Fix VideoDecoderShim to return correct decode_id for all codecs.

media/base/video_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_BASE_VIDEO_DECODER_H_
 #define MEDIA_BASE_VIDEO_DECODER_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 60 matching lines @@
   // GetMaxDecodeRequests() to get number of buffers that may be decoded in
   // parallel. Decoder must call |decode_cb| in the same order in which Decode()
   // is called.
   //
   // Implementations guarantee that the callback will not be called from within
   // this method and that |decode_cb| will not be blocked on the following
   // Decode() calls (i.e. |decode_cb| will be called even if Decode() is never
   // called again).
   //
   // After decoding is finished the decoder calls |output_cb| specified in
-  // Initialize() for each decoded frame. |output_cb| may be called before or
-  // after |decode_cb|.
+  // Initialize() for each decoded frame. In general |output_cb| may be called
+  // before or after |decode_cb|, but software decoders must always call
+  // |output_cb| before calling |decode_cb|.

[xhwang, 2015/01/15 21:35:10]

Hmm, is there any reason why VideoDecoderShim can't work with the current
VideoDecoder interface and has to have multiple exceptions?

[Sergey Ulanov, 2015/01/15 22:59:20]

PPB_VideoDecoder API needs to tell the plugin which frame is generated by which
input buffer. VideoDecoder interface currently doesn't support that.
VideoDecoder just calls OutputCB, but it's not possible to tell which buffer the
frame comes from. Another possible solution is to add decode_id parameter in
Decode() and OutputCB, but that would also require updating AudioDecoder
(because DecoderStream is shared), and would touch a lot of code, while this
feature is needed only in VideoDecoderShim

[xhwang, 2015/01/15 23:41:47]

Thanks for the info! But what exactly do you mean here by "software decoders
must always call |output_cb| before calling |decode_cb|"? What if the software
has decoding delay? e.g.

input 1, no output, decode_cb called
input 2, no output, decode_cb called
input 3, output 1, decode_cb called

In the last line, |output_cb| is called before |decode_cb|, is it okay? But the
|decode_cb| is also called before the future |output_cb| for input 3. So this is
kinda ambiguous...

   //
   // If |buffer| is an EOS buffer then the decoder must be flushed, i.e.
   // |output_cb| must be called for each frame pending in the queue and
   // |decode_cb| must be called after that. Callers will not call Decode()
   // again until after the flush completes.
   virtual void Decode(const scoped_refptr<DecoderBuffer>& buffer,
                       const DecodeCB& decode_cb) = 0;
 
   // Resets decoder state. All pending Decode() requests will be finished or
   // aborted before |closure| is called.
@@ skipping 12 matching lines @@
   // Returns maximum number of parallel decode requests.
   virtual int GetMaxDecodeRequests() const;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(VideoDecoder);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_VIDEO_DECODER_H_