https://github.com/python/cpython/commit/fdac87ae39ae0672be22837592f1fc4101e8432a
commit: fdac87ae39ae0672be22837592f1fc4101e8432a
branch: 3.13
author: Petr Viktorin <[email protected]>
committer: encukou <[email protected]>
date: 2026-01-26T16:31:52+01:00
summary:
[3.13] gh-127773: Disable attribute cache on incompatible MRO entries
(GH-127924) (GH-143729)
files:
A Misc/NEWS.d/next/Core and
Builtins/2024-12-13-15-21-45.gh-issue-127773.E-DZR4.rst
M Include/cpython/object.h
M Lib/test/test_metaclass.py
M Objects/typeobject.c
diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index 6cb2c40fe2eb71..5b3b890dcf3723 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -221,7 +221,9 @@ struct _typeobject {
PyObject *tp_weaklist; /* not used for static builtin types */
destructor tp_del;
- /* Type attribute cache version tag. Added in version 2.6 */
+ /* Type attribute cache version tag. Added in version 2.6.
+ * If zero, the cache is invalid and must be initialized.
+ */
unsigned int tp_version_tag;
destructor tp_finalize;
@@ -229,9 +231,17 @@ struct _typeobject {
/* bitset of which type-watchers care about this type */
unsigned char tp_watched;
+
+ /* Number of tp_version_tag values used.
+ * Set to _Py_ATTR_CACHE_UNUSED if the attribute cache is
+ * disabled for this type (e.g. due to custom MRO entries).
+ * Otherwise, limited to MAX_VERSIONS_PER_CLASS (defined elsewhere).
+ */
uint16_t tp_versions_used;
};
+#define _Py_ATTR_CACHE_UNUSED (30000) // (see tp_versions_used)
+
/* This struct is used by the specializer
* It should be treated as an opaque blob
* by code other than the specializer and interpreter. */
diff --git a/Lib/test/test_metaclass.py b/Lib/test/test_metaclass.py
index b37b7defe84d1c..07a333f98fa0a9 100644
--- a/Lib/test/test_metaclass.py
+++ b/Lib/test/test_metaclass.py
@@ -254,6 +254,33 @@
[...]
test.test_metaclass.ObscureException
+Test setting attributes with a non-base type in mro() (gh-127773).
+
+ >>> class Base:
+ ... value = 1
+ ...
+ >>> class Meta(type):
+ ... def mro(cls):
+ ... return (cls, Base, object)
+ ...
+ >>> class WeirdClass(metaclass=Meta):
+ ... pass
+ ...
+ >>> Base.value
+ 1
+ >>> WeirdClass.value
+ 1
+ >>> Base.value = 2
+ >>> Base.value
+ 2
+ >>> WeirdClass.value
+ 2
+ >>> Base.value = 3
+ >>> Base.value
+ 3
+ >>> WeirdClass.value
+ 3
+
"""
import sys
diff --git a/Misc/NEWS.d/next/Core and
Builtins/2024-12-13-15-21-45.gh-issue-127773.E-DZR4.rst b/Misc/NEWS.d/next/Core
and Builtins/2024-12-13-15-21-45.gh-issue-127773.E-DZR4.rst
new file mode 100644
index 00000000000000..7e68b3fecd1c08
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and
Builtins/2024-12-13-15-21-45.gh-issue-127773.E-DZR4.rst
@@ -0,0 +1 @@
+Do not use the type attribute cache for types with incompatible :term:`MRO`.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index ded0f22b4e10c8..0c16377034fbb7 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -967,6 +967,7 @@ static void
set_version_unlocked(PyTypeObject *tp, unsigned int version)
{
ASSERT_TYPE_LOCK_HELD();
+ assert(version == 0 || (tp->tp_versions_used != _Py_ATTR_CACHE_UNUSED));
#ifndef Py_GIL_DISABLED
if (version) {
tp->tp_versions_used++;
@@ -1109,6 +1110,10 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
PyObject *b = PyTuple_GET_ITEM(bases, i);
PyTypeObject *cls = _PyType_CAST(b);
+ if (cls->tp_versions_used >= _Py_ATTR_CACHE_UNUSED) {
+ goto clear;
+ }
+
if (!is_subtype_with_mro(lookup_tp_mro(type), type, cls)) {
goto clear;
}
@@ -1117,7 +1122,8 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
clear:
assert(!(type->tp_flags & _Py_TPFLAGS_STATIC_BUILTIN));
- set_version_unlocked(type, 0); /* 0 is not a valid version tag */
+ set_version_unlocked(type, 0); /* 0 is not a valid version tag */
+ type->tp_versions_used = _Py_ATTR_CACHE_UNUSED;
if (PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
// This field *must* be invalidated if the type is modified (see the
// comment on struct _specialization_cache):
@@ -1127,6 +1133,9 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
}
#define MAX_VERSIONS_PER_CLASS 1000
+#if _Py_ATTR_CACHE_UNUSED < MAX_VERSIONS_PER_CLASS
+#error "_Py_ATTR_CACHE_UNUSED must be bigger than max"
+#endif
static int
assign_version_tag(PyInterpreterState *interp, PyTypeObject *type)
@@ -1144,6 +1153,7 @@ assign_version_tag(PyInterpreterState *interp,
PyTypeObject *type)
return 0;
}
if (type->tp_versions_used >= MAX_VERSIONS_PER_CLASS) {
+ /* (this includes `tp_versions_used == _Py_ATTR_CACHE_UNUSED`) */
return 0;
}
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