Implementation of 'AudioContext.getOutputTimestamp' method

media/audio/alsa/alsa_output.cc

Index: media/audio/alsa/alsa_output.cc
diff --git a/media/audio/alsa/alsa_output.cc b/media/audio/alsa/alsa_output.cc
index c127fb4c2e93a9c6bfd000e41b9717bd181681e1..dbac2a624008499d18812c8aafa37e9a491b2586 100644
--- a/media/audio/alsa/alsa_output.cc
+++ b/media/audio/alsa/alsa_output.cc
@@ -161,6 +161,7 @@ AlsaPcmOutputStream::AlsaPcmOutputStream(const std::string& device_name,
       frames_per_packet_(packet_size_ / bytes_per_frame_),
       state_(kCreated),
       volume_(1.0f),
+      elapsed_written_frames_(0),
       source_callback_(NULL),
       audio_bus_(AudioBus::Create(params)),
       weak_factory_(this) {
@@ -289,6 +290,9 @@ void AlsaPcmOutputStream::Start(AudioSourceCallback* callback) {
   // device.
   buffer_->Clear();
 
+  // Invalidate written frames count.
+  elapsed_written_frames_ = 0;
+
   // When starting again, drop all packets in the device and prepare it again
   // in case we are restarting from a pause state and need to flush old data.
   int error = wrapper_->PcmDrop(playback_handle_);
@@ -466,6 +470,8 @@ void AlsaPcmOutputStream::WritePacket() {
       // Seek forward in the buffer after we've written some data to ALSA.
       buffer_->Seek(frames_written * bytes_per_output_frame_);
     }
+    if (frames_written)
+      elapsed_written_frames_ += frames_written;
   } else {
     // If nothing left to write and playback hasn't started yet, start it now.
     // This ensures that shorter sounds will still play.
@@ -783,8 +789,16 @@ int AlsaPcmOutputStream::RunDataCallback(AudioBus* audio_bus,
                                          uint32_t total_bytes_delay) {
   TRACE_EVENT0("audio", "AlsaPcmOutputStream::RunDataCallback");
 
+  AudioTimestamp output_timestamp;
+  // This is quite a raugh estimation, we need to consider using
+  // snd_pcm_status_get_audio* API when it is accessible.
+  output_timestamp.frames = elapsed_written_frames_ - GetCurrentDelay();
+  output_timestamp.ticks = base::TimeTicks::Now().ToInternalValue();
+  DCHECK(output_timestamp.frames >= 0);
+
   if (source_callback_)
-    return source_callback_->OnMoreData(audio_bus, total_bytes_delay, 0);
+    return source_callback_->OnMoreData(audio_bus, total_bytes_delay, 0,
+                                        output_timestamp);
 
   return 0;
 }