Add Mac OS X synchronized audio I/O back-end

media/base/audio_fifo.h

Index: media/base/audio_fifo.h
===================================================================
--- media/base/audio_fifo.h	(revision 157152)
+++ media/base/audio_fifo.h	(working copy)
@@ -5,6 +5,7 @@
 #ifndef MEDIA_BASE_AUDIO_FIFO_H_
 #define MEDIA_BASE_AUDIO_FIFO_H_
 
+#include "base/atomicops.h"
 #include "media/base/audio_bus.h"
 #include "media/base/media_export.h"
 
@@ -14,6 +15,8 @@
 // The maximum number of audio frames in the FIFO is set at construction and
 // can not be extended dynamically.  The allocated memory is utilized as a
 // ring buffer.
+// This class is thread-safe in the limited sense that one thread may call
+// Push(), while a second thread calls Consume().
 class MEDIA_EXPORT AudioFifo {
  public:
   // Creates a new AudioFifo and allocates |channels| of length |frames|.
@@ -35,11 +38,11 @@
   void Clear();
 
   // Number of actual audio frames in the FIFO.
-  int frames() const { return frames_; }
+  int frames() const;
 
- private:
   int max_frames() const { return max_frames_; }
 
+ private:
   // The actual FIFO is an audio bus implemented as a ring buffer.
   scoped_ptr<AudioBus> audio_bus_;
 
@@ -48,7 +51,8 @@
   const int max_frames_;
 
   // Number of actual elements in the FIFO.
-  int frames_;
+  volatile base::subtle::Atomic32 frames_pushed_;
+  volatile base::subtle::Atomic32 frames_consumed_;
 
   // Current read position.
   int read_pos_;