Allow creating WeakRefs outside the main Ractor

[rkh]

Fix for https://bugs.ruby-lang.org/issues/22105

WeakRef instance can move between Ractors (also moving the object they reference if it isn't shareable).

They cannot be made shareable, as ObjectSpace::WeakMap cannot be made shareable.

[byroot]

This very significantly increase memory usage, because now every WeakRef has its own WeakMap instance (208B for one entry).

[rkh]

Could keep one WeakRef per Ractor, as #2 does, but only if there was a mechanism to also move/copy the referenced object if necessary. This would be possible to implement but would significantly increase the complexity.

Alternatively, we could follow #2, but would need to somehow prevent WeakRefs from being moved.

[rkh]

I mean, what I would really want is a ractor-shareable, mutable WeakMap, but I guess the constraints around non-shareable values could not be resolved.

Or alternatively, a way to mutate an object before and after it has been sent over a port. That way it could hold on to the referenced object for transit.

Both of these are obviously out of scope for this PR.

[byroot]

What's unfortunate is that since a couple versions, thanks to @peterzhu2118 we have actual weak references in the GC, and WeakRef could be implemented without needing a WeakMap (I think Peter had a prototype).

But since it's all private API and weakref is a gem, it can't use those facilities.

My preferred solution (but not the easiest) would be to request weakref to be made core, so that it can become a real weakref.

It was extracted as a gem because that was the direction to turn the stdlib into gems for easier maintenance, but given it's barely 40 lines of codes, I'd think this is a bit of an overshoot.

[rkh]

Oh I agree, that would be a proper solution.