aio-libs / multidict / 34481271608

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Protect MultiDict update()/extend()/merge()/clear() with a critical section (#1433)

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## What do these changes do?

Protects `MultiDict.update()`, `.extend()`, `.merge()`, `.clear()`,
`.copy()`,
and the `MultiDict`/`CIMultiDict` constructors (including the fast-clone
path and, for an explicit re-init on an already-published multidict, the
reset step) in the C extension with a critical section, so each
operation
is atomic from the point of view of other threads. When
update()/extend()/
merge() take a plain dict argument, that dict is locked too (in addition
to self), since PyDict_Next() is not thread-safe against a foreign
thread
concurrently mutating the dict being iterated.

## Are there changes in behavior for the user?

No behavioral change for single-threaded code. On the free-threaded
build of CPython, this fixes a real segmentation fault and/or data
corruption.

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

No, it only touches the extend/update/merge/copy/clone/init
implementation
in the C extension and uses the critical section macros already vendored
via
`pythoncapi_compat.h`.

## Related issue number

Part of #1432

## 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`
- [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>

Reproduced the crash against `master` with a threaded stress script and
a
`gdb` backtrace pinpointing a NULL key/value read in
`_md_add_with_hash()`, using a genuine free-threaded CPython 3.13.7t
build (via pyenv), not just compiling the macros. Confirmed the fix
under the same build: 25+ stress runs and 3 `gdb`-driven runs wi... (continued)

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