Allow media track switching.

media/video/gpu_memory_buffer_video_frame_pool.h

 // Copyright 2015 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_VIDEO_GPU_MEMORY_BUFFER_VIDEO_FRAME_POOL_H_
 #define MEDIA_VIDEO_GPU_MEMORY_BUFFER_VIDEO_FRAME_POOL_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/task_runner.h"
 #include "media/base/video_frame.h"
 
 namespace base {
 class SingleThreadTaskRunner;
 }
 
 namespace media {
 class GpuVideoAcceleratorFactories;
 
 // Interface to a pool of GpuMemoryBuffers/textures/images that can be used to
 // transform software VideoFrames to VideoFrames backed by native textures.
 // The resources used by the VideoFrame created by the pool will be
 // automatically put back into the pool once the frame is destroyed.
 // The pool recycles resources to a void unnecessarily allocating and
 // destroying textures, images and GpuMemoryBuffer that could result
 // in a round trip to the browser/GPU process.
+// NOTE: Destroying the pool will not immediately invalidate outstanding video
+// frames. GPU memory buffers are ref-counted and will be kept alive for as long

[Daniele Castagna, 2017/03/31 00:58:13]

nit: Actually VideoFrames are ref-counted. The resources backing VideoFrames
stay alive until all VideoFrames are returned from the compositor. This includes
GMBs, images and textures.

[servolk, 2017/03/31 01:04:19]

Thanks for correcting me. How about this?
// NOTE: Destroying the pool will not immediately invalidate outstanding video
// frames. GPU memory buffers will be kept alive by video frames referencing
// them. Video frames themselves are ref-counted and will be released when they
// are no longer needed, potentially after the pool is destroyed.

[Daniele Castagna, 2017/03/31 01:20:54]

Thanks to you for adding this note.

The comment seems good. Maybe just remove the end of the second sentence where
you mention that video frames are referencing GMBs (just remove the "referencing
them" part), since it might seem they reference them directly.

The real story is that the release callbacks of videoframes are keeping alive
another ref counted object that contains all the resources needed. But I don't
think that belongs to this note.

I'm also OK adding an "indirectly" referencing them.

Thanks!

[servolk, 2017/03/31 01:23:21]

Done.

+// as the corresponding video frame is alive.
 class MEDIA_EXPORT GpuMemoryBufferVideoFramePool {
  public:
   GpuMemoryBufferVideoFramePool();
   GpuMemoryBufferVideoFramePool(
       const scoped_refptr<base::SingleThreadTaskRunner>& media_task_runner,
       const scoped_refptr<base::TaskRunner>& worker_task_runner,
       GpuVideoAcceleratorFactories* gpu_factories);
   virtual ~GpuMemoryBufferVideoFramePool();
 
   // Callback used by MaybeCreateHardwareFrame to deliver a new VideoFrame
@@ skipping 14 matching lines @@
  private:
   class PoolImpl;
   scoped_refptr<PoolImpl> pool_impl_;
 
   DISALLOW_COPY_AND_ASSIGN(GpuMemoryBufferVideoFramePool);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_VIDEO_GPU_MEMORY_BUFFER_VIDEO_FRAME_POOL_H_