Adding upstream version 124.0.1.upstream/124.0.1

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

11 files changed, 1495 insertions, 0 deletions

diff --git a/toolkit/components/extensions/storage/ExtensionStorageComponents.h b/toolkit/components/extensions/storage/ExtensionStorageComponents.h
new file mode 100644
index 0000000000..53af177432
--- /dev/null
+++ b/toolkit/components/extensions/storage/ExtensionStorageComponents.h
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_extensions_storage_ExtensionStorageComponents_h_
+#define mozilla_extensions_storage_ExtensionStorageComponents_h_
+
+#include "mozIExtensionStorageArea.h"
+#include "nsCOMPtr.h"
+
+extern "C" {
+
+// Implemented in Rust, in the `webext_storage_bridge` crate.
+nsresult NS_NewExtensionStorageSyncArea(mozIExtensionStorageArea** aResult);
+
+}  // extern "C"
+
+namespace mozilla {
+namespace extensions {
+namespace storage {
+
+// The C++ constructor for a `storage.sync` area. This wrapper exists because
+// `components.conf` requires a component class constructor to return an
+// `already_AddRefed<T>`, but Rust doesn't have such a type. So we call the
+// Rust constructor using a `nsCOMPtr` (which is compatible with Rust's
+// `xpcom::RefPtr`) out param, and return that.
+already_AddRefed<mozIExtensionStorageArea> NewSyncArea() {
+  nsCOMPtr<mozIExtensionStorageArea> storage;
+  nsresult rv = NS_NewExtensionStorageSyncArea(getter_AddRefs(storage));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return nullptr;
+  }
+  return storage.forget();
+}
+
+}  // namespace storage
+}  // namespace extensions
+}  // namespace mozilla
+
+#endif  // mozilla_extensions_storage_ExtensionStorageComponents_h_
diff --git a/toolkit/components/extensions/storage/ExtensionStorageComponents.sys.mjs b/toolkit/components/extensions/storage/ExtensionStorageComponents.sys.mjs
new file mode 100644
index 0000000000..4de62005a1
--- /dev/null
+++ b/toolkit/components/extensions/storage/ExtensionStorageComponents.sys.mjs
@@ -0,0 +1,118 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AsyncShutdown: "resource://gre/modules/AsyncShutdown.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+});
+
+const StorageSyncArea = Components.Constructor(
+  "@mozilla.org/extensions/storage/internal/sync-area;1",
+  "mozIConfigurableExtensionStorageArea",
+  "configure"
+);
+
+/**
+ * An XPCOM service for the WebExtension `storage.sync` API. The service manages
+ * a storage area for storing and syncing extension data.
+ *
+ * The service configures its storage area with the database path, and hands
+ * out references to the configured area via `getInterface`. It also registers
+ * a shutdown blocker to automatically tear down the area.
+ *
+ * ## What's the difference between `storage/internal/storage-sync-area;1` and
+ *    `storage/sync;1`?
+ *
+ * `components.conf` has two classes:
+ * `@mozilla.org/extensions/storage/internal/sync-area;1` and
+ * `@mozilla.org/extensions/storage/sync;1`.
+ *
+ * The `storage/internal/sync-area;1` class is implemented in Rust, and can be
+ * instantiated using `createInstance` and `Components.Constructor`. It's not
+ * a singleton, so creating a new instance will create a new `storage.sync`
+ * area, with its own database connection. It's useful for testing, but not
+ * meant to be used outside of this module.
+ *
+ * The `storage/sync;1` class is implemented in this file. It's a singleton,
+ * ensuring there's only one `storage.sync` area, with one database connection.
+ * The service implements `nsIInterfaceRequestor`, so callers can access the
+ * storage interface like this:
+ *
+ *     let storageSyncArea = Cc["@mozilla.org/extensions/storage/sync;1"]
+ *        .getService(Ci.nsIInterfaceRequestor)
+ *        .getInterface(Ci.mozIExtensionStorageArea);
+ *
+ * ...And the Sync interface like this:
+ *
+ *    let extensionStorageEngine = Cc["@mozilla.org/extensions/storage/sync;1"]
+ *       .getService(Ci.nsIInterfaceRequestor)
+ *       .getInterface(Ci.mozIBridgedSyncEngine);
+ *
+ * @class
+ */
+export function StorageSyncService() {
+  if (StorageSyncService._singleton) {
+    return StorageSyncService._singleton;
+  }
+
+  let file = new lazy.FileUtils.File(
+    PathUtils.join(PathUtils.profileDir, "storage-sync-v2.sqlite")
+  );
+  let kintoFile = new lazy.FileUtils.File(
+    PathUtils.join(PathUtils.profileDir, "storage-sync.sqlite")
+  );
+  this._storageArea = new StorageSyncArea(file, kintoFile);
+
+  // Register a blocker to close the storage connection on shutdown.
+  this._shutdownBound = () => this._shutdown();
+  lazy.AsyncShutdown.profileChangeTeardown.addBlocker(
+    "StorageSyncService: shutdown",
+    this._shutdownBound
+  );
+
+  StorageSyncService._singleton = this;
+}
+
+StorageSyncService._singleton = null;
+
+StorageSyncService.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["nsIInterfaceRequestor"]),
+
+  // Returns the storage and syncing interfaces. This just hands out a
+  // reference to the underlying storage area, with a quick check to make sure
+  // that callers are asking for the right interfaces.
+  getInterface(iid) {
+    if (
+      iid.equals(Ci.mozIExtensionStorageArea) ||
+      iid.equals(Ci.mozIBridgedSyncEngine)
+    ) {
+      return this._storageArea.QueryInterface(iid);
+    }
+    throw Components.Exception(
+      "This interface isn't implemented",
+      Cr.NS_ERROR_NO_INTERFACE
+    );
+  },
+
+  // Tears down the storage area and lifts the blocker so that shutdown can
+  // continue.
+  async _shutdown() {
+    try {
+      await new Promise((resolve, reject) => {
+        this._storageArea.teardown({
+          handleSuccess: resolve,
+          handleError(code, message) {
+            reject(Components.Exception(message, code));
+          },
+        });
+      });
+    } finally {
+      lazy.AsyncShutdown.profileChangeTeardown.removeBlocker(
+        this._shutdownBound
+      );
+    }
+  },
+};
diff --git a/toolkit/components/extensions/storage/components.conf b/toolkit/components/extensions/storage/components.conf
new file mode 100644
index 0000000000..a1d54fa542
--- /dev/null
+++ b/toolkit/components/extensions/storage/components.conf
@@ -0,0 +1,22 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Classes = [
+    {
+        'cid': '{f1e424f2-67fe-4f69-a8f8-3993a71f44fa}',
+        'contract_ids': ['@mozilla.org/extensions/storage/internal/sync-area;1'],
+        'type': 'mozIExtensionStorageArea',
+        'headers': ['mozilla/extensions/storage/ExtensionStorageComponents.h'],
+        'constructor': 'mozilla::extensions::storage::NewSyncArea',
+    },
+    {
+        'cid': '{5b7047b4-fe17-4661-8e13-871402bc2023}',
+        'contract_ids': ['@mozilla.org/extensions/storage/sync;1'],
+        'esModule': 'resource://gre/modules/ExtensionStorageComponents.sys.mjs',
+        'constructor': 'StorageSyncService',
+        'singleton': True,
+    },
+]
diff --git a/toolkit/components/extensions/storage/moz.build b/toolkit/components/extensions/storage/moz.build
new file mode 100644
index 0000000000..85f52cdadb
--- /dev/null
+++ b/toolkit/components/extensions/storage/moz.build
@@ -0,0 +1,33 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+with Files("**"):
+    BUG_COMPONENT = ("WebExtensions", "Storage")
+
+XPIDL_MODULE = "webextensions-storage"
+
+XPIDL_SOURCES += [
+    "mozIExtensionStorageArea.idl",
+]
+
+# Don't build the Rust `storage.sync` bridge for GeckoView, as it will expose
+# a delegate for consumers to use instead. Android Components can then provide
+# an implementation of the delegate that's backed by the Rust component. For
+# details, please see bug 1626506, comment 4.
