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GeezerVison is showing github.com — gh-123471: Make concurrent iteration over itertools.islice thread-safe using a critical section by eendebakpt · Pull Request #148348 · python/cpython

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@eendebakpt

@eendebakpt eendebakpt commented Apr 10, 2026 •
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In this PR we use a critical section. An alternative approach is to use atomics (#144528), but that is more complex.

  • Issue: Make concurrent iteration over pairwise, combinations, permutations, cwr, product, etc. from itertools safe under free-threading #123471

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@eendebakpt
Make itertools.islice safe in the FT build
0a581a1
@eendebakpt
eendebakpt requested a review from rhettinger as a code owner April 10, 2026 16:52
@bedevere-app bedevere-app Bot added the awaiting review label Apr 10, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Apr 10, 2026
Make concurrent iteration over pairwise, combinations, permutations, cwr, product, etc. from itertools safe under free-threading #123471
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@eendebakpt eendebakpt mentioned this pull request Apr 10, 2026
gh-123471: Make concurrent iteration over itertools.islice thread-safe #144528
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@eendebakpt
news entry
5ee09e1
@eendebakpt eendebakpt added the topic-free-threading label Apr 10, 2026
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github-actions Bot commented May 18, 2026

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This PR is stale because it has been open for 30 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label May 18, 2026
@eendebakpt
Merge branch 'main' into islice_ft_cs
1024151
@github-actions github-actions Bot removed the stale Stale PR or inactive for long period of time. label May 26, 2026
@eendebakpt eendebakpt mentioned this pull request Jun 12, 2026
gh-151409: add critical section for islice_next #151410
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@dpdani

dpdani commented Jul 24, 2026

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I could not reproduce the new test against main. I reproduced an assertion failure after changing the numbers.

    @threading_helper.reap_threads
    def test_islice(self):
        number_of_iterations = 600
        for _ in range(number_of_iterations):
            it = islice(tuple(range(100000)), 1, 8000000, 2)
            threading_helper.run_concurrently(work_iterator, nthreads=10, args=[it])

Which resulted in:

Python/gc_free_threading.c:1083: validate_refcounts: Assertion "_Py_REFCNT(((PyObject*)((op)))) > 0" failed: tracked objects must have a reference count > 0

I didn't check how long it takes to run this test, but we would ideally keep it short. Can you find a way to make it reliably fail on main?
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@ambv ambv mentioned this pull request Oct 20, 2026
gh-123471: Make concurrent iteration over itertools.pairwise thread-safe #144489
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github-actions Bot commented Oct 27, 2026

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This PR is stale because it has been open for 90 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label Oct 27, 2026
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@eendebakpt @dpdani
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