Issue 3499 - Create a clean messaging API for internal use

lib/messaging.js

Index: lib/messaging.js
===================================================================
new file mode 100644
--- /dev/null
+++ b/lib/messaging.js
@@ -0,0 +1,316 @@
+/*
+ * This file is part of Adblock Plus <https://adblockplus.org/>,
+ * Copyright (C) 2006-2016 Eyeo GmbH
+ *
+ * Adblock Plus is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * Adblock Plus is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+"use strict";
+
+const MESSAGE_NAME = "AdblockPlus:Message";
+const RESPONSE_NAME = "AdblockPlus:Response";
+
+function isPromise(value)
+{
+  // value instanceof Promise won't work - there can be different Promise
+  // classes (e.g. in different contexts) and there can also be promise-like
+  // classes (e.g. Task).
+  return (value && typeof value.then == "function");
+}
+
+function sendMessage(messageManager, messageName, payload, callbackID)
+{
+  let request = {messageName, payload, callbackID};
+  if (messageManager instanceof Ci.nsIMessageSender)
+  {
+    messageManager.sendAsyncMessage(MESSAGE_NAME, request);
+    return 1;
+  }
+  else if (messageManager instanceof Ci.nsIMessageBroadcaster)
+  {
+    messageManager.broadcastAsyncMessage(MESSAGE_NAME, request);
+    return messageManager.childCount;
+  }
+  else
+  {
+    Cu.reportError("Unexpected message manager, impossible to send message");
+    return 0;
+  }
+}
+
+function sendSyncMessage(messageManager, messageName, payload)
+{
+  let request = {messageName, payload};
+  let responses = messageManager.sendRpcMessage(MESSAGE_NAME, request);
+  return flattenResponses(responses, messageName);
+}
+
+function flattenResponses(responses, messageName)
+{
+  let result = undefined;
+  for (let response of responses)
+  {
+    if (typeof response == "undefined")
+      continue;
+
+    if (typeof result == "undefined")
+      result = response;
+    else
+      Cu.reportError("Got multiple responses to message '" + messageName + "', only first response was accepted.");
+  }
+  return result;
+}
+
+function getSender(origin)
+{
+  if (origin instanceof Ci.nsIDOMXULElement)
+    origin = origin.messageManager;
+
+  if (origin instanceof Ci.nsIMessageSender)
+    return new LightWeightPort(origin);
+  else
+    return null;
+}
+
+/**
+ * Lightweight communication port allowing only sending messages.
+ * @param {nsIMessageManager} messageManager
+ * @constructor
+ */
+function LightWeightPort(messageManager)
+{
+  this._messageManager = messageManager;
+}
+LightWeightPort.prototype =
+{
+  /**
+   * @see Port#emit
+   */
+  emit: function(messageName, payload)
+  {
+    sendMessage(this._messageManager, messageName, payload);
+  },
+
+  /**
+   * @see Port#emitSync
+   */
+  emitSync: function(messageName, payload)
+  {
+    return sendSyncMessage(this._messageManager, messageName, payload);
+  }
+};
+
+/**
+ * Communication port wrapping the message manager API to send and receive
+ * messages.
+ * @param {nsIMessageManager} messageManager
+ * @constructor
+ */
+function Port(messageManager)
+{
+  this._messageManager = messageManager;
+
+  this._callbacks = new Map();
+  this._responseCallbacks = new Map();
+  this._responseCallbackCounter = 0;
+
+  this._handleRequest = this._handleRequest.bind(this);
+  this._handleResponse = this._handleResponse.bind(this);
+  this._messageManager.addMessageListener(MESSAGE_NAME, this._handleRequest);
+  this._messageManager.addMessageListener(RESPONSE_NAME, this._handleResponse);
+}
+Port.prototype = {
+  /**
+   * Disables the port and makes it stop listening to incoming messages.
