Negative indexing in free-threading can access data that never lived at that index

[tom-pytel]

Bug report

Bug description:

The place that negative indices for lists are offset currently in the free-threaded build can cause data to be returned that never actually resided at that offset (negatively speaking) if the list is modified during the access (either before or after the modification). Is this an issue?

cpython/Objects/listobject.c

Line 3581 in 5326c27

i += PyList_GET_SIZE(self);

Reproducer (it will happen eventually):

import re
import threading


def ins1(b, l):
    b.wait()
    l.insert(0, 'potato')

def get_secret(b, l):
    b.wait()
    assert l[-1] != 'super secret password'


def check(funcs, *args):
    barrier = threading.Barrier(len(funcs))
    thrds = []

    for func in funcs:
        thrd = threading.Thread(target=func, args=(barrier, *args))

        thrds.append(thrd)
        thrd.start()

    for thrd in thrds:
        thrd.join()


if __name__ == "__main__":
    while True:
        check([ins1] + [get_secret] * 10, ['super secret password', 'tomato'])

Output:

$ ./python ../check.py 
Exception in thread Thread-38474 (get_secret):
Traceback (most recent call last):
  File "/home/tom/work/cpython/free/cp/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/home/tom/work/cpython/free/cp/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tom/work/cpython/free/cp/../check.py", line 11, in get_secret
    assert l[-1] != 'super secret password'
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[tom-pytel]

ping @colesbury

[colesbury]

We can document this, but I don't think we should change the implementation.