Bug 1159974 - Dispatch SetMinimizePrerollUntilPlabackStarts. r=jww

dom/media/MediaDecoder.cpp

@@ -751,14 +751,19 @@ void MediaDecoder::SetStateMachineParameters()
     mDecoderStateMachine->DispatchAudioCaptured();
   }
   if (mMinimizePreroll) {
-    mDecoderStateMachine->SetMinimizePrerollUntilPlaybackStarts();
+    mDecoderStateMachine->DispatchMinimizePrerollUntilPlaybackStarts();
   }
 }
 
 void MediaDecoder::SetMinimizePrerollUntilPlaybackStarts()
 {
+  DECODER_LOG("SetMinimizePrerollUntilPlaybackStarts()");
   MOZ_ASSERT(NS_IsMainThread());
   mMinimizePreroll = true;
+
+  // This needs to be called before we init the state machine, otherwise it will
+  // have no effect.
+  MOZ_DIAGNOSTIC_ASSERT(!mDecoderStateMachine);
 }
 
 nsresult MediaDecoder::ScheduleStateMachineThread()

dom/media/MediaDecoderStateMachine.cpp

@@ -3416,14 +3416,6 @@ void MediaDecoderStateMachine::PreservesPitchChanged()
   }
 }
 
-void
-MediaDecoderStateMachine::SetMinimizePrerollUntilPlaybackStarts()
-{
-  AssertCurrentThreadInMonitor();
-  DECODER_LOG("SetMinimizePrerollUntilPlaybackStarts()");
-  mMinimizePreroll = true;
-}
-
 bool MediaDecoderStateMachine::IsShutdown()
 {
   AssertCurrentThreadInMonitor();

dom/media/MediaDecoderStateMachine.h

@@ -399,7 +399,21 @@ public:
   // the state machine is free to return to prerolling normally. Note
   // "prerolling" in this context refers to when we decode and buffer decoded
   // samples in advance of when they're needed for playback.
-  void SetMinimizePrerollUntilPlaybackStarts();
+  void DispatchMinimizePrerollUntilPlaybackStarts()
+  {
+    nsRefPtr<MediaDecoderStateMachine> self = this;
+    nsCOMPtr<nsIRunnable> r = NS_NewRunnableFunction([self] () -> void
+    {
+      MOZ_ASSERT(self->OnTaskQueue());
+      ReentrantMonitorAutoEnter mon(self->mDecoder->GetReentrantMonitor());
+      self->mMinimizePreroll = true;
+
+      // Make sure that this arrives before playback starts, otherwise this won't
+      // have the intended effect.
+      MOZ_DIAGNOSTIC_ASSERT(self->mPlayState == MediaDecoder::PLAY_STATE_LOADING);
+    });
+    TaskQueue()->Dispatch(r.forget());
+  }
 
   void OnAudioDecoded(AudioData* aSample);
   void OnVideoDecoded(VideoData* aSample);