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diff --git a/toolkit/modules/Integration.jsm b/toolkit/modules/Integration.jsm
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+/* 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/. */
+
+/*
+ * Implements low-overhead integration between components of the application.
+ * This may have different uses depending on the component, including:
+ *
+ * - Providing product-specific implementations registered at startup.
+ * - Using alternative implementations during unit tests.
+ * - Allowing add-ons to change specific behaviors.
+ *
+ * Components may define one or more integration points, each defined by a
+ * root integration object whose properties and methods are the public interface
+ * and default implementation of the integration point. For example:
+ *
+ *   const DownloadIntegration = {
+ *     getTemporaryDirectory() {
+ *       return "/tmp/";
+ *     },
+ *
+ *     getTemporaryFile(name) {
+ *       return this.getTemporaryDirectory() + name;
+ *     },
+ *   };
+ *
+ * Other parts of the application may register overrides for some or all of the
+ * defined properties and methods. The component defining the integration point
+ * does not have to be loaded at this stage, because the name of the integration
+ * point is the only information required. For example, if the integration point
+ * is called "downloads":
+ *
+ *   Integration.downloads.register(base => ({
+ *     getTemporaryDirectory() {
+ *       return base.getTemporaryDirectory.call(this) + "subdir/";
+ *     },
+ *   }));
+ *
+ * When the component defining the integration point needs to call a method on
+ * the integration object, instead of using it directly the component would use
+ * the "getCombined" method to retrieve an object that includes all overrides.
+ * For example:
+ *
+ *   let combined = Integration.downloads.getCombined(DownloadIntegration);
+ *   Assert.is(combined.getTemporaryFile("file"), "/tmp/subdir/file");
+ *
+ * Overrides can be registered at startup or at any later time, so each call to
+ * "getCombined" may return a different object. The simplest way to create a
+ * reference to the combined object that stays updated to the latest version is
+ * to define the root object in a JSM and use the "defineModuleGetter" method.
+ *
+ * *** Registration ***
+ *
+ * Since the interface is not declared formally, the registrations can happen
+ * at startup without loading the component, so they do not affect performance.
+ *
+ * Hovever, this module does not provide a startup registry, this means that the
+ * code that registers and implements the override must be loaded at startup.
+ *
+ * If performance for the override code is a concern, you can take advantage of
+ * the fact that the function used to create the override is called lazily, and
+ * include only a stub loader for the final code in an existing startup module.
+ *
+ * The registration of overrides should be repeated for each process where the
+ * relevant integration methods will be called.
+ *
+ * *** Accessing base methods and properties ***
+ *
+ * Overrides are included in the prototype chain of the combined object in the
+ * same order they were registered, where the first is closest to the root.
+ *
+ * When defining overrides, you do not need to set the "__proto__" property of
+ * the objects you create, because their properties and methods are moved to a
+ * new object with the correct prototype. If you do, however, you can call base
+ * properties and methods using the "super" keyword. For example:
+ *
+ *   Integration.downloads.register(base => ({
+ *     __proto__: base,
+ *     getTemporaryDirectory() {
+ *       return super.getTemporaryDirectory() + "subdir/";
+ *     },
+ *   }));
+ *
+ * *** State handling ***
+ *
+ * Storing state directly on the combined integration object using the "this"
+ * reference is not recommended. When a new integration is registered, own
+ * properties stored on the old combined object are copied to the new combined
+ * object using a shallow copy, but the "this" reference for new invocations
+ * of the methods will be different.
+ *
+ * If the root object defines a property that always points to the same object,
+ * for example a "state" property, you can safely use it across registrations.
+ *
+ * Integration overrides provided by restartless add-ons should not use the
+ * "this" reference to store state, to avoid conflicts with other add-ons.
+ *
+ * *** Interaction with XPCOM ***
+ *
+ * Providing the combined object as an argument to any XPCOM method will
+ * generate a console error message, and will throw an exception where possible.
+ * For example, you cannot register observers directly on the combined object.
+ * This helps preventing mistakes due to the fact that the combined object
+ * reference changes when new integration overrides are registered.
+ */
+
+"use strict";
+
+var EXPORTED_SYMBOLS = ["Integration"];
+
+const { XPCOMUtils } = ChromeUtils.import(
+  "resource://gre/modules/XPCOMUtils.jsm"
+);
+
+/**
+ * Maps integration point names to IntegrationPoint objects.
+ */
+const gIntegrationPoints = new Map();
+
+/**
+ * This Proxy object creates IntegrationPoint objects using their name as key.
+ * The objects will be the same for the duration of the process. For example:
+ *
+ *   Integration.downloads.register(...);
+ *   Integration["addon-provided-integration"].register(...);
+ */
+var Integration = new Proxy(
+  {},
+  {
+    get(target, name) {
+      let integrationPoint = gIntegrationPoints.get(name);
+      if (!integrationPoint) {
+        integrationPoint = new IntegrationPoint();
+        gIntegrationPoints.set(name, integrationPoint);
+      }
+      return integrationPoint;
+    },
+  }
+);
+
+/**
+ * Individual integration point for which overrides can be registered.
