WeakRef requires rescuing RefError to avoid race condition

从 WeakRef API 开始，重点关注 weakref_alive?、委托访问、RefError 处理以及不推荐的 WeakMap 替代方案。阅读链接的 Stack Overflow 讨论，了解竞态条件的背景；完成的标准是就一个 API 达成一致并完成实现：对象存活时安全地返回一个具有强引用的对象，否则返回 nil。

There's an race condition in the implied usage of WeakRef.

The API only has weakref_alive?, and then delegated access to the referenced object. But the delegated access to the object cannot be protected by weakref_alive? since GC may occur between the check and the usage.

This means we basically always have to check for RefError, which basically makes weakref_alive? useless if we want to actually potentially use the object.

WeakMap usage is discouraged, leaving us with needing to add this functionality to WeakRef, which may break if WeakRef implementation changes (i.e., there is no good solution for this).

This is discussed at length here:

https://stackoverflow.com/questions/69185508/ruby-weakref-has-implicit-race-condition

I would recommend an addition to the API that will safely return a (non-weak) object if it's alive, or else nil, and obviously it's up to the user to realize that this will stop GC from happening on that object while they hold it.