failure to read from webcam

[Alessio Treglia]

This bug was reported to Debian (bts #555106)

The reporter said the following:

Package: arista
Version: 0.9.3-1

I'm trying to use a webcam to read a video. The webcam is supported with the
video-4-linux interface, so that part should not be the problem, xawtv for
example works.

Now, what I do is this:

  $ arista-transcode /dev/video0 ./video.out
  AttributeError: 'module' object has no attribute 'Element'
  Encoding video0 for computer (default)

From now on, the application just sits there, which isn't surprising, since
the input stream doesn't have an end. The confusing part is the
"AttributeError.." message, which may or may not be important. In any case, I
try to interrupt the thing with control-C:

  ^C
  Interrupt caught. Cleaning up... (Ctrl-C to force exit)
  Traceback (most recent call last):
    File "/usr/bin/arista-transcode", line 121, in check_interrupted
      source = transcoder.pipe.get_by_name("source")
  AttributeError: 'NoneType' object has no attribute 'get_by_name'

Hmm, interesting, I guess it shouldn't do that. Note that this is in
check_interrupted(), where it tries to access the message queue (pipe) of the
transcoder, it seems. It then sits there until I interrupt it with another
control-C.

What I tried also, after reading the source a bit, was to not use /dev/video0
but the (*otherwise undocumented!*) v4l:///dev/video0 syntax:

  $ arista-transcode v4l:///dev/video0 ./video.out
  AttributeError: 'module' object has no attribute 'Element'
  Encoding video0 for computer (default)
  No audio or video streams found in input!

I'm dropped back at the shell, with at least an error message. Problem there
is that I can also change the input specifier to "v4l://can/kiss/my/butt", the
error message remains the same, i.e. completely useless IMHO.

Okay, V4L version 1 is obsolete anyway, so let's try version 2:

  $ arista-transcode v4l2:///dev/video0 ./video.out
  AttributeError: 'module' object has no attribute 'Element'
  Encoding video0 for computer (default)
  Starting pass 1 of 1
  libv4l2: error reading: No such device

It then sits there, so I interrupt it with control-C again. Only this time, it
doesn't care, it keeps sitting there no matter what. I can kill the process
from a different shell or by putting it in the background though. A difference
to the first attempt is that it has managed to create the output file,
although it is empty. A difference to the second attempt is that it matters
which input device I select.

Further notes:
 * All above attempts were made using the root account, so permissions
shouldn't be the problem, unless you are trying to access an X session or
something like that.
 * If I try the v4l2:// URL from a different user account, I get the rather
meaningless "No audio or video streams found in input!" error message, just
like the v4l:// URL.
 * If I try the v4l2:// URL from the user running X, it gets to "Starting pass
1 of 1" and then sits there until I control-C out of it.

[Alessio Treglia (quadrispro)]

http://github.com/danielgtaylor/arista/issues/#issue/9