gh-144774: Add critical section in BaseException.__setstate__

Lib/test/test_exceptions.py

@@ -1954,7 +1954,6 @@ def test_exec_set_nomemory_hang(self):
         self.assertGreater(len(output), 0)  # At minimum, should not hang
         self.assertIn(b"MemoryError", output)
 
-
 class NameErrorTests(unittest.TestCase):
     def test_name_error_has_name(self):
         try:

Lib/test/test_free_threading/test_exceptions.py

@@ -0,0 +1,16 @@
+import unittest
+import copy
+
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+class ExceptionTests(unittest.TestCase):
+    def test_setstate_data_race(self):
+        E = Exception()
+
+        def func():
+            for i in range(100):
+                setattr(E, 'x', i)
+                copy.copy(E)
+
+        threading_helper.run_concurrently(func, nthreads=4)

Misc/NEWS.d/next/Core_and_Builtins/2026-05-29-17-51-11.gh-issue-144774.jPcixe.rst

@@ -0,0 +1 @@
+Fix data race in :meth:`BaseException.__setstate__` when an exception is copied while being mutated.

Objects/exceptions.c

@@ -225,15 +225,15 @@ BaseException___reduce___impl(PyBaseExceptionObject *self)
  */
 
 /*[clinic input]
-@critical_section
+@critical_section state
 BaseException.__setstate__
     state: object
     /
 [clinic start generated code]*/
 
 static PyObject *
 BaseException___setstate___impl(PyBaseExceptionObject *self, PyObject *state)
-/*[clinic end generated code: output=f3834889950453ab input=5524b61cfe9b9856]*/
+/*[clinic end generated code: output=f3834889950453ab input=2ddc941f42fc5bf6]*/
 {
     PyObject *d_key, *d_value;
     Py_ssize_t i = 0;