Rename Toolkit's webextensions component directory to better reflect what it is.

1 files changed, 797 insertions, 0 deletions

diff --git a/toolkit/components/webextensions/MessageChannel.jsm b/toolkit/components/webextensions/MessageChannel.jsm
new file mode 100644
index 000000000..c5b326405
--- /dev/null
+++ b/toolkit/components/webextensions/MessageChannel.jsm
@@ -0,0 +1,797 @@
+/* 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 strict";
+
+/**
+ * This module provides wrappers around standard message managers to
+ * simplify bidirectional communication. It currently allows a caller to
+ * send a message to a single listener, and receive a reply. If there
+ * are no matching listeners, or the message manager disconnects before
+ * a reply is received, the caller is returned an error.
+ *
+ * The listener end may specify filters for the messages it wishes to
+ * receive, and the sender end likewise may specify recipient tags to
+ * match the filters.
+ *
+ * The message handler on the listener side may return its response
+ * value directly, or may return a promise, the resolution or rejection
+ * of which will be returned instead. The sender end likewise receives a
+ * promise which resolves or rejects to the listener's response.
+ *
+ *
+ * A basic setup works something like this:
+ *
+ * A content script adds a message listener to its global
+ * nsIContentFrameMessageManager, with an appropriate set of filters:
+ *
+ *  {
+ *    init(messageManager, window, extensionID) {
+ *      this.window = window;
+ *
+ *      MessageChannel.addListener(
+ *        messageManager, "ContentScript:TouchContent",
+ *        this);
+ *
+ *      this.messageFilterStrict = {
+ *        innerWindowID: getInnerWindowID(window),
+ *        extensionID: extensionID,
+ *      };
+ *
+ *      this.messageFilterPermissive = {
+ *        outerWindowID: getOuterWindowID(window),
+ *      };
+ *    },
+ *
+ *    receiveMessage({ target, messageName, sender, recipient, data }) {
+ *      if (messageName == "ContentScript:TouchContent") {
+ *        return new Promise(resolve => {
+ *          this.touchWindow(data.touchWith, result => {
+ *            resolve({ touchResult: result });
+ *          });
+ *        });
+ *      }
+ *    },
+ *  };
+ *
+ * A script in the parent process sends a message to the content process
+ * via a tab message manager, including recipient tags to match its
+ * filter, and an optional sender tag to identify itself:
+ *
+ *  let data = { touchWith: "pencil" };
+ *  let sender = { extensionID, contextID };
+ *  let recipient = { innerWindowID: tab.linkedBrowser.innerWindowID, extensionID };
+ *
+ *  MessageChannel.sendMessage(
+ *    tab.linkedBrowser.messageManager, "ContentScript:TouchContent",
+ *    data, {recipient, sender}
+ *  ).then(result => {
+ *    alert(result.touchResult);
+ *  });
+ *
+ * Since the lifetimes of message senders and receivers may not always
+ * match, either side of the message channel may cancel pending
+ * responses which match its sender or recipient tags.
+ *
+ * For the above client, this might be done from an
+ * inner-window-destroyed observer, when its target scope is destroyed:
+ *
+ *  observe(subject, topic, data) {
+ *    if (topic == "inner-window-destroyed") {
+ *      let innerWindowID = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
+ *
+ *      MessageChannel.abortResponses({ innerWindowID });
+ *    }
+ *  },
+ *
+ * From the parent, it may be done when its context is being destroyed:
+ *
+ *  onDestroy() {
+ *    MessageChannel.abortResponses({
+ *      extensionID: this.extensionID,
+ *      contextID: this.contextID,
+ *    });
+ *  },
+ *
+ */
+
+this.EXPORTED_SYMBOLS = ["MessageChannel"];
+
+/* globals MessageChannel */
+
+const Ci = Components.interfaces;
+const Cc = Components.classes;
+const Cu = Components.utils;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "ExtensionUtils",
+                                  "resource://gre/modules/ExtensionUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PromiseUtils",
+                                  "resource://gre/modules/PromiseUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Task",
+                                  "resource://gre/modules/Task.jsm");
+
+XPCOMUtils.defineLazyGetter(this, "MessageManagerProxy",
+                            () => ExtensionUtils.MessageManagerProxy);
+
+/**
+ * Handles the mapping and dispatching of messages to their registered
+ * handlers. There is one broker per message manager and class of
+ * messages. Each class of messages is mapped to one native message
+ * name, e.g., "MessageChannel:Message", and is dispatched to handlers
+ * based on an internal message name, e.g., "Extension:ExecuteScript".
