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gh-144356: fix data race in setiter_len() under no-gil #144357

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gh-144356: fix data race in setiter_len() under no-gil #144357

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47d75fe
Fix data race in setiter_len() under no-gil
hyongtao-code Jan 31, 2026
3e3785c
Add test case
hyongtao-code Jan 31, 2026
229ced3
📜🤖 Added by blurb_it.
blurb-it[bot] Jan 31, 2026
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26 changes: 26 additions & 0 deletions Lib/test/test_free_threading/test_set.py
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Original file line number Diff line number Diff line change
Expand Up @@ -148,6 +148,32 @@ def read_set():
for t in threads:
t.join()

def test_iter_length_hint_mutate(self):
s = set(range(2000))
it = iter(s)
stop = Event()

def reader():
while not stop.is_set():
it.__length_hint__()

def writer():
i = 0
while not stop.is_set():
s.add(i)
s.discard(i - 1)
i += 1

threads = [Thread(target=reader) for _ in range(4)]
threads.append(Thread(target=writer))

for t in threads:
t.start()

stop.set()
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@eendebakpt eendebakpt Jan 31, 2026

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This means the threads will stop right after they have started. I would prefer the pattern that is used in some other tests in this file: set a constant NUM_LOOPS (determined so that the test < 0.1 seconds, but there still is a decent number of mutations)


for t in threads:
t.join()

@threading_helper.requires_working_threading()
class SmallSetTest(RaceTestBase, unittest.TestCase):
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-01-31-03-40-28.gh-issue-144356.otfq_X.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix a data race in ``set_iterator.__length_hint__`` under ``Py_GIL_DISABLED``.
3 changes: 2 additions & 1 deletion Objects/setobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1056,7 +1056,8 @@ setiter_len(PyObject *op, PyObject *Py_UNUSED(ignored))
{
setiterobject *si = (setiterobject*)op;
Py_ssize_t len = 0;
if (si->si_set != NULL && si->si_used == si->si_set->used)
PySetObject *so = si->si_set;
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@eendebakpt eendebakpt Jan 31, 2026

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Here so is a borrowed reference to si->si_set. But si->si_set can be cleared in setiter_iternext (if the iterator is exhausted) outside the critical section.

This is a different mechanism than the corresponding issue, so maybe something to address in another PR. But solving both together is something to consider.

if (so != NULL && si->si_used == FT_ATOMIC_LOAD_SSIZE_RELAXED(so->used))
len = si->len;
return PyLong_FromSsize_t(len);
}
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