aio-libs / multidict / 33994420898

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Fix segfault in `add()` with a keyword-only required argument (#1376)

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## What do these changes do?

`MultiDict().add(key="k")` — a required argument supplied only by
keyword, with the other missing —
segfaulted the interpreter instead of raising `TypeError`. The
two-argument FASTCALL parser `parse2()`
(`_multilib/parser.h`) did not enforce `minargs` in its keyword branch,
so `value` reached the caller as
a NULL pointer that was then dereferenced. Calls with more than two
keyword arguments were also
mis-routed into the one-keyword path.

`parse2()` now enforces `minargs` when the only supplied argument comes
by keyword, and rejects
more-than-two keyword arguments. The pure-Python backend already raised
the correct `TypeError`s for
these calls, so this only brings the C extension in line — no
pure-Python change is needed.

## Are there changes in behavior for the user?

Yes: `MultiDict().add(key="k")` and `add(key="k", value="v", extra="e")`
now raise `TypeError` (the same
messages the pure-Python implementation raises) instead of crashing /
silently misbehaving.

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

No. A localized fix in one argument-parser function, plus a parametrized
regression test.

## Related issue number

Same argument-parser family as the `setdefault()` segfault #1159 (fixed
narrowly by #1160); `add()` was
still affected. Surfaced by the C-extension analysis report #1306.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
(`test_add_missing_required_argument`, both backends)
- [x] Documentation reflects the changes (no public API change)
- [ ] N/A — no `CONTRIBUTORS.txt` in this repo
- [x] Add a new news fragment into the `CHANGES/` folder

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

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