tsan-006: itertoolsmodule.c: count.__repr__ plain-reads cnt

[johng]

Bug description:

Follows from #153852 for data race on plain read of cnt in repr

itertoolsmodule.c: count.repr plain-reads cnt while count_next writes it with an atomic CAS — the writer was hardened, the reader missed

import itertools
import threading

STOP = False


def advance(it):
    while not STOP:
        next(it)


def read_repr(it):
    while not STOP:
        repr(it)


def main():
    global STOP
    it = itertools.count()
    threads = [threading.Thread(target=advance, args=(it,)) for _ in range(2)]
    threads += [threading.Thread(target=read_repr, args=(it,)) for _ in range(4)]
    for t in threads:
        t.start()
    # Let the race run briefly; TSan reports on the first conflicting pair.
    threading.Event().wait(2.0)
    STOP = True
    for t in threads:
        t.join()
    print("done (no race detected)")


if __name__ == "__main__":
    main()

PYTHON_GIL=0 ./python.exe repro_tsan_0006_count.py
SUMMARY: ThreadSanitizer: data race itertoolsmodule.c:3680 in count_repr
==================
done (no race detected)
ThreadSanitizer: reported 2 warnings
[1]    55630 abort      PYTHON_GIL=0 ./python.exe repro_tsan_0006_count.py

CPython versions tested on:

CPython main branch

Operating systems tested on:

Macos 26.3.2

Linked PRs

• gh-153908: Fix datarace in itertools.count().repr() #153917
• [3.15] gh-153908: Fix data race in itertools.count.__repr__ (GH-153917) #153951
• [3.14] gh-153908: Fix data race in itertools.count.__repr__ (GH-153917) #153952

[ZeroIntensity]

Backports on automerge