Ensure that sendMessage's callback is called.

extensions/renderer/resources/messaging.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // chrome.runtime.messaging API implementation.
 
   // TODO(kalman): factor requiring chrome out of here.
   var chrome = requireNative('chrome').GetChrome();
   var Event = require('event_bindings').Event;
   var lastError = require('lastError');
@@ skipping 317 matching lines @@
 
     if (port.name == kMessageChannel && !responseCallback) {
       // TODO(mpcomplete): Do this for the old sendRequest API too, after
       // verifying it doesn't break anything.
       // Go ahead and disconnect immediately if the sender is not expecting
       // a response.
       port.disconnect();
       return;
     }
 
-    // Ensure the callback exists for the older sendRequest API.
-    if (!responseCallback)
-      responseCallback = function() {};
+    function sendResponseAndClearCallback(response) {
+      // Save a reference so that we don't re-entrantly call responseCallback.
+      var sendResponse = responseCallback;
+      responseCallback = null;
+      sendResponse(response);
+    }
+
 
     // Note: make sure to manually remove the onMessage/onDisconnect listeners
     // that we added before destroying the Port, a workaround to a bug in Port
     // where any onMessage/onDisconnect listeners added but not removed will
     // be leaked when the Port is destroyed.
     // http://crbug.com/320723 tracks a sustainable fix.
 
     function disconnectListener() {
-      // For onDisconnects, we only notify the callback if there was an error.
-      if (chrome.runtime && chrome.runtime.lastError)
-        responseCallback();
+      if (!responseCallback)

[Devlin, 2016/01/08 18:20:01]

nit: Maybe add a super short comment.
if (!responseCallback)  // Callback was already called; our work is done.

[robwu, 2016/01/08 18:22:44]

Or the user did not supply a callback in the first place (the callback is
optional). Anyway, this is so trivial that I think that a comment is not
necessary.

[Devlin, 2016/01/08 21:48:09]

Fair enough.

+        return;
+
+      if (lastError.hasError(chrome)) {
+        sendResponseAndClearCallback();
+      } else {
+        lastError.set(
+            port.name, 'The message port closed before a reponse was received.',
+            null, chrome);
+        try {
+          sendResponseAndClearCallback();
+        } finally {
+          lastError.clear(chrome);
+        }
+      }
     }
 
     function messageListener(response) {
       try {
-        responseCallback(response);
+        if (responseCallback)
+          sendResponseAndClearCallback(response);
       } finally {
         port.disconnect();
       }
     }
 
     privates(port).impl.onDestroy_ = function() {
       port.onDisconnect.removeListener(disconnectListener);
       port.onMessage.removeListener(messageListener);
     };
     port.onDisconnect.addListener(disconnectListener);
@@ skipping 26 matching lines @@
 exports.$set('Port', Port);
 exports.$set('createPort', createPort);
 exports.$set('sendMessageImpl', sendMessageImpl);
 exports.$set('sendMessageUpdateArguments', sendMessageUpdateArguments);
 
 // For C++ code to call.
 exports.$set('hasPort', hasPort);
 exports.$set('dispatchOnConnect', dispatchOnConnect);
 exports.$set('dispatchOnDisconnect', dispatchOnDisconnect);
 exports.$set('dispatchOnMessage', dispatchOnMessage);