+ */
+class FilteringMessageManager {
+  /**
+   * @param {string} messageName
+   *     The name of the native message this broker listens for.
+   * @param {function} callback
+   *     A function which is called for each message after it has been
+   *     mapped to its handler. The function receives two arguments:
+   *
+   *       result:
+   *         An object containing either a `handler` or an `error` property.
+   *         If no error occurs, `handler` will be a matching handler that
+   *         was registered by `addHandler`. Otherwise, the `error` property
+   *         will contain an object describing the error.
+   *
+   *        data:
+   *          An object describing the message, as defined in
+   *          `MessageChannel.addListener`.
+   * @param {nsIMessageListenerManager} messageManager
+   */
+  constructor(messageName, callback, messageManager) {
+    this.messageName = messageName;
+    this.callback = callback;
+    this.messageManager = messageManager;
+
+    this.messageManager.addMessageListener(this.messageName, this, true);
+
+    this.handlers = new Map();
+  }
+
+  /**
+   * Receives a message from our message manager, maps it to a handler, and
+   * passes the result to our message callback.
+   */
+  receiveMessage({data, target}) {
+    let handlers = Array.from(this.getHandlers(data.messageName, data.sender, data.recipient));
+
+    data.target = target;
+    this.callback(handlers, data);
+  }
+
+  /**
+   * Iterates over all handlers for the given message name. If `recipient`
+   * is provided, only iterates over handlers whose filters match it.
+   *
+   * @param {string|number} messageName
+   *     The message for which to return handlers.
+   * @param {object} sender
+   *     The sender data on which to filter handlers.
+   * @param {object} recipient
+   *     The recipient data on which to filter handlers.
+   */
+  * getHandlers(messageName, sender, recipient) {
+    let handlers = this.handlers.get(messageName) || new Set();
+    for (let handler of handlers) {
+      if (MessageChannel.matchesFilter(handler.messageFilterStrict || {}, recipient) &&
+          MessageChannel.matchesFilter(handler.messageFilterPermissive || {}, recipient, false) &&
+          (!handler.filterMessage || handler.filterMessage(sender, recipient))) {
+        yield handler;
+      }
+    }
+  }
+
+  /**
+   * Registers a handler for the given message.
+   *
+   * @param {string} messageName
+   *     The internal message name for which to register the handler.
+   * @param {object} handler
+   *     An opaque handler object. The object may have a
+   *     `messageFilterStrict` and/or a `messageFilterPermissive`
+   *     property and/or a `filterMessage` method on which to filter messages.
+   *
+   *     Final dispatching is handled by the message callback passed to
+   *     the constructor.
+   */
+  addHandler(messageName, handler) {
+    if (!this.handlers.has(messageName)) {
+      this.handlers.set(messageName, new Set());
+    }
+
+    this.handlers.get(messageName).add(handler);
+  }
+
+  /**
+   * Unregisters a handler for the given message.
+   *
+   * @param {string} messageName
+   *     The internal message name for which to unregister the handler.
+   * @param {object} handler
+   *     The handler object to unregister.
+   */
+  removeHandler(messageName, handler) {
+    this.handlers.get(messageName).delete(handler);
+  }
+}
+
+/**
+ * Manages mappings of message managers to their corresponding message
+ * brokers. Brokers are lazily created for each message manager the
+ * first time they are accessed. In the case of content frame message
+ * managers, they are also automatically destroyed when the frame
+ * unload event fires.
+ */
+class FilteringMessageManagerMap extends Map {
+  // Unfortunately, we can't use a WeakMap for this, because message
+  // managers do not support preserved wrappers.
+
+  /**
+   * @param {string} messageName
+   *     The native message name passed to `FilteringMessageManager` constructors.
+   * @param {function} callback
+   *     The message callback function passed to
+   *     `FilteringMessageManager` constructors.
+   */
+  constructor(messageName, callback) {
+    super();
+
+    this.messageName = messageName;
+    this.callback = callback;
+  }
+
+  /**
+   * Returns, and possibly creates, a message broker for the given
+   * message manager.
+   *
+   * @param {nsIMessageListenerManager} target
+   *     The message manager for which to return a broker.
