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gh-151722: Defer GC tracking in frozendict.copy() #152230

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gh-151722: Defer GC tracking in frozendict.copy()
vstinner Jun 24, 2026
9db06a4
Fix dict_merge() assertion in Free Threading
vstinner Jun 25, 2026
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109 changes: 72 additions & 37 deletions Objects/dictobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -305,16 +305,20 @@ static inline int
can_modify_dict(PyDictObject *mp)
{
if (PyFrozenDict_Check(mp)) {
// gh-151722: A frozendict must not be tracked by the GC
// when it's being modified.
assert(!_PyObject_GC_IS_TRACKED(mp));

// No locking required to modify a newly created frozendict
// since it's only accessible from the current thread.
return PyUnstable_Object_IsUniquelyReferenced(_PyObject_CAST(mp));
assert(PyUnstable_Object_IsUniquelyReferenced(_PyObject_CAST(mp)));
}
else {
// Locking is only required if the dictionary is not
// uniquely referenced.
ASSERT_DICT_LOCKED(mp);
return 1;
}
return 1;
}
#endif

Expand Down Expand Up @@ -937,7 +941,7 @@ free_values(PyDictValues *values, bool use_qsbr)
static inline PyObject *
new_dict_impl(PyDictObject *mp, PyDictKeysObject *keys,
PyDictValues *values, Py_ssize_t used,
int free_values_on_failure, int frozendict)
int free_values_on_failure, int frozendict, int gc_track)
{
assert(keys != NULL);
if (mp == NULL) {
Expand All @@ -956,12 +960,14 @@ new_dict_impl(PyDictObject *mp, PyDictKeysObject *keys,
((PyFrozenDictObject *)mp)->ma_hash = -1;
}
ASSERT_CONSISTENT(mp);
_PyObject_GC_TRACK(mp);
if (gc_track) {
_PyObject_GC_TRACK(mp);
}
return (PyObject *)mp;
}

/* Consumes a reference to the keys object */
static PyObject *
static PyObject*
new_dict(PyDictKeysObject *keys, PyDictValues *values,
Py_ssize_t used, int free_values_on_failure)
{
Expand All @@ -971,16 +977,30 @@ new_dict(PyDictKeysObject *keys, PyDictValues *values,
}
assert(mp == NULL || Py_IS_TYPE(mp, &PyDict_Type));

return new_dict_impl(mp, keys, values, used, free_values_on_failure, 0);
return new_dict_impl(mp, keys, values, used, free_values_on_failure, 0, 1);
}

/* Consumes a reference to the keys object */
static PyObject *
new_frozendict(PyDictKeysObject *keys, PyDictValues *values,
Py_ssize_t used, int free_values_on_failure)
static PyObject*
new_dict_untracked(PyDictKeysObject *keys, PyDictValues *values,
Py_ssize_t used, int free_values_on_failure)
{
PyDictObject *mp = _Py_FREELIST_POP(PyDictObject, dicts);
if (mp == NULL) {
mp = PyObject_GC_New(PyDictObject, &PyDict_Type);
}
assert(mp == NULL || Py_IS_TYPE(mp, &PyDict_Type));

return new_dict_impl(mp, keys, values, used, free_values_on_failure, 0, 0);
}

/* Consumes a reference to the keys object */
static PyObject*
new_frozendict_untracked(PyDictKeysObject *keys, PyDictValues *values,
Py_ssize_t used, int free_values_on_failure)
{
PyDictObject *mp = PyObject_GC_New(PyDictObject, &PyFrozenDict_Type);
return new_dict_impl(mp, keys, values, used, free_values_on_failure, 1);
return new_dict_impl(mp, keys, values, used, free_values_on_failure, 1, 0);
}

static PyObject *
Expand Down Expand Up @@ -3471,7 +3491,6 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
}
if (PyFrozenDict_Check(d)) {
assert(can_modify_dict((PyDictObject*)d));
assert(!_PyObject_GC_IS_TRACKED(d));
}

if (PyDict_CheckExact(d)) {
Expand Down Expand Up @@ -4076,6 +4095,7 @@ merge_from_seq2_lock_held(PyObject *d, PyObject *seq2, int override)
assert(d != NULL);
assert(PyAnyDict_Check(d));
assert(seq2 != NULL);
assert(can_modify_dict((PyDictObject*)d));

it = PyObject_GetIter(seq2);
if (it == NULL)
Expand Down Expand Up @@ -4277,11 +4297,13 @@ dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey)
assert(0 <= override && override <= 2);

