b=987976 make AudioNodeExternalInputStream output channel count match stream, unless > MaxChannelCount r=padenot

The difference from Blink here is that Blink plays silence for if element
channel counts are > 32, but here more channels are down-mixed.  Media stream
channel counts are also fixed to 2 in Blink, but that restriction is not
applied here.

Leaving the "inline" const static/class member initialization of
MaxChannelCount left missing symbols with gcc 4.7.3.

--HG--
extra : transplant_source : %C9%BA%84%0F%E8%89%E2%85p%B8%28%EF%E9M%CC%81%B9ob/

content/media/AudioNodeExternalInputStream.cpp

@@ -58,7 +58,8 @@ AudioNodeExternalInputStream::GetTrackMapEntry(const StreamBuffer::Track& aTrack
   // Create a speex resampler with the same sample rate and number of channels
   // as the track.
   SpeexResamplerState* resampler = nullptr;
-  uint32_t channelCount = (*ci).mChannelData.Length();
+  uint32_t channelCount = std::min((*ci).mChannelData.Length(),
+                                   WebAudioUtils::MaxChannelCount);
   if (aTrack.GetRate() != mSampleRate) {
     resampler = speex_resampler_init(channelCount,
       aTrack.GetRate(), mSampleRate, SPEEX_RESAMPLER_QUALITY_DEFAULT, nullptr);
@@ -436,10 +437,8 @@ AudioNodeExternalInputStream::ProcessInput(GraphTime aFrom, GraphTime aTo,
     }
   }
 
-  uint32_t outputChannels = ComputeFinalOuputChannelCount(inputChannels);
-
-  if (outputChannels) {
-    AllocateAudioBlock(outputChannels, &mLastChunks[0]);
+  if (inputChannels) {
+    AllocateAudioBlock(inputChannels, &mLastChunks[0]);
     nsAutoTArray<float,GUESS_AUDIO_CHANNELS*WEBAUDIO_BLOCK_SIZE> downmixBuffer;
     for (uint32_t i = 0; i < audioSegments.Length(); ++i) {
       AudioChunk tmpChunk;

content/media/webaudio/WebAudioUtils.cpp

@@ -14,6 +14,10 @@ namespace mozilla {
 
 namespace dom {
 
+// 32 is the minimum required by the spec and matches what is used by blink.
+// The limit protects against large memory allocations.
+const uint32_t WebAudioUtils::MaxChannelCount = 32;
+
 struct ConvertTimeToTickHelper
 {
   AudioNodeStream* mSourceStream;

content/media/webaudio/WebAudioUtils.h

@@ -26,9 +26,7 @@ namespace dom {
 class AudioParamTimeline;
 
 struct WebAudioUtils {
-  // This is an arbitrary large number used to protect against OOMs.
-  // We can adjust it later if needed.
-  static const uint32_t MaxChannelCount = 32;
+  static const uint32_t MaxChannelCount;
 
   static bool FuzzyEqual(float v1, float v2)
   {

content/media/webaudio/test/test_mediaStreamAudioSourceNode.html

@@ -15,7 +15,7 @@ function createBuffer(context, delay) {
   var buffer = context.createBuffer(2, 2048, context.sampleRate);
   for (var i = 0; i < 2048 - delay; ++i) {
     buffer.getChannelData(0)[i + delay] = Math.sin(440 * 2 * Math.PI * i / context.sampleRate);
-    buffer.getChannelData(1)[i + delay] = buffer.getChannelData(0)[i + delay];
+    buffer.getChannelData(1)[i + delay] = -buffer.getChannelData(0)[i + delay];
   }
   return buffer;
 }
@@ -32,6 +32,9 @@ var gTest = {
     source.start(0);
 
     var mediaStreamSource = context.createMediaStreamSource(dest.stream);
+    // channelCount and channelCountMode should have no effect
+    mediaStreamSource.channelCount = 1;
+    mediaStreamSource.channelCountMode = "explicit";
     return mediaStreamSource;
   },
   createExpectedBuffers: function(context) {