aio-libs / multidict / 38161442554

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Check a CIMultiDict key for a borrowed identity only once (#1770)

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## What do these changes do?

CodSpeed on #1769 flagged `test_cimultidict_add_str_mixed_case[c]` 9%
slower. A key that cannot lend its identity, such as a `CIMultiDict`
`str` key holding uppercase letters, first went through the failed
borrow attempt and then through `md_calc_identity_hash()`, which
repeated the istr, ASCII and uppercase tests; with more callers that
helper also fell out of line.

`md_identity_or_new()` runs each test once and returns either the
borrowed identity or a new reference, and `md_identity_is_new()` tells
the two apart without an ownership flag or an out-pointer. Each of
`add()`, `md[key] = value`, `del md[key]` and `getall()` now calls its
body once. The #1769 news fragment is updated to the resulting numbers.
The pure-Python backend is unchanged.

## Are there changes in behavior for the user?

No.

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

No, it replaces the per-operation `_owned` helpers #1769 added with one
identity helper.

## Related issue number

Follow-up to #1769.

## 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>

Mixed-case `CIMultiDict` keys, Ir per call on GIL 3.14.7 (callgrind,
includes the lowering), before #1769 / with #1769 / this PR:

| op | before #1769 | #1769 | this PR |
|---|---|---|---|
| add | 1891 | 1966 | 1907 |
| `md[k] = v` | 1881 | 1935 | 1879 |
| `del md[k]` | 1614 | 1664 | 1606 |
| getall | 2137 | 2194 |... (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