`_py_abc` Python implementation of `abc` is not thread-safe

Bug report

The update to the invalidation counter is not thread-safe and can lose updates in some Python implementations:

Failures seen on:

• Python 3.14t
• Python 3.9
• pypy3.10
• pypy3.11

But not on Python 3.10-3.14 with GIL due to limited GIL switch opportunities.

https://github.com/python/cpython/blob/05e89c34bd8389f87bd6c9462d5a06ef9e1a65ab/Lib/_py_abc.py#L54-L70

For example, consider the following repro, adapted from test_abc.test_registration_basics:

import _py_abc as abc # Use Python implementation of ABCs!!
import threading
import os
import sys

sys.setswitchinterval(1e-6)

N = 5

def run(b):
    b.wait()

    class A(metaclass=abc.ABCMeta):
        pass
    A.register(int)
    if not isinstance(42, A):
        print("Oops!")
        os._exit(1)

def main():
    for _ in range(10000):
        threads = []
        b = threading.Barrier(N)
        for _ in range(N):
            t = threading.Thread(target=run, args=(b,))
            threads.append(t)
            t.start()
        for t in threads:
            t.join()


if __name__ == "__main__":
    main()

Linked PRs

• gh-130131

Start with Lib/_py_abc.py around the invalidation-counter update and reproduce the race using the threaded example in the issue. Run Lib/test/test_abc.py, especially test_registration_basics; done means concurrent registrations no longer lose invalidation updates and the affected Python implementations pass the relevant tests.