Issue #8428: Fix a race condition in multiprocessing.Pool when terminating

worker processes: new processes would be spawned while the pool is being
shut down.  Patch by Charles-François Natali.

1 files changed, 7 insertions, 2 deletions

diff --git a/Lib/multiprocessing/pool.py b/Lib/multiprocessing/pool.py
index ff7c29c..efa0056 100644
--- a/Lib/multiprocessing/pool.py
+++ b/Lib/multiprocessing/pool.py
@@ -322,6 +322,8 @@ class Pool(object):
         while pool._worker_handler._state == RUN and pool._state == RUN:
             pool._maintain_pool()
             time.sleep(0.1)
+        # send sentinel to stop workers
+        pool._taskqueue.put(None)
         debug('worker handler exiting')
 
     @staticmethod
@@ -440,7 +442,6 @@ class Pool(object):
         if self._state == RUN:
             self._state = CLOSE
             self._worker_handler._state = CLOSE
-            self._taskqueue.put(None)
 
     def terminate(self):
         debug('terminating pool')
@@ -474,7 +475,6 @@ class Pool(object):
 
         worker_handler._state = TERMINATE
         task_handler._state = TERMINATE
-        taskqueue.put(None)                 # sentinel
 
         debug('helping task handler/workers to finish')
         cls._help_stuff_finish(inqueue, task_handler, len(pool))
@@ -484,6 +484,11 @@ class Pool(object):
         result_handler._state = TERMINATE
         outqueue.put(None)                  # sentinel
 
+        # We must wait for the worker handler to exit before terminating
+        # workers because we don't want workers to be restarted behind our back.
+        debug('joining worker handler')
+        worker_handler.join()
+
         # Terminate workers which haven't already finished.
         if pool and hasattr(pool[0], 'terminate'):
             debug('terminating workers')