gh-91051: allow setting a callback hook on PyType_Modified (GH-97875)

Doc/c-api/type.rst

@@ -57,6 +57,55 @@ Type Objects
    modification of the attributes or base classes of the type.
 
 
+.. c:function:: int PyType_AddWatcher(PyType_WatchCallback callback)
+
+   Register *callback* as a type watcher. Return a non-negative integer ID
+   which must be passed to future calls to :c:func:`PyType_Watch`. In case of
+   error (e.g. no more watcher IDs available), return ``-1`` and set an
+   exception.
+
+   .. versionadded:: 3.12
+
+
+.. c:function:: int PyType_ClearWatcher(int watcher_id)
+
+   Clear watcher identified by *watcher_id* (previously returned from
+   :c:func:`PyType_AddWatcher`). Return ``0`` on success, ``-1`` on error (e.g.
+   if *watcher_id* was never registered.)
+
+   An extension should never call ``PyType_ClearWatcher`` with a *watcher_id*
+   that was not returned to it by a previous call to
+   :c:func:`PyType_AddWatcher`.
+
+   .. versionadded:: 3.12
+
+
+.. c:function:: int PyType_Watch(int watcher_id, PyObject *type)
+
+   Mark *type* as watched. The callback granted *watcher_id* by
+   :c:func:`PyType_AddWatcher` will be called whenever
+   :c:func:`PyType_Modified` reports a change to *type*. (The callback may be
+   called only once for a series of consecutive modifications to *type*, if
+   :c:func:`PyType_Lookup` is not called on *type* between the modifications;
+   this is an implementation detail and subject to change.)
+
+   An extension should never call ``PyType_Watch`` with a *watcher_id* that was
+   not returned to it by a previous call to :c:func:`PyType_AddWatcher`.
+
+   .. versionadded:: 3.12
+
+
+.. c:type:: int (*PyType_WatchCallback)(PyObject *type)
+
+   Type of a type-watcher callback function.
+
+   The callback must not modify *type* or cause :c:func:`PyType_Modified` to be
+   called on *type* or any type in its MRO; violating this rule could cause
+   infinite recursion.
+
+   .. versionadded:: 3.12
+
+
 .. c:function:: int PyType_HasFeature(PyTypeObject *o, int feature)
 
    Return non-zero if the type object *o* sets the feature *feature*.