[Bug 758703] New: v4l2src: gst_v4l2_set_attribute warning messages cause infinite loop with .dot dump

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=758703

            Bug ID: 758703
           Summary: v4l2src: gst_v4l2_set_attribute warning messages cause
                    infinite loop with .dot dump
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: All
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: patcherwork at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

I am developing a gstreamer pipeline on a v4l2 device using gstreamer. While
trying to dump the .dot files using gst-launch I ran into an infinite message
loop. When a dot file is dumped from such a pipeline, it iterates over the
properties of the v4l2src, amongst which are the Hue, Saturation, Brightness
and Contrast controls. the dot dump tries to query the controls, which in my
case are not available. This causes the ioctl to fail, issuing a warning on the
bus. the new warning causes a new dot dump which causes a new set of warnings
to get posted. 

Trying to query the controls for unavailable ones is also not an option, since
according to the v4l2 API for VIDIOC_QUERYCTRL: "Drivers may return EINVAL if a
control in this range is not supported". Thus filtering the controls that are
not available is also not possible. 

A simple fix presented here is to change the warning handling so it doesn't
post on the bus.

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