Issue #10332: multiprocessing: fix a race condition when a Pool is closed

before all tasks have completed.

2 files changed, 19 insertions, 1 deletions

diff --git a/Lib/multiprocessing/pool.py b/Lib/multiprocessing/pool.py
index e450319..04e7c44 100644
--- a/Lib/multiprocessing/pool.py
+++ b/Lib/multiprocessing/pool.py
@@ -321,7 +321,11 @@ class Pool(object):
 
     @staticmethod
     def _handle_workers(pool):
-        while pool._worker_handler._state == RUN and pool._state == RUN:
+        thread = threading.current_thread()
+
+        # Keep maintaining workers until the cache gets drained, unless the pool
+        # is terminated.
+        while thread._state == RUN or (pool._cache and thread._state != TERMINATE):
             pool._maintain_pool()
             time.sleep(0.1)
         # send sentinel to stop workers
diff --git a/Lib/test/test_multiprocessing.py b/Lib/test/test_multiprocessing.py
index 6940d0e..45bf454 100644
--- a/Lib/test/test_multiprocessing.py
+++ b/Lib/test/test_multiprocessing.py
@@ -1217,6 +1217,20 @@ class _TestPoolWorkerLifetime(BaseTestCase):
         p.close()
         p.join()
 
+    def test_pool_worker_lifetime_early_close(self):
+        # Issue #10332: closing a pool whose workers have limited lifetimes
+        # before all the tasks completed would make join() hang.
+        p = multiprocessing.Pool(3, maxtasksperchild=1)
+        results = []
+        for i in range(6):
+            results.append(p.apply_async(sqr, (i, 0.3)))
+        p.close()
+        p.join()
+        # check the results
+        for (j, res) in enumerate(results):
+            self.assertEqual(res.get(), sqr(j))
+
+
 #
 # Test that manager has expected number of shared objects left
 #