Bug 1274221 - Ensure MediaStreamListeners are always notified of created and ended tracks. r=ctai, r=jesup

This means that when a MediaStreamListener is added to a stream, we'll call
NotifyQueuedTrackChanges with TRACK_EVENT_CREATE for all tracks that already
exist.

Likewise, we'll call NotifyQueuedTrackChanges with TRACK_EVENT_ENDED for all
tracks that exist and have ended.

This fixes potential race conditions where a track was created and/or ended
before the listener was asynchronously added.

MozReview-Commit-ID: G3juhfiZMtg

--HG--
extra : rebase_source : 6f4fe68b888b6dd0d9ee370b6a390ff17da5f1de
extra : source : 70e8c5a785a9866e6205b54bd1c45faaf834717d

dom/media/MediaStreamGraph.cpp

@@ -2380,6 +2380,31 @@ MediaStream::AddListenerImpl(already_AddRefed<MediaStreamListener> aListener)
   MediaStreamListener* listener = *mListeners.AppendElement() = aListener;
   listener->NotifyBlockingChanged(GraphImpl(),
     mNotifiedBlocked ? MediaStreamListener::BLOCKED : MediaStreamListener::UNBLOCKED);
+
+  for (StreamTracks::TrackIter it(mTracks); !it.IsEnded(); it.Next()) {
+    MediaStream* inputStream = nullptr;
+    TrackID inputTrackID = TRACK_INVALID;
+    if (ProcessedMediaStream* ps = AsProcessedStream()) {
+      // The only ProcessedMediaStream where we should have listeners is
+      // TrackUnionStream - it's what's used as owned stream in DOMMediaStream,
+      // the only main-thread exposed stream type.
+      // TrackUnionStream guarantees that each of its tracks has an input track.
+      // Other types do not implement GetInputStreamFor() and will return null.
+      inputStream = ps->GetInputStreamFor(it->GetID());
+      MOZ_ASSERT(inputStream);
+      inputTrackID = ps->GetInputTrackIDFor(it->GetID());
+      MOZ_ASSERT(IsTrackIDExplicit(inputTrackID));
+    }
+
+    uint32_t flags = MediaStreamListener::TRACK_EVENT_CREATED;
+    if (it->IsEnded()) {
+      flags |= MediaStreamListener::TRACK_EVENT_ENDED;
+    }
+    nsAutoPtr<MediaSegment> segment(it->GetSegment()->CreateEmptyClone());
+    listener->NotifyQueuedTrackChanges(Graph(), it->GetID(), it->GetEnd(),
+                                       flags, *segment,
+                                       inputStream, inputTrackID);
+  }
   if (mNotifiedFinished) {
     listener->NotifyEvent(GraphImpl(), MediaStreamListener::EVENT_FINISHED);
   }

dom/media/MediaStreamGraph.h

@@ -1439,6 +1439,8 @@ public:
   {
     return mInputs.Length();
   }
+  virtual MediaStream* GetInputStreamFor(TrackID aTrackID) { return nullptr; }
+  virtual TrackID GetInputTrackIDFor(TrackID aTrackID) { return TRACK_NONE; }
   void DestroyImpl() override;
   /**
    * This gets called after we've computed the blocking states for all

dom/media/TrackUnionStream.cpp

@@ -374,6 +374,30 @@ TrackUnionStream::SetTrackEnabledImpl(TrackID aTrackID, bool aEnabled) {
   MediaStream::SetTrackEnabledImpl(aTrackID, aEnabled);
 }
 
+MediaStream*
+TrackUnionStream::GetInputStreamFor(TrackID aTrackID)
+{
+  for (TrackMapEntry& entry : mTrackMap) {
+    if (entry.mOutputTrackID == aTrackID && entry.mInputPort) {
+      return entry.mInputPort->GetSource();
+    }
+  }
+
+  return nullptr;
+}
+
+TrackID
+TrackUnionStream::GetInputTrackIDFor(TrackID aTrackID)
+{
+  for (TrackMapEntry& entry : mTrackMap) {
+    if (entry.mOutputTrackID == aTrackID) {
+      return entry.mInputTrackID;
+    }
+  }
+
+  return TRACK_NONE;
+}
+
 void
 TrackUnionStream::AddDirectTrackListenerImpl(already_AddRefed<MediaStreamTrackDirectListener> aListener,
                                              TrackID aTrackID)

dom/media/TrackUnionStream.h

@@ -23,6 +23,9 @@ public:
 
   void SetTrackEnabledImpl(TrackID aTrackID, bool aEnabled) override;
 
+  MediaStream* GetInputStreamFor(TrackID aTrackID) override;
+  TrackID GetInputTrackIDFor(TrackID aTrackID) override;
+
 protected:
   // Only non-ended tracks are allowed to persist in this map.
   struct TrackMapEntry {