Race in stgdict PyCStructUnionType_update_stginfo under free-threading

[vfdev-5]

Bug report

Bug description:

I built cpython (3.13 branch) with free-threading and TSAN. The following python code reports TSAN warnings:

import ctypes
import concurrent.futures
import threading


def make_nd_memref_descriptor(rank, dtype):
    class MemRefDescriptor(ctypes.Structure):
        _fields_ = [
            ("aligned", ctypes.POINTER(dtype)),
        ]

    return MemRefDescriptor


class F32(ctypes.Structure):
    _fields_ = [("f32", ctypes.c_float)]


def test_ctypes():
    arg1_ndim = 1
    ctp = F32
    ctypes.pointer(
        ctypes.pointer(make_nd_memref_descriptor(arg1_ndim, ctp)())
    )


if __name__ == "__main__":
    num_workers = 20

    barrier = threading.Barrier(num_workers)

    def closure():
        barrier.wait()
        test_ctypes()

    with concurrent.futures.ThreadPoolExecutor(max_workers=num_workers) as executor:
        futures = []
        for i in range(num_workers):
            futures.append(executor.submit(closure))
        assert len(list(f.result() for f in futures)) == num_workers

TSAN report extract:

WARNING: ThreadSanitizer: data race (pid=264065)
  Read of size 4 at 0x7fffb80a0430 by thread T17:
    #0 PyCStructUnionType_update_stginfo /project/cpython/./Modules/_ctypes/stgdict.c:438:19 (_ctypes.cpython-313t-x86_64-linux-gnu.so+0x20976) (BuildId: 6a7727f376cee83a3124898fe6eb84e45e3a54bc)
    #1 PyCStructType_setattro /project/cpython/./Modules/_ctypes/_ctypes.c:1096:16 (_ctypes.cpython-313t-x86_64-linux-gnu.so+0xc809) (BuildId: 6a7727f376cee83a3124898fe6eb84e45e3a54bc)
    #2 PyObject_SetAttr /project/cpython/Objects/object.c:1415:15 (python3.13t+0x294fba) (BuildId: 65e52348ca9319985f9f5e265fd43aab2c867d59)
    #3 StructUnionType_init /project/cpython/./Modules/_ctypes/_ctypes.c:685:13 (_ctypes.cpython-313t-x86_64-linux-gnu.so+0xe222) (BuildId: 6a7727f376cee83a3124898fe6eb84e45e3a54bc)
    #4 PyCStructType_init /project/cpython/./Modules/_ctypes/_ctypes.c:715:12 (_ctypes.cpython-313t-x86_64-linux-gnu.so+0xc84a) (BuildId: 6a7727f376cee83a3124898fe6eb84e45e3a54bc)

Full traceback

Related to #127945

CPython versions tested on:

3.13

Operating systems tested on:

Linux