The media Renderer interface and implementations have gone through many changes since it's born and over time it's less clear about what the API expects.

Here are some issues that I see:

1. Historically we require all one-time callbacks to be fired asynchronously (after the call is returned). The current comment is ambiguous on this now [1]:

  // |init_cb| must only be run after this
  // method has returned.  Firing |init_cb| may result in the immediate
  // destruction of the caller, so it must be run only prior to returning.

Also, I think we disallow OnError() to be called during initialization. Now this is missing. Actually, shall we disallow any RendererClient() method to be called during initialization?

2. Now we have the RendererCient interface. It's very natural that the Renderer calls into RendererClient at any time. This can easily cause reentrance into the caller given that typically the caller implements the RendererClient as well, e.g. caller -> Renderer::Foo() -> RendererClient::Bar() -> caller. This is fine in most cases but is definitely error prone. Shall we mention this in the API, either somehow prevent this from happening, or at least let people know this could happen?

3. Since RendererClient and Renderer are defined in two files. It's less clear about the interaction between them, especially for error handling. For example, after RendererClient::OnError() is called, can the caller call into Renderer again? If so, will the calls just be no-op, or fail (for calls that returns results synchronously, or returns results through callbacks asynchronously)? Today looking at RendererImpl, it seems we allow calling Renderer after OnError(). This was partly because OnError() is posted so we cannot guarantee Renderer() is not called before OnError() is executed. But then in RendererImpl::Flush(), if we are in the error state, we just return without even fire the |flush_cb|, which seems wrong [2].

[1] https://cs.chromium.org/chromium/src/media/base/renderer.h?rcl=0&l=34

[2] https://cs.chromium.org/chromium/src/media/renderers/renderer_impl.cc?rcl=0&l=173