pythongh-112062: Make _struct module thread-safe in --disable-gil b…

…uilds

* use `PyDict_GetItemRef` instead of `PyDict_GetItemWithError` in `cache_struct_converter` to avoid usage of borrowed ref with explicit incref
* clear cache dict but preserve the reference in `_clearcache_impl`
* create cache dict in `_structmodule_exec`

Modules/_struct.c

@@ -2250,20 +2250,12 @@ 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;
+    if (PyDict_GetItemRef(state->cache, fmt, ptr) < 0) {
+        return 0;
     }
-
-    s_object = PyDict_GetItemWithError(state->cache, fmt);
-    if (s_object != NULL) {
-        *ptr = (PyStructObject *)Py_NewRef(s_object);
+    if (*ptr != NULL) {
         return Py_CLEANUP_SUPPORTED;
     }
-    else if (PyErr_Occurred()) {
-        return 0;
-    }
 
     s_object = PyObject_CallOneArg(state->PyStructType, fmt);
     if (s_object != NULL) {
@@ -2288,7 +2280,7 @@ 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;
 }
 
@@ -2512,6 +2504,11 @@ _structmodule_exec(PyObject *m)
 {
     _structmodulestate *state = get_struct_state(m);
 
+    state->cache = PyDict_New();
+    if (state->cache == NULL) {
+        return -1;
+    }
+
     state->PyStructType = PyType_FromModuleAndSpec(
         m, &PyStructType_spec, NULL);
     if (state->PyStructType == NULL) {