+   *
+   * @returns {FilteringMessageManager}
+   */
+  get(target) {
+    if (this.has(target)) {
+      return super.get(target);
+    }
+
+    let broker = new FilteringMessageManager(this.messageName, this.callback, target);
+    this.set(target, broker);
+
+    if (target instanceof Ci.nsIDOMEventTarget) {
+      let onUnload = event => {
+        target.removeEventListener("unload", onUnload);
+        this.delete(target);
+      };
+      target.addEventListener("unload", onUnload);
+    }
+
+    return broker;
+  }
+}
+
+const MESSAGE_MESSAGE = "MessageChannel:Message";
+const MESSAGE_RESPONSE = "MessageChannel:Response";
+
+this.MessageChannel = {
+  init() {
+    Services.obs.addObserver(this, "message-manager-close", false);
+    Services.obs.addObserver(this, "message-manager-disconnect", false);
+
+    this.messageManagers = new FilteringMessageManagerMap(
+      MESSAGE_MESSAGE, this._handleMessage.bind(this));
+
+    this.responseManagers = new FilteringMessageManagerMap(
+      MESSAGE_RESPONSE, this._handleResponse.bind(this));
+
+    /**
+     * Contains a list of pending responses, either waiting to be
+     * received or waiting to be sent. @see _addPendingResponse
+     */
+    this.pendingResponses = new Set();
+  },
+
+  RESULT_SUCCESS: 0,
+  RESULT_DISCONNECTED: 1,
+  RESULT_NO_HANDLER: 2,
+  RESULT_MULTIPLE_HANDLERS: 3,
+  RESULT_ERROR: 4,
+  RESULT_NO_RESPONSE: 5,
+
+  REASON_DISCONNECTED: {
+    result: this.RESULT_DISCONNECTED,
+    message: "Message manager disconnected",
+  },
+
+  /**
+   * Specifies that only a single listener matching the specified
+   * recipient tag may be listening for the given message, at the other
+   * end of the target message manager.
+   *
+   * If no matching listeners exist, a RESULT_NO_HANDLER error will be
+   * returned. If multiple matching listeners exist, a
+   * RESULT_MULTIPLE_HANDLERS error will be returned.
+   */
+  RESPONSE_SINGLE: 0,
+
+  /**
+   * If multiple message managers matching the specified recipient tag
+   * are listening for a message, all listeners are notified, but only
+   * the first response or error is returned.
+   *
+   * Only handlers which return a value other than `undefined` are
+   * considered to have responded. Returning a Promise which evaluates
+   * to `undefined` is interpreted as an explicit response.
+   *
+   * If no matching listeners exist, a RESULT_NO_HANDLER error will be
+   * returned. If no listeners return a response, a RESULT_NO_RESPONSE
+   * error will be returned.
+   */
+  RESPONSE_FIRST: 1,
+
+  /**
+   * If multiple message managers matching the specified recipient tag
+   * are listening for a message, all listeners are notified, and all
+   * responses are returned as an array, once all listeners have
+   * replied.
+   */
+  RESPONSE_ALL: 2,
+
+  /**
+   * Fire-and-forget: The sender of this message does not expect a reply.
+   */
+  RESPONSE_NONE: 3,
+
+  /**
+   * Initializes message handlers for the given message managers if needed.
+   *
+   * @param {Array<nsIMessageListenerManager>} messageManagers
+   */
+  setupMessageManagers(messageManagers) {
+    for (let mm of messageManagers) {
+      // This call initializes a FilteringMessageManager for |mm| if needed.
+      // The FilteringMessageManager must be created to make sure that senders
+      // of messages that expect a reply, such as MessageChannel:Message, do
+      // actually receive a default reply even if there are no explicit message
+      // handlers.
+      this.messageManagers.get(mm);
+    }
+  },
+
+  /**
+   * Returns true if the properties of the `data` object match those in
+   * the `filter` object. Matching is done on a strict equality basis,
+   * and the behavior varies depending on the value of the `strict`
+   * parameter.
+   *
+   * @param {object} filter
+   *    The filter object to match against.
+   * @param {object} data
+   *    The data object being matched.
+   * @param {boolean} [strict=false]
+   *    If true, all properties in the `filter` object have a
+   *    corresponding property in `data` with the same value. If
+   *    false, properties present in both objects must have the same
+   *    value.
+   * @returns {boolean} True if the objects match.
+   */
+  matchesFilter(filter, data, strict = true) {
+    if (strict) {
+      return Object.keys(filter).every(key => {
+        return key in data && data[key] === filter[key];
+      });
+    }
+    return Object.keys(filter).every(key => {
+      return !(key in data) || data[key] === filter[key];
+    });
+  },
+
+  /**
+   * Adds a message listener to the given message manager.
