Bug 907162 - Fix MediaDecoderStateMachine might dispatch MediaDecoder::PlaybackEnded more than once and trigger multiple 'ended' events in HTMLMediaElement. r=cpearce

content/media/MediaDecoderStateMachine.cpp

@@ -1521,7 +1521,7 @@ void
 MediaDecoderStateMachine::DispatchDecodeTasksIfNeeded()
 {
   AssertCurrentThreadInMonitor();
-  
+
   // NeedToDecodeAudio() can go from false to true while we hold the
   // monitor, but it can't go from true to false. This can happen because
   // NeedToDecodeAudio() takes into account the amount of decoded audio
@@ -2274,9 +2274,16 @@ nsresult MediaDecoderStateMachine::RunStateMachine()
         int64_t clockTime = std::max(mEndTime, std::max(videoTime, GetAudioClock()));
         UpdatePlaybackPosition(clockTime);
 
-        nsCOMPtr<nsIRunnable> event =
-          NS_NewRunnableMethod(mDecoder, &MediaDecoder::PlaybackEnded);
-        NS_DispatchToMainThread(event, NS_DISPATCH_NORMAL);
+        {
+          // Wait for the state change is completed in the main thread,
+          // otherwise we might see |mDecoder->GetState() == MediaDecoder::PLAY_STATE_PLAYING|
+          // in next loop and send |MediaDecoder::PlaybackEnded| again to trigger 'ended'
+          // event twice in the media element.
+          ReentrantMonitorAutoExit exitMon(mDecoder->GetReentrantMonitor());
+          nsCOMPtr<nsIRunnable> event =
+            NS_NewRunnableMethod(mDecoder, &MediaDecoder::PlaybackEnded);
+          NS_DispatchToMainThread(event, NS_DISPATCH_SYNC);
+        }
       }
       return NS_OK;
     }
@@ -2756,6 +2763,12 @@ nsresult MediaDecoderStateMachine::ScheduleStateMachine(int64_t aUsecs) {
     return NS_OK;
   }
 
+  // Since there is already a pending task that will run immediately,
+  // we don't need to schedule a timer task.
+  if (mRunAgain) {
+    return NS_OK;
+  }
+
   mTimeout = timeout;
 
   nsresult res;