Fixed the audio backgrounding bug

content/browser/renderer_host/media/audio_renderer_host.cc

Index: content/browser/renderer_host/media/audio_renderer_host.cc
diff --git a/content/browser/renderer_host/media/audio_renderer_host.cc b/content/browser/renderer_host/media/audio_renderer_host.cc
index dddeee21626a31f30aff5b4f749c396fcd5024b8..f71dcdf920592003b7daa7c8371891744944de86 100644
--- a/content/browser/renderer_host/media/audio_renderer_host.cc
+++ b/content/browser/renderer_host/media/audio_renderer_host.cc
@@ -61,6 +61,26 @@ void NotifyResourceDispatcherOfAudioStateChange(int render_process_id,
       render_process_id, render_view_id, is_playing);
 }
 
+void NotifyRenderProcessHostThatAudioStarted(int render_process_id) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  RenderProcessHost* render_process_host =
+      RenderProcessHost::FromID(render_process_id);
+
+  if (render_process_host)
+    render_process_host->AudioStarted();
+}
+
+void NotifyRenderProcessHostThatAudioStopped(int render_process_id) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  RenderProcessHost* render_process_host =
+      RenderProcessHost::FromID(render_process_id);
+
+  if (render_process_host)
+    render_process_host->AudioStopped();
+}
+
 }  // namespace
 
 class AudioRendererHost::AudioEntry
@@ -651,11 +671,28 @@ void AudioRendererHost::UpdateNumPlayingStreams(AudioEntry* entry,
         !RenderFrameHasActiveAudio(entry->render_frame_id());
     entry->set_playing(true);
     base::AtomicRefCountInc(&num_playing_streams_);
+
+    // Inform the renderer that audio started playing for the first time so the

[gab, 2015/07/02 13:11:53]

s/renderer/renderer host/

[gab, 2015/07/02 13:11:53]

Since this comment is outside the "if", I'd s/that audio started/when audio
starts/ (i.e. outside the "if" it is not always the case "that audio started").

[sebsg, 2015/07/06 18:24:53]

Done.

[sebsg, 2015/07/06 18:24:53]

Done.

+    // process can be un-backgrounded if necessary.

[gab, 2015/07/02 13:11:53]

Remove " so the process can be...". What the RenderProcessHost does in response
is its business and shouldn't be documented here.

[sebsg, 2015/07/06 18:24:53]

Done.

+    if (base::AtomicRefCountIsOne(&num_playing_streams_)) {

[gab, 2015/07/06 17:24:09]

Note that this depends on this thread being the only writer (or at least on the
writes being otherwise sequenced), otherwise you could go from two threads
seeing 0 and then both seeing 2.

It would also be possible for this thread to see 1 but for another thread to
decrement immediately after, resulting in a race of start/stop notifications.

This is not a problem in practice because these writes are sequenced on a single
thread, but perhaps we should make a note on |num_playing_streams_|:

  // The number of streams in the playing state. Atomic read safe from any
  // thread, but should only be updated from the IO thread. 
  base::AtomicRefCount num_playing_streams_;

[sebsg, 2015/07/06 18:24:53]

Done.

+      BrowserThread::PostTask(
+          BrowserThread::UI, FROM_HERE,
+          base::Bind(&NotifyRenderProcessHostThatAudioStarted,
+                     render_process_id_));
+    }
   } else {
     entry->set_playing(false);
     should_alert_resource_scheduler =
         !RenderFrameHasActiveAudio(entry->render_frame_id());
-    base::AtomicRefCountDec(&num_playing_streams_);
+
+    // Inform the renderer that there is no more audio playing so the process

[gab, 2015/07/02 13:11:53]

s/that there is/when there is/

[sebsg, 2015/07/06 18:24:53]

Done.

+    // can be backgrounded if possible.

[gab, 2015/07/02 13:11:53]

Remove " so the process (...)".

[sebsg, 2015/07/06 18:24:53]

Done.

+    if (!base::AtomicRefCountDec(&num_playing_streams_)) {
+      BrowserThread::PostTask(
+          BrowserThread::UI, FROM_HERE,
+          base::Bind(&NotifyRenderProcessHostThatAudioStopped,
+                     render_process_id_));
+    }
   }
 
   if (should_alert_resource_scheduler && ResourceDispatcherHostImpl::Get()) {