+   *
+   * @param {nsIMessageListenerManager|Array<nsIMessageListenerManager>} targets
+   *    The message managers on which to listen.
+   * @param {string|number} messageName
+   *    The name of the message to listen for.
+   * @param {MessageReceiver} handler
+   *    The handler to dispatch to. Must be an object with the following
+   *    properties:
+   *
+   *      receiveMessage:
+   *        A method which is called for each message received by the
+   *        listener. The method takes one argument, an object, with the
+   *        following properties:
+   *
+   *          messageName:
+   *            The internal message name, as passed to `sendMessage`.
+   *
+   *          target:
+   *            The message manager which received this message.
+   *
+   *          channelId:
+   *            The internal ID of the transaction, used to map responses to
+   *            the original sender.
+   *
+   *          sender:
+   *            An object describing the sender, as passed to `sendMessage`.
+   *
+   *          recipient:
+   *            An object describing the recipient, as passed to
+   *            `sendMessage`.
+   *
+   *          data:
+   *            The contents of the message, as passed to `sendMessage`.
+   *
+   *        The method may return any structured-clone-compatible
+   *        object, which will be returned as a response to the message
+   *        sender. It may also instead return a `Promise`, the
+   *        resolution or rejection value of which will likewise be
+   *        returned to the message sender.
+   *
+   *      messageFilterStrict:
+   *        An object containing arbitrary properties on which to filter
+   *        received messages. Messages will only be dispatched to this
+   *        object if the `recipient` object passed to `sendMessage`
+   *        matches this filter, as determined by `matchesFilter` with
+   *        `strict=true`.
+   *
+   *      messageFilterPermissive:
+   *        An object containing arbitrary properties on which to filter
+   *        received messages. Messages will only be dispatched to this
+   *        object if the `recipient` object passed to `sendMessage`
+   *        matches this filter, as determined by `matchesFilter` with
+   *        `strict=false`.
+   *
+   *      filterMessage:
+   *        An optional function that prevents the handler from handling a
+   *        message by returning `false`. See `getHandlers` for the parameters.
+   */
+  addListener(targets, messageName, handler) {
+    for (let target of [].concat(targets)) {
+      this.messageManagers.get(target).addHandler(messageName, handler);
+    }
+  },
+
+  /**
+   * Removes a message listener from the given message manager.
+   *
+   * @param {nsIMessageListenerManager|Array<nsIMessageListenerManager>} targets
+   *    The message managers on which to stop listening.
+   * @param {string|number} messageName
+   *    The name of the message to stop listening for.
+   * @param {MessageReceiver} handler
+   *    The handler to stop dispatching to.
+   */
+  removeListener(targets, messageName, handler) {
+    for (let target of [].concat(targets)) {
+      if (this.messageManagers.has(target)) {
+        this.messageManagers.get(target).removeHandler(messageName, handler);
+      }
+    }
+  },
+
+  /**
+   * Sends a message via the given message manager. Returns a promise which
+   * resolves or rejects with the return value of the message receiver.
+   *
+   * The promise also rejects if there is no matching listener, or the other
+   * side of the message manager disconnects before the response is received.
+   *
+   * @param {nsIMessageSender} target
+   *    The message manager on which to send the message.
+   * @param {string} messageName
+   *    The name of the message to send, as passed to `addListener`.
+   * @param {object} data
+   *    A structured-clone-compatible object to send to the message
+   *    recipient.
+   * @param {object} [options]
+   *    An object containing any of the following properties:
+   * @param {object} [options.recipient]
+   *    A structured-clone-compatible object to identify the message
+   *    recipient. The object must match the `messageFilterStrict` and
+   *    `messageFilterPermissive` filters defined by recipients in order
+   *    for the message to be received.
+   * @param {object} [options.sender]
+   *    A structured-clone-compatible object to identify the message
+   *    sender. This object may also be used to avoid delivering the
+   *    message to the sender, and as a filter to prematurely
+   *    abort responses when the sender is being destroyed.
+   *    @see `abortResponses`.
+   * @param {integer} [options.responseType=RESPONSE_SINGLE]
+   *    Specifies the type of response expected. See the `RESPONSE_*`
+   *    contents for details.
