler.scheduleNextSync(requestedInterval);
+  Assert.ok(scheduler.nextSync <= Date.now() + requestedInterval);
+  Assert.equal(scheduler.syncTimer.delay, requestedInterval);
+
+  // Request a sync at the smallest possible interval (0 triggers now).
+  scheduler.scheduleNextSync(1);
+  Assert.ok(scheduler.nextSync <= Date.now() + 1);
+  Assert.equal(scheduler.syncTimer.delay, 1);
+
+  await cleanUpAndGo();
+});
+
+add_task(async function test_scheduleNextSync_future_backoff() {
+  enableValidationPrefs();
+
+  _("scheduleNextSync() will honour backoff in all scheduling requests.");
+  // Let's take a backoff interval that's bigger than the default sync interval.
+  const BACKOFF = 7337;
+  Status.backoffInterval = scheduler.syncInterval + BACKOFF;
+
+  _("Test setting sync interval when nextSync == 0");
+  scheduler.nextSync = 0;
+  scheduler.scheduleNextSync();
+
+  // nextSync - Date.now() might be smaller than expectedInterval
+  // since some time has passed since we called scheduleNextSync().
+  Assert.ok(scheduler.nextSync - Date.now() <= Status.backoffInterval);
+  Assert.equal(scheduler.syncTimer.delay, Status.backoffInterval);
+
+  _("Test setting sync interval when nextSync != 0");
+  scheduler.nextSync = Date.now() + scheduler.singleDeviceInterval;
+  scheduler.scheduleNextSync();
+
+  // nextSync - Date.now() might be smaller than expectedInterval
+  // since some time has passed since we called scheduleNextSync().
+  Assert.ok(scheduler.nextSync - Date.now() <= Status.backoffInterval);
+  Assert.ok(scheduler.syncTimer.delay <= Status.backoffInterval);
+
+  // Request a sync at a longer interval. The sync that's already scheduled
+  // for sooner takes precedence.
+  let nextSync = scheduler.nextSync;
+  let timerDelay = scheduler.syncTimer.delay;
+  let requestedInterval = scheduler.syncInterval * 10;
+  Assert.ok(requestedInterval > Status.backoffInterval);
+  scheduler.scheduleNextSync(requestedInterval);
+  Assert.equal(scheduler.nextSync, nextSync);
+  Assert.equal(scheduler.syncTimer.delay, timerDelay);
+
+  // We can schedule anything we want if there isn't a sync scheduled.
+  scheduler.nextSync = 0;
+  scheduler.scheduleNextSync(requestedInterval);
+  Assert.ok(scheduler.nextSync <= Date.now() + requestedInterval);
+  Assert.equal(scheduler.syncTimer.delay, requestedInterval);
+
+  // Request a sync at the smallest possible interval (0 triggers now).
+  scheduler.scheduleNextSync(1);
+  Assert.ok(scheduler.nextSync <= Date.now() + Status.backoffInterval);
+  Assert.equal(scheduler.syncTimer.delay, Status.backoffInterval);
+
+  await cleanUpAndGo();
+});
+
+add_task(async function test_handleSyncError() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Force sync to fail.
+  Svc.PrefBranch.setStringPref("firstSync", "notReady");
+
+  _("Ensure expected initial environment.");
+  Assert.equal(scheduler._syncErrors, 0);
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.equal(Status.backoffInterval, 0);
+
+  // Trigger sync with an error several times & observe
+  // functionality of handleSyncError()
+  _("Test first error calls scheduleNextSync on default interval");
+  await Service.sync();
+  Assert.ok(scheduler.nextSync <= Date.now() + scheduler.singleDeviceInterval);
+  Assert.equal(scheduler.syncTimer.delay, scheduler.singleDeviceInterval);
+  Assert.equal(scheduler._syncErrors, 1);
+  Assert.ok(!Status.enforceBackoff);
+  scheduler.syncTimer.clear();
+
+  _("Test second error still calls scheduleNextSync on default interval");
+  await Service.sync();
+  Assert.ok(scheduler.nextSync <= Date.now() + scheduler.singleDeviceInterval);
+  Assert.equal(scheduler.syncTimer.delay, scheduler.singleDeviceInterval);
+  Assert.equal(scheduler._syncErrors, 2);
+  Assert.ok(!Status.enforceBackoff);
+  scheduler.syncTimer.clear();
+
+  _("Test third error sets Status.enforceBackoff and calls scheduleAtInterval");
+  await Service.sync();
+  let maxInterval = scheduler._syncErrors * (2 * MINIMUM_BACKOFF_INTERVAL);
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.ok(scheduler.nextSync <= Date.now() + maxInterval);
+  Assert.ok(scheduler.syncTimer.delay <= maxInterval);
+  Assert.equal(scheduler._syncErrors, 3);
+  Assert.ok(Status.enforceBackoff);
+
+  // Status.enforceBackoff is false but there are still errors.
+  Status.resetBackoff();
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(scheduler._syncErrors, 3);
+  scheduler.syncTimer.clear();
+
+  _(
+    "Test fourth error still calls scheduleAtInterval even if enforceBackoff was reset"
+  );
+  await Service.sync();
+  maxInterval = scheduler._syncErrors * (2 * MINIMUM_BACKOFF_INTERVAL);
+  Assert.ok(scheduler.nextSync <= Date.now() + maxInterval);
+  Assert.ok(scheduler.syncTimer.delay <= maxInterval);
+  Assert.equal(scheduler._syncErrors, 4);
+  Assert.ok(Status.enforceBackoff);
+  scheduler.syncTimer.clear();
+
+  _("Arrange for a successful sync to reset the scheduler error count");
+  let promiseObserved = promiseOneObserver("weave:service:sync:finish");
+  Svc.PrefBranch.setStringPref("firstSync", "wipeRemote");
+  scheduler.scheduleNextSync(-1);
+  await promiseObserved;
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_client_sync_finish_updateClientMode() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Confirm defaults.
+  Assert.equal(scheduler.syncThreshold, SINGLE_USER_THRESHOLD);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.ok(!scheduler.idle);
+
+  // Trigger a change in interval & threshold by adding a client.
+  await clientsEngine._store.create({
+    id: "foo",
+    cleartext: { os: "mobile", version: "0.01", type: "desktop" },
+  });
+  Assert.equal(false, scheduler.numClients > 1);
+  scheduler.updateClientMode();
+  await Service.sync();
+
+  Assert.equal(scheduler.syncThreshold, MULTI_DEVICE_THRESHOLD);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+  Assert.ok(scheduler.numClients > 1);
+  Assert.ok(!scheduler.idle);
+
+  // Resets the number of clients to 0.
+  await clientsEngine.resetClient();
+  // Also re-init the server, or we suck our "foo" client back down.
+  await setUp(server);
+
+  await Service.sync();
+
+  // Goes back to single user if # clients is 1.
+  Assert.equal(scheduler.numClients, 1);
+  Assert.equal(scheduler.syncThreshold, SINGLE_USER_THRESHOLD);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+  Assert.equal(false, scheduler.numClients > 1);
+  Assert.ok(!scheduler.idle);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_autoconnect_nextSync_past() {
+  enableValidationPrefs();
+
+  let promiseObserved = promiseOneObserver("weave:service:sync:finish");
+  // nextSync will be 0 by default, so it's way in the past.
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  scheduler.autoConnect();
+  await promiseObserved;
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_autoconnect_nextSync_future() {
+  enableValidationPrefs();
+
+  let previousSync = Date.now() + scheduler.syncInterval / 2;
+  scheduler.nextSync = previousSync;
+  // nextSync rounds to the nearest second.
+  let expectedSync = scheduler.nextSync;
+  let expectedInterval = expectedSync - Date.now() - 1000;
+
+  // Ensure we don't actually try to sync (or log in for that matter).
+  function onLoginStart() {
+    do_throw("Should not get here!");
+  }
+  Svc.Obs.add("weave:service:login:start", onLoginStart);
+
+  await configureIdentity({ username: "johndoe@mozilla.com" });
+  scheduler.autoConnect();
+  await promiseZeroTimer();
+
+  Assert.equal(scheduler.nextSync, expectedSync);
+  Assert.ok(scheduler.syncTimer.delay >= expectedInterval);
+
+  Svc.Obs.remove("weave:service:login:start", onLoginStart);
+  await cleanUpAndGo();
+});
+
+add_task(async function test_autoconnect_mp_locked() {
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Pretend user did not unlock master password.
+  let origLocked = Utils.mpLocked;
+  Utils.mpLocked = () => true;
+
+  let origEnsureMPUnlocked = Utils.ensureMPUnlocked;
+  Utils.ensureMPUnlocked = () => {
+    _("Faking Master Password entry cancelation.");
+    return false;
+  };
+  let origFxA = Service.identity._fxaService;
+  Service.identity._fxaService = new FxAccounts({
+    currentAccountState: {
+      getUserAccountData(...args) {
+        return origFxA._internal.currentAccountState.getUserAccountData(
+          ...args
+        );
+      },
+    },
+    keys: {
+      canGetKeyForScope() {
+        return false;
+      },
+    },
+  });
+  // A locked master password will still trigger a sync, but then we'll hit
+  // MASTER_PASSWORD_LOCKED and hence MASTER_PASSWORD_LOCKED_RETRY_INTERVAL.
+  let promiseObserved = promiseOneObserver("weave:service:login:error");
+
+  scheduler.autoConnect();
+  await promiseObserved;
+
+  await Async.promiseYield();
+
+  Assert.equal(Status.login, MASTER_PASSWORD_LOCKED);
+
+  Utils.mpLocked = origLocked;
+  Utils.ensureMPUnlocked = origEnsureMPUnlocked;
+  Service.identity._fxaService = origFxA;
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_no_autoconnect_during_wizard() {
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Simulate the Sync setup wizard.
+  Svc.PrefBranch.setStringPref("firstSync", "notReady");
+
+  // Ensure we don't actually try to sync (or log in for that matter).
+  function onLoginStart() {
+    do_throw("Should not get here!");
+  }
+  Svc.Obs.add("weave:service:login:start", onLoginStart);
+
+  scheduler.autoConnect(0);
+  await promiseZeroTimer();
+  Svc.Obs.remove("weave:service:login:start", onLoginStart);
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_no_autoconnect_status_not_ok() {
+  let server = await sync_httpd_setup();
+  Status.__authManager = Service.identity = new SyncAuthManager();
+
+  // Ensure we don't actually try to sync (or log in for that matter).
+  function onLoginStart() {
+    do_throw("Should not get here!");
+  }
+  Svc.Obs.add("weave:service:login:start", onLoginStart);
+
+  scheduler.autoConnect();
+  await promiseZeroTimer();
+  Svc.Obs.remove("weave:service:login:start", onLoginStart);
+
+  Assert.equal(Status.service, CLIENT_NOT_CONFIGURED);
+  Assert.equal(Status.login, LOGIN_FAILED_NO_USERNAME);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_idle_adjustSyncInterval() {
+  // Confirm defaults.
+  Assert.equal(scheduler.idle, false);
+
+  // Single device: nothing changes.
+  scheduler.observe(
+    null,
+    "idle",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.equal(scheduler.idle, true);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+
+  // Multiple devices: switch to idle interval.
+  scheduler.idle = false;
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.updateClientMode();
+  scheduler.observe(
+    null,
+    "idle",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.equal(scheduler.idle, true);
+  Assert.equal(scheduler.syncInterval, scheduler.idleInterval);
+
+  await cleanUpAndGo();
+});
+
+add_task(async function test_back_triggersSync() {
+  // Confirm defaults.
+  Assert.ok(!scheduler.idle);
+  Assert.equal(Status.backoffInterval, 0);
+
+  // Set up: Define 2 clients and put the system in idle.
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.observe(
+    null,
+    "idle",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.ok(scheduler.idle);
+
+  // We don't actually expect the sync (or the login, for that matter) to
+  // succeed. We just want to ensure that it was attempted.
+  let promiseObserved = promiseOneObserver("weave:service:login:error");
+
+  // Send an 'active' event to trigger sync soonish.
+  scheduler.observe(
+    null,
+    "active",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  await promiseObserved;
+  await cleanUpAndGo();
+});
+
+add_task(async function test_active_triggersSync_observesBackoff() {
+  // Confirm defaults.
+  Assert.ok(!scheduler.idle);
+
+  // Set up: Set backoff, define 2 clients and put the system in idle.
+  const BACKOFF = 7337;
+  Status.backoffInterval = scheduler.idleInterval + BACKOFF;
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.observe(
+    null,
+    "idle",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.equal(scheduler.idle, true);
+
+  function onLoginStart() {
+    do_throw("Shouldn't have kicked off a sync!");
+  }
+  Svc.Obs.add("weave:service:login:start", onLoginStart);
+
+  let promiseTimer = promiseNamedTimer(
+    IDLE_OBSERVER_BACK_DELAY * 1.5,
+    {},
+    "timer"
+  );
+
+  // Send an 'active' event to try to trigger sync soonish.
+  scheduler.observe(
+    null,
+    "active",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  await promiseTimer;
+  Svc.Obs.remove("weave:service:login:start", onLoginStart);
+
+  Assert.ok(scheduler.nextSync <= Date.now() + Status.backoffInterval);
+  Assert.equal(scheduler.syncTimer.delay, Status.backoffInterval);
+
+  await cleanUpAndGo();
+});
+
+add_task(async function test_back_debouncing() {
+  _(
+    "Ensure spurious back-then-idle events, as observed on OS X, don't trigger a sync."
+  );
+
+  // Confirm defaults.
+  Assert.equal(scheduler.idle, false);
+
+  // Set up: Define 2 clients and put the system in idle.
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.observe(
+    null,
+    "idle",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  Assert.equal(scheduler.idle, true);
+
+  function onLoginStart() {
+    do_throw("Shouldn't have kicked off a sync!");
+  }
+  Svc.Obs.add("weave:service:login:start", onLoginStart);
+
+  // Create spurious back-then-idle events as observed on OS X:
+  scheduler.observe(
+    null,
+    "active",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+  scheduler.observe(
+    null,
+    "idle",
+    Svc.PrefBranch.getIntPref("scheduler.idleTime")
+  );
+
+  await promiseNamedTimer(IDLE_OBSERVER_BACK_DELAY * 1.5, {}, "timer");
+  Svc.Obs.remove("weave:service:login:start", onLoginStart);
+  await cleanUpAndGo();
+});
+
+add_task(async function test_no_sync_node() {
+  enableValidationPrefs();
+
+  // Test when Status.sync == NO_SYNC_NODE_FOUND
+  // it is not overwritten on sync:finish
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  let oldfc = Service.identity._findCluster;
+  Service.identity._findCluster = () => null;
+  Service.clusterURL = "";
+  try {
+    await Service.sync();
+    Assert.equal(Status.sync, NO_SYNC_NODE_FOUND);
+    Assert.equal(scheduler.syncTimer.delay, NO_SYNC_NODE_INTERVAL);
+
+    await cleanUpAndGo(server);
+  } finally {
+    Service.identity._findCluster = oldfc;
+  }
+});
+
+add_task(async function test_sync_failed_partial_500s() {
+  enableValidationPrefs();
+
+  _("Test a 5xx status calls handleSyncError.");
+  scheduler._syncErrors = MAX_ERROR_COUNT_BEFORE_BACKOFF;
+  let server = await sync_httpd_setup();
+
+  let engine = Service.engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = { status: 500 };
+
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+  Assert.ok(await setUp(server));
+
+  await Service.sync();
+
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+
+  let maxInterval = scheduler._syncErrors * (2 * MINIMUM_BACKOFF_INTERVAL);
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.ok(Status.enforceBackoff);
+  Assert.equal(scheduler._syncErrors, 4);
+  Assert.ok(scheduler.nextSync <= Date.now() + maxInterval);
+  Assert.ok(scheduler.syncTimer.delay <= maxInterval);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_sync_failed_partial_noresync() {
+  enableValidationPrefs();
+  let server = await sync_httpd_setup();
+
+  let engine = Service.engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = "Bad news";
+  engine._tracker._score = MULTI_DEVICE_THRESHOLD + 1;
+
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+  Assert.ok(await setUp(server));
+
+  let resyncDoneObserver = promiseOneObserver("weave:service:resyncs-finished");
+
+  await Service.sync();
+
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+
+  function onSyncStarted() {
+    do_throw("Should not start resync when previous sync failed");
+  }
+
+  Svc.Obs.add("weave:service:sync:start", onSyncStarted);
+  await resyncDoneObserver;
+
+  Svc.Obs.remove("weave:service:sync:start", onSyncStarted);
+  engine._tracker._store = 0;
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_sync_failed_partial_400s() {
+  enableValidationPrefs();
+
+  _("Test a non-5xx status doesn't call handleSyncError.");
+  scheduler._syncErrors = MAX_ERROR_COUNT_BEFORE_BACKOFF;
+  let server = await sync_httpd_setup();
+
+  let engine = Service.engineManager.get("catapult");
+  engine.enabled = true;
+  engine.exception = { status: 400 };
+
+  // Have multiple devices for an active interval.
+  await clientsEngine._store.create({
+    id: "foo",
+    cleartext: { os: "mobile", version: "0.01", type: "desktop" },
+  });
+
+  Assert.equal(Status.sync, SYNC_SUCCEEDED);
+
+  Assert.ok(await setUp(server));
+
+  await Service.sync();
+
+  Assert.equal(Status.service, SYNC_FAILED_PARTIAL);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(scheduler._syncErrors, 0);
+  Assert.ok(scheduler.nextSync <= Date.now() + scheduler.activeInterval);
+  Assert.ok(scheduler.syncTimer.delay <= scheduler.activeInterval);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_sync_X_Weave_Backoff() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Use an odd value on purpose so that it doesn't happen to coincide with one
+  // of the sync intervals.
+  const BACKOFF = 7337;
+
+  // Extend info/collections so that we can put it into server maintenance mode.
+  const INFO_COLLECTIONS = "/1.1/johndoe@mozilla.com/info/collections";
+  let infoColl = server._handler._overridePaths[INFO_COLLECTIONS];
+  let serverBackoff = false;
+  function infoCollWithBackoff(request, response) {
+    if (serverBackoff) {
+      response.setHeader("X-Weave-Backoff", "" + BACKOFF);
+    }
+    infoColl(request, response);
+  }
+  server.registerPathHandler(INFO_COLLECTIONS, infoCollWithBackoff);
+
+  // Pretend we have two clients so that the regular sync interval is
+  // sufficiently low.
+  await clientsEngine._store.create({
+    id: "foo",
+    cleartext: { os: "mobile", version: "0.01", type: "desktop" },
+  });
+  let rec = await clientsEngine._store.createRecord("foo", "clients");
+  await rec.encrypt(Service.collectionKeys.keyForCollection("clients"));
+  await rec.upload(Service.resource(clientsEngine.engineURL + rec.id));
+
+  // Sync once to log in and get everything set up. Let's verify our initial
+  // values.
+  await Service.sync();
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.equal(Status.minimumNextSync, 0);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+  Assert.ok(scheduler.nextSync <= Date.now() + scheduler.syncInterval);
+  // Sanity check that we picked the right value for BACKOFF:
+  Assert.ok(scheduler.syncInterval < BACKOFF * 1000);
+
+  // Turn on server maintenance and sync again.
+  serverBackoff = true;
+  await Service.sync();
+
+  Assert.ok(Status.backoffInterval >= BACKOFF * 1000);
+  // Allowing 20 seconds worth of of leeway between when Status.minimumNextSync
+  // was set and when this line gets executed.
+  let minimumExpectedDelay = (BACKOFF - 20) * 1000;
+  Assert.ok(Status.minimumNextSync >= Date.now() + minimumExpectedDelay);
+
+  // Verify that the next sync is actually going to wait that long.
+  Assert.ok(scheduler.nextSync >= Date.now() + minimumExpectedDelay);
+  Assert.ok(scheduler.syncTimer.delay >= minimumExpectedDelay);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_sync_503_Retry_After() {
+  enableValidationPrefs();
+
+  let server = await sync_httpd_setup();
+  await setUp(server);
+
+  // Use an odd value on purpose so that it doesn't happen to coincide with one
+  // of the sync intervals.
+  const BACKOFF = 7337;
+
+  // Extend info/collections so that we can put it into server maintenance mode.
+  const INFO_COLLECTIONS = "/1.1/johndoe@mozilla.com/info/collections";
+  let infoColl = server._handler._overridePaths[INFO_COLLECTIONS];
+  let serverMaintenance = false;
+  function infoCollWithMaintenance(request, response) {
+    if (!serverMaintenance) {
+      infoColl(request, response);
+      return;
+    }
+    response.setHeader("Retry-After", "" + BACKOFF);
+    response.setStatusLine(request.httpVersion, 503, "Service Unavailable");
+  }
+  server.registerPathHandler(INFO_COLLECTIONS, infoCollWithMaintenance);
+
+  // Pretend we have two clients so that the regular sync interval is
+  // sufficiently low.
+  await clientsEngine._store.create({
+    id: "foo",
+    cleartext: { os: "mobile", version: "0.01", type: "desktop" },
+  });
+  let rec = await clientsEngine._store.createRecord("foo", "clients");
+  await rec.encrypt(Service.collectionKeys.keyForCollection("clients"));
+  await rec.upload(Service.resource(clientsEngine.engineURL + rec.id));
+
+  // Sync once to log in and get everything set up. Let's verify our initial
+  // values.
+  await Service.sync();
+  Assert.ok(!Status.enforceBackoff);
+  Assert.equal(Status.backoffInterval, 0);
+  Assert.equal(Status.minimumNextSync, 0);
+  Assert.equal(scheduler.syncInterval, scheduler.activeInterval);
+  Assert.ok(scheduler.nextSync <= Date.now() + scheduler.syncInterval);
+  // Sanity check that we picked the right value for BACKOFF:
+  Assert.ok(scheduler.syncInterval < BACKOFF * 1000);
+
+  // Turn on server maintenance and sync again.
+  serverMaintenance = true;
+  await Service.sync();
+
+  Assert.ok(Status.enforceBackoff);
+  Assert.ok(Status.backoffInterval >= BACKOFF * 1000);
+  // Allowing 3 seconds worth of of leeway between when Status.minimumNextSync
+  // was set and when this line gets executed.
+  let minimumExpectedDelay = (BACKOFF - 3) * 1000;
+  Assert.ok(Status.minimumNextSync >= Date.now() + minimumExpectedDelay);
+
+  // Verify that the next sync is actually going to wait that long.
+  Assert.ok(scheduler.nextSync >= Date.now() + minimumExpectedDelay);
+  Assert.ok(scheduler.syncTimer.delay >= minimumExpectedDelay);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_loginError_recoverable_reschedules() {
+  _("Verify that a recoverable login error schedules a new sync.");
+  await configureIdentity({ username: "johndoe@mozilla.com" });
+  Service.clusterURL = "http://localhost:1234/";
+  Status.resetSync(); // reset Status.login
+
+  let promiseObserved = promiseOneObserver("weave:service:login:error");
+
+  // Let's set it up so that a sync is overdue, both in terms of previously
+  // scheduled syncs and the global score. We still do not expect an immediate
+  // sync because we just tried (duh).
+  scheduler.nextSync = Date.now() - 100000;
+  scheduler.globalScore = SINGLE_USER_THRESHOLD + 1;
+  function onSyncStart() {
+    do_throw("Shouldn't have started a sync!");
+  }
+  Svc.Obs.add("weave:service:sync:start", onSyncStart);
+
+  // Sanity check.
+  Assert.equal(scheduler.syncTimer, null);
+  Assert.equal(Status.checkSetup(), STATUS_OK);
+  Assert.equal(Status.login, LOGIN_SUCCEEDED);
+
+  scheduler.scheduleNextSync(0);
+  await promiseObserved;
+  await Async.promiseYield();
+
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR);
+
+  let expectedNextSync = Date.now() + scheduler.syncInterval;
+  Assert.ok(scheduler.nextSync > Date.now());
+  Assert.ok(scheduler.nextSync <= expectedNextSync);
+  Assert.ok(scheduler.syncTimer.delay > 0);
+  Assert.ok(scheduler.syncTimer.delay <= scheduler.syncInterval);
+
+  Svc.Obs.remove("weave:service:sync:start", onSyncStart);
+  await cleanUpAndGo();
+});
+
+add_task(async function test_loginError_fatal_clearsTriggers() {
+  _("Verify that a fatal login error clears sync triggers.");
+  await configureIdentity({ username: "johndoe@mozilla.com" });
+
+  let server = httpd_setup({
+    "/1.1/johndoe@mozilla.com/info/collections": httpd_handler(
+      401,
+      "Unauthorized"
+    ),
+  });
+
+  Service.clusterURL = server.baseURI + "/";
+  Status.resetSync(); // reset Status.login
+
+  let promiseObserved = promiseOneObserver("weave:service:login:error");
+
+  // Sanity check.
+  Assert.equal(scheduler.nextSync, 0);
+  Assert.equal(scheduler.syncTimer, null);
+  Assert.equal(Status.checkSetup(), STATUS_OK);
+  Assert.equal(Status.login, LOGIN_SUCCEEDED);
+
+  scheduler.scheduleNextSync(0);
+  await promiseObserved;
+  await Async.promiseYield();
+
+  // For the FxA identity, a 401 on info/collections means a transient
+  // error, probably due to an inability to fetch a token.
+  Assert.equal(Status.login, LOGIN_FAILED_NETWORK_ERROR);
+  // syncs should still be scheduled.
+  Assert.ok(scheduler.nextSync > Date.now());
+  Assert.ok(scheduler.syncTimer.delay > 0);
+
+  await cleanUpAndGo(server);
+});
+
+add_task(async function test_proper_interval_on_only_failing() {
+  _("Ensure proper behavior when only failed records are applied.");
+
+  // If an engine reports that no records succeeded, we shouldn't decrease the
+  // sync interval.
+  Assert.ok(!scheduler.hasIncomingItems);
+  const INTERVAL = 10000000;
+  scheduler.syncInterval = INTERVAL;
+
+  Svc.Obs.notify("weave:service:sync:applied", {
+    applied: 2,
+    succeeded: 0,
+    failed: 2,
+    newFailed: 2,
+    reconciled: 0,
+  });
+
+  await Async.promiseYield();
+  scheduler.adjustSyncInterval();
+  Assert.ok(!scheduler.hasIncomingItems);
+  Assert.equal(scheduler.syncInterval, scheduler.singleDeviceInterval);
+});
+
+add_task(async function test_link_status_change() {
+  _("Check that we only attempt to sync when link status is up");
+  try {
+    sinon.spy(scheduler, "scheduleNextSync");
+
+    Svc.Obs.notify("network:link-status-changed", null, "down");
+    equal(scheduler.scheduleNextSync.callCount, 0);
+
+    Svc.Obs.notify("network:link-status-changed", null, "change");
+    equal(scheduler.scheduleNextSync.callCount, 0);
+
+    Svc.Obs.notify("network:link-status-changed", null, "up");
+    equal(scheduler.scheduleNextSync.callCount, 1);
+
+    Svc.Obs.notify("network:link-status-changed", null, "change");
+    equal(scheduler.scheduleNextSync.callCount, 1);
+  } finally {
+    scheduler.scheduleNextSync.restore();
+  }
+});
diff --git a/services/sync/tests/unit/test_tab_engine.js b/services/sync/tests/unit/test_tab_engine.js
new file mode 100644
index 0000000000..5b1e61871e
--- /dev/null
+++ b/services/sync/tests/unit/test_tab_engine.js
@@ -0,0 +1,226 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { TabProvider } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/tabs.sys.mjs"
+);
+const { WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+let engine;
+// We'll need the clients engine for testing as tabs is closely related
+let clientsEngine;
+
+async function syncClientsEngine(server) {
+  clientsEngine._lastFxADevicesFetch = 0;
+  clientsEngine.lastModified = server.getCollection("foo", "clients").timestamp;
+  await clientsEngine._sync();
+}
+
+async function makeRemoteClients() {
+  let server = await serverForFoo(clientsEngine);
+  await configureIdentity({ username: "foo" }, server);
+  await Service.login();
+
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let remoteId = Utils.makeGUID();
+  let remoteId2 = Utils.makeGUID();
+  let collection = server.getCollection("foo", "clients");
+
+  _("Create remote client records");
+  collection.insertRecord({
+    id: remoteId,
+    name: "Remote client",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    fxaDeviceId: remoteId,
+    fxaDeviceName: "Fxa - Remote client",
+    protocols: ["1.5"],
+  });
+
+  collection.insertRecord({
+    id: remoteId2,
+    name: "Remote client 2",
+    type: "desktop",
+    commands: [],
+    version: "48",
+    fxaDeviceId: remoteId2,
+    fxaDeviceName: "Fxa - Remote client 2",
+    protocols: ["1.5"],
+  });
+
+  let fxAccounts = clientsEngine.fxAccounts;
+  clientsEngine.fxAccounts = {
+    notifyDevices() {
+      return Promise.resolve(true);
+    },
+    device: {
+      getLocalId() {
+        return fxAccounts.device.getLocalId();
+      },
+      getLocalName() {
+        return fxAccounts.device.getLocalName();
+      },
+      getLocalType() {
+        return fxAccounts.device.getLocalType();
+      },
+      recentDeviceList: [{ id: remoteId, name: "remote device" }],
+      refreshDeviceList() {
+        return Promise.resolve(true);
+      },
+    },
+    _internal: {
+      now() {
+        return Date.now();
+      },
+    },
+  };
+
+  await syncClientsEngine(server);
+}
+
+add_task(async function setup() {
+  clientsEngine = Service.clientsEngine;
+  // Make some clients to test with
+  await makeRemoteClients();
+
+  // Make the tabs engine for all the tests to use
+  engine = Service.engineManager.get("tabs");
+  await engine.initialize();
+
+  // Since these are xpcshell tests, we'll need to mock this
+  TabProvider.shouldSkipWindow = mockShouldSkipWindow;
+});
+
+add_task(async function test_tab_engine_skips_incoming_local_record() {
+  _("Ensure incoming records that match local client ID are never applied.");
+
+  let localID = clientsEngine.localID;
+  let collection = new ServerCollection();
+
+  _("Creating remote tab record with local client ID");
+  let localRecord = encryptPayload({
+    id: localID,
+    clientName: "local",
+    tabs: [
+      {
+        title: "title",
+        urlHistory: ["http://foo.com/"],
+        icon: "",
+        lastUsed: 2000,
+      },
+    ],
+  });
+  collection.insert(localID, localRecord);
+
+  _("Creating remote tab record with a different client ID");
+  let remoteID = "fake-guid-00"; // remote should match one of the test clients
+  let remoteRecord = encryptPayload({
+    id: remoteID,
+    clientName: "not local",
+    tabs: [
+      {
+        title: "title2",
+        urlHistory: ["http://bar.com/"],
+        icon: "",
+        lastUsed: 3000,
+      },
+    ],
+  });
+  collection.insert(remoteID, remoteRecord);
+
+  _("Setting up Sync server");
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/tabs": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = {
+    tabs: { version: engine.version, syncID },
+  };
+
+  await generateNewKeys(Service.collectionKeys);
+
+  let promiseFinished = new Promise(resolve => {
+    let syncFinish = engine._syncFinish;
+    engine._syncFinish = async function () {
+      let remoteTabs = await engine._rustStore.getAll();
+      equal(
+        remoteTabs.length,
+        1,
+        "Remote client record was applied and local wasn't"
+      );
+      let record = remoteTabs[0];
+      equal(record.clientId, remoteID, "Remote client ID matches");
+
+      _("Ensure getAllClients returns the correct shape");
+      let clients = await engine.getAllClients();
+      equal(clients.length, 1);
+      let client = clients[0];
+      equal(client.id, "fake-guid-00");
+      equal(client.name, "Remote client");
+      equal(client.type, "desktop");
+      Assert.ok(client.lastModified); // lastModified should be filled in once serverModified is populated from the server
+      deepEqual(client.tabs, [
+        {
+          title: "title2",
+          urlHistory: ["http://bar.com/"],
+          icon: "",
+          inactive: false,
+          lastUsed: 3000,
+        },
+      ]);
+      await syncFinish.call(engine);
+      resolve();
+    };
+  });
+
+  _("Start sync");
+  Service.scheduler.hasIncomingItems = false;
+  await engine._sync();
+  await promiseFinished;
+  // Bug 1800185 - we don't want the sync scheduler to see these records as incoming.
+  Assert.ok(!Service.scheduler.hasIncomingItems);
+});
+
+// Ensure we trim tabs in the case of going past the max payload size allowed
+add_task(async function test_too_many_tabs() {
+  let a_lot_of_tabs = [];
+
+  for (let i = 0; i < 4000; ++i) {
+    a_lot_of_tabs.push(
+      `http://example${i}.com/some-super-long-url-chain-to-help-with-bytes`
+    );
+  }
+
+  TabProvider.getWindowEnumerator = mockGetWindowEnumerator.bind(
+    this,
+    a_lot_of_tabs
+  );
+
+  let encoder = Utils.utf8Encoder;
+  // see tryfitItems(..) in util.js
+  const computeSerializedSize = records =>
+    encoder.encode(JSON.stringify(records)).byteLength;
+
+  const maxPayloadSize = Service.getMaxRecordPayloadSize();
+  const maxSerializedSize = (maxPayloadSize / 4) * 3 - 1500;
+  // We are over max payload size
+  Assert.ok(computeSerializedSize(a_lot_of_tabs) > maxSerializedSize);
+  let tabs = await engine.getTabsWithinPayloadSize();
+  // We are now under max payload size
+  Assert.ok(computeSerializedSize(tabs) < maxSerializedSize);
+});
diff --git a/services/sync/tests/unit/test_tab_provider.js b/services/sync/tests/unit/test_tab_provider.js
new file mode 100644
index 0000000000..bbf68dea33
--- /dev/null
+++ b/services/sync/tests/unit/test_tab_provider.js
@@ -0,0 +1,64 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { TabProvider } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/tabs.sys.mjs"
+);
+
+add_task(async function test_getAllTabs() {
+  let provider = TabProvider;
+  provider.shouldSkipWindow = mockShouldSkipWindow;
+
+  let tabs;
+
+  provider.getWindowEnumerator = mockGetWindowEnumerator.bind(this, [
+    "http://bar.com",
+  ]);
+
+  _("Get all tabs.");
+  tabs = await provider.getAllTabsWithEstimatedMax(
+    false,
+    Number.MAX_SAFE_INTEGER
+  );
+  _("Tabs: " + JSON.stringify(tabs));
+  equal(tabs.length, 1);
+  equal(tabs[0].title, "title");
+  equal(tabs[0].urlHistory.length, 1);
+  equal(tabs[0].urlHistory[0], "http://bar.com/");
+  equal(tabs[0].icon, "");
+  equal(tabs[0].lastUsed, 2); // windowenumerator returns in ms but the getAllTabs..() returns in seconds
+
+  _("Get all tabs, and check that filtering works.");
+  provider.getWindowEnumerator = mockGetWindowEnumerator.bind(this, [
+    "http://foo.com",
+    "about:foo",
+  ]);
+  tabs = await provider.getAllTabsWithEstimatedMax(
+    true,
+    Number.MAX_SAFE_INTEGER
+  );
+  _("Filtered: " + JSON.stringify(tabs));
+  equal(tabs.length, 1);
+
+  _("Get all tabs, and check that they are properly sorted");
+  provider.getWindowEnumerator = mockGetWindowEnumerator.bind(this, [
+    "http://foo.com",
+    "http://bar.com",
+  ]);
+  tabs = await provider.getAllTabsWithEstimatedMax(
+    true,
+    Number.MAX_SAFE_INTEGER
+  );
+  _("Ordered: " + JSON.stringify(tabs));
+  equal(tabs[0].lastUsed > tabs[1].lastUsed, true);
+
+  // reader mode URLs are provided.
