gh-123471: Make itertools.zip_longest safe in the FT build - #146033
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kumaraditya303 merged 4 commits intoMar 27, 2026
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gh-123471: Make itertools.zip_longest safe in the FT build#146033kumaraditya303 merged 4 commits into
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…132814) (GH-135689) (GH-144402) (GH-146033) (GH-142957) (GH-153791) Combined backport of PRs from main branch, fixing free-threading data-races in itertools: * gh-123471: make concurrent iteration over `itertools.cycle` safe under free-threading (gh-131212) * gh-123471: Make itertools.product and itertools.combinations thread-safe (GH-132814) * gh-123471: Make itertools.chain thread-safe (gh-135689) * gh-123471: Make concurrent iteration over `itertools.permutations` and `itertools.combinations_with_replacement` thread-safe (gh-144402) * gh-123471: make concurrent iteration over itertools.accumulate thread-safe (gh-144486) * gh-123471: Make `itertools.zip_longest` safe in the FT build (gh-146033) (cherry picked from commit 26a1cd4) (cherry picked from commit 847d1c2) (cherry picked from commit 0533c1f) (cherry picked from commit 009c8c0) (cherry picked from commit 3a24856) (cherry picked from commit 9214e3f) Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com> Co-authored-by: Kumar Aditya <kumaraditya@python.org>
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ziplongest_next is not thread safe due to setting the iterators in lz->ittuple to NULL on exhaustion. In addition, the lz->numactive is mutable state.
The usual approach to making the iterators in this module thread-safe by not setting the underlying iterator to NULL is not working here, since the NULL value is used as a signal (e.g. the check it == NULL). We therefore make the iterator safe using a critical section.