aio-libs / multidict / 38159918890

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Borrow the key's identity in add, setitem, delitem and getall (#1769)

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## What do these changes do?

`md[key]` borrows the identity of an exact `str` key (or of an exact
`istr`'s canonical form) and computes an owned one only for other keys.
`add()`, `md[key] = value`, `del md[key]` and `getall()` still computed
an owned identity on every call, paying an incref and a decref. They now
borrow it the same way, with a separate fallback for the other keys.
`md_calc_identity_hash_fits()` lost its last caller and is removed.

The caller's reference to the key keeps a borrowed identity alive across
the finalizers a delete or replace can run;
`tests/test_finalizer_reentrancy.py` already covers those paths for both
classes. The pure-Python backend has no equivalent cost and is
unchanged.

## Are there changes in behavior for the user?

No.

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

No, it repeats the pattern `md_get_one()` already uses.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist (existing tests; no behaviour
change)
- [x] Documentation reflects the changes (N/A, no API change)
- [ ] 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`

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

Instructions per operation, `benchmarks/callgrind_driver.py
--include-multidict-only`, CPython 3.14.7 and 3.14.7t, master 709f2600
vs this branch:

| operation | GIL MultiDict | GIL CIMultiDict | FT MultiDict | FT
CIMultiDict |
|---|---|---|---|---|
| delitem | 265.5 → 247.5 (-6.8%) | 315.9 → 293.9 (-7.0%) | +1.1% |
+2.6% |
| setitem_replace | 323.3 → 314.3 (-2.8%) | 363.5 → 352.5 (-3.0%) |
-2.7% | -2.9% |
| setitem_insert | ... (continued)

832 of 1664 branches covered (50.0%)

Branch coverage included in aggregate %.

9757 of 10641 relevant lines covered (91.69%)

1.83 hits per line