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Maybe can not shutdown ThreadPoolExecutor when call the method shutdown.
Though the variable of _shutdown is set to true in the method of shutdown, it may also reads the variable of _shutdown from cpu cache in the method of _worker, and the worst case is that it could see an out-of-date value of _shutdown forever. so need to acquire lock before reading the variable of _shutdown to make sure see an up-to-date value.
the following is the new code:
def _worker(executor_reference, work_queue):
 try:
 while True:
 work_item = work_queue.get(block=True)
 if work_item is not None:
 work_item.run()
 continue
 executor = executor_reference()
 
 shutdown = False
 with executor._shutdown_lock.acquire():
 shutdown = executor._shutdown
 
 # Exit if:
 # - The interpreter is shutting down OR
 # - The executor that owns the worker has been collected OR
 # - The executor that owns the worker has been shutdown.
 if _shutdown or executor is None or shutdown:
 # Notice other workers
 work_queue.put(None)
 return
 del executor
 except BaseException:
 _base.LOGGER.critical('Exception in worker', exc_info=True)
 def shutdown(self, wait=True):
 with self._shutdown_lock:
 self._shutdown = True
 self._work_queue.put(None)
 if wait:
 for t in self._threads:
 t.join()