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..06266b0f557e320e76497c3d7a76cbaa948d7133 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 +140,20 @@ void VirtualAudioInputStream::ReadAudio() {
                     params_.GetBytesPerBuffer(),
                     1.0);
 
+  // Need to account for time spent here due to renderer side mixing as well as
+  // the imprecision of PostDelayedTask.
+  next_read_time_ += buffer_duration_ms_;
+  base::TimeDelta delay = next_read_time_ - base::Time::Now();
+  if (delay < base::TimeDelta()) {
+    // Reset the next read time if we end up getting too far behind. We'll just
+    // slow down playback to avoid using up all the CPU.
+    delay = base::TimeDelta();
+    next_read_time_ = base::Time::Now();

[DaleCurtis, 2013/01/17 00:10:51]

I'd save base::Time::Now() from the first call instead of calling it twice. This
still seems like it might be prone to cycles. Have you tried logging how
frequently callbacks fall behind?

[justinlin, 2013/01/17 22:21:05]

Done. Updated this: delay should be set to the buffer duration (i.e. we'll just
schedule it at normal time if we're really behind).

Yea, I put logging to see how often this happens and it's pretty much almost
always behind by a little. Roughly ~200us due to posttask's imprecision and
~400us-ish for the time in the audio conversions.

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