'Delete any image files in the invalid directory and any
files in the incomplete directory that are older than a
configurable amount of time.'

def clean(self, stall_time=None):

    self.delete_invalid_files()
    if (stall_time is None):
        stall_time = CONF.image_cache_stall_time
    now = time.time()
    older_than = (now - stall_time)
    self.delete_stalled_files(older_than)
