Data race in the `codec.errors` setter (`codecctx_errors_set`) on a shared stateful codec

Bug report

Bug description:

Bug description:

In the free-threaded build, the codecctx_errors_set decrefs the old handler(ERROR_DECREF(self->errors)) and stores the new one (self->errors = cb) without any synchronization

https://github.com/python/cpython/blob/ecdef1773006529b0fea6639d0effeecbb41679c/Modules/cjkcodecs/multibytecodec.c#L170-L197

For any handler name other than the strict/ignore/replace sentinels (e.g. backslashreplace), self->errors holds a real refcounted PyUnicode.

The decref-then-assign is not atomic, so concurrent codec.errors = drops references incorrectly and can free a handler while it is still referenced.

Reproducer:

import codecs
import random
from threading import Thread
shared = codecs.getincrementalencoder('shift_jis')()
NAMES = ['backslashreplace', 'xmlcharrefreplace', 'namereplace']

def thread1():
    for _ in range(20000):
        try:
            shared.errors = random.choice(NAMES)
        except Exception:
            pass

if __name__ == "__main__":
    threads = [Thread(target=thread1) for _ in range(8)]
    for t in threads: t.start()
    for t in threads: t.join()

TSAN Report:

==================
WARNING: ThreadSanitizer: data race (pid=3576762)
  Read of size 8 at 0x7fffb65a1cc0 by thread T2:
    #0 codecctx_errors_set /cpython/./Modules/cjkcodecs/multibytecodec.c:194:5 
    #1 getset_set /cpython/Objects/descrobject.c:250:16 
    #2 _PyObject_GenericSetAttrWithDict /cpython/Objects/object.c:2049:19
    #3 PyObject_GenericSetAttr /cpython/Objects/object.c:2120:12
    #4 PyObject_SetAttr /cpython/Objects/object.c:1533:15
    #5 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:12146:27 
...

  Previous write of size 8 at 0x7fffb65a1cc0 by thread T1:
    #0 codecctx_errors_set /cpython/./Modules/cjkcodecs/multibytecodec.c:195:18
    #1 getset_set /cpython/Objects/descrobject.c:250:16 
    #2 _PyObject_GenericSetAttrWithDict /cpython/Objects/object.c:2049:19
    #3 PyObject_GenericSetAttr /cpython/Objects/object.c:2120:12 
    #4 PyObject_SetAttr /cpython/Objects/object.c:1533:15 
    #5 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:12146:27 
...

SUMMARY: ThreadSanitizer: data race /cpython/./Modules/cjkcodecs/multibytecodec.c:194:5 in codecctx_errors_set
==================

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-153000

Start in Modules/cjkcodecs/multibytecodec.c at codecctx_errors_set, then run the supplied threaded reproducer under the free-threaded build with ThreadSanitizer. Compare the behavior with linked PR gh-153000; done means concurrent codec.errors assignments no longer produce the reported race or incorrect handler lifetime behavior.