Data race on `kqueue.kqfd` between `close()` and the `closed` getter with free-threading build

Bug report

Bug description:

select.kqueue accesses its internal kqfd file-descriptor field with no synchronization. So kqueue.close() can write the kqfd,

https://github.com/python/cpython/blob/b18168cb32d545ed976b760983478cbd5dde5bdf/Modules/selectmodule.c#L2160-L2165

while the closed getter reads it.

https://github.com/python/cpython/blob/b18168cb32d545ed976b760983478cbd5dde5bdf/Modules/selectmodule.c#L2276-L2279

Reproducer:

import select
from threading import Thread

slot = [select.kqueue()]

def reader():
    for _ in range(200000):
        try:
            _ = slot[0].closed
        except Exception:
            pass

def churner():
    for _ in range(200000):
        kq = select.kqueue()
        slot[0] = kq
        kq.close()

threads  = [Thread(target=reader)  for _ in range(4)]
threads += [Thread(target=churner) for _ in range(4)]
for t in threads: t.start()
for t in threads: t.join()

TSAN Report:

==================
WARNING: ThreadSanitizer: data race (pid=28784)
  Write of size 4 at 0x00011c040140 by thread T5:
    #0 kqueue_queue_internal_close selectmodule.c:2165
    #1 select_kqueue_close selectmodule.c.h:1189
    #2 method_vectorcall_NOARGS descrobject.c:448
    #3 PyObject_Vectorcall call.c:327
    #4 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:766
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:1846
...

  Previous read of size 4 at 0x00011c040140 by thread T4:
    #0 kqueue_queue_get_closed selectmodule.c:2279
    #1 getset_get descrobject.c:194
    #2 _PyObject_GenericGetAttrWithDict object.c
    #3 PyObject_GenericGetAttr object.c:2012
    #4 _PyObject_GetAttrStackRef object.c
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:8312

SUMMARY: ThreadSanitizer: data race selectmodule.c:2165 in kqueue_queue_internal_close
==================

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-151376

Start in Modules/selectmodule.c at kqueue_queue_internal_close around line 2165 and kqueue_queue_get_closed around line 2279, then review the reproducer and TSAN report. Check linked PR gh-151376 for work already underway; done means the concurrent close() and closed access no longer produce the reported race or descriptor hazard.