Land Henrik's patch: http://codereview.chromium.org/6721027/

content/renderer/audio_device.h

Index: content/renderer/audio_device.h
===================================================================
--- content/renderer/audio_device.h	(revision 79311)
+++ content/renderer/audio_device.h	(working copy)
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Copyright (c) 2011 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.
 
@@ -14,16 +14,20 @@
 #include "base/threading/simple_thread.h"
 #include "content/renderer/audio_message_filter.h"
 
+struct AudioParameters;
+
 // Each instance of AudioDevice corresponds to one host stream.
 // This class is not thread-safe, so its methods must be called from
 // the same thread.
 class AudioDevice : public AudioMessageFilter::Delegate,
-                    public base::DelegateSimpleThread::Delegate {
+                    public base::DelegateSimpleThread::Delegate,
+                    public base::RefCountedThreadSafe<AudioDevice> {
  public:
   class RenderCallback {
    public:
     virtual void Render(const std::vector<float*>& audio_data,
-                        size_t number_of_frames) = 0;
+                        size_t number_of_frames,
+                        size_t audio_delay_milliseconds) = 0;
    protected:
     virtual ~RenderCallback() {}
   };
@@ -35,11 +39,30 @@
               RenderCallback* callback);
   virtual ~AudioDevice();
 
-  // Returns |true| on success.
+  // Starts audio playback. Returns |true| on success.
   bool Start();
+
+  // Stops audio playback. Returns |true| on success.
   bool Stop();
 
+  // Sets the playback volume, with range [0.0, 1.0] inclusive.
+  // Returns |true| on success.
+  bool SetVolume(double volume);
+
+  // Gets the playback volume, with range [0.0, 1.0] inclusive.
+  // Returns |true| on success.
+  bool GetVolume(double* volume);
+
+  double sample_rate() const { return sample_rate_; }
+  size_t buffer_size() const { return buffer_size_; }
+
  private:
+  // I/O thread backends to above functions.
+  void InitializeOnIOThread(const AudioParameters& params);
+  void StartOnIOThread();
+  void ShutDownOnIOThread();
+  void SetVolumeOnIOThread(double volume);
+
   // AudioMessageFilter::Delegate implementation.
   virtual void OnRequestPacket(AudioBuffersState buffers_state);
   virtual void OnStateChanged(AudioStreamState state);
@@ -48,7 +71,6 @@
                                    base::SyncSocket::Handle socket_handle,
                                    uint32 length);
   virtual void OnVolume(double volume);
-  virtual void OnDestroy();
 
   // DelegateSimpleThread::Delegate implementation.
   virtual void Run();
@@ -56,6 +78,7 @@
   // Format
   size_t buffer_size_;  // in sample-frames
   int channels_;
+  int bits_per_sample_;
   double sample_rate_;
 
   // Calls the client's callback for rendering audio.
@@ -65,6 +88,14 @@
   // The client callback renders audio into here.
   std::vector<float*> audio_data_;
 
+  // The client stores the last reported audio delay in this member.
+  // The delay shall reflect the amount of audio which still resides in
+  // the output buffer, i.e., the expected audio output delay.
+  int audio_delay_milliseconds_;
+
+  // The current volume scaling [0.0, 1.0] of the audio stream.
+  double volume_;
+
   // Callbacks for rendering audio occur on this thread.
   scoped_ptr<base::DelegateSimpleThread> audio_thread_;
 
@@ -73,8 +104,14 @@
   base::SyncSocket* socket() { return socket_.get(); }
   void* shared_memory_data() { return shared_memory()->memory(); }
 
+  // MessageFilter used to send/receive IPC. THIS MUST ONLY BE ACCESSED ON THE
+  // I/O thread except to send messages and get the message loop.
   static scoped_refptr<AudioMessageFilter> filter_;
+
+  // Our ID on the message filter. THIS MUST ONLY BE ACCESSED ON THE I/O THREAD
+  // or else you could race with the initialize function which sets it.
   int32 stream_id_;
+
   scoped_ptr<base::SharedMemory> shared_memory_;
   scoped_ptr<base::SyncSocket> socket_;