python · kumaraditya303 · Jan 9, 2023 · May 16, 2019 · May 16, 2019 · Jan 1, 2023
diff --git a/Lib/ctypes/test/test_refcounts.py b/Lib/ctypes/test/test_refcounts.py
@@ -97,5 +97,31 @@ def func(a, b):
         f(1, 2)
         self.assertEqual(sys.getrefcount(ctypes.c_int), a)
 
+    @support.refcount_test
+    def test_callback_py_object_none_return(self):
+        """Test that returning ``None`` from a ``py_object`` callback
+        does not affect ``None``'s refcount (bpo-36880)."""
+
+        import sys
+
+        for FUNCTYPE in (ctypes.CFUNCTYPE, ctypes.PYFUNCTYPE):
+            with self.subTest(FUNCTYPE=FUNCTYPE):
+                proto = FUNCTYPE(ctypes.py_object)
+                @proto
+                def func():
+                    return None
+
+                # Check that calling func does not affect None's refcount.
+                none_refcount = sys.getrefcount(None)
+                # Because None's refcount can also change for other reasons,
+                # we call func in a loop to ensure that any effects on None's
+                # refcount are clearly visible.
+                for _ in range(10000):
+                    func()
+                # Allow for small variations in None's refcount from other
+                # sources.
+                self.assertAlmostEqual(
+                    sys.getrefcount(None), none_refcount, delta=50)
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2019-05-13-11-37-30.bpo-36880.ZgBgH0.rst b/Misc/NEWS.d/next/Library/2019-05-13-11-37-30.bpo-36880.ZgBgH0.rst
@@ -0,0 +1,2 @@
+Fix a reference counting issue when a :mod:`ctypes` callback with return
+type :class:`~ctypes.py_object` returns ``None``, which could cause crashes.
diff --git a/Modules/_ctypes/callbacks.c b/Modules/_ctypes/callbacks.c
@@ -270,15 +270,16 @@ if (x == NULL) _PyTraceback_Add(what, "_ctypes/callbacks.c", __LINE__ - 1), PyEr
            be the result.  EXCEPT when restype is py_object - Python
            itself knows how to manage the refcount of these objects.
         */
-        if (keep == NULL) /* Could not convert callback result. */
+        if (keep == NULL) { /* Could not convert callback result. */
             PyErr_WriteUnraisable(callable);
-        else if (keep == Py_None) /* Nothing to keep */
-            Py_DECREF(keep);
-        else if (setfunc != _ctypes_get_fielddesc("O")->setfunc) {
-            if (-1 == PyErr_WarnEx(PyExc_RuntimeWarning,
-                                   "memory leak in callback function.",
-                                   1))
+        } else if (setfunc != _ctypes_get_fielddesc("O")->setfunc) {
+            if (keep == Py_None) { /* Nothing to keep */
+                Py_DECREF(keep);
+            } else if (PyErr_WarnEx(PyExc_RuntimeWarning,
+                                    "memory leak in callback function.",
+                                    1) == -1) {
                 PyErr_WriteUnraisable(callable);
+            }
         }
     }
     Py_XDECREF(result);
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix a reference counting issue when a :mod:`ctypes` callback with return
		type :class:`~ctypes.py_object` returns ``None``, which could cause crashes.