_struct: fix race condition in cache_struct_converter

colesbury · colesbury · commit ada9b73feba9 · 2023-04-23T12:51:05.000-07:00
diff --git a/Modules/_struct.c b/Modules/_struct.c
@@ -2239,15 +2239,9 @@ cache_struct_converter(PyObject *module, PyObject *fmt, PyStructObject **ptr)
         return 1;
     }
 
-    if (state->cache == NULL) {
-        state->cache = PyDict_New();
-        if (state->cache == NULL)
-            return 0;
-    }
-
-    s_object = PyDict_GetItemWithError(state->cache, fmt);
+    s_object = PyDict_FetchItemWithError(state->cache, fmt);
     if (s_object != NULL) {
-        *ptr = (PyStructObject *)Py_NewRef(s_object);
+        *ptr = (PyStructObject *)s_object;
         return Py_CLEANUP_SUPPORTED;
     }
     else if (PyErr_Occurred()) {
@@ -2277,7 +2271,7 @@ static PyObject *
 _clearcache_impl(PyObject *module)
 /*[clinic end generated code: output=ce4fb8a7bf7cb523 input=463eaae04bab3211]*/
 {
-    Py_CLEAR(get_struct_state(module)->cache);
+    PyDict_Clear(get_struct_state(module)->cache);
     Py_RETURN_NONE;
 }
 
@@ -2506,6 +2500,12 @@ _structmodule_exec(PyObject *m)
     if (state->PyStructType == NULL) {
         return -1;
     }
+
+    state->cache = PyDict_New();
+    if (state->cache == NULL) {
+        return -1;
+    }
+
     if (PyModule_AddType(m, (PyTypeObject *)state->PyStructType) < 0) {
         return -1;
     }
