[Bug 745377] New: v4l2src: Camera restarts when used with decodebin

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=745377

            Bug ID: 745377
           Summary: v4l2src: Camera restarts when used with decodebin
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gstreamer (core)
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: nicolas.dufresne at collabora.co.uk
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

When using decodebin, v4l2src receives a reconfigure-event. Even though it
won't renegotiate, a new allocation query is run. In the decide allocation,
v4l2 sets the negotiated configuration on the pool but the pool is active. This
configuation is different because the pool do small adjustments.

Currently the pool check for equality of the structure, and try to stop the
pool in order to set the new configuration. I think the pool should not try to
stop the configuration. Stopping the pool, cause the streaming to stop and
start (which takes a bit of time with UVC cameras). In the end, the
configuration didn't change, so there was no reason to stop the pool.

I think start/stop the pool should be left to the user of the pool and not
implicit. Instead, what the pool could do (and this is according to the doc) is
simply return FALSE on the set_config. The caller will then read back the pool
configuration (the active one) and check if it's usable.

This way moves back the task of start/stop of the pool the controlling element
instead of being implicit.

Pipeline to reproduce:
v4l2src ! image/jpeg ! decodebin ! autovideosink

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