Lazily initialize MessageLoop for faster thread startup

media/audio/audio_manager_base.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 "media/audio/audio_manager_base.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 77 matching lines @@
   // CoreAudio calls must occur on the main thread of the process, which in our
   // case is sadly the browser UI thread.  Failure to execute calls on the right
   // thread leads to crashes and odd behavior.  See http://crbug.com/158170.
   // TODO(dalecurtis): We should require the message loop to be passed in.
   if (base::MessageLoopForUI::IsCurrent()) {
     task_runner_ = base::MessageLoopProxy::current();
     return;
   }
 #endif
 
+#if defined(OS_WIN)
+  // We check COM initializer status right after calling this, so wait here

[DaleCurtis, 2015/03/26 00:12:38]

Can you elaborate on why this is necessary? CoreAudioUtil::IsSupported() was
previously running on the UI thread, what piece of code runs after this and
checks the COM status from the UI thread of the audio thread? 

...which as I think about it may not actually be correct. +tommi for his
recollection on why we initialize the device enumerator on the UI thread instead
of the audio thread. I seem to remember some race condition somewhere, but I
don't properly recall.

[DaleCurtis, 2015/04/16 16:41:07]

Did you have a chance to dig into this further? If not do you remember which
tests were failing? I'll try and patch it in today or tomorrow and see why this
is necessary.

+  // until we finish audio thread initialization (where we also run COM
+  // initializer).
+  CHECK(audio_thread_.StartAndWait());
+#else
   CHECK(audio_thread_.Start());
+#endif
   task_runner_ = audio_thread_.message_loop_proxy();
 }
 
 AudioManagerBase::~AudioManagerBase() {
   // The platform specific AudioManager implementation must have already
   // stopped the audio thread. Otherwise, we may destroy audio streams before
   // stopping the thread, resulting an unexpected behavior.
   // This way we make sure activities of the audio streams are all stopped
   // before we destroy them.
   CHECK(!audio_thread_.IsRunning());
@@ skipping 289 matching lines @@
 scoped_ptr<AudioLog> AudioManagerBase::CreateAudioLog(
     AudioLogFactory::AudioComponent component) {
   return audio_log_factory_->CreateAudioLog(component);
 }
 
 void AudioManagerBase::SetHasKeyboardMic() {
   NOTREACHED();
 }
 
 }  // namespace media