playbin about-to-finish signal thread locking

[Robert Rosengren]

Hi,

I have observed a thread deadlocking problem in my application using playbin,  and having it connected to about-to-finish signal. From my application's perspective I need to protect data with mutex since the signal will run in a gstreamer thread while application otherwise use glib mainloop. 

So, if application mainloop currently holds mutex and trying to stop pipeline by setting to NULL state, at the same time as the signal callback is received trying to take same mutex. Internally in gstreamer there is also a mutex that both threads wants, the signal callback holds it while my application mainloop wants to take it.

Should it not be possible to use mutex from within about-to-finish callback? 

Internal gstreamer lock involved in gstpad:
#4  0xb6d71f9e in post_activate (new_mode=GST_PAD_MODE_NONE, pad=0x15c5a10) at ../gstreamer-1.16.2/gst/gstpad.c:1046

Kind of simple to trigger in a small application, just add g_idle_add from about-to-finish callback with a function that takes a mutex and tries to set NULL state, and make sure callback waits for it to happen and then take same mutex :)

Any ideas? Is it a bug in gstreamer not unlocking before calling signal callback, or my application is doing something it shouldn't?

Thanks and regards,
Robert