+if CONFIG["MOZ_WIDGET_TOOLKIT"] != "android":
+    EXPORTS.mozilla.extensions.storage += [
+        "ExtensionStorageComponents.h",
+    ]
+
+    EXTRA_JS_MODULES += [
+        "ExtensionStorageComponents.sys.mjs",
+    ]
+
+    XPCOM_MANIFESTS += [
+        "components.conf",
+    ]
+
+FINAL_LIBRARY = "xul"
diff --git a/toolkit/components/extensions/storage/mozIExtensionStorageArea.idl b/toolkit/components/extensions/storage/mozIExtensionStorageArea.idl
new file mode 100644
index 0000000000..b3dcaa2479
--- /dev/null
+++ b/toolkit/components/extensions/storage/mozIExtensionStorageArea.idl
@@ -0,0 +1,127 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface mozIExtensionStorageCallback;
+interface nsIFile;
+interface nsIVariant;
+
+// Implements the operations needed to support the `StorageArea` WebExtension
+// API.
+[scriptable, uuid(d8eb3ff1-9b4b-435a-99ca-5b8cbaba2420)]
+interface mozIExtensionStorageArea : nsISupports {
+    // These constants are exposed by the rust crate, but it's not worth the
+    // effort of jumping through the hoops to get them exposed to the JS
+    // code in a sane way - so we just duplicate them here. We should consider a
+    // test that checks they match the rust code.
+    // This interface is agnostic WRT the area, so we prefix the constants with
+    // the area - it's the consumer of this interface which knows what to use.
+    const unsigned long SYNC_QUOTA_BYTES = 102400;
+    const unsigned long SYNC_QUOTA_BYTES_PER_ITEM = 8192;
+    const unsigned long SYNC_MAX_ITEMS = 512;
+
+    // Sets one or more key-value pairs specified in `json` for the
+    // `extensionId`. If the `callback` implements
+    // `mozIExtensionStorageListener`, its `onChange`
+    // method will be called with the new and old values.
+    void set(in AUTF8String extensionId,
+             in AUTF8String json,
+             in mozIExtensionStorageCallback callback);
+
+    // Returns the value for the `key` in the storage area for the
+    // `extensionId`. `key` must be a JSON string containing either `null`,
+    // an array of string key names, a single string key name, or an object
+    // where the properties are the key names, and the values are the defaults
+    // if the key name doesn't exist in the storage area.
+    //
+    // If `get()` fails due to the quota being exceeded, the exception will
+    // have a result code of NS_ERROR_DOM_QUOTA_EXCEEDED_ERR (==0x80530016)
+    void get(in AUTF8String extensionId,
+             in AUTF8String key,
+             in mozIExtensionStorageCallback callback);
+
+    // Removes the `key` from the storage area for the `extensionId`. If `key`
+    // exists and the `callback` implements `mozIExtensionStorageListener`, its
+    // `onChanged` method will be called with the removed key-value pair.
+    void remove(in AUTF8String extensionId,
+                in AUTF8String key,
+                in mozIExtensionStorageCallback callback);
+
+    // Removes all keys from the storage area for the `extensionId`. If
+    // `callback` implements `mozIExtensionStorageListener`, its `onChange`
+    // method will be called with all removed key-value pairs.
+    void clear(in AUTF8String extensionId,
+               in mozIExtensionStorageCallback callback);
+
+    // Gets the number of bytes in use for the specified keys.
+    void getBytesInUse(in AUTF8String extensionId,
+                       in AUTF8String keys,
+                       in mozIExtensionStorageCallback callback);
+
+    // Gets and clears the information about the migration from the kinto
+    // database into the rust one. As "and clears" indicates, this will
+    // only produce a non-empty the first time it's called after a
+    // migration (which, hopefully, should only happen once).
+    void takeMigrationInfo(in mozIExtensionStorageCallback callback);
+};
+
+// Implements additional methods for setting up and tearing down the underlying
+// database connection for a storage area. This is a separate interface because
+// these methods are not part of the `StorageArea` API, and have restrictions on
+// when they can be called.
+[scriptable, uuid(2b008295-1bcc-4610-84f1-ad4cab2fa9ee)]
+interface mozIConfigurableExtensionStorageArea : nsISupports {
+    // Sets up the storage area. An area can only be configured once; calling
+    // `configure` multiple times will throw. `configure` must also be called
+    // before any of the `mozIExtensionStorageArea` methods, or they'll fail
+    // with errors.
+    // The second param is the path to the kinto database file from which we
+    // should migrate. This should always be specified even when there's a
+    // chance the file doesn't exist.
+    void configure(in nsIFile databaseFile, in nsIFile kintoFile);
+
+    // Tears down the storage area, closing the backing database connection.
+    // This is called automatically when Firefox shuts down. Once a storage area
+    // has been shut down, all its methods will fail with errors. If `configure`
+    // hasn't been called for this area yet, `teardown` is a no-op.
+    void teardown(in mozIExtensionStorageCallback callback);
+};
+
+// Implements additional methods for syncing a storage area. This is a separate
+// interface because these methods are not part of the `StorageArea` API, and
+// have restrictions on when they can be called.
+[scriptable, uuid(6dac82c9-1d8a-4893-8c0f-6e626aef802c)]
+interface mozISyncedExtensionStorageArea : nsISupports {
+    // If a sync is in progress, this method fetches pending change
+    // notifications for all extensions whose storage areas were updated.
+    // `callback` should implement `mozIExtensionStorageListener` to forward
+    // the records to `storage.onChanged` listeners. This method should only
+    // be called by Sync, after `mozIBridgedSyncEngine.apply` and before
+    // `syncFinished`. It fetches nothing if called at any other time.
+    void fetchPendingSyncChanges(in mozIExtensionStorageCallback callback);
+};
+
+// A listener for storage area notifications.
+[scriptable, uuid(8cb3c7e4-d0ca-4353-bccd-2673b4e11510)]
+interface mozIExtensionStorageListener : nsISupports {
+    // Notifies that an operation has data to pass to `storage.onChanged`
+    // listeners for the given `extensionId`. `json` is a JSON array of listener
+    // infos. If an operation affects multiple extensions, this method will be
+    // called multiple times, once per extension.
+    void onChanged(in AUTF8String extensionId, in AUTF8String json);
+};
+
+// A generic callback for a storage operation. Either `handleSuccess` or
+// `handleError` is guaranteed to be called once.
