Adding upstream version 124.0.1.upstream/124.0.1

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

149 files changed, 37156 insertions, 0 deletions

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.