Add a test for #327 (#334)

tests/cloudpickle_test.py

@@ -108,6 +108,28 @@ def setUp(self):
     def tearDown(self):
         shutil.rmtree(self.tmpdir)
 
+    @pytest.mark.skipif(
+            platform.python_implementation() != "CPython" or
+            (sys.version_info >= (3, 8, 0) and sys.version_info < (3, 8, 2)),
+            reason="Underlying bug fixed upstream starting Python 3.8.2")
+    def test_reducer_override_reference_cycle(self):
+        # Early versions of Python 3.8 introduced a reference cycle between a
+        # Pickler and it's reducer_override method. Because a Pickler
+        # object references every object it has pickled through its memo, this
+        # cycle prevented the garbage-collection of those external pickled
+        # objects. See #327 as well as https://bugs.python.org/issue39492
+        # This bug was fixed in Python 3.8.2, but is still present using
+        # cloudpickle and Python 3.8.0/1, hence the skipif directive.
+        class MyClass:
+            pass
+
+        my_object = MyClass()
+        wr = weakref.ref(my_object)
+
+        cloudpickle.dumps(my_object)
+        del my_object
+        assert wr() is None, "'del'-ed my_object has not been collected"
+
     def test_itemgetter(self):
         d = range(10)
         getter = itemgetter(1)