[Mojo Video Capture] Retire buffers when Android Chromium goes to the background

media/capture/video/video_frame_receiver.h

 // Copyright 2016 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_CAPTURE_VIDEO_VIDEO_FRAME_RECEIVER_H_
 #define MEDIA_CAPTURE_VIDEO_VIDEO_FRAME_RECEIVER_H_
 
 #include "media/capture/capture_export.h"
 #include "media/capture/video/video_capture_device.h"
 
 namespace media {
 
 // Callback interface for VideoCaptureDeviceClient to communicate with its
 // clients.
 class CAPTURE_EXPORT VideoFrameReceiver {
  public:
   virtual ~VideoFrameReceiver(){};
 
   virtual void OnIncomingCapturedVideoFrame(
       media::VideoCaptureDevice::Client::Buffer buffer,
       scoped_refptr<media::VideoFrame> frame) = 0;
   virtual void OnError() = 0;
   virtual void OnLog(const std::string& message) = 0;
-  virtual void OnBufferDestroyed(int buffer_id_to_drop) = 0;
+
+  // Tells the VideoFrameReceiver that the producer is no longer going to use
+  // the buffer with id |buffer_id| for frame delivery. This may be called even
+  // while the receiver is still consuming the buffer from a call to
+  // OnIncomingCapturedVideoFrame(). In that case, it means that the
+  // caller is asking the VideoFrameReceiver to release the buffer
+  // at its earliest convenience.

[miu, 2017/01/31 01:18:51]

nit: Does "at its earliest convenience" really mean "once there are no consumers
referencing the frame?"

[chfremer, 2017/01/31 18:54:28]

In case of the concrete implementation VideoCaptureController this is indeed how
we do it. 

However, this information belongs with the concrete class VideoCaptureController
and not with this interface VideoFrameReceiver. That is the reason why I
formulated it in such loose terms. The code operating the VideoFrameReceiver
should not need to know what the implementation is, and it should not care how
long that implementation holds on to retired buffers.

+  // The producer guarantees that it does not reuse a retired |buffer_id| before
+  // having cycled through all other possible values.
+  virtual void OnBufferRetired(int buffer_id) = 0;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_CAPTURE_VIDEO_VIDEO_FRAME_RECEIVER_H_