Add lock to fix race in AudioRendererMixerInput.

media/base/audio_renderer_mixer_input.cc

Index: media/base/audio_renderer_mixer_input.cc
diff --git a/media/base/audio_renderer_mixer_input.cc b/media/base/audio_renderer_mixer_input.cc
index 5743700f04b0a62c1c3c6eaf0f2d0bfa54aef0f5..a2627da37edef192aede7f48f8aa564ba54b3ee4 100644
--- a/media/base/audio_renderer_mixer_input.cc
+++ b/media/base/audio_renderer_mixer_input.cc
@@ -32,12 +32,14 @@ AudioRendererMixerInput::AudioRendererMixerInput(
                            base::Unretained(this))) {}
 
 AudioRendererMixerInput::~AudioRendererMixerInput() {
+  DCHECK(!started_);
   DCHECK(!mixer_);
 }
 
 void AudioRendererMixerInput::Initialize(
     const AudioParameters& params,
     AudioRendererSink::RenderCallback* callback) {
+  DCHECK(!started_);
   DCHECK(!mixer_);
   DCHECK(callback);
 
@@ -106,6 +108,7 @@ void AudioRendererMixerInput::Pause() {
 }
 
 bool AudioRendererMixerInput::SetVolume(double volume) {
+  base::AutoLock auto_lock(volume_lock_);
   volume_ = volume;
   return true;
 }
@@ -190,7 +193,14 @@ double AudioRendererMixerInput::ProvideInput(AudioBus* audio_bus,
         frames_filled, audio_bus->frames() - frames_filled);
   }
 
-  return frames_filled > 0 ? volume_ : 0;
+  // We're reading |volume_| from the audio device thread and must avoid racing
+  // with the main/media thread calls to SetVolume(). This is the only method
+  // called on the audio device thread. For other methods, thread safety
+  // should be enforced by the caller. See the lock in AudioRendererMixer.

[o1ka, 2016/03/16 14:31:18]

It's not quite clear to me how the lock in AudioRendererMixer is related here?
And can we add a header comment about caller responsibility for thread safety?

[chcunningham, 2016/03/17 17:59:45]

Woops! AudioRendererMixer should have been WebAudioSourceProviderImpl. I've
simplified this comment and added a more detailed comment in the header. 

+  {
+    base::AutoLock auto_lock(volume_lock_);
+    return frames_filled > 0 ? volume_ : 0;
+  }
 }
 
 void AudioRendererMixerInput::OnRenderError() {