aio-libs / multidict / 38260712021

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Release a key's identity through md_identity_release() (#1780)

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## What do these changes do?

`md_identity_is_new()` was always followed by `Py_DECREF(identity)`.
Replaces it with `md_identity_release()`, which does the check and the
decref, named to pair with `md_identity_or_new()`. The C extension only;
the pure-Python backend has no equivalent.

Callgrind, 3.14.7, against master, rows that moved by 0.1% or more:

| GIL | before | after | |
| --- | ---: | ---: | ---: |
| `MultiDict.update()` | 37110.0 | 35513.0 | -4.30% |
| `key in MultiDict` | 145.8 | 144.8 | -0.69% |
| `MultiDict.pop()` | 389.2 | 396.2 | +1.80% |
| `CIMultiDict.update()` | 42012.0 | 40515.0 | -3.56% |
| `CIMultiDict.pop()` | 446.9 | 450.4 | +0.78% |

No row moved on FT. Before #1781, the same change made `MultiDict[key]`
2.5% and `key in MultiDict` 2.2% slower on GIL, from GCC laying out the
probe loop in `_md_get_one_locked()` differently; that is gone now.

## Are there changes in behavior for the user?

No.

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

No.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist (N/A, no behavior change; covered
by the existing suite)
- [x] Documentation reflects the changes
- [ ] 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>

Tests: `pytest -q` on 3.11.13, C extension 4262 passed, pure Python
(`MULTIDICT_NO_EXTENSIONS=1`, `--no-c-extensions`) 1692 passed.
Inlining: `tools/check_inlining.py` on 3.14.7 and 3.14.7t, ok.
Callgrind: `benchmarks/callgrind_driver.py --include-multidict-only`,
`--impl multidict_c` and `cimultidict_c`, GIL and FT venvs per t... (continued)

842 of 1684 branches covered (50.0%)

Branch coverage included in aggregate %.

9796 of 10704 relevant lines covered (91.52%)

1.83 hits per line