Fix deadlock on macOS when running GStreamer in the main thread

[amyspark]

Hi,

This addresses the issue in #83 (comment) where CEF would deadlock on launch on macOS. I traced it to an incorrect assumption in my code, that GStreamer would always run in a separate thread; in the supplied app, GStreamer is handled from the Cocoa event loop.

While this was already addressed for the CEF startup itself, it didn't account for GStreamer's code flow before CEF initialization (GstElement's start handler runs before the call to run_cef) and on shutdown (there was no check for main thread, so the deadlock would've occurred on the call to dispatch_async_f).

Let me know if this is OK; I tested it locally with gst-launch-1.0 and observed no issues.

cc @reinismu , can you check with your Nix recipe?

[amyspark]

I found another one, this one comes from existing code. CefPostTask does not seem to if it's running in the main thread, so it'll post the task asynchronously. Then the semaphore will wait endlessly. I've added the same checking code as in src_start and src_stop.

With that I get the app running, it seems to be waiting for an outside connection -- I take that's expected @reinismu .

[sdroege]

set_caps() and set_property() are calling browser APIs but those would be called from arbitrary threads. Is that OK?

[amyspark]

set_caps() and set_property() are calling browser APIs but those would be called from arbitrary threads. Is that OK?

Yes, that's OK. They get processed by separate threads within CEF.

[sdroege]

Yes, that's OK. They get processed by separate threads within CEF.

This API is very confusing :)

[sdroege]

The g_cond_wait() 10 lines above would also wait forever if we end up here on macOS on the main thread or not?

[amyspark]

Not after this MR. If GStreamer is managed from the main thread, this function gets called by line 762 above, by which time either CEF has initialized (skipping the wait altogether), or it has failed (and this function will not be called at all). The wait is still necessary if the app handles CEF in a different event loop from the GStreamer pipeline's, like with gst-launch-1.0.