Close #18469: Replace PyDict_GetItemString() with _PyDict_GetItemId() in structseq.c

_PyDict_GetItemId() is more efficient: it only builds the Unicode string once.
Identifiers (dictionary keys) are now created at Python initialization, and if
the creation failed, Python does exit with a fatal error.

Before, PyDict_GetItemString() failure was not handled: structseq_new() could
call PyObject_GC_NewVar() with a negative size, and structseq_dealloc() could
also crash.

Objects/structseq.c

@@ -11,17 +11,20 @@ static char unnamed_fields_key[] = "n_unnamed_fields";
 /* Fields with this name have only a field index, not a field name.
    They are only allowed for indices < n_visible_fields. */
 char *PyStructSequence_UnnamedField = "unnamed field";
+_Py_IDENTIFIER(n_sequence_fields);
+_Py_IDENTIFIER(n_fields);
+_Py_IDENTIFIER(n_unnamed_fields);
 
 #define VISIBLE_SIZE(op) Py_SIZE(op)
 #define VISIBLE_SIZE_TP(tp) PyLong_AsLong( \
-                      PyDict_GetItemString((tp)->tp_dict, visible_length_key))
+                      _PyDict_GetItemId((tp)->tp_dict, &PyId_n_sequence_fields))
 
 #define REAL_SIZE_TP(tp) PyLong_AsLong( \
-                      PyDict_GetItemString((tp)->tp_dict, real_length_key))
+                      _PyDict_GetItemId((tp)->tp_dict, &PyId_n_fields))
 #define REAL_SIZE(op) REAL_SIZE_TP(Py_TYPE(op))
 
 #define UNNAMED_FIELDS_TP(tp) PyLong_AsLong( \
-                      PyDict_GetItemString((tp)->tp_dict, unnamed_fields_key))
+                      _PyDict_GetItemId((tp)->tp_dict, &PyId_n_unnamed_fields))
 #define UNNAMED_FIELDS(op) UNNAMED_FIELDS_TP(Py_TYPE(op))
 
 
@@ -382,9 +385,21 @@ PyStructSequence_InitType(PyTypeObject *type, PyStructSequence_Desc *desc)
 PyTypeObject*
 PyStructSequence_NewType(PyStructSequence_Desc *desc)
 {
-    PyTypeObject *result = (PyTypeObject*)PyType_GenericAlloc(&PyType_Type, 0);
+    PyTypeObject *result;
+
+    result = (PyTypeObject*)PyType_GenericAlloc(&PyType_Type, 0);
     if (result != NULL) {
         PyStructSequence_InitType(result, desc);
     }
     return result;
 }
+
+int _PyStructSequence_Init(void)
+{
+    if (_PyUnicode_FromId(&PyId_n_sequence_fields) == NULL
+        || _PyUnicode_FromId(&PyId_n_fields) == NULL
+        || _PyUnicode_FromId(&PyId_n_unnamed_fields) == NULL)
+        return -1;
+
+    return 0;
+}

Python/pythonrun.c

@@ -86,6 +86,7 @@ static void call_py_exitfuncs(void);
 static void wait_for_thread_shutdown(void);
 static void call_ll_exitfuncs(void);
 extern int _PyUnicode_Init(void);
+extern int _PyStructSequence_Init(void);
 extern void _PyUnicode_Fini(void);
 extern int _PyLong_Init(void);
 extern void PyLong_Fini(void);
@@ -336,6 +337,8 @@ _Py_InitializeEx_Private(int install_sigs, int install_importlib)
     /* Init Unicode implementation; relies on the codec registry */
     if (_PyUnicode_Init() < 0)
         Py_FatalError("Py_Initialize: can't initialize unicode");
+    if (_PyStructSequence_Init() < 0)
+        Py_FatalError("Py_Initialize: can't initialize structseq");
 
     bimod = _PyBuiltin_Init();
     if (bimod == NULL)