aio-libs / multidict / 34788665032

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Make pure-Python MultiDict safe under free-threading (#1447)

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## What do these changes do?

Adds per-instance locking to the pure-Python `MultiDict`/`CIMultiDict`
fallback (`multidict/_multidict_py.py`) so it gives the same
crash-free, corruption-free guarantee as the C extension under a
free-threaded (no-GIL) build. Every method that touches an instance's
hash table takes that instance's own `RLock` (created once in
`__new__`, reused across a later re-init); two-object operations
(`update()`, `extend()`, `merge()`, `__eq__()`, copy construction) lock
both objects together in a fixed, deadlock-safe order; view iterators
take the lock per step rather than for the whole iteration, so a
concurrent mutation between steps surfaces as the same "changed during
iteration" `RuntimeError` a single-threaded caller already gets today
instead of a deadlock; and the version counter shared by every
instance gets its own lock. On a regular GIL-enabled interpreter the
locked methods are the exact same function objects as before this
change -- no wrapper, no lock, no measurable overhead.

## Are there changes in behavior for the user?

Yes, but only on a free-threaded build: concurrent use of a
pure-Python `MultiDict`/`CIMultiDict` from multiple threads is now
safe against crashes and data corruption, matching the C extension.
On a regular (GIL-enabled) build there is no behavior or performance
change.

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

No. The change is self-contained to `_multidict_py.py`, gated entirely
behind a runtime free-threading check computed once at import, and
mirrors the locking approach already reviewed and merged for the C
extension (#1438, #1441, #1445, #1446). The C extension itself is
unaffected; this only touches the pure-Python fallback.

## Related issue number

N/A -- no linked issue. Continues the free-threading hardening already
done for the C extension.

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