+   * @returns {Promise}
+   */
+  sendMessage(target, messageName, data, options = {}) {
+    let sender = options.sender || {};
+    let recipient = options.recipient || {};
+    let responseType = options.responseType || this.RESPONSE_SINGLE;
+
+    let channelId = ExtensionUtils.getUniqueId();
+    let message = {messageName, channelId, sender, recipient, data, responseType};
+
+    if (responseType == this.RESPONSE_NONE) {
+      try {
+        target.sendAsyncMessage(MESSAGE_MESSAGE, message);
+      } catch (e) {
+        // Caller is not expecting a reply, so dump the error to the console.
+        Cu.reportError(e);
+        return Promise.reject(e);
+      }
+      return Promise.resolve();  // Not expecting any reply.
+    }
+
+    let deferred = PromiseUtils.defer();
+    deferred.sender = recipient;
+    deferred.messageManager = target;
+
+    this._addPendingResponse(deferred);
+
+    // The channel ID is used as the message name when routing responses.
+    // Add a message listener to the response broker, and remove it once
+    // we've gotten (or canceled) a response.
+    let broker = this.responseManagers.get(target);
+    broker.addHandler(channelId, deferred);
+
+    let cleanup = () => {
+      broker.removeHandler(channelId, deferred);
+    };
+    deferred.promise.then(cleanup, cleanup);
+
+    try {
+      target.sendAsyncMessage(MESSAGE_MESSAGE, message);
+    } catch (e) {
+      deferred.reject(e);
+    }
+    return deferred.promise;
+  },
+
+  _callHandlers(handlers, data) {
+    let responseType = data.responseType;
+
+    // At least one handler is required for all response types but
+    // RESPONSE_ALL.
+    if (handlers.length == 0 && responseType != this.RESPONSE_ALL) {
+      return Promise.reject({result: MessageChannel.RESULT_NO_HANDLER,
+                             message: "No matching message handler"});
+    }
+
+    if (responseType == this.RESPONSE_SINGLE) {
+      if (handlers.length > 1) {
+        return Promise.reject({result: MessageChannel.RESULT_MULTIPLE_HANDLERS,
+                               message: `Multiple matching handlers for ${data.messageName}`});
+      }
+
+      // Note: We use `new Promise` rather than `Promise.resolve` here
+      // so that errors from the handler are trapped and converted into
+      // rejected promises.
+      return new Promise(resolve => {
+        resolve(handlers[0].receiveMessage(data));
+      });
+    }
+
+    let responses = handlers.map(handler => {
+      try {
+        return handler.receiveMessage(data);
+      } catch (e) {
+        return Promise.reject(e);
+      }
+    });
+    responses = responses.filter(response => response !== undefined);
+
+    switch (responseType) {
+      case this.RESPONSE_FIRST:
+        if (responses.length == 0) {
+          return Promise.reject({result: MessageChannel.RESULT_NO_RESPONSE,
+                                 message: "No handler returned a response"});
+        }
+
+        return Promise.race(responses);
+
+      case this.RESPONSE_ALL:
+        return Promise.all(responses);
+    }
+    return Promise.reject({message: "Invalid response type"});
+  },
+
+  /**
+   * Handles dispatching message callbacks from the message brokers to their
+   * appropriate `MessageReceivers`, and routing the responses back to the
+   * original senders.
+   *
+   * Each handler object is a `MessageReceiver` object as passed to
+   * `addListener`.
+   *
+   * @param {Array<MessageHandler>} handlers
+   * @param {object} data
+   * @param {nsIMessageSender|{messageManager:nsIMessageSender}} data.target
+   */
+  _handleMessage(handlers, data) {
+    if (data.responseType == this.RESPONSE_NONE) {
+      handlers.forEach(handler => {
+        // The sender expects no reply, so dump any errors to the console.
+        new Promise(resolve => {
+          resolve(handler.receiveMessage(data));
+        }).catch(e => {
+          Cu.reportError(e.stack ? `${e}\n${e.stack}` : e.message || e);
+        });
+      });
+      // Note: Unhandled messages are silently dropped.
+      return;
+    }
+
+    let target = new MessageManagerProxy(data.target);
+
+    let deferred = {
+      sender: data.sender,
+      messageManager: target,
+    };
+    deferred.promise = new Promise((resolve, reject) => {
+      deferred.reject = reject;
+
+      this._callHandlers(handlers, data).then(resolve, reject);
+    }).then(
+      value => {
+        let response = {
+          result: this.RESULT_SUCCESS,
+          messageName: data.channelId,
+          recipient: {},
+          value,
+        };
+
+        target.sendAsyncMessage(MESSAGE_RESPONSE, response);
+      },
+      error => {
+        let response = {
+          result: this.RESULT_ERROR,
+          messageName: data.channelId,
+          recipient: {},
+          error: {},
+        };
+
+        if (error && typeof(error) == "object") {
+          if (error.result) {
+            response.result = error.result;
+          }
+          // Error objects are not structured-clonable, so just copy
+          // over the important properties.