+  provider.getWindowEnumerator = mockGetWindowEnumerator.bind(this, [
+    "about:reader?url=http%3A%2F%2Ffoo.com%2F",
+  ]);
+  tabs = await provider.getAllTabsWithEstimatedMax(
+    true,
+    Number.MAX_SAFE_INTEGER
+  );
+  equal(tabs[0].urlHistory[0], "http://foo.com/");
+});
diff --git a/services/sync/tests/unit/test_tab_quickwrite.js b/services/sync/tests/unit/test_tab_quickwrite.js
new file mode 100644
index 0000000000..2a1c75c8c6
--- /dev/null
+++ b/services/sync/tests/unit/test_tab_quickwrite.js
@@ -0,0 +1,204 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+ChromeUtils.importESModule("resource://services-sync/engines/tabs.sys.mjs");
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const { TabProvider } = ChromeUtils.importESModule(
+  "resource://services-sync/engines/tabs.sys.mjs"
+);
+
+const FAR_FUTURE = 4102405200000; // 2100/01/01
+
+add_task(async function setup() {
+  // Since these are xpcshell tests, we'll need to mock ui features
+  TabProvider.shouldSkipWindow = mockShouldSkipWindow;
+  TabProvider.getWindowEnumerator = mockGetWindowEnumerator.bind(this, [
+    "http://foo.com",
+  ]);
+});
+
+async function prepareServer() {
+  _("Setting up Sync server");
+  Service.serverConfiguration = {
+    max_post_records: 100,
+  };
+
+  let server = new SyncServer();
+  server.start();
+  await SyncTestingInfrastructure(server, "username");
+  server.registerUser("username");
+
+  let collection = server.createCollection("username", "tabs");
+  await generateNewKeys(Service.collectionKeys);
+
+  let engine = Service.engineManager.get("tabs");
+  await engine.initialize();
+
+  return { server, collection, engine };
+}
+
+async function withPatchedValue(object, name, patchedVal, fn) {
+  _(`patching ${name}=${patchedVal}`);
+  let old = object[name];
+  object[name] = patchedVal;
+  try {
+    await fn();
+  } finally {
+    object[name] = old;
+  }
+}
+
+add_task(async function test_tab_quickwrite_works() {
+  _("Ensure a simple quickWrite works.");
+  let { server, collection, engine } = await prepareServer();
+  Assert.equal(collection.count(), 0, "starting with 0 tab records");
+  Assert.ok(await engine.quickWrite());
+  // Validate we didn't bork lastSync
+  let lastSync = await engine.getLastSync();
+  Assert.ok(lastSync < FAR_FUTURE);
+  Assert.equal(collection.count(), 1, "tab record was written");
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_tab_bad_status() {
+  _("Ensure quickWrite silently aborts when we aren't setup correctly.");
+  let { server, engine } = await prepareServer();
+  // Store the original lock to reset it back after this test
+  let lock = engine.lock;
+  // Arrange for this test to fail if it tries to take the lock.
+  engine.lock = function () {
+    throw new Error("this test should abort syncing before locking");
+  };
+  let quickWrite = engine.quickWrite.bind(engine); // lol javascript.
+
+  await withPatchedValue(engine, "enabled", false, quickWrite);
+  await withPatchedValue(Service, "serverConfiguration", null, quickWrite);
+
+  Services.prefs.clearUserPref("services.sync.username");
+  await quickWrite();
+  // Validate we didn't bork lastSync
+  let lastSync = await engine.getLastSync();
+  Assert.ok(lastSync < FAR_FUTURE);
+  Service.status.resetSync();
+  engine.lock = lock;
+  await promiseStopServer(server);
+});
+
+add_task(async function test_tab_quickwrite_lock() {
+  _("Ensure we fail to quickWrite if the engine is locked.");
+  let { server, collection, engine } = await prepareServer();
+
+  Assert.equal(collection.count(), 0, "starting with 0 tab records");
+  engine.lock();
+  Assert.ok(!(await engine.quickWrite()));
+  Assert.equal(collection.count(), 0, "didn't sync due to being locked");
+  engine.unlock();
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_tab_quickwrite_keeps_old_tabs() {
+  _("Ensure we don't delete other tabs on quickWrite (bug 1801295).");
+  let { server, engine } = await prepareServer();
+
+  // need a first sync to ensure everything is setup correctly.
+  await Service.sync({ engines: ["tabs"] });
+
+  const id = "fake-guid-99";
+  let remoteRecord = encryptPayload({
+    id,
+    clientName: "not local",
+    tabs: [
+      {
+        title: "title2",
+        urlHistory: ["http://bar.com/"],
+        icon: "",
+        lastUsed: 3000,
+      },
+    ],
+  });
+
+  let collection = server.getCollection("username", "tabs");
+  collection.insert(id, remoteRecord);
+
+  await Service.sync({ engines: ["tabs"] });
+
+  // collection should now have 2 records - ours and the pretend remote one we inserted.
+  Assert.equal(collection.count(), 2, "starting with 2 tab records");
+
+  // So fxAccounts.device.recentDeviceList is not null.
+  engine.service.clientsEngine.fxAccounts.device._deviceListCache = {
+    devices: [],
+  };
+  // trick the clients engine into thinking it has a remote client with the same guid.
+  engine.service.clientsEngine._store._remoteClients = {};
+  engine.service.clientsEngine._store._remoteClients[id] = {
+    id,
+    fxaDeviceId: id,
+  };
+
+  let clients = await engine.getAllClients();
+  Assert.equal(clients.length, 1);
+
+  _("Doing a quick-write");
+  Assert.ok(await engine.quickWrite());
+
+  // Should still have our client after a quickWrite.
+  _("Grabbing clients after the quick-write");
+  clients = await engine.getAllClients();
+  Assert.equal(clients.length, 1);
+
+  engine.service.clientsEngine._store._remoteClients = {};
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_tab_lastSync() {
+  _("Ensure we restore the lastSync timestamp after a quick-write.");
+  let { server, collection, engine } = await prepareServer();
+
+  await engine.initialize();
+  await engine.service.clientsEngine.initialize();
+
+  let origLastSync = engine.lastSync;
+  Assert.ok(await engine.quickWrite());
+  Assert.equal(engine.lastSync, origLastSync);
+  Assert.equal(collection.count(), 1, "successful sync");
+  engine.unlock();
+
+  await promiseStopServer(server);
+});
+
+add_task(async function test_tab_quickWrite_telemetry() {
+  _("Ensure we record the telemetry we expect.");
+  // hook into telemetry
+  let telem = get_sync_test_telemetry();
+  telem.payloads = [];
+  let oldSubmit = telem.submit;
+  let submitPromise = new Promise((resolve, reject) => {
+    telem.submit = function (ping) {
+      telem.submit = oldSubmit;
+      resolve(ping);
+    };
+  });
+
+  let { server, collection, engine } = await prepareServer();
+
+  Assert.equal(collection.count(), 0, "starting with 0 tab records");
+  Assert.ok(await engine.quickWrite());
+  Assert.equal(collection.count(), 1, "tab record was written");
+
+  let ping = await submitPromise;
+  let syncs = ping.syncs;
+  Assert.equal(syncs.length, 1);
+  let sync = syncs[0];
+  Assert.equal(sync.why, "quick-write");
+  Assert.equal(sync.engines.length, 1);
+  Assert.equal(sync.engines[0].name, "tabs");
+
+  await promiseStopServer(server);
+});
diff --git a/services/sync/tests/unit/test_tab_tracker.js b/services/sync/tests/unit/test_tab_tracker.js
new file mode 100644
index 0000000000..8bb71a898a
--- /dev/null
+++ b/services/sync/tests/unit/test_tab_tracker.js
@@ -0,0 +1,371 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+ChromeUtils.importESModule("resource://services-sync/engines/tabs.sys.mjs");
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+const { SyncScheduler } = ChromeUtils.importESModule(
+  "resource://services-sync/policies.sys.mjs"
+);
+
+var scheduler = new SyncScheduler(Service);
+let clientsEngine;
+
+add_task(async function setup() {
+  await Service.promiseInitialized;
+  clientsEngine = Service.clientsEngine;
+
+  scheduler.setDefaults();
+});
+
+function fakeSvcWinMediator() {
+  // actions on windows are captured in logs
+  let logs = [];
+  delete Services.wm;
+
+  function getNext() {
+    let elt = { addTopics: [], remTopics: [], numAPL: 0, numRPL: 0 };
+    logs.push(elt);
+    return {
+      addEventListener(topic) {
+        elt.addTopics.push(topic);
+      },
+      removeEventListener(topic) {
+        elt.remTopics.push(topic);
+      },
+      gBrowser: {
+        addProgressListener() {
+          elt.numAPL++;
+        },
+        removeProgressListener() {
+          elt.numRPL++;
+        },
+      },
+    };
+  }
+
+  Services.wm = {
+    getEnumerator() {
+      return [getNext(), getNext()];
+    },
+  };
+  return logs;
+}
+
+function fakeGetTabState(tab) {
+  return tab;
+}
+
+function clearQuickWriteTimer(tracker) {
+  if (tracker.tabsQuickWriteTimer) {
+    tracker.tabsQuickWriteTimer.clear();
+  }
+}
+
+add_task(async function run_test() {
+  let engine = Service.engineManager.get("tabs");
+  await engine.initialize();
+  _("We assume that tabs have changed at startup.");
+  let tracker = engine._tracker;
+  tracker.getTabState = fakeGetTabState;
+
+  Assert.ok(tracker.modified);
+  Assert.ok(
+    Utils.deepEquals(Object.keys(await engine.getChangedIDs()), [
+      clientsEngine.localID,
+    ])
+  );
+
+  let logs;
+
+  _("Test listeners are registered on windows");
+  logs = fakeSvcWinMediator();
+  tracker.start();
+  Assert.equal(logs.length, 2);
+  for (let log of logs) {
+    Assert.equal(log.addTopics.length, 3);
+    Assert.ok(log.addTopics.includes("TabOpen"));
+    Assert.ok(log.addTopics.includes("TabClose"));
+    Assert.ok(log.addTopics.includes("unload"));
+    Assert.equal(log.remTopics.length, 0);
+    Assert.equal(log.numAPL, 1, "Added 1 progress listener");
+    Assert.equal(log.numRPL, 0, "Didn't remove a progress listener");
+  }
+
+  _("Test listeners are unregistered on windows");
+  logs = fakeSvcWinMediator();
+  await tracker.stop();
+  Assert.equal(logs.length, 2);
+  for (let log of logs) {
+    Assert.equal(log.addTopics.length, 0);
+    Assert.equal(log.remTopics.length, 3);
+    Assert.ok(log.remTopics.includes("TabOpen"));
+    Assert.ok(log.remTopics.includes("TabClose"));
+    Assert.ok(log.remTopics.includes("unload"));
+    Assert.equal(log.numAPL, 0, "Didn't add a progress listener");
+    Assert.equal(log.numRPL, 1, "Removed 1 progress listener");
+  }
+
+  _("Test tab listener");
+  for (let evttype of ["TabOpen", "TabClose"]) {
+    // Pretend we just synced.
+    await tracker.clearChangedIDs();
+    Assert.ok(!tracker.modified);
+
+    // Send a fake tab event
+    tracker.onTab({
+      type: evttype,
+      originalTarget: evttype,
+      target: { entries: [], currentURI: "about:config" },
+    });
+    Assert.ok(tracker.modified);
+    Assert.ok(
+      Utils.deepEquals(Object.keys(await engine.getChangedIDs()), [
+        clientsEngine.localID,
+      ])
+    );
+  }
+
+  // Pretend we just synced.
+  await tracker.clearChangedIDs();
+  Assert.ok(!tracker.modified);
+
+  tracker.onTab({
+    type: "TabOpen",
+    originalTarget: "TabOpen",
+    target: { entries: [], currentURI: "about:config" },
+  });
+  Assert.ok(
+    Utils.deepEquals(Object.keys(await engine.getChangedIDs()), [
+      clientsEngine.localID,
+    ])
+  );
+
+  // Pretend we just synced and saw some progress listeners.
+  await tracker.clearChangedIDs();
+  Assert.ok(!tracker.modified);
+  tracker.onLocationChange({ isTopLevel: false }, undefined, undefined, 0);
+  Assert.ok(!tracker.modified, "non-toplevel request didn't flag as modified");
+
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("https://www.mozilla.org"),
+    Ci.nsIWebProgressListener.LOCATION_CHANGE_SAME_DOCUMENT
+  );
+  Assert.ok(
+    tracker.modified,
+    "location change within the same document request did flag as modified"
+  );
+
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("https://www.mozilla.org")
+  );
+  Assert.ok(
+    tracker.modified,
+    "location change for a new top-level document flagged as modified"
+  );
+  Assert.ok(
+    Utils.deepEquals(Object.keys(await engine.getChangedIDs()), [
+      clientsEngine.localID,
+    ])
+  );
+});
+
+add_task(async function run_sync_on_tab_change_test() {
+  let testPrefDelay = 20000;
+
+  // This is the pref that determines sync delay after tab change
+  Svc.PrefBranch.setIntPref(
+    "syncedTabs.syncDelayAfterTabChange",
+    testPrefDelay
+  );
+  // We should only be syncing on tab change if
+  // the user has > 1 client
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.numClients, 2);
+
+  let engine = Service.engineManager.get("tabs");
+
+  _("We assume that tabs have changed at startup.");
+  let tracker = engine._tracker;
+  tracker.getTabState = fakeGetTabState;
+
+  Assert.ok(tracker.modified);
+  Assert.ok(
+    Utils.deepEquals(Object.keys(await engine.getChangedIDs()), [
+      clientsEngine.localID,
+    ])
+  );
+
+  _("Test sync is scheduled after a tab change");
+  for (let evttype of ["TabOpen", "TabClose"]) {
+    // Pretend we just synced
+    await tracker.clearChangedIDs();
+    clearQuickWriteTimer(tracker);
+
+    // Send a fake tab event
+    tracker.onTab({
+      type: evttype,
+      originalTarget: evttype,
+      target: { entries: [], currentURI: "about:config" },
+    });
+    // Ensure the tracker fired
+    Assert.ok(tracker.modified);
+    // We should be more delayed at or more than what the pref is set at
+    let nextSchedule = tracker.tabsQuickWriteTimer.delay;
+    Assert.ok(nextSchedule >= testPrefDelay);
+  }
+
+  _("Test sync is NOT scheduled after an unsupported tab open");
+  for (let evttype of ["TabOpen"]) {
+    // Send a fake tab event
+    tracker.onTab({
+      type: evttype,
+      originalTarget: evttype,
+      target: { entries: ["about:newtab"], currentURI: null },
+    });
+    // Ensure the tracker fired
+    Assert.ok(tracker.modified);
+    // We should be scheduling <= pref value
+    Assert.ok(scheduler.nextSync - Date.now() <= testPrefDelay);
+  }
+
+  _("Test navigating within the same tab does NOT trigger a sync");
+  // Pretend we just synced
+  await tracker.clearChangedIDs();
+  clearQuickWriteTimer(tracker);
+
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("https://www.mozilla.org"),
+    Ci.nsIWebProgressListener.LOCATION_CHANGE_RELOAD
+  );
+  Assert.ok(
+    !tracker.modified,
+    "location change for reloading doesn't trigger a sync"
+  );
+  Assert.ok(!tracker.tabsQuickWriteTimer, "reload does not trigger a sync");
+
+  // Pretend we just synced
+  await tracker.clearChangedIDs();
+  clearQuickWriteTimer(tracker);
+
+  _("Test navigating to an about page does trigger sync");
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("about:config")
+  );
+  Assert.ok(tracker.modified, "about page does not trigger a tab modified");
+  Assert.ok(
+    tracker.tabsQuickWriteTimer,
+    "about schema should trigger a sync happening soon"
+  );
+
+  _("Test adjusting the filterScheme pref works");
+  // Pretend we just synced
+  await tracker.clearChangedIDs();
+  clearQuickWriteTimer(tracker);
+
+  Svc.PrefBranch.setStringPref(
+    "engine.tabs.filteredSchemes",
+    // Removing the about scheme for this test
+    "resource|chrome|file|blob|moz-extension"
+  );
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("about:config")
+  );
+  Assert.ok(
+    tracker.modified,
+    "about page triggers a modified after we changed the pref"
+  );
+  Assert.ok(
+    tracker.tabsQuickWriteTimer,
+    "about page should schedule a quickWrite sync soon after we changed the pref"
+  );
+
+  _("Test no sync after tab change for accounts with <= 1 clients");
+  // Pretend we just synced
+  await tracker.clearChangedIDs();
+  clearQuickWriteTimer(tracker);
+  // Setting clients to only 1 so we don't sync after a tab change
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 0);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.numClients, 1);
+
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("https://www.mozilla.org")
+  );
+  Assert.ok(
+    tracker.modified,
+    "location change for a new top-level document flagged as modified"
+  );
+  Assert.ok(
+    !tracker.tabsQuickWriteTimer,
+    "We should NOT be syncing shortly because there is only one client"
+  );
+
+  _("Changing the pref adjusts the sync schedule");
+  Svc.PrefBranch.setIntPref("syncedTabs.syncDelayAfterTabChange", 10000); // 10seconds
+  let delayPref = Svc.PrefBranch.getIntPref(
+    "syncedTabs.syncDelayAfterTabChange"
+  );
+  let evttype = "TabOpen";
+  Assert.equal(delayPref, 10000); // ensure our pref is at 10s
+  // Only have task continuity if we have more than 1 device
+  Svc.PrefBranch.setIntPref("clients.devices.desktop", 1);
+  Svc.PrefBranch.setIntPref("clients.devices.mobile", 1);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.numClients, 2);
+  clearQuickWriteTimer(tracker);
+
+  // Fire ontab event
+  tracker.onTab({
+    type: evttype,
+    originalTarget: evttype,
+    target: { entries: [], currentURI: "about:config" },
+  });
+
+  // Ensure the tracker fired
+  Assert.ok(tracker.modified);
+  // We should be scheduling <= preference value
+  Assert.equal(tracker.tabsQuickWriteTimer.delay, delayPref);
+
+  _("We should not have a sync scheduled if pref is at 0");
+
+  Svc.PrefBranch.setIntPref("syncedTabs.syncDelayAfterTabChange", 0);
+  // Pretend we just synced
+  await tracker.clearChangedIDs();
+  clearQuickWriteTimer(tracker);
+
+  // Fire ontab event
+  evttype = "TabOpen";
+  tracker.onTab({
+    type: evttype,
+    originalTarget: evttype,
+    target: { entries: [], currentURI: "about:config" },
+  });
+  // Ensure the tracker fired
+  Assert.ok(tracker.modified);
+
+  // We should NOT be scheduled for a sync soon
+  Assert.ok(!tracker.tabsQuickWriteTimer);
+
+  scheduler.setDefaults();
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+});
diff --git a/services/sync/tests/unit/test_telemetry.js b/services/sync/tests/unit/test_telemetry.js
new file mode 100644
index 0000000000..961e96a01b
--- /dev/null
+++ b/services/sync/tests/unit/test_telemetry.js
@@ -0,0 +1,1462 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { WBORecord } = ChromeUtils.importESModule(
+  "resource://services-sync/record.sys.mjs"
+);
+const { Resource } = ChromeUtils.importESModule(
+  "resource://services-sync/resource.sys.mjs"
+);
+const { RotaryEngine } = ChromeUtils.importESModule(
+  "resource://testing-common/services/sync/rotaryengine.sys.mjs"
+);
+const { getFxAccountsSingleton } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccounts.sys.mjs"
+);
+const fxAccounts = getFxAccountsSingleton();
+
+function SteamStore(engine) {
+  Store.call(this, "Steam", engine);
+}
+Object.setPrototypeOf(SteamStore.prototype, Store.prototype);
+
+function SteamTracker(name, engine) {
+  LegacyTracker.call(this, name || "Steam", engine);
+}
+Object.setPrototypeOf(SteamTracker.prototype, LegacyTracker.prototype);
+
+function SteamEngine(service) {
+  SyncEngine.call(this, "steam", service);
+}
+
+SteamEngine.prototype = {
+  _storeObj: SteamStore,
+  _trackerObj: SteamTracker,
+  _errToThrow: null,
+  problemsToReport: null,
+  async _sync() {
+    if (this._errToThrow) {
+      throw this._errToThrow;
+    }
+  },
+  getValidator() {
+    return new SteamValidator();
+  },
+};
+Object.setPrototypeOf(SteamEngine.prototype, SyncEngine.prototype);
+
+function BogusEngine(service) {
+  SyncEngine.call(this, "bogus", service);
+}
+
+BogusEngine.prototype = Object.create(SteamEngine.prototype);
+
+class SteamValidator {
+  async canValidate() {
+    return true;
+  }
+
+  async validate(engine) {
+    return {
+      problems: new SteamValidationProblemData(engine.problemsToReport),
+      version: 1,
+      duration: 0,
+      recordCount: 0,
+    };
+  }
+}
+
+class SteamValidationProblemData {
+  constructor(problemsToReport = []) {
+    this.problemsToReport = problemsToReport;
+  }
+
+  getSummary() {
+    return this.problemsToReport;
+  }
+}
+
+async function cleanAndGo(engine, server) {
+  await engine._tracker.clearChangedIDs();
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  syncTestLogging();
+  Service.recordManager.clearCache();
+  await promiseStopServer(server);
+}
+
+add_task(async function setup() {
+  // Avoid addon manager complaining about not being initialized
+  await Service.engineManager.unregister("addons");
+  await Service.engineManager.unregister("extension-storage");
+});
+
+add_task(async function test_basic() {
+  enableValidationPrefs();
+
+  let helper = track_collections_helper();
+  let upd = helper.with_updated_collection;
+
+  let handlers = {
+    "/1.1/johndoe/info/collections": helper.handler,
+    "/1.1/johndoe/storage/crypto/keys": upd(
+      "crypto",
+      new ServerWBO("keys").handler()
+    ),
+    "/1.1/johndoe/storage/meta/global": upd(
+      "meta",
+      new ServerWBO("global").handler()
+    ),
+  };
+
+  let collections = [
+    "clients",
+    "bookmarks",
+    "forms",
+    "history",
+    "passwords",
+    "prefs",
+    "tabs",
+  ];
+
+  for (let coll of collections) {
+    handlers["/1.1/johndoe/storage/" + coll] = upd(
+      coll,
+      new ServerCollection({}, true).handler()
+    );
+  }
+
+  let server = httpd_setup(handlers);
+  await configureIdentity({ username: "johndoe" }, server);
+
+  let ping = await wait_for_ping(() => Service.sync(), true, true);
+
+  // Check the "os" block - we can't really check specific values, but can
+  // check it smells sane.
+  ok(ping.os, "there is an OS block");
+  ok("name" in ping.os, "there is an OS name");
+  ok("version" in ping.os, "there is an OS version");
+  ok("locale" in ping.os, "there is an OS locale");
+
+  for (const pref of Svc.PrefBranch.getChildList("")) {
+    Svc.PrefBranch.clearUserPref(pref);
+  }
+  await promiseStopServer(server);
+});
+
+add_task(async function test_processIncoming_error() {
+  let engine = Service.engineManager.get("bookmarks");
+  await engine.initialize();
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let collection = server.user("foo").collection("bookmarks");
+  try {
+    // Create a bogus record that when synced down will provoke a
+    // network error which in turn provokes an exception in _processIncoming.
+    const BOGUS_GUID = "zzzzzzzzzzzz";
+    let bogus_record = collection.insert(BOGUS_GUID, "I'm a bogus record!");
+    bogus_record.get = function get() {
+      throw new Error("Sync this!");
+    };
+    // Make the 10 minutes old so it will only be synced in the toFetch phase.
+    bogus_record.modified = Date.now() / 1000 - 60 * 10;
+    await engine.setLastSync(Date.now() / 1000 - 60);
+    engine.toFetch = new SerializableSet([BOGUS_GUID]);
+
+    let error, pingPayload, fullPing;
+    try {
+      await sync_engine_and_validate_telem(
+        engine,
+        true,
+        (errPing, fullErrPing) => {
+          pingPayload = errPing;
+          fullPing = fullErrPing;
+        }
+      );
+    } catch (ex) {
+      error = ex;
+    }
+    ok(!!error);
+    ok(!!pingPayload);
+
+    equal(fullPing.uid, "f".repeat(32)); // as setup by SyncTestingInfrastructure
+    deepEqual(pingPayload.failureReason, {
+      name: "httperror",
+      code: 500,
+    });
+
+    equal(pingPayload.engines.length, 1);
+
+    equal(pingPayload.engines[0].name, "bookmarks-buffered");
+    deepEqual(pingPayload.engines[0].failureReason, {
+      name: "httperror",
+      code: 500,
+    });
+  } finally {
+    await store.wipe();
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_uploading() {
+  let engine = Service.engineManager.get("bookmarks");
+  await engine.initialize();
+  let store = engine._store;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  let bmk = await PlacesUtils.bookmarks.insert({
+    parentGuid: PlacesUtils.bookmarks.toolbarGuid,
+    url: "http://getfirefox.com/",
+    title: "Get Firefox!",
+  });
+
+  try {
+    let ping = await sync_engine_and_validate_telem(engine, false);
+    ok(!!ping);
+    equal(ping.engines.length, 1);
+    equal(ping.engines[0].name, "bookmarks-buffered");
+    ok(!!ping.engines[0].outgoing);
+    greater(ping.engines[0].outgoing[0].sent, 0);
+    ok(!ping.engines[0].incoming);
+
+    await PlacesUtils.bookmarks.update({
+      guid: bmk.guid,
+      title: "New Title",
+    });
+
+    await store.wipe();
+    await engine.resetClient();
+    // We don't sync via the service, so don't re-hit info/collections, so
+    // lastModified remaning at zero breaks things subtly...
+    engine.lastModified = null;
+
+    ping = await sync_engine_and_validate_telem(engine, false);
+    equal(ping.engines.length, 1);
+    equal(ping.engines[0].name, "bookmarks-buffered");
+    equal(ping.engines[0].outgoing.length, 1);
+    ok(!!ping.engines[0].incoming);
+  } finally {
+    // Clean up.
+    await store.wipe();
+    await cleanAndGo(engine, server);
+  }
+});
+
+add_task(async function test_upload_failed() {
+  let collection = new ServerCollection();
+  collection._wbos.flying = new ServerWBO("flying");
+
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+
+  await SyncTestingInfrastructure(server);
+  await configureIdentity({ username: "foo" }, server);
+
+  let engine = new RotaryEngine(Service);
+  engine._store.items = {
+    flying: "LNER Class A3 4472",
+    scotsman: "Flying Scotsman",
+    peppercorn: "Peppercorn Class",
+  };
+  const FLYING_CHANGED = 12345;
+  const SCOTSMAN_CHANGED = 23456;
+  const PEPPERCORN_CHANGED = 34567;
+  await engine._tracker.addChangedID("flying", FLYING_CHANGED);
+  await engine._tracker.addChangedID("scotsman", SCOTSMAN_CHANGED);
+  await engine._tracker.addChangedID("peppercorn", PEPPERCORN_CHANGED);
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    await engine.setLastSync(123); // needs to be non-zero so that tracker is queried
+    let changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_upload_failed: Rotary tracker contents at first sync: ${JSON.stringify(
+        changes
+      )}`
+    );
+    engine.enabled = true;
+    let ping = await sync_engine_and_validate_telem(engine, true);
+    ok(!!ping);
+    equal(ping.engines.length, 1);
+    equal(ping.engines[0].incoming, null);
+    deepEqual(ping.engines[0].outgoing, [
+      {
+        sent: 3,
+        failed: 2,
+        failedReasons: [
+          { name: "scotsman", count: 1 },
+          { name: "peppercorn", count: 1 },
+        ],
+      },
+    ]);
+    await engine.setLastSync(123);
+
+    changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_upload_failed: Rotary tracker contents at second sync: ${JSON.stringify(
+        changes
+      )}`
+    );
+    ping = await sync_engine_and_validate_telem(engine, true);
+    ok(!!ping);
+    equal(ping.engines.length, 1);
+    deepEqual(ping.engines[0].outgoing, [
+      {
+        sent: 2,
+        failed: 2,
+        failedReasons: [
+          { name: "scotsman", count: 1 },
+          { name: "peppercorn", count: 1 },
+        ],
+      },
+    ]);
+  } finally {
+    await cleanAndGo(engine, server);
+    await engine.finalize();
+  }
+});
+
+add_task(async function test_sync_partialUpload() {
+  let collection = new ServerCollection();
+  let server = sync_httpd_setup({
+    "/1.1/foo/storage/rotary": collection.handler(),
+  });
+  await SyncTestingInfrastructure(server);
+  await generateNewKeys(Service.collectionKeys);
+
+  let engine = new RotaryEngine(Service);
+  await engine.setLastSync(123);
+
+  // Create a bunch of records (and server side handlers)
+  for (let i = 0; i < 234; i++) {
+    let id = "record-no-" + i;
+    engine._store.items[id] = "Record No. " + i;
+    await engine._tracker.addChangedID(id, i);
+    // Let two items in the first upload batch fail.
+    if (i != 23 && i != 42) {
+      collection.insert(id);
+    }
+  }
+
+  let syncID = await engine.resetLocalSyncID();
+  let meta_global = Service.recordManager.set(
+    engine.metaURL,
+    new WBORecord(engine.metaURL)
+  );
+  meta_global.payload.engines = { rotary: { version: engine.version, syncID } };
+
+  try {
+    let changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_sync_partialUpload: Rotary tracker contents at first sync: ${JSON.stringify(
+        changes
+      )}`
+    );
+    engine.enabled = true;
+    let ping = await sync_engine_and_validate_telem(engine, true);
+
+    ok(!!ping);
+    ok(!ping.failureReason);
+    equal(ping.engines.length, 1);
+    equal(ping.engines[0].name, "rotary");
+    ok(!ping.engines[0].incoming);
+    ok(!ping.engines[0].failureReason);
+    deepEqual(ping.engines[0].outgoing, [
+      {
+        sent: 234,
+        failed: 2,
+        failedReasons: [
+          { name: "record-no-23", count: 1 },
+          { name: "record-no-42", count: 1 },
+        ],
+      },
+    ]);
+    collection.post = function () {
+      throw new Error("Failure");
+    };
+
+    engine._store.items["record-no-1000"] = "Record No. 1000";
+    await engine._tracker.addChangedID("record-no-1000", 1000);
+    collection.insert("record-no-1000", 1000);
+
+    await engine.setLastSync(123);
+    ping = null;
+
+    changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_sync_partialUpload: Rotary tracker contents at second sync: ${JSON.stringify(
+        changes
+      )}`
+    );
+    try {
+      // should throw
+      await sync_engine_and_validate_telem(
+        engine,
+        true,
+        errPing => (ping = errPing)
+      );
+    } catch (e) {}
+    // It would be nice if we had a more descriptive error for this...
+    let uploadFailureError = {
+      name: "httperror",
+      code: 500,
+    };
+
+    ok(!!ping);
+    deepEqual(ping.failureReason, uploadFailureError);
+    equal(ping.engines.length, 1);
+    equal(ping.engines[0].name, "rotary");
+    deepEqual(ping.engines[0].incoming, {
+      failed: 1,
+      failedReasons: [{ name: "No ciphertext: nothing to decrypt?", count: 1 }],
+    });
+    ok(!ping.engines[0].outgoing);
+    deepEqual(ping.engines[0].failureReason, uploadFailureError);
+  } finally {
+    await cleanAndGo(engine, server);
+    await engine.finalize();
+  }
+});
+
+add_task(async function test_generic_engine_fail() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let e = new Error("generic failure message");
+  engine._errToThrow = e;
+
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_generic_engine_fail: Steam tracker contents: ${JSON.stringify(
+        changes
+      )}`
+    );
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      deepEqual(ping.engines.find(err => err.name === "steam").failureReason, {
+        name: "unexpectederror",
+        error: String(e),
+      });
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_engine_fail_weird_errors() {
+  enableValidationPrefs();
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  try {
+    let msg = "Bad things happened!";
+    engine._errToThrow = { message: msg };
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      deepEqual(ping.engines.find(err => err.name === "steam").failureReason, {
+        name: "unexpectederror",
+        error: "Bad things happened!",
+      });
+    });
+    let e = { msg };
+    engine._errToThrow = e;
+    await sync_and_validate_telem(ping => {
+      deepEqual(ping.engines.find(err => err.name === "steam").failureReason, {
+        name: "unexpectederror",
+        error: JSON.stringify(e),
+      });
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_overrideTelemetryName() {
+  enableValidationPrefs(["steam"]);
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.overrideTelemetryName = "steam-but-better";
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+
+  const problemsToReport = [
+    { name: "someProblem", count: 123 },
+    { name: "anotherProblem", count: 456 },
+  ];
+
+  try {
+    info("Sync with validation problems");
+    engine.problemsToReport = problemsToReport;
+    await sync_and_validate_telem(ping => {
+      let enginePing = ping.engines.find(e => e.name === "steam-but-better");
+      ok(enginePing);
+      ok(!ping.engines.find(e => e.name === "steam"));
+      deepEqual(
+        enginePing.validation,
+        {
+          version: 1,
+          checked: 0,
+          problems: problemsToReport,
+        },
+        "Should include validation report with overridden name"
+      );
+    });
+
+    info("Sync without validation problems");
+    engine.problemsToReport = null;
+    await sync_and_validate_telem(ping => {
+      let enginePing = ping.engines.find(e => e.name === "steam-but-better");
+      ok(enginePing);
+      ok(!ping.engines.find(e => e.name === "steam"));
+      ok(
+        !enginePing.validation,
+        "Should not include validation report when there are no problems"
+      );
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_engine_fail_ioerror() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  // create an IOError to re-throw as part of Sync.
+  try {
+    // (Note that fakeservices.js has replaced Utils.jsonMove etc, but for
+    // this test we need the real one so we get real exceptions from the
+    // filesystem.)
+    await Utils._real_jsonMove("file-does-not-exist", "anything", {});
+  } catch (ex) {
+    engine._errToThrow = ex;
+  }
+  ok(engine._errToThrow, "expecting exception");
+
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_engine_fail_ioerror: Steam tracker contents: ${JSON.stringify(
+        changes
+      )}`
+    );
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      let failureReason = ping.engines.find(
+        e => e.name === "steam"
+      ).failureReason;
+      equal(failureReason.name, "unexpectederror");
+      // ensure the profile dir in the exception message has been stripped.
+      ok(
+        !failureReason.error.includes(PathUtils.profileDir),
+        failureReason.error
+      );
+      ok(failureReason.error.includes("[profileDir]"), failureReason.error);
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_error_detections() {
+  let telem = get_sync_test_telemetry();
+
+  // Non-network NS_ERROR_ codes get their own category.
+  Assert.deepEqual(
+    telem.transformError(Components.Exception("", Cr.NS_ERROR_FAILURE)),
+    { name: "nserror", code: Cr.NS_ERROR_FAILURE }
+  );
+
+  // Some NS_ERROR_ code in the "network" module are treated as http errors.
+  Assert.deepEqual(
+    telem.transformError(Components.Exception("", Cr.NS_ERROR_UNKNOWN_HOST)),
+    { name: "httperror", code: Cr.NS_ERROR_UNKNOWN_HOST }
+  );
+  // Some NS_ERROR_ABORT is treated as network by our telemetry.
+  Assert.deepEqual(
+    telem.transformError(Components.Exception("", Cr.NS_ERROR_ABORT)),
+    { name: "httperror", code: Cr.NS_ERROR_ABORT }
+  );
+});
+
+add_task(async function test_clean_urls() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  engine._errToThrow = new TypeError(
+    "http://www.google .com is not a valid URL."
+  );
+
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(`test_clean_urls: Steam tracker contents: ${JSON.stringify(changes)}`);
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      let failureReason = ping.engines.find(
+        e => e.name === "steam"
+      ).failureReason;
+      equal(failureReason.name, "unexpectederror");
+      equal(failureReason.error, "<URL> is not a valid URL.");
+    });
+    // Handle other errors that include urls.
+    engine._errToThrow =
+      "Other error message that includes some:url/foo/bar/ in it.";
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      let failureReason = ping.engines.find(
+        e => e.name === "steam"
+      ).failureReason;
+      equal(failureReason.name, "unexpectederror");
+      equal(
+        failureReason.error,
+        "Other error message that includes <URL> in it."
+      );
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+// Test sanitizing guid-related errors with the pattern of <guid: {guid}>
+add_task(async function test_sanitize_bookmarks_guid() {
+  let { ErrorSanitizer } = ChromeUtils.importESModule(
+    "resource://services-sync/telemetry.sys.mjs"
+  );
+
+  for (let [original, expected] of [
+    [
+      "Can't insert Bookmark <guid: sknD84IdnSY2> into Folder <guid: odfninDdi93_3>",
+      "Can't insert Bookmark <GUID> into Folder <GUID>",
+    ],
+    [
+      "Merge Error: Item <guid: H6fmPA16gZs9> can't contain itself",
+      "Merge Error: Item <GUID> can't contain itself",
+    ],
+  ]) {
+    const sanitized = ErrorSanitizer.cleanErrorMessage(original);
+    Assert.equal(sanitized, expected);
+  }
+});
+
+// Test sanitization of some hard-coded error strings.
+add_task(async function test_clean_errors() {
+  let { ErrorSanitizer } = ChromeUtils.importESModule(
+    "resource://services-sync/telemetry.sys.mjs"
+  );
+
+  for (let [message, name, expected] of [
+    [
+      `Could not open the file at ${PathUtils.join(
+        PathUtils.profileDir,
+        "weave",
+        "addonsreconciler.json"
+      )} for writing`,
+      "NotFoundError",
+      "OS error [File/Path not found] Could not open the file at [profileDir]/weave/addonsreconciler.json for writing",
+    ],
+    [
+      `Could not get info for the file at ${PathUtils.join(
+        PathUtils.profileDir,
+        "weave",
+        "addonsreconciler.json"
+      )}`,
+      "NotAllowedError",
+      "OS error [Permission denied] Could not get info for the file at [profileDir]/weave/addonsreconciler.json",
+    ],
+  ]) {
+    const error = new DOMException(message, name);
+    const sanitized = ErrorSanitizer.cleanErrorMessage(message, error);
+    Assert.equal(sanitized, expected);
+  }
+});
+
+// Arrange for a sync to hit a "real" OS error during a sync and make sure it's sanitized.
