[webkit-dev] Making MockMediaPlayerMediaSource and other MediaPlayerPrivateInterface subclasses work together

[Enrique Ocaña González]

Hi,

These days I've been working on improving the layout test passrate of our new 
Media Source Extensions GStreamer platform implementation 
(MediaPlayerPrivateGStreamerMSE), but I'm having problems with the selection 
of the right MediaPlayerPrivateInterface implementation for each use case.

As I understand it (please correct me if I'm wrong), under normal 
circumstances (no MEDIA_SOURCE enabled) MediaPlayer tries to find the best 
media engine (ie: MediaPlayerPrivateInterface implementation) available to 
play a content. It does so by asking each engine if they support (yes/no/
maybe) the particular mime type of the video. For the "maybe" case, the engine 
is instantiated, loading goes forward and the networkState is set to 
FormatError in case something goes wrong. In that case MediaPlayer tries the 
next available engine.

Things work different for MSE. No matter what support the engine reports, all 
the engines are tried and the content loading is attempted. Setting 
FormatError in networkState is the only way in which an engine can reject 
being selected. Unfortunately, it's impossible for 
MediaPlayerPrivateGStreamerMSE to take that decision at loading time, because 
usually the load happens before the MediaSource has been configured with 
SourceBuffers (the ones specifying a mime type). Therefore, the MSE player 
private must always succeed blindly on loading if it wants to have any 
opportunity. This works fine for real world use cases.

My issue is related to MockMediaPlayerMediaSource, the test engine which 
should take care of the "video/mock" content used in some layout tests. Our 
MSE player private gets selected and performs the loading (of an empty 
MediaSource). However, when a video/mock SourceBuffer is added, it's too late 
for the MSE player private to reject being in charge. Both MediaSource and 
SourceBuffer are already using the GStreamer-related subclasses as their 
MediaSourcePrivate and SourceBufferPrivate counterparts and they can't be hot-
swapped with the Mock-related subclasses. Returning NotSupported in 
MediaSourceGStreamer::addSourceBuffer() only makes things worse. The 
JavaScript code triggering that call expects addSourceBuffer() to be handled 
by the mock engine and to succeed on the first attempt. The JS code isn't 
supposed to retry the call to addSourceBuffer().

I wonder what's the right way to manage the competitive selection between the 
platform player private and the mock player private to make real world use 
cases and test use cases work together. In particular, I wonder how it can 
successfully work in the Mac implementation.

The MediaPlayerPrivateMediaSourceAVFObjC implementation declares that it 
doesn't support and empty mimetype (irrelevant here, as it'll always be tried 
by MediaPlayer when MEDIA_SOURCE is enabled). Then, on load, it doesn't check 
anything so apparently succeeds.

I would be really grateful if Jer Noble or anybody else with knowledge on the 
matter could devote some minutes to shed some light about the right way to 
make the mock player private and the MSE player private live together.

Thank you.

-- 
Enrique Ocaña González