nogil inconsistent empty list while sorting

Bug report

Bug description:

Hi,

We're a research group focused on testing concurrent runtimes. Our work-in-progress prototype found a violation of atomicity on the current nogil build when using concurrent operations on the same list. The program below shows the wrong behavior.

from threading import Thread,Barrier,Lock

def t1(b1,l,r):
    b1.wait()
    r[0] = l.__len__()

def t2(b1,l,r):
    b1.wait()
    r[0] = l.__str__()

def t3(b1,l):
    b1.wait()
    l.sort()

def normalSetTest(i):

    sharedList = [1,2,3] * 100
    barrier = Barrier(2)
    res = [ None ]
    threads = [
            Thread(target= t1, args=(barrier,sharedList,res,)),
            #Thread(target= t2, args=(barrier,sharedList,res,)),
            Thread(target= t3, args=(barrier,sharedList,)),
            ]


    for t in threads:
        t.start()
    for t in threads:
        t.join()

    if res[0] in [ 0, '[]' ]:
        print("\tfound bug: " + str(res[0]))

print("test begin...")

for n in range(0,10):
    threads = []

    for i in range(0,1):
        threads.append(Thread(target= normalSetTest, args=(n,)))

    for t in threads:
        t.start()

    for t in threads:
        t.join()

print("test Done")

A list with 300 integers is sorted in t3 while another thread either gets the size of the list t1 or turns the list into a string t2. Running the code above will show threads t1 and t2 finding an inconsistent empty list with length 0 or turned into the string "[]".
Our tool did not find any other interesting values: either it's an expected value (original list or sorted list) OR it's empty.

Sample output:

test begin...
        found bug: 0
        found bug: 0
        found bug: 0
        found bug: 0
        found bug: 0
test Done

We're happy to provide more details about this bug, and to help developers reproducing it.

Output of python -VV: Python 3.14.0a1+ experimental free-threading build (heads/main:faa3272fb8d, Oct 29 2024, 09:14:25) [GCC 14.2.1 20240805]

@flypoodles and @overlorde are part of the team, adding them so they get notified about further discussion.

I believe this issue is part of the ongoing conversation on #126136 about acceptable behaviors of containers operated by many threads concurrently, described in the original nogil PEP: https://peps.python.org/pep-0703/#container-thread-safety

CPython versions tested on:

3.13, 3.14, CPython main branch

Operating systems tested on:

Linux

Bắt đầu bằng cách chạy reproducer được cung cấp trên một bản build CPython free-threading, sau đó đọc phần về tính an toàn luồng của container trong PEP 703 và cuộc thảo luận trong issue #126136. Cần xác định hành vi mong đợi đối với việc sắp xếp list đồng thời, lấy độ dài và chuyển đổi sang string trước khi xác định xem có cần bản sửa lỗi và regression test hay không.