Py_(X)SETREF should have atomic versions on free-threaded builds

Feature or enhancement

Proposal:

Right now, Py_SETREF is implemented in non-atomic terms, which makes it subject to operation reordering issues on weakly ordered architectures (which caused a problem fixing gh-149816, where even switching from separate Py_CLEAR and later assignment to set_sni_cb to a single "atomic" Py_SETREF didn't fix the issue on ARM, because SSL's callback was being invoked asynchronously).

Py_SETREF is naturally implemented in terms of an atomic exchange, and at least for the free-threaded builds, it seems like it should be implemented as such to avoid such raciness, supplementing the existing:

#ifdef _Py_TYPEOF
#define Py_SETREF(dst, src) \
    do { \
        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
        _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
        *_tmp_dst_ptr = (src); \
        Py_DECREF(_tmp_old_dst); \
    } while (0)
#else
#define Py_SETREF(dst, src) \
    do { \
        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
        PyObject *_tmp_src = _PyObject_CAST(src); \
        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
        Py_DECREF(_tmp_old_dst); \
    } while (0)
#endif

// Similarly long nonsense for Py_XSETREF

with a new atomic-based approach for free-threaded builds:

#define Py_SETREF(dst, src) Py_DECREF(_Py_atomic_exchange_ptr(&(dst), (src)))
#define Py_XSETREF(dst, src) Py_XDECREF(_Py_atomic_exchange_ptr(&(dst), (src)))

Please poke holes in this approach. I have a lot of experience with writing multithreaded C code, but will never be convinced I've covered every race condition no matter how simple the code.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Locate the CPython C API definitions of Py_SETREF and Py_XSETREF, then inspect how free-threaded builds select their implementations. Evaluate the proposed atomic exchange against the ownership and memory-ordering concerns described here; done means reaching a reviewed decision and implementing or rejecting the approach with appropriate validation.