Segfault in bytes.join for mutating list

[eendebakpt]

Crash report

In the FT build bytes.join can crash on a mutating list.

import threading

lst = [bytes(10) for _ in range(100)]

def mutate():
    while True:
        for i in range(100):
            lst[i] = bytes(10)      # new object; the old one is freed here

threading.Thread(target=mutate, daemon=True).start()
while True:
    b''.join(lst)

The issue seems to be on line

cpython/Objects/stringlib/join.h

Line 63 in 9d22a53

item = PySequence_Fast_GET_ITEM(seq, i);

where a borrowed ref is used.

A solution could be to add a Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST like in #119315

#119315

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Output from running 'python -VV' on the command line:

No response