gh-120198: Fix race condition when editing __class__ with an audit hook active (GH-120195)

(cherry picked from commit 203565b2f9)

Co-authored-by: Ken Jin <kenjin@python.org>

Lib/test/test_super.py

@@ -1,9 +1,10 @@
 """Unit tests for zero-argument super() & related machinery."""
 
 import textwrap
+import threading
 import unittest
 from unittest.mock import patch
-from test.support import import_helper
+from test.support import import_helper, threading_helper
 
 
 ADAPTIVE_WARMUP_DELAY = 2
@@ -505,6 +506,38 @@ class TestSuper(unittest.TestCase):
         for _ in range(ADAPTIVE_WARMUP_DELAY):
             C.some(C)
 
+    @threading_helper.requires_working_threading()
+    def test___class___modification_multithreaded(self):
+        """ Note: this test isn't actually testing anything on its own.
+        It requires a sys audithook to be set to crash on older Python.
+        This should be the case anyways as our test suite sets
+        an audit hook.
+        """
+        class Foo:
+            pass
+
+        class Bar:
+            pass
+
+        thing = Foo()
+        def work():
+            foo = thing
+            for _ in range(5000):
+                foo.__class__ = Bar
+                type(foo)
+                foo.__class__ = Foo
+                type(foo)
+
+
+        threads = []
+        for _ in range(6):
+            thread = threading.Thread(target=work)
+            thread.start()
+            threads.append(thread)
+
+        for thread in threads:
+            thread.join()
+
 
 if __name__ == "__main__":
     unittest.main()