From 09b90a037d18f5d4acdf1b14082e57bda78e85d3 Mon Sep 17 00:00:00 2001
From: Pablo Galindo <Pablogsal@gmail.com>
Date: Mon, 29 Mar 2021 23:38:51 +0100
Subject: [PATCH] bpo-43660: Fix crash when displaying exceptions with custom
 values for sys.stderr (GH-25075)

---
 Lib/test/test_sys.py                              | 15 +++++++++++++++
 .../2021-03-29-19-50-34.bpo-43660.scTgag.rst      |  3 +++
 Python/pythonrun.c                                |  3 ++-
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2021-03-29-19-50-34.bpo-43660.scTgag.rst

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 5b004c2b52d..ee39375af31 100644
--- a/Lib/test/test_sys.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-03-29-19-50-34.bpo-43660.scTgag.rst b/Misc/NEWS.d/next/Core and Builtins/2021-03-29-19-50-34.bpo-43660.scTgag.rst
new file mode 100644
index 00000000000..98419501d9e
--- /dev/null
+++ b/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.
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index b562875f8a4..1715cde49dd 100644
--- a/Python/pythonrun.c
+++ b/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 *