Add lock to fix race in AudioRendererMixerInput.

Clusterfuzz identified a race between the media thread calling SetVolume
and the audio device thread calling ProvideInput. Add a lock to
synchronize access to |volume_| between threads.

Also adds some thread_checkers to just firm up the contract that the
majority of these methods are called only by the media thread.

BUG=588992

Committed: https://crrev.com/57c6958a99f2c62a4408d2a9d262a22129309106
Cr-Commit-Position: refs/heads/master@{#379349}

Review URL: https://codereview.chromium.org/1748183006

Cr-Commit-Position: refs/heads/master@{#381763}

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 +193,13 @@ 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(). See thread safety comment
+  // in the header file.
+  {
+    base::AutoLock auto_lock(volume_lock_);
+    return frames_filled > 0 ? volume_ : 0;
+  }
 }
 
 void AudioRendererMixerInput::OnRenderError() {

media/base/audio_renderer_mixer_input.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+//
+// THREAD SAFETY
+//
+// This class is generally not thread safe. Callers should ensure thread safety.
+// For instance, the |sink_lock_| in WebAudioSourceProvider synchronizes access
+// to this object across the main thread (for WebAudio APIs) and the
+// media thread (for HTMLMediaElement APIs).
+//
+// The one exception is protection for |volume_| via |volume_lock_|. This lock
+// prevents races between SetVolume() (called on any thread) and ProvideInput
+// (called on audio device thread). See http://crbug.com/588992.
 
 #ifndef MEDIA_BASE_AUDIO_RENDERER_MIXER_INPUT_H_
 #define MEDIA_BASE_AUDIO_RENDERER_MIXER_INPUT_H_
@@ -9,6 +20,7 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/synchronization/lock.h"
 #include "media/base/audio_converter.h"
 #include "media/base/audio_renderer_sink.h"
 #include "media/base/output_device.h"
@@ -69,6 +81,10 @@ class MEDIA_EXPORT AudioRendererMixerInput
  private:
   friend class AudioRendererMixerInputTest;
 
+  // Protect |volume_|, accessed by separate threads in ProvideInput() and
+  // SetVolume().
+  base::Lock volume_lock_;
+
   bool started_;
   bool playing_;
   double volume_;