Fixed the audio backgrounding bug

content/browser/renderer_host/media/audio_renderer_host.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/media/audio_renderer_host.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/logging.h"
 #include "base/memory/shared_memory.h"
@@ skipping 43 matching lines @@
                                                 bool is_playing,
                                                 int render_view_id) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   if (render_view_id == MSG_ROUTING_NONE || !ResourceDispatcherHostImpl::Get())
     return;
 
   ResourceDispatcherHostImpl::Get()->OnAudioRenderHostStreamStateChanged(
       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
     : public media::AudioOutputController::EventHandler {
  public:
   AudioEntry(AudioRendererHost* host,
              int stream_id,
              int render_frame_id,
              const media::AudioParameters& params,
              const std::string& output_device_id,
@@ skipping 570 matching lines @@
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   if (entry->playing() == is_playing)
     return;
 
   bool should_alert_resource_scheduler;
   if (is_playing) {
     should_alert_resource_scheduler =
         !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()) {
     BrowserThread::PostTaskAndReplyWithResult(
         BrowserThread::UI, FROM_HERE,
         base::Bind(&RenderFrameIdToRenderViewId, render_process_id_,
                    entry->render_frame_id()),
         base::Bind(&NotifyResourceDispatcherOfAudioStateChange,
                    render_process_id_, is_playing));
   }
 }
 
 bool AudioRendererHost::HasActiveAudio() {
   return !base::AtomicRefCountIsZero(&num_playing_streams_);
 }
 
 bool AudioRendererHost::RenderFrameHasActiveAudio(int render_frame_id) const {
   for (AudioEntryMap::const_iterator it = audio_entries_.begin();
        it != audio_entries_.end();
        ++it) {
     AudioEntry* entry = it->second;
     if (entry->render_frame_id() == render_frame_id && entry->playing())
       return true;
   }
   return false;
 }
 
 }  // namespace content