create one WebAudioSource for each CreateMediaStreamSource() call.

Source/modules/webaudio/AudioContext.cpp

 /*
  * Copyright (C) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 375 matching lines @@
     if (!mediaStream) {
         exceptionState.throwDOMException(
             InvalidStateError,
             "invalid MediaStream source");
         return nullptr;
     }
 
     ASSERT(isMainThread());
     lazyInitialize();
 
-    AudioSourceProvider* provider = 0;
-
     MediaStreamTrackVector audioTracks = mediaStream->getAudioTracks();
-    RefPtr<MediaStreamTrack> audioTrack;
-
-    // FIXME: get a provider for non-local MediaStreams (like from a remote peer).
-    for (size_t i = 0; i < audioTracks.size(); ++i) {
-        audioTrack = audioTracks[i];
-        if (audioTrack->component()->audioSourceProvider()) {
-            provider = audioTrack->component()->audioSourceProvider();
-            break;
-        }
+    if (audioTracks.isEmpty()) {
+        exceptionState.throwDOMException(
+            InvalidStateError,
+            "MediaStream has no audio track");
+        return nullptr;
     }
 
-    RefPtr<MediaStreamAudioSourceNode> node = MediaStreamAudioSourceNode::create(this, mediaStream, audioTrack.get(), provider);
+    // Use the first audio track in the media stream.
+    RefPtr<MediaStreamTrack> audioTrack = audioTracks[0];
+    OwnPtr<AudioSourceProvider> provider = audioTrack->createWebAudioSource();

[Raymond Toy, 2014/03/25 22:35:41]

Are lines 405-406 here the same as the original lines 402-408? The original
finds the first entry in audioTracks[] that has an audioSourceProvider, but the
new code just uses audioTracks[0]. Are these the same?

[no longer working on chromium, 2014/03/26 13:31:11]

Yes, they are the same.
Chrome guarantees that the all the local track will have a source provider. So
the previous code just uses the source provider in the first track of the
stream. And there will be no source provider on the remote track.

The new code will use the first track to create a source provider.
For the local audio track, we create the source provider; but for remote audio
track, it will be NULL since we don't support it yet.

If there is only on CreateMediaStreamSource(), the behaviour will be exactly the
same as before; if there are multiple, the new code will work while the old code
won't.

[Raymond Toy, 2014/03/26 17:04:46]

Thanks for the very detailed explanation. This sounds excellent.

+    RefPtr<MediaStreamAudioSourceNode> node = MediaStreamAudioSourceNode::create(this, mediaStream, audioTrack.get(), provider.release());
 
     // FIXME: Only stereo streams are supported right now. We should be able to accept multi-channel streams.
     node->setFormat(2, sampleRate());
 
     refNode(node.get()); // context keeps reference until node is disconnected
     return node;
 }
 
 PassRefPtr<MediaStreamAudioDestinationNode> AudioContext::createMediaStreamDestination()
 {
@@ skipping 617 matching lines @@
 }
 
 void AudioContext::decrementActiveSourceCount()
 {
     atomicDecrement(&m_activeSourceCount);
 }
 
 } // namespace WebCore
 
 #endif // ENABLE(WEB_AUDIO)