bpo-46417: Add _PyType_GetSubclasses() function (GH-30761)

Add a new _PyType_GetSubclasses() function to get type's subclasses.

_PyType_GetSubclasses(type) returns a list which holds strong
refererences to subclasses. It is safer than iterating on
type->tp_subclasses which yields weak references and can be modified
in the loop.

_PyType_GetSubclasses(type) now holds a reference to the tp_subclasses
dict while creating the list of subclasses.

set_collection_flag_recursive() of _abc.c now uses
_PyType_GetSubclasses().

1 files changed, 6 insertions, 5 deletions

diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index 0348563..be308cd 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -220,11 +220,12 @@ static inline PyObject **_PyObject_ManagedDictPointer(PyObject *obj)
     return ((PyObject **)obj)-3;
 }
 
-PyObject ** _PyObject_DictPointer(PyObject *);
-int _PyObject_VisitInstanceAttributes(PyObject *self, visitproc visit, void *arg);
-void _PyObject_ClearInstanceAttributes(PyObject *self);
-void _PyObject_FreeInstanceAttributes(PyObject *self);
-int _PyObject_IsInstanceDictEmpty(PyObject *);
+extern PyObject ** _PyObject_DictPointer(PyObject *);
+extern int _PyObject_VisitInstanceAttributes(PyObject *self, visitproc visit, void *arg);
+extern void _PyObject_ClearInstanceAttributes(PyObject *self);
+extern void _PyObject_FreeInstanceAttributes(PyObject *self);
+extern int _PyObject_IsInstanceDictEmpty(PyObject *);
+extern PyObject* _PyType_GetSubclasses(PyTypeObject *);
 
 #ifdef __cplusplus
 }