+[scriptable, uuid(870dca40-6602-4748-8493-c4253eb7f322)]
+interface mozIExtensionStorageCallback : nsISupports {
+    // Called when the operation completes. Operations that return a result,
+    // like `get`, will pass a `UTF8String` variant. Those that don't return
+    // anything, like `set` or `remove`, will pass a `null` variant.
+    void handleSuccess(in nsIVariant result);
+
+    // Called when the operation fails.
+    void handleError(in nsresult code, in AUTF8String message);
+};
diff --git a/toolkit/components/extensions/storage/webext_storage_bridge/Cargo.toml b/toolkit/components/extensions/storage/webext_storage_bridge/Cargo.toml
new file mode 100644
index 0000000000..39c5bf92c6
--- /dev/null
+++ b/toolkit/components/extensions/storage/webext_storage_bridge/Cargo.toml
@@ -0,0 +1,25 @@
+[package]
+name = "webext_storage_bridge"
+description = "The WebExtension `storage.sync` bindings for Firefox"
+version = "0.1.0"
+authors = ["The Firefox Sync Developers <sync-team@mozilla.com>"]
+edition = "2018"
+license = "MPL-2.0"
+
+[dependencies]
+anyhow = "1.0"
+atomic_refcell = "0.1"
+cstr = "0.2"
+golden_gate = { path = "../../../../../services/sync/golden_gate" }
+interrupt-support = "0.1"
+moz_task = { path = "../../../../../xpcom/rust/moz_task" }
+nserror = { path = "../../../../../xpcom/rust/nserror" }
+nsstring = { path = "../../../../../xpcom/rust/nsstring" }
+once_cell = "1"
+thin-vec = { version = "0.2.1", features = ["gecko-ffi"] }
+xpcom = { path = "../../../../../xpcom/rust/xpcom" }
+serde = "1"
+serde_json = "1"
+storage_variant = { path = "../../../../../storage/variant" }
+sql-support = "0.1"
+webext-storage = "0.1"
diff --git a/toolkit/components/extensions/storage/webext_storage_bridge/src/area.rs b/toolkit/components/extensions/storage/webext_storage_bridge/src/area.rs
new file mode 100644
index 0000000000..1418ccca29
--- /dev/null
+++ b/toolkit/components/extensions/storage/webext_storage_bridge/src/area.rs
@@ -0,0 +1,484 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{
+    cell::{Ref, RefCell},
+    convert::TryInto,
+    ffi::OsString,
+    mem,
+    path::PathBuf,
+    str,
+    sync::Arc,
+};
+
+use golden_gate::{ApplyTask, BridgedEngine, FerryTask};
+use moz_task::{self, DispatchOptions, TaskRunnable};
+use nserror::{nsresult, NS_OK};
+use nsstring::{nsACString, nsCString, nsString};
+use thin_vec::ThinVec;
+use webext_storage::STORAGE_VERSION;
+use xpcom::{
+    interfaces::{
+        mozIBridgedSyncEngineApplyCallback, mozIBridgedSyncEngineCallback,
+        mozIExtensionStorageCallback, mozIServicesLogSink, nsIFile, nsISerialEventTarget,
+    },
+    RefPtr,
+};
+
+use crate::error::{Error, Result};
+use crate::punt::{Punt, PuntTask, TeardownTask};
+use crate::store::{LazyStore, LazyStoreConfig};
+
+fn path_from_nsifile(file: &nsIFile) -> Result<PathBuf> {
+    let mut raw_path = nsString::new();
+    // `nsIFile::GetPath` gives us a UTF-16-encoded version of its
+    // native path, which we must turn back into a platform-native
+    // string. We can't use `nsIFile::nativePath()` here because
+    // it's marked as `nostdcall`, which Rust doesn't support.
+    unsafe { file.GetPath(&mut *raw_path) }.to_result()?;
+    let native_path = {
+        // On Windows, we can create a native string directly from the
+        // encoded path.
+        #[cfg(windows)]
+        {
+            use std::os::windows::prelude::*;
+            OsString::from_wide(&raw_path)
+        }
+        // On other platforms, we must first decode the raw path from
+        // UTF-16, and then create our native string.
+        #[cfg(not(windows))]
+        OsString::from(String::from_utf16(&raw_path)?)
+    };
+    Ok(native_path.into())
+}
+
+/// An XPCOM component class for the Rust extension storage API. This class
+/// implements the interfaces needed for syncing and storage.
+///
+/// This class can be created on any thread, but must not be shared between
+/// threads. In Rust terms, it's `Send`, but not `Sync`.
+#[xpcom(
+    implement(
+        mozIExtensionStorageArea,
+        mozIConfigurableExtensionStorageArea,
+        mozISyncedExtensionStorageArea,
+        mozIInterruptible,
+        mozIBridgedSyncEngine
+    ),
+    nonatomic
+)]
+pub struct StorageSyncArea {
+    /// A background task queue, used to run all our storage operations on a
+    /// thread pool. Using a serial event target here means that all operations
+    /// will execute sequentially.
+    queue: RefPtr<nsISerialEventTarget>,
+    /// The store is lazily initialized on the task queue the first time it's
+    /// used.
+    store: RefCell<Option<Arc<LazyStore>>>,
+}
+
+/// `mozIExtensionStorageArea` implementation.
+impl StorageSyncArea {
+    /// Creates a storage area and its task queue.
+    pub fn new() -> Result<RefPtr<StorageSyncArea>> {
+        let queue = moz_task::create_background_task_queue(cstr!("StorageSyncArea"))?;
+        Ok(StorageSyncArea::allocate(InitStorageSyncArea {
+            queue,
+            store: RefCell::new(Some(Arc::default())),
+        }))
+    }
+
+    /// Returns the store for this area, or an error if it's been torn down.
+    fn store(&self) -> Result<Ref<'_, Arc<LazyStore>>> {
+        let maybe_store = self.store.borrow();
+        if maybe_store.is_some() {
+            Ok(Ref::map(maybe_store, |s| s.as_ref().unwrap()))
+        } else {
+            Err(Error::AlreadyTornDown)
+        }
+    }
+
+    /// Dispatches a task for a storage operation to the task queue.
+    fn dispatch(&self, punt: Punt, callback: &mozIExtensionStorageCallback) -> Result<()> {
+        let name = punt.name();
+        let task = PuntTask::new(Arc::downgrade(&*self.store()?), punt, callback)?;
+        let runnable = TaskRunnable::new(name, Box::new(task))?;
+        // `may_block` schedules the runnable on a dedicated I/O pool.
+        TaskRunnable::dispatch_with_options(
+            runnable,
+            self.queue.coerce(),
+            DispatchOptions::new().may_block(true),
+        )?;
+        Ok(())
+    }
+
+    xpcom_method!(
+        configure => Configure(
+            database_file: *const nsIFile,
+            kinto_file: *const nsIFile
+        )
+    );
+    /// Sets up the storage area.
