Create video and audio decoder threads on demand.

media/filters/ffmpeg_audio_decoder.h

 // 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.
 
 #ifndef MEDIA_FILTERS_FFMPEG_AUDIO_DECODER_H_
 #define MEDIA_FILTERS_FFMPEG_AUDIO_DECODER_H_
 
 #include <list>
 
 #include "base/message_loop.h"
 #include "media/base/audio_decoder.h"
 
 struct AVCodecContext;
 
 namespace media {
 
 class DataBuffer;
+class MessageLoopFactory;
 
 class MEDIA_EXPORT FFmpegAudioDecoder : public AudioDecoder {
  public:
-  explicit FFmpegAudioDecoder(MessageLoop* message_loop);
+  FFmpegAudioDecoder(MessageLoopFactory* loop_factory,

[Ami GONE FROM CHROMIUM, 2012/03/08 16:32:07]

You could avoid the need for the MessageLoopFactory changes and the need for
MessageLoopFactoryForTest by instead passing a Callback<MessageLoop*()> to the
decoders, built with something like:
base::Bind(&MessageLoopFactory::GetMessageLoop, loop_factory,
"FooDecoderThread")
for the "real" callsite and 
template<class T> T Identity(T t) { return t; }
base::Bind(&Identity, MessageLoop::current())
for the test callsites.

[tommi (sloooow) - chröme, 2012/03/09 10:07:15]

That's so clever it can't possibly work! hehe, much better. Done!

+                     const std::string& loop_name);
   virtual ~FFmpegAudioDecoder();
 
   // AudioDecoder implementation.
   virtual void Initialize(const scoped_refptr<DemuxerStream>& stream,
                           const PipelineStatusCB& callback,
                           const StatisticsCallback& stats_callback) OVERRIDE;
   virtual void Read(const ReadCB& callback) OVERRIDE;
   virtual int bits_per_channel() OVERRIDE;
   virtual ChannelLayout channel_layout() OVERRIDE;
   virtual int samples_per_second() OVERRIDE;
@@ skipping 16 matching lines @@
   // buffer. Will fall back to an estimated timestamp if the input lacks a
   // valid timestamp.
   void UpdateDurationAndTimestamp(const Buffer* input, DataBuffer* output);
 
   // Calculates duration based on size of decoded audio bytes.
   base::TimeDelta CalculateDuration(int size);
 
   // Delivers decoded samples to |read_cb_| and resets the callback.
   void DeliverSamples(const scoped_refptr<Buffer>& samples);
 
+  MessageLoop* EnsureMessageLoop();

[Ami GONE FROM CHROMIUM, 2012/03/08 16:32:07]

This idiom seems to me either racy or unnecessary.
If one is guaranteed that Initialize() is called before any other method, and
that that "before" carries memory-barrier implication, then every method other
than Initialize can simply trust & use message_loop_. 
If there is no such happens-before relationship then nothing prevents
EnsureMessageLoop() from running concurrently in different threads and returning
different values.

I believe the former is the case (and all methods other than Initialize() in the
.cc file can be reverted, and EML() removed).

[tommi (sloooow) - chröme, 2012/03/09 10:07:15]

Removed EML and now only initialize message_loop_ inside Initialize.
EML wasn't racy though since MessageLoopFactory is thread safe and always
returns the same pointer given the same key.

+
+  MessageLoopFactory* message_loop_factory_;
+  const std::string loop_name_;
   MessageLoop* message_loop_;
 
   scoped_refptr<DemuxerStream> demuxer_stream_;
   StatisticsCallback stats_callback_;
   AVCodecContext* codec_context_;
 
   // Decoded audio format.
   int bits_per_channel_;
   ChannelLayout channel_layout_;
   int samples_per_second_;
 
   base::TimeDelta estimated_next_timestamp_;
 
   // Holds decoded audio. As required by FFmpeg, input/output buffers should
   // be allocated with suitable padding and alignment. av_malloc() provides
   // us that guarantee.
   const int decoded_audio_size_;
   uint8* decoded_audio_;  // Allocated via av_malloc().
 
   ReadCB read_cb_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(FFmpegAudioDecoder);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_FILTERS_FFMPEG_AUDIO_DECODER_H_