aio-libs / multidict / 36311301009

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Make the pure-Python version bump atomic on GIL builds (#1610)

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## What do these changes do?

On GIL builds the pure-Python `_incr_version()` read, incremented and
stored the shared version counter across several bytecodes, so two
threads mutating different multidicts at once could be handed the same
version. The counter is now an `itertools.count`, and the GIL build
takes `next()` from it, which is a single C call. The free-threaded
build keeps `_VERSION_LOCK` around the same `next()`, since
`itertools.count` is not documented as thread-safe there.

`test_pure_python_version_thread_safety` no longer carries
`_gil_build_race_skip`. It lowers the switch interval to 1e-4 s while
its threads run, which fails reliably on PyPy before the fix. CPython
3.10 and 3.14 GIL builds never showed the race, since they only switch
threads at calls and backward jumps and the old increment had neither in
the window.

The C extension is unaffected; its counter was made atomic in #1446.

## Are there changes in behavior for the user?

No, apart from pure-Python versions no longer colliding across threads.

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

No; the change is three lines and makes the increment simpler.

## Related issue number

Related to #1605, whose pure-Python GIL-build skip in
`test_versions_unique_across_threads` can be removed once both land.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes (N/A, no documented behaviour
changed)
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` (N/A)
- [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`

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

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