+    fn configure(&self, database_file: &nsIFile, kinto_file: &nsIFile) -> Result<()> {
+        self.store()?.configure(LazyStoreConfig {
+            path: path_from_nsifile(database_file)?,
+            kinto_path: path_from_nsifile(kinto_file)?,
+        })?;
+        Ok(())
+    }
+
+    xpcom_method!(
+        set => Set(
+            ext_id: *const ::nsstring::nsACString,
+            json: *const ::nsstring::nsACString,
+            callback: *const mozIExtensionStorageCallback
+        )
+    );
+    /// Sets one or more key-value pairs.
+    fn set(
+        &self,
+        ext_id: &nsACString,
+        json: &nsACString,
+        callback: &mozIExtensionStorageCallback,
+    ) -> Result<()> {
+        self.dispatch(
+            Punt::Set {
+                ext_id: str::from_utf8(ext_id)?.into(),
+                value: serde_json::from_str(str::from_utf8(json)?)?,
+            },
+            callback,
+        )?;
+        Ok(())
+    }
+
+    xpcom_method!(
+        get => Get(
+            ext_id: *const ::nsstring::nsACString,
+            json: *const ::nsstring::nsACString,
+            callback: *const mozIExtensionStorageCallback
+        )
+    );
+    /// Gets values for one or more keys.
+    fn get(
+        &self,
+        ext_id: &nsACString,
+        json: &nsACString,
+        callback: &mozIExtensionStorageCallback,
+    ) -> Result<()> {
+        self.dispatch(
+            Punt::Get {
+                ext_id: str::from_utf8(ext_id)?.into(),
+                keys: serde_json::from_str(str::from_utf8(json)?)?,
+            },
+            callback,
+        )
+    }
+
+    xpcom_method!(
+        remove => Remove(
+            ext_id: *const ::nsstring::nsACString,
+            json: *const ::nsstring::nsACString,
+            callback: *const mozIExtensionStorageCallback
+        )
+    );
+    /// Removes one or more keys and their values.
+    fn remove(
+        &self,
+        ext_id: &nsACString,
+        json: &nsACString,
+        callback: &mozIExtensionStorageCallback,
+    ) -> Result<()> {
+        self.dispatch(
+            Punt::Remove {
+                ext_id: str::from_utf8(ext_id)?.into(),
+                keys: serde_json::from_str(str::from_utf8(json)?)?,
+            },
+            callback,
+        )
+    }
+
+    xpcom_method!(
+        clear => Clear(
+            ext_id: *const ::nsstring::nsACString,
+            callback: *const mozIExtensionStorageCallback
+        )
+    );
+    /// Removes all keys and values for the specified extension.
+    fn clear(&self, ext_id: &nsACString, callback: &mozIExtensionStorageCallback) -> Result<()> {
+        self.dispatch(
+            Punt::Clear {
+                ext_id: str::from_utf8(ext_id)?.into(),
+            },
+            callback,
+        )
+    }
+
+    xpcom_method!(
+        getBytesInUse => GetBytesInUse(
+            ext_id: *const ::nsstring::nsACString,
+            keys: *const ::nsstring::nsACString,
+            callback: *const mozIExtensionStorageCallback
+        )
+    );
+    /// Obtains the count of bytes in use for the specified key or for all keys.
+    fn getBytesInUse(
+        &self,
+        ext_id: &nsACString,
+        keys: &nsACString,
+        callback: &mozIExtensionStorageCallback,
+    ) -> Result<()> {
+        self.dispatch(
+            Punt::GetBytesInUse {
+                ext_id: str::from_utf8(ext_id)?.into(),
+                keys: serde_json::from_str(str::from_utf8(keys)?)?,
+            },
+            callback,
+        )
+    }
+
+    xpcom_method!(teardown => Teardown(callback: *const mozIExtensionStorageCallback));
+    /// Tears down the storage area, closing the backing database connection.
+    fn teardown(&self, callback: &mozIExtensionStorageCallback) -> Result<()> {
+        // Each storage task holds a `Weak` reference to the store, which it
+        // upgrades to an `Arc` (strong reference) when the task runs on the
+        // background queue. The strong reference is dropped when the task
+        // finishes. When we tear down the storage area, we relinquish our one
+        // owned strong reference to the `TeardownTask`. Because we're using a
+        // task queue, when the `TeardownTask` runs, it should have the only
+        // strong reference to the store, since all other tasks that called
+        // `Weak::upgrade` will have already finished. The `TeardownTask` can
+        // then consume the `Arc` and destroy the store.
+        let mut maybe_store = self.store.borrow_mut();
+        match mem::take(&mut *maybe_store) {
+            Some(store) => {
+                // Interrupt any currently-running statements.
+                store.interrupt();
+                // If dispatching the runnable fails, we'll leak the store
+                // without closing its database connection.
+                teardown(&self.queue, store, callback)?;
+            }
+            None => return Err(Error::AlreadyTornDown),
+        }
+        Ok(())
+    }
+
+    xpcom_method!(takeMigrationInfo => TakeMigrationInfo(callback: *const mozIExtensionStorageCallback));
+
+    /// Fetch-and-delete (e.g. `take`) information about the migration from the
+    /// kinto-based extension-storage to the rust-based storage.
+    fn takeMigrationInfo(&self, callback: &mozIExtensionStorageCallback) -> Result<()> {
+        self.dispatch(Punt::TakeMigrationInfo, callback)
+    }
+}
+
+fn teardown(
+    queue: &nsISerialEventTarget,
+    store: Arc<LazyStore>,
+    callback: &mozIExtensionStorageCallback,
+) -> Result<()> {
+    let task = TeardownTask::new(store, callback)?;
+    let runnable = TaskRunnable::new(TeardownTask::name(), Box::new(task))?;
+    TaskRunnable::dispatch_with_options(
+        runnable,
+        queue.coerce(),
+        DispatchOptions::new().may_block(true),
+    )?;
+    Ok(())
+}
+
+/// `mozISyncedExtensionStorageArea` implementation.
+impl StorageSyncArea {
+    xpcom_method!(
+        fetch_pending_sync_changes => FetchPendingSyncChanges(callback: *const mozIExtensionStorageCallback)
+    );
+    fn fetch_pending_sync_changes(&self, callback: &mozIExtensionStorageCallback) -> Result<()> {
+        self.dispatch(Punt::FetchPendingSyncChanges, callback)
+    }
+}
+
+/// `mozIInterruptible` implementation.
+impl StorageSyncArea {
+    xpcom_method!(
+        interrupt => Interrupt()
+    );
+    /// Interrupts any operations currently running on the background task
+    /// queue.
+    fn interrupt(&self) -> Result<()> {
+        self.store()?.interrupt();
+        Ok(())
+    }
+}
+
+/// `mozIBridgedSyncEngine` implementation.
+impl StorageSyncArea {
+    xpcom_method!(get_logger => GetLogger() -> *const mozIServicesLogSink);
+    fn get_logger(&self) -> Result<RefPtr<mozIServicesLogSink>> {
+        Err(NS_OK)?
