pythongh-129983: fix data race in compile_template in sre.c (python#130015)

Author: tom-pytel

Misc/NEWS.d/next/Core_and_Builtins/2025-02-11-20-38-37.gh-issue-129983._1Fujo.rst

@@ -0,0 +1 @@
+Fix data race in compile_template in :file:`sre.c`.

Modules/_sre/sre.c

@@ -1167,13 +1167,21 @@ compile_template(_sremodulestate *module_state,
                  PatternObject *pattern, PyObject *template)
 {
     /* delegate to Python code */
-    PyObject *func = module_state->compile_template;
+    PyObject *func = FT_ATOMIC_LOAD_PTR(module_state->compile_template);
     if (func == NULL) {
         func = PyImport_ImportModuleAttrString("re", "_compile_template");
         if (func == NULL) {
             return NULL;
         }
+#ifdef Py_GIL_DISABLED
+        PyObject *other_func = NULL;
+        if (!_Py_atomic_compare_exchange_ptr(&module_state->compile_template, &other_func, func))  {
+            Py_DECREF(func);
+            func = other_func;
+        }
+#else
         Py_XSETREF(module_state->compile_template, func);
+#endif
     }
 
     PyObject *args[] = {(PyObject *)pattern, template};