code: def _schedule_slice(cls,
                        shard_state,
                        tstate,
                        queue_name=None,
                        eta=None,
                        countdown=None):
      
      queue_name = queue_name or os.environ.get("HTTP_X_APPENGINE_QUEUENAME",
                                                "default")
      task = cls._state_to_task(tstate, shard_state, eta, countdown)
      cls._add_task(task, tstate.mapreduce_spec, queue_name)
docs: Schedule slice scanning by adding it to the task queue.
  
      Args:
        shard_state: An instance of ShardState.
        tstate: An instance of TransientShardState.
        queue_name: Optional queue to run on; uses the current queue of
          execution or the default queue if unspecified.
        eta: Absolute time when the MR should execute. May not be specified
          if 'countdown' is also supplied. This may be timezone-aware or
          timezone-naive.
        countdown: Time in seconds into the future that this MR should execute.
          Defaults to zero.