+    }
+
+    xpcom_method!(set_logger => SetLogger(logger: *const mozIServicesLogSink));
+    fn set_logger(&self, _logger: Option<&mozIServicesLogSink>) -> Result<()> {
+        Ok(())
+    }
+
+    xpcom_method!(get_storage_version => GetStorageVersion() -> i32);
+    fn get_storage_version(&self) -> Result<i32> {
+        Ok(STORAGE_VERSION.try_into().unwrap())
+    }
+
+    // It's possible that migration, or even merging, will result in records
+    // too large for the server. We tolerate that (and hope that the addons do
+    // too :)
+    xpcom_method!(get_allow_skipped_record => GetAllowSkippedRecord() -> bool);
+    fn get_allow_skipped_record(&self) -> Result<bool> {
+        Ok(true)
+    }
+
+    xpcom_method!(
+        get_last_sync => GetLastSync(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn get_last_sync(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_last_sync(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        set_last_sync => SetLastSync(
+            last_sync_millis: i64,
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn set_last_sync(
+        &self,
+        last_sync_millis: i64,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<()> {
+        Ok(
+            FerryTask::for_set_last_sync(self.new_bridge()?, last_sync_millis, callback)?
+                .dispatch(&self.queue)?,
+        )
+    }
+
+    xpcom_method!(
+        get_sync_id => GetSyncId(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn get_sync_id(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_sync_id(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        reset_sync_id => ResetSyncId(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn reset_sync_id(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_reset_sync_id(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        ensure_current_sync_id => EnsureCurrentSyncId(
+            new_sync_id: *const nsACString,
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn ensure_current_sync_id(
+        &self,
+        new_sync_id: &nsACString,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<()> {
+        Ok(
+            FerryTask::for_ensure_current_sync_id(self.new_bridge()?, new_sync_id, callback)?
+                .dispatch(&self.queue)?,
+        )
+    }
+
+    xpcom_method!(
+        sync_started => SyncStarted(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn sync_started(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_sync_started(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        store_incoming => StoreIncoming(
+            incoming_envelopes_json: *const ThinVec<::nsstring::nsCString>,
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn store_incoming(
+        &self,
+        incoming_envelopes_json: Option<&ThinVec<nsCString>>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<()> {
+        Ok(FerryTask::for_store_incoming(
+            self.new_bridge()?,
+            incoming_envelopes_json.map(|v| v.as_slice()).unwrap_or(&[]),
+            callback,
+        )?
+        .dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(apply => Apply(callback: *const mozIBridgedSyncEngineApplyCallback));
+    fn apply(&self, callback: &mozIBridgedSyncEngineApplyCallback) -> Result<()> {
+        Ok(ApplyTask::new(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        set_uploaded => SetUploaded(
+            server_modified_millis: i64,
+            uploaded_ids: *const ThinVec<::nsstring::nsCString>,
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn set_uploaded(
+        &self,
+        server_modified_millis: i64,
+        uploaded_ids: Option<&ThinVec<nsCString>>,
+        callback: &mozIBridgedSyncEngineCallback,
+    ) -> Result<()> {
+        Ok(FerryTask::for_set_uploaded(
+            self.new_bridge()?,
+            server_modified_millis,
+            uploaded_ids.map(|v| v.as_slice()).unwrap_or(&[]),
+            callback,
+        )?
+        .dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        sync_finished => SyncFinished(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn sync_finished(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_sync_finished(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        reset => Reset(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn reset(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_reset(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    xpcom_method!(
+        wipe => Wipe(
+            callback: *const mozIBridgedSyncEngineCallback
+        )
+    );
+    fn wipe(&self, callback: &mozIBridgedSyncEngineCallback) -> Result<()> {
+        Ok(FerryTask::for_wipe(self.new_bridge()?, callback)?.dispatch(&self.queue)?)
+    }
+
+    fn new_bridge(&self) -> Result<Box<dyn BridgedEngine>> {
+        Ok(Box::new(self.store()?.get()?.bridged_engine()))
+    }
+}
diff --git a/toolkit/components/extensions/storage/webext_storage_bridge/src/error.rs b/toolkit/components/extensions/storage/webext_storage_bridge/src/error.rs
new file mode 100644
index 0000000000..877b2b21a8
--- /dev/null
+++ b/toolkit/components/extensions/storage/webext_storage_bridge/src/error.rs
@@ -0,0 +1,124 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{error, fmt, result, str::Utf8Error, string::FromUtf16Error};
+
+use golden_gate::Error as GoldenGateError;
+use nserror::{
+    nsresult, NS_ERROR_ALREADY_INITIALIZED, NS_ERROR_CANNOT_CONVERT_DATA,
+    NS_ERROR_DOM_QUOTA_EXCEEDED_ERR, NS_ERROR_FAILURE, NS_ERROR_INVALID_ARG,
+    NS_ERROR_NOT_IMPLEMENTED, NS_ERROR_NOT_INITIALIZED, NS_ERROR_UNEXPECTED,
+};
+use serde_json::error::Error as JsonError;
+use webext_storage::error::Error as WebextStorageError;
+
+/// A specialized `Result` type for extension storage operations.
+pub type Result<T> = result::Result<T, Error>;
+
+/// The error type for extension storage operations. Errors can be converted
+/// into `nsresult` codes, and include more detailed messages that can be passed
+/// to callbacks.
+#[derive(Debug)]
+pub enum Error {
+    Nsresult(nsresult),
+    WebextStorage(WebextStorageError),
+    MigrationFailed(WebextStorageError),
+    GoldenGate(GoldenGateError),
+    MalformedString(Box<dyn error::Error + Send + Sync + 'static>),
+    AlreadyConfigured,
+    NotConfigured,
+    AlreadyRan(&'static str),
+    DidNotRun(&'static str),
+    AlreadyTornDown,
+    NotImplemented,
+}
+
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match self {
+            Error::MalformedString(error) => Some(error.as_ref()),
+            _ => None,
+        }
+    }
+}
+
+impl From<nsresult> for Error {
+    fn from(result: nsresult) -> Error {
+        Error::Nsresult(result)
+    }
+}
+
+impl From<WebextStorageError> for Error {
+    fn from(error: WebextStorageError) -> Error {
+        Error::WebextStorage(error)
+    }
+}
+
+impl From<GoldenGateError> for Error {
+    fn from(error: GoldenGateError) -> Error {
+        Error::GoldenGate(error)
+    }
+}
+
+impl From<Utf8Error> for Error {
+    fn from(error: Utf8Error) -> Error {
+        Error::MalformedString(error.into())
+    }
+}
+
+impl From<FromUtf16Error> for Error {
+    fn from(error: FromUtf16Error) -> Error {
+        Error::MalformedString(error.into())
+    }
+}
+
+impl From<JsonError> for Error {
+    fn from(error: JsonError) -> Error {
+        Error::MalformedString(error.into())
+    }
+}
+
+impl From<Error> for nsresult {
+    fn from(error: Error) -> nsresult {
+        match error {
+            Error::Nsresult(result) => result,
+            Error::WebextStorage(e) => match e {
+                WebextStorageError::QuotaError(_) => NS_ERROR_DOM_QUOTA_EXCEEDED_ERR,
+                _ => NS_ERROR_FAILURE,
+            },
+            Error::MigrationFailed(_) => NS_ERROR_CANNOT_CONVERT_DATA,
+            Error::GoldenGate(error) => error.into(),
+            Error::MalformedString(_) => NS_ERROR_INVALID_ARG,
+            Error::AlreadyConfigured => NS_ERROR_ALREADY_INITIALIZED,
+            Error::NotConfigured => NS_ERROR_NOT_INITIALIZED,
+            Error::AlreadyRan(_) => NS_ERROR_UNEXPECTED,
+            Error::DidNotRun(_) => NS_ERROR_UNEXPECTED,
+            Error::AlreadyTornDown => NS_ERROR_UNEXPECTED,
+            Error::NotImplemented => NS_ERROR_NOT_IMPLEMENTED,
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Error::Nsresult(result) => write!(f, "Operation failed with {result}"),
+            Error::WebextStorage(error) => error.fmt(f),
+            Error::MigrationFailed(error) => write!(f, "Migration failed with {error}"),
+            Error::GoldenGate(error) => error.fmt(f),
+            Error::MalformedString(error) => error.fmt(f),
+            Error::AlreadyConfigured => write!(f, "The storage area is already configured"),
+            Error::NotConfigured => write!(
+                f,
+                "The storage area must be configured by calling `configure` first"
+            ),
+            Error::AlreadyRan(what) => write!(f, "`{what}` already ran on the background thread"),
+            Error::DidNotRun(what) => write!(f, "`{what}` didn't run on the background thread"),
+            Error::AlreadyTornDown => {
+                write!(f, "Can't use a storage area that's already torn down")
+            }
+            Error::NotImplemented => write!(f, "Operation not implemented"),
+        }
+    }
+}
diff --git a/toolkit/components/extensions/storage/webext_storage_bridge/src/lib.rs b/toolkit/components/extensions/storage/webext_storage_bridge/src/lib.rs
new file mode 100644
index 0000000000..94133ef1e9
--- /dev/null
+++ b/toolkit/components/extensions/storage/webext_storage_bridge/src/lib.rs
@@ -0,0 +1,65 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#![allow(non_snake_case)]
+
+//! This crate bridges the WebExtension storage area interfaces in Firefox
+//! Desktop to the extension storage Rust component in Application Services.
