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diff --git a/toolkit/modules/IndexedDB.sys.mjs b/toolkit/modules/IndexedDB.sys.mjs
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+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+/* 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/. */
+
+/**
+ * @file
+ *
+ * This module provides Promise-based wrappers around ordinarily
+ * IDBRequest-based IndexedDB methods and classes.
+ */
+
+/**
+ * Wraps the given request object, and returns a Promise which resolves when
+ * the requests succeeds or rejects when it fails.
+ *
+ * @param {IDBRequest} request
+ *        An IndexedDB request object to wrap.
+ * @returns {Promise}
+ */
+function wrapRequest(request) {
+  return new Promise((resolve, reject) => {
+    request.onsuccess = () => {
+      resolve(request.result);
+    };
+    request.onerror = () => {
+      reject(request.error);
+    };
+  });
+}
+
+/**
+ * Forwards a set of getter properties from a wrapper class to the wrapped
+ * object.
+ *
+ * @param {function} cls
+ *        The class constructor for which to forward the getters.
+ * @param {string} target
+ *        The name of the property which contains the wrapped object to which
+ *        to forward the getters.
+ * @param {Array<string>} props
+ *        A list of property names to forward.
+ */
+function forwardGetters(cls, target, props) {
+  for (let prop of props) {
+    Object.defineProperty(cls.prototype, prop, {
+      get() {
+        return this[target][prop];
+      },
+    });
+  }
+}
+
+/**
+ * Forwards a set of getter and setter properties from a wrapper class to the
+ * wrapped object.
+ *
+ * @param {function} cls
+ *        The class constructor for which to forward the properties.
+ * @param {string} target
+ *        The name of the property which contains the wrapped object to which
+ *        to forward the properties.
+ * @param {Array<string>} props
+ *        A list of property names to forward.
+ */
+function forwardProps(cls, target, props) {
+  for (let prop of props) {
+    Object.defineProperty(cls.prototype, prop, {
+      get() {
+        return this[target][prop];
+      },
+      set(value) {
+        this[target][prop] = value;
+      },
+    });
+  }
+}
+
+/**
+ * Wraps a set of IDBRequest-based methods via {@link wrapRequest} and
+ * forwards them to the equivalent methods on the wrapped object.
+ *
+ * @param {function} cls
+ *        The class constructor for which to forward the methods.
+ * @param {string} target
+ *        The name of the property which contains the wrapped object to which
+ *        to forward the methods.
+ * @param {Array<string>} methods
+ *        A list of method names to forward.
+ */
+function wrapMethods(cls, target, methods) {
+  for (let method of methods) {
+    cls.prototype[method] = function (...args) {
+      return wrapRequest(this[target][method](...args));
+    };
+  }
+}
+
+/**
+ * Forwards a set of methods from a wrapper class to the wrapped object.
+ *
+ * @param {function} cls
+ *        The class constructor for which to forward the getters.
+ * @param {string} target
+ *        The name of the property which contains the wrapped object to which
+ *        to forward the methods.
+ * @param {Array<string>} methods
+ *        A list of method names to forward.
+ */
+function forwardMethods(cls, target, methods) {
+  for (let method of methods) {
+    cls.prototype[method] = function (...args) {
+      return this[target][method](...args);
+    };
+  }
+}
+
+class Cursor {
+  constructor(cursorRequest, source) {
+    this.cursorRequest = cursorRequest;
+    this.source = source;
+    this.cursor = null;
+  }
+
+  get done() {
+    return !this.cursor;
+  }
+
+  // This method is used internally to wait the cursor's IDBRequest to have been
+  // completed and the internal cursor has been updated (used when we initially
+  // create the cursor from Cursed.openCursor/openKeyCursor, and in the method
+  // of this class defined by defineCursorUpdateMethods).
+  async awaitRequest() {
+    this.cursor = await wrapRequest(this.cursorRequest);
+    return this;
+  }
+}
+
+/**
+ * Define the Cursor class methods that update the cursor (continue, continuePrimaryKey
+ * and advance) as async functions that call the related IDBCursor methods and
+ * await the cursor's IDBRequest to be completed.
+ *
+ * @param {function} cls
+ *        The class constructor for which to define the cursor update methods.
