Fix VirtualAudioInputStream callback timing issues.

media/audio/virtual_audio_input_stream.cc

Index: media/audio/virtual_audio_input_stream.cc
diff --git a/media/audio/virtual_audio_input_stream.cc b/media/audio/virtual_audio_input_stream.cc
index 4f2f9bd04fb84a4619766597777c75665b5641c6..4b2cd9ed4ac86df301dccbcfd5172eed222bd136 100644
--- a/media/audio/virtual_audio_input_stream.cc
+++ b/media/audio/virtual_audio_input_stream.cc
@@ -85,6 +85,7 @@ bool VirtualAudioInputStream::Open() {
 void VirtualAudioInputStream::Start(AudioInputCallback* callback) {
   DCHECK(message_loop_->BelongsToCurrentThread());
   callback_ = callback;
+  next_read_time_ = base::Time::Now();
   on_more_data_cb_.Reset(base::Bind(&VirtualAudioInputStream::ReadAudio,
                                     base::Unretained(this)));
   audio_manager_->GetMessageLoop()->PostTask(FROM_HERE,
@@ -139,9 +140,15 @@ void VirtualAudioInputStream::ReadAudio() {
                     params_.GetBytesPerBuffer(),
                     1.0);
 
+  // Need to account for time spent here due to renderer side mixing.
+  next_read_time_ += buffer_duration_ms_;
+  base::TimeDelta delay = next_read_time_ - base::Time::Now();
+  if (delay < base::TimeDelta())

[DaleCurtis, 2013/01/15 21:00:08]

I'd worry about drift over time here. Can you limit this calculation to the
delta between ReadAudio() calls? I.e. reset next_read_time_ based on
base::Time::Now() in each call.

[justinlin, 2013/01/15 22:36:21]

I tested this and actually the comment I left there isn't quite right.
PostDelayedTask actually contributes to the imprecision as well. I think a
combination of the time spent in audio conversions and the imprecision of
PostDelayedTask pushes it just over the edge to produce a pretty noticeable
framerate drop (i.e. the posttask delay ends up fluctuating between 8 and 9ms on
my machine if we do account for both things). Accounting for only the audio
conversions is not enough.

As for drift towards the delay getting shorter and shorter and eventually using
up all the cpu, how about we just reset the next read time in that case. We'll
get a video slow down but at least it won't end up spinning.

+    delay = base::TimeDelta();
+
   message_loop_->PostDelayedTask(FROM_HERE,
                                  on_more_data_cb_.callback(),
-                                 buffer_duration_ms_);
+                                 delay);
 }
 
 void VirtualAudioInputStream::Close() {