Video pipeline occasionally freezing

[Louis Simons]

I've been working on a video pipeline to create a video switcher (multiple
inputs, program and preview outputs).  It seems to work most of the time,
but occasionally refuses to start playing.  Based on the pipeline graphs (
http://imgur.com/a/460eY, same when working or frozen) it seems like the
graph is properly connected.

When the pipeline works, I get the following terminal output:

0:00:00.665432750  6088      FIXME                default
gstutils.c:3622:gst_pad_create_stream_id_printf_valist:<videotestsrc0:src>
Creating random stream-id, consider implementing a deterministic way of
creating a stream-id
0:00:00.665691820  6088      FIXME                default
gstutils.c:3622:gst_pad_create_stream_id_printf_valist:<videotestsrc1:src>
Creating random stream-id, consider implementing a deterministic way of
creating a stream-id
0:00:00.666065013  6088      FIXME                default
gstutils.c:3622:gst_pad_create_stream_id_printf_valist:<videotestsrc2:src>
Creating random stream-id, consider implementing a deterministic way of
creating a stream-id
0:00:00.674592283  6088      WARN                 default
gstvideopool.c:171:video_buffer_pool_set_config:<videobufferpool0> no caps
in config
0:00:00.675071739  6088      WARN                 default
gstvideopool.c:171:video_buffer_pool_set_config:<videobufferpool1> no caps
in config
0:00:00.675642853  6088      WARN                 default
gstvideopool.c:171:video_buffer_pool_set_config:<videobufferpool2> no caps
in config

When it is frozen:

0:00:00.704295405  6109      FIXME                default
gstutils.c:3622:gst_pad_create_stream_id_printf_valist:<videotestsrc0:src>
Creating random stream-id, consider implementing a deterministic way of
creating a stream-id
0:00:00.704414897  6109      FIXME                default
gstutils.c:3622:gst_pad_create_stream_id_printf_valist:<videotestsrc1:src>
Creating random stream-id, consider implementing a deterministic way of
creating a stream-id
0:00:00.704839956  6109      FIXME                default
gstutils.c:3622:gst_pad_create_stream_id_printf_valist:<videotestsrc2:src>
Creating random stream-id, consider implementing a deterministic way of
creating a stream-id
0:00:00.708171947  6109      WARN                 default
gstvideopool.c:171:video_buffer_pool_set_config:<videobufferpool0> no caps
in config
0:00:00.711269706  6109      WARN                GST_PADS
gstpad.c:3577:gst_pad_peer_query:<videotestsrc0:src> could not send sticky
events
0:00:00.711462476  6109      WARN                 default
gstvideopool.c:171:video_buffer_pool_set_config:<videobufferpool1> no caps
in config
0:00:00.711749323  6109      WARN                GST_PADS
gstpad.c:3577:gst_pad_peer_query:<videotestsrc2:src> could not send sticky
events
0:00:00.711973080  6109      WARN                 default
gstvideopool.c:171:video_buffer_pool_set_config:<videobufferpool2> no caps
in config
0:00:00.717317973  6109     WARN                 basesrc
gstbasesrc.c:2809:gst_base_src_loop:<videotestsrc0> error: Internal data
flow error.
0:00:00.717347425  6109      WARN                 basesrc
gstbasesrc.c:2809:gst_base_src_loop:<videotestsrc0> error: streaming task
paused, reason not-negotiated (-4)
0:00:00.720445073  6109      WARN                 basesrc
gstbasesrc.c:2809:gst_base_src_loop:<videotestsrc2> error: Internal data
flow error.
0:00:00.720553042  6109      WARN                 basesrc
gstbasesrc.c:2809:gst_base_src_loop:<videotestsrc2> error: streaming task
paused, reason not-negotiated (-4)

it sure sounds like somethings not negotiating.  I heard from tim on IRC
that it might be a race condition (and that this tends to happen with
videomixer), but short of randomly adding videoconvert elements (and I put
in a capsfilter to try and make it more deterministic) , I'm not sure the
proper way to debug where the negotiation is failing.  How does one go
about finding the root so as to create a stable and efficient pipeline?

Thanks,
Louis
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