+ * @param {Array<string>} methods
+ *        A list of "cursor update" method names to define.
+ */
+function defineCursorUpdateMethods(cls, methods) {
+  for (let method of methods) {
+    cls.prototype[method] = async function (...args) {
+      const promise = this.awaitRequest();
+      this.cursor[method](...args);
+      await promise;
+    };
+  }
+}
+
+defineCursorUpdateMethods(Cursor, [
+  "advance",
+  "continue",
+  "continuePrimaryKey",
+]);
+
+forwardGetters(Cursor, "cursor", ["direction", "key", "primaryKey"]);
+wrapMethods(Cursor, "cursor", ["delete", "update"]);
+
+class CursorWithValue extends Cursor {}
+
+forwardGetters(CursorWithValue, "cursor", ["value"]);
+
+class Cursed {
+  constructor(cursed) {
+    this.cursed = cursed;
+  }
+
+  openCursor(...args) {
+    const cursor = new CursorWithValue(this.cursed.openCursor(...args), this);
+    return cursor.awaitRequest();
+  }
+
+  openKeyCursor(...args) {
+    const cursor = new Cursor(this.cursed.openKeyCursor(...args), this);
+    return cursor.awaitRequest();
+  }
+}
+
+wrapMethods(Cursed, "cursed", [
+  "count",
+  "get",
+  "getAll",
+  "getAllKeys",
+  "getKey",
+]);
+
+class Index extends Cursed {
+  constructor(index, objectStore) {
+    super(index);
+
+    this.objectStore = objectStore;
+    this.index = index;
+  }
+}
+
+forwardGetters(Index, "index", [
+  "isAutoLocale",
+  "keyPath",
+  "locale",
+  "multiEntry",
+  "name",
+  "unique",
+]);
+
+class ObjectStore extends Cursed {
+  constructor(store) {
+    super(store);
+
+    this.store = store;
+  }
+
+  createIndex(...args) {
+    return new Index(this.store.createIndex(...args), this);
+  }
+
+  index(...args) {
+    return new Index(this.store.index(...args), this);
+  }
+}
+
+wrapMethods(ObjectStore, "store", ["add", "clear", "delete", "put"]);
+
+forwardMethods(ObjectStore, "store", ["deleteIndex"]);
+
+class Transaction {
+  constructor(transaction) {
+    this.transaction = transaction;
+
+    this._completionPromise = new Promise((resolve, reject) => {
+      transaction.oncomplete = resolve;
+      transaction.onerror = () => {
+        reject(transaction.error);
+      };
+      transaction.onabort = () => {
+        const error =
+          transaction.error ||
+          new DOMException("The operation has been aborted", "AbortError");
+        reject(error);
+      };
+    });
+  }
+
+  objectStore(name) {
+    return new ObjectStore(this.transaction.objectStore(name));
+  }
+
+  /**
+   * Returns a Promise which resolves when the transaction completes, or
+   * rejects when a transaction error or abort occurs.
+   *
+   * @returns {Promise}
+   */
+  promiseComplete() {
+    return this._completionPromise;
+  }
+}
+
+forwardGetters(Transaction, "transaction", [
+  "db",
+  "mode",
+  "error",
+  "objectStoreNames",
+]);
+
+forwardMethods(Transaction, "transaction", ["abort"]);
+
+export class IndexedDB {
+  /**
+   * Opens the database with the given name, and returns a Promise which
+   * resolves to an IndexedDB instance when the operation completes.
+   *
+   * @param {string} dbName
+   *        The name of the database to open.
+   * @param {object} options
+   *        The options with which to open the database.
+   * @param {integer} options.version
+   *        The schema version with which the database needs to be opened. If
+   *        the database does not exist, or its current schema version does
+   *        not match, the `onupgradeneeded` function will be called.
+   * @param {function} [onupgradeneeded]
+   *        A function which will be called with an IndexedDB object as its
+   *        first parameter when the database needs to be created, or its
+   *        schema needs to be upgraded. If this function is not provided, the
+   *        {@link #onupgradeneeded} method will be called instead.
