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gh-151722: Defer GC tracking of frozendict to end of construction by corona10 · Pull Request #151740 · python/cpython · GitHub

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gh-151722: Defer GC tracking of frozendict to end of construction #151740

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Merged
corona10 merged 6 commits into from
Jun 22, 2026
Conversation 21 Commits 6 Checks 54 Files changed
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gh-151722: Defer GC tracking of frozendict to end of construction #151740

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gh-151722: Defer GC tracking of frozendict to end of construction
corona10 Jun 19, 2026
399d810
Address code review
corona10 Jun 20, 2026
286eafd
fix
corona10 Jun 20, 2026
d74be81
fix
corona10 Jun 20, 2026
897bdec
Update
corona10 Jun 20, 2026
69a6316
Address code review
corona10 Jun 22, 2026
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-06-20-00-30-47.gh-issue-151722.RPMPIY.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Defer GC tracking of :class:`frozendict` to end of construction. Patch by
Donghee Na.
53 changes: 38 additions & 15 deletions Objects/dictobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,7 @@ As a consequence of this, split keys have a maximum size of 16.
// Forward declarations
static PyObject* frozendict_new(PyTypeObject *type, PyObject *args,
PyObject *kwds);
static PyObject* frozendict_new_untracked(PyTypeObject *type);
static PyObject* dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
static int dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey);
static int dict_contains(PyObject *op, PyObject *key);
Expand Down Expand Up @@ -4146,12 +4147,9 @@ dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override, PyObject **
set_keys(mp, keys);
STORE_USED(mp, other->ma_used);
ASSERT_CONSISTENT(mp);

if (_PyObject_GC_IS_TRACKED(other) && !_PyObject_GC_IS_TRACKED(mp)) {

corona10 Jun 20, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

#127027

No more lazy track from here, we can remove this logic safely.

vstinner Jun 22, 2026

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This removal makes me a little bit nervous. You may add the following assertion at the top of dict_dict_merge() (after other assertions).

    if (PyDict_Check(mp)) {
        assert(_PyObject_GC_IS_TRACKED(mp));
    }

/* Maintain tracking. */
_PyObject_GC_TRACK(mp);
if (PyDict_Check(mp)) {
assert(_PyObject_GC_IS_TRACKED(mp));
}

return 0;
}
}
Expand Down Expand Up @@ -5242,14 +5240,13 @@ static PyNumberMethods dict_as_number = {
};

static PyObject *
dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
dict_new_untracked(PyTypeObject *type)
{
assert(type != NULL);
assert(type->tp_alloc != NULL);
// dict subclasses must implement the GC protocol
assert(_PyType_IS_GC(type));

PyObject *self = type->tp_alloc(type, 0);
PyObject *self = _PyType_AllocNoTrack(type, 0);
if (self == NULL) {
return NULL;
}
Expand All @@ -5262,9 +5259,19 @@ dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
d->ma_keys = Py_EMPTY_KEYS;
d->ma_values = NULL;
ASSERT_CONSISTENT(d);
if (!_PyObject_GC_IS_TRACKED(d)) {
_PyObject_GC_TRACK(d);
return self;
}

static PyObject *
dict_new(PyTypeObject *type, PyObject *Py_UNUSED(args), PyObject *Py_UNUSED(kwds))
{
/* tp_new ignores args/kwds; args/kwds are consumed by dict_init (tp_init). */
PyObject *self = dict_new_untracked(type);
if (self == NULL) {
return NULL;
}
assert(!_PyObject_GC_IS_TRACKED(self));
_PyObject_GC_TRACK(self);
return self;
}

Expand Down Expand Up @@ -5323,7 +5330,9 @@ frozendict_vectorcall(PyObject *type, PyObject * const*args,
return Py_NewRef(args[0]);
}

PyObject *self = frozendict_new(_PyType_CAST(type), NULL, NULL);
/* gh-151722: Keep the frozendict untracked until it is fully built,
so a half-built object is never reachable from another thread (using the gc module). */
PyObject *self = frozendict_new_untracked(_PyType_CAST(type));
if (self == NULL) {
return NULL;
}
Expand All @@ -5343,6 +5352,8 @@ frozendict_vectorcall(PyObject *type, PyObject * const*args,
}
}
}
assert(!_PyObject_GC_IS_TRACKED(self));
_PyObject_GC_TRACK(self);
return self;
}

Expand Down Expand Up @@ -8361,17 +8372,27 @@ frozendict_hash(PyObject *op)
}


/* Allocate an empty, GC-untracked frozendict; the constructor tracks it once
fully built. */
static PyObject *
frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
frozendict_new_untracked(PyTypeObject *type)
{
PyObject *d = dict_new(type, args, kwds);
PyObject *d = dict_new_untracked(type);
if (d == NULL) {
return NULL;
}
assert(can_modify_dict(_PyAnyDict_CAST(d)));
_PyFrozenDictObject_CAST(d)->ma_hash = -1;
return d;
}

PyFrozenDictObject *self = _PyFrozenDictObject_CAST(d);
self->ma_hash = -1;
static PyObject *
frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
PyObject *d = frozendict_new_untracked(type);
if (d == NULL) {
return NULL;
}

if (args != NULL) {
if (dict_update_common(d, args, kwds, "frozendict") < 0) {
Expand All @@ -8383,6 +8404,8 @@ frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
assert(kwds == NULL);
}

assert(!_PyObject_GC_IS_TRACKED(d));
_PyObject_GC_TRACK(d);
return d;
}

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