WeakRef requires rescuing RefError to avoid race condition

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.

Inizia con l’API WeakRef, in particolare con weakref_alive?, l’accesso delegato, la gestione di RefError e l’alternativa sconsigliata WeakMap. Leggi la discussione collegata su Stack Overflow per il contesto della race condition; il lavoro è concluso quando si è concordata e implementata un’API che restituisca in sicurezza un oggetto con un riferimento forte quando è ancora vivo, oppure nil in caso contrario.