+add_task(async function test_clean_real_os_error() {
+  enableValidationPrefs();
+
+  // Simulate a real error.
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let path = PathUtils.join(PathUtils.profileDir, "no", "such", "path.json");
+  try {
+    await IOUtils.readJSON(path);
+    throw new Error("should fail to read the file");
+  } catch (ex) {
+    engine._errToThrow = ex;
+  }
+
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(`test_clean_urls: Steam tracker contents: ${JSON.stringify(changes)}`);
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      let failureReason = ping.engines.find(
+        e => e.name === "steam"
+      ).failureReason;
+      equal(failureReason.name, "unexpectederror");
+      equal(
+        failureReason.error,
+        "OS error [File/Path not found] Could not open the file at [profileDir]/no/such/path.json"
+      );
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_initial_sync_engines() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  // These are the only ones who actually have things to sync at startup.
+  let telemetryEngineNames = ["clients", "prefs", "tabs", "bookmarks-buffered"];
+  let server = await serverForEnginesWithKeys(
+    { foo: "password" },
+    ["bookmarks", "prefs", "tabs"].map(name => Service.engineManager.get(name))
+  );
+  await SyncTestingInfrastructure(server);
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_initial_sync_engines: Steam tracker contents: ${JSON.stringify(
+        changes
+      )}`
+    );
+    let ping = await wait_for_ping(() => Service.sync(), true);
+
+    equal(ping.engines.find(e => e.name === "clients").outgoing[0].sent, 1);
+    equal(ping.engines.find(e => e.name === "tabs").outgoing[0].sent, 1);
+
+    // for the rest we don't care about specifics
+    for (let e of ping.engines) {
+      if (!telemetryEngineNames.includes(engine.name)) {
+        continue;
+      }
+      greaterOrEqual(e.took, 1);
+      ok(!!e.outgoing);
+      equal(e.outgoing.length, 1);
+      notEqual(e.outgoing[0].sent, undefined);
+      equal(e.outgoing[0].failed, undefined);
+      equal(e.outgoing[0].failedReasons, undefined);
+    }
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_nserror() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  engine._errToThrow = Components.Exception(
+    "NS_ERROR_UNKNOWN_HOST",
+    Cr.NS_ERROR_UNKNOWN_HOST
+  );
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(`test_nserror: Steam tracker contents: ${JSON.stringify(changes)}`);
+    await sync_and_validate_telem(ping => {
+      deepEqual(ping.status, {
+        service: SYNC_FAILED_PARTIAL,
+        sync: LOGIN_FAILED_NETWORK_ERROR,
+      });
+      let enginePing = ping.engines.find(e => e.name === "steam");
+      deepEqual(enginePing.failureReason, {
+        name: "httperror",
+        code: Cr.NS_ERROR_UNKNOWN_HOST,
+      });
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_sync_why() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(SteamEngine);
+  let engine = Service.engineManager.get("steam");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  await SyncTestingInfrastructure(server);
+  let e = new Error("generic failure message");
+  engine._errToThrow = e;
+
+  try {
+    const changes = await engine._tracker.getChangedIDs();
+    _(
+      `test_generic_engine_fail: Steam tracker contents: ${JSON.stringify(
+        changes
+      )}`
+    );
+    let ping = await wait_for_ping(
+      () => Service.sync({ why: "user" }),
+      true,
+      false
+    );
+    _(JSON.stringify(ping));
+    equal(ping.why, "user");
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_discarding() {
+  enableValidationPrefs();
+
+  let helper = track_collections_helper();
+  let upd = helper.with_updated_collection;
+  let telem = get_sync_test_telemetry();
+  telem.maxPayloadCount = 2;
+  telem.submissionInterval = Infinity;
+  let oldSubmit = telem.submit;
+
+  let server;
+  try {
+    let handlers = {
+      "/1.1/johndoe/info/collections": helper.handler,
+      "/1.1/johndoe/storage/crypto/keys": upd(
+        "crypto",
+        new ServerWBO("keys").handler()
+      ),
+      "/1.1/johndoe/storage/meta/global": upd(
+        "meta",
+        new ServerWBO("global").handler()
+      ),
+    };
+
+    let collections = [
+      "clients",
+      "bookmarks",
+      "forms",
+      "history",
+      "passwords",
+      "prefs",
+      "tabs",
+    ];
+
+    for (let coll of collections) {
+      handlers["/1.1/johndoe/storage/" + coll] = upd(
+        coll,
+        new ServerCollection({}, true).handler()
+      );
+    }
+
+    server = httpd_setup(handlers);
+    await configureIdentity({ username: "johndoe" }, server);
+    telem.submit = p =>
+      ok(
+        false,
+        "Submitted telemetry ping when we should not have" + JSON.stringify(p)
+      );
+
+    for (let i = 0; i < 5; ++i) {
+      await Service.sync();
+    }
+    telem.submit = oldSubmit;
+    telem.submissionInterval = -1;
+    let ping = await wait_for_ping(() => Service.sync(), true, true); // with this we've synced 6 times
+    equal(ping.syncs.length, 2);
+    equal(ping.discarded, 4);
+  } finally {
+    telem.maxPayloadCount = 500;
+    telem.submissionInterval = -1;
+    telem.submit = oldSubmit;
+    if (server) {
+      await promiseStopServer(server);
+    }
+  }
+});
+
+add_task(async function test_submit_interval() {
+  let telem = get_sync_test_telemetry();
+  let oldSubmit = telem.submit;
+  let numSubmissions = 0;
+  telem.submit = function () {
+    numSubmissions += 1;
+  };
+
+  function notify(what, data = null) {
+    Svc.Obs.notify(what, JSON.stringify(data));
+  }
+
+  try {
+    // submissionInterval is set such that each sync should submit
+    notify("weave:service:sync:start", { why: "testing" });
+    notify("weave:service:sync:finish");
+    Assert.equal(numSubmissions, 1, "should submit this ping due to interval");
+
+    // As should each event outside of a sync.
+    Service.recordTelemetryEvent("object", "method");
+    Assert.equal(numSubmissions, 2);
+
+    // But events while we are syncing should not.
+    notify("weave:service:sync:start", { why: "testing" });
+    Service.recordTelemetryEvent("object", "method");
+    Assert.equal(numSubmissions, 2, "no submission for this event");
+    notify("weave:service:sync:finish");
+    Assert.equal(numSubmissions, 3, "was submitted after sync finish");
+  } finally {
+    telem.submit = oldSubmit;
+  }
+});
+
+add_task(async function test_no_foreign_engines_in_error_ping() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+  engine._errToThrow = new Error("Oh no!");
+  await SyncTestingInfrastructure(server);
+  try {
+    await sync_and_validate_telem(ping => {
+      equal(ping.status.service, SYNC_FAILED_PARTIAL);
+      ok(ping.engines.every(e => e.name !== "bogus"));
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_no_foreign_engines_in_success_ping() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  try {
+    await sync_and_validate_telem(ping => {
+      ok(ping.engines.every(e => e.name !== "bogus"));
+    });
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_events() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+
+  let telem = get_sync_test_telemetry();
+  telem.submissionInterval = Infinity;
+
+  try {
+    let serverTime = Resource.serverTime;
+    Service.recordTelemetryEvent("object", "method", "value", { foo: "bar" });
+    let ping = await wait_for_ping(() => Service.sync(), true, true);
+    equal(ping.events.length, 1);
+    let [timestamp, category, method, object, value, extra] = ping.events[0];
+    ok(typeof timestamp == "number" && timestamp > 0); // timestamp.
+    equal(category, "sync");
+    equal(method, "method");
+    equal(object, "object");
+    equal(value, "value");
+    deepEqual(extra, { foo: "bar", serverTime: String(serverTime) });
+    ping = await wait_for_ping(
+      () => {
+        // Test with optional values.
+        Service.recordTelemetryEvent("object", "method");
+      },
+      false,
+      true
+    );
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 4);
+
+    ping = await wait_for_ping(
+      () => {
+        Service.recordTelemetryEvent("object", "method", "extra");
+      },
+      false,
+      true
+    );
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 5);
+
+    ping = await wait_for_ping(
+      () => {
+        Service.recordTelemetryEvent("object", "method", undefined, {
+          foo: "bar",
+        });
+      },
+      false,
+      true
+    );
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 6);
+    [timestamp, category, method, object, value, extra] = ping.events[0];
+    equal(value, null);
+
+    // Fake a submission due to shutdown.
+    ping = await wait_for_ping(
+      () => {
+        telem.submissionInterval = Infinity;
+        Service.recordTelemetryEvent("object", "method", undefined, {
+          foo: "bar",
+        });
+        telem.finish("shutdown");
+      },
+      false,
+      true
+    );
+    equal(ping.syncs.length, 0);
+    equal(ping.events.length, 1);
+    equal(ping.events[0].length, 6);
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_histograms() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  try {
+    let histId = "TELEMETRY_TEST_LINEAR";
+    Services.obs.notifyObservers(null, "weave:telemetry:histogram", histId);
+    let ping = await wait_for_ping(() => Service.sync(), true, true);
+    equal(Object.keys(ping.histograms).length, 1);
+    equal(ping.histograms[histId].sum, 0);
+    equal(ping.histograms[histId].histogram_type, 1);
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_invalid_events() {
+  enableValidationPrefs();
+
+  await Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+
+  async function checkNotRecorded(...args) {
+    Service.recordTelemetryEvent.call(args);
+    let ping = await wait_for_ping(() => Service.sync(), false, true);
+    equal(ping.events, undefined);
+  }
+
+  await SyncTestingInfrastructure(server);
+  try {
+    let long21 = "l".repeat(21);
+    let long81 = "l".repeat(81);
+    let long86 = "l".repeat(86);
+    await checkNotRecorded("object");
+    await checkNotRecorded("object", 2);
+    await checkNotRecorded(2, "method");
+    await checkNotRecorded("object", "method", 2);
+    await checkNotRecorded("object", "method", "value", 2);
+    await checkNotRecorded("object", "method", "value", { foo: 2 });
+    await checkNotRecorded(long21, "method", "value");
+    await checkNotRecorded("object", long21, "value");
+    await checkNotRecorded("object", "method", long81);
+    let badextra = {};
+    badextra[long21] = "x";
+    await checkNotRecorded("object", "method", "value", badextra);
+    badextra = { x: long86 };
+    await checkNotRecorded("object", "method", "value", badextra);
+    for (let i = 0; i < 10; i++) {
+      badextra["name" + i] = "x";
+    }
+    await checkNotRecorded("object", "method", "value", badextra);
+  } finally {
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_no_ping_for_self_hosters() {
+  enableValidationPrefs();
+
+  let telem = get_sync_test_telemetry();
+  let oldSubmit = telem.submit;
+
+  await Service.engineManager.register(BogusEngine);
+  let engine = Service.engineManager.get("bogus");
+  engine.enabled = true;
+  let server = await serverForFoo(engine);
+
+  await SyncTestingInfrastructure(server);
+  try {
+    let submitPromise = new Promise(resolve => {
+      telem.submit = function () {
+        let result = oldSubmit.apply(this, arguments);
+        resolve(result);
+      };
+    });
+    await Service.sync();
+    let pingSubmitted = await submitPromise;
+    // The Sync testing infrastructure already sets up a custom token server,
+    // so we don't need to do anything to simulate a self-hosted user.
+    ok(!pingSubmitted, "Should not submit ping with custom token server URL");
+  } finally {
+    telem.submit = oldSubmit;
+    await cleanAndGo(engine, server);
+    await Service.engineManager.unregister(engine);
+  }
+});
+
+add_task(async function test_fxa_device_telem() {
+  let t = get_sync_test_telemetry();
+  let syncEnabled = true;
+  let oldGetClientsEngineRecords = t.getClientsEngineRecords;
+  let oldGetFxaDevices = t.getFxaDevices;
+  let oldSyncIsEnabled = t.syncIsEnabled;
+  let oldSanitizeFxaDeviceId = t.sanitizeFxaDeviceId;
+  t.syncIsEnabled = () => syncEnabled;
+  t.sanitizeFxaDeviceId = id => `So clean: ${id}`;
+  try {
+    let keep0 = Utils.makeGUID();
+    let keep1 = Utils.makeGUID();
+    let keep2 = Utils.makeGUID();
+    let curdev = Utils.makeGUID();
+
+    let keep1Sync = Utils.makeGUID();
+    let keep2Sync = Utils.makeGUID();
+    let curdevSync = Utils.makeGUID();
+    let fxaDevices = [
+      {
+        id: curdev,
+        isCurrentDevice: true,
+        lastAccessTime: Date.now() - 1000 * 60 * 60 * 24 * 1,
+        pushEndpointExpired: false,
+        type: "desktop",
+        name: "current device",
+      },
+      {
+        id: keep0,
+        isCurrentDevice: false,
+        lastAccessTime: Date.now() - 1000 * 60 * 60 * 24 * 10,
+        pushEndpointExpired: false,
+        type: "mobile",
+        name: "dupe",
+      },
+      // Valid 2
+      {
+        id: keep1,
+        isCurrentDevice: false,
+        lastAccessTime: Date.now() - 1000 * 60 * 60 * 24 * 1,
+        pushEndpointExpired: false,
+        type: "desktop",
+        name: "valid2",
+      },
+      // Valid 3
+      {
+        id: keep2,
+        isCurrentDevice: false,
+        lastAccessTime: Date.now() - 1000 * 60 * 60 * 24 * 5,
+        pushEndpointExpired: false,
+        type: "desktop",
+        name: "valid3",
+      },
+    ];
+    let clientInfo = [
+      {
+        id: keep1Sync,
+        fxaDeviceId: keep1,
+        os: "Windows 30",
+        version: "Firefox 1 million",
+      },
+      {
+        id: keep2Sync,
+        fxaDeviceId: keep2,
+        os: "firefox, but an os",
+        verison: "twelve",
+      },
+      {
+        id: Utils.makeGUID(),
+        fxaDeviceId: null,
+        os: "apparently ios used to keep write these IDs as null.",
+        version: "Doesn't seem to anymore",
+      },
+      {
+        id: curdevSync,
+        fxaDeviceId: curdev,
+        os: "emacs",
+        version: "22",
+      },
+      {
+        id: Utils.makeGUID(),
+        fxaDeviceId: Utils.makeGUID(),
+        os: "not part of the fxa device set at all",
+        version: "foo bar baz",
+      },
+      // keep0 intententionally omitted.
+    ];
+    t.getClientsEngineRecords = () => clientInfo;
+    let devInfo = t.updateFxaDevices(fxaDevices);
+    equal(devInfo.deviceID, t.sanitizeFxaDeviceId(curdev));
+    for (let d of devInfo.devices) {
+      ok(d.id.startsWith("So clean:"));
+      if (d.syncID) {
+        ok(d.syncID.startsWith("So clean:"));
+      }
+    }
+    equal(devInfo.devices.length, 4);
+    let k0 = devInfo.devices.find(d => d.id == t.sanitizeFxaDeviceId(keep0));
+    let k1 = devInfo.devices.find(d => d.id == t.sanitizeFxaDeviceId(keep1));
+    let k2 = devInfo.devices.find(d => d.id == t.sanitizeFxaDeviceId(keep2));
+
+    deepEqual(k0, {
+      id: t.sanitizeFxaDeviceId(keep0),
+      type: "mobile",
+      os: undefined,
+      version: undefined,
+      syncID: undefined,
+    });
+    deepEqual(k1, {
+      id: t.sanitizeFxaDeviceId(keep1),
+      type: "desktop",
+      os: clientInfo[0].os,
+      version: clientInfo[0].version,
+      syncID: t.sanitizeFxaDeviceId(keep1Sync),
+    });
+    deepEqual(k2, {
+      id: t.sanitizeFxaDeviceId(keep2),
+      type: "desktop",
+      os: clientInfo[1].os,
+      version: clientInfo[1].version,
+      syncID: t.sanitizeFxaDeviceId(keep2Sync),
+    });
+    let newCurId = Utils.makeGUID();
+    // Update the ID
+    fxaDevices[0].id = newCurId;
+
+    let keep3 = Utils.makeGUID();
+    fxaDevices.push({
+      id: keep3,
+      isCurrentDevice: false,
+      lastAccessTime: Date.now() - 1000 * 60 * 60 * 24 * 1,
+      pushEndpointExpired: false,
+      type: "desktop",
+      name: "valid 4",
+    });
+    devInfo = t.updateFxaDevices(fxaDevices);
+
+    let afterSubmit = [keep0, keep1, keep2, keep3, newCurId]
+      .map(id => t.sanitizeFxaDeviceId(id))
+      .sort();
+    deepEqual(devInfo.devices.map(d => d.id).sort(), afterSubmit);
+
+    // Reset this, as our override doesn't check for sync being enabled.
+    t.sanitizeFxaDeviceId = oldSanitizeFxaDeviceId;
+    syncEnabled = false;
+    fxAccounts.telemetry._setHashedUID(false);
+    devInfo = t.updateFxaDevices(fxaDevices);
+    equal(devInfo.deviceID, undefined);
+    equal(devInfo.devices.length, 5);
+    for (let d of devInfo.devices) {
+      equal(d.os, undefined);
+      equal(d.version, undefined);
+      equal(d.syncID, undefined);
+      // Type should still be present.
+      notEqual(d.type, undefined);
+    }
+  } finally {
+    t.getClientsEngineRecords = oldGetClientsEngineRecords;
+    t.getFxaDevices = oldGetFxaDevices;
+    t.syncIsEnabled = oldSyncIsEnabled;
+    t.sanitizeFxaDeviceId = oldSanitizeFxaDeviceId;
+  }
+});
+
+add_task(async function test_sanitize_fxa_device_id() {
+  let t = get_sync_test_telemetry();
+  fxAccounts.telemetry._setHashedUID(false);
+  sinon.stub(t, "syncIsEnabled").callsFake(() => true);
+  const rawDeviceId = "raw one two three";
+  try {
+    equal(t.sanitizeFxaDeviceId(rawDeviceId), null);
+    fxAccounts.telemetry._setHashedUID("mock uid");
+    const sanitizedDeviceId = t.sanitizeFxaDeviceId(rawDeviceId);
+    ok(sanitizedDeviceId);
+    notEqual(sanitizedDeviceId, rawDeviceId);
+  } finally {
+    t.syncIsEnabled.restore();
+    fxAccounts.telemetry._setHashedUID(false);
+  }
+});
+
+add_task(async function test_no_node_type() {
+  let server = sync_httpd_setup({});
+  await configureIdentity(null, server);
+
+  await sync_and_validate_telem(ping => {
+    ok(ping.syncNodeType === undefined);
+  }, true);
+  await promiseStopServer(server);
+});
+
+add_task(async function test_node_type() {
+  Service.identity.logout();
+  let server = sync_httpd_setup({});
+  await configureIdentity({ node_type: "the-node-type" }, server);
+
+  await sync_and_validate_telem(ping => {
+    equal(ping.syncNodeType, "the-node-type");
+  }, true);
+  await promiseStopServer(server);
+});
+
+add_task(async function test_node_type_change() {
+  let pingPromise = wait_for_pings(2);
+
+  Service.identity.logout();
+  let server = sync_httpd_setup({});
+  await configureIdentity({ node_type: "first-node-type" }, server);
+  // Default to submitting each hour - we should still submit on node change.
+  let telem = get_sync_test_telemetry();
+  telem.submissionInterval = 60 * 60 * 1000;
+  // reset the node type from previous test or our first sync will submit.
+  telem.lastSyncNodeType = null;
+  // do 2 syncs with the same node type.
+  await Service.sync();
+  await Service.sync();
+  // then another with a different node type.
+  Service.identity.logout();
+  await configureIdentity({ node_type: "second-node-type" }, server);
+  await Service.sync();
+  telem.finish();
+
+  let pings = await pingPromise;
+  equal(pings.length, 2);
+  equal(pings[0].syncs.length, 2, "2 syncs in first ping");
+  equal(pings[0].syncNodeType, "first-node-type");
+  equal(pings[1].syncs.length, 1, "1 sync in second ping");
+  equal(pings[1].syncNodeType, "second-node-type");
+  await promiseStopServer(server);
+});
+
+add_task(async function test_ids() {
+  let telem = get_sync_test_telemetry();
+  Assert.ok(!telem._shouldSubmitForDataChange());
+  fxAccounts.telemetry._setHashedUID("new_uid");
+  Assert.ok(telem._shouldSubmitForDataChange());
+  telem.maybeSubmitForDataChange();
+  // now it's been submitted the new uid is current.
+  Assert.ok(!telem._shouldSubmitForDataChange());
+});
+
+add_task(async function test_deletion_request_ping() {
+  async function assertRecordedSyncDeviceID(expected) {
+    // The scalar gets updated asynchronously, so wait a tick before checking.
+    await Promise.resolve();
+    const scalars =
+      Services.telemetry.getSnapshotForScalars("deletion-request").parent || {};
+    equal(scalars["deletion.request.sync_device_id"], expected);
+  }
+
+  const MOCK_HASHED_UID = "00112233445566778899aabbccddeeff";
+  const MOCK_DEVICE_ID1 = "ffeeddccbbaa99887766554433221100";
+  const MOCK_DEVICE_ID2 = "aabbccddeeff99887766554433221100";
+
+  // Calculated by hand using SHA256(DEVICE_ID + HASHED_UID)[:32]
+  const SANITIZED_DEVICE_ID1 = "dd7c845006df9baa1c6d756926519c8c";
+  const SANITIZED_DEVICE_ID2 = "0d06919a736fc029007e1786a091882c";
+
+  let currentDeviceID = null;
+  sinon.stub(fxAccounts.device, "getLocalId").callsFake(() => {
+    return Promise.resolve(currentDeviceID);
+  });
+  let telem = get_sync_test_telemetry();
+  sinon.stub(telem, "isProductionSyncUser").callsFake(() => true);
+  fxAccounts.telemetry._setHashedUID(false);
+  try {
+    // The scalar should start out undefined, since no user is actually logged in.
+    await assertRecordedSyncDeviceID(undefined);
+
+    // If we start up without knowing the hashed UID, it should stay undefined.
+    telem.observe(null, "weave:service:ready");
+    await assertRecordedSyncDeviceID(undefined);
+
+    // But now let's say we've discovered the hashed UID from the server.
+    fxAccounts.telemetry._setHashedUID(MOCK_HASHED_UID);
+    currentDeviceID = MOCK_DEVICE_ID1;
+
+    // Now when we load up, we'll record the sync device id.
+    telem.observe(null, "weave:service:ready");
+    await assertRecordedSyncDeviceID(SANITIZED_DEVICE_ID1);
+
+    // When the device-id changes we'll update it.
+    currentDeviceID = MOCK_DEVICE_ID2;
+    telem.observe(null, "fxaccounts:new_device_id");
+    await assertRecordedSyncDeviceID(SANITIZED_DEVICE_ID2);
+
+    // When the user signs out we'll clear it.
+    telem.observe(null, "fxaccounts:onlogout");
+    await assertRecordedSyncDeviceID("");
+  } finally {
+    fxAccounts.telemetry._setHashedUID(false);
+    telem.isProductionSyncUser.restore();
+    fxAccounts.device.getLocalId.restore();
+  }
+});
diff --git a/services/sync/tests/unit/test_tracker_addChanged.js b/services/sync/tests/unit/test_tracker_addChanged.js
new file mode 100644
index 0000000000..7f510794fc
--- /dev/null
+++ b/services/sync/tests/unit/test_tracker_addChanged.js
@@ -0,0 +1,59 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function test_tracker_basics() {
+  let tracker = new LegacyTracker("Tracker", Service);
+
+  let id = "the_id!";
+
+  _("Make sure nothing exists yet..");
+  let changes = await tracker.getChangedIDs();
+  Assert.equal(changes[id], null);
+
+  _("Make sure adding of time 0 works");
+  await tracker.addChangedID(id, 0);
+  changes = await tracker.getChangedIDs();
+  Assert.equal(changes[id], 0);
+
+  _("A newer time will replace the old 0");
+  await tracker.addChangedID(id, 10);
+  changes = await tracker.getChangedIDs();
+  Assert.equal(changes[id], 10);
+
+  _("An older time will not replace the newer 10");
+  await tracker.addChangedID(id, 5);
+  changes = await tracker.getChangedIDs();
+  Assert.equal(changes[id], 10);
+
+  _("Adding without time defaults to current time");
+  await tracker.addChangedID(id);
+  changes = await tracker.getChangedIDs();
+  Assert.ok(changes[id] > 10);
+});
+
+add_task(async function test_tracker_persistence() {
+  let tracker = new LegacyTracker("Tracker", Service);
+  let id = "abcdef";
+
+  let promiseSave = new Promise((resolve, reject) => {
+    let save = tracker._storage._save;
+    tracker._storage._save = function () {
+      save.call(tracker._storage).then(resolve, reject);
+    };
+  });
+
+  await tracker.addChangedID(id, 5);
+
+  await promiseSave;
+
+  _("IDs saved.");
+  const changes = await tracker.getChangedIDs();
+  Assert.equal(5, changes[id]);
+
+  let json = await Utils.jsonLoad(["changes", "tracker"], tracker);
+  Assert.equal(5, json[id]);
+});
diff --git a/services/sync/tests/unit/test_uistate.js b/services/sync/tests/unit/test_uistate.js
new file mode 100644
index 0000000000..cb1ff1979e
--- /dev/null
+++ b/services/sync/tests/unit/test_uistate.js
@@ -0,0 +1,324 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { UIState } = ChromeUtils.importESModule(
+  "resource://services-sync/UIState.sys.mjs"
+);
+
+const UIStateInternal = UIState._internal;
+
+add_task(async function test_isReady_unconfigured() {
+  UIState.reset();
+
+  let refreshState = sinon.spy(UIStateInternal, "refreshState");
+
+  // On the first call, returns false
+  // Does trigger a refresh of the state - even though services.sync.username
+  // is undefined we still need to check the account state.
+  ok(!UIState.isReady());
+  // resfreshState is called when idle - so only check after idle.
+  await new Promise(resolve => {
+    Services.tm.idleDispatchToMainThread(resolve);
+  });
+  ok(refreshState.called);
+  refreshState.resetHistory();
+
+  // On subsequent calls, only return true
+  ok(UIState.isReady());
+  ok(!refreshState.called);
+
+  refreshState.restore();
+});
+
+add_task(async function test_isReady_signedin() {
+  UIState.reset();
+  Services.prefs.setStringPref("services.sync.username", "foo");
+
+  let refreshState = sinon.spy(UIStateInternal, "refreshState");
+
+  // On the first call, returns false and triggers a refresh of the state
+  ok(!UIState.isReady());
+  await new Promise(resolve => {
+    Services.tm.idleDispatchToMainThread(resolve);
+  });
+  ok(refreshState.calledOnce);
+  refreshState.resetHistory();
+
+  // On subsequent calls, only return true
+  ok(UIState.isReady());
+  ok(!refreshState.called);
+
+  refreshState.restore();
+});
+
+add_task(async function test_refreshState_signedin() {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  const now = new Date().toString();
+  Services.prefs.setStringPref("services.sync.lastSync", now);
+  UIStateInternal.syncing = false;
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () =>
+      Promise.resolve({
+        verified: true,
+        uid: "123",
+        email: "foo@bar.com",
+        displayName: "Foo Bar",
+        avatar: "https://foo/bar",
+      }),
+    hasLocalSession: () => Promise.resolve(true),
+  };
+
+  let state = await UIState.refresh();
+
+  equal(state.status, UIState.STATUS_SIGNED_IN);
+  equal(state.uid, "123");
+  equal(state.email, "foo@bar.com");
+  equal(state.displayName, "Foo Bar");
+  equal(state.avatarURL, "https://foo/bar");
+  equal(state.lastSync, now);
+  equal(state.syncing, false);
+
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+});
+
+add_task(async function test_refreshState_syncButNoFxA() {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  const now = new Date().toString();
+  Services.prefs.setStringPref("services.sync.lastSync", now);
+  Services.prefs.setStringPref("services.sync.username", "test@test.com");
+  UIStateInternal.syncing = false;
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () => Promise.resolve(null),
+  };
+
+  let state = await UIState.refresh();
+
+  equal(state.status, UIState.STATUS_LOGIN_FAILED);
+  equal(state.uid, undefined);
+  equal(state.email, "test@test.com");
+  equal(state.displayName, undefined);
+  equal(state.avatarURL, undefined);
+  equal(state.lastSync, undefined); // only set when STATUS_SIGNED_IN.
+  equal(state.syncing, false);
+
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+  Services.prefs.clearUserPref("services.sync.lastSync");
+  Services.prefs.clearUserPref("services.sync.username");
+});
+
+add_task(async function test_refreshState_signedin_profile_unavailable() {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  const now = new Date().toString();
+  Services.prefs.setStringPref("services.sync.lastSync", now);
+  Services.prefs.setStringPref("services.sync.username", "test@test.com");
+  UIStateInternal.syncing = false;
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () =>
+      Promise.resolve({ verified: true, uid: "123", email: "foo@bar.com" }),
+    hasLocalSession: () => Promise.resolve(true),
+    _internal: {
+      profile: {
+        getProfile: () => {
+          return Promise.reject(new Error("Profile unavailable"));
+        },
+      },
+    },
+  };
+
+  let state = await UIState.refresh();
+
+  equal(state.status, UIState.STATUS_SIGNED_IN);
+  equal(state.uid, "123");
+  equal(state.email, "foo@bar.com");
+  equal(state.displayName, undefined);
+  equal(state.avatarURL, undefined);
+  equal(state.lastSync, now);
+  equal(state.syncing, false);
+
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+  Services.prefs.clearUserPref("services.sync.lastSync");
+  Services.prefs.clearUserPref("services.sync.username");
+});
+
+add_task(async function test_refreshState_unverified() {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () =>
+      Promise.resolve({ verified: false, uid: "123", email: "foo@bar.com" }),
+    hasLocalSession: () => Promise.resolve(true),
+  };
+
+  let state = await UIState.refresh();
+
+  equal(state.status, UIState.STATUS_NOT_VERIFIED);
+  equal(state.uid, "123");
+  equal(state.email, "foo@bar.com");
+  equal(state.displayName, undefined);
+  equal(state.avatarURL, undefined);
+  equal(state.lastSync, undefined);
+
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+});
+
+add_task(async function test_refreshState_unverified_nosession() {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () =>
+      Promise.resolve({ verified: false, uid: "123", email: "foo@bar.com" }),
+    hasLocalSession: () => Promise.resolve(false),
+  };
+
+  let state = await UIState.refresh();
+
+  // No session should "win" over the unverified state.
+  equal(state.status, UIState.STATUS_LOGIN_FAILED);
+  equal(state.uid, "123");
+  equal(state.email, "foo@bar.com");
+  equal(state.displayName, undefined);
+  equal(state.avatarURL, undefined);
+  equal(state.lastSync, undefined);
+
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+});
+
+add_task(async function test_refreshState_loginFailed() {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  let loginFailed = sinon.stub(UIStateInternal, "_loginFailed");
+  loginFailed.returns(true);
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () =>
+      Promise.resolve({ verified: true, uid: "123", email: "foo@bar.com" }),
+  };
+
+  let state = await UIState.refresh();
+
+  equal(state.status, UIState.STATUS_LOGIN_FAILED);
+  equal(state.uid, "123");
+  equal(state.email, "foo@bar.com");
+  equal(state.displayName, undefined);
+  equal(state.avatarURL, undefined);
+  equal(state.lastSync, undefined);
+
+  loginFailed.restore();
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+});
+
+add_task(async function test_observer_refreshState() {
+  let refreshState = sinon.spy(UIStateInternal, "refreshState");
+
+  let shouldRefresh = [
+    "weave:service:login:got-hashed-id",
+    "weave:service:login:error",
+    "weave:service:ready",
+    "fxaccounts:onverified",
+    "fxaccounts:onlogin",
+    "fxaccounts:onlogout",
+    "fxaccounts:profilechange",
+  ];
+
+  for (let topic of shouldRefresh) {
+    let uiUpdateObserved = observeUIUpdate();
+    Services.obs.notifyObservers(null, topic);
+    await uiUpdateObserved;
+    ok(refreshState.calledOnce);
+    refreshState.resetHistory();
+  }
+
+  refreshState.restore();
+});
+
+// Drive the UIState in a configured state.
