Finish plumbing MojoVideoDecoder.

media/mojo/interfaces/video_decoder.mojom

Index: media/mojo/interfaces/video_decoder.mojom
diff --git a/media/mojo/interfaces/video_decoder.mojom b/media/mojo/interfaces/video_decoder.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..777999425eafaa82801364a80dadeb7ab5544d04
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+++ b/media/mojo/interfaces/video_decoder.mojom
@@ -0,0 +1,55 @@
+// 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.
+
+module media.interfaces;
+
+import "media/mojo/interfaces/media_types.mojom";
+
+interface VideoDecoder {
+  // Initialize the decoder. This must be called before any other method.
+  //
+  // |decoder_buffer_pipe| will be used to transfer encoded data for each
+  // DecoderBuffer.
+  //
+  // TODO(sandersd): Rename to Initialize() if/when
+  // media::VideoDecoder::Initialize() is renamed to Configure().
+  Construct(VideoDecoderClient client,
+            handle<data_pipe_consumer> decoder_buffer_pipe);
+
+  // Configure (or reconfigure) the decoder. This must be called before decoding
+  // any frames, and must not be called while there are pending Initialize(),
+  // Decode(), or Reset() requests.
+  //
+  // If |low_delay| is true, the decoder must output frames as soon as possible;
+  // in particular, it must not wait for another Decode() request, except as
+  // required for frame reordering.
+  Initialize(VideoDecoderConfig config, bool low_delay) => (bool success);

[dcheng, 2016/05/11 07:37:45]

It's a bit weird to have a Construct() and Initialize() on the same interface.
Can we combine them?

[sandersd (OOO until July 31), 2016/05/11 18:20:18]

They have significantly different semantics. As the comments describe, the
current names are poor, but match media::VideoDecoder.

Specifically, Constuct() should be called Initialize(), and does what you
expect.

Initialize() should be called Configure(), and may be called multiple times
throughout decoding.

If they were combined, the result would be all nullable parameters, with two
distinct calling styles.

[dcheng, 2016/05/11 22:15:06]

Can we at least fix the names in the mojom interface? it might be confusing in
the interim period, but I think we should eventually strive to fix these names.
It's pretty confusing as-is.

[sandersd (OOO until July 31), 2016/05/11 23:00:50]

I'll have to defer this discussion to xhwang@, as I had the same request. How
about it Xiaohan?

[sandersd (OOO until July 31), 2016/05/17 23:05:00]

xhwang@: ping.

[xhwang, 2016/05/17 23:15:37]

dcheng: 

We already have C++ VideoDecoder::Initialize() today:
https://code.google.com/p/chromium/codesearch#chromium/src/media/base/video_d...

I agree the naming is not ideal. I was trying to avoid the confusion where mojo
VideoDecoder::Initialize() does something, then mojo VideoDecoder::Config()
calls C++ VideoDecoder::Initialize().

Does that make some sense?

+
+  // Request decoding of exactly one frame or an EOS buffer. This must not be
+  // called while there are pending Configure(), Reset(), or Decode() requests.
+  //
+  // Implementations must eventually execute the callback, even if Decode() is
+  // not called again. It is not required that the decode status match the
+  // actual result of decoding a frame; only that decode errors are eventually
+  // reported (such as at EOS). The purpose of the callback is primarily for
+  // Decode() rate control.
+  //
+  // If |buffer| is an EOS buffer, implementations execute all other pending
+  // Decode() callbacks and output all pending frames before executing the EOS
+  // buffer Decode() callback. (That is, they must flush.)
+  //
+  // TODO(sandersd): Plumb GetMaxDecodeRequests() so that parallel Decode()
+  // requests can be allowed.
+  Decode(DecoderBuffer buffer) => (DecodeStatus status);
+
+  // Reset the decoder. All ongoing Decode() requests must be completed or
+  // aborted before executing the callback. This must not be called while there
+  // is a pending Initialize() request.
+  Reset() => ();
+};
+
+interface VideoDecoderClient {
+  // Output a decoded frame. Frames must be output in presentation order.
+  OnVideoFrameDecoded(VideoFrame frame);
+};