[Bug 788754] gl: wayland: sometimes block pipeline at PREROLLED

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=788754

--- Comment #14 from Matthew Waters (ystreet00) <ystreet00 at gmail.com> ---
(In reply to Nicolas Dufresne (stormer) from comment #12)
> Now, if this is racing, it means that external compoenent are also doing
> some stuff on the queue from other threads. This is miss-use of wayland
> queues really.

Precisely.

> But it also waits if there is nothing to process, and that seems like the
> issue we are facing.

"nothing to process" is not quite correct. The wl_callback that is installed is
"something to process".  It would only hang if the wl_callback is processed
somewhere else.

> But it also waits if there is nothing to process, and that seems like the
> issue we are facing.

Which is why it's only safe to be called from the thread that will be reading
events from the specified wl_queue (as mentioned in the comment above
_roundtrip()) and thus my response in comment 9 asking for a backtrace if this
was the case or not when the hang occurs and if this is a problem in GStreamer
itself.

(In reply to Nicolas Dufresne (stormer) from comment #13)
> Btw, if you can find back such an application that would show that a
> roundtrip inside _show() call is sometimes needed, that could unblock this
> issue. I think user-input in full app is what makes this case less visible,
> but could easily appear in kiosk or digital signage applications. This
> random roundtrip call looks generally harmful and random.

IIRC, the gtk videooverlay examples in -bad exhibited the hang sporadically.

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