Add lock to fix race in AudioRendererMixerInput.

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_
 
 #include <string>
 
 #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"
 #include "url/origin.h"
 
 namespace media {
 
 class AudioRendererMixer;
 
 class MEDIA_EXPORT AudioRendererMixerInput
@@ skipping 40 matching lines @@
 
   // Called by AudioRendererMixer when an error occurs.
   void OnRenderError();
 
  protected:
   ~AudioRendererMixerInput() override;
 
  private:
   friend class AudioRendererMixerInputTest;
 
+  // Protect |volume_|, accessed by separate threads in ProvideInput() and
+  // SetVolume().
+  base::Lock volume_lock_;
+
   bool started_;
   bool playing_;
   double volume_;
 
   // AudioConverter::InputCallback implementation.
   double ProvideInput(AudioBus* audio_bus,
                       base::TimeDelta buffer_delay) override;
 
   // Callbacks provided during construction which allow AudioRendererMixerInput
   // to retrieve a mixer during Initialize() and notify when it's done with it.
@@ skipping 26 matching lines @@
   SwitchOutputDeviceCB pending_switch_callback_;
   std::string pending_switch_device_id_;
   url::Origin pending_switch_security_origin_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioRendererMixerInput);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_AUDIO_RENDERER_MIXER_INPUT_H_