+   *
+   * @returns {Promise<IndexedDB>}
+   */
+  static open(dbName, options, onupgradeneeded = null) {
+    let request = indexedDB.open(dbName, options);
+    return this._wrapOpenRequest(request, onupgradeneeded);
+  }
+
+  /**
+   * Opens the database for a given principal and with the given name, returns
+   * a Promise which resolves to an IndexedDB instance when the operation completes.
+   *
+   * @param {nsIPrincipal} principal
+   *        The principal to open the database for.
+   * @param {string} dbName
+   *        The name of the database to open.
+   * @param {object} options
+   *        The options with which to open the database.
+   * @param {integer} options.version
+   *        The schema version with which the database needs to be opened. If
+   *        the database does not exist, or its current schema version does
+   *        not match, the `onupgradeneeded` function will be called.
+   * @param {function} [onupgradeneeded]
+   *        A function which will be called with an IndexedDB object as its
+   *        first parameter when the database needs to be created, or its
+   *        schema needs to be upgraded. If this function is not provided, the
+   *        {@link #onupgradeneeded} method will be called instead.
+   *
+   * @returns {Promise<IndexedDB>}
+   */
+  static openForPrincipal(principal, dbName, options, onupgradeneeded = null) {
+    const request = indexedDB.openForPrincipal(principal, dbName, options);
+    return this._wrapOpenRequest(request, onupgradeneeded);
+  }
+
+  static _wrapOpenRequest(request, onupgradeneeded = null) {
+    request.onupgradeneeded = event => {
+      let db = new this(request.result);
+      if (onupgradeneeded) {
+        onupgradeneeded(db, event);
+      } else {
+        db.onupgradeneeded(event);
+      }
+    };
+
+    return wrapRequest(request).then(db => new this(db));
+  }
+
+  constructor(db) {
+    this.db = db;
+  }
+
+  onupgradeneeded() {}
+
+  /**
+   * Opens a transaction for the given object stores.
+   *
+   * @param {Array<string>} storeNames
+   *        The names of the object stores for which to open a transaction.
+   * @param {string} [mode = "readonly"]
+   *        The mode in which to open the transaction.
+   * @param {function} [callback]
+   *        An optional callback function. If provided, the function will be
+   *        called with the Transaction, and a Promise will be returned, which
+   *        will resolve to the callback's return value when the transaction
+   *        completes.
+   * @returns {Transaction|Promise}
+   */
+  transaction(storeNames, mode, callback = null) {
+    let transaction = new Transaction(this.db.transaction(storeNames, mode));
+
+    if (callback) {
+      let result = new Promise(resolve => {
+        resolve(callback(transaction));
+      });
+      return transaction.promiseComplete().then(() => result);
+    }
+
+    return transaction;
+  }
+
+  /**
+   * Opens a transaction for a single object store, and returns that object
+   * store.
+   *
+   * @param {string} storeName
+   *        The name of the object store to open.
+   * @param {string} [mode = "readonly"]
+   *        The mode in which to open the transaction.
+   * @param {function} [callback]
+   *        An optional callback function. If provided, the function will be
+   *        called with the ObjectStore, and a Promise will be returned, which
+   *        will resolve to the callback's return value when the transaction
+   *        completes.
+   * @returns {ObjectStore|Promise}
+   */
+  objectStore(storeName, mode, callback = null) {
+    let transaction = this.transaction([storeName], mode);
+    let objectStore = transaction.objectStore(storeName);
+
+    if (callback) {
+      let result = new Promise(resolve => {
+        resolve(callback(objectStore));
+      });
+      return transaction.promiseComplete().then(() => result);
+    }
+
+    return objectStore;
+  }
+
+  createObjectStore(...args) {
+    return new ObjectStore(this.db.createObjectStore(...args));
+  }
+}
+
+for (let method of ["cmp", "deleteDatabase"]) {
+  IndexedDB[method] = function (...args) {
+    return indexedDB[method](...args);
+  };
+}
+
+forwardMethods(IndexedDB, "db", [
+  "addEventListener",
+  "close",
+  "deleteObjectStore",
+  "hasEventListener",
+  "removeEventListener",
+]);
+
+forwardGetters(IndexedDB, "db", ["name", "objectStoreNames", "version"]);
+
+forwardProps(IndexedDB, "db", [
+  "onabort",
+  "onclose",
+  "onerror",
+  "onversionchange",
+]);