aio-libs / multidict / 34869375795

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Fix update() not invalidating iterators when overwriting an existing key (#1458)

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## What do these changes do?

In the C extension, `update()` (and `merge()`, which shares the same
code path) overwrites an already-present key's value in place without
bumping the hash table's version counter. `__setitem__` bumps it on the
equivalent overwrite, and `update()` already bumps it when a key is
newly added, so a live `keys()`/`items()`/`values()` iterator (or
`iter(md)`) silently kept yielding stale entries instead of raising
`RuntimeError` after an `update()` call that only touched existing
keys. The fix bumps the version once per batch in `md_post_update()`,
matching pure Python's `_post_update()`, which already does this
unconditionally.

## Are there changes in behavior for the user?

Yes. `update()`/`merge()` calls that only overwrite existing keys in
the C extension now correctly invalidate any iterator in progress,
raising `RuntimeError` on the next advance, matching `__setitem__` and
matching the pure-Python implementation's existing behavior.

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

No. The fix is a single version bump in an already-existing,
already-locked cleanup helper; no new public API or code path.

## Related issue number

No linked issue; found via Hypothesis-based fuzz testing of the
iteration guard.

## 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`
- [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` passed (1953 passed, 10 skipped) under both the C
extension and `MULTIDICT_NO_EXTENSIONS=1` pure-Python ... (continued)

727 of 1454 branches covered (50.0%)

Branch coverage included in aggregate %.

27 of 27 new or added lines in 1 file covered. (100.0%)

5322 of 5569 relevant lines covered (95.56%)

1.91 hits per line