gh-144774: Add critical section in BaseException.__setstate__

Lib/test/test_exceptions.py

@@ -1954,6 +1954,24 @@ def test_exec_set_nomemory_hang(self):
         self.assertGreater(len(output), 0)  # At minimum, should not hang
         self.assertIn(b"MemoryError", output)
 
+    def test_data_race(self):
+        from threading import Thread
+        import copy
+
+        E = Exception()
+
+        def func():
+            for i in range(100):
+                setattr(E, 'x', i)
+                copy.copy(E)
+
+        threads = [Thread(target=func) for _ in range(4)]
+
+        for t in threads:
+            t.start()
+
+        for t in threads:
+            t.join()
 
 class NameErrorTests(unittest.TestCase):
     def test_name_error_has_name(self):

Objects/exceptions.c

@@ -243,16 +243,23 @@ BaseException___setstate___impl(PyBaseExceptionObject *self, PyObject *state)
             PyErr_SetString(PyExc_TypeError, "state is not a dictionary");
             return NULL;
         }
+        int error = 0;
+        Py_BEGIN_CRITICAL_SECTION(state);
         while (PyDict_Next(state, &i, &d_key, &d_value)) {
             Py_INCREF(d_key);
             Py_INCREF(d_value);
             int res = PyObject_SetAttr((PyObject *)self, d_key, d_value);
             Py_DECREF(d_value);
             Py_DECREF(d_key);
             if (res < 0) {
-                return NULL;
+                error = 1;
+                break;
             }
         }
+        Py_END_CRITICAL_SECTION();
+        if (error) {
+            return NULL;
+        }
     }
     Py_RETURN_NONE;
 }