gh-111926: Update _weakref to be threadsafe in --disable-gil build

[corona10]

• Issue: Make weakref thread-safe without the GIL #111926

[corona10]

I am going to work on weakref object separately.

[corona10]

There was some misunderstanding with my PR, I will re-send a patch for this :)

[corona10]

@colesbury Hmm, I am not sure this was what you intended from #111926.

IIUC, There might be two kinds of task that should be done

• From the side of the modules: This PR might be an example.
• From the weakreference object side: For example, PyWeakref_GetRef or _PyWeakref_GET_REF should be updated to be thread-safe?

Please let me know if there is my misunderstanding.
The reason why I don't use AC for this PR is that AC generate the critical section based on module object rather than target object.

[colesbury]

This looks good, other than the issue I mention below.

We will want to make both the module-level code (like this) and the operations on the weakref object thread-safe. I think it makes sense to split them up as separate PRs as you are doing.

Regarding Argument Clinic: maybe the @critical_section directive should take an optional argument specifying which parameter name to lock. For now, I think writing the critical sections manually is fine.

The tricky bit will be making _PyWeakref_GET_REF thread-safe because the lock is in the pointed-to object (ref->wr_object), but that might change or even get deallocated. I think that will need an operation _Py_TryXFetchRef from nogil-3.12 that isn't upstreamed yet. It may be worth doing the other bits and leaving _PyWeakref_GET_REF for last