aio-libs / multidict / 34397817863

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Protect MultiDict repr() with a critical section and fix recursion guards (#1431)

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## What do these changes do?

Protects `repr()` of `MultiDict`, `MultiDictProxy`, and their
items/keys/values
views in the C extension with `Py_BEGIN_CRITICAL_SECTION`, avoiding data
races
when another thread mutates the multidict during iteration on the
free-threaded build of CPython. Also adds the missing `Py_ReprEnter`/
`Py_ReprLeave` recursion guard to `MultiDictProxy` and `KeysView` repr
in the
C extension (already present on `MultiDict`, `ItemsView`, and
`ValuesView`),
consolidates that guard plus the `__name__` lookup into the shared
`md_repr()` helper, and adds the same missing guard to the pure-Python
`_KeysView.__repr__`, which never had it either.

## Are there changes in behavior for the user?

Yes: a self-referential `MultiDictProxy` or `KeysView` now prints `...`
for
the cyclic reference instead of recursing until `RecursionError`,
matching
the existing behavior of `MultiDict`, `ItemsView`, and `ValuesView`, on
both
the C extension and the pure-Python implementation.

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

No, it only touches the `repr()` implementations and uses the critical
section macros already vendored via `pythoncapi_compat.h`.

## Related issue number

#1432 

## 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`
- [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`

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Tests: `pytest -q` (1808 passed, C extension + pure-Python legs) and
`MULTIDICT_NO_EXTENSIONS=1 pytest -q --no-c-extensions` (917 passed,
pure-Python-only leg)
Lint: `make lint` (r... (continued)

634 of 1268 branches covered (50.0%)

Branch coverage included in aggregate %.

16 of 16 new or added lines in 2 files covered. (100.0%)

4242 of 4459 relevant lines covered (95.13%)

1.9 hits per line