[Bug 796640] Fix for a synchronization bug in 1.14.1 ext/qt/gstqsgtexture.cc

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796640

Matthew Waters (ystreet00) <ystreet00 at gmail.com> changed:

           What    |Removed                     |Added
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 Attachment #372742|none                        |needs-work
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--- Comment #11 from Matthew Waters (ystreet00) <ystreet00 at gmail.com> ---
Review of attachment 372742:
 --> (https://bugzilla.gnome.org/review?bug=796640&attachment=372742)

What you might be seeing is a use-before-release where qmlglsink releases the
buffer before Qt has drawn with it and could be written too by upstream. 
Fixing that involves keeping the buffer and/or gst_video_frame_map () alive
between successive bind()  calls (or error/destruction).

::: ext/qt/gstqsgtexture.cc
@@ +152,3 @@
+  *  since the memory in buffer_ is used by other threads
+  */
+  gst_gl_sync_meta_wait_cpu (sync_meta, this->qt_context_);

This is actually the wrong API to cause a wait and will cause a glFinish() (or
equivalent in GL fences) which is way to heavy handed for this.  The fact that
they're used by other threads/contexts does not matter in GPU space.

You only need to wait for the GPU to complete (of which there is already code
to do that).

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