aio-libs / multidict / 34026452283

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Fix reference/resource leaks on three C-extension error paths (#1381)

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## What do these changes do?

Three leaks on error paths in the C extension:

1. `items`-view `__contains__` (`_multilib/views.h`) fetched and owned
element 0 of a candidate pair,
   then returned on an element-1 failure without releasing element 0.
2. `md_repr` (`_multilib/hashtable.h`) did a bare `return NULL` on the
"changed during iteration" guard,
   leaking its `PyUnicodeWriter`; it now `goto fail` to discard it.
3. `md_next` (`_multilib/hashtable.h`) leaked the identity reference
when key materialisation failed
   under low memory.

C-extension-only concerns (the pure-Python backend relies on the GC); no
pure-Python change is needed.

## Are there changes in behavior for the user?

No user-visible behavior change; these are leak fixes on error paths.

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

No. Small, localized changes.

## Related issue number

Findings from the C-extension analysis report #1306.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist (deterministic refcount test for
(1), functional test for (2))
- [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: Andrew Svetlov <andrew.svetlov@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

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