Changing source without stopping pipeline

[superlou]

I have a simplified test pipeline consisting of:
v4l2src -> ffmpegcolorspace --> videoscale --> ximagesink

I would like to detect a loss of signal from the v4lsrc (currently doing by
waiting for the EOS signal, though the v42lsrc provides better warnings on
camera disconnection if I can figure out how to get them).  For now, I'm
trying to achieve the simpler case of simply switching the source element to
a videotestsrc while the pipeline is playing.

I've read the information about pad blocking
(http://cgit.freedesktop.org/gstreamer/gstreamer/tree/docs/design/part-block.txt),
but it doesn't provide any indication on what to do for changing the source
element.  I read that it may be possible to simply unlink the v4l2src,
remove it, add the videotestsrc, and link it.  This sort of works in that
eventually you see the videotestsrc, but it updates the output every few
seconds (like a very slow stuttering).  If I stop and play, the output is
smooth.

I was wondering if anyone could point me towards an example or help me find
the search terms to find the proper way to change out the source element on
a playing pipeline.

Thanks,
Louis

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