Clear potential ref cycle between Process and Process target (#2470)

* Clear potential ref cycle between Process and Process target

Besides Process.join() not being called, this was an indirect cause of bpo-30775.
The threading module already does this.

* Add issue reference

Lib/test/_test_multiprocessing.py

@@ -191,6 +191,12 @@ def get_value(self):
 # Testcases
 #
 
+class DummyCallable:
+    def __call__(self, q, c):
+        assert isinstance(c, DummyCallable)
+        q.put(5)
+
+
 class _TestProcess(BaseTestCase):
 
     ALLOWED_TYPES = ('processes', 'threads')
@@ -469,6 +475,18 @@ class _TestProcess(BaseTestCase):
             for p in procs:
                 self.assertEqual(p.exitcode, -signal.SIGTERM)
 
+    def test_lose_target_ref(self):
+        c = DummyCallable()
+        wr = weakref.ref(c)
+        q = self.Queue()
+        p = self.Process(target=c, args=(q, c))
+        del c
+        p.start()
+        p.join()
+        self.assertIs(wr(), None)
+        self.assertEqual(q.get(), 5)
+
+
 #
 #
 #