gh-144356: Make set iterator `__length_hint__` and `iternext` race-safe under no-gil by hyongtao-code · Pull Request #144357 · python/cpython · GitHub

Long log:

In free-threaded builds, setiter_len() and setiter_iternext() could race with concurrent set mutation and iterator exhaustion.

We now serialize operations on the iterator itself (and keep the set stable) using a critical section, and ensure the iterator’s exhaustion state is handled consistently.

The non-free-threaded build keeps the existing semantics. Add free-threading-style concurrency tests (similar to the itertools FT tests) to exercise __length_hint__ and iternext exhaustion under concurrent execution.

setiter_len() was reading so->used without atomic access while concurrent
mutations update it atomically under Py_GIL_DISABLED.

In free-threaded builds, setiter_len() could race with concurrent set
mutation and iterator exhaustion.

Use an atomic load for so->used to avoid a data race. This preserves the
existing semantics of __length_hint__ while making the access thread-safe.

Signed-off-by: Yongtao Huang yongtaoh2022@gmail.com

Issue: Data race in set iterator length_hint under no-gil #144356

setiter_len() was reading so->used without atomic access while concurrent mutations update it atomically under Py_GIL_DISABLED. Use an atomic load for so->used to avoid a data race. This preserves the existing semantics of __length_hint__ while making the access thread-safe. Signed-off-by: Yongtao Huang <yongtaoh2022@gmail.com>

Thanks for the review. I’ve decided to address both issues in this PR. I also added a corresponding test case for the issue you pointed out.

I left some more review comments. I think we can get this right, but a simpler approach here would be to put a critical section on the set iterator itself.

put a critical section on the set iterator

Thanks for the suggestion. I simplified the implementation by protecting both the iterator and the set with Py_BEGIN_CRITICAL_SECTION2(self, so), so concurrent use of the same iterator is properly serialized. This keeps the logic aligned with other iterators and avoids the races.

It looks like there is an unrelated flaky test case.

@hyongtao-code Thanks for your work on this! The core idea of the PR has shifted a bit, which is why I have some more review comments, but we are getting closer I believe.

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

This reverts commit 78241a8.

Can you move the decref to the section

       /* exhausted */
        si->si_pos = -1;
        si->len = 0;

?

I’d prefer not to move the Py_DECREF into the Py_BEGIN_CRITICAL_SECTION block.

We went through several iterations, but currently the PR:

Adds free-threading tests for the behavior of set iterators
Makes the set iterator safe in the FT build by using locks and not freeing the underlying set

Remaining suggestions are about the style, not about the actual implementation so I am approving.

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

On normal builds Py_BEGIN_CRITICAL_SECTION is a no-op, you either want Py_BEGIN_CRITICAL_SECTION or Py_BEGIN_CRITICAL_SECTION2, not both.

Hi, thanks for your review. I switched this to always use Py_BEGIN_CRITICAL_SECTION2(self, so) / Py_END_CRITICAL_SECTION2().

The remaining #ifdef Py_GIL_DISABLED around the done label is a bit awkward, but it preserves the existing control flow.

This PR is stale because it has been open for 30 days with no activity.

Hi @colesbury @kumaraditya303 — a gentle ping on this PR. I think all the review points have now been addressed:

colesbury : si->si_set is no longer cleared on exhaustion.
kumaraditya303: the mixed Py_BEGIN_CRITICAL_SECTION / Py_BEGIN_CRITICAL_SECTION2 usage is fixed.

Would you mind taking another look?

On a related note: I found other builtin iterators have similar race issues (discussed here) — I'm planning to send follow-up fixes.

Thanks!

hyongtao-code added 2 commits January 31, 2026 10:51

Add test case

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hyongtao-code requested a review from rhettinger as a code owner January 31, 2026 03:34

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Data race in set iterator length_hint under no-gil #144356

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hyongtao-code changed the title gh-144356: fix data race in setiter_len() under no-gil gh-144356: Avoid races when computing set_iterator.__length_hint__ under no-gil Feb 1, 2026

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Update Lib/test/test_free_threading/test_set.py

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Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

Revert "Update Lib/test/test_free_threading/test_set.py"

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This reverts commit 78241a8.

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hyongtao-code changed the title gh-144356: Avoid races when computing set_iterator.__length_hint__ under no-gil gh-144356: Make set iterator __length_hint__ and iternext race-safe under no-gil Feb 10, 2026

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This was referenced Jul 21, 2026

Strategy for Iterators in Free Threading #124397

Open

Sharing a contextvars.Context iterator across threads crashes (HAMT iterator cursor corruption) under free-threading #154535

Open

gianghungtien mentioned this pull request Jul 24, 2026

gh-154535: Fix crash when a contextvars.Context iterator is shared between threads #154586

Open

                   if (key == NULL) {
-                      si->si_set = NULL;
+                      /* exhausted */
                       Py_DECREF(so);

+              #ifdef Py_GIL_DISABLED
+                  Py_BEGIN_CRITICAL_SECTION2(self, so);
+              #else
                   Py_BEGIN_CRITICAL_SECTION(so);

gh-144356: Make set iterator `__length_hint__` and `iternext` race-safe under no-gil by hyongtao-code · Pull Request #144357 · python/cpython · GitHub

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gh-144356: Make set iterator __length_hint__ and iternext race-safe under no-gil - #144357

gh-144356: Make set iterator __length_hint__ and iternext race-safe under no-gil#144357hyongtao-code wants to merge 24 commits intopython:mainpython/cpython:mainfrom hyongtao-code:fix-data-racehyongtao-code/cpython:fix-data-raceCopy head branch name to clipboard

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gh-144356: Make set iterator __length_hint__ and iternext race-safe under no-gil#144357
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