[Bug 753899] baseparse: push sticky events to downstreams for initial prerolling

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=753899

Edward Hervey <bilboed at bilboed.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |---

--- Comment #40 from Edward Hervey <bilboed at bilboed.com> ---
Reopening this bug report because it does introduce a regression. There is no
way for subclasses to tell the parent class that it doesn't have enough
information to negotiate to at least a compatible format.

For example:
* mpegaudioparse receives "audio/mpeg, mpegversion=1" (without any other
fields)
* A gap event comes in
* the base class negotiates to "audio/mpeg, mpegversion=1, layer=1,
mpegaudioversion=1, rate=8000, channels=1, parsed=true"
* the caps event get pushed downstream and a decoder gets selected for MP1 (say
avdec_mp1float)
* eventually data comes into mpegaudioparse and it negotiates to the *actual*
format (mpegaudioversion=3, i.e. mp3)
* the decoder fails to negotiate

While we do want to not delay gap events in parsers, we should only allow this
to happen if we do have enough information from upstream caps to negotiate to
something which will later be compatible with the actual data stream.

In this case, mpegaudioparse should *not* push caps event if it hasn't at least
got a clue as to which mpegaudioversion the stream corresponds to.

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