Bug 1554075 - Move MediaDecoderStateMachine to its own thread pool. r=jya

The MDSM currently uses the "MediaPlayback" thread pool. This is the same
thread pool used by the demuxers. If all the threads in the pool are in
use demuxing, we can end up not being able to run the A/V sync logic in
the MDSM's VideoSink. This means we end up not presenting frames we could
have potentially presented.

So move the MDSM's TaskQueue to its own SharedThreadPool of size 1. This
should allow the state transition tasks to run more independently from
the demuxing tasks.

Differential Revision: https://phabricator.services.mozilla.com/D33869

--HG--
extra : moz-landing-system : lando

dom/media/MediaDecoderStateMachine.cpp

@@ -2615,7 +2615,7 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
       mAbstractMainThread(aDecoder->AbstractMainThread()),
       mFrameStats(&aDecoder->GetFrameStatistics()),
       mVideoFrameContainer(aDecoder->GetVideoFrameContainer()),
-      mTaskQueue(new TaskQueue(GetMediaThreadPool(MediaThreadType::PLAYBACK),
+      mTaskQueue(new TaskQueue(GetMediaThreadPool(MediaThreadType::MDSM),
                                "MDSM::mTaskQueue",
                                /* aSupportsTailDispatch = */ true)),
       mWatchManager(this, mTaskQueue),

dom/media/VideoUtils.cpp

@@ -206,6 +206,7 @@ bool IsValidVideoRegion(const gfx::IntSize& aFrame,
 
 already_AddRefed<SharedThreadPool> GetMediaThreadPool(MediaThreadType aType) {
   const char* name;
+  uint32_t threads = 4;
   switch (aType) {
     case MediaThreadType::PLATFORM_DECODER:
       name = "MediaPDecoder";
@@ -216,6 +217,10 @@ already_AddRefed<SharedThreadPool> GetMediaThreadPool(MediaThreadType aType) {
     case MediaThreadType::WEBRTC_DECODER:
       name = "WebRTCPD";
       break;
+    case MediaThreadType::MDSM:
+      name = "MediaDecoderStateMachine";
+      threads = 1;
+      break;
     default:
       MOZ_FALLTHROUGH_ASSERT("Unexpected MediaThreadType");
     case MediaThreadType::PLAYBACK:
@@ -223,9 +228,8 @@ already_AddRefed<SharedThreadPool> GetMediaThreadPool(MediaThreadType aType) {
       break;
   }
 
-  static const uint32_t kMediaThreadPoolDefaultCount = 4;
-  RefPtr<SharedThreadPool> pool = SharedThreadPool::Get(
-      nsDependentCString(name), kMediaThreadPoolDefaultCount);
+  RefPtr<SharedThreadPool> pool =
+      SharedThreadPool::Get(nsDependentCString(name), threads);
 
   // Ensure a larger stack for platform decoder threads
   if (aType == MediaThreadType::PLATFORM_DECODER) {

dom/media/VideoUtils.h

@@ -184,7 +184,8 @@ enum class MediaThreadType {
   PLAYBACK,          // MediaDecoderStateMachine and MediaFormatReader
   PLATFORM_DECODER,  // MediaDataDecoder
   MSG_CONTROL,
-  WEBRTC_DECODER
+  WEBRTC_DECODER,
+  MDSM,
 };
 // Returns the thread pool that is shared amongst all decoder state machines
 // for decoding streams.