1209408 - NotifyStoppedPlayback/NotifyStartedPlayback races with suspended AudioDestinationNode

Status: NEW

(Core :: Web Audio, defect, P3)

Description

[Karl Tomlinson (:karlt)]

AudioDestinationNode triggers NotifyStartedPlaying/NotifyStoppedPlaying from
two different threads which makes it difficult to keep the notifications in
order.  The following scenario appears possible:

1. An AudioContext is started on the main thread with an audio node to provide
   sound to the context.

2. suspend() is called on the audio context (main thread).
   This triggers NotifyStoppedPlayback() and sends a message to the
   rendering thread to suspend.

3. The rendering thread processes sound from the audio node and
   DestinationNodeEngine::ProcessBlock() dispatches InputMutedRunnable().

4. The rendering thread receives the suspend message and blocks/suspends the
   stream.  ProcessBlock() is not called again for this thread.

5. The main thread processes the InputMutedRunnable() and calls
   NotifyStartedPlayback().

The last notification is NotifyStartedPlayback, even
though the stream is suspended.

Comment 1

[Karl Tomlinson (:karlt)]

(In reply to Ehsan Akhgari (don't ask for review please) from bug 1198386 comment #547)
> Do you mind filing a bug with some details on how the stream can get
> suspended?

When a stream is suspended, it is removed from the mStreams array:
https://hg.mozilla.org/mozilla-central/annotate/031db40e2b558c7e4dd0b4c565db4a992c1627c8/dom/media/MediaStreamGraph.cpp#l2846

Processing of streams occurs only for streams in that array:
https://hg.mozilla.org/mozilla-central/annotate/031db40e2b558c7e4dd0b4c565db4a992c1627c8/dom/media/MediaStreamGraph.cpp#l975

The mechanism was somewhat different before bug 1189506 landed, but the
behavior was similar AIUI.

Comment 2

[(no longer active)]

Hmm, do we receive a notification on the main thread when a stream gets suspended?  What can cause this to happen?

Comment 3

[Karl Tomlinson (:karlt)]

The graph doesn't provide any stream-specific notification, but there is a StateChangeTask for the AudioContext, in reply to the message generated in ApplyAudioContextOperation().

https://hg.mozilla.org/mozilla-central/annotate/acdb22976ff86539dc10413c5f366e1fb429a680/dom/media/MediaStreamGraph.cpp#l2903

Comment 4

[Bulk Bug Changes for mreavy's org]

Mass change P2->P3 to align with new Mozilla triage process.