+//!
+//! ## How are the WebExtension storage APIs implemented in Firefox?
+//!
+//! There are three storage APIs available for WebExtensions:
+//! `storage.local`, which is stored locally in an IndexedDB database and never
+//! synced to other devices, `storage.sync`, which is stored in a local SQLite
+//! database and synced to all devices signed in to the same Firefox Account,
+//! and `storage.managed`, which is provisioned in a native manifest and
+//! read-only.
+//!
+//! * `storage.local` is implemented in `ExtensionStorageIDB.jsm`.
+//! * `storage.sync` is implemented in a Rust component, `webext_storage`. This
+//!   Rust component is vendored in m-c, and exposed to JavaScript via an XPCOM
+//!   API in `webext_storage_bridge` (this crate). Eventually, we'll change
+//!   `ExtensionStorageSync.jsm` to call the XPCOM API instead of using the
+//!   old Kinto storage adapter.
+//! * `storage.managed` is implemented directly in `parent/ext-storage.js`.
+//!
+//! `webext_storage_bridge` implements the `mozIExtensionStorageArea`
+//! (and, eventually, `mozIBridgedSyncEngine`) interface for `storage.sync`. The
+//! implementation is in `area::StorageSyncArea`, and is backed by the
+//! `webext_storage` component.
+
+#[macro_use]
+extern crate cstr;
+#[macro_use]
+extern crate xpcom;
+
+mod area;
+mod error;
+mod punt;
+mod store;
+
+use nserror::{nsresult, NS_OK};
+use xpcom::{interfaces::mozIExtensionStorageArea, RefPtr};
+
+use crate::area::StorageSyncArea;
+
+/// The constructor for a `storage.sync` area. This uses C linkage so that it
+/// can be called from C++. See `ExtensionStorageComponents.h` for the C++
+/// constructor that's passed to the component manager.
+///
+/// # Safety
+///
+/// This function is unsafe because it dereferences `result`.
+#[no_mangle]
+pub unsafe extern "C" fn NS_NewExtensionStorageSyncArea(
+    result: *mut *const mozIExtensionStorageArea,
+) -> nsresult {
+    match StorageSyncArea::new() {
+        Ok(bridge) => {
+            RefPtr::new(bridge.coerce::<mozIExtensionStorageArea>()).forget(&mut *result);
+            NS_OK
+        }
+        Err(err) => err.into(),
+    }
+}
diff --git a/toolkit/components/extensions/storage/webext_storage_bridge/src/punt.rs b/toolkit/components/extensions/storage/webext_storage_bridge/src/punt.rs
new file mode 100644
index 0000000000..4740237942
--- /dev/null
+++ b/toolkit/components/extensions/storage/webext_storage_bridge/src/punt.rs
@@ -0,0 +1,321 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{
+    borrow::Borrow,
+    fmt::Write,
+    mem, result, str,
+    sync::{Arc, Weak},
+};
+
+use atomic_refcell::AtomicRefCell;
+use moz_task::{Task, ThreadPtrHandle, ThreadPtrHolder};
+use nserror::nsresult;
+use nsstring::nsCString;
+use serde::Serialize;
+use serde_json::Value as JsonValue;
+use storage_variant::VariantType;
+use xpcom::{
+    interfaces::{mozIExtensionStorageCallback, mozIExtensionStorageListener},
+    RefPtr, XpCom,
+};
+
+use crate::error::{Error, Result};
+use crate::store::LazyStore;
+
+/// A storage operation that's punted from the main thread to the background
+/// task queue.
+pub enum Punt {
+    /// Get the values of the keys for an extension.
+    Get { ext_id: String, keys: JsonValue },
+    /// Set a key-value pair for an extension.
+    Set { ext_id: String, value: JsonValue },
+    /// Remove one or more keys for an extension.
+    Remove { ext_id: String, keys: JsonValue },
+    /// Clear all keys and values for an extension.
+    Clear { ext_id: String },
+    /// Returns the bytes in use for the specified, or all, keys.
+    GetBytesInUse { ext_id: String, keys: JsonValue },
+    /// Fetches all pending Sync change notifications to pass to
+    /// `storage.onChanged` listeners.
+    FetchPendingSyncChanges,
+    /// Fetch-and-delete (e.g. `take`) information about the migration from the
+    /// kinto-based extension-storage to the rust-based storage.
+    ///
+    /// This data is stored in the database instead of just being returned by
+    /// the call to `migrate`, as we may migrate prior to telemetry being ready.
+    TakeMigrationInfo,
+}
+
+impl Punt {
+    /// Returns the operation name, used to label the task runnable and report
+    /// errors.
+    pub fn name(&self) -> &'static str {
+        match self {
+            Punt::Get { .. } => "webext_storage::get",
+            Punt::Set { .. } => "webext_storage::set",
+            Punt::Remove { .. } => "webext_storage::remove",
+            Punt::Clear { .. } => "webext_storage::clear",
+            Punt::GetBytesInUse { .. } => "webext_storage::get_bytes_in_use",
+            Punt::FetchPendingSyncChanges => "webext_storage::fetch_pending_sync_changes",
+            Punt::TakeMigrationInfo => "webext_storage::take_migration_info",
+        }
+    }
+}
+
+/// A storage operation result, punted from the background queue back to the
+/// main thread.
+#[derive(Default)]
+struct PuntResult {
+    changes: Vec<Change>,
+    value: Option<String>,
+}
+
+/// A change record for an extension.
