_py_abc Python implementation of abc is not thread-safe #130095
Description
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.
Lines 54 to 70 in 05e89c3
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()