aio-libs / multidict / 34846098293

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Enable sanitizers in the debug build and add a ThreadSanitizer CI job (#1455)

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## What do these changes do?

`MULTIDICT_DEBUG_BUILD=1` now also builds the C extension with
AddressSanitizer and UndefinedBehaviorSanitizer (skipped on Windows).
A new `MULTIDICT_TSAN_BUILD=1` switches to ThreadSanitizer instead,
since it can't share a binary with ASan/UBSan. AGENTS.md documents
exact usage, including the `LD_PRELOAD`/`PYTHONMALLOC=malloc` dance
ASan needs and why TSan needs a CPython actually built with
`--with-thread-sanitizer` to avoid false positives from the
interpreter's own locks.

A CI job (`tsan`, alongside `benchmark` at the same level) builds such
a free-threaded CPython from the official source tarball, installs
multidict against it with `MULTIDICT_TSAN_BUILD=1`, and runs the suite
with `halt_on_error=1` so a detected race fails the job outright.

## Are there changes in behavior for the user?

No. Both sanitizer modes are opt-in via environment variables; the
default build is unaffected.

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

No. The sanitizer flags only apply under `MULTIDICT_DEBUG_BUILD=1`,
and the new CI job is self-contained and independent of the rest of
the test matrix.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] 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>

Verified locally (GitHub Actions itself wasn't exercised, only the
equivalent commands and interpreters):

- ASan+UBSan: full suite passes (`pytest -q`, C extension) except
  `test_leaks.py`, which fails under ASan for unrelated... (continued)

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