+          for (let key of ["fileName", "filename", "lineNumber",
+                           "columnNumber", "message", "stack", "result"]) {
+            if (key in error) {
+              response.error[key] = error[key];
+            }
+          }
+        }
+
+        target.sendAsyncMessage(MESSAGE_RESPONSE, response);
+      }).catch(e => {
+        Cu.reportError(e);
+      }).then(() => {
+        target.dispose();
+      });
+
+    this._addPendingResponse(deferred);
+  },
+
+  /**
+   * Handles message callbacks from the response brokers.
+   *
+   * Each handler object is a deferred object created by `sendMessage`, and
+   * should be resolved or rejected based on the contents of the response.
+   *
+   * @param {Array<MessageHandler>} handlers
+   * @param {object} data
+   * @param {nsIMessageSender|{messageManager:nsIMessageSender}} data.target
+   */
+  _handleResponse(handlers, data) {
+    // If we have an error at this point, we have handler to report it to,
+    // so just log it.
+    if (handlers.length == 0) {
+      Cu.reportError(`No matching message response handler for ${data.messageName}`);
+    } else if (handlers.length > 1) {
+      Cu.reportError(`Multiple matching response handlers for ${data.messageName}`);
+    } else if (data.result === this.RESULT_SUCCESS) {
+      handlers[0].resolve(data.value);
+    } else {
+      handlers[0].reject(data.error);
+    }
+  },
+
+  /**
+   * Adds a pending response to the the `pendingResponses` list.
+   *
+   * The response object must be a deferred promise with the following
+   * properties:
+   *
+   *  promise:
+   *    The promise object which resolves or rejects when the response
+   *    is no longer pending.
+   *
+   *  reject:
+   *    A function which, when called, causes the `promise` object to be
+   *    rejected.
+   *
+   *  sender:
+   *    A sender object, as passed to `sendMessage.
+   *
+   *  messageManager:
+   *    The message manager the response will be sent or received on.
+   *
+   * When the promise resolves or rejects, it will be removed from the
+   * list.
+   *
+   * These values are used to clear pending responses when execution
+   * contexts are destroyed.
+   *
+   * @param {Deferred} deferred
+   */
+  _addPendingResponse(deferred) {
+    let cleanup = () => {
+      this.pendingResponses.delete(deferred);
+    };
+    this.pendingResponses.add(deferred);
+    deferred.promise.then(cleanup, cleanup);
+  },
+
+  /**
+   * Aborts any pending message responses to senders matching the given
+   * filter.
+   *
+   * @param {object} sender
+   *    The object on which to filter senders, as determined by
+   *    `matchesFilter`.
+   * @param {object} [reason]
+   *    An optional object describing the reason the response was aborted.
+   *    Will be passed to the promise rejection handler of all aborted
+   *    responses.
+   */
+  abortResponses(sender, reason = this.REASON_DISCONNECTED) {
+    for (let response of this.pendingResponses) {
+      if (this.matchesFilter(sender, response.sender)) {
+        response.reject(reason);
+      }
+    }
+  },
+
+  /**
+   * Aborts any pending message responses to the broker for the given
+   * message manager.
+   *
+   * @param {nsIMessageListenerManager} target
+   *    The message manager for which to abort brokers.
+   * @param {object} reason
+   *    An object describing the reason the responses were aborted.
+   *    Will be passed to the promise rejection handler of all aborted
+   *    responses.
+   */
+  abortMessageManager(target, reason) {
+    for (let response of this.pendingResponses) {
+      if (MessageManagerProxy.matches(response.messageManager, target)) {
+        response.reject(reason);
+      }
+    }
+  },
+
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "message-manager-close":
+      case "message-manager-disconnect":
+        try {
+          if (this.responseManagers.has(subject)) {
+            this.abortMessageManager(subject, this.REASON_DISCONNECTED);
+          }
+        } finally {
+          this.responseManagers.delete(subject);
+          this.messageManagers.delete(subject);
+        }
+        break;
+    }
+  },
+};
+
+MessageChannel.init();