+async function configureUIState(syncing, lastSync = new Date()) {
+  UIState.reset();
+  const fxAccountsOrig = UIStateInternal.fxAccounts;
+
+  UIStateInternal._syncing = syncing;
+  Services.prefs.setStringPref("services.sync.lastSync", lastSync.toString());
+  Services.prefs.setStringPref("services.sync.username", "test@test.com");
+
+  UIStateInternal.fxAccounts = {
+    getSignedInUser: () =>
+      Promise.resolve({ verified: true, uid: "123", email: "foo@bar.com" }),
+    hasLocalSession: () => Promise.resolve(true),
+  };
+  await UIState.refresh();
+  UIStateInternal.fxAccounts = fxAccountsOrig;
+}
+
+add_task(async function test_syncStarted() {
+  await configureUIState(false);
+
+  const oldState = Object.assign({}, UIState.get());
+  ok(!oldState.syncing);
+
+  let uiUpdateObserved = observeUIUpdate();
+  Services.obs.notifyObservers(null, "weave:service:sync:start");
+  await uiUpdateObserved;
+
+  const newState = Object.assign({}, UIState.get());
+  ok(newState.syncing);
+});
+
+add_task(async function test_syncFinished() {
+  let yesterday = new Date();
+  yesterday.setDate(yesterday.getDate() - 1);
+  await configureUIState(true, yesterday);
+
+  const oldState = Object.assign({}, UIState.get());
+  ok(oldState.syncing);
+
+  let uiUpdateObserved = observeUIUpdate();
+  Services.prefs.setStringPref("services.sync.lastSync", new Date().toString());
+  Services.obs.notifyObservers(null, "weave:service:sync:finish");
+  await uiUpdateObserved;
+
+  const newState = Object.assign({}, UIState.get());
+  ok(!newState.syncing);
+  ok(new Date(newState.lastSync) > new Date(oldState.lastSync));
+});
+
+add_task(async function test_syncError() {
+  let yesterday = new Date();
+  yesterday.setDate(yesterday.getDate() - 1);
+  await configureUIState(true, yesterday);
+
+  const oldState = Object.assign({}, UIState.get());
+  ok(oldState.syncing);
+
+  let uiUpdateObserved = observeUIUpdate();
+  Services.obs.notifyObservers(null, "weave:service:sync:error");
+  await uiUpdateObserved;
+
+  const newState = Object.assign({}, UIState.get());
+  ok(!newState.syncing);
+  deepEqual(newState.lastSync, oldState.lastSync);
+});
+
+function observeUIUpdate() {
+  return new Promise(resolve => {
+    let obs = (aSubject, aTopic, aData) => {
+      Services.obs.removeObserver(obs, aTopic);
+      const state = UIState.get();
+      resolve(state);
+    };
+    Services.obs.addObserver(obs, UIState.ON_UPDATE);
+  });
+}
diff --git a/services/sync/tests/unit/test_utils_catch.js b/services/sync/tests/unit/test_utils_catch.js
new file mode 100644
index 0000000000..590d04527f
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_catch.js
@@ -0,0 +1,119 @@
+const { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+
+add_task(async function run_test() {
+  _("Make sure catch when copied to an object will correctly catch stuff");
+  let ret, rightThis, didCall, didThrow, wasCovfefe, wasLocked;
+  let obj = {
+    _catch: Utils.catch,
+    _log: {
+      debug(str) {
+        didThrow = str.search(/^Exception/) == 0;
+      },
+      info(str) {
+        wasLocked = str.indexOf("Cannot start sync: already syncing?") == 0;
+      },
+    },
+
+    func() {
+      return this._catch(async function () {
+        rightThis = this == obj;
+        didCall = true;
+        return 5;
+      })();
+    },
+
+    throwy() {
+      return this._catch(async function () {
+        rightThis = this == obj;
+        didCall = true;
+        throw new Error("covfefe");
+      })();
+    },
+
+    callbacky() {
+      return this._catch(
+        async function () {
+          rightThis = this == obj;
+          didCall = true;
+          throw new Error("covfefe");
+        },
+        async function (ex) {
+          wasCovfefe = ex && ex.message == "covfefe";
+        }
+      )();
+    },
+
+    lockedy() {
+      return this._catch(async function () {
+        rightThis = this == obj;
+        didCall = true;
+        Utils.throwLockException(null);
+      })();
+    },
+
+    lockedy_chained() {
+      return this._catch(async function () {
+        rightThis = this == obj;
+        didCall = true;
+        Utils.throwLockException(null);
+      })();
+    },
+  };
+
+  _("Make sure a normal call will call and return");
+  rightThis = didCall = didThrow = wasLocked = false;
+  ret = await obj.func();
+  Assert.equal(ret, 5);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  Assert.ok(!didThrow);
+  Assert.equal(wasCovfefe, undefined);
+  Assert.ok(!wasLocked);
+
+  _(
+    "Make sure catch/throw results in debug call and caller doesn't need to handle exception"
+  );
+  rightThis = didCall = didThrow = wasLocked = false;
+  ret = await obj.throwy();
+  Assert.equal(ret, undefined);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  Assert.ok(didThrow);
+  Assert.equal(wasCovfefe, undefined);
+  Assert.ok(!wasLocked);
+
+  _("Test callback for exception testing.");
+  rightThis = didCall = didThrow = wasLocked = false;
+  ret = await obj.callbacky();
+  Assert.equal(ret, undefined);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  Assert.ok(didThrow);
+  Assert.ok(wasCovfefe);
+  Assert.ok(!wasLocked);
+
+  _("Test the lock-aware catch that Service uses.");
+  obj._catch = Service._catch;
+  rightThis = didCall = didThrow = wasLocked = false;
+  wasCovfefe = undefined;
+  ret = await obj.lockedy();
+  Assert.equal(ret, undefined);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  Assert.ok(didThrow);
+  Assert.equal(wasCovfefe, undefined);
+  Assert.ok(wasLocked);
+
+  _("Test the lock-aware catch that Service uses with a chained promise.");
+  rightThis = didCall = didThrow = wasLocked = false;
+  wasCovfefe = undefined;
+  ret = await obj.lockedy_chained();
+  Assert.equal(ret, undefined);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  Assert.ok(didThrow);
+  Assert.equal(wasCovfefe, undefined);
+  Assert.ok(wasLocked);
+});
diff --git a/services/sync/tests/unit/test_utils_deepEquals.js b/services/sync/tests/unit/test_utils_deepEquals.js
new file mode 100644
index 0000000000..218cc21b72
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_deepEquals.js
@@ -0,0 +1,51 @@
+_("Make sure Utils.deepEquals correctly finds items that are deeply equal");
+
+Services.prefs.setBoolPref("security.allow_eval_with_system_principal", true);
+registerCleanupFunction(() => {
+  Services.prefs.clearUserPref("security.allow_eval_with_system_principal");
+});
+
+function run_test() {
+  let data =
+    '[NaN, undefined, null, true, false, Infinity, 0, 1, "a", "b", {a: 1}, {a: "a"}, [{a: 1}], [{a: true}], {a: 1, b: 2}, [1, 2], [1, 2, 3]]';
+  _("Generating two copies of data:", data);
+  /* eslint-disable no-eval */
+  let d1 = eval(data);
+  let d2 = eval(data);
+  /* eslint-enable no-eval */
+
+  d1.forEach(function (a) {
+    _("Testing", a, typeof a, JSON.stringify([a]));
+    let numMatch = 0;
+
+    d2.forEach(function (b) {
+      if (Utils.deepEquals(a, b)) {
+        numMatch++;
+        _("Found a match", b, typeof b, JSON.stringify([b]));
+      }
+    });
+
+    let expect = 1;
+    if (isNaN(a) && typeof a == "number") {
+      expect = 0;
+      _("Checking NaN should result in no matches");
+    }
+
+    _("Making sure we found the correct # match:", expect);
+    _("Actual matches:", numMatch);
+    Assert.equal(numMatch, expect);
+  });
+
+  _("Make sure adding undefined properties doesn't affect equalness");
+  let a = {};
+  let b = { a: undefined };
+  Assert.ok(Utils.deepEquals(a, b));
+  a.b = 5;
+  Assert.ok(!Utils.deepEquals(a, b));
+  b.b = 5;
+  Assert.ok(Utils.deepEquals(a, b));
+  a.c = undefined;
+  Assert.ok(Utils.deepEquals(a, b));
+  b.d = undefined;
+  Assert.ok(Utils.deepEquals(a, b));
+}
diff --git a/services/sync/tests/unit/test_utils_deferGetSet.js b/services/sync/tests/unit/test_utils_deferGetSet.js
new file mode 100644
index 0000000000..6db812b844
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_deferGetSet.js
@@ -0,0 +1,50 @@
+_(
+  "Make sure various combinations of deferGetSet arguments correctly defer getting/setting properties to another object"
+);
+
+function run_test() {
+  let base = function () {};
+  base.prototype = {
+    dst: {},
+
+    get a() {
+      return "a";
+    },
+    set b(val) {
+      this.dst.b = val + "!!!";
+    },
+  };
+  let src = new base();
+
+  _("get/set a single property");
+  Utils.deferGetSet(base, "dst", "foo");
+  src.foo = "bar";
+  Assert.equal(src.dst.foo, "bar");
+  Assert.equal(src.foo, "bar");
+
+  _("editing the target also updates the source");
+  src.dst.foo = "baz";
+  Assert.equal(src.dst.foo, "baz");
+  Assert.equal(src.foo, "baz");
+
+  _("handle multiple properties");
+  Utils.deferGetSet(base, "dst", ["p1", "p2"]);
+  src.p1 = "v1";
+  src.p2 = "v2";
+  Assert.equal(src.p1, "v1");
+  Assert.equal(src.dst.p1, "v1");
+  Assert.equal(src.p2, "v2");
+  Assert.equal(src.dst.p2, "v2");
+
+  _("make sure existing getter keeps its functionality");
+  Utils.deferGetSet(base, "dst", "a");
+  src.a = "not a";
+  Assert.equal(src.dst.a, "not a");
+  Assert.equal(src.a, "a");
+
+  _("make sure existing setter keeps its functionality");
+  Utils.deferGetSet(base, "dst", "b");
+  src.b = "b";
+  Assert.equal(src.dst.b, "b!!!");
+  Assert.equal(src.b, "b!!!");
+}
diff --git a/services/sync/tests/unit/test_utils_json.js b/services/sync/tests/unit/test_utils_json.js
new file mode 100644
index 0000000000..5bf26b2361
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_json.js
@@ -0,0 +1,95 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+
+add_task(async function test_roundtrip() {
+  _("Do a simple write of an array to json and read");
+  await Utils.jsonSave("foo", {}, ["v1", "v2"]);
+
+  let foo = await Utils.jsonLoad("foo", {});
+  Assert.equal(typeof foo, "object");
+  Assert.equal(foo.length, 2);
+  Assert.equal(foo[0], "v1");
+  Assert.equal(foo[1], "v2");
+});
+
+add_task(async function test_string() {
+  _("Try saving simple strings");
+  await Utils.jsonSave("str", {}, "hi");
+
+  let str = await Utils.jsonLoad("str", {});
+  Assert.equal(typeof str, "string");
+  Assert.equal(str.length, 2);
+  Assert.equal(str[0], "h");
+  Assert.equal(str[1], "i");
+});
+
+add_task(async function test_number() {
+  _("Try saving a number");
+  await Utils.jsonSave("num", {}, 42);
+
+  let num = await Utils.jsonLoad("num", {});
+  Assert.equal(typeof num, "number");
+  Assert.equal(num, 42);
+});
+
+add_task(async function test_nonexistent_file() {
+  _("Try loading a non-existent file.");
+  let val = await Utils.jsonLoad("non-existent", {});
+  Assert.equal(val, undefined);
+});
+
+add_task(async function test_save_logging() {
+  _("Verify that writes are logged.");
+  let trace;
+  await Utils.jsonSave(
+    "log",
+    {
+      _log: {
+        trace(msg) {
+          trace = msg;
+        },
+      },
+    },
+    "hi"
+  );
+  Assert.ok(!!trace);
+});
+
+add_task(async function test_load_logging() {
+  _("Verify that reads and read errors are logged.");
+
+  // Write a file with some invalid JSON
+  let file = await IOUtils.getFile(PathUtils.profileDir, "weave", "log.json");
+  let fos = Cc["@mozilla.org/network/file-output-stream;1"].createInstance(
+    Ci.nsIFileOutputStream
+  );
+  let flags =
+    FileUtils.MODE_WRONLY | FileUtils.MODE_CREATE | FileUtils.MODE_TRUNCATE;
+  fos.init(file, flags, FileUtils.PERMS_FILE, fos.DEFER_OPEN);
+  let stream = Cc["@mozilla.org/intl/converter-output-stream;1"].createInstance(
+    Ci.nsIConverterOutputStream
+  );
+  stream.init(fos, "UTF-8");
+  stream.writeString("invalid json!");
+  stream.close();
+
+  let trace, debug;
+  let obj = {
+    _log: {
+      trace(msg) {
+        trace = msg;
+      },
+      debug(msg) {
+        debug = msg;
+      },
+    },
+  };
+  let val = await Utils.jsonLoad("log", obj);
+  Assert.ok(!val);
+  Assert.ok(!!trace);
+  Assert.ok(!!debug);
+});
diff --git a/services/sync/tests/unit/test_utils_keyEncoding.js b/services/sync/tests/unit/test_utils_keyEncoding.js
new file mode 100644
index 0000000000..30a8a4f2aa
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_keyEncoding.js
@@ -0,0 +1,23 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function run_test() {
+  Assert.equal(
+    Utils.encodeKeyBase32("foobarbafoobarba"),
+    "mzxw6ytb9jrgcztpn5rgc4tcme"
+  );
+  Assert.equal(
+    Utils.decodeKeyBase32("mzxw6ytb9jrgcztpn5rgc4tcme"),
+    "foobarbafoobarba"
+  );
+  Assert.equal(
+    Utils.encodeKeyBase32(
+      "\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01"
+    ),
+    "aeaqcaibaeaqcaibaeaqcaibae"
+  );
+  Assert.equal(
+    Utils.decodeKeyBase32("aeaqcaibaeaqcaibaeaqcaibae"),
+    "\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01"
+  );
+}
diff --git a/services/sync/tests/unit/test_utils_lock.js b/services/sync/tests/unit/test_utils_lock.js
new file mode 100644
index 0000000000..71d6486ff9
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_lock.js
@@ -0,0 +1,76 @@
+_("Make sure lock prevents calling with a shared lock");
+
+// Utility that we only use here.
+
+function do_check_begins(thing, startsWith) {
+  if (!(thing && thing.indexOf && thing.indexOf(startsWith) == 0)) {
+    do_throw(thing + " doesn't begin with " + startsWith);
+  }
+}
+
+add_task(async function run_test() {
+  let ret, rightThis, didCall;
+  let state, lockState, lockedState, unlockState;
+  let obj = {
+    _lock: Utils.lock,
+    lock() {
+      lockState = ++state;
+      if (this._locked) {
+        lockedState = ++state;
+        return false;
+      }
+      this._locked = true;
+      return true;
+    },
+    unlock() {
+      unlockState = ++state;
+      this._locked = false;
+    },
+
+    func() {
+      return this._lock("Test utils lock", async function () {
+        rightThis = this == obj;
+        didCall = true;
+        return 5;
+      })();
+    },
+
+    throwy() {
+      return this._lock("Test utils lock throwy", async function () {
+        rightThis = this == obj;
+        didCall = true;
+        return this.throwy();
+      })();
+    },
+  };
+
+  _("Make sure a normal call will call and return");
+  rightThis = didCall = false;
+  state = 0;
+  ret = await obj.func();
+  Assert.equal(ret, 5);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  Assert.equal(lockState, 1);
+  Assert.equal(unlockState, 2);
+  Assert.equal(state, 2);
+
+  _("Make sure code that calls locked code throws");
+  ret = null;
+  rightThis = didCall = false;
+  try {
+    ret = await obj.throwy();
+    do_throw("throwy internal call should have thrown!");
+  } catch (ex) {
+    // Should throw an Error, not a string.
+    do_check_begins(ex.message, "Could not acquire lock");
+  }
+  Assert.equal(ret, null);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+  _("Lock should be called twice so state 3 is skipped");
+  Assert.equal(lockState, 4);
+  Assert.equal(lockedState, 5);
+  Assert.equal(unlockState, 6);
+  Assert.equal(state, 6);
+});
diff --git a/services/sync/tests/unit/test_utils_makeGUID.js b/services/sync/tests/unit/test_utils_makeGUID.js
new file mode 100644
index 0000000000..b1104c1114
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_makeGUID.js
@@ -0,0 +1,44 @@
+const base64url =
+  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_";
+
+function run_test() {
+  _("Make sure makeGUID makes guids of the right length/characters");
+  _("Create a bunch of guids to make sure they don't conflict");
+  let guids = [];
+  for (let i = 0; i < 1000; i++) {
+    let newGuid = Utils.makeGUID();
+    _("Generated " + newGuid);
+
+    // Verify that the GUID's length is correct, even when it's URL encoded.
+    Assert.equal(newGuid.length, 12);
+    Assert.equal(encodeURIComponent(newGuid).length, 12);
+
+    // Verify that the GUID only contains base64url characters
+    Assert.ok(
+      Array.prototype.every.call(newGuid, function (chr) {
+        return base64url.includes(chr);
+      })
+    );
+
+    // Verify that Utils.checkGUID() correctly identifies them as valid.
+    Assert.ok(Utils.checkGUID(newGuid));
+
+    // Verify uniqueness within our sample of 1000. This could cause random
+    // failures, but they should be extremely rare. Otherwise we'd have a
+    // problem with GUID collisions.
+    Assert.ok(
+      guids.every(function (g) {
+        return g != newGuid;
+      })
+    );
+    guids.push(newGuid);
+  }
+
+  _("Make sure checkGUID fails for invalid GUIDs");
+  Assert.ok(!Utils.checkGUID(undefined));
+  Assert.ok(!Utils.checkGUID(null));
+  Assert.ok(!Utils.checkGUID(""));
+  Assert.ok(!Utils.checkGUID("blergh"));
+  Assert.ok(!Utils.checkGUID("ThisGUIDisWayTooLong"));
+  Assert.ok(!Utils.checkGUID("Invalid!!!!!"));
+}
diff --git a/services/sync/tests/unit/test_utils_notify.js b/services/sync/tests/unit/test_utils_notify.js
new file mode 100644
index 0000000000..5c0c3702a6
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_notify.js
@@ -0,0 +1,97 @@
+_("Make sure notify sends out the right notifications");
+add_task(async function run_test() {
+  let ret, rightThis, didCall;
+  let obj = {
+    notify: Utils.notify("foo:"),
+    _log: {
+      trace() {},
+    },
+
+    func() {
+      return this.notify("bar", "baz", async function () {
+        rightThis = this == obj;
+        didCall = true;
+        return 5;
+      })();
+    },
+
+    throwy() {
+      return this.notify("bad", "one", async function () {
+        rightThis = this == obj;
+        didCall = true;
+        throw new Error("covfefe");
+      })();
+    },
+  };
+
+  let state = 0;
+  let makeObs = function (topic) {
+    let obj2 = {
+      observe(subject, obsTopic, data) {
+        this.state = ++state;
+        this.subject = subject;
+        this.topic = obsTopic;
+        this.data = data;
+      },
+    };
+
+    Svc.Obs.add(topic, obj2);
+    return obj2;
+  };
+
+  _("Make sure a normal call will call and return with notifications");
+  rightThis = didCall = false;
+  let fs = makeObs("foo:bar:start");
+  let ff = makeObs("foo:bar:finish");
+  let fe = makeObs("foo:bar:error");
+  ret = await obj.func();
+  Assert.equal(ret, 5);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+
+  Assert.equal(fs.state, 1);
+  Assert.equal(fs.subject, undefined);
+  Assert.equal(fs.topic, "foo:bar:start");
+  Assert.equal(fs.data, "baz");
+
+  Assert.equal(ff.state, 2);
+  Assert.equal(ff.subject, 5);
+  Assert.equal(ff.topic, "foo:bar:finish");
+  Assert.equal(ff.data, "baz");
+
+  Assert.equal(fe.state, undefined);
+  Assert.equal(fe.subject, undefined);
+  Assert.equal(fe.topic, undefined);
+  Assert.equal(fe.data, undefined);
+
+  _("Make sure a throwy call will call and throw with notifications");
+  ret = null;
+  rightThis = didCall = false;
+  let ts = makeObs("foo:bad:start");
+  let tf = makeObs("foo:bad:finish");
+  let te = makeObs("foo:bad:error");
+  try {
+    ret = await obj.throwy();
+    do_throw("throwy should have thrown!");
+  } catch (ex) {
+    Assert.equal(ex.message, "covfefe");
+  }
+  Assert.equal(ret, null);
+  Assert.ok(rightThis);
+  Assert.ok(didCall);
+
+  Assert.equal(ts.state, 3);
+  Assert.equal(ts.subject, undefined);
+  Assert.equal(ts.topic, "foo:bad:start");
+  Assert.equal(ts.data, "one");
+
+  Assert.equal(tf.state, undefined);
+  Assert.equal(tf.subject, undefined);
+  Assert.equal(tf.topic, undefined);
+  Assert.equal(tf.data, undefined);
+
+  Assert.equal(te.state, 4);
+  Assert.equal(te.subject.message, "covfefe");
+  Assert.equal(te.topic, "foo:bad:error");
+  Assert.equal(te.data, "one");
+});
diff --git a/services/sync/tests/unit/test_utils_passphrase.js b/services/sync/tests/unit/test_utils_passphrase.js
new file mode 100644
index 0000000000..fa58086113
--- /dev/null
+++ b/services/sync/tests/unit/test_utils_passphrase.js
@@ -0,0 +1,45 @@
+/* eslint no-tabs:"off" */
+
+function run_test() {
+  _("Normalize passphrase recognizes hyphens.");
+  const pp = "26ect2thczm599m2ffqarbicjq";
+  const hyphenated = "2-6ect2-thczm-599m2-ffqar-bicjq";
+  Assert.equal(Utils.normalizePassphrase(hyphenated), pp);
+
+  _("Skip whitespace.");
+  Assert.equal(
+    "aaaaaaaaaaaaaaaaaaaaaaaaaa",
+    Utils.normalizePassphrase("aaaaaaaaaaaaaaaaaaaaaaaaaa  ")
+  );
+  Assert.equal(
+    "aaaaaaaaaaaaaaaaaaaaaaaaaa",
+    Utils.normalizePassphrase("	 aaaaaaaaaaaaaaaaaaaaaaaaaa")
+  );
+  Assert.equal(
+    "aaaaaaaaaaaaaaaaaaaaaaaaaa",
+    Utils.normalizePassphrase("    aaaaaaaaaaaaaaaaaaaaaaaaaa  ")
+  );
+  Assert.equal(
+    "aaaaaaaaaaaaaaaaaaaaaaaaaa",
+    Utils.normalizePassphrase("    a-aaaaa-aaaaa-aaaaa-aaaaa-aaaaa  ")
+  );
+  Assert.ok(Utils.isPassphrase("aaaaaaaaaaaaaaaaaaaaaaaaaa  "));
+  Assert.ok(Utils.isPassphrase("	 aaaaaaaaaaaaaaaaaaaaaaaaaa"));
+  Assert.ok(Utils.isPassphrase("    aaaaaaaaaaaaaaaaaaaaaaaaaa  "));
+  Assert.ok(Utils.isPassphrase("    a-aaaaa-aaaaa-aaaaa-aaaaa-aaaaa  "));
+  Assert.ok(!Utils.isPassphrase("    -aaaaa-aaaaa-aaaaa-aaaaa-aaaaa  "));
+
+  _("Normalizing 20-char passphrases.");
+  Assert.equal(
+    Utils.normalizePassphrase("abcde-abcde-abcde-abcde"),
+    "abcdeabcdeabcdeabcde"
+  );
+  Assert.equal(
+    Utils.normalizePassphrase("a-bcde-abcde-abcde-abcde"),
+    "a-bcde-abcde-abcde-abcde"
+  );
+  Assert.equal(
+    Utils.normalizePassphrase(" abcde-abcde-abcde-abcde "),
+    "abcdeabcdeabcdeabcde"
+  );
+}
diff --git a/services/sync/tests/unit/xpcshell.toml b/services/sync/tests/unit/xpcshell.toml
new file mode 100644
index 0000000000..e958c8a738
--- /dev/null
+++ b/services/sync/tests/unit/xpcshell.toml
@@ -0,0 +1,304 @@
+[DEFAULT]
+head = "head_appinfo.js ../../../common/tests/unit/head_helpers.js head_helpers.js head_http_server.js head_errorhandler_common.js"
+firefox-appdir = "browser"
+prefs = ["identity.fxaccounts.enabled=true"]
+support-files = [
+  "addon1-search.json",
+  "bootstrap1-search.json",
+  "missing-sourceuri.json",
+  "missing-xpi-search.json",
+  "rewrite-search.json",
+  "sync_ping_schema.json",
+  "systemaddon-search.json",
+  "!/services/common/tests/unit/head_helpers.js",
+  "!/toolkit/components/extensions/test/xpcshell/head_sync.js",
+]
+
+# The manifest is roughly ordered from low-level to high-level. When making
+# systemic sweeping changes, this makes it easier to identify errors closer to
+# the source.
+
+# Ensure we can import everything.
+
+["test_412.js"]
+
+["test_addon_utils.js"]
+run-sequentially = "Restarts server, can't change pref."
+tags = "addons"
+
+["test_addons_engine.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+tags = "addons"
+
+["test_addons_reconciler.js"]
+skip-if = ["appname == 'thunderbird'"]
+tags = "addons"
+
+["test_addons_store.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+tags = "addons"
+
+["test_addons_tracker.js"]
+tags = "addons"
+
+["test_addons_validator.js"]
+tags = "addons"
+
+["test_bookmark_batch_fail.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_bookmark_decline_undecline.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_bookmark_engine.js"]
+skip-if = [
+  "appname == 'thunderbird'",
+  "tsan", # Runs unreasonably slow on TSan, bug 1612707
+]
+
+["test_bookmark_order.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_bookmark_places_query_rewriting.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_bookmark_record.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_bookmark_store.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_bookmark_tracker.js"]
+skip-if = [
+  "appname == 'thunderbird'",
+  "tsan", # Runs unreasonably slow on TSan, bug 1612707
+]
+requesttimeoutfactor = 4
+
+["test_bridged_engine.js"]
+
+["test_clients_engine.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_clients_escape.js"]
+
+["test_collection_getBatched.js"]
+
+["test_collections_recovery.js"]
+
+["test_corrupt_keys.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_declined.js"]
+
+["test_disconnect_shutdown.js"]
+
+["test_engine.js"]
+
+["test_engine_abort.js"]
+
+["test_engine_changes_during_sync.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_enginemanager.js"]
+
+["test_errorhandler_1.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_errorhandler_2.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_errorhandler_filelog.js"]
+
+["test_errorhandler_sync_checkServerError.js"]
+
+["test_extension_storage_engine.js"]
+skip-if = ["appname == 'thunderbird'"]
+run-sequentially = "extension-storage migration happens only once, and must be tested first."
+
+["test_extension_storage_engine_kinto.js"]
+skip-if = ["appname == 'thunderbird'"]
+run-sequentially = "extension-storage migration happens only once, and must be tested first."
+
+["test_extension_storage_migration_telem.js"]
+skip-if = ["appname == 'thunderbird'"]
+run-sequentially = "extension-storage migration happens only once, and must be tested first."
+
+["test_extension_storage_tracker_kinto.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_form_validator.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_forms_store.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_forms_tracker.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_fxa_node_reassignment.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_fxa_service_cluster.js"]
+# Finally, we test each engine.
+
+["test_history_engine.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_history_store.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_history_tracker.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_hmac_error.js"]
+
+["test_httpd_sync_server.js"]
+# HTTP layers.
+
+["test_interval_triggers.js"]
+
+["test_keys.js"]
+
+["test_load_modules.js"]
+# util contains a bunch of functionality used throughout.
+
+["test_node_reassignment.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_password_engine.js"]
+
+["test_password_store.js"]
+
+["test_password_tracker.js"]
+
+["test_password_validator.js"]
+
+["test_postqueue.js"]
+# Synced tabs.
+
+["test_prefs_engine.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_prefs_store.js"]
+skip-if = ["appname == 'thunderbird'"]
+support-files = ["prefs_test_prefs_store.js"]
+
+["test_prefs_tracker.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_records_crypto.js"]
+
+["test_records_wbo.js"]
+
+["test_resource.js"]
+
+["test_resource_header.js"]
+
+["test_resource_ua.js"]
+# Generic Sync types.
+
+["test_score_triggers.js"]
+
+["test_service_attributes.js"]
+# Bug 752243: Profile cleanup frequently fails
+skip-if = [
+  "os == 'mac'",
+  "os == 'linux'",
+]
+
+["test_service_cluster.js"]
+
+["test_service_detect_upgrade.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_service_login.js"]
+
+["test_service_startOver.js"]
+
+["test_service_startup.js"]
+
+["test_service_sync_401.js"]
+
+["test_service_sync_locked.js"]
+
+["test_service_sync_remoteSetup.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_service_sync_specified.js"]
+
+["test_service_sync_updateEnabledEngines.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_service_verifyLogin.js"]
+
+["test_service_wipeClient.js"]
+
+["test_service_wipeServer.js"]
+# Bug 752243: Profile cleanup frequently fails
+skip-if = [
+  "os == 'mac'",
+  "os == 'linux'",
+]
+
+["test_status.js"]
+
+["test_status_checkSetup.js"]
+
+["test_sync_auth_manager.js"]
+# Engine APIs.
+
+["test_syncedtabs.js"]
+
+["test_syncengine.js"]
+
+["test_syncengine_sync.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+
+["test_syncscheduler.js"]
+run-sequentially = "Frequent timeouts, bug 1395148"
+# Firefox Accounts specific tests
+
+["test_tab_engine.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_tab_provider.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_tab_quickwrite.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_tab_tracker.js"]
+skip-if = ["appname == 'thunderbird'"]
+
+["test_telemetry.js"]
+skip-if = [
+  "appname == 'thunderbird'",
+  "tsan", # Unreasonably slow, bug 1612707
+]
+requesttimeoutfactor = 4
+
+["test_tracker_addChanged.js"]
+# Service semantics.
+
+["test_uistate.js"]
+
+["test_utils_catch.js"]
+
+["test_utils_deepEquals.js"]
+
+["test_utils_deferGetSet.js"]
+
+["test_utils_json.js"]
+
+["test_utils_keyEncoding.js"]
+
+["test_utils_lock.js"]
+
+["test_utils_makeGUID.js"]
+run-sequentially = "Disproportionately slows down full test run, bug 1450316"
+
+["test_utils_notify.js"]
+
+["test_utils_passphrase.js"]
+# We have a number of other libraries that are pretty much standalone.
diff --git a/services/sync/tps/extensions/tps/api.js b/services/sync/tps/extensions/tps/api.js
new file mode 100644
index 0000000000..0843376630
--- /dev/null
+++ b/services/sync/tps/extensions/tps/api.js
@@ -0,0 +1,77 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+
+/* globals ExtensionAPI, Services, XPCOMUtils */
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "resProto",
+  "@mozilla.org/network/protocol;1?name=resource",
+  "nsISubstitutingProtocolHandler"
+);
+
+async function tpsStartup() {
+  try {
+    var { TPS } = ChromeUtils.importESModule("resource://tps/tps.sys.mjs");
+    let { goQuitApplication } = ChromeUtils.importESModule(
+      "resource://tps/quit.sys.mjs"
+    );
+    TPS.goQuitApplication = goQuitApplication;
+
+    let testFile = Services.prefs.getStringPref("testing.tps.testFile", "");
+    let testPhase = Services.prefs.getStringPref("testing.tps.testPhase", "");
+    if (!testFile || !testPhase) {
+      // Note: this quits.
+      TPS.DumpError(
+        "TPS no longer takes arguments from the command line. " +
+          "instead you need to pass preferences  `testing.tps.{testFile,testPhase}` " +
+          "and optionally `testing.tps.{logFile,ignoreUnusedEngines}`.\n"
+      );
+    }
+
+    let logFile = Services.prefs.getStringPref("testing.tps.logFile", "");
+    let ignoreUnusedEngines = Services.prefs.getBoolPref(
+      "testing.tps.ignoreUnusedEngines",
+      false
+    );
+    let options = { ignoreUnusedEngines };
+    let testFileUri = Services.io.newFileURI(new FileUtils.File(testFile)).spec;
+
+    try {
+      await TPS.RunTestPhase(testFileUri, testPhase, logFile, options);
+    } catch (err) {
+      TPS.DumpError("TestPhase failed", err);
+    }
+  } catch (e) {
+    if (typeof TPS != "undefined") {
+      // Note: This calls quit() under the hood
+      TPS.DumpError("Test initialization failed", e);
+    }
+    dump(`TPS test initialization failed: ${e} - ${e.stack}\n`);
+    // Try and quit right away, no reason to wait around for python
+    // to kill us if initialization failed.
+    Services.startup.quit(Ci.nsIAppStartup.eForceQuit);
+  }
+}
+
+this.tps = class extends ExtensionAPI {
+  onStartup() {
+    resProto.setSubstitution(
+      "tps",
+      Services.io.newURI("resource/", null, this.extension.rootURI)
+    );
+    /* Ignore the platform's online/offline status while running tests. */
+    Services.io.manageOfflineStatus = false;
+    Services.io.offline = false;
+    tpsStartup();
+  }
+
+  onShutdown() {
+    resProto.setSubstitution("tps", null);
+  }
+};
diff --git a/services/sync/tps/extensions/tps/manifest.json b/services/sync/tps/extensions/tps/manifest.json
new file mode 100644
index 0000000000..c961e76506
--- /dev/null
+++ b/services/sync/tps/extensions/tps/manifest.json
@@ -0,0 +1,23 @@
+{
+  "manifest_version": 2,
+  "name": "TPS",
+  "version": "1.0",
+
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "tps@mozilla.org"
+    }
+  },
+
+  "experiment_apis": {
+    "tps": {
+      "schema": "schema.json",
+      "parent": {
+        "scopes": ["addon_parent"],
+        "script": "api.js",
+        "paths": [["tps"]],
+        "events": ["startup"]
+      }
+    }
+  }
+}
diff --git a/services/sync/tps/extensions/tps/resource/auth/fxaccounts.sys.mjs b/services/sync/tps/extensions/tps/resource/auth/fxaccounts.sys.mjs
new file mode 100644
index 0000000000..81c0fd578a
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/auth/fxaccounts.sys.mjs
@@ -0,0 +1,209 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { Log } from "resource://gre/modules/Log.sys.mjs";
+import { clearTimeout, setTimeout } from "resource://gre/modules/Timer.sys.mjs";
+
+import { getFxAccountsSingleton } from "resource://gre/modules/FxAccounts.sys.mjs";
+
+const fxAccounts = getFxAccountsSingleton();
+import { FxAccountsClient } from "resource://gre/modules/FxAccountsClient.sys.mjs";
+import { FxAccountsConfig } from "resource://gre/modules/FxAccountsConfig.sys.mjs";
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+/**
+ * Helper object for Firefox Accounts authentication
+ */
+export var Authentication = {
+  /**
+   * Check if an user has been logged in
+   */
+  async isLoggedIn() {
+    return !!(await this.getSignedInUser());
+  },
+
+  async isReady() {
+    let user = await this.getSignedInUser();
+    return user && user.verified;
+  },
+
+  _getRestmailUsername(user) {
+    const restmailSuffix = "@restmail.net";
+    if (user.toLowerCase().endsWith(restmailSuffix)) {
+      return user.slice(0, -restmailSuffix.length);
+    }
+    return null;
+  },
+
+  async shortWaitForVerification(ms) {
+    let userData = await this.getSignedInUser();
+    let timeoutID;
+    let timeoutPromise = new Promise(resolve => {
+      timeoutID = setTimeout(() => {
+        Logger.logInfo(`Warning: no verification after ${ms}ms.`);
+        resolve();
+      }, ms);
+    });
+    await Promise.race([
+      fxAccounts.whenVerified(userData).finally(() => clearTimeout(timeoutID)),
+      timeoutPromise,
+    ]);
+    userData = await this.getSignedInUser();
+    return userData && userData.verified;
+  },
+
+  async _openVerificationPage(uri) {
+    let mainWindow = Services.wm.getMostRecentWindow("navigator:browser");
+    let newtab = mainWindow.gBrowser.addWebTab(uri);
+    let win = mainWindow.gBrowser.getBrowserForTab(newtab);
+    await new Promise(resolve => {
+      win.addEventListener("loadend", resolve, { once: true });
+    });
+    let didVerify = await this.shortWaitForVerification(10000);
+    mainWindow.gBrowser.removeTab(newtab);
+    return didVerify;
+  },
+
+  async _completeVerification(user) {
+    let username = this._getRestmailUsername(user);
+    if (!username) {
+      Logger.logInfo(
+        `Username "${user}" isn't a restmail username so can't complete verification`
+      );
+      return false;
+    }
+    Logger.logInfo("Fetching mail (from restmail) for user " + username);
+    let restmailURI = `https://www.restmail.net/mail/${encodeURIComponent(
+      username
+    )}`;
+    let triedAlready = new Set();
+    const tries = 10;
+    const normalWait = 2000;
+    for (let i = 0; i < tries; ++i) {
+      let resp = await fetch(restmailURI);
+      let messages = await resp.json();
+      // Sort so that the most recent emails are first.
+      messages.sort((a, b) => new Date(b.receivedAt) - new Date(a.receivedAt));
+      for (let m of messages) {
+        // We look for a link that has a x-link that we haven't yet tried.
+        if (!m.headers["x-link"] || triedAlready.has(m.headers["x-link"])) {
+          continue;
+        }
+        let confirmLink = m.headers["x-link"];
+        triedAlready.add(confirmLink);
+        Logger.logInfo("Trying confirmation link " + confirmLink);
+        try {
+          if (await this._openVerificationPage(confirmLink)) {
+            return true;
+          }
+        } catch (e) {
+          Logger.logInfo(
+            "Warning: Failed to follow confirmation link: " +
+              Log.exceptionStr(e)
+          );
+        }
+      }
+      if (i === 0) {
+        // first time through after failing we'll do this.
+        await fxAccounts.resendVerificationEmail();
+      }
+      if (await this.shortWaitForVerification(normalWait)) {
+        return true;
+      }
+    }
+    // One last try.
+    return this.shortWaitForVerification(normalWait);
+  },
+
+  async deleteEmail(user) {
+    let username = this._getRestmailUsername(user);
+    if (!username) {
+      Logger.logInfo("Not a restmail username, can't delete");
+      return false;
+    }
+    Logger.logInfo("Deleting mail (from restmail) for user " + username);
+    let restmailURI = `https://www.restmail.net/mail/${encodeURIComponent(
+      username
+    )}`;
+    try {
+      // Clean up after ourselves.
+      let deleteResult = await fetch(restmailURI, { method: "DELETE" });
+      if (!deleteResult.ok) {
+        Logger.logInfo(
+          `Warning: Got non-success status ${deleteResult.status} when deleting emails`
+        );
+        return false;
+      }
+    } catch (e) {
+      Logger.logInfo(
+        "Warning: Failed to delete old emails: " + Log.exceptionStr(e)
+      );
+      return false;
+    }
+    return true;
+  },
+
+  /**
+   * Wrapper to retrieve the currently signed in user
+   *
+   * @returns Information about the currently signed in user
+   */
+  async getSignedInUser() {
+    try {
+      return await fxAccounts.getSignedInUser();
+    } catch (error) {
+      Logger.logError(
+        "getSignedInUser() failed with: " + JSON.stringify(error)
+      );
+      throw error;
+    }
+  },
+
+  /**
+   * Wrapper to synchronize the login of a user
+   *
+   * @param account
+   *        Account information of the user to login
+   * @param account.username
+   *        The username for the account (utf8)
+   * @param account.password
+   *        The user's password
+   */
+  async signIn(account) {
+    Logger.AssertTrue(account.username, "Username has been found");
+    Logger.AssertTrue(account.password, "Password has been found");
+
+    Logger.logInfo("Login user: " + account.username);
+
+    try {
+      // Required here since we don't go through the real login page
+      await FxAccountsConfig.ensureConfigured();
+
+      let client = new FxAccountsClient();
+      let credentials = await client.signIn(
+        account.username,
+        account.password,
+        true
+      );
+      await fxAccounts._internal.setSignedInUser(credentials);
+      if (!credentials.verified) {
+        await this._completeVerification(account.username);
+      }
+
+      return true;
+    } catch (error) {
+      throw new Error("signIn() failed with: " + error.message);
+    }
+  },
+
+  /**
+   * Sign out of Firefox Accounts.
