gh-150490: Raise PyType_Modified for insertion into split dictionary (take 2)

Include/internal/pycore_dict.h

@@ -90,6 +90,7 @@ typedef struct {
 } PyDictUnicodeEntry;
 
 extern PyDictKeysObject *_PyDict_NewKeysForClass(PyHeapTypeObject *);
+extern void _PyDict_RemoveKeysForClass(PyHeapTypeObject *);
 extern PyObject *_PyDict_FromKeys(PyObject *, PyObject *, PyObject *);
 
 /* Implementations of the `|` and `|=` operators for dict, used by the
@@ -239,6 +240,17 @@ struct _dictkeysobject {
        see the DK_ENTRIES() / DK_UNICODE_ENTRIES() functions below */
 };
 
+struct _instancekeysobject {
+    PyTypeObject* dsk_owning_type;
+    struct _dictkeysobject dsk_keys;
+};
+
+static inline struct _instancekeysobject *_PyDictKeys_AsSharedKeys(struct _dictkeysobject *keys)
+{
+    assert(keys->dk_kind == DICT_KEYS_SPLIT);
+    return _Py_CONTAINER_OF(keys, struct _instancekeysobject, dsk_keys);
+}
+
 /* This must be no more than 250, for the prefix size to fit in one byte. */
 #define SHARED_KEYS_MAX_SIZE 30
 #define NEXT_LOG2_SHARED_KEYS_MAX_SIZE 6

Include/internal/pycore_typeobject.h

@@ -148,6 +148,9 @@ extern int _PyType_AddMethod(PyTypeObject *, PyMethodDef *);
 extern void _PyType_SetFlagsRecursive(PyTypeObject *self, unsigned long mask,
                                       unsigned long flags);
 
+// Raise PyType_Modified with the type lock already held.
+extern void _PyType_Modified_Unlocked(PyTypeObject *type);
+
 PyAPI_FUNC(void) _PyType_SetVersion(PyTypeObject *tp, unsigned int version);
 PyTypeObject *_PyType_LookupByVersion(unsigned int version);