MediaDecoderReader::OwnerThread() should be made protected or private so that the client code doesn't know too much about its internal thread model.

IIRC, there are methods in the PDMs that uses MediaDecoderReader::OwnerThread() in order to get access to a non-flushable task queue (as you can't use promises with flushable task queues). I believe that the main reason for this is because you can't create a MozTaskQueue outside of the main thread ; and that's only because the MozTaskQueue constructor access a preference to find out the default number of threads to use.

Can you point me to the code?

http://mxr.mozilla.org/mozilla-central/source/dom/media/MediaFormatReader.h#177 Used for the implementation of http://mxr.mozilla.org/mozilla-central/source/dom/media/platforms/PlatformDecoderModule.h#132

We can make DecoderCallback a friend of MediaFormatReader because they are tightly coupled. So OnTaskQueue() doesn't need to be exposed from MediaDecoderReader.

Btw, I just found out: MediaCodecReader::AsyncReadMetadata -> MediaCodecReader::ReallocateExtractorResources -> MediaCodecReader::CreateTaskQueues -> CreateMediaDecodeTaskQueue -> GetMediaThreadPool would access pref off the main thread. However, B2G allows you to access pref off the main thread. This inconsistency is bad and obscure the code.

This is already done by MediaDecoderReaderWrapper.