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August 2026

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gh-151627: fix thread safety of OrderedDict iterator creation (#151688)
by kumaraditya303 Aug. 1, 2026

Aug. 1, 2026

https://github.com/python/cpython/commit/86dab7c55bb85269b764886ceda1f85e3f… commit: 86dab7c55bb85269b764886ceda1f85e3fdc7a7d branch: main author: Pedram Karimi <147748351+pedramkarimii(a)users.noreply.github.com> committer: kumaraditya303 <kumaraditya(a)python.org> date: 2026-08-01T15:27:14+05:30 summary: gh-151627: fix thread safety of OrderedDict iterator creation (#151688) files: A Misc/NEWS.d/next/Library/2026-06-19-08-30-00.gh-issue-151627.xY7kLm.rst M Lib/test/test_free_threading/test_collections.py M Objects/odictobject.c diff --git a/Lib/test/test_free_threading/test_collections.py b/Lib/test/test_free_threading/test_collections.py index 849b0480e232fc2..a0a73c965d072a2 100644 --- a/Lib/test/test_free_threading/test_collections.py +++ b/Lib/test/test_free_threading/test_collections.py @@ -1,5 +1,5 @@ import unittest -from collections import deque +from collections import OrderedDict, deque from copy import copy from test.support import threading_helper @@ -49,5 +49,30 @@ def mutate(): ) +class TestOrderedDict(unittest.TestCase): + def test_iterator_update_clear_race(self): + # gh-151627: OrderedDict iterator construction must not race with + # concurrent clear()/update() operations that mutate the linked list. + od = OrderedDict((i, i) for i in range(100)) + + def mutate(): + for i in range(5000): + od.clear() + od.update(((i, i), (i + 1, i + 1), (i + 2, i + 2))) + + def iterate(): + for _ in range(5000): + try: + for _ in od: + pass + list(reversed(od)) + except RuntimeError: + pass + + threading_helper.run_concurrently( + [mutate, *[iterate for _ in range(8)]], + ) + + if __name__ == "__main__": unittest.main() diff --git a/Misc/NEWS.d/next/Library/2026-06-19-08-30-00.gh-issue-151627.xY7kLm.rst b/Misc/NEWS.d/next/Library/2026-06-19-08-30-00.gh-issue-151627.xY7kLm.rst new file mode 100644 index 000000000000000..2c5d39eec12347b --- /dev/null +++ b/Misc/NEWS.d/next/Library/2026-06-19-08-30-00.gh-issue-151627.xY7kLm.rst @@ -0,0 +1,2 @@ +Fix a crash in :class:`collections.OrderedDict` iterators in free-threaded +builds when the dictionary is concurrently cleared or updated. diff --git a/Objects/odictobject.c b/Objects/odictobject.c index c51fd4b7195da42..a860ade3b148e5f 100644 --- a/Objects/odictobject.c +++ b/Objects/odictobject.c @@ -1958,11 +1958,13 @@ odictiter_new(PyODictObject *od, int kind) } di->kind = kind; + Py_BEGIN_CRITICAL_SECTION(od); node = reversed ? _odict_LAST(od) : _odict_FIRST(od); di->di_current = node ? Py_NewRef(_odictnode_KEY(node)) : NULL; di->di_size = PyODict_SIZE(od); di->di_state = od->od_state; di->di_odict = (PyODictObject*)Py_NewRef(od); + Py_END_CRITICAL_SECTION(); _PyObject_GC_TRACK(di); return (PyObject *)di;

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