gh-126434: Use RLock for multiprocessing Event to avoid deadlock when handle system signal

[ivarref]

Please see issue #126434 for details about bug and code to reproduce deadlock.

Kind regards.

• Issue: multiprocessing.Event.set() can be deadlocked by .is_set when called by a sigterm handler #126434

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[Zheaoli]

Mostly LGTM

BTW this PR title need be changed

[ivarref]

Mostly LGTM

BTW this PR title need be changed

I've changed the title @Zheaoli . What do you think? Thanks again.

[Zheaoli]

LGTM

[tomasr8]

Suggested change

	Use RLock for :mod:`multiprocessing` ``Event`` to avoid deadlock when handle system signal
	Use :class:`multiprocessing.RLock` for :class:`multiprocessing.Event` to avoid deadlock when handle system signal

[Zheaoli]

This patch is failed in free threading TSAN test. need to figure out the root cause

[tomasr8]

This test seems to fail in other PRs as well, I restarted the job again to see if it fails again.

[colesbury]

The Event implementation is not correct with an RLock because that allows reentrancy (from the same thread) during operations that are not written to be reentrant. For example:

    def is_set(self):
        with self._cond:
            if self._flag.acquire(False):
                self._flag.release()
                return True
            return False

After the self._flag.acquire(False) before the self._flag.release(), reentrant is_set() calls will incorrectly appear to be false.

Event is not written to be safely called from a Python signal handler.

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[Zheaoli]

Event is not written to be safely called from a Python signal handler.

If this is the origin design behavior. Should we update the docs?

[ZeroIntensity]

This needs a test, anyway.