+   */
+  async signOut() {
+    if (await Authentication.isLoggedIn()) {
+      // Note: This will clean up the device ID.
+      await fxAccounts.signOut();
+    }
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/logger.sys.mjs b/services/sync/tps/extensions/tps/resource/logger.sys.mjs
new file mode 100644
index 0000000000..a1995f88b6
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/logger.sys.mjs
@@ -0,0 +1,168 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+    ChromeUtils.import() and acts as a singleton.
+    Only the following listed symbols will exposed on import, and only when
+    and where imported. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  ObjectUtils: "resource://gre/modules/ObjectUtils.sys.mjs",
+});
+
+export var Logger = {
+  _foStream: null,
+  _converter: null,
+  _potentialError: null,
+
+  init(path) {
+    if (this._converter != null) {
+      // we're already open!
+      return;
+    }
+
+    if (path) {
+      Services.prefs.setStringPref("tps.logfile", path);
+    } else {
+      path = Services.prefs.getStringPref("tps.logfile");
+    }
+
+    this._file = Cc["@mozilla.org/file/local;1"].createInstance(Ci.nsIFile);
+    this._file.initWithPath(path);
+    var exists = this._file.exists();
+
+    // Make a file output stream and converter to handle it.
+    this._foStream = Cc[
+      "@mozilla.org/network/file-output-stream;1"
+    ].createInstance(Ci.nsIFileOutputStream);
+    // If the file already exists, append it, otherwise create it.
+    var fileflags = exists ? 0x02 | 0x08 | 0x10 : 0x02 | 0x08 | 0x20;
+
+    this._foStream.init(this._file, fileflags, 0o666, 0);
+    this._converter = Cc[
+      "@mozilla.org/intl/converter-output-stream;1"
+    ].createInstance(Ci.nsIConverterOutputStream);
+    this._converter.init(this._foStream, "UTF-8");
+  },
+
+  write(data) {
+    if (this._converter == null) {
+      console.error("TPS Logger.write called with _converter == null!");
+      return;
+    }
+    this._converter.writeString(data);
+  },
+
+  close() {
+    if (this._converter != null) {
+      this._converter.close();
+      this._converter = null;
+      this._foStream = null;
+    }
+  },
+
+  AssertTrue(bool, msg, showPotentialError) {
+    if (bool) {
+      return;
+    }
+
+    if (showPotentialError && this._potentialError) {
+      msg += "; " + this._potentialError;
+      this._potentialError = null;
+    }
+    throw new Error("ASSERTION FAILED! " + msg);
+  },
+
+  AssertFalse(bool, msg, showPotentialError) {
+    return this.AssertTrue(!bool, msg, showPotentialError);
+  },
+
+  AssertEqual(got, expected, msg) {
+    if (!lazy.ObjectUtils.deepEqual(got, expected)) {
+      throw new Error(
+        "ASSERTION FAILED! " +
+          msg +
+          "; expected " +
+          JSON.stringify(expected) +
+          ", got " +
+          JSON.stringify(got)
+      );
+    }
+  },
+
+  log(msg, withoutPrefix) {
+    dump(msg + "\n");
+    if (withoutPrefix) {
+      this.write(msg + "\n");
+    } else {
+      function pad(n, len) {
+        let s = "0000" + n;
+        return s.slice(-len);
+      }
+
+      let now = new Date();
+      let year = pad(now.getFullYear(), 4);
+      let month = pad(now.getMonth() + 1, 2);
+      let day = pad(now.getDate(), 2);
+      let hour = pad(now.getHours(), 2);
+      let minutes = pad(now.getMinutes(), 2);
+      let seconds = pad(now.getSeconds(), 2);
+      let ms = pad(now.getMilliseconds(), 3);
+
+      this.write(
+        year +
+          "-" +
+          month +
+          "-" +
+          day +
+          " " +
+          hour +
+          ":" +
+          minutes +
+          ":" +
+          seconds +
+          "." +
+          ms +
+          " " +
+          msg +
+          "\n"
+      );
+    }
+  },
+
+  clearPotentialError() {
+    this._potentialError = null;
+  },
+
+  logPotentialError(msg) {
+    this._potentialError = msg;
+  },
+
+  logLastPotentialError(msg) {
+    var message = msg;
+    if (this._potentialError) {
+      message = this._poentialError;
+      this._potentialError = null;
+    }
+    this.log("CROSSWEAVE ERROR: " + message);
+  },
+
+  logError(msg) {
+    this.log("CROSSWEAVE ERROR: " + msg);
+  },
+
+  logInfo(msg, withoutPrefix) {
+    if (withoutPrefix) {
+      this.log(msg, true);
+    } else {
+      this.log("CROSSWEAVE INFO: " + msg);
+    }
+  },
+
+  logPass(msg) {
+    this.log("CROSSWEAVE TEST PASS: " + msg);
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/addons.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/addons.sys.mjs
new file mode 100644
index 0000000000..596f942a06
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/addons.sys.mjs
@@ -0,0 +1,93 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { AddonManager } from "resource://gre/modules/AddonManager.sys.mjs";
+import { AddonUtils } from "resource://services-sync/addonutils.sys.mjs";
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+export const STATE_ENABLED = 1;
+export const STATE_DISABLED = 2;
+
+export function Addon(TPS, id) {
+  this.TPS = TPS;
+  this.id = id;
+}
+
+Addon.prototype = {
+  addon: null,
+
+  async uninstall() {
+    // find our addon locally
+    let addon = await AddonManager.getAddonByID(this.id);
+    Logger.AssertTrue(
+      !!addon,
+      "could not find addon " + this.id + " to uninstall"
+    );
+    await AddonUtils.uninstallAddon(addon);
+  },
+
+  async find(state) {
+    let addon = await AddonManager.getAddonByID(this.id);
+
+    if (!addon) {
+      Logger.logInfo("Could not find add-on with ID: " + this.id);
+      return false;
+    }
+
+    this.addon = addon;
+
+    Logger.logInfo(
+      "add-on found: " + addon.id + ", enabled: " + !addon.userDisabled
+    );
+    if (state == STATE_ENABLED) {
+      Logger.AssertFalse(addon.userDisabled, "add-on is disabled: " + addon.id);
+      return true;
+    } else if (state == STATE_DISABLED) {
+      Logger.AssertTrue(addon.userDisabled, "add-on is enabled: " + addon.id);
+      return true;
+    } else if (state) {
+      throw new Error("Don't know how to handle state: " + state);
+    } else {
+      // No state, so just checking that it exists.
+      return true;
+    }
+  },
+
+  async install() {
+    // For Install, the id parameter initially passed is really the filename
+    // for the addon's install .xml; we'll read the actual id from the .xml.
+
+    const result = await AddonUtils.installAddons([
+      { id: this.id, requireSecureURI: false },
+    ]);
+
+    Logger.AssertEqual(
+      1,
+      result.installedIDs.length,
+      "Exactly 1 add-on was installed."
+    );
+    Logger.AssertEqual(
+      this.id,
+      result.installedIDs[0],
+      "Add-on was installed successfully: " + this.id
+    );
+  },
+
+  async setEnabled(flag) {
+    Logger.AssertTrue(await this.find(), "Add-on is available.");
+
+    let userDisabled;
+    if (flag == STATE_ENABLED) {
+      userDisabled = false;
+    } else if (flag == STATE_DISABLED) {
+      userDisabled = true;
+    } else {
+      throw new Error("Unknown flag to setEnabled: " + flag);
+    }
+
+    AddonUtils.updateUserDisabled(this.addon, userDisabled);
+
+    return true;
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/bookmarkValidator.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/bookmarkValidator.sys.mjs
new file mode 100644
index 0000000000..a7724c6aaa
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/bookmarkValidator.sys.mjs
@@ -0,0 +1,1063 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This file was moved to tps from the main production code as it was unused
+// after removal of the non-mirror bookmarks engine.
+// It used to have a test before it was moved:
+// https://searchfox.org/mozilla-central/rev/b1a5802e0f73bfd6d2096e5fefc2b47831a50b2d/services/sync/tests/unit/test_bookmark_validator.js
+
+import { CommonUtils } from "resource://services-common/utils.sys.mjs";
+import { Utils } from "resource://services-sync/util.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Async: "resource://services-common/async.sys.mjs",
+  PlacesSyncUtils: "resource://gre/modules/PlacesSyncUtils.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+});
+
+const QUERY_PROTOCOL = "place:";
+
+function areURLsEqual(a, b) {
+  if (a === b) {
+    return true;
+  }
+  if (a.startsWith(QUERY_PROTOCOL) != b.startsWith(QUERY_PROTOCOL)) {
+    return false;
+  }
+  // Tag queries are special because we rewrite them to point to the
+  // local tag folder ID. It's expected that the folders won't match,
+  // but all other params should.
+  let aParams = new URLSearchParams(a.slice(QUERY_PROTOCOL.length));
+  let aType = +aParams.get("type");
+  if (aType != Ci.nsINavHistoryQueryOptions.RESULTS_AS_TAG_CONTENTS) {
+    return false;
+  }
+  let bParams = new URLSearchParams(b.slice(QUERY_PROTOCOL.length));
+  let bType = +bParams.get("type");
+  if (bType != Ci.nsINavHistoryQueryOptions.RESULTS_AS_TAG_CONTENTS) {
+    return false;
+  }
+  let aKeys = new Set(aParams.keys());
+  let bKeys = new Set(bParams.keys());
+  if (aKeys.size != bKeys.size) {
+    return false;
+  }
+  // Tag queries shouldn't reference multiple folders, or named folders like
+  // "TOOLBAR" or "BOOKMARKS_MENU". Just in case, we make sure all folder IDs
+  // are numeric. If they are, we ignore them when comparing the query params.
+  if (aKeys.has("folder") && aParams.getAll("folder").every(isFinite)) {
+    aKeys.delete("folder");
+  }
+  if (bKeys.has("folder") && bParams.getAll("folder").every(isFinite)) {
+    bKeys.delete("folder");
+  }
+  for (let key of aKeys) {
+    if (!bKeys.has(key)) {
+      return false;
+    }
+    if (
+      !CommonUtils.arrayEqual(
+        aParams.getAll(key).sort(),
+        bParams.getAll(key).sort()
+      )
+    ) {
+      return false;
+    }
+  }
+  for (let key of bKeys) {
+    if (!aKeys.has(key)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+const BOOKMARK_VALIDATOR_VERSION = 1;
+
+/**
+ * Result of bookmark validation. Contains the following fields which describe
+ * server-side problems unless otherwise specified.
+ *
+ * - missingIDs (number): # of objects with missing ids
+ * - duplicates (array of ids): ids seen more than once
+ * - parentChildMismatches (array of {parent: parentid, child: childid}):
+ *   instances where the child's parentid and the parent's children array
+ *   do not match
+ * - cycles (array of array of ids). List of cycles found in the server-side tree.
+ * - clientCycles (array of array of ids). List of cycles found in the client-side tree.
+ * - orphans (array of {id: string, parent: string}): List of nodes with
+ *   either no parentid, or where the parent could not be found.
+ * - missingChildren (array of {parent: id, child: id}):
+ *   List of parent/children where the child id couldn't be found
+ * - deletedChildren (array of { parent: id, child: id }):
+ *   List of parent/children where child id was a deleted item (but still showed up
+ *   in the children array)
+ * - multipleParents (array of {child: id, parents: array of ids}):
+ *   List of children that were part of multiple parent arrays
+ * - deletedParents (array of ids) : List of records that aren't deleted but
+ *   had deleted parents
+ * - childrenOnNonFolder (array of ids): list of non-folders that still have
+ *   children arrays
+ * - duplicateChildren (array of ids): list of records who have the same
+ *   child listed multiple times in their children array
+ * - parentNotFolder (array of ids): list of records that have parents that
+ *   aren't folders
+ * - rootOnServer (boolean): true if the root came from the server
+ * - badClientRoots (array of ids): Contains any client-side root ids where
+ *   the root is missing or isn't a (direct) child of the places root.
+ *
+ * - clientMissing: Array of ids on the server missing from the client
+ * - serverMissing: Array of ids on the client missing from the server
+ * - serverDeleted: Array of ids on the client that the server had marked as deleted.
+ * - serverUnexpected: Array of ids that appear on the server but shouldn't
+ *   because the client attempts to never upload them.
+ * - differences: Array of {id: string, differences: string array} recording
+ *   the non-structural properties that are differente between the client and server
+ * - structuralDifferences: As above, but contains the items where the differences were
+ *   structural, that is, they contained childGUIDs or parentid
+ */
+export class BookmarkProblemData {
+  constructor() {
+    this.rootOnServer = false;
+    this.missingIDs = 0;
+
+    this.duplicates = [];
+    this.parentChildMismatches = [];
+    this.cycles = [];
+    this.clientCycles = [];
+    this.orphans = [];
+    this.missingChildren = [];
+    this.deletedChildren = [];
+    this.multipleParents = [];
+    this.deletedParents = [];
+    this.childrenOnNonFolder = [];
+    this.duplicateChildren = [];
+    this.parentNotFolder = [];
+
+    this.badClientRoots = [];
+    this.clientMissing = [];
+    this.serverMissing = [];
+    this.serverDeleted = [];
+    this.serverUnexpected = [];
+    this.differences = [];
+    this.structuralDifferences = [];
+  }
+
+  /**
+   * Convert ("difference", [{ differences: ["tags", "name"] }, { differences: ["name"] }]) into
+   * [{ name: "difference:tags", count: 1}, { name: "difference:name", count: 2 }], etc.
+   */
+  _summarizeDifferences(prefix, diffs) {
+    let diffCounts = new Map();
+    for (let { differences } of diffs) {
+      for (let type of differences) {
+        let name = prefix + ":" + type;
+        let count = diffCounts.get(name) || 0;
+        diffCounts.set(name, count + 1);
+      }
+    }
+    return [...diffCounts].map(([name, count]) => ({ name, count }));
+  }
+
+  /**
+   * Produce a list summarizing problems found. Each entry contains {name, count},
+   * where name is the field name for the problem, and count is the number of times
+   * the problem was encountered.
+   *
+   * Validation has failed if all counts are not 0.
+   *
+   * If the `full` argument is truthy, we also include information about which
+   * properties we saw structural differences in. Currently, this means either
+   * "sdiff:parentid" and "sdiff:childGUIDS" may be present.
+   */
+  getSummary(full) {
+    let result = [
+      { name: "clientMissing", count: this.clientMissing.length },
+      { name: "serverMissing", count: this.serverMissing.length },
+      { name: "serverDeleted", count: this.serverDeleted.length },
+      { name: "serverUnexpected", count: this.serverUnexpected.length },
+
+      {
+        name: "structuralDifferences",
+        count: this.structuralDifferences.length,
+      },
+      { name: "differences", count: this.differences.length },
+
+      { name: "missingIDs", count: this.missingIDs },
+      { name: "rootOnServer", count: this.rootOnServer ? 1 : 0 },
+
+      { name: "duplicates", count: this.duplicates.length },
+      {
+        name: "parentChildMismatches",
+        count: this.parentChildMismatches.length,
+      },
+      { name: "cycles", count: this.cycles.length },
+      { name: "clientCycles", count: this.clientCycles.length },
+      { name: "badClientRoots", count: this.badClientRoots.length },
+      { name: "orphans", count: this.orphans.length },
+      { name: "missingChildren", count: this.missingChildren.length },
+      { name: "deletedChildren", count: this.deletedChildren.length },
+      { name: "multipleParents", count: this.multipleParents.length },
+      { name: "deletedParents", count: this.deletedParents.length },
+      { name: "childrenOnNonFolder", count: this.childrenOnNonFolder.length },
+      { name: "duplicateChildren", count: this.duplicateChildren.length },
+      { name: "parentNotFolder", count: this.parentNotFolder.length },
+    ];
+    if (full) {
+      let structural = this._summarizeDifferences(
+        "sdiff",
+        this.structuralDifferences
+      );
+      result.push.apply(result, structural);
+    }
+    return result;
+  }
+}
+
+// Defined lazily to avoid initializing PlacesUtils.bookmarks too soon.
+ChromeUtils.defineLazyGetter(lazy, "SYNCED_ROOTS", () => [
+  lazy.PlacesUtils.bookmarks.menuGuid,
+  lazy.PlacesUtils.bookmarks.toolbarGuid,
+  lazy.PlacesUtils.bookmarks.unfiledGuid,
+  lazy.PlacesUtils.bookmarks.mobileGuid,
+]);
+
+// Maps root GUIDs to their query folder names from
+// toolkit/components/places/nsNavHistoryQuery.cpp. We follow queries that
+// reference existing folders in the client tree, and detect cycles where a
+// query references its containing folder.
+ChromeUtils.defineLazyGetter(lazy, "ROOT_GUID_TO_QUERY_FOLDER_NAME", () => ({
+  [lazy.PlacesUtils.bookmarks.rootGuid]: "PLACES_ROOT",
+  [lazy.PlacesUtils.bookmarks.menuGuid]: "BOOKMARKS_MENU",
+
+  // Tags should never show up in our client tree, and never form cycles, but we
+  // report them just in case.
+  [lazy.PlacesUtils.bookmarks.tagsGuid]: "TAGS",
+
+  [lazy.PlacesUtils.bookmarks.unfiledGuid]: "UNFILED_BOOKMARKS",
+  [lazy.PlacesUtils.bookmarks.toolbarGuid]: "TOOLBAR",
+  [lazy.PlacesUtils.bookmarks.mobileGuid]: "MOBILE_BOOKMARKS",
+}));
+
+async function detectCycles(records) {
+  // currentPath and pathLookup contain the same data. pathLookup is faster to
+  // query, but currentPath gives is the order of traversal that we need in
+  // order to report the members of the cycles.
+  let pathLookup = new Set();
+  let currentPath = [];
+  let cycles = [];
+  let seenEver = new Set();
+  const yieldState = lazy.Async.yieldState();
+
+  const traverse = async node => {
+    if (pathLookup.has(node)) {
+      let cycleStart = currentPath.lastIndexOf(node);
+      let cyclePath = currentPath.slice(cycleStart).map(n => n.id);
+      cycles.push(cyclePath);
+      return;
+    } else if (seenEver.has(node)) {
+      // If we're checking the server, this is a problem, but it should already be reported.
+      // On the client, this could happen due to including `node.concrete` in the child list.
+      return;
+    }
+    seenEver.add(node);
+    let children = node.children || [];
+    if (node.concreteItems) {
+      children.push(...node.concreteItems);
+    }
+    if (children.length) {
+      pathLookup.add(node);
+      currentPath.push(node);
+      await lazy.Async.yieldingForEach(children, traverse, yieldState);
+      currentPath.pop();
+      pathLookup.delete(node);
+    }
+  };
+
+  await lazy.Async.yieldingForEach(
+    records,
+    async record => {
+      if (!seenEver.has(record)) {
+        await traverse(record);
+      }
+    },
+    yieldState
+  );
+
+  return cycles;
+}
+
+class ServerRecordInspection {
+  constructor() {
+    this.serverRecords = null;
+    this.liveRecords = [];
+
+    this.folders = [];
+
+    this.root = null;
+
+    this.idToRecord = new Map();
+
+    this.deletedIds = new Set();
+    this.deletedRecords = [];
+
+    this.problemData = new BookmarkProblemData();
+
+    // These are handled outside of problemData
+    this._orphans = new Map();
+    this._multipleParents = new Map();
+
+    this.yieldState = lazy.Async.yieldState();
+  }
+
+  static async create(records) {
+    return new ServerRecordInspection().performInspection(records);
+  }
+
+  async performInspection(records) {
+    await this._setRecords(records);
+    await this._linkParentIDs();
+    await this._linkChildren();
+    await this._findOrphans();
+    await this._finish();
+    return this;
+  }
+
+  // We don't set orphans in this.problemData. Instead, we walk the tree at the
+  // end to find unreachable items.
+  _noteOrphan(id, parentId = undefined) {
+    // This probably shouldn't be called with a parentId twice, but if it
+    // happens we take the most recent one.
+    if (parentId || !this._orphans.has(id)) {
+      this._orphans.set(id, parentId);
+    }
+  }
+
+  noteParent(child, parent) {
+    let parents = this._multipleParents.get(child);
+    if (!parents) {
+      this._multipleParents.set(child, [parent]);
+    } else {
+      parents.push(parent);
+    }
+  }
+
+  noteMismatch(child, parent) {
+    let exists = this.problemData.parentChildMismatches.some(
+      match => match.child == child && match.parent == parent
+    );
+    if (!exists) {
+      this.problemData.parentChildMismatches.push({ child, parent });
+    }
+  }
+
+  // - Populates `this.deletedIds`, `this.folders`, and `this.idToRecord`
+  // - calls `_initRoot` (thus initializing `this.root`).
+  async _setRecords(records) {
+    if (this.serverRecords) {
+      // In general this class is expected to be created, have
+      // `performInspection` called, and then only read from from that point on.
+      throw new Error("Bug: ServerRecordInspection can't `setRecords` twice");
+    }
+    this.serverRecords = records;
+    let rootChildren = [];
+
+    await lazy.Async.yieldingForEach(
+      this.serverRecords,
+      async record => {
+        if (!record.id) {
+          ++this.problemData.missingIDs;
+          return;
+        }
+
+        if (record.deleted) {
+          this.deletedIds.add(record.id);
+        }
+        if (this.idToRecord.has(record.id)) {
+          this.problemData.duplicates.push(record.id);
+          return;
+        }
+
+        this.idToRecord.set(record.id, record);
+
+        if (!record.deleted) {
+          this.liveRecords.push(record);
+
+          if (record.parentid == "places") {
+            rootChildren.push(record);
+          }
+        }
+
+        if (!record.children) {
+          return;
+        }
+
+        if (record.type != "folder") {
+          // Due to implementation details in engines/bookmarks.js, (Livemark
+          // subclassing BookmarkFolder) Livemarks will have a children array,
+          // but it should still be empty.
+          if (!record.children.length) {
+            return;
+          }
+          // Otherwise we mark it as an error and still try to resolve the children
+          this.problemData.childrenOnNonFolder.push(record.id);
+        }
+
+        this.folders.push(record);
+
+        if (new Set(record.children).size !== record.children.length) {
+          this.problemData.duplicateChildren.push(record.id);
+        }
+
+        // After we're through with them, folder records store 3 (ugh) arrays that
+        // represent their folder information. The final fields looks like:
+        //
+        // - childGUIDs: The original `children` array, which is an array of
+        //   record IDs.
+        //
+        // - unfilteredChildren: Contains more or less `childGUIDs.map(id =>
+        //   idToRecord.get(id))`, without the nulls for missing children. It will
+        //   still have deleted, duplicate, mismatching, etc. children.
+        //
+        // - children: This is the 'cleaned' version of the child records that are
+        //   safe to iterate over, etc.. If there are no reported problems, it should
+        //   be identical to unfilteredChildren.
+        //
+        // The last two are left alone until later `this._linkChildren`, however.
+        record.childGUIDs = record.children;
+
+        await lazy.Async.yieldingForEach(
+          record.childGUIDs,
+          id => {
+            this.noteParent(id, record.id);
+          },
+          this.yieldState
+        );
+
+        record.children = [];
+      },
+      this.yieldState
+    );
+
+    // Finish up some parts we can easily do now that we have idToRecord.
+    this.deletedRecords = Array.from(this.deletedIds, id =>
+      this.idToRecord.get(id)
+    );
+
+    this._initRoot(rootChildren);
+  }
+
+  _initRoot(rootChildren) {
+    let serverRoot = this.idToRecord.get("places");
+    if (serverRoot) {
+      this.root = serverRoot;
+      this.problemData.rootOnServer = true;
+      return;
+    }
+
+    // Fabricate a root. We want to be able to remember that it's fake, but
+    // would like to avoid it needing too much special casing, so we come up
+    // with children for it too (we just get these while we're iterating over
+    // the records to avoid needing two passes over a potentially large number
+    // of records).
+
+    this.root = {
+      id: "places",
+      fake: true,
+      children: rootChildren,
+      childGUIDs: rootChildren.map(record => record.id),
+      type: "folder",
+      title: "",
+    };
+    this.liveRecords.push(this.root);
+    this.idToRecord.set("places", this.root);
+  }
+
+  // Adds `parent` to all records it can that have `parentid`
+  async _linkParentIDs() {
+    await lazy.Async.yieldingForEach(
+      this.idToRecord,
+      ([id, record]) => {
+        if (record == this.root || record.deleted) {
+          return false;
+        }
+
+        // Check and update our orphan map.
+        let parentID = record.parentid;
+        let parent = this.idToRecord.get(parentID);
+        if (!parentID || !parent) {
+          this._noteOrphan(id, parentID);
+          return false;
+        }
+
+        record.parent = parent;
+
+        if (parent.deleted) {
+          this.problemData.deletedParents.push(id);
+          return true;
+        } else if (parent.type != "folder") {
+          this.problemData.parentNotFolder.push(record.id);
+          return true;
+        }
+
+        if (parent.id !== "place" || this.problemData.rootOnServer) {
+          if (!parent.childGUIDs.includes(record.id)) {
+            this.noteMismatch(record.id, parent.id);
+          }
+        }
+
+        if (parent.deleted && !record.deleted) {
+          this.problemData.deletedParents.push(record.id);
+        }
+
+        // Note: We used to check if the parentName on the server matches the
+        // actual local parent name, but given this is used only for de-duping a
+        // record the first time it is seen and expensive to keep up-to-date, we
+        // decided to just stop recording it. See bug 1276969 for more.
+        return false;
+      },
+      this.yieldState
+    );
+  }
+
+  // Build the children and unfilteredChildren arrays, (which are of record
+  // objects, not ids)
+  async _linkChildren() {
+    // Check that we aren't missing any children.
+    await lazy.Async.yieldingForEach(
+      this.folders,
+      async folder => {
+        folder.children = [];
+        folder.unfilteredChildren = [];
+
+        let idsThisFolder = new Set();
+
+        await lazy.Async.yieldingForEach(
+          folder.childGUIDs,
+          childID => {
+            let child = this.idToRecord.get(childID);
+
+            if (!child) {
+              this.problemData.missingChildren.push({
+                parent: folder.id,
+                child: childID,
+              });
+              return;
+            }
+
+            if (child.deleted) {
+              this.problemData.deletedChildren.push({
+                parent: folder.id,
+                child: childID,
+              });
+              return;
+            }
+
+            if (child.parentid != folder.id) {
+              this.noteMismatch(childID, folder.id);
+              return;
+            }
+
+            if (idsThisFolder.has(childID)) {
+              // Already recorded earlier, we just don't want to mess up `children`
+              return;
+            }
+            folder.children.push(child);
+          },
+          this.yieldState
+        );
+      },
+      this.yieldState
+    );
+  }
+
+  // Finds the orphans in the tree using something similar to a `mark and sweep`
+  // strategy. That is, we iterate over the children from the root, remembering
+  // which items we've seen. Then, we iterate all items, and know the ones we
+  // haven't seen are orphans.
+  async _findOrphans() {
+    let seen = new Set([this.root.id]);
+
+    const inCycle = await lazy.Async.yieldingForEach(
+      Utils.walkTree(this.root),
+      ([node]) => {
+        if (seen.has(node.id)) {
+          // We're in an infloop due to a cycle.
+          // Return early to avoid reporting false positives for orphans.
+          return true;
+        }
+        seen.add(node.id);
+
+        return false;
+      },
+      this.yieldState
+    );
+
+    if (inCycle) {
+      return;
+    }
+
+    await lazy.Async.yieldingForEach(
+      this.liveRecords,
+      (record, i) => {
+        if (!seen.has(record.id)) {
+          // We intentionally don't record the parentid here, since we only record
+          // that if the record refers to a parent that doesn't exist, which we
+          // have already handled (when linking parentid's).
+          this._noteOrphan(record.id);
+        }
+      },
+      this.yieldState
+    );
+
+    await lazy.Async.yieldingForEach(
+      this._orphans,
+      ([id, parent]) => {
+        this.problemData.orphans.push({ id, parent });
+      },
+      this.yieldState
+    );
+  }
+
+  async _finish() {
+    this.problemData.cycles = await detectCycles(this.liveRecords);
+
+    for (const [child, recordedParents] of this._multipleParents) {
+      let parents = new Set(recordedParents);
+      if (parents.size > 1) {
+        this.problemData.multipleParents.push({ child, parents: [...parents] });
+      }
+    }
+    // Dedupe simple arrays in the problem data, so that we don't have to worry
+    // about it in the code
+    const idArrayProps = [
+      "duplicates",
+      "deletedParents",
+      "childrenOnNonFolder",
+      "duplicateChildren",
+      "parentNotFolder",
+    ];
+    for (let prop of idArrayProps) {
+      this.problemData[prop] = [...new Set(this.problemData[prop])];
+    }
+  }
+}
+
+export class BookmarkValidator {
+  constructor() {
+    this.yieldState = lazy.Async.yieldState();
+  }
+
+  async canValidate() {
+    return !(await lazy.PlacesSyncUtils.bookmarks.havePendingChanges());
+  }
+
+  async _followQueries(recordsByQueryId) {
+    await lazy.Async.yieldingForEach(
+      recordsByQueryId.values(),
+      entry => {
+        if (
+          entry.type !== "query" &&
+          (!entry.bmkUri || !entry.bmkUri.startsWith(QUERY_PROTOCOL))
+        ) {
+          return;
+        }
+        let params = new URLSearchParams(
+          entry.bmkUri.slice(QUERY_PROTOCOL.length)
+        );
+        // Queries with `excludeQueries` won't form cycles because they'll
+        // exclude all queries, including themselves, from the result set.
+        let excludeQueries = params.get("excludeQueries");
+        if (excludeQueries === "1" || excludeQueries === "true") {
+          // `nsNavHistoryQuery::ParseQueryBooleanString` allows `1` and `true`.
+          return;
+        }
+        entry.concreteItems = [];
+        let queryIds = params.getAll("folder");
+        for (let queryId of queryIds) {
+          let concreteItem = recordsByQueryId.get(queryId);
+          if (concreteItem) {
+            entry.concreteItems.push(concreteItem);
+          }
+        }
+      },
+      this.yieldState
+    );
+  }
+
+  async createClientRecordsFromTree(clientTree) {
+    // Iterate over the treeNode, converting it to something more similar to what
+    // the server stores.
+    let records = [];
+    // A map of local IDs and well-known query folder names to records. Unlike
+    // GUIDs, local IDs aren't synced, since they're not stable across devices.
+    // New Places APIs use GUIDs to refer to bookmarks, but the legacy APIs
+    // still use local IDs. We use this mapping to parse `place:` queries that
+    // refer to folders via their local IDs.
+    let recordsByQueryId = new Map();
+    let syncedRoots = lazy.SYNCED_ROOTS;
+
+    const traverse = async (treeNode, synced) => {
+      if (!synced) {
+        synced = syncedRoots.includes(treeNode.guid);
+      }
+      let localId = treeNode.id;
+      let guid = lazy.PlacesSyncUtils.bookmarks.guidToRecordId(treeNode.guid);
+      let itemType = "item";
+      treeNode.ignored = !synced;
+      treeNode.id = guid;
+      switch (treeNode.type) {
+        case lazy.PlacesUtils.TYPE_X_MOZ_PLACE:
+          if (treeNode.uri.startsWith(QUERY_PROTOCOL)) {
+            itemType = "query";
+          } else {
+            itemType = "bookmark";
+          }
+          break;
+        case lazy.PlacesUtils.TYPE_X_MOZ_PLACE_CONTAINER:
+          let isLivemark = false;
+          if (treeNode.annos) {
+            for (let anno of treeNode.annos) {
+              if (anno.name === lazy.PlacesUtils.LMANNO_FEEDURI) {
+                isLivemark = true;
+                treeNode.feedUri = anno.value;
+              } else if (anno.name === lazy.PlacesUtils.LMANNO_SITEURI) {
+                isLivemark = true;
+                treeNode.siteUri = anno.value;
+              }
+            }
+          }
+          itemType = isLivemark ? "livemark" : "folder";
+          break;
+        case lazy.PlacesUtils.TYPE_X_MOZ_PLACE_SEPARATOR:
+          itemType = "separator";
+          break;
+      }
+
+      if (treeNode.tags) {
+        treeNode.tags = treeNode.tags.split(",");
+      } else {
+        treeNode.tags = [];
+      }
+      treeNode.type = itemType;
+      treeNode.pos = treeNode.index;
+      treeNode.bmkUri = treeNode.uri;
+      records.push(treeNode);
+      if (treeNode.guid in lazy.ROOT_GUID_TO_QUERY_FOLDER_NAME) {
+        let queryId = lazy.ROOT_GUID_TO_QUERY_FOLDER_NAME[treeNode.guid];
+        recordsByQueryId.set(queryId, treeNode);
+      }
+      if (localId) {
+        // Always add the ID, since it's still possible for a query to
+        // reference a root without using the well-known name. For example,
+        // `place:folder=${PlacesUtils.mobileFolderId}` and
+        // `place:folder=MOBILE_BOOKMARKS` are equivalent.
+        recordsByQueryId.set(localId.toString(10), treeNode);
+      }
+      if (treeNode.type === "folder") {
+        treeNode.childGUIDs = [];
+        if (!treeNode.children) {
+          treeNode.children = [];
+        }
+
+        await lazy.Async.yieldingForEach(
+          treeNode.children,
+          async child => {
+            await traverse(child, synced);
+            child.parent = treeNode;
+            child.parentid = guid;
+            treeNode.childGUIDs.push(child.guid);
+          },
+          this.yieldState
+        );
+      }
+    };
+
+    await traverse(clientTree, false);
+
+    clientTree.id = "places";
+    await this._followQueries(recordsByQueryId);
+    return records;
+  }
+
+  /**
+   * Process the server-side list. Mainly this builds the records into a tree,
+   * but it also records information about problems, and produces arrays of the
+   * deleted and non-deleted nodes.
+   *
+   * Returns an object containing:
+   * - records:Array of non-deleted records. Each record contains the following
+   *    properties
+   *     - childGUIDs (array of strings, only present if type is 'folder'): the
+   *       list of child GUIDs stored on the server.
+   *     - children (array of records, only present if type is 'folder'):
+   *       each record has these same properties. This may differ in content
+   *       from what you may expect from the childGUIDs list, as it won't
+   *       contain any records that could not be found.
+   *     - parent (record): The parent to this record.
+   *     - Unchanged properties send down from the server: id, title, type,
+   *       parentName, parentid, bmkURI, keyword, tags, pos, queryId
+   * - root: Root of the server-side bookmark tree. Has the same properties as
+   *   above.
+   * - deletedRecords: As above, but only contains items that the server sent
+   *   where it also sent indication that the item should be deleted.
+   * - problemData: a BookmarkProblemData object, with the caveat that
+   *   the fields describing client/server relationship will not have been filled
+   *   out yet.
+   */
+  async inspectServerRecords(serverRecords) {
+    const data = await ServerRecordInspection.create(serverRecords);
+    return {
+      deletedRecords: data.deletedRecords,
+      records: data.liveRecords,
+      problemData: data.problemData,
+      root: data.root,
+    };
+  }
+
+  // Perform client-side sanity checking that doesn't involve server data
+  async _validateClient(problemData, clientRecords) {
+    problemData.clientCycles = await detectCycles(clientRecords);
+    for (let rootGUID of lazy.SYNCED_ROOTS) {
+      let record = clientRecords.find(record => record.guid === rootGUID);
+      if (!record || record.parentid !== "places") {
+        problemData.badClientRoots.push(rootGUID);
+      }
+    }
+  }
+
+  async _computeUnifiedRecordMap(serverRecords, clientRecords) {
+    let allRecords = new Map();
+    await lazy.Async.yieldingForEach(
+      serverRecords,
+      sr => {
+        if (sr.fake) {
+          return;
+        }
+        allRecords.set(sr.id, { client: null, server: sr });
+      },
+      this.yieldState
+    );
+
+    await lazy.Async.yieldingForEach(
+      clientRecords,
+      cr => {
+        let unified = allRecords.get(cr.id);
+        if (!unified) {
+          allRecords.set(cr.id, { client: cr, server: null });
+        } else {
+          unified.client = cr;
+        }
+      },
+      this.yieldState
+    );
+
+    return allRecords;
+  }
+
+  _recordMissing(problems, id, clientRecord, serverRecord, serverTombstones) {
+    if (!clientRecord && serverRecord) {
+      problems.clientMissing.push(id);
+    }
+    if (!serverRecord && clientRecord) {
+      if (serverTombstones.has(id)) {
+        problems.serverDeleted.push(id);
+      } else if (!clientRecord.ignored && clientRecord.id != "places") {
+        problems.serverMissing.push(id);
+      }
+    }
+  }
+
+  _compareRecords(client, server) {
+    let structuralDifferences = [];
+    let differences = [];
+
+    // Don't bother comparing titles of roots. It's okay if locally it's
+    // "Mobile Bookmarks", but the server thinks it's "mobile".
