`context_aware_warnings` isn't thread-safe when `showwarning` is patched

Bug report

Bug description:

When showwarning is patched (most commonly by setting logging.captureWarnings(True), the warnings.catch_warnings context manager still isn't thread safe with context_aware_warnings enabled.

In Python 3.14 even with context_aware_warnings flag enabled, there is a bug in asyncio / multithread contexts when using warnings.catch_warnings and logging.captureWarnings(True) since both attempt to patch warnings.showwarning. Write me a minimal reproducible example of this issue to share with the cpython team.
"""
MRE: catch_warnings + captureWarnings race on warnings.showwarning

Python 3.14+ with context_aware_warnings enabled.

Both warnings.catch_warnings() and logging.captureWarnings(True) save/restore
warnings.showwarning by *value at call time* — a plain reference copy.  Under
concurrent execution the save/restore pairs interleave, leaking the wrong
showwarning into the global module state.

Scenario that demonstrates the interleaving:

    T1 catch_warnings.__enter__   → saved = showwarning      (= warn_orig)
    T1 captureWarnings(True)      → saved = showwarning      (= warn_orig)
                                   showwarning = log_showwarning
    T2 catch_warnings.__enter__   → saved = showwarning      (= log_showwarning ← T1's patch!)
    T2 captureWarnings(True)      → showwarning = log_showwarning
    T1 captureWarnings(False)     → showwarning = warn_orig  (correct)
    T1 catch_warnings.__exit__    → showwarning = warn_orig  (correct, saved in step 1)
    T2 captureWarnings(False)     → showwarning = log_showwarning  (BUG — saved T1's patch)
    T2 catch_warnings.__exit__    → showwarning = log_showwarning  (BUG — persists)

After all threads exit, warnings.showwarning is still logging.showwarning even
though every thread called captureWarnings(False).

Two reproductions:
  1. ThreadPoolExecutor (preemptive — natural race)
  2. asyncio with explicit yield points (cooperative — deterministic)
"""

import asyncio
import concurrent.futures
import logging
import threading
import warnings
import sys

try:
    warnings._context_aware_warnings = True
except AttributeError:
    sys.exit("Python 3.14+ required for context_aware_warnings")

logging.basicConfig(level=logging.WARNING, force=True)

ORIGINAL_SHOWWARNING = warnings.showwarning


# ── Reproduction 1: threads (preemptive) ──────────────────────────────

_thread_errors: list[str] = []
_thread_lock = threading.Lock()


def _thread_worker(n: int) -> None:
    with warnings.catch_warnings():
        warnings.simplefilter("always")
        logging.captureWarnings(True)
        warnings.warn(f"thread worker {n}", UserWarning)
        logging.captureWarnings(False)

    if warnings.showwarning is not ORIGINAL_SHOWWARNING:
        with _thread_lock:
            _thread_errors.append(
                f"worker {n}: showwarning is {warnings.showwarning!r} "
                f"instead of {ORIGINAL_SHOWWARNING!r}"
            )


def reproduce_with_threads() -> None:
    for _ in range(200):
        with concurrent.futures.ThreadPoolExecutor(max_workers=8) as pool:
            list(pool.map(_thread_worker, range(32)))


# ── Reproduction 2: asyncio (cooperative with explicit yield points) ──

_async_errors: list[str] = []
_async_lock = threading.Lock()

# Barriers to orchestrate the interleaving shown at the top of this file.
_barrier_1 = asyncio.Event()  # T2 waits for T1 to finish captureWarnings(True)
_barrier_2 = asyncio.Event()  # T1 waits for T2 to finish catch_warnings __enter__
_barrier_3 = asyncio.Event()  # T2 waits for T1 to exit catch_warnings


async def _async_worker_a(n: int) -> None:
    """Task A: the "early" task that sets captureWarnings before B enters."""
    with warnings.catch_warnings():
        warnings.simplefilter("always")
        logging.captureWarnings(True)
        _barrier_1.set()           # T1: captureWarnings(True) done
        await asyncio.sleep(0)     # yield so B enters catch_warnings

        await _barrier_2.wait()    # wait until B has done __enter__
        logging.captureWarnings(False)
        # __exit__ restores showwarning to what it was on __enter__ (= original)
    _barrier_3.set()               # signal B to continue


async def _async_worker_b(n: int) -> None:
    """Task B: enters catch_warnings while A's captureWarnings(True) is active."""
    await _barrier_1.wait()        # wait for A's captureWarnings(True)
    with warnings.catch_warnings():
        # __enter__ saves showwarning — which is currently A's logging.showwarning!
        _barrier_2.set()           # signal A to continue
        await asyncio.sleep(0)     # yield so A calls captureWarnings(False) + __exit__

        await _barrier_3.wait()    # A is done
        logging.captureWarnings(True)
        await asyncio.sleep(0)
        logging.captureWarnings(False)
        # __exit__ restores showwarning to what it was on __enter__ (= logging.showwarning)
    # At this point showwarning is logging.showwarning — even though
    # captureWarnings(False) was called.  This is wrong.

    if warnings.showwarning is not ORIGINAL_SHOWWARNING:
        async with _async_lock:
            _async_errors.append(
                f"async scenario: showwarning is {warnings.showwarning!r} "
                f"instead of {ORIGINAL_SHOWWARNING!r}"
            )


async def reproduce_with_asyncio() -> None:
    _barrier_1.clear()
    _barrier_2.clear()
    _barrier_3.clear()
    await asyncio.gather(_async_worker_a(0), _async_worker_b(0))


# ── Report ────────────────────────────────────────────────────────────

def _report(backend: str, errors: list[str]) -> None:
    print(f"\n{'─'*70}")
    print(f"[{backend}]")
    print(f"{'─'*70}")
    if errors:
        print(f"  BUG REPRODUCED — {len(errors)} occurrence(s):")
        for err in errors:
            print(f"    • {err}")
    else:
        print("  No bug detected (race is timing-dependent; try again)")
    sw = warnings.showwarning
    print(f"  warnings.showwarning: original={ORIGINAL_SHOWWARNING!r}, current={sw!r}")
    print(f"  {'✓ match' if sw is ORIGINAL_SHOWWARNING else '✗ LEAKED'}")


def main() -> None:
    reproduce_with_threads()
    _report("ThreadPoolExecutor (200×32 workers)", _thread_errors)

    _async_errors.clear()
    for _ in range(20):
        asyncio.run(reproduce_with_asyncio())
    _report("asyncio (20 deterministic interleavings)", _async_errors)


if __name__ == "__main__":
    main()

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Bắt đầu bằng cách chạy các bản tái hiện được cung cấp trong các entry point warnings.catch_warnings và logging.captureWarnings với context_aware_warnings được bật, sau đó kiểm tra cách mỗi entry point lưu và khôi phục warnings.showwarning. Issue không nêu tệp mã nguồn hoặc test nào và không chỉ rõ thay đổi mong muốn; để hoàn thành, cần có một bản sửa lỗi hoặc regression test do maintainer xác định, chứng minh rằng global handler được khôi phục sau cả hai interleaving.