bpo-31699 Deadlocks in concurrent.futures.ProcessPoolExecutor with pickling error (#3895)

Fix deadlocks in :class:`concurrent.futures.ProcessPoolExecutor` when task arguments or results cause pickling or unpickling errors.
This should make sure that calls to the :class:`ProcessPoolExecutor` API always eventually return.

Lib/test/_test_multiprocessing.py

@@ -1029,6 +1029,43 @@ class _TestQueue(BaseTestCase):
             self.assertTrue(q.get(timeout=1.0))
             close_queue(q)
 
+    def test_queue_feeder_on_queue_feeder_error(self):
+        # bpo-30006: verify feeder handles exceptions using the
+        # _on_queue_feeder_error hook.
+        if self.TYPE != 'processes':
+            self.skipTest('test not appropriate for {}'.format(self.TYPE))
+
+        class NotSerializable(object):
+            """Mock unserializable object"""
+            def __init__(self):
+                self.reduce_was_called = False
+                self.on_queue_feeder_error_was_called = False
+
+            def __reduce__(self):
+                self.reduce_was_called = True
+                raise AttributeError
+
+        class SafeQueue(multiprocessing.queues.Queue):
+            """Queue with overloaded _on_queue_feeder_error hook"""
+            @staticmethod
+            def _on_queue_feeder_error(e, obj):
+                if (isinstance(e, AttributeError) and
+                        isinstance(obj, NotSerializable)):
+                    obj.on_queue_feeder_error_was_called = True
+
+        not_serializable_obj = NotSerializable()
+        # The captured_stderr reduces the noise in the test report
+        with test.support.captured_stderr():
+            q = SafeQueue(ctx=multiprocessing.get_context())
+            q.put(not_serializable_obj)
+
+            # Verify that q is still functionning correctly
+            q.put(True)
+            self.assertTrue(q.get(timeout=1.0))
+
+        # Assert that the serialization and the hook have been called correctly
+        self.assertTrue(not_serializable_obj.reduce_was_called)
+        self.assertTrue(not_serializable_obj.on_queue_feeder_error_was_called)
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