+    // TODO: We probably should be handing other localized bookmarks (e.g.
+    // default bookmarks) here as well, see bug 1316041.
+    if (!lazy.SYNCED_ROOTS.includes(client.guid)) {
+      // We want to treat undefined, null and an empty string as identical
+      if ((client.title || "") !== (server.title || "")) {
+        differences.push("title");
+      }
+    }
+
+    if (client.parentid || server.parentid) {
+      if (client.parentid !== server.parentid) {
+        structuralDifferences.push("parentid");
+      }
+    }
+
+    if (client.tags || server.tags) {
+      let cl = client.tags ? [...client.tags].sort() : [];
+      let sl = server.tags ? [...server.tags].sort() : [];
+      if (!CommonUtils.arrayEqual(cl, sl)) {
+        differences.push("tags");
+      }
+    }
+
+    let sameType = client.type === server.type;
+    if (!sameType) {
+      if (
+        server.type === "query" &&
+        client.type === "bookmark" &&
+        client.bmkUri.startsWith(QUERY_PROTOCOL)
+      ) {
+        sameType = true;
+      }
+    }
+
+    if (!sameType) {
+      differences.push("type");
+    } else {
+      switch (server.type) {
+        case "bookmark":
+        case "query":
+          if (!areURLsEqual(server.bmkUri, client.bmkUri)) {
+            differences.push("bmkUri");
+          }
+          break;
+        case "separator":
+          if (server.pos != client.pos) {
+            differences.push("pos");
+          }
+          break;
+        case "livemark":
+          if (server.feedUri != client.feedUri) {
+            differences.push("feedUri");
+          }
+          if (server.siteUri != client.siteUri) {
+            differences.push("siteUri");
+          }
+          break;
+        case "folder":
+          if (server.id === "places" && server.fake) {
+            // It's the fabricated places root. It won't have the GUIDs, but
+            // it doesn't matter.
+            break;
+          }
+          if (client.childGUIDs || server.childGUIDs) {
+            let cl = client.childGUIDs || [];
+            let sl = server.childGUIDs || [];
+            if (!CommonUtils.arrayEqual(cl, sl)) {
+              structuralDifferences.push("childGUIDs");
+            }
+          }
+          break;
+      }
+    }
+    return { differences, structuralDifferences };
+  }
+
+  /**
+   * Compare the list of server records with the client tree.
+   *
+   * Returns the same data as described in the inspectServerRecords comment,
+   * with the following additional fields.
+   * - clientRecords: an array of client records in a similar format to
+   *   the .records (ie, server records) entry.
+   * - problemData is the same as for inspectServerRecords, except all properties
+   *   will be filled out.
+   */
+  async compareServerWithClient(serverRecords, clientTree) {
+    let clientRecords = await this.createClientRecordsFromTree(clientTree);
+    let inspectionInfo = await this.inspectServerRecords(serverRecords);
+    inspectionInfo.clientRecords = clientRecords;
+
+    // Mainly do this to remove deleted items and normalize child guids.
+    serverRecords = inspectionInfo.records;
+    let problemData = inspectionInfo.problemData;
+
+    await this._validateClient(problemData, clientRecords);
+
+    let allRecords = await this._computeUnifiedRecordMap(
+      serverRecords,
+      clientRecords
+    );
+
+    let serverDeleted = new Set(inspectionInfo.deletedRecords.map(r => r.id));
+
+    await lazy.Async.yieldingForEach(
+      allRecords,
+      ([id, { client, server }]) => {
+        if (!client || !server) {
+          this._recordMissing(problemData, id, client, server, serverDeleted);
+          return;
+        }
+        if (server && client && client.ignored) {
+          problemData.serverUnexpected.push(id);
+        }
+        let { differences, structuralDifferences } = this._compareRecords(
+          client,
+          server
+        );
+
+        if (differences.length) {
+          problemData.differences.push({ id, differences });
+        }
+        if (structuralDifferences.length) {
+          problemData.structuralDifferences.push({
+            id,
+            differences: structuralDifferences,
+          });
+        }
+      },
+      this.yieldState
+    );
+
+    return inspectionInfo;
+  }
+
+  async _getServerState(engine) {
+    let collection = engine.itemSource();
+    let collectionKey = engine.service.collectionKeys.keyForCollection(
+      engine.name
+    );
+    collection.full = true;
+    let result = await collection.getBatched();
+    if (!result.response.success) {
+      throw result.response;
+    }
+    let cleartexts = [];
+    await lazy.Async.yieldingForEach(
+      result.records,
+      async record => {
+        await record.decrypt(collectionKey);
+        cleartexts.push(record.cleartext);
+      },
+      this.yieldState
+    );
+    return cleartexts;
+  }
+
+  async validate(engine) {
+    let start = Date.now();
+    let clientTree = await lazy.PlacesUtils.promiseBookmarksTree("", {
+      includeItemIds: true,
+    });
+    let serverState = await this._getServerState(engine);
+    let serverRecordCount = serverState.length;
+    let result = await this.compareServerWithClient(serverState, clientTree);
+    let end = Date.now();
+    let duration = end - start;
+
+    engine._log.debug(`Validated bookmarks in ${duration}ms`);
+    engine._log.debug(`Problem summary`);
+    for (let { name, count } of result.problemData.getSummary()) {
+      engine._log.debug(`  ${name}: ${count}`);
+    }
+
+    return {
+      duration,
+      version: this.version,
+      problems: result.problemData,
+      recordCount: serverRecordCount,
+    };
+  }
+}
+
+BookmarkValidator.prototype.version = BOOKMARK_VALIDATOR_VERSION;
diff --git a/services/sync/tps/extensions/tps/resource/modules/bookmarks.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/bookmarks.sys.mjs
new file mode 100644
index 0000000000..e4aac948b5
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/bookmarks.sys.mjs
@@ -0,0 +1,833 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+ * ChromeUtils.import() and acts as a singleton. Only the following
+ * listed symbols will exposed on import, and only when and where imported.
+ */
+
+import { PlacesBackups } from "resource://gre/modules/PlacesBackups.sys.mjs";
+
+import { PlacesSyncUtils } from "resource://gre/modules/PlacesSyncUtils.sys.mjs";
+import { PlacesUtils } from "resource://gre/modules/PlacesUtils.sys.mjs";
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+export async function DumpBookmarks() {
+  let [bookmarks] = await PlacesBackups.getBookmarksTree();
+  Logger.logInfo(
+    "Dumping Bookmarks...\n" + JSON.stringify(bookmarks, undefined, 2) + "\n\n"
+  );
+}
+
+/**
+ * extend, causes a child object to inherit from a parent
+ */
+function extend(child, supertype) {
+  Object.setPrototypeOf(child.prototype, supertype.prototype);
+}
+/**
+ * PlacesItemProps object, holds properties for places items
+ */
+function PlacesItemProps(props) {
+  this.location = null;
+  this.uri = null;
+  this.keyword = null;
+  this.title = null;
+  this.after = null;
+  this.before = null;
+  this.folder = null;
+  this.position = null;
+  this.delete = false;
+  this.tags = null;
+  this.last_item_pos = null;
+  this.type = null;
+
+  for (var prop in props) {
+    if (prop in this) {
+      this[prop] = props[prop];
+    }
+  }
+}
+
+/**
+ * PlacesItem object.  Base class for places items.
+ */
+export function PlacesItem(props) {
+  this.props = new PlacesItemProps(props);
+  if (this.props.location == null) {
+    this.props.location = "menu";
+  }
+  if ("changes" in props) {
+    this.updateProps = new PlacesItemProps(props.changes);
+  } else {
+    this.updateProps = null;
+  }
+}
+
+/**
+ * Instance methods for generic places items.
+ */
+PlacesItem.prototype = {
+  // an array of possible root folders for places items
+  _bookmarkFolders: {
+    places: PlacesUtils.bookmarks.rootGuid,
+    menu: PlacesUtils.bookmarks.menuGuid,
+    tags: PlacesUtils.bookmarks.tagsGuid,
+    unfiled: PlacesUtils.bookmarks.unfiledGuid,
+    toolbar: PlacesUtils.bookmarks.toolbarGuid,
+    mobile: PlacesUtils.bookmarks.mobileGuid,
+  },
+
+  _typeMap: new Map([
+    [PlacesUtils.TYPE_X_MOZ_PLACE_CONTAINER, PlacesUtils.bookmarks.TYPE_FOLDER],
+    [
+      PlacesUtils.TYPE_X_MOZ_PLACE_SEPARATOR,
+      PlacesUtils.bookmarks.TYPE_SEPARATOR,
+    ],
+    [PlacesUtils.TYPE_X_MOZ_PLACE, PlacesUtils.bookmarks.TYPE_BOOKMARK],
+  ]),
+
+  toString() {
+    var that = this;
+    var props = ["uri", "title", "location", "folder"];
+    var string =
+      (this.props.type ? this.props.type + " " : "") +
+      "(" +
+      (function () {
+        var ret = [];
+        for (var i in props) {
+          if (that.props[props[i]]) {
+            ret.push(props[i] + ": " + that.props[props[i]]);
+          }
+        }
+        return ret;
+      })().join(", ") +
+      ")";
+    return string;
+  },
+
+  /**
+   * GetPlacesChildGuid
+   *
+   * Finds the guid of the an item with the specified properties in the places
+   * database under the specified parent.
+   *
+   * @param folder The guid of the folder to search
+   * @param type The type of the item to find, or null to match any item;
+   *        this is one of the PlacesUtils.bookmarks.TYPE_* values
+   * @param title The title of the item to find, or null to match any title
+   * @param uri The uri of the item to find, or null to match any uri
+   *
+   * @return the node id if the item was found, otherwise null
+   */
+  async GetPlacesChildGuid(folder, type, title, uri) {
+    let children = (await PlacesUtils.promiseBookmarksTree(folder)).children;
+    if (!children) {
+      return null;
+    }
+    let guid = null;
+    for (let node of children) {
+      if (node.title == title) {
+        let nodeType = this._typeMap.get(node.type);
+        if (type == null || type == undefined || nodeType == type) {
+          if (uri == undefined || uri == null || node.uri.spec == uri.spec) {
+            // Note that this is suspect as we return the *last* matching
+            // child, which some tests rely on (ie, an early-return here causes
+            // at least 1 test to fail). But that's a yak for another day.
+            guid = node.guid;
+          }
+        }
+      }
+    }
+    return guid;
+  },
+
+  /**
+   * IsAdjacentTo
+   *
+   * Determines if this object is immediately adjacent to another.
+   *
+   * @param itemName The name of the other object; this may be any kind of
+   *        places item
+   * @param relativePos The relative position of the other object.  If -1,
+   *        it means the other object should precede this one, if +1,
+   *        the other object should come after this one
+   * @return true if this object is immediately adjacent to the other object,
+   *         otherwise false
+   */
+  async IsAdjacentTo(itemName, relativePos) {
+    Logger.AssertTrue(
+      this.props.folder_id != -1 && this.props.guid != null,
+      "Either folder_id or guid was invalid"
+    );
+    let otherGuid = await this.GetPlacesChildGuid(
+      this.props.parentGuid,
+      null,
+      itemName
+    );
+    Logger.AssertTrue(otherGuid, "item " + itemName + " not found");
+    let other_pos = (await PlacesUtils.bookmarks.fetch(otherGuid)).index;
+    let this_pos = (await PlacesUtils.bookmarks.fetch(this.props.guid)).index;
+    if (other_pos + relativePos != this_pos) {
+      Logger.logPotentialError(
+        "Invalid position - " +
+          (this.props.title ? this.props.title : this.props.folder) +
+          " not " +
+          (relativePos == 1 ? "after " : "before ") +
+          itemName +
+          " for " +
+          this.toString()
+      );
+      return false;
+    }
+    return true;
+  },
+
+  /**
+   * GetItemIndex
+   *
+   * Gets the item index for this places item.
+   *
+   * @return the item index, or -1 if there's an error
+   */
+  async GetItemIndex() {
+    if (this.props.guid == null) {
+      return -1;
+    }
+    return (await PlacesUtils.bookmarks.fetch(this.props.guid)).index;
+  },
+
+  /**
+   * GetFolder
+   *
+   * Gets the folder guid for the specified bookmark folder
+   *
+   * @param location The full path of the folder, which must begin
+   *        with one of the bookmark root folders
+   * @return the folder guid if the folder is found, otherwise null
+   */
+  async GetFolder(location) {
+    let folder_parts = location.split("/");
+    if (!(folder_parts[0] in this._bookmarkFolders)) {
+      return null;
+    }
+    let folderGuid = this._bookmarkFolders[folder_parts[0]];
+    for (let i = 1; i < folder_parts.length; i++) {
+      let guid = await this.GetPlacesChildGuid(
+        folderGuid,
+        PlacesUtils.bookmarks.TYPE_FOLDER,
+        folder_parts[i]
+      );
+      if (guid == null) {
+        return null;
+      }
+      folderGuid = guid;
+    }
+    return folderGuid;
+  },
+
+  /**
+   * CreateFolder
+   *
+   * Creates a bookmark folder.
+   *
+   * @param location The full path of the folder, which must begin
+   *        with one of the bookmark root folders
+   * @return the folder id if the folder was created, otherwise -1
+   */
+  async CreateFolder(location) {
+    let folder_parts = location.split("/");
+    if (!(folder_parts[0] in this._bookmarkFolders)) {
+      return -1;
+    }
+    let folderGuid = this._bookmarkFolders[folder_parts[0]];
+    for (let i = 1; i < folder_parts.length; i++) {
+      let subfolderGuid = await this.GetPlacesChildGuid(
+        folderGuid,
+        PlacesUtils.bookmarks.TYPE_FOLDER,
+        folder_parts[i]
+      );
+      if (subfolderGuid == null) {
+        let { guid } = await PlacesUtils.bookmarks.insert({
+          parentGuid: folderGuid,
+          name: folder_parts[i],
+          type: PlacesUtils.bookmarks.TYPE_FOLDER,
+        });
+        folderGuid = guid;
+      } else {
+        folderGuid = subfolderGuid;
+      }
+    }
+    return folderGuid;
+  },
+
+  /**
+   * GetOrCreateFolder
+   *
+   * Locates the specified folder; if not found it is created.
+   *
+   * @param location The full path of the folder, which must begin
+   *        with one of the bookmark root folders
+   * @return the folder id if the folder was found or created, otherwise -1
+   */
+  async GetOrCreateFolder(location) {
+    let parentGuid = await this.GetFolder(location);
+    if (parentGuid == null) {
+      parentGuid = await this.CreateFolder(location);
+    }
+    return parentGuid;
+  },
+
+  /**
+   * CheckPosition
+   *
+   * Verifies the position of this places item.
+   *
+   * @param before The name of the places item that this item should be
+            before, or null if this check should be skipped
+   * @param after The name of the places item that this item should be
+            after, or null if this check should be skipped
+   * @param last_item_pos The index of the places item above this one,
+   *        or null if this check should be skipped
+   * @return true if this item is in the correct position, otherwise false
+   */
+  async CheckPosition(before, after, last_item_pos) {
+    if (after) {
+      if (!(await this.IsAdjacentTo(after, 1))) {
+        return false;
+      }
+    }
+    if (before) {
+      if (!(await this.IsAdjacentTo(before, -1))) {
+        return false;
+      }
+    }
+    if (last_item_pos != null && last_item_pos > -1) {
+      let index = await this.GetItemIndex();
+      if (index != last_item_pos + 1) {
+        Logger.logPotentialError(
+          "Item not found at the expected index, got " +
+            index +
+            ", expected " +
+            (last_item_pos + 1) +
+            " for " +
+            this.toString()
+        );
+        return false;
+      }
+    }
+    return true;
+  },
+
+  /**
+   * SetLocation
+   *
+   * Moves this places item to a different folder.
+   *
+   * @param location The full path of the folder to which to move this
+   *        places item, which must begin with one of the bookmark root
+   *        folders; if null, no changes are made
+   * @return nothing if successful, otherwise an exception is thrown
+   */
+  async SetLocation(location) {
+    if (location != null) {
+      let newfolderGuid = await this.GetOrCreateFolder(location);
+      Logger.AssertTrue(
+        newfolderGuid,
+        "Location " + location + " doesn't exist; can't change item's location"
+      );
+      await PlacesUtils.bookmarks.update({
+        guid: this.props.guid,
+        parentGuid: newfolderGuid,
+        index: PlacesUtils.bookmarks.DEFAULT_INDEX,
+      });
+      this.props.parentGuid = newfolderGuid;
+    }
+  },
+
+  /**
+   * SetPosition
+   *
+   * Updates the position of this places item within this item's current
+   * folder.  Use SetLocation to change folders.
+   *
+   * @param position The new index this item should be moved to; if null,
+   *        no changes are made; if -1, this item is moved to the bottom of
+   *        the current folder. Otherwise, must be a string which is the
+   *        title of an existing item in the folder, who's current position
+   *        is used as the index.
+   * @return nothing if successful, otherwise an exception is thrown
+   */
+  async SetPosition(position) {
+    if (position == null) {
+      return;
+    }
+    let index = -1;
+    if (position != -1) {
+      let existingGuid = await this.GetPlacesChildGuid(
+        this.props.parentGuid,
+        null,
+        position
+      );
+      if (existingGuid) {
+        index = (await PlacesUtils.bookmarks.fetch(existingGuid)).index;
+      }
+      Logger.AssertTrue(
+        index != -1,
+        "position " + position + " is invalid; unable to change position"
+      );
+    }
+    await PlacesUtils.bookmarks.update({ guid: this.props.guid, index });
+  },
+
+  /**
+   * Update the title of this places item
+   *
+   * @param title The new title to set for this item; if null, no changes
+   *        are made
+   * @return nothing
+   */
+  async SetTitle(title) {
+    if (title != null) {
+      await PlacesUtils.bookmarks.update({ guid: this.props.guid, title });
+    }
+  },
+};
+
+/**
+ * Bookmark class constructor.  Initializes instance properties.
+ */
+export function Bookmark(props) {
+  PlacesItem.call(this, props);
+  if (this.props.title == null) {
+    this.props.title = this.props.uri;
+  }
+  this.props.type = "bookmark";
+}
+
+/**
+ * Bookmark instance methods.
+ */
+Bookmark.prototype = {
+  /**
+   * SetKeyword
+   *
+   * Update this bookmark's keyword.
+   *
+   * @param keyword The keyword to set for this bookmark; if null, no
+   *        changes are made
+   * @return nothing
+   */
+  async SetKeyword(keyword) {
+    if (keyword != null) {
+      // Mirror logic from PlacesSyncUtils's updateBookmarkMetadata
+      let entry = await PlacesUtils.keywords.fetch({ url: this.props.uri });
+      if (entry) {
+        await PlacesUtils.keywords.remove(entry);
+      }
+      await PlacesUtils.keywords.insert({ keyword, url: this.props.uri });
+    }
+  },
+
+  /**
+   * SetUri
+   *
+   * Updates this bookmark's URI.
+   *
+   * @param uri The new URI to set for this boomark; if null, no changes
+   *        are made
+   * @return nothing
+   */
+  async SetUri(uri) {
+    if (uri) {
+      let url = Services.io.newURI(uri);
+      await PlacesUtils.bookmarks.update({ guid: this.props.guid, url });
+    }
+  },
+
+  /**
+   * SetTags
+   *
+   * Updates this bookmark's tags.
+   *
+   * @param tags An array of tags which should be associated with this
+   *        bookmark; any previous tags are removed; if this param is null,
+   *        no changes are made.  If this param is an empty array, all
+   *        tags are removed from this bookmark.
+   * @return nothing
+   */
+  SetTags(tags) {
+    if (tags != null) {
+      let URI = Services.io.newURI(this.props.uri);
+      PlacesUtils.tagging.untagURI(URI, null);
+      if (tags.length) {
+        PlacesUtils.tagging.tagURI(URI, tags);
+      }
+    }
+  },
+
+  /**
+   * Create
+   *
+   * Creates the bookmark described by this object's properties.
+   *
+   * @return the id of the created bookmark
+   */
+  async Create() {
+    this.props.parentGuid = await this.GetOrCreateFolder(this.props.location);
+    Logger.AssertTrue(
+      this.props.parentGuid,
+      "Unable to create " +
+        "bookmark, error creating folder " +
+        this.props.location
+    );
+    let bookmarkURI = Services.io.newURI(this.props.uri);
+    let { guid } = await PlacesUtils.bookmarks.insert({
+      parentGuid: this.props.parentGuid,
+      url: bookmarkURI,
+      title: this.props.title,
+    });
+    this.props.guid = guid;
+    await this.SetKeyword(this.props.keyword);
+    await this.SetTags(this.props.tags);
+    return this.props.guid;
+  },
+
+  /**
+   * Update
+   *
+   * Updates this bookmark's properties according the properties on this
+   * object's 'updateProps' property.
+   *
+   * @return nothing
+   */
+  async Update() {
+    Logger.AssertTrue(this.props.guid, "Invalid guid during Update");
+    await this.SetTitle(this.updateProps.title);
+    await this.SetUri(this.updateProps.uri);
+    await this.SetKeyword(this.updateProps.keyword);
+    await this.SetTags(this.updateProps.tags);
+    await this.SetLocation(this.updateProps.location);
+    await this.SetPosition(this.updateProps.position);
+  },
+
+  /**
+   * Find
+   *
+   * Locates the bookmark which corresponds to this object's properties.
+   *
+   * @return the bookmark guid if the bookmark was found, otherwise null
+   */
+  async Find() {
+    this.props.parentGuid = await this.GetFolder(this.props.location);
+
+    if (this.props.parentGuid == null) {
+      Logger.logError("Unable to find folder " + this.props.location);
+      return null;
+    }
+    let bookmarkTitle = this.props.title;
+    this.props.guid = await this.GetPlacesChildGuid(
+      this.props.parentGuid,
+      null,
+      bookmarkTitle,
+      this.props.uri
+    );
+
+    if (!this.props.guid) {
+      Logger.logPotentialError(this.toString() + " not found");
+      return null;
+    }
+    if (this.props.keyword != null) {
+      let { keyword } = await PlacesSyncUtils.bookmarks.fetch(this.props.guid);
+      if (keyword != this.props.keyword) {
+        Logger.logPotentialError(
+          "Incorrect keyword - expected: " +
+            this.props.keyword +
+            ", actual: " +
+            keyword +
+            " for " +
+            this.toString()
+        );
+        return null;
+      }
+    }
+    if (this.props.tags != null) {
+      try {
+        let URI = Services.io.newURI(this.props.uri);
+        let tags = PlacesUtils.tagging.getTagsForURI(URI);
+        tags.sort();
+        this.props.tags.sort();
+        if (JSON.stringify(tags) != JSON.stringify(this.props.tags)) {
+          Logger.logPotentialError(
+            "Wrong tags - expected: " +
+              JSON.stringify(this.props.tags) +
+              ", actual: " +
+              JSON.stringify(tags) +
+              " for " +
+              this.toString()
+          );
+          return null;
+        }
+      } catch (e) {
+        Logger.logPotentialError("error processing tags " + e);
+        return null;
+      }
+    }
+    if (
+      !(await this.CheckPosition(
+        this.props.before,
+        this.props.after,
+        this.props.last_item_pos
+      ))
+    ) {
+      return null;
+    }
+    return this.props.guid;
+  },
+
+  /**
+   * Remove
+   *
+   * Removes this bookmark.  The bookmark should have been located previously
+   * by a call to Find.
+   *
+   * @return nothing
+   */
+  async Remove() {
+    Logger.AssertTrue(this.props.guid, "Invalid guid during Remove");
+    await PlacesUtils.bookmarks.remove(this.props.guid);
+  },
+};
+
+extend(Bookmark, PlacesItem);
+
+/**
+ * BookmarkFolder class constructor. Initializes instance properties.
+ */
+export function BookmarkFolder(props) {
+  PlacesItem.call(this, props);
+  this.props.type = "folder";
+}
+
+/**
+ * BookmarkFolder instance methods
+ */
+BookmarkFolder.prototype = {
+  /**
+   * Create
+   *
+   * Creates the bookmark folder described by this object's properties.
+   *
+   * @return the id of the created bookmark folder
+   */
+  async Create() {
+    this.props.parentGuid = await this.GetOrCreateFolder(this.props.location);
+    Logger.AssertTrue(
+      this.props.parentGuid,
+      "Unable to create " +
+        "folder, error creating parent folder " +
+        this.props.location
+    );
+    let { guid } = await PlacesUtils.bookmarks.insert({
+      parentGuid: this.props.parentGuid,
+      title: this.props.folder,
+      index: PlacesUtils.bookmarks.DEFAULT_INDEX,
+      type: PlacesUtils.bookmarks.TYPE_FOLDER,
+    });
+    this.props.guid = guid;
+    return this.props.parentGuid;
+  },
+
+  /**
+   * Find
+   *
+   * Locates the bookmark folder which corresponds to this object's
+   * properties.
+   *
+   * @return the folder guid if the folder was found, otherwise null
+   */
+  async Find() {
+    this.props.parentGuid = await this.GetFolder(this.props.location);
+    if (this.props.parentGuid == null) {
+      Logger.logError("Unable to find folder " + this.props.location);
+      return null;
+    }
+    this.props.guid = await this.GetPlacesChildGuid(
+      this.props.parentGuid,
+      PlacesUtils.bookmarks.TYPE_FOLDER,
+      this.props.folder
+    );
+    if (this.props.guid == null) {
+      return null;
+    }
+    if (
+      !(await this.CheckPosition(
+        this.props.before,
+        this.props.after,
+        this.props.last_item_pos
+      ))
+    ) {
+      return null;
+    }
+    return this.props.guid;
+  },
+
+  /**
+   * Remove
+   *
+   * Removes this folder.  The folder should have been located previously
+   * by a call to Find.
+   *
+   * @return nothing
+   */
+  async Remove() {
+    Logger.AssertTrue(this.props.guid, "Invalid guid during Remove");
+    await PlacesUtils.bookmarks.remove(this.props.guid);
+  },
+
+  /**
+   * Update
+   *
+   * Updates this bookmark's properties according the properties on this
+   * object's 'updateProps' property.
+   *
+   * @return nothing
+   */
+  async Update() {
+    Logger.AssertTrue(this.props.guid, "Invalid guid during Update");
+    await this.SetLocation(this.updateProps.location);
+    await this.SetPosition(this.updateProps.position);
+    await this.SetTitle(this.updateProps.folder);
+  },
+};
+
+extend(BookmarkFolder, PlacesItem);
+
+/**
+ * Separator class constructor. Initializes instance properties.
+ */
+export function Separator(props) {
+  PlacesItem.call(this, props);
+  this.props.type = "separator";
+}
+
+/**
+ * Separator instance methods.
+ */
+Separator.prototype = {
+  /**
+   * Create
+   *
+   * Creates the bookmark separator described by this object's properties.
+   *
+   * @return the id of the created separator
+   */
+  async Create() {
+    this.props.parentGuid = await this.GetOrCreateFolder(this.props.location);
+    Logger.AssertTrue(
+      this.props.parentGuid,
+      "Unable to create " +
+        "folder, error creating parent folder " +
+        this.props.location
+    );
+    let { guid } = await PlacesUtils.bookmarks.insert({
+      parentGuid: this.props.parentGuid,
+      type: PlacesUtils.bookmarks.TYPE_SEPARATOR,
+    });
+    this.props.guid = guid;
+    return guid;
+  },
+
+  /**
+   * Find
+   *
+   * Locates the bookmark separator which corresponds to this object's
+   * properties.
+   *
+   * @return the item guid if the separator was found, otherwise null
+   */
+  async Find() {
+    this.props.parentGuid = await this.GetFolder(this.props.location);
+    if (this.props.parentGuid == null) {
+      Logger.logError("Unable to find folder " + this.props.location);
+      return null;
+    }
+    if (this.props.before == null && this.props.last_item_pos == null) {
+      Logger.logPotentialError(
+        "Separator requires 'before' attribute if it's the" +
+          "first item in the list"
+      );
+      return null;
+    }
+    let expected_pos = -1;
+    if (this.props.before) {
+      let otherGuid = this.GetPlacesChildGuid(
+        this.props.parentGuid,
+        null,
+        this.props.before
+      );
+      if (otherGuid == null) {
+        Logger.logPotentialError(
+          "Can't find places item " +
+            this.props.before +
+            " for locating separator"
+        );
+        return null;
+      }
+      expected_pos = (await PlacesUtils.bookmarks.fetch(otherGuid)).index - 1;
+    } else {
+      expected_pos = this.props.last_item_pos + 1;
+    }
+    // Note these are IDs instead of GUIDs.
+    let children = await PlacesSyncUtils.bookmarks.fetchChildRecordIds(
+      this.props.parentGuid
+    );
+    this.props.guid = children[expected_pos];
+    if (this.props.guid == null) {
+      Logger.logPotentialError(
+        "No separator found at position " + expected_pos
+      );
+      return null;
+    }
+    let info = await PlacesUtils.bookmarks.fetch(this.props.guid);
+    if (info.type != PlacesUtils.bookmarks.TYPE_SEPARATOR) {
+      Logger.logPotentialError(
+        "Places item at position " + expected_pos + " is not a separator"
+      );
+      return null;
+    }
+    return this.props.guid;
+  },
+
+  /**
+   * Update
+   *
+   * Updates this separator's properties according the properties on this
+   * object's 'updateProps' property.
+   *
+   * @return nothing
+   */
+  async Update() {
+    Logger.AssertTrue(this.props.guid, "Invalid guid during Update");
+    await this.SetLocation(this.updateProps.location);
+    await this.SetPosition(this.updateProps.position);
+    return true;
+  },
+
+  /**
+   * Remove
+   *
+   * Removes this separator.  The separator should have been located
+   * previously by a call to Find.
+   *
+   * @return nothing
+   */
+  async Remove() {
+    Logger.AssertTrue(this.props.guid, "Invalid guid during Update");
+    await PlacesUtils.bookmarks.remove(this.props.guid);
+  },
+};
+
+extend(Separator, PlacesItem);
diff --git a/services/sync/tps/extensions/tps/resource/modules/formautofill.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/formautofill.sys.mjs
new file mode 100644
index 0000000000..587d7668f4
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/formautofill.sys.mjs
@@ -0,0 +1,128 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+ * ChromeUtils.import() and acts as a singleton. Only the following
+ * listed symbols will exposed on import, and only when and where imported.
+ */
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  OSKeyStore: "resource://gre/modules/OSKeyStore.sys.mjs",
+  formAutofillStorage: "resource://autofill/FormAutofillStorage.sys.mjs",
+});
+
+class FormAutofillBase {
+  constructor(props, subStorageName, fields) {
+    this._subStorageName = subStorageName;
+    this._fields = fields;
+
+    this.props = {};
+    this.updateProps = null;
+    if ("changes" in props) {
+      this.updateProps = props.changes;
+    }
+    for (const field of this._fields) {
+      this.props[field] = field in props ? props[field] : null;
+    }
+  }
+
+  async getStorage() {
+    await lazy.formAutofillStorage.initialize();
+    return lazy.formAutofillStorage[this._subStorageName];
+  }
+
+  async Create() {
+    const storage = await this.getStorage();
+    await storage.add(this.props);
+  }
+
+  async Find() {
+    const storage = await this.getStorage();
+    return storage._data.find(entry =>
+      this._fields.every(field => entry[field] === this.props[field])
+    );
+  }
+
+  async Update() {
+    const storage = await this.getStorage();
+    const { guid } = await this.Find();
+    await storage.update(guid, this.updateProps, true);
+  }
+
+  async Remove() {
+    const storage = await this.getStorage();
+    const { guid } = await this.Find();
+    storage.remove(guid);
+  }
+}
+
+async function DumpStorage(subStorageName) {
+  await lazy.formAutofillStorage.initialize();
+  Logger.logInfo(`\ndumping ${subStorageName} list\n`, true);
+  const entries = lazy.formAutofillStorage[subStorageName]._data;
+  for (const entry of entries) {
+    Logger.logInfo(JSON.stringify(entry), true);
+  }
+  Logger.logInfo(`\n\nend ${subStorageName} list\n`, true);
+}
+
+const ADDRESS_FIELDS = [
+  "given-name",
+  "additional-name",
+  "family-name",
+  "organization",
+  "street-address",
+  "address-level2",
+  "address-level1",
+  "postal-code",
+  "country",
+  "tel",
+  "email",
+];
+
+export class Address extends FormAutofillBase {
+  constructor(props) {
+    super(props, "addresses", ADDRESS_FIELDS);
+  }
+}
+
+export async function DumpAddresses() {
+  await DumpStorage("addresses");
+}
+
+const CREDIT_CARD_FIELDS = [
+  "cc-name",
+  "cc-number",
+  "cc-exp-month",
+  "cc-exp-year",
+];
+
+export class CreditCard extends FormAutofillBase {
+  constructor(props) {
+    super(props, "creditCards", CREDIT_CARD_FIELDS);
+  }
+
+  async Find() {
+    const storage = await this.getStorage();
+    await Promise.all(
+      storage._data.map(
+        async entry =>
+          (entry["cc-number"] = await lazy.OSKeyStore.decrypt(
+            entry["cc-number-encrypted"]
+          ))
+      )
+    );
+    return storage._data.find(entry => {
+      return this._fields.every(field => entry[field] === this.props[field]);
+    });
+  }
+}
+
+export async function DumpCreditCards() {
+  await DumpStorage("creditCards");
+}
diff --git a/services/sync/tps/extensions/tps/resource/modules/forms.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/forms.sys.mjs
new file mode 100644
index 0000000000..35b5f5c03b
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/forms.sys.mjs
@@ -0,0 +1,205 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+   ChromeUtils.import() and acts as a singleton. Only the following
+   listed symbols will exposed on import, and only when and where imported.
+  */
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+import { FormHistory } from "resource://gre/modules/FormHistory.sys.mjs";
+
+/**
+ * FormDB
+ *
+ * Helper object containing methods to interact with the FormHistory module.
+ */
+var FormDB = {
+  async _update(data) {
+    await FormHistory.update(data);
+  },
+
+  /**
+   * insertValue
+   *
+   * Adds the specified value for the specified fieldname into form history.
+   *
+   * @param fieldname The form fieldname to insert
+   * @param value The form value to insert
+   * @param us The time, in microseconds, to use for the lastUsed
+   *        and firstUsed columns
+   * @return Promise<undefined>
+   */
+  insertValue(fieldname, value, us) {
+    let data = {
+      op: "add",
+      fieldname,
+      value,
+      timesUsed: 1,
+      firstUsed: us,
+      lastUsed: us,
+    };
+    return this._update(data);
+  },
+
+  /**
+   * updateValue
+   *
+   * Updates a row in the moz_formhistory table with a new value.
+   *
+   * @param id The id of the row to update
+   * @param newvalue The new value to set
+   * @return Promise<undefined>
+   */
+  updateValue(id, newvalue) {
+    return this._update({ op: "update", guid: id, value: newvalue });
+  },
+
+  /**
+   * getDataForValue
+   *
+   * Retrieves a set of values for a row in the database that
+   * corresponds to the given fieldname and value.
+   *
+   * @param fieldname The fieldname of the row to query
+   * @param value The value of the row to query
+   * @return Promise<null if no row is found with the specified fieldname and value,
+   *         or an object containing the row's guid, lastUsed, and firstUsed
+   *         values>
+   */
+  async getDataForValue(fieldname, value) {
+    let results = await FormHistory.search(["guid", "lastUsed", "firstUsed"], {
+      fieldname,
+      value,
+    });
+    if (results.length > 1) {
+      throw new Error("more than 1 result for this query");
+    }
+    return results;
+  },
+
+  /**
+   * remove
+   *
+   * Removes the specified GUID from the database.
+   *
+   * @param guid The guid of the item to delete
+   * @return Promise<>
+   */
+  remove(guid) {
+    return this._update({ op: "remove", guid });
+  },
+};
+
+/**
+ * FormData class constructor
+ *
+ * Initializes instance properties.
+ */
+export function FormData(props, msSinceEpoch) {
+  this.fieldname = null;
+  this.value = null;
+  this.date = 0;
+  this.newvalue = null;
+  this.usSinceEpoch = msSinceEpoch * 1000;
+
+  for (var prop in props) {
+    if (prop in this) {
+      this[prop] = props[prop];
+    }
+  }
+}
+
+/**
+ * FormData instance methods
+ */
+FormData.prototype = {
+  /**
+   * hours_to_us
+   *
+   * Converts hours since present to microseconds since epoch.
+   *
+   * @param hours The number of hours since the present time (e.g., 0 is
+   *        'now', and -1 is 1 hour ago)
+   * @return the corresponding number of microseconds since the epoch
+   */
+  hours_to_us(hours) {
+    return this.usSinceEpoch + hours * 60 * 60 * 1000 * 1000;
+  },
+
+  /**
+   * Create
+   *
+   * If this FormData object doesn't exist in the moz_formhistory database,
+   * add it.  Throws on error.