+struct Change {
+    ext_id: String,
+    json: String,
+}
+
+impl PuntResult {
+    /// Creates a result with a single change to pass to `onChanged`, and no
+    /// return value for `handleSuccess`. The `Borrow` bound lets this method
+    /// take either a borrowed reference or an owned value.
+    fn with_change<T: Borrow<S>, S: Serialize>(ext_id: &str, changes: T) -> Result<Self> {
+        Ok(PuntResult {
+            changes: vec![Change {
+                ext_id: ext_id.into(),
+                json: serde_json::to_string(changes.borrow())?,
+            }],
+            value: None,
+        })
+    }
+
+    /// Creates a result with changes for multiple extensions to pass to
+    /// `onChanged`, and no return value for `handleSuccess`.
+    fn with_changes(changes: Vec<Change>) -> Self {
+        PuntResult {
+            changes,
+            value: None,
+        }
+    }
+
+    /// Creates a result with no changes to pass to `onChanged`, and a return
+    /// value for `handleSuccess`.
+    fn with_value<T: Borrow<S>, S: Serialize>(value: T) -> Result<Self> {
+        Ok(PuntResult {
+            changes: Vec::new(),
+            value: Some(serde_json::to_string(value.borrow())?),
+        })
+    }
+}
+
+/// A generic task used for all storage operations. Punts the operation to the
+/// background task queue, receives a result back on the main thread, and calls
+/// the callback with it.
+pub struct PuntTask {
+    name: &'static str,
+    /// Storage tasks hold weak references to the store, which they upgrade
+    /// to strong references when running on the background queue. This
+    /// ensures that pending storage tasks don't block teardown (for example,
+    /// if a consumer calls `get` and then `teardown`, without waiting for
+    /// `get` to finish).
+    store: Weak<LazyStore>,
+    punt: AtomicRefCell<Option<Punt>>,
+    callback: ThreadPtrHandle<mozIExtensionStorageCallback>,
+    result: AtomicRefCell<Result<PuntResult>>,
+}
+
+impl PuntTask {
+    /// Creates a storage task that punts an operation to the background queue.
+    /// Returns an error if the task couldn't be created because the thread
+    /// manager is shutting down.
+    pub fn new(
+        store: Weak<LazyStore>,
+        punt: Punt,
+        callback: &mozIExtensionStorageCallback,
+    ) -> Result<Self> {
+        let name = punt.name();
+        Ok(Self {
+            name,
+            store,
+            punt: AtomicRefCell::new(Some(punt)),
+            callback: ThreadPtrHolder::new(
+                cstr!("mozIExtensionStorageCallback"),
+                RefPtr::new(callback),
+            )?,
+            result: AtomicRefCell::new(Err(Error::DidNotRun(name))),
+        })
+    }
+
+    /// Upgrades the task's weak `LazyStore` reference to a strong one. Returns
+    /// an error if the store has been torn down.
+    ///
+    /// It's important that this is called on the background queue, after the
+    /// task has been dispatched. Storage tasks shouldn't hold strong references
+    /// to the store on the main thread, because then they might block teardown.
+    fn store(&self) -> Result<Arc<LazyStore>> {
+        match self.store.upgrade() {
+            Some(store) => Ok(store),
+            None => Err(Error::AlreadyTornDown),
+        }
+    }
+
+    /// Runs this task's storage operation on the background queue.
+    fn inner_run(&self, punt: Punt) -> Result<PuntResult> {
+        Ok(match punt {
+            Punt::Set { ext_id, value } => {
+                PuntResult::with_change(&ext_id, self.store()?.get()?.set(&ext_id, value)?)?
+            }
+            Punt::Get { ext_id, keys } => {
+                PuntResult::with_value(self.store()?.get()?.get(&ext_id, keys)?)?
+            }
+            Punt::Remove { ext_id, keys } => {
+                PuntResult::with_change(&ext_id, self.store()?.get()?.remove(&ext_id, keys)?)?
+            }
+            Punt::Clear { ext_id } => {
+                PuntResult::with_change(&ext_id, self.store()?.get()?.clear(&ext_id)?)?
+            }
+            Punt::GetBytesInUse { ext_id, keys } => {
+                PuntResult::with_value(self.store()?.get()?.get_bytes_in_use(&ext_id, keys)?)?
+            }
+            Punt::FetchPendingSyncChanges => PuntResult::with_changes(
+                self.store()?
+                    .get()?
+                    .get_synced_changes()?
+                    .into_iter()
+                    .map(|info| Change {
+                        ext_id: info.ext_id,
+                        json: info.changes,
+                    })
+                    .collect(),
+            ),
+            Punt::TakeMigrationInfo => {
+                PuntResult::with_value(self.store()?.get()?.take_migration_info()?)?
+            }
+        })
+    }
+}
+
+impl Task for PuntTask {
+    fn run(&self) {
+        *self.result.borrow_mut() = match self.punt.borrow_mut().take() {
+            Some(punt) => self.inner_run(punt),
+            // A task should never run on the background queue twice, but we
+            // return an error just in case.
+            None => Err(Error::AlreadyRan(self.name)),
+        };
+    }
+
+    fn done(&self) -> result::Result<(), nsresult> {
+        let callback = self.callback.get().unwrap();
+        // As above, `done` should never be called multiple times, but we handle
+        // that by returning an error.
+        match mem::replace(
+            &mut *self.result.borrow_mut(),
+            Err(Error::AlreadyRan(self.name)),
+        ) {
+            Ok(PuntResult { changes, value }) => {
+                // If we have change data, and the callback implements the
+                // listener interface, notify about it first.
+                if let Some(listener) = callback.query_interface::<mozIExtensionStorageListener>() {
+                    for Change { ext_id, json } in changes {
+                        // Ignore errors.
+                        let _ = unsafe {
+                            listener.OnChanged(&*nsCString::from(ext_id), &*nsCString::from(json))
+                        };
+                    }
+                }
+                let result = value.map(nsCString::from).into_variant();
+                unsafe { callback.HandleSuccess(result.coerce()) }
+            }
+            Err(err) => {
+                let mut message = nsCString::new();
+                write!(message, "{err}").unwrap();
+                unsafe { callback.HandleError(err.into(), &*message) }
+            }
+        }
+        .to_result()
+    }
+}
+
+/// A task to tear down the store on the background task queue.
+pub struct TeardownTask {
+    /// Unlike storage tasks, the teardown task holds a strong reference to
+    /// the store, which it drops on the background queue. This is the only
+    /// task that should do that.
+    store: AtomicRefCell<Option<Arc<LazyStore>>>,
+    callback: ThreadPtrHandle<mozIExtensionStorageCallback>,
+    result: AtomicRefCell<Result<()>>,
+}
+
+impl TeardownTask {
+    /// Creates a teardown task. This should only be created and dispatched
+    /// once, to clean up the store at shutdown. Returns an error if the task
+    /// couldn't be created because the thread manager is shutting down.
+    pub fn new(store: Arc<LazyStore>, callback: &mozIExtensionStorageCallback) -> Result<Self> {
+        Ok(Self {
+            store: AtomicRefCell::new(Some(store)),
+            callback: ThreadPtrHolder::new(
+                cstr!("mozIExtensionStorageCallback"),
+                RefPtr::new(callback),
+            )?,
+            result: AtomicRefCell::new(Err(Error::DidNotRun(Self::name()))),
+        })
+    }
+
+    /// Returns the task name, used to label its runnable and report errors.
