Bug 1188643. Buffer more audio in audio capture mode to avoid glitches. r=cpearce.

dom/media/MediaDecoderStateMachine.cpp

@@ -205,7 +205,7 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mQuickBufferingLowDataThresholdUsecs(detail::QUICK_BUFFERING_LOW_DATA_USECS),
   mIsAudioPrerolling(false),
   mIsVideoPrerolling(false),
-  mAudioCaptured(false),
+  mAudioCaptured(false, "MediaDecoderStateMachine::mAudioCaptured"),
   mAudioCompleted(false, "MediaDecoderStateMachine::mAudioCompleted"),
   mNotifyMetadataBeforeFirstFrame(false),
   mDispatchedEventToDecode(false),
@@ -219,7 +219,7 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mCorruptFrames(60),
   mDecodingFirstFrame(true),
   mSentLoadedMetadataEvent(false),
-  mSentFirstFrameLoadedEvent(false),
+  mSentFirstFrameLoadedEvent(false, "MediaDecoderStateMachine::mSentFirstFrameLoadedEvent"),
   mSentPlaybackEndedEvent(false),
   mStreamSink(new DecodedStream(mTaskQueue, mAudioQueue, mVideoQueue)),
   mResource(aDecoder->GetResource()),
@@ -350,6 +350,8 @@ MediaDecoderStateMachine::InitializationTask()
   mWatchManager.Watch(mPlayState, &MediaDecoderStateMachine::PlayStateChanged);
   mWatchManager.Watch(mLogicallySeeking, &MediaDecoderStateMachine::LogicallySeekingChanged);
   mWatchManager.Watch(mSameOriginMedia, &MediaDecoderStateMachine::SameOriginMediaChanged);
+  mWatchManager.Watch(mSentFirstFrameLoadedEvent, &MediaDecoderStateMachine::AdjustAudioThresholds);
+  mWatchManager.Watch(mAudioCaptured, &MediaDecoderStateMachine::AdjustAudioThresholds);
 
   // Propagate mSameOriginMedia to mDecodedStream.
   SameOriginMediaChanged();
@@ -2068,6 +2070,28 @@ MediaDecoderStateMachine::IsDecodingFirstFrame()
   return mState == DECODER_STATE_DECODING && mDecodingFirstFrame;
 }
 
+void
+MediaDecoderStateMachine::AdjustAudioThresholds()
+{
+  MOZ_ASSERT(OnTaskQueue());
+  AssertCurrentThreadInMonitor();
+
+  // Experiments show that we need to buffer more if audio is captured to avoid
+  // audio glitch. See bug 1188643 comment 16 for the details.
+  auto divisor = mAudioCaptured ? NO_VIDEO_AMPLE_AUDIO_DIVISOR / 2
+                                : NO_VIDEO_AMPLE_AUDIO_DIVISOR;
+
+  // We're playing audio only. We don't need to worry about slow video
+  // decodes causing audio underruns, so don't buffer so much audio in
+  // order to reduce memory usage.
+  if (HasAudio() && !HasVideo() && mSentFirstFrameLoadedEvent) {
+    mAmpleAudioThresholdUsecs = detail::AMPLE_AUDIO_USECS / divisor;
+    mLowAudioThresholdUsecs = detail::LOW_AUDIO_USECS / divisor;
+    mQuickBufferingLowDataThresholdUsecs =
+      detail::QUICK_BUFFERING_LOW_DATA_USECS / divisor;
+  }
+}
+
 void
 MediaDecoderStateMachine::FinishDecodeFirstFrame()
 {
@@ -2088,15 +2112,6 @@ MediaDecoderStateMachine::FinishDecodeFirstFrame()
               "transportSeekable=%d, mediaSeekable=%d",
               Duration().ToMicroseconds(), mResource->IsTransportSeekable(), mMediaSeekable.Ref());
 
-  if (HasAudio() && !HasVideo() && !mSentFirstFrameLoadedEvent) {
-    // We're playing audio only. We don't need to worry about slow video
-    // decodes causing audio underruns, so don't buffer so much audio in
-    // order to reduce memory usage.
-    mAmpleAudioThresholdUsecs /= NO_VIDEO_AMPLE_AUDIO_DIVISOR;
-    mLowAudioThresholdUsecs /= NO_VIDEO_AMPLE_AUDIO_DIVISOR;
-    mQuickBufferingLowDataThresholdUsecs /= NO_VIDEO_AMPLE_AUDIO_DIVISOR;
-  }
-
   // Get potentially updated metadata
   {
     ReentrantMonitorAutoExit exitMon(mDecoder->GetReentrantMonitor());

dom/media/MediaDecoderStateMachine.h

@@ -654,6 +654,8 @@ private:
   // play time.
   bool NeedToSkipToNextKeyframe();
 
+  void AdjustAudioThresholds();
+
   // The decoder object that created this state machine. The state machine
   // holds a strong reference to the decoder to ensure that the decoder stays
   // alive once media element has started the decoder shutdown process, and has
@@ -1136,7 +1138,7 @@ private:
   // True if we shouldn't play our audio (but still write it to any capturing
   // streams). When this is true, the audio thread will never start again after
   // it has stopped.
-  bool mAudioCaptured;
+  Watchable<bool> mAudioCaptured;
 
   // True if the audio playback thread has finished. It is finished
   // when either all the audio frames have completed playing, or we've moved
@@ -1228,7 +1230,7 @@ private:
   // SetStartTime because the mStartTime already set before. Also we don't need
   // to decode any audio/video since the MediaDecoder will trigger a seek
   // operation soon.
-  bool mSentFirstFrameLoadedEvent;
+  Watchable<bool> mSentFirstFrameLoadedEvent;
 
   bool mSentPlaybackEndedEvent;