'Spawns a clean task if not running.'

@utils.synchronized('clean_cache')
def _spawn_clean_cache_job(self):

    if getattr(self, 'cleaning', None):
        LOG.debug('Image    cache    cleaning    in    progress.    Returning...    ')
        return
    else:
        self.cleaning = True
        t = threading.Timer(0, self._clean_image_cache)
        t.start()