+   */
+  disconnect: function()
+  {
+    this._messageManager.removeMessageListener(MESSAGE_NAME, this._handleRequest);
+    this._messageManager.removeMessageListener(RESPONSE_NAME, this._handleResponse);
+  },
+
+  _sendResponse: function(sender, callbackID, payload)
+  {
+    if (!sender || typeof callbackID == "undefined")
+      return;
+
+    let response = {callbackID, payload};
+    sender._messageManager.sendAsyncMessage(RESPONSE_NAME, response);
+  },
+
+  _handleRequest: function(message)
+  {
+    let sender = getSender(message.target);
+    let {callbackID, messageName, payload} = message.data;
+
+    let result = this._dispatch(messageName, payload, sender);
+    if (isPromise(result))
+    {
+      // This is a promise - asynchronous response
+      if (message.sync)
+      {
+        Cu.reportError("Asynchronous response to the synchronous message '" + messageName + "' is not possible");
+        return undefined;
+      }
+
+      result.then(result =>
+      {
+        this._sendResponse(sender, callbackID, result)
+      }, e =>
+      {
+        Cu.reportError(e);
+        this._sendResponse(sender, callbackID, undefined);
+      });
+    }
+    else
+      this._sendResponse(sender, callbackID, result);
+
+    return result;
+  },
+
+  _handleResponse: function(message)
+  {
+    let {callbackID, payload} = message.data;
+    if (!this._responseCallbacks.has(callbackID))
+      return;
+
+    let [callback, messageName, responses, expectedResponses] =
+        this._responseCallbacks.get(callbackID);
+    responses.push(payload);
+    if (responses.length == expectedResponses)
+    {
+      this._responseCallbacks.delete(callbackID);
+      callback(flattenResponses(responses, messageName));
+    }
+  },
+
+  _dispatch: function(messageName, payload, sender)
+  {
+    let callbacks = this._callbacks.get(messageName);
+    if (!callbacks)
+      return undefined;
+
+    callbacks = callbacks.slice();
+    let responses = [];
+    for (let callback of callbacks)
+    {
+      try
+      {
+        responses.push(callback(payload, sender));
+      }
+      catch (e)
+      {
+        Cu.reportError(e);
+      }
+    }
+    return flattenResponses(responses, messageName);
+  },
+
+  /**
+   * Function to be called when a particular message is received
+   * @callback Port~messageHandler
+   * @param payload data attached to the message if any
+   * @param {LightWeightPort} sender object that can be used to communicate with
+   *      the sender of the message, could be null
+   * @return the handler can return undefined (no response), a value (response
+   *      to be sent to sender immediately) or a promise (asynchronous
+   *      response).
+   */
+
+  /**
+   * Adds a handler for the specified message.
+   * @param {string} messageName message that would trigger the callback
+   * @param {Port~messageHandler} callback
+   */
+  on: function(messageName, callback)
+  {
+    let callbacks = this._callbacks.get(messageName);
+    if (callbacks)
+      callbacks.push(callback);
+    else
+      this._callbacks.set(messageName, [callback]);
+  },
+
+  /**
+   * Removes a handler for the specified message.
+   * @param {string} messageName message that would trigger the callback
+   * @param {Port~messageHandler} callback
+   */
+  off: function(messageName, callback)
+  {
+    let callbacks = this._callbacks.get(messageName);
+    if (!callbacks)
+      return;
+
+    let index = callbacks.indexOf(callback);
+    if (index >= 0)
+      callbacks.splice(index, 1);
+  },
+
+  /**
+   * Sends a message.
+   * @param {string} messageName message identifier
+   * @param [payload] data to attach to the message
+   */
+  emit: function(messageName, payload)
+  {
+    sendMessage(this._messageManager, messageName, payload, undefined);
+  },
+
+  /**
+   * Sends a message and expects a response.
+   * @param {string} messageName message identifier
+   * @param [payload] data to attach to the message
+   * @return {Promise} promise that will be resolved with the response
+   */
+  emitWithResponse: function(messageName, payload)
+  {
+    let callbackID = ++this._responseCallbackCounter;
+    let expectedResponses = sendMessage(
+        this._messageManager, messageName, payload, callbackID);
+    return new Promise((resolve, reject) =>
+    {
+      this._responseCallbacks.set(callbackID,
+          [resolve, messageName, [], expectedResponses]);
+    });
+  },
+
+  /**
+   * Sends a synchonous message (DO NOT USE unless absolutely unavoidable).
+   * @param {string} messageName message identifier
+   * @param [payload] data to attach to the message
+   * @return response returned by the handler
+   */
+  emitSync: function(messageName, payload)
+  {
+    return sendSyncMessage(this._messageManager, messageName, payload);
+  }
+};
+exports.Port = Port;
+
+let messageManager;
+try
+{
+  // Child
+  messageManager = require("messageManager");
+}
+catch (e)
+{
+  // Parent
+  messageManager = Cc["@mozilla.org/parentprocessmessagemanager;1"]
+                     .getService(Ci.nsIMessageListenerManager);
+}
+
+let port = new Port(messageManager);
+onShutdown.add(() => port.disconnect());
+exports.port = port;