aio-libs / multidict / 36439719872

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Release the pairs del and popall() remove only after the scan (#1627)

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## What do these changes do?

`del md[key]` and `popall()` decref'd each match as soon as they
unlinked it. A finalizer of a removed key or value could add the key
back mid-scan; the scan then restarted (the restart #1512 added for
concurrent resizes) and removed that new pair too, and `popall()`
returned its value. The removed pairs are now held until every match is
gone: the first in place, which covers the usual single match, any more
in a `reflist_t`. With nothing in the loops able to run Python, the
restart is gone.

The pure-Python `__delitem__()`, `popall()` and `__setitem__()` freed
each match mid-scan as well, and with a finalizer that replaced the
table they raised `IndexError` or miscounted `len()`; they now keep the
removed entries alive until the scan ends. The C `__setitem__()` already
deferred its decrefs.

## Are there changes in behavior for the user?

Yes: pairs a finalizer adds during `del md[key]` or `popall()` stay in
the multidict.

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

No.

## Related issue number

Found by the finalizer audit in #1631.

## 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` (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>

New `test_remove_all_keeps_what_a_finalizer_adds` (delitem, popall,
setitem x key/value side x both classes x both backends): against master
6 C and 6 pure-Python delitem/popall cases fail, and the 4 pure-Python
setitem ones.

Tests, one venv and tree copy per leg, full suite: GIL 3.14.7 (2923
passed), FT 3... (continued)

790 of 1580 branches covered (50.0%)

Branch coverage included in aggregate %.

40 of 40 new or added lines in 2 files covered. (100.0%)

8646 of 9267 relevant lines covered (93.3%)

1.87 hits per line