[3.14] gh-153062: Fix a crash iterating itertools.tee on the free-threaded build (gh-153063)#153476
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…ee-threaded build (pythongh-153063) itertools.tee branches share a linked list of teedataobject cells. On the free-threaded build, iterating one branch from multiple threads, or iterating sibling branches concurrently, raced on the shared cells and each branch's position, corrupting refcounts and crashing. Lock each teedataobject while reading, extending, or clearing it, and snapshot each branch's position under the tee object's own lock, revalidating before advancing, so the two locks are never nested. Concurrent iteration of one tee is undefined and may raise RuntimeError as documented, but no longer crashes. The free-threading snapshot and revalidation add per-element overhead on the default build, where the GIL already serializes access. Guard that path under Py_GIL_DISABLED so the default build keeps the original iteration and the free-threaded path is unchanged. (cherry picked from commit 6c389c4) Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com> Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>
This was referenced Jul 10, 2026
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itertools.tee branches share a linked list of teedataobject cells. On the free-threaded build, iterating one branch from multiple threads, or iterating sibling branches concurrently, raced on the shared cells and each branch's position, corrupting refcounts and crashing.
Lock each teedataobject while reading, extending, or clearing it, and snapshot each branch's position under the tee object's own lock, revalidating before advancing, so the two locks are never nested. Concurrent iteration of one tee is undefined and may raise RuntimeError as documented, but no longer crashes.
The free-threading snapshot and revalidation add per-element overhead on the default build, where the GIL already serializes access. Guard that path under Py_GIL_DISABLED so the default build keeps the original iteration and the free-threaded path is unchanged.
(cherry picked from commit 6c389c4)