+   *
+   * @return nothing
+   */
+  Create() {
+    Logger.AssertTrue(
+      this.fieldname != null && this.value != null,
+      "Must specify both fieldname and value"
+    );
+
+    return FormDB.getDataForValue(this.fieldname, this.value).then(formdata => {
+      if (!formdata) {
+        // this item doesn't exist yet in the db, so we need to insert it
+        return FormDB.insertValue(
+          this.fieldname,
+          this.value,
+          this.hours_to_us(this.date)
+        );
+      }
+      /* Right now, we ignore this case.  If bug 552531 is ever fixed,
+         we might need to add code here to update the firstUsed or
+         lastUsed fields, as appropriate.
+       */
+      return null;
+    });
+  },
+
+  /**
+   * Find
+   *
+   * Attempts to locate an entry in the moz_formhistory database that
+   * matches the fieldname and value for this FormData object.
+   *
+   * @return true if this entry exists in the database, otherwise false
+   */
+  Find() {
+    return FormDB.getDataForValue(this.fieldname, this.value).then(formdata => {
+      let status = formdata != null;
+      if (status) {
+        /*
+        //form history dates currently not synced!  bug 552531
+        let us = this.hours_to_us(this.date);
+        status = Logger.AssertTrue(
+          us >= formdata.firstUsed && us <= formdata.lastUsed,
+          "No match for with that date value");
+
+        if (status)
+        */
+        this.id = formdata.guid;
+      }
+      return status;
+    });
+  },
+
+  /**
+   * Remove
+   *
+   * Removes the row represented by this FormData instance from the
+   * moz_formhistory database.
+   *
+   * @return nothing
+   */
+  async Remove() {
+    const formdata = await FormDB.getDataForValue(this.fieldname, this.value);
+    if (!formdata) {
+      return;
+    }
+    await FormDB.remove(formdata.guid);
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/history.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/history.sys.mjs
new file mode 100644
index 0000000000..845bab3aa9
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/history.sys.mjs
@@ -0,0 +1,158 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+ * ChromeUtils.import() and acts as a singleton. Only the following
+ * listed symbols will exposed on import, and only when and where imported.
+ */
+
+import { PlacesUtils } from "resource://gre/modules/PlacesUtils.sys.mjs";
+
+import { PlacesSyncUtils } from "resource://gre/modules/PlacesSyncUtils.sys.mjs";
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+export var DumpHistory = async function TPS_History__DumpHistory() {
+  let query = PlacesUtils.history.getNewQuery();
+  let options = PlacesUtils.history.getNewQueryOptions();
+  let root = PlacesUtils.history.executeQuery(query, options).root;
+  root.containerOpen = true;
+  Logger.logInfo("\n\ndumping history\n", true);
+  for (var i = 0; i < root.childCount; i++) {
+    let node = root.getChild(i);
+    let uri = node.uri;
+    let guid = await PlacesSyncUtils.history
+      .fetchGuidForURL(uri)
+      .catch(() => "?".repeat(12));
+    let curvisits = await PlacesSyncUtils.history.fetchVisitsForURL(uri);
+    for (var visit of curvisits) {
+      Logger.logInfo(
+        `GUID: ${guid}, URI: ${uri}, type=${visit.type}, date=${visit.date}`,
+        true
+      );
+    }
+  }
+  root.containerOpen = false;
+  Logger.logInfo("\nend history dump\n", true);
+};
+
+/**
+ * HistoryEntry object
+ *
+ * Contains methods for manipulating browser history entries.
+ */
+export var HistoryEntry = {
+  /**
+   * Add
+   *
+   * Adds visits for a uri to the history database.  Throws on error.
+   *
+   * @param item An object representing one or more visits to a specific uri
+   * @param usSinceEpoch The number of microseconds from Epoch to
+   *        the time the current Crossweave run was started
+   * @return nothing
+   */
+  async Add(item, msSinceEpoch) {
+    Logger.AssertTrue(
+      "visits" in item && "uri" in item,
+      "History entry in test file must have both 'visits' " +
+        "and 'uri' properties"
+    );
+    let place = {
+      url: item.uri,
+      visits: [],
+    };
+    for (let visit of item.visits) {
+      let date = new Date(
+        Math.round(msSinceEpoch + visit.date * 60 * 60 * 1000)
+      );
+      place.visits.push({ date, transition: visit.type });
+    }
+    if ("title" in item) {
+      place.title = item.title;
+    }
+    return PlacesUtils.history.insert(place);
+  },
+
+  /**
+   * Find
+   *
+   * Finds visits for a uri to the history database.  Throws on error.
+   *
+   * @param item An object representing one or more visits to a specific uri
+   * @param usSinceEpoch The number of microseconds from Epoch to
+   *        the time the current Crossweave run was started
+   * @return true if all the visits for the uri are found, otherwise false
+   */
+  async Find(item, msSinceEpoch) {
+    Logger.AssertTrue(
+      "visits" in item && "uri" in item,
+      "History entry in test file must have both 'visits' " +
+        "and 'uri' properties"
+    );
+    let curvisits = await PlacesSyncUtils.history.fetchVisitsForURL(item.uri);
+    for (let visit of curvisits) {
+      for (let itemvisit of item.visits) {
+        // Note: in microseconds.
+        let expectedDate =
+          itemvisit.date * 60 * 60 * 1000 * 1000 + msSinceEpoch * 1000;
+        if (visit.type == itemvisit.type) {
+          if (itemvisit.date === undefined || visit.date == expectedDate) {
+            itemvisit.found = true;
+          }
+        }
+      }
+    }
+
+    let all_items_found = true;
+    for (let itemvisit of item.visits) {
+      all_items_found = all_items_found && "found" in itemvisit;
+      Logger.logInfo(
+        `History entry for ${item.uri}, type: ${itemvisit.type}, date: ${itemvisit.date}` +
+          `(${
+            itemvisit.date * 60 * 60 * 1000 * 1000
+          }), found = ${!!itemvisit.found}`
+      );
+    }
+    return all_items_found;
+  },
+
+  /**
+   * Delete
+   *
+   * Removes visits from the history database. Throws on error.
+   *
+   * @param item An object representing items to delete
+   * @param usSinceEpoch The number of microseconds from Epoch to
+   *        the time the current Crossweave run was started
+   * @return nothing
+   */
+  async Delete(item, msSinceEpoch) {
+    if ("uri" in item) {
+      let removedAny = await PlacesUtils.history.remove(item.uri);
+      if (!removedAny) {
+        Logger.log("Warning: Removed 0 history visits for uri " + item.uri);
+      }
+    } else if ("host" in item) {
+      await PlacesUtils.history.removeByFilter({ host: item.host });
+    } else if ("begin" in item && "end" in item) {
+      let filter = {
+        beginDate: new Date(msSinceEpoch + item.begin * 60 * 60 * 1000),
+        endDate: new Date(msSinceEpoch + item.end * 60 * 60 * 1000),
+      };
+      let removedAny = await PlacesUtils.history.removeVisitsByFilter(filter);
+      if (!removedAny) {
+        Logger.log(
+          "Warning: Removed 0 history visits with " +
+            JSON.stringify({ item, filter })
+        );
+      }
+    } else {
+      Logger.AssertTrue(
+        false,
+        "invalid entry in delete history " + JSON.stringify(item)
+      );
+    }
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/passwords.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/passwords.sys.mjs
new file mode 100644
index 0000000000..976755e989
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/passwords.sys.mjs
@@ -0,0 +1,187 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+ * ChromeUtils.import() and acts as a singleton. Only the following
+ * listed symbols will exposed on import, and only when and where imported.
+ */
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+var nsLoginInfo = new Components.Constructor(
+  "@mozilla.org/login-manager/loginInfo;1",
+  Ci.nsILoginInfo,
+  "init"
+);
+
+export var DumpPasswords = async function TPS__Passwords__DumpPasswords() {
+  let logins = await Services.logins.getAllLogins();
+  Logger.logInfo("\ndumping password list\n", true);
+  for (var i = 0; i < logins.length; i++) {
+    Logger.logInfo(
+      "* origin=" +
+        logins[i].origin +
+        ", formActionOrigin=" +
+        logins[i].formActionOrigin +
+        ", realm=" +
+        logins[i].httpRealm +
+        ", password=" +
+        logins[i].password +
+        ", passwordField=" +
+        logins[i].passwordField +
+        ", username=" +
+        logins[i].username +
+        ", usernameField=" +
+        logins[i].usernameField,
+      true
+    );
+  }
+  Logger.logInfo("\n\nend password list\n", true);
+};
+
+/**
+ * PasswordProps object; holds password properties.
+ */
+function PasswordProps(props) {
+  this.hostname = null;
+  this.submitURL = null;
+  this.realm = null;
+  this.username = "";
+  this.password = "";
+  this.usernameField = "";
+  this.passwordField = "";
+  this.delete = false;
+
+  for (var prop in props) {
+    if (prop in this) {
+      this[prop] = props[prop];
+    }
+  }
+}
+
+/**
+ * Password class constructor. Initializes instance properties.
+ */
+export function Password(props) {
+  this.props = new PasswordProps(props);
+  if ("changes" in props) {
+    this.updateProps = new PasswordProps(props);
+    for (var prop in props.changes) {
+      if (prop in this.updateProps) {
+        this.updateProps[prop] = props.changes[prop];
+      }
+    }
+  } else {
+    this.updateProps = null;
+  }
+}
+
+/**
+ * Password instance methods.
+ */
+Password.prototype = {
+  /**
+   * Create
+   *
+   * Adds a password entry to the login manager for the password
+   * represented by this object's properties. Throws on error.
+   *
+   * @return the new login guid
+   */
+  async Create() {
+    let login = new nsLoginInfo(
+      this.props.hostname,
+      this.props.submitURL,
+      this.props.realm,
+      this.props.username,
+      this.props.password,
+      this.props.usernameField,
+      this.props.passwordField
+    );
+    await Services.logins.addLoginAsync(login);
+    login.QueryInterface(Ci.nsILoginMetaInfo);
+    return login.guid;
+  },
+
+  /**
+   * Find
+   *
+   * Finds a password entry in the login manager, for the password
+   * represented by this object's properties.
+   *
+   * @return the guid of the password if found, otherwise -1
+   */
+  async Find() {
+    let logins = await Services.logins.searchLoginsAsync({
+      origin: this.props.hostname,
+      formActionOrigin: this.props.submitURL,
+      httpRealm: this.props.realm,
+    });
+    for (var i = 0; i < logins.length; i++) {
+      if (
+        logins[i].username == this.props.username &&
+        logins[i].password == this.props.password &&
+        logins[i].usernameField == this.props.usernameField &&
+        logins[i].passwordField == this.props.passwordField
+      ) {
+        logins[i].QueryInterface(Ci.nsILoginMetaInfo);
+        return logins[i].guid;
+      }
+    }
+    return -1;
+  },
+
+  /**
+   * Update
+   *
+   * Updates an existing password entry in the login manager with
+   * new properties. Throws on error.  The 'old' properties are this
+   * object's properties, the 'new' properties are the properties in
+   * this object's 'updateProps' object.
+   *
+   * @return nothing
+   */
+  Update() {
+    let oldlogin = new nsLoginInfo(
+      this.props.hostname,
+      this.props.submitURL,
+      this.props.realm,
+      this.props.username,
+      this.props.password,
+      this.props.usernameField,
+      this.props.passwordField
+    );
+    let newlogin = new nsLoginInfo(
+      this.updateProps.hostname,
+      this.updateProps.submitURL,
+      this.updateProps.realm,
+      this.updateProps.username,
+      this.updateProps.password,
+      this.updateProps.usernameField,
+      this.updateProps.passwordField
+    );
+    Services.logins.modifyLogin(oldlogin, newlogin);
+  },
+
+  /**
+   * Remove
+   *
+   * Removes an entry from the login manager for a password which
+   * matches this object's properties. Throws on error.
+   *
+   * @return nothing
+   */
+  Remove() {
+    let login = new nsLoginInfo(
+      this.props.hostname,
+      this.props.submitURL,
+      this.props.realm,
+      this.props.username,
+      this.props.password,
+      this.props.usernameField,
+      this.props.passwordField
+    );
+    Services.logins.removeLogin(login);
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/prefs.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/prefs.sys.mjs
new file mode 100644
index 0000000000..c629ef4a73
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/prefs.sys.mjs
@@ -0,0 +1,122 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+   ChromeUtils.import() and acts as a singleton.
+   Only the following listed symbols will exposed on import, and only when
+   and where imported. */
+
+const WEAVE_PREF_PREFIX = "services.sync.prefs.sync.";
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+/**
+ * Preference class constructor
+ *
+ * Initializes instance properties.
+ */
+export function Preference(props) {
+  Logger.AssertTrue(
+    "name" in props && "value" in props,
+    "Preference must have both name and value"
+  );
+
+  this.name = props.name;
+  this.value = props.value;
+}
+
+/**
+ * Preference instance methods
+ */
+Preference.prototype = {
+  /**
+   * Modify
+   *
+   * Sets the value of the preference this.name to this.value.
+   * Throws on error.
+   *
+   * @return nothing
+   */
+  Modify() {
+    // Determine if this pref is actually something Weave even looks at.
+    let weavepref = WEAVE_PREF_PREFIX + this.name;
+    try {
+      let syncPref = Services.prefs.getBoolPref(weavepref);
+      if (!syncPref) {
+        Services.prefs.setBoolPref(weavepref, true);
+      }
+    } catch (e) {
+      Logger.AssertTrue(false, "Weave doesn't sync pref " + this.name);
+    }
+
+    // Modify the pref; throw an exception if the pref type is different
+    // than the value type specified in the test.
+    let prefType = Services.prefs.getPrefType(this.name);
+    switch (prefType) {
+      case Ci.nsIPrefBranch.PREF_INT:
+        Logger.AssertEqual(
+          typeof this.value,
+          "number",
+          "Wrong type used for preference value"
+        );
+        Services.prefs.setIntPref(this.name, this.value);
+        break;
+      case Ci.nsIPrefBranch.PREF_STRING:
+        Logger.AssertEqual(
+          typeof this.value,
+          "string",
+          "Wrong type used for preference value"
+        );
+        Services.prefs.setStringPref(this.name, this.value);
+        break;
+      case Ci.nsIPrefBranch.PREF_BOOL:
+        Logger.AssertEqual(
+          typeof this.value,
+          "boolean",
+          "Wrong type used for preference value"
+        );
+        Services.prefs.setBoolPref(this.name, this.value);
+        break;
+    }
+  },
+
+  /**
+   * Find
+   *
+   * Verifies that the preference this.name has the value
+   * this.value. Throws on error, or if the pref's type or value
+   * doesn't match.
+   *
+   * @return nothing
+   */
+  Find() {
+    // Read the pref value.
+    let value;
+    try {
+      let prefType = Services.prefs.getPrefType(this.name);
+      switch (prefType) {
+        case Ci.nsIPrefBranch.PREF_INT:
+          value = Services.prefs.getIntPref(this.name);
+          break;
+        case Ci.nsIPrefBranch.PREF_STRING:
+          value = Services.prefs.getStringPref(this.name);
+          break;
+        case Ci.nsIPrefBranch.PREF_BOOL:
+          value = Services.prefs.getBoolPref(this.name);
+          break;
+      }
+    } catch (e) {
+      Logger.AssertTrue(false, "Error accessing pref " + this.name);
+    }
+
+    // Throw an exception if the current and expected values aren't of
+    // the same type, or don't have the same values.
+    Logger.AssertEqual(
+      typeof value,
+      typeof this.value,
+      "Value types don't match"
+    );
+    Logger.AssertEqual(value, this.value, "Preference values don't match");
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/tabs.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/tabs.sys.mjs
new file mode 100644
index 0000000000..8ea8f3b780
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/tabs.sys.mjs
@@ -0,0 +1,92 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+   ChromeUtils.import() and acts as a singleton.
+   Only the following listed symbols will exposed on import, and only when
+   and where imported. */
+
+import { Weave } from "resource://services-sync/main.sys.mjs";
+
+import { Logger } from "resource://tps/logger.sys.mjs";
+
+// Unfortunately, due to where TPS is run, we can't directly reuse the logic from
+// BrowserTestUtils.sys.mjs. Moreover, we can't resolve the URI it loads the content
+// frame script from ("chrome://mochikit/content/tests/BrowserTestUtils/content-utils.js"),
+// hence the hackiness here and in BrowserTabs.Add.
+Services.mm.loadFrameScript(
+  "data:application/javascript;charset=utf-8," +
+    encodeURIComponent(`
+  addEventListener("load", function(event) {
+    let subframe = event.target != content.document;
+    sendAsyncMessage("tps:loadEvent", {subframe: subframe, url: event.target.documentURI});
+  }, true)`),
+  true,
+  true
+);
+
+export var BrowserTabs = {
+  /**
+   * Add
+   *
+   * Opens a new tab in the current browser window for the
+   * given uri. Rejects on error.
+   *
+   * @param uri The uri to load in the new tab
+   * @return Promise
+   */
+  async Add(uri) {
+    let mainWindow = Services.wm.getMostRecentWindow("navigator:browser");
+    let browser = mainWindow.gBrowser;
+    let newtab = browser.addTrustedTab(uri);
+
+    // Wait for the tab to load.
+    await new Promise(resolve => {
+      let mm = browser.ownerGlobal.messageManager;
+      mm.addMessageListener("tps:loadEvent", function onLoad(msg) {
+        mm.removeMessageListener("tps:loadEvent", onLoad);
+        resolve();
+      });
+    });
+
+    browser.selectedTab = newtab;
+  },
+
+  /**
+   * Find
+   *
+   * Finds the specified uri and title in Weave's list of remote tabs
+   * for the specified profile.
+   *
+   * @param uri The uri of the tab to find
+   * @param title The page title of the tab to find
+   * @param profile The profile to search for tabs
+   * @return true if the specified tab could be found, otherwise false
+   */
+  async Find(uri, title, profile) {
+    // Find the uri in Weave's list of tabs for the given profile.
+    let tabEngine = Weave.Service.engineManager.get("tabs");
+    for (let client of Weave.Service.clientsEngine.remoteClients) {
+      let tabClients = await tabEngine.getAllClients();
+      let tabClient = tabClients.find(x => x.id === client.id);
+      if (!tabClient || !tabClient.tabs) {
+        continue;
+      }
+      for (let key in tabClient.tabs) {
+        let tab = tabClient.tabs[key];
+        let weaveTabUrl = tab.urlHistory[0];
+        if (uri == weaveTabUrl && profile == client.name) {
+          if (title == undefined || title == tab.title) {
+            return true;
+          }
+        }
+      }
+      Logger.logInfo(
+        `Dumping tabs for ${client.name}...\n` +
+          JSON.stringify(tabClient.tabs, null, 2)
+      );
+    }
+    return false;
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/modules/windows.sys.mjs b/services/sync/tps/extensions/tps/resource/modules/windows.sys.mjs
new file mode 100644
index 0000000000..b0798b9031
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/modules/windows.sys.mjs
@@ -0,0 +1,32 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+   ChromeUtils.import() and acts as a singleton.
+   Only the following listed symbols will exposed on import, and only when
+   and where imported. */
+
+export var BrowserWindows = {
+  /**
+   * Add
+   *
+   * Opens a new window. Throws on error.
+   *
+   * @param aPrivate The private option.
+   * @return nothing
+   */
+  Add(aPrivate, fn) {
+    return new Promise(resolve => {
+      let mainWindow = Services.wm.getMostRecentWindow("navigator:browser");
+      let win = mainWindow.OpenBrowserWindow({ private: aPrivate });
+      win.addEventListener(
+        "load",
+        function () {
+          resolve(win);
+        },
+        { once: true }
+      );
+    });
+  },
+};
diff --git a/services/sync/tps/extensions/tps/resource/quit.sys.mjs b/services/sync/tps/extensions/tps/resource/quit.sys.mjs
new file mode 100644
index 0000000000..e2cb8d8c22
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/quit.sys.mjs
@@ -0,0 +1,38 @@
+/* -*- indent-tabs-mode: nil -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+  From mozilla/toolkit/content
+  These files did not have a license
+*/
+function canQuitApplication() {
+  try {
+    var cancelQuit = Cc["@mozilla.org/supports-PRBool;1"].createInstance(
+      Ci.nsISupportsPRBool
+    );
+    Services.obs.notifyObservers(cancelQuit, "quit-application-requested");
+
+    // Something aborted the quit process.
+    if (cancelQuit.data) {
+      return false;
+    }
+  } catch (ex) {}
+
+  return true;
+}
+
+export function goQuitApplication() {
+  if (!canQuitApplication()) {
+    return false;
+  }
+
+  try {
+    Services.startup.quit(Ci.nsIAppStartup.eForceQuit);
+  } catch (ex) {
+    throw new Error(`goQuitApplication: ${ex.message}`);
+  }
+
+  return true;
+}
diff --git a/services/sync/tps/extensions/tps/resource/tps.sys.mjs b/services/sync/tps/extensions/tps/resource/tps.sys.mjs
new file mode 100644
index 0000000000..2c4a5994a6
--- /dev/null
+++ b/services/sync/tps/extensions/tps/resource/tps.sys.mjs
@@ -0,0 +1,1583 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* This is a JavaScript module (JSM) to be imported via
+ * ChromeUtils.import() and acts as a singleton. Only the following
+ * listed symbols will exposed on import, and only when and where imported.
+ */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Addon: "resource://tps/modules/addons.sys.mjs",
+  AddonValidator: "resource://services-sync/engines/addons.sys.mjs",
+  Address: "resource://tps/modules/formautofill.sys.mjs",
+  Async: "resource://services-common/async.sys.mjs",
+  Authentication: "resource://tps/auth/fxaccounts.sys.mjs",
+  Bookmark: "resource://tps/modules/bookmarks.sys.mjs",
+  BookmarkFolder: "resource://tps/modules/bookmarks.sys.mjs",
+  BookmarkValidator: "resource://tps/modules/bookmarkValidator.sys.mjs",
+  BrowserTabs: "resource://tps/modules/tabs.sys.mjs",
+  BrowserWindows: "resource://tps/modules/windows.sys.mjs",
+  CommonUtils: "resource://services-common/utils.sys.mjs",
+  CreditCard: "resource://tps/modules/formautofill.sys.mjs",
+  DumpAddresses: "resource://tps/modules/formautofill.sys.mjs",
+  DumpBookmarks: "resource://tps/modules/bookmarks.sys.mjs",
+  DumpCreditCards: "resource://tps/modules/formautofill.sys.mjs",
+  DumpHistory: "resource://tps/modules/history.sys.mjs",
+  DumpPasswords: "resource://tps/modules/passwords.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+  FormData: "resource://tps/modules/forms.sys.mjs",
+  FormValidator: "resource://services-sync/engines/forms.sys.mjs",
+  HistoryEntry: "resource://tps/modules/history.sys.mjs",
+  JsonSchema: "resource://gre/modules/JsonSchema.sys.mjs",
+  Livemark: "resource://tps/modules/bookmarks.sys.mjs",
+  Log: "resource://gre/modules/Log.sys.mjs",
+  Logger: "resource://tps/logger.sys.mjs",
+  NetUtil: "resource://gre/modules/NetUtil.sys.mjs",
+  Password: "resource://tps/modules/passwords.sys.mjs",
+  PasswordValidator: "resource://services-sync/engines/passwords.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+  Preference: "resource://tps/modules/prefs.sys.mjs",
+  STATUS_OK: "resource://services-sync/constants.sys.mjs",
+  Separator: "resource://tps/modules/bookmarks.sys.mjs",
+  SessionStore: "resource:///modules/sessionstore/SessionStore.sys.mjs",
+  Svc: "resource://services-sync/util.sys.mjs",
+  SyncTelemetry: "resource://services-sync/telemetry.sys.mjs",
+  WEAVE_VERSION: "resource://services-sync/constants.sys.mjs",
+  Weave: "resource://services-sync/main.sys.mjs",
+  extensionStorageSync: "resource://gre/modules/ExtensionStorageSync.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "fileProtocolHandler", () => {
+  let fileHandler = Services.io.getProtocolHandler("file");
+  return fileHandler.QueryInterface(Ci.nsIFileProtocolHandler);
+});
+
+ChromeUtils.defineLazyGetter(lazy, "gTextDecoder", () => {
+  return new TextDecoder();
+});
+
+// Options for wiping data during a sync
+const SYNC_RESET_CLIENT = "resetClient";
+const SYNC_WIPE_CLIENT = "wipeClient";
+const SYNC_WIPE_REMOTE = "wipeRemote";
+
+// Actions a test can perform
+const ACTION_ADD = "add";
+const ACTION_DELETE = "delete";
+const ACTION_MODIFY = "modify";
+const ACTION_SET_ENABLED = "set-enabled";
+const ACTION_SYNC = "sync";
+const ACTION_SYNC_RESET_CLIENT = SYNC_RESET_CLIENT;
+const ACTION_SYNC_WIPE_CLIENT = SYNC_WIPE_CLIENT;
+const ACTION_SYNC_WIPE_REMOTE = SYNC_WIPE_REMOTE;
+const ACTION_VERIFY = "verify";
+const ACTION_VERIFY_NOT = "verify-not";
+
+const OBSERVER_TOPICS = [
+  "fxaccounts:onlogin",
+  "fxaccounts:onlogout",
+  "profile-before-change",
+  "weave:service:tracking-started",
+  "weave:service:tracking-stopped",
+  "weave:service:login:error",
+  "weave:service:setup-complete",
+  "weave:service:sync:finish",
+  "weave:service:sync:delayed",
+  "weave:service:sync:error",
+  "weave:service:sync:start",
+  "weave:service:resyncs-finished",
+  "places-browser-init-complete",
+];
+
+export var TPS = {
+  _currentAction: -1,
+  _currentPhase: -1,
+  _enabledEngines: null,
+  _errors: 0,
+  _isTracking: false,
+  _phaseFinished: false,
+  _phaselist: {},
+  _setupComplete: false,
+  _syncActive: false,
+  _syncCount: 0,
+  _syncsReportedViaTelemetry: 0,
+  _syncErrors: 0,
+  _syncWipeAction: null,
+  _tabsAdded: 0,
+  _tabsFinished: 0,
+  _test: null,
+  _triggeredSync: false,
+  _msSinceEpoch: 0,
+  _requestedQuit: false,
+  shouldValidateAddons: false,
+  shouldValidateBookmarks: false,
+  shouldValidatePasswords: false,
+  shouldValidateForms: false,
+  _placesInitDeferred: Promise.withResolvers(),
+  ACTIONS: [
+    ACTION_ADD,
+    ACTION_DELETE,
+    ACTION_MODIFY,
+    ACTION_SET_ENABLED,
+    ACTION_SYNC,
+    ACTION_SYNC_RESET_CLIENT,
+    ACTION_SYNC_WIPE_CLIENT,
+    ACTION_SYNC_WIPE_REMOTE,
+    ACTION_VERIFY,
+    ACTION_VERIFY_NOT,
+  ],
+
+  _init: function TPS__init() {
+    this.delayAutoSync();
+
+    OBSERVER_TOPICS.forEach(function (aTopic) {
+      Services.obs.addObserver(this, aTopic, true);
+    }, this);
+
+    // Some engines bump their score during their sync, which then causes
+    // another sync immediately (notably, prefs and addons). We don't want
+    // this to happen, and there's no obvious preference to kill it - so
+    // we do this nasty hack to ensure the global score is always zero.
+    Services.prefs.addObserver("services.sync.globalScore", () => {
+      if (lazy.Weave.Service.scheduler.globalScore != 0) {
+        lazy.Weave.Service.scheduler.globalScore = 0;
+      }
+    });
+  },
+
+  DumpError(msg, exc = null) {
+    this._errors++;
+    let errInfo;
+    if (exc) {
+      errInfo = lazy.Log.exceptionStr(exc); // includes details and stack-trace.
+    } else {
+      // always write a stack even if no error passed.
+      errInfo = lazy.Log.stackTrace(new Error());
+    }
+    lazy.Logger.logError(`[phase ${this._currentPhase}] ${msg} - ${errInfo}`);
+    this.quit();
+  },
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIObserver",
+    "nsISupportsWeakReference",
+  ]),
+
+  observe: function TPS__observe(subject, topic, data) {
+    try {
+      lazy.Logger.logInfo("----------event observed: " + topic);
+
+      switch (topic) {
+        case "profile-before-change":
+          OBSERVER_TOPICS.forEach(function (topic) {
+            Services.obs.removeObserver(this, topic);
+          }, this);
+
+          lazy.Logger.close();
+
+          break;
+
+        case "places-browser-init-complete":
+          this._placesInitDeferred.resolve();
+          break;
+
+        case "weave:service:setup-complete":
+          this._setupComplete = true;
+
+          if (this._syncWipeAction) {
+            lazy.Weave.Svc.PrefBranch.setStringPref(
+              "firstSync",
+              this._syncWipeAction
+            );
+            this._syncWipeAction = null;
+          }
+
+          break;
+
+        case "weave:service:sync:error":
+          this._syncActive = false;
+
+          this.delayAutoSync();
+
+          // If this is the first sync error, retry...
+          if (this._syncErrors === 0) {
+            lazy.Logger.logInfo("Sync error; retrying...");
+            this._syncErrors++;
+            lazy.CommonUtils.nextTick(() => {
+              this.RunNextTestAction().catch(err => {
+                this.DumpError("RunNextTestActionFailed", err);
+              });
+            });
+          } else {
+            this._triggeredSync = false;
+            this.DumpError("Sync error; aborting test");
+          }
+
+          break;
+
+        case "weave:service:resyncs-finished":
+          this._syncActive = false;
+          this._syncErrors = 0;
+          this._triggeredSync = false;
+
+          this.delayAutoSync();
+          break;
+
+        case "weave:service:sync:start":
+          // Ensure that the sync operation has been started by TPS
+          if (!this._triggeredSync) {
+            this.DumpError(
+              "Automatic sync got triggered, which is not allowed."
+            );
+          }
+
+          this._syncActive = true;
+          break;
+
+        case "weave:service:tracking-started":
+          this._isTracking = true;
+          break;
+
+        case "weave:service:tracking-stopped":
+          this._isTracking = false;
+          break;
+
+        case "fxaccounts:onlogin":
+          // A user signed in - for TPS that always means sync - so configure
+          // that.
+          lazy.Weave.Service.configure().catch(e => {
+            this.DumpError("Configuring sync failed.", e);
+          });
+          break;
+
+        default:
+          lazy.Logger.logInfo(`unhandled event: ${topic}`);
+      }
+    } catch (e) {
+      this.DumpError("Observer failed", e);
+    }
+  },
+
+  /**
+   * Given that we cannot completely disable the automatic sync operations, we
+   * massively delay the next sync. Sync operations have to only happen when
+   * directly called via TPS.Sync()!
+   */
+  delayAutoSync: function TPS_delayAutoSync() {
+    lazy.Weave.Svc.PrefBranch.setIntPref("scheduler.immediateInterval", 7200);
+    lazy.Weave.Svc.PrefBranch.setIntPref("scheduler.idleInterval", 7200);
+    lazy.Weave.Svc.PrefBranch.setIntPref("scheduler.activeInterval", 7200);
+    lazy.Weave.Svc.PrefBranch.setIntPref("syncThreshold", 10000000);
+  },
+
+  quit: function TPS__quit() {
+    lazy.Logger.logInfo("quitting");
+    this._requestedQuit = true;
+    this.goQuitApplication();
+  },
+
+  async HandleWindows(aWindow, action) {
+    lazy.Logger.logInfo(
+      "executing action " +
+        action.toUpperCase() +
+        " on window " +
+        JSON.stringify(aWindow)
+    );
+    switch (action) {
+      case ACTION_ADD:
+        await lazy.BrowserWindows.Add(aWindow.private);
+        break;
+    }
+    lazy.Logger.logPass(
+      "executing action " + action.toUpperCase() + " on windows"
+    );
+  },
+
+  async HandleTabs(tabs, action) {
+    for (let tab of tabs) {
+      lazy.Logger.logInfo(
+        "executing action " +
+          action.toUpperCase() +
+          " on tab " +
+          JSON.stringify(tab)
+      );
+      switch (action) {
+        case ACTION_ADD:
+          await lazy.BrowserTabs.Add(tab.uri);
+          break;
+        case ACTION_VERIFY:
+          lazy.Logger.AssertTrue(
+            typeof tab.profile != "undefined",
+            "profile must be defined when verifying tabs"
+          );
+          lazy.Logger.AssertTrue(
+            await lazy.BrowserTabs.Find(tab.uri, tab.title, tab.profile),
+            "error locating tab"
+          );
+          break;
+        case ACTION_VERIFY_NOT:
+          lazy.Logger.AssertTrue(
+            typeof tab.profile != "undefined",
+            "profile must be defined when verifying tabs"
+          );
+          lazy.Logger.AssertTrue(
+            await !lazy.BrowserTabs.Find(tab.uri, tab.title, tab.profile),
+            "tab found which was expected to be absent"
+          );
+          break;
+        default:
+          lazy.Logger.AssertTrue(false, "invalid action: " + action);
+      }
+    }
+    lazy.Logger.logPass(
+      "executing action " + action.toUpperCase() + " on tabs"
+    );
+  },
+
+  async HandlePrefs(prefs, action) {
+    for (let pref of prefs) {
+      lazy.Logger.logInfo(
+        "executing action " +
+          action.toUpperCase() +
+          " on pref " +
+          JSON.stringify(pref)
+      );
+      let preference = new lazy.Preference(pref);
+      switch (action) {
+        case ACTION_MODIFY:
+          preference.Modify();
+          break;
+        case ACTION_VERIFY:
+          preference.Find();
+          break;
+        default:
+          lazy.Logger.AssertTrue(false, "invalid action: " + action);
+      }
+    }
+    lazy.Logger.logPass(
+      "executing action " + action.toUpperCase() + " on pref"
+    );
+  },
+
+  async HandleForms(data, action) {
+    this.shouldValidateForms = true;
+    for (let datum of data) {
+      lazy.Logger.logInfo(
+        "executing action " +
+          action.toUpperCase() +
+          " on form entry " +
+          JSON.stringify(datum)
+      );
+      let formdata = new lazy.FormData(datum, this._msSinceEpoch);
+      switch (action) {
+        case ACTION_ADD:
+          await formdata.Create();
+          break;
+        case ACTION_DELETE:
+          await formdata.Remove();
+          break;
+        case ACTION_VERIFY:
+          lazy.Logger.AssertTrue(await formdata.Find(), "form data not found");
+          break;
+        case ACTION_VERIFY_NOT:
+          lazy.Logger.AssertTrue(
+            !(await formdata.Find()),
+            "form data found, but it shouldn't be present"
+          );
+          break;
+        default:
+          lazy.Logger.AssertTrue(false, "invalid action: " + action);
+      }
+    }
+    lazy.Logger.logPass(
+      "executing action " + action.toUpperCase() + " on formdata"
+    );
+  },
+
+  async HandleHistory(entries, action) {
+    try {
+      for (let entry of entries) {
+        const entryString = JSON.stringify(entry);
+        lazy.Logger.logInfo(
+          "executing action " +
+            action.toUpperCase() +
+            " on history entry " +
+            entryString
+        );
+        switch (action) {
+          case ACTION_ADD:
+            await lazy.HistoryEntry.Add(entry, this._msSinceEpoch);
+            break;
+          case ACTION_DELETE:
+            await lazy.HistoryEntry.Delete(entry, this._msSinceEpoch);
+            break;
+          case ACTION_VERIFY:
+            lazy.Logger.AssertTrue(
+              await lazy.HistoryEntry.Find(entry, this._msSinceEpoch),
+              "Uri visits not found in history database: " + entryString
+            );
+            break;
+          case ACTION_VERIFY_NOT:
+            lazy.Logger.AssertTrue(
+              !(await lazy.HistoryEntry.Find(entry, this._msSinceEpoch)),
+              "Uri visits found in history database, but they shouldn't be: " +
+                entryString
+            );
+            break;
+          default:
+            lazy.Logger.AssertTrue(false, "invalid action: " + action);
+        }
+      }
+      lazy.Logger.logPass(
+        "executing action " + action.toUpperCase() + " on history"
+      );
+    } catch (e) {
+      await lazy.DumpHistory();
+      throw e;
+    }
+  },
+
+  async HandlePasswords(passwords, action) {
+    this.shouldValidatePasswords = true;
+    try {
+      for (let password of passwords) {
+        lazy.Logger.logInfo(
+          "executing action " +
+            action.toUpperCase() +
+            " on password " +
+            JSON.stringify(password)
+        );
+        let passwordOb = new lazy.Password(password);
+        switch (action) {
+          case ACTION_ADD:
+            lazy.Logger.AssertTrue(
+              (await passwordOb.Create()) > -1,
+              "error adding password"
+            );
+            break;
+          case ACTION_VERIFY:
+            lazy.Logger.AssertTrue(
+              (await passwordOb.Find()) != -1,
+              "password not found"
+            );
+            break;
+          case ACTION_VERIFY_NOT:
+            lazy.Logger.AssertTrue(
+              (await passwordOb.Find()) == -1,
+              "password found, but it shouldn't exist"
+            );
+            break;
+          case ACTION_DELETE:
+            lazy.Logger.AssertTrue(
+              (await passwordOb.Find()) != -1,
+              "password not found"
+            );
+            passwordOb.Remove();
+            break;
+          case ACTION_MODIFY:
+            if (passwordOb.updateProps != null) {
+              lazy.Logger.AssertTrue(
+                (await passwordOb.Find()) != -1,
+                "password not found"
+              );
+              passwordOb.Update();
+            }
+            break;
+          default:
+            lazy.Logger.AssertTrue(false, "invalid action: " + action);
+        }
+      }
+      lazy.Logger.logPass(
+        "executing action " + action.toUpperCase() + " on passwords"
+      );
+    } catch (e) {
+      await lazy.DumpPasswords();
+      throw e;
+    }
+  },
+
+  async HandleAddons(addons, action, state) {
+    this.shouldValidateAddons = true;
+    for (let entry of addons) {
+      lazy.Logger.logInfo(
+        "executing action " +
+          action.toUpperCase() +
+          " on addon " +
+          JSON.stringify(entry)
+      );
+      let addon = new lazy.Addon(this, entry);
+      switch (action) {
+        case ACTION_ADD:
+          await addon.install();
+          break;
+        case ACTION_DELETE:
+          await addon.uninstall();
+          break;
+        case ACTION_VERIFY:
+          lazy.Logger.AssertTrue(
+            await addon.find(state),
+            "addon " + addon.id + " not found"
+          );
+          break;
+        case ACTION_VERIFY_NOT:
+          lazy.Logger.AssertFalse(
+            await addon.find(state),
+            "addon " + addon.id + " is present, but it shouldn't be"
+          );
+          break;
+        case ACTION_SET_ENABLED:
+          lazy.Logger.AssertTrue(
+            await addon.setEnabled(state),
+            "addon " + addon.id + " not found"
+          );
+          break;
+        default:
+          throw new Error("Unknown action for add-on: " + action);
+      }
+    }
+    lazy.Logger.logPass(
+      "executing action " + action.toUpperCase() + " on addons"
+    );
+  },
+
+  async HandleBookmarks(bookmarks, action) {
+    // wait for default bookmarks to be created.
