Implement AudioRendererMixerInput::CurrentThreadIsRenderingThread().

media/base/audio_renderer_mixer_input.cc

Index: media/base/audio_renderer_mixer_input.cc
diff --git a/media/base/audio_renderer_mixer_input.cc b/media/base/audio_renderer_mixer_input.cc
index 3d2263f87bc62914f35fc9126eb678913367caec..30e738013d159285963e93ea28f811f0431ba01e 100644
--- a/media/base/audio_renderer_mixer_input.cc
+++ b/media/base/audio_renderer_mixer_input.cc
@@ -123,8 +123,11 @@ OutputDeviceInfo AudioRendererMixerInput::GetOutputDeviceInfo() {
 }
 
 bool AudioRendererMixerInput::CurrentThreadIsRenderingThread() {
-  NOTIMPLEMENTED();
-  return false;
+  // The users of this class are required to handle thread safety. In
+  // particular, the AudioConverter uses a lock when calling Stop(), where
+  // |mixer_| is cleared, and also in its render call, where this function is
+  // called.
+  return mixer_ && mixer_->CurrentThreadIsRenderingThread();

[o1ka, 2016/06/21 12:43:09]

I would say something like:
This method is expected to be called in callback_->Render().
|mixer_| access on rendering thread is safe:
AudioRendererMixer::AddInput()/RemoveInput()/Render() take lock for
AudioConverter operations, which guarantees that there are no
ARMI::ProvideInput()(=>callback_->Render()) calls coming before/during
AddInput() and during/after RemoveInput().

[DaleCurtis, 2016/06/21 19:50:03]

Can we just DCHECK(mixer_)?

[Henrik Grunell, 2016/06/23 06:19:55]

No need to DCHECK since it'll crash when dereferencing. And as a means to
document the code, it doesn't document why it's safe throughout the execution of
the function. So I think a a comment is better, but "mixer_ &&" can be removed.

[Henrik Grunell, 2016/06/23 06:19:55]

I'd prefer a more general and shorter comment. :) Since the details are about
something outside of this class, which can change. What about describing the
contract in the header?

 }
 
 void AudioRendererMixerInput::SwitchOutputDevice(