aio-libs / multidict / 37742297551

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Drop the extra key and value references in md_update_from_dict() (#1747)

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## What do these changes do?

Removes the `owned` bookkeeping from `md_update_from_dict()`. It held an
extra reference to each key and value of a `str` subclass key, because
that key's `lower()` could clear the dict being read. Since #1746
computing an identity runs no user code, so the dict keeps both alive
for every key type, as it already did for exact `str` keys. `extend()`
still takes its two references right before handing them to
`md_add_with_hash_steal_refs()`.

Also corrects the comment on the list re-read in the sequence path,
which still named `lower()`; the re-read itself stays, since an item's
`__getitem__()` or a `__del__` can still shrink the list.

The pure-Python backend has no equivalent bookkeeping.

## Are there changes in behavior for the user?

No.

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

No; it removes code.

## Related issue number

N/A; follow-up to #1746.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes (N/A)
- [ ] 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 3.14.7 (4076 passed) and 3.14.7t (4061 passed);
pure-Python leg `MULTIDICT_NO_EXTENSIONS=1 pytest -q --no-c-extensions`
on 3.14.7 (1603 passed); `pytest -q -m "not leaks and not hypothesis and
not capi"` on 3.10.17 (3899 passed). The new
`test_str_subclass_keys_from_dict_keep_refcounts` fails when an extra
incref is injected into the merge case.
Inlining: `tools/check_inlining.py` ok on 3.14.7 and 3.14.7t with GCC
15, and with GCC 13.3 on 3.14.7 and 3.14.8t... (continued)

793 of 1586 branches covered (50.0%)

Branch coverage included in aggregate %.

12 of 15 new or added lines in 1 file covered. (80.0%)

9326 of 10194 relevant lines covered (91.49%)

1.83 hits per line