aio-libs / multidict / 34782403362

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Fix a data race on entry->hash between getall()/update() and lock-free reads (#1448)

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## What do these changes do?

`getall()`/`popall()` (via `md_find_next()`/`md_finder_cleanup()`) and
`update()`/`extend()`/`merge()` (via `md_post_update()`) mark and unmark
a matching entry's hash while holding `self`'s critical section. That
excludes other mutators but not a lock-free reader: `__contains__` and
`get()`/`getone()`/`__getitem__` load the same field with an atomic op
and no lock at all. The mark/unmark writes were plain, non-atomic
stores, racing that atomic load under the free-threaded build. This
routes them through the existing `_md_entry_store_hash()` helper,
matching the pattern `_md_replace()`/`_md_update()` already use for the
same mark bit.

Note this is unrelated to and much cheaper than the approach tried in
#1442 (which replaced the marking scheme entirely and was withdrawn for
a large performance regression): the fix here keeps the existing
in-place marking and only makes the one racy write atomic, at the cost
of a relaxed atomic store in place of a plain one.

## Are there changes in behavior for the user?

No. This only affects the free-threaded build's internal memory-model
correctness; there is no change to observable behavior.

## Is it a substantial burden for the maintainers to support this?

No, it's a small, self-contained fix mirroring the existing atomic-op
pattern already used elsewhere in `hashtable.h`.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` (N/A: no `CONTRIBUTORS.txt` in this repo)
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

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