scrapy-redis框架中，reids存储的xxx:requests已经爬取完毕，但程序仍然一直运行，如何自动停止程序，而不是一直在空跑？

2017-07-03 09:17:06 [scrapy.extensions.logstats] INFO: Crawled 0 pages (at 0 pages/min), scraped 0 items (at 0 items/min)
2017-07-03 09:18:06 [scrapy.extensions.logstats] INFO: Crawled 0 pages (at 0 pages/min), scraped 0 items (at 0 items/min)

可以通过engine.close_spider(spider, 'reason')来停止程序的运行。

    def next_request(self):
        block_pop_timeout = self.idle_before_close
        request = self.queue.pop(block_pop_timeout)
        if request and self.stats:
            self.stats.inc_value('scheduler/dequeued/redis', spider=self.spider)
        if request is None:
            self.spider.crawler.engine.close_spider(self.spider, 'queue is empty')
        return request

还有一个问题不明白：
当通过engine.close_spider(spider, 'reason')来关闭spider时，会出现几个错误之后才能关闭。

# 正常关闭
2017-07-03 18:02:38 [scrapy.core.engine] INFO: Closing spider (queue is empty)
2017-07-03 18:02:38 [scrapy.statscollectors] INFO: Dumping Scrapy stats:
{'finish_reason': 'queue is empty',
 'finish_time': datetime.datetime(2017, 7, 3, 10, 2, 38, 616021),
 'log_count/INFO': 8,
 'start_time': datetime.datetime(2017, 7, 3, 10, 2, 38, 600382)}
2017-07-03 18:02:38 [scrapy.core.engine] INFO: Spider closed (queue is empty)
# 之后还会出现几个错误才关闭spider，难道spider刚启动时会启动多个线程一起抓取， 
# 然后其中一个线程关闭了spider，其他线程就找不到spider才会报错！
Unhandled Error
Traceback (most recent call last):
  File "D:/papp/project/launch.py", line 37, in <module>
    process.start()
  File "D:\Program Files\python3\lib\site-packages\scrapy\crawler.py", line 285, in start
    reactor.run(installSignalHandlers=False)  # blocking call
  File "D:\Program Files\python3\lib\site-packages\twisted\internet\base.py", line 1243, in run
    self.mainLoop()
  File "D:\Program Files\python3\lib\site-packages\twisted\internet\base.py", line 1252, in mainLoop
    self.runUntilCurrent()
--- <exception caught here> ---
  File "D:\Program Files\python3\lib\site-packages\twisted\internet\base.py", line 878, in runUntilCurrent
    call.func(*call.args, **call.kw)
  File "D:\Program Files\python3\lib\site-packages\scrapy\utils\reactor.py", line 41, in __call__
    return self._func(*self._a, **self._kw)
  File "D:\Program Files\python3\lib\site-packages\scrapy\core\engine.py", line 137, in _next_request
    if self.spider_is_idle(spider) and slot.close_if_idle:
  File "D:\Program Files\python3\lib\site-packages\scrapy\core\engine.py", line 189, in spider_is_idle
    if self.slot.start_requests is not None:
builtins.AttributeError: 'NoneType' object has no attribute 'start_requests'