PyDictObject *mp = _PyAnyDict_CAST(a);

int res = 0;
if (PyAnyDict_Check(b) && (Py_TYPE(b)->tp_iter == dict_iter)) {
PyDictObject *other = (PyDictObject*)b;
int res;
Py_BEGIN_CRITICAL_SECTION2(a, b);
assert(can_modify_dict(mp));
res = dict_dict_merge((PyDictObject *)a, other, override, dupkey);
ASSERT_CONSISTENT(a);
Py_END_CRITICAL_SECTION2();
Expand All @@ -4290,6 +4312,8 @@ dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey)
else {
/* Do it the generic, slower way */
Py_BEGIN_CRITICAL_SECTION(a);
assert(can_modify_dict(mp));

PyObject *keys = PyMapping_Keys(b);
PyObject *iter;
PyObject *key, *value;
Expand Down Expand Up @@ -4382,9 +4406,7 @@ dict_merge_api(PyObject *a, PyObject *b, int override, PyObject **dupkey)
}

int res = dict_merge(a, b, override, dupkey);
if (PyDict_Check(a)) {
assert(_PyObject_GC_IS_TRACKED(a));
}
assert(_PyObject_GC_IS_TRACKED(a));
return res;
}

Expand Down Expand Up @@ -4442,7 +4464,7 @@ copy_values(PyDictValues *values)
}

static PyObject *
copy_lock_held(PyObject *o, int as_frozendict)
copy_lock_held_untracked(PyObject *o, int as_frozendict)
{
PyObject *copy;
PyDictObject *mp;
Expand All @@ -4455,12 +4477,15 @@ copy_lock_held(PyObject *o, int as_frozendict)
mp = (PyDictObject *)o;
if (mp->ma_used == 0) {
/* The dict is empty; just return a new dict. */
PyObject *d;
if (as_frozendict) {
return PyFrozenDict_New(NULL);
d = frozendict_new_untracked(&PyFrozenDict_Type);
}
else {
return PyDict_New();
d = dict_new_untracked(&PyDict_Type);
}
assert(!_PyObject_GC_IS_TRACKED(d));
return d;
}

if (_PyDict_HasSplitTable(mp)) {
Expand Down Expand Up @@ -4492,7 +4517,7 @@ copy_lock_held(PyObject *o, int as_frozendict)
PyFrozenDictObject *frozen = (PyFrozenDictObject *)split_copy;
frozen->ma_hash = -1;
}
_PyObject_GC_TRACK(split_copy);
assert(!_PyObject_GC_IS_TRACKED(split_copy));
return (PyObject *)split_copy;
}

Expand Down Expand Up @@ -4520,10 +4545,10 @@ copy_lock_held(PyObject *o, int as_frozendict)
}
PyDictObject *new;
if (as_frozendict) {
new = (PyDictObject *)new_frozendict(keys, NULL, 0, 0);
new = (PyDictObject *)new_frozendict_untracked(keys, NULL, 0, 0);
}
else {
new = (PyDictObject *)new_dict(keys, NULL, 0, 0);
new = (PyDictObject *)new_dict_untracked(keys, NULL, 0, 0);
}
if (new == NULL) {
/* In case of an error, new_dict()/new_frozendict() takes care of
Expand All @@ -4533,14 +4558,15 @@ copy_lock_held(PyObject *o, int as_frozendict)

new->ma_used = mp->ma_used;
ASSERT_CONSISTENT(new);
assert(!_PyObject_GC_IS_TRACKED(new));
return (PyObject *)new;
}

if (as_frozendict) {
copy = PyFrozenDict_New(NULL);
copy = frozendict_new_untracked(&PyFrozenDict_Type);
}
else {
copy = PyDict_New();
copy = dict_new_untracked(&PyDict_Type);
}
if (copy == NULL)
return NULL;
Expand All @@ -4549,9 +4575,7 @@ copy_lock_held(PyObject *o, int as_frozendict)
return NULL;
}

if (PyDict_Check(copy)) {
assert(_PyObject_GC_IS_TRACKED(copy));
}
assert(!_PyObject_GC_IS_TRACKED(copy));
return copy;
}

