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gh-161767: Fix crash in PyDict_AsFrozenDictAndClear() with inline values #161768

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gh-161767: Fix crash in PyDict_AsFrozenDictAndClear() with inline values #161768

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gh-161767: Fix crash in PyDict_AsFrozenDictAndClear() with inline values
iRELDevz Nov 10, 2026
a3112a5
Merge branch 'main' into gh-161767-frozendict-inline-values
iRELDevz Nov 11, 2026
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11 changes: 11 additions & 0 deletions Lib/test/test_capi/test_dict.py
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Original file line number Diff line number Diff line change
Expand Up @@ -635,6 +635,17 @@ class DictSubtype(dict): ...
self.assertIs(type(d), DictSubtype)
self.assertEqual(d, DictSubtype({}))

# instance dict that still has inline values
class A: ...
a = A()
a.x = 1
f = as_frozendict(a.__dict__)
a.x = 2
self.assertEqual(f, frozendict(x=1))
self.assertEqual(vars(a), {'x': 2})
del a
self.assertEqual(f, frozendict(x=1))

for wrong_input in (frozendict(), [], None):
with self.subTest(wrong_input=wrong_input):
with self.assertRaises(SystemError):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/C_API/2026-11-10-18-00-09.gh-issue-161767.2EoShw.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix a crash in :c:func:`PyDict_AsFrozenDictAndClear` when called on an
instance :attr:`~object.__dict__` that still uses inline values.
40 changes: 30 additions & 10 deletions Objects/dictobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -8662,33 +8662,48 @@ frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
return d;
}

static void
static int
transfer_keys_and_values_lock_held(PyObject *res, PyObject *dict)
{
PyDictObject *new = (PyDictObject *)res;
PyDictObject *old = (PyDictObject *)dict;
assert(can_modify_dict(old));

// Fast path: do nothing on an empty dict:
if (old->ma_keys == Py_EMPTY_KEYS) {
return;
return 0;
}

Py_ssize_t used = old->ma_used;
PyDictKeysObject *keys = old->ma_keys;
PyDictValues *values = old->ma_values;

// Clear the old dict keys and values, but do not decref them:
clear_common(old);
set_keys(old, Py_EMPTY_KEYS);
set_values(old, NULL);
ASSERT_CONSISTENT(old);
if (values && values->embedded) {
// values are inline in the object, copy them and clear the object's copy
PyDictValues *v = values;
values = copy_values(v);
if (!values) {
PyErr_NoMemory();
return -1;
}
dictkeys_incref(keys);
clear_common(old);
for (Py_ssize_t i = 0; i < keys->dk_nentries; i++) {
FT_ATOMIC_STORE_PTR_RELEASE(v->values[i], NULL);
}
FT_ATOMIC_STORE_UINT8_RELEASE(v->size, 0);
}
else {
clear_common(old);
set_keys(old, Py_EMPTY_KEYS);
set_values(old, NULL);
ASSERT_CONSISTENT(old);
}

// Transfer keys and values from dict to frozendict:
new->ma_used = used;
new->ma_keys = keys;
new->ma_values = values;
ASSERT_CONSISTENT(new);
return 0;
}

PyObject *
Expand All @@ -8704,9 +8719,14 @@ PyDict_AsFrozenDictAndClear(PyObject *dict)
return NULL;
}

int err;
Py_BEGIN_CRITICAL_SECTION(dict);
transfer_keys_and_values_lock_held(res, dict);
err = transfer_keys_and_values_lock_held(res, dict);
Py_END_CRITICAL_SECTION();
if (err < 0) {
Py_DECREF(res);
return NULL;
}

_PyObject_GC_TRACK(res);
assert(_PyFrozenDictObject_CAST(res)->ma_hash == -1);
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