Slightly optimize Pool._taskqueue_buffersize_semaphores terminate

Author: obaltian

Lib/multiprocessing/pool.py

@@ -726,9 +726,10 @@ def _terminate_pool(cls, taskqueue, inqueue, outqueue, pool, change_notifier,
 
         task_handler._state = TERMINATE
         # Release all semaphores to wake up task_handler to stop.
-        for job_id, sema in tuple(taskqueue_buffersize_semaphores.items()):
-            taskqueue_buffersize_semaphores.pop(job_id, None)
-            sema.release()
+        for job_id in tuple(taskqueue_buffersize_semaphores.keys()):
+            sema = taskqueue_buffersize_semaphores.pop(job_id, None)
+            if sema is not None:
+                sema.release()
 
         util.debug('helping task handler/workers to finish')
         cls._help_stuff_finish(inqueue, task_handler, len(pool))