Expand All @@ -4565,25 +4589,28 @@ PyDict_Copy(PyObject *o)

PyObject *res;
Py_BEGIN_CRITICAL_SECTION(o);
res = copy_lock_held(o, 0);
res = copy_lock_held_untracked(o, 0);
Py_END_CRITICAL_SECTION();
if (res != NULL) {
_PyObject_GC_TRACK(res);
}
return res;
}

// Similar to PyDict_Copy(), but return a frozendict if the argument
// is a frozendict.
static PyObject *
anydict_copy(PyObject *o)
anydict_copy_untracked(PyObject *o)
{
assert(PyAnyDict_Check(o));

PyObject *res;
if (PyFrozenDict_Check(o)) {
res = copy_lock_held(o, 1);
res = copy_lock_held_untracked(o, 1);
}
else {
Py_BEGIN_CRITICAL_SECTION(o);
res = copy_lock_held(o, 0);
res = copy_lock_held_untracked(o, 0);
Py_END_CRITICAL_SECTION();
}
return res;
Expand All @@ -4598,13 +4625,16 @@ _PyDict_CopyAsDict(PyObject *o)

PyObject *res;
if (PyFrozenDict_Check(o)) {
res = copy_lock_held(o, 0);
res = copy_lock_held_untracked(o, 0);
}
else {
Py_BEGIN_CRITICAL_SECTION(o);
res = copy_lock_held(o, 0);
res = copy_lock_held_untracked(o, 0);
Py_END_CRITICAL_SECTION();
}
if (res != NULL) {
_PyObject_GC_TRACK(res);
}
return res;
}

Expand Down Expand Up @@ -5157,14 +5187,15 @@ _PyDict_Or(PyObject *self, PyObject *other)
if (!PyAnyDict_Check(self) || !PyAnyDict_Check(other)) {
Py_RETURN_NOTIMPLEMENTED;
}
PyObject *new = anydict_copy(self);
PyObject *new = anydict_copy_untracked(self);
if (new == NULL) {
return NULL;
}
if (dict_update_arg(new, other)) {
Py_DECREF(new);
return NULL;
}
_PyObject_GC_TRACK(new);
return new;
}

Expand Down Expand Up @@ -5352,7 +5383,6 @@ dict_new(PyTypeObject *type, PyObject *Py_UNUSED(args), PyObject *Py_UNUSED(kwds
if (self == NULL) {
return NULL;
}
assert(!_PyObject_GC_IS_TRACKED(self));
_PyObject_GC_TRACK(self);
return self;
}
Expand Down Expand Up @@ -5434,7 +5464,7 @@ frozendict_vectorcall(PyObject *type, PyObject * const*args,
}
}
}
assert(!_PyObject_GC_IS_TRACKED(self));

_PyObject_GC_TRACK(self);
return self;
}
Expand Down Expand Up @@ -6753,10 +6783,12 @@ dictitems_xor_lock_held(PyObject *d1, PyObject *d2)
ASSERT_DICT_LOCKED(d1);
ASSERT_DICT_LOCKED(d2);

PyObject *temp_dict = copy_lock_held(d1, 0);
PyObject *temp_dict = copy_lock_held_untracked(d1, 0);
if (temp_dict == NULL) {
return NULL;
}
_PyObject_GC_TRACK(temp_dict);

PyObject *result_set = PySet_New(NULL);
if (result_set == NULL) {
Py_CLEAR(temp_dict);
Expand Down Expand Up @@ -8488,7 +8520,6 @@ frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
assert(kwds == NULL);
}

assert(!_PyObject_GC_IS_TRACKED(d));
_PyObject_GC_TRACK(d);
return d;
}
Expand Down Expand Up @@ -8533,7 +8564,11 @@ frozendict_copy_impl(PyFrozenDictObject *self)
return Py_NewRef(self);
}

return anydict_copy((PyObject*)self);
PyObject *copy = anydict_copy_untracked((PyObject*)self);
if (copy != NULL) {
_PyObject_GC_TRACK(copy);
}
return copy;
}


Expand Down
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