problematic gstreamer frame extraction

get_video_frame.py

def get_video_frame(full_file_name: str,
                    offset: Optional[float]=5.0,
                    caps=Gst.Caps.from_string('image/png')) -> Optional[bytes]:
    """
    Source: https://gist.github.com/dplanella/5563018

    :param full_file_name: file and path of the video
    :param offset:
    :param caps:
    :return: gstreamer buffer
    """
    logging.debug("Using gstreamer to generate thumbnail from %s", full_file_name)
    pipeline = Gst.parse_launch('playbin')
    pipeline.props.uri = 'file://{}'.format(pathname2url(os.path.abspath(full_file_name)))
    pipeline.props.audio_sink = Gst.ElementFactory.make('fakesink', 'fakeaudio')
    pipeline.props.video_sink = Gst.ElementFactory.make('fakesink', 'fakevideo')
    pipeline.set_state(Gst.State.PAUSED)
    # Wait for state change to finish.
    pipeline.get_state(Gst.CLOCK_TIME_NONE)

    # Seek offset seconds into the video, if the video is long enough
    # If video is shorter than offset, seek 0.25 seconds less than the duration,
    # but always at least zero.
    offset = max(
        min(
            pipeline.query_duration(Gst.Format.TIME)[1] - Gst.SECOND / 4, offset * Gst.SECOND
        ), 0
    )

    try:
        assert pipeline.seek_simple(Gst.Format.TIME, Gst.SeekFlags.FLUSH, offset)
    except AssertionError:
        return None
    # Wait for seek to finish.
    pipeline.get_state(Gst.CLOCK_TIME_NONE)
    sample = pipeline.emit('convert-sample', caps)
    if sample is not None:
        buffer = sample.get_buffer()
        pipeline.set_state(Gst.State.NULL)
        return buffer.extract_dup(0, buffer.get_size())
    else:
        return None