Merge pull request #2 from encukou/dont_modify_refcount

_PyType_GetBaseByToken_Borrow can't fail; adjust comments

Author: DinoV

Include/internal/pycore_typeobject.h

@@ -155,6 +155,8 @@ extern int _PyType_CacheGetItemForSpecialization(PyHeapTypeObject *ht, PyObject
 // Precalculates count of non-unique slots and fills wrapperbase.name_count.
 extern int _PyType_InitSlotDefs(PyInterpreterState *interp);
 
+// Like PyType_GetBaseByToken, but does not modify refcounts.
+// Cannot fail; arguments must be valid.
 PyAPI_FUNC(int)
 _PyType_GetBaseByToken_Borrow(PyTypeObject *type, void *token, PyTypeObject **result);
 

Misc/NEWS.d/next/Library/2025-12-10-11-20-05.gh-issue-123241.oYg2n7.rst

@@ -0,0 +1,2 @@
+Avoid reference count operations in garbage collection of :mod:`ctypes`
+objects.

Modules/_ctypes/ctypes.h

@@ -608,7 +608,8 @@ PyStgInfo_FromAny(ctypes_state *state, PyObject *obj, StgInfo **result)
     return _stginfo_from_type(state, Py_TYPE(obj), result);
 }
 
-/* A variant of PyStgInfo_FromType that doesn't need the state,
+/* A variant of PyStgInfo_FromType that doesn't need the state
+ * and doesn't modify any refcounts,
  * so it can be called from finalization functions when the module
  * state is torn down.
  */

Objects/typeobject.c

@@ -5879,21 +5879,13 @@ get_base_by_token_recursive(PyObject *bases, void *token)
 int
 _PyType_GetBaseByToken_Borrow(PyTypeObject *type, void *token, PyTypeObject **result)
 {
+    assert(token != NULL);
+    assert(PyType_Check(type));
+
     if (result != NULL) {
         *result = NULL;
     }
 
-    if (token == NULL) {
-        PyErr_Format(PyExc_SystemError,
-                     "PyType_GetBaseByToken called with token=NULL");
-        return -1;
-    }
-    if (!PyType_Check(type)) {
-        PyErr_Format(PyExc_TypeError,
-                     "expected a type, got a '%T' object", type);
-        return -1;
-    }
-
     if (!_PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
         // No static type has a heaptype superclass,
         // which is ensured by type_ready_mro().
@@ -5939,6 +5931,17 @@ _PyType_GetBaseByToken_Borrow(PyTypeObject *type, void *token, PyTypeObject **re
 int
 PyType_GetBaseByToken(PyTypeObject *type, void *token, PyTypeObject **result)
 {
+    if (token == NULL) {
+        PyErr_Format(PyExc_SystemError,
+                     "PyType_GetBaseByToken called with token=NULL");
+        return -1;
+    }
+    if (!PyType_Check(type)) {
+        PyErr_Format(PyExc_TypeError,
+                     "expected a type, got a '%T' object", type);
+        return -1;
+    }
+
     int res = _PyType_GetBaseByToken_Borrow(type, token, result);
     if (res > 0 && result) {
         Py_INCREF(*result);