+    pub fn name() -> &'static str {
+        "webext_storage::teardown"
+    }
+
+    /// Tears down and drops the store on the background queue.
+    fn inner_run(&self, store: Arc<LazyStore>) -> Result<()> {
+        // At this point, we should be holding the only strong reference
+        // to the store, since 1) `StorageSyncArea` gave its one strong
+        // reference to our task, and 2) we're running on a background
+        // task queue, which runs all tasks sequentially...so no other
+        // `PuntTask`s should be running and trying to upgrade their
+        // weak references. So we can unwrap the `Arc` and take ownership
+        // of the store.
+        match Arc::try_unwrap(store) {
+            Ok(store) => store.teardown(),
+            Err(_) => {
+                // If unwrapping the `Arc` fails, someone else must have
+                // a strong reference to the store. We could sleep and
+                // try again, but this is so unexpected that it's easier
+                // to just leak the store, and return an error to the
+                // callback. Except in tests, we only call `teardown` at
+                // shutdown, so the resources will get reclaimed soon,
+                // anyway.
+                Err(Error::DidNotRun(Self::name()))
+            }
+        }
+    }
+}
+
+impl Task for TeardownTask {
+    fn run(&self) {
+        *self.result.borrow_mut() = match self.store.borrow_mut().take() {
+            Some(store) => self.inner_run(store),
+            None => Err(Error::AlreadyRan(Self::name())),
+        };
+    }
+
+    fn done(&self) -> result::Result<(), nsresult> {
+        let callback = self.callback.get().unwrap();
+        match mem::replace(
+            &mut *self.result.borrow_mut(),
+            Err(Error::AlreadyRan(Self::name())),
+        ) {
+            Ok(()) => unsafe { callback.HandleSuccess(().into_variant().coerce()) },
+            Err(err) => {
+                let mut message = nsCString::new();
+                write!(message, "{err}").unwrap();
+                unsafe { callback.HandleError(err.into(), &*message) }
+            }
+        }
+        .to_result()
+    }
+}
diff --git a/toolkit/components/extensions/storage/webext_storage_bridge/src/store.rs b/toolkit/components/extensions/storage/webext_storage_bridge/src/store.rs
new file mode 100644
index 0000000000..cb1ce07784
--- /dev/null
+++ b/toolkit/components/extensions/storage/webext_storage_bridge/src/store.rs
@@ -0,0 +1,136 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{fs::remove_file, path::PathBuf, sync::Arc};
+
+use interrupt_support::SqlInterruptHandle;
+use once_cell::sync::OnceCell;
+use webext_storage::store::WebExtStorageStore as Store;
+
+use crate::error::{self, Error};
+
+/// Options for an extension storage area.
+pub struct LazyStoreConfig {
+    /// The path to the database file for this storage area.
+    pub path: PathBuf,
+    /// The path to the old kinto database. If it exists, we should attempt to
+    /// migrate from this database as soon as we open our DB. It's not Option<>
+    /// because the caller will not have checked whether it exists or not, so
+    /// will assume it might.
+    pub kinto_path: PathBuf,
+}
+
+/// A lazy store is automatically initialized on a background thread with its
+/// configuration the first time it's used.
+#[derive(Default)]
+pub struct LazyStore {
+    store: OnceCell<InterruptStore>,
+    config: OnceCell<LazyStoreConfig>,
+}
+
+/// An `InterruptStore` wraps an inner extension store, and its interrupt
+/// handle.
+struct InterruptStore {
+    inner: Store,
+    handle: Arc<SqlInterruptHandle>,
+}
+
+impl LazyStore {
+    /// Configures the lazy store. Returns an error if the store has already
+    /// been configured. This method should be called from the main thread.
+    pub fn configure(&self, config: LazyStoreConfig) -> error::Result<()> {
+        self.config
+            .set(config)
+            .map_err(|_| Error::AlreadyConfigured)
+    }
+
+    /// Interrupts all pending operations on the store. If a database statement
+    /// is currently running, this will interrupt that statement. If the
+    /// statement is a write inside an active transaction, the entire
+    /// transaction will be rolled back. This method should be called from the
+    /// main thread.
+    pub fn interrupt(&self) {
+        if let Some(outer) = self.store.get() {
+            outer.handle.interrupt();
+        }
+    }
+
+    /// Returns the underlying store, initializing it if needed. This method
+    /// should only be called from a background thread or task queue, since
+    /// opening the database does I/O.
+    pub fn get(&self) -> error::Result<&Store> {
+        Ok(&self
+            .store
+            .get_or_try_init(|| match self.config.get() {
+                Some(config) => {
+                    let store = init_store(config)?;
+                    let handle = store.interrupt_handle();
+                    Ok(InterruptStore {
+                        inner: store,
+                        handle,
+                    })
+                }
+                None => Err(Error::NotConfigured),
+            })?
+            .inner)
+    }
+
+    /// Tears down the store. If the store wasn't initialized, this is a no-op.
+    /// This should only be called from a background thread or task queue,
+    /// because closing the database also does I/O.
+    pub fn teardown(self) -> error::Result<()> {
+        if let Some(store) = self.store.into_inner() {
+            store.inner.close()?;
+        }
+        Ok(())
+    }
+}
+
+// Initialize the store, performing a migration if necessary.
+// The requirements for migration are, roughly:
+// * If kinto_path doesn't exist, we don't try to migrate.
+// * If our DB path exists, we assume we've already migrated and don't try again
+// * If the migration fails, we close our store and delete the DB, then return
+//   a special error code which tells our caller about the failure. It's then
+//   expected to fallback to the "old" kinto store and we'll try next time.
+// Note that the migrate() method on the store is written such that is should
+// ignore all "read" errors from the source, but propagate "write" errors on our
+// DB - the intention is that things like corrupted source databases never fail,
+// but disk-space failures on our database does.
+fn init_store(config: &LazyStoreConfig) -> error::Result<Store> {
+    let should_migrate = config.kinto_path.exists() && !config.path.exists();
+    let store = Store::new(&config.path)?;
+    if should_migrate {
+        match store.migrate(&config.kinto_path) {
+            // It's likely to be too early for us to stick the MigrationInfo
+            // into the sync telemetry, a separate call to `take_migration_info`
+            // must be made to the store (this is done by telemetry after it's
+            // ready to submit the data).
+            Ok(()) => {
+                // need logging, but for now let's print to stdout.
+                println!("extension-storage: migration complete");
+                Ok(store)
+            }
+            Err(e) => {
+                println!("extension-storage: migration failure: {e}");
+                if let Err(e) = store.close() {
+                    // welp, this probably isn't going to end well...
+                    println!(
+                        "extension-storage: failed to close the store after migration failure: {e}"
+                    );
+                }
+                if let Err(e) = remove_file(&config.path) {
+                    // this is bad - if it happens regularly it will defeat
+                    // out entire migration strategy - we'll assume it
+                    // worked.
+                    // So it's desirable to make noise if this happens.
+                    println!("Failed to remove file after failed migration: {e}");
+                }
+                Err(Error::MigrationFailed(e))
+            }
+        }
+    } else {
+        Ok(store)
+    }
+}