[PPAPI] Pepper MediaStream API audio track implementation and example.

ppapi/shared_impl/media_stream_frame_buffer.h

Index: ppapi/shared_impl/media_stream_frame_buffer.h
diff --git a/ppapi/shared_impl/media_stream_frame_buffer.h b/ppapi/shared_impl/media_stream_frame_buffer.h
index e95084642c03bc20a2ae32b452df4f0a88ca8c4d..ab46b272a55ec0ddc4c1e789c1454d6e4e441dfb 100644
--- a/ppapi/shared_impl/media_stream_frame_buffer.h
+++ b/ppapi/shared_impl/media_stream_frame_buffer.h
@@ -11,6 +11,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/shared_memory.h"
+#include "base/synchronization/lock.h"
 #include "ppapi/shared_impl/media_stream_frame.h"
 #include "ppapi/shared_impl/ppapi_shared_export.h"
 
@@ -48,9 +49,15 @@ class PPAPI_SHARED_EXPORT MediaStreamFrameBuffer {
 
   ~MediaStreamFrameBuffer();
 
-  int32_t number_of_frames() const { return number_of_frames_; }
+  int32_t number_of_frames() const {
+    base::AutoLock lock(mutex_);
+    return number_of_frames_;
+  }
 
-  int32_t frame_size() const { return frame_size_; }
+  int32_t frame_size() const {
+    base::AutoLock lock(mutex_);
+    return frame_size_;
+  }
 
   // Initializes shared memory for frames transmission.
   bool SetFrames(int32_t number_of_frames,
@@ -70,6 +77,9 @@ class PPAPI_SHARED_EXPORT MediaStreamFrameBuffer {
  private:
   Delegate* delegate_;
 
+  // A mutex to protect below fields.
+  mutable base::Lock mutex_;

[dmichael (off chromium), 2014/01/16 22:34:20]

Can you explain more about why you've done this? We try to avoid explicit
locking as much as possible in Chromium. FWIW, on the plugin side of the proxy,
there is already a lock around everything.

On the host side, we should find a different approach. Usually it involves
posting tasks to an appropriate thread. If you can outline which functions are
on which threads currently in the host side, maybe we can help you come up with
a good approach for dealing with the threading.

[Peng, 2014/01/17 00:23:49]

All audio frames are received from an audio thread (|OnData()|). I know we could
post it to main thread. But it will add some latency.:( And samples data passed
to OnData is not refcounted. If we want to post task to main thread |OnData()|,
we also need find a way how to handle it (copied it to a temp buffer, or call
Dequeue() to get a free frame ready in main thread before OnData() is called).

FYI, This Lock protects operations on the frame_queue_. Most operations are
enqueue and dequeue. They are not big time consuming operations.
The lock's impact is minor I think. 

I have add more inline comments on AudioTrackHost, please take a look.

+
   // A queue of frame indexes.
   std::deque<int32_t> frame_queue_;