gh-158973: Reset the warnings lock after fork in the child

[nascheme]

Reset the warnings lock in the child after fork, before clearing other threads’ states. This prevents deadlocks when their finalizers issue warnings and the lock was held by a thread that no longer exists. Record ownership before fork so the child can preserve the lock’s recursion depth and update its owner ID if the forking thread held it. Adds regression tests for child-cleanup warnings and recursive ownership across fork.

The PR was developed with assistance from GPT 6.1 Sol.

• Issue: os.fork() child can hang inside PyOS_AfterFork_Child() on the warnings lock (3.14 regression) #158973

[leandrodamascena]

I was able to reproduce this independently and validate the fix.

With a probe similar to test_fork_other_thread_holds_lock (a worker holds the warnings lock and keeps an object with a warning-emitting finalizer in thread-local storage, then the main thread forks), the child hangs inside os.fork() in every attempt on main before this PR. It stays stuck until it's killed, even with a 10 s timeout. With this PR, the child runs the finalizer, writes its marker and exits with status 0 every time, and the parent's worker releases the lock and joins cleanly.

	before this PR	with this PR
child hangs and is killed by the watchdog	100/100	0/100
child exits 0 and the finalizer runs	0/100	100/100

I saw the same on AWS Lambda (x86_64, PGO+LTO build), where this matters since the default runtime is a regular GIL build: every attempt hit the watchdog before the fix, and none did after it. test_warnings, test_fork1 and test_os pass with the change. Thanks for fixing this!

[ngoldbaum]

I think it's safe to store the bool in a static global: https://github.com/nascheme/cpython/compare/gh-158973-reset-warnings-lock...ngoldbaum:cpython:gh-158973-reset-warnings-lock?expand=1

This is being maximally cautions about avoiding ABI changes where they're not needed. I'm not sure if it's actually a problem in-practice. The static global is a bit of a code smell.

[ngoldbaum]

I think this is good to merge for main but I'm not sure about the backport, see my comment.

[miss-islington-app]

Thanks @nascheme for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @nascheme, I could not cleanly backport this to 3.14 due to a conflict.

Please backport manually with cherry_picker, see the devguide for more information.

cherry_picker 801377967fd7bfa573c0b4c48e31b69d50c1da2f 3.14

[bedevere-app]

GH-159613 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-159614 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-159615 is a backport of this pull request to the 3.14 branch.