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

media/base/audio_fifo.cc

Index: media/base/audio_fifo.cc
===================================================================
--- media/base/audio_fifo.cc	(revision 156920)
+++ media/base/audio_fifo.cc	(working copy)
@@ -6,6 +6,10 @@
 
 #include "base/logging.h"
 
+using base::subtle::Atomic32;
+using base::subtle::NoBarrier_AtomicExchange;
+using base::subtle::MemoryBarrier;
+
 namespace media {
 
 // Given current position in the FIFO, the maximum number of elements in the
@@ -40,7 +44,8 @@
 AudioFifo::AudioFifo(int channels, int frames)
     : audio_bus_(AudioBus::Create(channels, frames)),
       max_frames_(frames),
-      frames_(0),
+      frames_pushed_(0),
+      frames_consumed_(0),
       read_pos_(0),
       write_pos_(0) {}
 
@@ -52,7 +57,7 @@
 
   // Ensure that there is space for the new data in the FIFO.
   const int source_size = source->frames();
-  CHECK_LE(source_size + frames_, max_frames_);
+  CHECK_LE(source_size + frames(), max_frames_);
 
   // Figure out if wrapping is needed and if so what segment sizes we need
   // when adding the new audio bus content to the FIFO.
@@ -73,8 +78,13 @@
     }
   }
 
-  frames_ += source_size;
-  DCHECK_LE(frames_, max_frames());
+  // Ensure the data is *really* written before updating |frames_pushed_|.
+  MemoryBarrier();
+
+  Atomic32 new_frames_pushed = frames_pushed_ + source_size;

[scherkus (not reviewing), 2012/09/17 14:51:17]

is the intent of this change to make this class lock-free?

I don't see any mention of it in the CL description

[Chris Rogers, 2012/09/17 20:44:23]

Added section in description about thread-safety.

+  NoBarrier_AtomicExchange(&frames_pushed_, new_frames_pushed);

[jbauman, 2012/09/15 00:33:52]

This (and the next one) should be NoBarrier_Store, because you're not doing
anything with the return value.

[Chris Rogers, 2012/09/17 20:44:23]

Done.

+
+  DCHECK_LE(frames(), max_frames());
   write_pos_ = UpdatePos(write_pos_, source_size, max_frames());
 }
 
@@ -85,7 +95,7 @@
   DCHECK_EQ(destination->channels(), audio_bus_->channels());
 
   // It is not possible to ask for more data than what is available in the FIFO.
-  CHECK_LE(frames_to_consume, frames_);
+  CHECK_LE(frames_to_consume, frames());
 
   // A copy from the FIFO to |destination| will only be performed if the
   // allocated memory in |destination| is sufficient.
@@ -113,12 +123,15 @@
     }
   }
 
-  frames_ -= frames_to_consume;
+  Atomic32 new_frames_consumed = frames_consumed_ + frames_to_consume;
+  NoBarrier_AtomicExchange(&frames_consumed_, new_frames_consumed);
+
   read_pos_ = UpdatePos(read_pos_, frames_to_consume, max_frames());
 }
 
 void AudioFifo::Clear() {

[no longer working on chromium, 2012/09/17 20:47:46]

when this Clear() is called? is there any potential race with Consume and
Push()?

[Chris Rogers, 2012/09/17 22:00:42]

Yes there is, but I already mention in the comments that this class is
thread-safe in a "limited sense" for Push() and Consume(), this thread-safety
does not extend to Clear().

-  frames_ = 0;
+  frames_pushed_ = 0;
+  frames_consumed_ = 0;
   read_pos_ = 0;
   write_pos_ = 0;
 }