[Bug 772457] Delayed linking failed with decodebin on mp4/H264 file

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=772457

--- Comment #1 from Florent Thiéry <florent.thiery at ubicast.eu> ---
The following pipeline is cleaner and reproduces the problem (sorry for the
bogus one in my initial report):

gst-launch-1.0 filesrc location=bbb-1920-1080-30.mp4 ! decodebin !
video/x-raw\,\ format\=\(string\)I420\,\ width\=\(int\)1920\,\
height\=\(int\)1080\,\ framerate\=\(fraction\)30/1 ! fakesink

This is equivalent to:
gst-launch-1.0 filesrc location=bbb-1920-1080-30.mp4 ! decodebin ! xvimagesink

ximagesink, xvimagesink, glimagesink fails; fakesink and vaapisink are the only
working sinks, so it.

Apparently, it is the NV12 to I420 conversion requested by downstream caps from
decodebin that seems to fail. Adding videoconvert after decodebin fixes it.

Interestingly, vaapih264dec succeeds in providing I420 data:
gst-launch-1.0 filesrc location=bbb-1920-1080-30.mp4 ! qtdemux ! vaapih264dec !
video/x-raw\,\ format\=\(string\)I420\,\ width\=\(int\)1920\,\
height\=\(int\)1080\,\ framerate\=\(fraction\)30/1 ! fakesink -v

Using vaapidecodebin does work partially (it seems to add vaapipostproc after
the decoder), but this freezes my machine heavily, which you can replicate
using:

gst-launch-1.0 filesrc location=bbb-1920-1080-30.mp4 ! qtdemux ! vaapih264dec !
vaapipostproc  ! video/x-raw\,\ format\=\(string\)I420\,\ width\=\(int\)1920\,\
height\=\(int\)1080\,\ framerate\=\(fraction\)30/1 ! fakesink -v

NB: this is on haswell

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