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gh-145036: Fix data race for list capacity #145365

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gh-145036: Fix data race for list capacity #145365

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51a185e
Fix data race for list capacity
bkap123 Feb 28, 2026
ffe1efb
📜🤖 Added by blurb_it.
blurb-it[bot] Feb 28, 2026
9a5991e
Update Misc/NEWS.d/next/Core_and_Builtins/2026-02-28-18-42-36.gh-issu…
bkap123 Mar 1, 2026
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25 changes: 25 additions & 0 deletions Lib/test/test_list.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
from test.support.import_helper import import_module
from test.support.script_helper import assert_python_failure, assert_python_ok
import pickle
from threading import Thread
import unittest

class ListTest(list_tests.CommonTest):
Expand Down Expand Up @@ -381,6 +382,30 @@ def foo(x):

self.assertEqual(foo(list(range(10))), 45)

# gh-145036: race condition with list.__sizeof__()
def test_list_sizeof_free_threaded_build(self):
L = []

def test1():
for _ in range(100):
L.append(1)
L.pop()

def test2():
for _ in range(100):
L.__sizeof__()

threads = []
for _ in range(4):
threads.append(Thread(target=test1))
threads.append(Thread(target=test2))

for t in threads:
t.start()
for t in threads:
t.join()

self.assertEqual(len(L), 0)

if __name__ == "__main__":
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-02-28-18-42-36.gh-issue-145036.70Kbfz.rst
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@@ -0,0 +1 @@
In free-threaded build, no longer change of race condition when calling :meth:`!__sizeof__` on a :class:`list`
10 changes: 8 additions & 2 deletions Objects/listobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3558,8 +3558,14 @@ list___sizeof___impl(PyListObject *self)
/*[clinic end generated code: output=3417541f95f9a53e input=b8030a5d5ce8a187]*/
{
size_t res = _PyObject_SIZE(Py_TYPE(self));
Py_ssize_t allocated = FT_ATOMIC_LOAD_SSIZE_RELAXED(self->allocated);
res += (size_t)allocated * sizeof(void*);
#ifdef Py_GIL_DISABLED
PyObject **ob_item = _Py_atomic_load_ptr(&self->ob_item);
if (ob_item != NULL) {
res += list_capacity(ob_item) * sizeof(PyObject *);
}
#else
res += (size_t)self->allocated * sizeof(PyObject *);
#endif
return PyLong_FromSize_t(res);
}

Expand Down
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