+    await this._placesInitDeferred.promise;
+    this.shouldValidateBookmarks = true;
+    try {
+      let items = [];
+      for (let folder in bookmarks) {
+        let last_item_pos = -1;
+        for (let bookmark of bookmarks[folder]) {
+          lazy.Logger.clearPotentialError();
+          let placesItem;
+          bookmark.location = folder;
+
+          if (last_item_pos != -1) {
+            bookmark.last_item_pos = last_item_pos;
+          }
+          let itemGuid = null;
+
+          if (action != ACTION_MODIFY && action != ACTION_DELETE) {
+            lazy.Logger.logInfo(
+              "executing action " +
+                action.toUpperCase() +
+                " on bookmark " +
+                JSON.stringify(bookmark)
+            );
+          }
+
+          if ("uri" in bookmark) {
+            placesItem = new lazy.Bookmark(bookmark);
+          } else if ("folder" in bookmark) {
+            placesItem = new lazy.BookmarkFolder(bookmark);
+          } else if ("livemark" in bookmark) {
+            placesItem = new lazy.Livemark(bookmark);
+          } else if ("separator" in bookmark) {
+            placesItem = new lazy.Separator(bookmark);
+          }
+
+          if (action == ACTION_ADD) {
+            itemGuid = await placesItem.Create();
+          } else {
+            itemGuid = await placesItem.Find();
+            if (action == ACTION_VERIFY_NOT) {
+              lazy.Logger.AssertTrue(
+                itemGuid == null,
+                "places item exists but it shouldn't: " +
+                  JSON.stringify(bookmark)
+              );
+            } else {
+              lazy.Logger.AssertTrue(itemGuid, "places item not found", true);
+            }
+          }
+
+          last_item_pos = await placesItem.GetItemIndex();
+          items.push(placesItem);
+        }
+      }
+
+      if (action == ACTION_DELETE || action == ACTION_MODIFY) {
+        for (let item of items) {
+          lazy.Logger.logInfo(
+            "executing action " +
+              action.toUpperCase() +
+              " on bookmark " +
+              JSON.stringify(item)
+          );
+          switch (action) {
+            case ACTION_DELETE:
+              await item.Remove();
+              break;
+            case ACTION_MODIFY:
+              if (item.updateProps != null) {
+                await item.Update();
+              }
+              break;
+          }
+        }
+      }
+
+      lazy.Logger.logPass(
+        "executing action " + action.toUpperCase() + " on bookmarks"
+      );
+    } catch (e) {
+      await lazy.DumpBookmarks();
+      throw e;
+    }
+  },
+
+  async HandleAddresses(addresses, action) {
+    try {
+      for (let address of addresses) {
+        lazy.Logger.logInfo(
+          "executing action " +
+            action.toUpperCase() +
+            " on address " +
+            JSON.stringify(address)
+        );
+        let addressOb = new lazy.Address(address);
+        switch (action) {
+          case ACTION_ADD:
+            await addressOb.Create();
+            break;
+          case ACTION_MODIFY:
+            await addressOb.Update();
+            break;
+          case ACTION_VERIFY:
+            lazy.Logger.AssertTrue(await addressOb.Find(), "address not found");
+            break;
+          case ACTION_VERIFY_NOT:
+            lazy.Logger.AssertTrue(
+              !(await addressOb.Find()),
+              "address found, but it shouldn't exist"
+            );
+            break;
+          case ACTION_DELETE:
+            lazy.Logger.AssertTrue(await addressOb.Find(), "address not found");
+            await addressOb.Remove();
+            break;
+          default:
+            lazy.Logger.AssertTrue(false, "invalid action: " + action);
+        }
+      }
+      lazy.Logger.logPass(
+        "executing action " + action.toUpperCase() + " on addresses"
+      );
+    } catch (e) {
+      await lazy.DumpAddresses();
+      throw e;
+    }
+  },
+
+  async HandleCreditCards(creditCards, action) {
+    try {
+      for (let creditCard of creditCards) {
+        lazy.Logger.logInfo(
+          "executing action " +
+            action.toUpperCase() +
+            " on creditCard " +
+            JSON.stringify(creditCard)
+        );
+        let creditCardOb = new lazy.CreditCard(creditCard);
+        switch (action) {
+          case ACTION_ADD:
+            await creditCardOb.Create();
+            break;
+          case ACTION_MODIFY:
+            await creditCardOb.Update();
+            break;
+          case ACTION_VERIFY:
+            lazy.Logger.AssertTrue(
+              await creditCardOb.Find(),
+              "creditCard not found"
+            );
+            break;
+          case ACTION_VERIFY_NOT:
+            lazy.Logger.AssertTrue(
+              !(await creditCardOb.Find()),
+              "creditCard found, but it shouldn't exist"
+            );
+            break;
+          case ACTION_DELETE:
+            lazy.Logger.AssertTrue(
+              await creditCardOb.Find(),
+              "creditCard not found"
+            );
+            await creditCardOb.Remove();
+            break;
+          default:
+            lazy.Logger.AssertTrue(false, "invalid action: " + action);
+        }
+      }
+      lazy.Logger.logPass(
+        "executing action " + action.toUpperCase() + " on creditCards"
+      );
+    } catch (e) {
+      await lazy.DumpCreditCards();
+      throw e;
+    }
+  },
+
+  async Cleanup() {
+    try {
+      await this.WipeServer();
+    } catch (ex) {
+      lazy.Logger.logError(
+        "Failed to wipe server: " + lazy.Log.exceptionStr(ex)
+      );
+    }
+    try {
+      if (await lazy.Authentication.isLoggedIn()) {
+        // signout and wait for Sync to completely reset itself.
+        lazy.Logger.logInfo("signing out");
+        let waiter = this.promiseObserver("weave:service:start-over:finish");
+        await lazy.Authentication.signOut();
+        await waiter;
+        lazy.Logger.logInfo("signout complete");
+      }
+      await lazy.Authentication.deleteEmail(this.config.fx_account.username);
+    } catch (e) {
+      lazy.Logger.logError("Failed to sign out: " + lazy.Log.exceptionStr(e));
+    }
+  },
+
+  /**
+   * Use Sync's bookmark validation code to see if we've corrupted the tree.
+   */
+  async ValidateBookmarks() {
+    let getServerBookmarkState = async () => {
+      let bookmarkEngine = lazy.Weave.Service.engineManager.get("bookmarks");
+      let collection = bookmarkEngine.itemSource();
+      let collectionKey =
+        bookmarkEngine.service.collectionKeys.keyForCollection(
+          bookmarkEngine.name
+        );
+      collection.full = true;
+      let items = [];
+      let resp = await collection.get();
+      for (let json of resp.obj) {
+        let record = new collection._recordObj();
+        record.deserialize(json);
+        await record.decrypt(collectionKey);
+        items.push(record.cleartext);
+      }
+      return items;
+    };
+    let serverRecordDumpStr;
+    try {
+      lazy.Logger.logInfo("About to perform bookmark validation");
+      let clientTree = await lazy.PlacesUtils.promiseBookmarksTree("", {
+        includeItemIds: true,
+      });
+      let serverRecords = await getServerBookmarkState();
+      // We can't wait until catch to stringify this, since at that point it will have cycles.
+      serverRecordDumpStr = JSON.stringify(serverRecords);
+
+      let validator = new lazy.BookmarkValidator();
+      let { problemData } = await validator.compareServerWithClient(
+        serverRecords,
+        clientTree
+      );
+
+      for (let { name, count } of problemData.getSummary()) {
+        // Exclude mobile showing up on the server hackily so that we don't
+        // report it every time, see bug 1273234 and 1274394 for more information.
+        if (
+          name === "serverUnexpected" &&
+          problemData.serverUnexpected.includes("mobile")
+        ) {
+          --count;
+        }
+        if (count) {
+          // Log this out before we assert. This is useful in the context of TPS logs, since we
+          // can see the IDs in the test files.
+          lazy.Logger.logInfo(
+            `Validation problem: "${name}": ${JSON.stringify(
+              problemData[name]
+            )}`
+          );
+        }
+        lazy.Logger.AssertEqual(
+          count,
+          0,
+          `Bookmark validation error of type ${name}`
+        );
+      }
+    } catch (e) {
+      // Dump the client records (should always be doable)
+      lazy.DumpBookmarks();
+      // Dump the server records if gotten them already.
+      if (serverRecordDumpStr) {
+        lazy.Logger.logInfo(
+          "Server bookmark records:\n" + serverRecordDumpStr + "\n"
+        );
+      }
+      this.DumpError("Bookmark validation failed", e);
+    }
+    lazy.Logger.logInfo("Bookmark validation finished");
+  },
+
+  async ValidateCollection(engineName, ValidatorType) {
+    let serverRecordDumpStr;
+    let clientRecordDumpStr;
+    try {
+      lazy.Logger.logInfo(`About to perform validation for "${engineName}"`);
+      let engine = lazy.Weave.Service.engineManager.get(engineName);
+      let validator = new ValidatorType(engine);
+      let serverRecords = await validator.getServerItems(engine);
+      let clientRecords = await validator.getClientItems();
+      try {
+        // This substantially improves the logs for addons while not making a
+        // substantial difference for the other two
+        clientRecordDumpStr = JSON.stringify(
+          clientRecords.map(r => {
+            let res = validator.normalizeClientItem(r);
+            delete res.original; // Try and prevent cyclic references
+            return res;
+          })
+        );
+      } catch (e) {
+        // ignore the error, the dump string is just here to make debugging easier.
+        clientRecordDumpStr = "<Cyclic value>";
+      }
+      try {
+        serverRecordDumpStr = JSON.stringify(serverRecords);
+      } catch (e) {
+        // as above
+        serverRecordDumpStr = "<Cyclic value>";
+      }
+      let { problemData } = await validator.compareClientWithServer(
+        clientRecords,
+        serverRecords
+      );
+      for (let { name, count } of problemData.getSummary()) {
+        if (count) {
+          lazy.Logger.logInfo(
+            `Validation problem: "${name}": ${JSON.stringify(
+              problemData[name]
+            )}`
+          );
+        }
+        lazy.Logger.AssertEqual(
+          count,
+          0,
+          `Validation error for "${engineName}" of type "${name}"`
+        );
+      }
+    } catch (e) {
+      // Dump the client records if possible
+      if (clientRecordDumpStr) {
+        lazy.Logger.logInfo(
+          `Client state for ${engineName}:\n${clientRecordDumpStr}\n`
+        );
+      }
+      // Dump the server records if gotten them already.
+      if (serverRecordDumpStr) {
+        lazy.Logger.logInfo(
+          `Server state for ${engineName}:\n${serverRecordDumpStr}\n`
+        );
+      }
+      this.DumpError(`Validation failed for ${engineName}`, e);
+    }
+    lazy.Logger.logInfo(`Validation finished for ${engineName}`);
+  },
+
+  ValidatePasswords() {
+    return this.ValidateCollection("passwords", lazy.PasswordValidator);
+  },
+
+  ValidateForms() {
+    return this.ValidateCollection("forms", lazy.FormValidator);
+  },
+
+  ValidateAddons() {
+    return this.ValidateCollection("addons", lazy.AddonValidator);
+  },
+
+  async RunNextTestAction() {
+    lazy.Logger.logInfo("Running next test action");
+    try {
+      if (this._currentAction >= this._phaselist[this._currentPhase].length) {
+        // Run necessary validations and then finish up
+        lazy.Logger.logInfo("No more actions - running validations...");
+        if (this.shouldValidateBookmarks) {
+          await this.ValidateBookmarks();
+        }
+        if (this.shouldValidatePasswords) {
+          await this.ValidatePasswords();
+        }
+        if (this.shouldValidateForms) {
+          await this.ValidateForms();
+        }
+        if (this.shouldValidateAddons) {
+          await this.ValidateAddons();
+        }
+        // Force this early so that we run the validation and detect missing pings
+        // *before* we start shutting down, since if we do it after, the python
+        // code won't notice the failure.
+        lazy.SyncTelemetry.shutdown();
+        // we're all done
+        lazy.Logger.logInfo(
+          "test phase " +
+            this._currentPhase +
+            ": " +
+            (this._errors ? "FAIL" : "PASS")
+        );
+        this._phaseFinished = true;
+        this.quit();
+        return;
+      }
+      this.seconds_since_epoch = Services.prefs.getIntPref(
+        "tps.seconds_since_epoch"
+      );
+      if (this.seconds_since_epoch) {
+        // Places dislikes it if we add visits in the future. We pretend the
+        // real time is 1 minute ago to avoid issues caused by places using a
+        // different clock than the one that set the seconds_since_epoch pref.
+        this._msSinceEpoch = (this.seconds_since_epoch - 60) * 1000;
+      } else {
+        this.DumpError("seconds-since-epoch not set");
+        return;
+      }
+
+      let phase = this._phaselist[this._currentPhase];
+      let action = phase[this._currentAction];
+      lazy.Logger.logInfo("starting action: " + action[0].name);
+      await action[0].apply(this, action.slice(1));
+
+      this._currentAction++;
+    } catch (e) {
+      if (lazy.Async.isShutdownException(e)) {
+        if (this._requestedQuit) {
+          lazy.Logger.logInfo("Sync aborted due to requested shutdown");
+        } else {
+          this.DumpError(
+            "Sync aborted due to shutdown, but we didn't request it"
+          );
+        }
+      } else {
+        this.DumpError("RunNextTestAction failed", e);
+      }
+      return;
+    }
+    await this.RunNextTestAction();
+  },
+
+  _getFileRelativeToSourceRoot(testFileURL, relativePath) {
+    let file = lazy.fileProtocolHandler.getFileFromURLSpec(testFileURL);
+    let root = file.parent.parent.parent.parent.parent; // <root>/services/sync/tests/tps/test_foo.js // <root>/services/sync/tests/tps // <root>/services/sync/tests // <root>/services/sync // <root>/services // <root>
+    root.appendRelativePath(relativePath);
+    root.normalize();
+    return root;
+  },
+
+  _pingValidator: null,
+
+  // Default ping validator that always says the ping passes. This should be
+  // overridden unless the `testing.tps.skipPingValidation` pref is true.
+  get pingValidator() {
+    return this._pingValidator
+      ? this._pingValidator
+      : {
+          validate() {
+            lazy.Logger.logInfo(
+              "Not validating ping -- disabled by pref or failure to load schema"
+            );
+            return { valid: true, errors: [] };
+          },
+        };
+  },
+
+  // Attempt to load the sync_ping_schema.json and initialize `this.pingValidator`
+  // based on the source of the tps file. Assumes that it's at "../unit/sync_ping_schema.json"
+  // relative to the directory the tps test file (testFile) is contained in.
+  _tryLoadPingSchema(testFile) {
+    if (Services.prefs.getBoolPref("testing.tps.skipPingValidation", false)) {
+      return;
+    }
+    try {
+      let schemaFile = this._getFileRelativeToSourceRoot(
+        testFile,
+        "services/sync/tests/unit/sync_ping_schema.json"
+      );
+
+      let stream = Cc[
+        "@mozilla.org/network/file-input-stream;1"
+      ].createInstance(Ci.nsIFileInputStream);
+
+      stream.init(
+        schemaFile,
+        lazy.FileUtils.MODE_RDONLY,
+        lazy.FileUtils.PERMS_FILE,
+        0
+      );
+
+      let bytes = lazy.NetUtil.readInputStream(stream, stream.available());
+      let schema = JSON.parse(lazy.gTextDecoder.decode(bytes));
+      lazy.Logger.logInfo("Successfully loaded schema");
+
+      this._pingValidator = new lazy.JsonSchema.Validator(schema);
+    } catch (e) {
+      this.DumpError(
+        `Failed to load ping schema relative to "${testFile}".`,
+        e
+      );
+    }
+  },
+
+  /**
+   * Runs a single test phase.
+   *
+   * This is the main entry point for each phase of a test. The TPS command
+   * line driver loads this module and calls into the function with the
+   * arguments from the command line.
+   *
+   * When a phase is executed, the file is loaded as JavaScript into the
+   * current object.
+   *
+   * The following keys in the options argument have meaning:
+   *
+   *   - ignoreUnusedEngines  If true, unused engines will be unloaded from
+   *                          Sync. This makes output easier to parse and is
+   *                          useful for debugging test failures.
+   *
+   * @param  file
+   *         String URI of the file to open.
+   * @param  phase
+   *         String name of the phase to run.
+   * @param  logpath
+   *         String path of the log file to write to.
+   * @param  options
+   *         Object defining addition run-time options.
+   */
+  async RunTestPhase(file, phase, logpath, options) {
+    try {
+      let settings = options || {};
+
+      lazy.Logger.init(logpath);
+      lazy.Logger.logInfo("Sync version: " + lazy.WEAVE_VERSION);
+      lazy.Logger.logInfo("Firefox buildid: " + Services.appinfo.appBuildID);
+      lazy.Logger.logInfo("Firefox version: " + Services.appinfo.version);
+      lazy.Logger.logInfo(
+        "Firefox source revision: " +
+          (AppConstants.SOURCE_REVISION_URL || "unknown")
+      );
+      lazy.Logger.logInfo("Firefox platform: " + AppConstants.platform);
+
+      // do some sync housekeeping
+      if (lazy.Weave.Service.isLoggedIn) {
+        this.DumpError("Sync logged in on startup...profile may be dirty");
+        return;
+      }
+
+      // Wait for Sync service to become ready.
+      if (!lazy.Weave.Status.ready) {
+        this.waitForEvent("weave:service:ready");
+      }
+
+      await lazy.Weave.Service.promiseInitialized;
+
+      // We only want to do this if we modified the bookmarks this phase.
+      this.shouldValidateBookmarks = false;
+
+      // Always give Sync an extra tick to initialize. If we waited for the
+      // service:ready event, this is required to ensure all handlers have
+      // executed.
+      await lazy.Async.promiseYield();
+      await this._executeTestPhase(file, phase, settings);
+    } catch (e) {
+      this.DumpError("RunTestPhase failed", e);
+    }
+  },
+
+  /**
+   * Executes a single test phase.
+   *
+   * This is called by RunTestPhase() after the environment is validated.
+   */
+  async _executeTestPhase(file, phase, settings) {
+    try {
+      this.config = JSON.parse(Services.prefs.getStringPref("tps.config"));
+      // parse the test file
+      Services.scriptloader.loadSubScript(file, this);
+      this._currentPhase = phase;
+      // cleanup phases are in the format `cleanup-${profileName}`.
+      if (this._currentPhase.startsWith("cleanup-")) {
+        let profileToClean = this._currentPhase.slice("cleanup-".length);
+        this.phases[this._currentPhase] = profileToClean;
+        this.Phase(this._currentPhase, [[this.Cleanup]]);
+      } else {
+        // Don't bother doing this for cleanup phases.
+        this._tryLoadPingSchema(file);
+      }
+      let this_phase = this._phaselist[this._currentPhase];
+
+      if (this_phase == undefined) {
+        this.DumpError("invalid phase " + this._currentPhase);
+        return;
+      }
+
+      if (this.phases[this._currentPhase] == undefined) {
+        this.DumpError("no profile defined for phase " + this._currentPhase);
+        return;
+      }
+
+      // If we have restricted the active engines, unregister engines we don't
+      // care about.
+      if (settings.ignoreUnusedEngines && Array.isArray(this._enabledEngines)) {
+        let names = {};
+        for (let name of this._enabledEngines) {
+          names[name] = true;
+        }
+        for (let engine of lazy.Weave.Service.engineManager.getEnabled()) {
+          if (!(engine.name in names)) {
+            lazy.Logger.logInfo("Unregistering unused engine: " + engine.name);
+            await lazy.Weave.Service.engineManager.unregister(engine);
+          }
+        }
+      }
+      lazy.Logger.logInfo("Starting phase " + this._currentPhase);
+
+      lazy.Logger.logInfo(
+        "setting client.name to " + this.phases[this._currentPhase]
+      );
+      lazy.Weave.Svc.PrefBranch.setStringPref(
+        "client.name",
+        this.phases[this._currentPhase]
+      );
+
+      this._interceptSyncTelemetry();
+
+      // start processing the test actions
+      this._currentAction = 0;
+      await lazy.SessionStore.promiseAllWindowsRestored;
+      await this.RunNextTestAction();
+    } catch (e) {
+      this.DumpError("_executeTestPhase failed", e);
+    }
+  },
+
+  /**
+   * Override sync telemetry functions so that we can detect errors generating
+   * the sync ping, and count how many pings we report.
+   */
+  _interceptSyncTelemetry() {
+    let originalObserve = lazy.SyncTelemetry.observe;
+    let self = this;
+    lazy.SyncTelemetry.observe = function () {
+      try {
+        originalObserve.apply(this, arguments);
+      } catch (e) {
+        self.DumpError("Error when generating sync telemetry", e);
+      }
+    };
+    lazy.SyncTelemetry.submit = record => {
+      lazy.Logger.logInfo(
+        "Intercepted sync telemetry submission: " + JSON.stringify(record)
+      );
+      this._syncsReportedViaTelemetry +=
+        record.syncs.length + (record.discarded || 0);
+      if (record.discarded) {
+        if (record.syncs.length != lazy.SyncTelemetry.maxPayloadCount) {
+          this.DumpError(
+            "Syncs discarded from ping before maximum payload count reached"
+          );
+        }
+      }
+      // If this is the shutdown ping, check and see that the telemetry saw all the syncs.
+      if (record.why === "shutdown") {
+        // If we happen to sync outside of tps manually causing it, its not an
+        // error in the telemetry, so we only complain if we didn't see all of them.
+        if (this._syncsReportedViaTelemetry < this._syncCount) {
+          this.DumpError(
+            `Telemetry missed syncs: Saw ${this._syncsReportedViaTelemetry}, should have >= ${this._syncCount}.`
+          );
+        }
+      }
+      if (!record.syncs.length) {
+        // Note: we're overwriting submit, so this is called even for pings that
+        // may have no data (which wouldn't be submitted to telemetry and would
+        // fail validation).
+        return;
+      }
+      // Our ping may have some undefined values, which we rely on JSON stripping
+      // out as part of the ping submission - but our validator fails with them,
+      // so round-trip via JSON here to avoid that.
+      record = JSON.parse(JSON.stringify(record));
+      const result = this.pingValidator.validate(record);
+      if (!result.valid) {
+        // Note that we already logged the record.
+        this.DumpError(
+          "Sync ping validation failed with errors: " +
+            JSON.stringify(result.errors)
+        );
+      }
+    };
+  },
+
+  /**
+   * Register a single phase with the test harness.
+   *
+   * This is called when loading individual test files.
+   *
+   * @param  phasename
+   *         String name of the phase being loaded.
+   * @param  fnlist
+   *         Array of functions/actions to perform.
+   */
+  Phase: function Test__Phase(phasename, fnlist) {
+    if (Object.keys(this._phaselist).length === 0) {
+      // This is the first phase we should force a log in
+      fnlist.unshift([this.Login]);
+    }
+    this._phaselist[phasename] = fnlist;
+  },
+
+  /**
+   * Restrict enabled Sync engines to a specified set.
+   *
+   * This can be called by a test to limit what engines are enabled. It is
+   * recommended to call it to reduce the overhead and log clutter for the
+   * test.
+   *
+   * The "clients" engine is special and is always enabled, so there is no
+   * need to specify it.
+   *
+   * @param  names
+   *         Array of Strings for engines to make active during the test.
+   */
+  EnableEngines: function EnableEngines(names) {
+    if (!Array.isArray(names)) {
+      throw new Error(
+        "Argument to RestrictEngines() is not an array: " + typeof names
+      );
+    }
+
+    this._enabledEngines = names;
+  },
+
+  /**
+   * Returns a promise that resolves when a specific observer notification is
+   * resolved. This is similar to the various waitFor* functions, although is
+   * typically safer if you need to do some other work that may make the event
+   * fire.
+   *
+   * eg:
+   *    doSomething(); // causes the event to be fired.
+   *    await promiseObserver("something");
+   * is risky as the call to doSomething may trigger the event before the
+   * promiseObserver call is made. Contrast with:
+   *
+   *   let waiter = promiseObserver("something");
+   *   doSomething(); // causes the event to be fired.
+   *   await waiter;  // will return as soon as the event fires, even if it fires
+   *                  // before this function is called.
+   *
+   * @param aEventName
+   *        String event to wait for.
+   */
+  promiseObserver(aEventName) {
+    return new Promise(resolve => {
+      lazy.Logger.logInfo("Setting up wait for " + aEventName + "...");
+      let handler = () => {
+        lazy.Logger.logInfo("Observed " + aEventName);
+        lazy.Svc.Obs.remove(aEventName, handler);
+        resolve();
+      };
+      lazy.Svc.Obs.add(aEventName, handler);
+    });
+  },
+
+  /**
+   * Wait for the named event to be observed.
+   *
+   * Note that in general, you should probably use promiseObserver unless you
+   * are 100% sure that the event being waited on can only be sent after this
+   * call adds the listener.
+   *
+   * @param aEventName
+   *        String event to wait for.
+   */
+  async waitForEvent(aEventName) {
+    await this.promiseObserver(aEventName);
+  },
+
+  /**
+   * Waits for Sync to logged in before returning
+   */
+  async waitForSetupComplete() {
+    if (!this._setupComplete) {
+      await this.waitForEvent("weave:service:setup-complete");
+    }
+  },
+
+  /**
+   * Waits for Sync to be finished before returning
+   */
+  async waitForSyncFinished() {
+    if (lazy.Weave.Service.locked) {
+      await this.waitForEvent("weave:service:resyncs-finished");
+    }
+  },
+
+  /**
+   * Waits for Sync to start tracking before returning.
+   */
+  async waitForTracking() {
+    if (!this._isTracking) {
+      await this.waitForEvent("weave:service:tracking-started");
+    }
+  },
+
+  /**
+   * Login on the server
+   */
+  async Login() {
+    if (await lazy.Authentication.isReady()) {
+      return;
+    }
+
+    lazy.Logger.logInfo("Setting client credentials and login.");
+    await lazy.Authentication.signIn(this.config.fx_account);
+    await this.waitForSetupComplete();
+    lazy.Logger.AssertEqual(
+      lazy.Weave.Status.service,
+      lazy.STATUS_OK,
+      "Weave status OK"
+    );
+    await this.waitForTracking();
+  },
+
+  /**
+   * Triggers a sync operation
+   *
+   * @param {String} [wipeAction]
+   *        Type of wipe to perform (resetClient, wipeClient, wipeRemote)
+   *
+   */
+  async Sync(wipeAction) {
+    if (this._syncActive) {
+      this.DumpError("Sync currently active which should be impossible");
+      return;
+    }
+    lazy.Logger.logInfo(
+      "Executing Sync" + (wipeAction ? ": " + wipeAction : "")
+    );
+
+    // Force a wipe action if requested. In case of an initial sync the pref
+    // will be overwritten by Sync itself (see bug 992198), so ensure that we
+    // also handle it via the "weave:service:setup-complete" notification.
+    if (wipeAction) {
+      this._syncWipeAction = wipeAction;
+      lazy.Weave.Svc.PrefBranch.setStringPref("firstSync", wipeAction);
+    } else {
+      lazy.Weave.Svc.PrefBranch.clearUserPref("firstSync");
+    }
+    if (!(await lazy.Weave.Service.login())) {
+      // We need to complete verification.
+      lazy.Logger.logInfo("Logging in before performing sync");
+      await this.Login();
+    }
+    ++this._syncCount;
+
+    lazy.Logger.logInfo(
+      "Executing Sync" + (wipeAction ? ": " + wipeAction : "")
+    );
+    this._triggeredSync = true;
+    await lazy.Weave.Service.sync();
+    lazy.Logger.logInfo("Sync is complete");
+    // wait a second for things to settle...
+    await new Promise(resolve => {
+      lazy.CommonUtils.namedTimer(resolve, 1000, this, "postsync");
+    });
+  },
+
+  async WipeServer() {
+    lazy.Logger.logInfo("Wiping data from server.");
+
+    await this.Login();
+    await lazy.Weave.Service.login();
+    await lazy.Weave.Service.wipeServer();
+  },
+
+  /**
+   * Action which ensures changes are being tracked before returning.
+   */
+  async EnsureTracking() {
+    await this.Login();
+    await this.waitForTracking();
+  },
+
+  Addons: {
+    async install(addons) {
+      await TPS.HandleAddons(addons, ACTION_ADD);
+    },
+    async setEnabled(addons, state) {
+      await TPS.HandleAddons(addons, ACTION_SET_ENABLED, state);
+    },
+    async uninstall(addons) {
+      await TPS.HandleAddons(addons, ACTION_DELETE);
+    },
+    async verify(addons, state) {
+      await TPS.HandleAddons(addons, ACTION_VERIFY, state);
+    },
+    async verifyNot(addons) {
+      await TPS.HandleAddons(addons, ACTION_VERIFY_NOT);
+    },
+    skipValidation() {
+      TPS.shouldValidateAddons = false;
+    },
+  },
+
+  Addresses: {
+    async add(addresses) {
+      await this.HandleAddresses(addresses, ACTION_ADD);
+    },
+    async modify(addresses) {
+      await this.HandleAddresses(addresses, ACTION_MODIFY);
+    },
+    async delete(addresses) {
+      await this.HandleAddresses(addresses, ACTION_DELETE);
+    },
+    async verify(addresses) {
+      await this.HandleAddresses(addresses, ACTION_VERIFY);
+    },
+    async verifyNot(addresses) {
+      await this.HandleAddresses(addresses, ACTION_VERIFY_NOT);
+    },
+  },
+
+  Bookmarks: {
+    async add(bookmarks) {
+      await TPS.HandleBookmarks(bookmarks, ACTION_ADD);
+    },
+    async modify(bookmarks) {
+      await TPS.HandleBookmarks(bookmarks, ACTION_MODIFY);
+    },
+    async delete(bookmarks) {
+      await TPS.HandleBookmarks(bookmarks, ACTION_DELETE);
+    },
+    async verify(bookmarks) {
+      await TPS.HandleBookmarks(bookmarks, ACTION_VERIFY);
+    },
+    async verifyNot(bookmarks) {
+      await TPS.HandleBookmarks(bookmarks, ACTION_VERIFY_NOT);
+    },
+    skipValidation() {
+      TPS.shouldValidateBookmarks = false;
+    },
+  },
+  CreditCards: {
+    async add(creditCards) {
+      await this.HandleCreditCards(creditCards, ACTION_ADD);
+    },
+    async modify(creditCards) {
+      await this.HandleCreditCards(creditCards, ACTION_MODIFY);
+    },
+    async delete(creditCards) {
+      await this.HandleCreditCards(creditCards, ACTION_DELETE);
+    },
+    async verify(creditCards) {
+      await this.HandleCreditCards(creditCards, ACTION_VERIFY);
+    },
+    async verifyNot(creditCards) {
+      await this.HandleCreditCards(creditCards, ACTION_VERIFY_NOT);
+    },
+  },
+
+  Formdata: {
+    async add(formdata) {
+      await this.HandleForms(formdata, ACTION_ADD);
+    },
+    async delete(formdata) {
+      await this.HandleForms(formdata, ACTION_DELETE);
+    },
+    async verify(formdata) {
+      await this.HandleForms(formdata, ACTION_VERIFY);
+    },
+    async verifyNot(formdata) {
+      await this.HandleForms(formdata, ACTION_VERIFY_NOT);
+    },
+  },
+  History: {
+    async add(history) {
+      await this.HandleHistory(history, ACTION_ADD);
+    },
+    async delete(history) {
+      await this.HandleHistory(history, ACTION_DELETE);
+    },
+    async verify(history) {
+      await this.HandleHistory(history, ACTION_VERIFY);
+    },
+    async verifyNot(history) {
+      await this.HandleHistory(history, ACTION_VERIFY_NOT);
+    },
+  },
+  Passwords: {
+    async add(passwords) {
+      await this.HandlePasswords(passwords, ACTION_ADD);
+    },
+    async modify(passwords) {
+      await this.HandlePasswords(passwords, ACTION_MODIFY);
+    },
+    async delete(passwords) {
+      await this.HandlePasswords(passwords, ACTION_DELETE);
+    },
+    async verify(passwords) {
+      await this.HandlePasswords(passwords, ACTION_VERIFY);
+    },
+    async verifyNot(passwords) {
+      await this.HandlePasswords(passwords, ACTION_VERIFY_NOT);
+    },
+    skipValidation() {
+      TPS.shouldValidatePasswords = false;
+    },
+  },
+  Prefs: {
+    async modify(prefs) {
+      await TPS.HandlePrefs(prefs, ACTION_MODIFY);
+    },
+    async verify(prefs) {
+      await TPS.HandlePrefs(prefs, ACTION_VERIFY);
+    },
+  },
+  Tabs: {
+    async add(tabs) {
+      await TPS.HandleTabs(tabs, ACTION_ADD);
+    },
+    async verify(tabs) {
+      await TPS.HandleTabs(tabs, ACTION_VERIFY);
+    },
+    async verifyNot(tabs) {
+      await TPS.HandleTabs(tabs, ACTION_VERIFY_NOT);
+    },
+  },
+  Windows: {
+    async add(aWindow) {
+      await TPS.HandleWindows(aWindow, ACTION_ADD);
+    },
+  },
+
+  // Jumping through loads of hoops via calling back into a "HandleXXX" method
+  // and adding an ACTION_XXX indirection adds no value - let's KISS!
+  // eslint-disable-next-line no-unused-vars
+  ExtStorage: {
+    async set(id, data) {
+      lazy.Logger.logInfo(`setting data for '${id}': ${data}`);
+      await lazy.extensionStorageSync.set({ id }, data);
+    },
+    async verify(id, keys, data) {
+      let got = await lazy.extensionStorageSync.get({ id }, keys);
+      lazy.Logger.AssertEqual(got, data, `data for '${id}'/${keys}`);
+    },
+  },
+};
+
+// Initialize TPS
+TPS._init();
diff --git a/services/sync/tps/extensions/tps/schema.json b/services/sync/tps/extensions/tps/schema.json
new file mode 100644
index 0000000000..fe51488c70
--- /dev/null
+++ b/services/sync/tps/extensions/tps/schema.json
@@ -0,0 +1 @@
+[]