bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr

Lib/test/test_sys.py

@@ -1512,6 +1512,21 @@ def test_asyncgen_hooks(self):
         self.assertIsNone(cur.firstiter)
         self.assertIsNone(cur.finalizer)
 
+    def test_changing_sys_stderr_and_removing_reference(self):
+        # If the default displayhook doesn't take a strong reference
+        # to sys.stderr the following code can crash. See bpo-43660
+        # for more details.
+        code = textwrap.dedent('''
+            import sys
+            class MyStderr:
+                def write(self, s):
+                    sys.stderr = None
+            sys.stderr = MyStderr()
+            1/0
+        ''')
+        rc, out, err = assert_python_failure('-c', code)
+        self.assertEqual(out, b"")
+        self.assertEqual(err, b"")
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2021-03-29-19-50-34.bpo-43660.scTgag.rst

@@ -0,0 +1,3 @@
+Fix crash that happens when replacing ``sys.stderr`` with a callable that
+can remove the object while an exception is being printed. Patch by Pablo
+Galindo.

Python/pythonrun.c

@@ -1082,8 +1082,9 @@ PyErr_Display(PyObject *exception, PyObject *value, PyObject *tb)
     if (file == Py_None) {
         return;
     }
-
+    Py_INCREF(file);
     _PyErr_Display(file, exception, value, tb);
+    Py_DECREF(file);
 }
 
 PyObject *