+ */
+var IntegrationPoint = function() {
+  this._overrideFns = new Set();
+  this._combined = {
+    // eslint-disable-next-line mozilla/use-chromeutils-generateqi
+    QueryInterface() {
+      let ex = new Components.Exception(
+        "Integration objects should not be used with XPCOM because" +
+          " they change when new overrides are registered.",
+        Cr.NS_ERROR_NO_INTERFACE
+      );
+      Cu.reportError(ex);
+      throw ex;
+    },
+  };
+};
+
+IntegrationPoint.prototype = {
+  /**
+   * Ordered set of registered functions defining integration overrides.
+   */
+  _overrideFns: null,
+
+  /**
+   * Combined integration object. When this reference changes, properties
+   * defined directly on this object are copied to the new object.
+   *
+   * Initially, the only property of this object is a "QueryInterface" method
+   * that throws an exception, to prevent misuse as a permanent XPCOM listener.
+   */
+  _combined: null,
+
+  /**
+   * Indicates whether the integration object is current based on the list of
+   * registered integration overrides.
+   */
+  _combinedIsCurrent: false,
+
+  /**
+   * Registers new overrides for the integration methods. For example:
+   *
+   *   Integration.nameOfIntegrationPoint.register(base => ({
+   *     asyncMethod: Task.async(function* () {
+   *       return yield base.asyncMethod.apply(this, arguments);
+   *     }),
+   *   }));
+   *
+   * @param overrideFn
+   *        Function returning an object defining the methods that should be
+   *        overridden. Its only parameter is an object that contains the base
+   *        implementation of all the available methods.
+   *
+   * @note The override function is called every time the list of registered
+   *       override functions changes. Thus, it should not have any side
+   *       effects or do any other initialization.
+   */
+  register(overrideFn) {
+    this._overrideFns.add(overrideFn);
+    this._combinedIsCurrent = false;
+  },
+
+  /**
+   * Removes a previously registered integration override.
+   *
+   * Overrides don't usually need to be unregistered, unless they are added by a
+   * restartless add-on, in which case they should be unregistered when the
+   * add-on is disabled or uninstalled.
+   *
+   * @param overrideFn
+   *        This must be the same function object passed to "register".
+   */
+  unregister(overrideFn) {
+    this._overrideFns.delete(overrideFn);
+    this._combinedIsCurrent = false;
+  },
+
+  /**
+   * Retrieves the dynamically generated object implementing the integration
+   * methods. Platform-specific code and add-ons can override methods of this
+   * object using the "register" method.
+   */
+  getCombined(root) {
+    if (this._combinedIsCurrent) {
+      return this._combined;
+    }
+
+    // In addition to enumerating all the registered integration overrides in
+    // order, we want to keep any state that was previously stored in the
+    // combined object using the "this" reference in integration methods.
+    let overrideFnArray = [...this._overrideFns, () => this._combined];
+
+    let combined = root;
+    for (let overrideFn of overrideFnArray) {
+      try {
+        // Obtain a new set of methods from the next override function in the
+        // list, specifying the current combined object as the base argument.
+        let override = overrideFn(combined);
+
+        // Retrieve a list of property descriptors from the returned object, and
+        // use them to build a new combined object whose prototype points to the
+        // previous combined object.
+        let descriptors = {};
+        for (let name of Object.getOwnPropertyNames(override)) {
+          descriptors[name] = Object.getOwnPropertyDescriptor(override, name);
+        }
+        combined = Object.create(combined, descriptors);
+      } catch (ex) {
+        // Any error will result in the current override being skipped.
+        Cu.reportError(ex);
+      }
+    }
+
+    this._combinedIsCurrent = true;
+    return (this._combined = combined);
+  },
+
+  /**
+   * Defines a getter to retrieve the dynamically generated object implementing
+   * the integration methods, loading the root implementation lazily from the
+   * specified JSM module. For example:
+   *
+   *   Integration.test.defineModuleGetter(this, "TestIntegration",
+   *                    "resource://testing-common/TestIntegration.jsm");
+   *
+   * @param targetObject
+   *        The object on which the lazy getter will be defined.
+   * @param name
+   *        The name of the getter to define.
+   * @param moduleUrl
+   *        The URL used to obtain the module.
+   * @param symbol [optional]
+   *        The name of the symbol exported by the module. This can be omitted
+   *        if the name of the exported symbol is equal to the getter name.
+   */
+  defineModuleGetter(targetObject, name, moduleUrl, symbol) {
+    let moduleHolder = {};
+    XPCOMUtils.defineLazyModuleGetter(moduleHolder, name, moduleUrl, symbol);
+    Object.defineProperty(targetObject, name, {
+      get: () => this.getCombined(moduleHolder[name]),
+      configurable: true,
+      enumerable: true,
+    });
+  },
+};