8000 GH-91636: Clear weakrefs created by finalizers. by nascheme · Pull Request #136401 · python/cpython · GitHub
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GH-91636: Clear weakrefs created by finalizers. #136401

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nascheme
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@nascheme nascheme commented Jul 8, 2025

Weakrefs to unreachable garbage that are created during running of finalizers need to be cleared. This avoids exposing objects that have tp_clear called on them to Python-level code.

Note that this is part of the GH-136189 change, split out to make it easier for back-porting. This change is conceptually quite simple and is a candidate to backport to all older Python versions (this bug existed for a long time, perhaps in version 2.3). There is some duplicated code between handle_finalizers and clear_finalizers but I think it's simpler just to leave that duplication. In the "main" branch I'll clean that up.

Weakrefs to unreachable garbage that are created during running of
finalizers need to be cleared.  This avoids exposing objects that
have `tp_clear` called on them to Python-level code.
@nascheme nascheme added 3.13 bugs and security fixes 3.14 bugs and security fixes 3.15 new features, bugs and security fixes labels Jul 8, 2025
@nascheme nascheme marked this pull request as ready for review July 8, 2025 01:15
@nascheme nascheme requested review from colesbury and pablogsal July 8, 2025 01:15
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nascheme commented Jul 8, 2025

@colesbury In case you want to look at the free_threading_gc changes.

@sergey-miryanov
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Looks good to me.

Add a boolean flag to indicate that callbacks should be enabled.  This
allows the same function to be used before and after finalizer
execution.
if (!allow_callbacks) {
return 0;
}

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Maybe it is worth to add an assert here for old != NULL?

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Similar question: The old and allow_callbacks arguments are orthogonal even in older versions?

@nascheme nascheme merged commit b6b99bf into python:main Jul 8, 2025
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