Influencing playbin2/decodebin2 auto-plugging

[Tim-Philipp Müller]

Hi,

> I am seeing a weird interaction between using autoplug-select and vaapi. The 
> attached reproducer does not transition into playing state for me unless I 
> remove the line where I connect my message handler to autoplug-select. 
> Interesting thing is that my handler in this configuration does not deviate 
> from the default behaviour ie. it always returns GST_AUTOPLUG_SELECT_TRY. 
> 
> In the no VA-API setup, with use_vaapi set to False at the top of the program, 
> playback works fine even with the autoplug-select handler installed.
> 
> Seems weird, any ideas?

I think playbin2 hooks up some of these signals as well
("autoplug-factories", "autoplug-select", "autoplug-continue"), so the
default for playbin2 would be whatever playbin2 does there.

Cheers
 -Tim