aio-libs / multidict / 33997113399

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Fix segfault from an uninitialised MultiDict/CIMultiDict created via `__new__` (#1378)

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## What do these changes do?

`MultiDict.__new__(MultiDict)` (or a subclass that skips
`super().__init__()`) left `md->state` and
`md->keys` as NULL, and the first method call dereferenced them and
segfaulted. The container now has a
`tp_new` that initialises the object to a valid empty state (`state`
set, `keys` = the shared
`&empty_htkeys` sentinel), with `is_ci` derived from the type so
`CIMultiDict` stays case-insensitive.
The sentinel is not heap-allocated, so the `md_init()` that `tp_init`
runs on normal construction does
not leak.

Scope: this fixes the container types. The proxy types
(`MultiDictProxy`/`CIMultiDictProxy`) have the
same latent issue (`self->md` left NULL by `__new__`) but need a
different fix (moving argument
validation into `tp_new`); I'll send that as a follow-up rather than
fold a construction-protocol change
in here. The pure-Python backend raises `AttributeError` for the same
`__new__`-without-`__init__` case
(no crash); aligning it to also yield a valid empty mapping can be a
small follow-up if you'd prefer
exact parity.

## Are there changes in behavior for the user?

Yes (mild): `MultiDict.__new__(MultiDict)` and subclasses that skip
`super().__init__()` now behave as a
valid empty mapping instead of crashing.

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

No. One `tp_new` function shared by both container types.

## Related issue number

Generalises the point fixes #1159 (`setdefault`) and #1161/#1163 (view
types) to the containers.
Surfaced by the analysis report #1306.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
(`test_new_without_init_is_valid_empty`, both container types + a
subclass)
- [x] Documentation reflects the changes (no public API change)
- [ ] N/A — no `CONTRIBUTORS.txt` in this repo
- [x] Add a new news fr... (continued)

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