Issue #15408 has been updated by Eregon (Benoit Daloze).


Agreed we should remove ObjectSpace._id2ref, since it's fundamentally broken on MRI with the current MRI object_id semantics (could get another object if that memory address is reused after GC'ing the old object).

Removing #object_id doesn't make sense to me if we remove _id2ref: #object_id has the same semantics as System.identityHashCode(), and it is idempotent, but not unique (at least on MRI).
The Ruby documentation doesn't guarantee uniqueness:

> The same number will be returned on all calls to object_id for a given object,
> and no two active objects will share an id.

That's the same as System.identityHashCode().
I think we do need a identity-based hash for hashtables and hashing arbitrary objects which do not redefine #hash.

So, how about deprecating _id2ref in 2.6 or 2.7, and removing it in 3.0?

It seems only drb is using _id2ref in the standard library. We will need to find a replacement for that usage.

Aside: I wonder how _id2ref works in MRI if objects are moved by the GC (since #object_id is just the address), or maybe objects are never moved in MRI?

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Feature #15408: Deprecate object_id and _id2ref
https://bugs.ruby-lang.org/issues/15408#change-75662

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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Ruby currently provides the object_id method to get a "identifier" for a given object. According to the documentation, this ID is the same for every object_id call against a given object, and guaranteed not to be the same as any other active (i.e. alive) object. However, no guarantee is made about the ID being reused for a future object after the original has been garbage collected.

As a result, object_id can't be used to uniquely identify any object that might be garbage collected, since that ID may be associated with a completely different object in the future.

Ruby also provides a method to go from an object_id to the object reference itself: ObjectSpace._id2ref. This method has been in Ruby for decades and is often used to implement a weak hashmap from ID to reference, since holding the ID will not keep the object alive. However due to the problems with object_id not actually being unique, it's possible for _id2ref to return a different object than originally had that ID as object slots are reused in the heap.

The only way to implement object_id safely (with idempotency guarantees) would be to assign to all objects a monotonically-increasing ID. Alternatively, this ID could be assigned lazily only for those objects on which the code calls object_id. JRuby implements object_id in this way currently.

The only way to implement _id2ref safely would be to have a mapping in memory from those monotonically-increasing IDs to the actual objects. This would have to be a weak mapping to prevent the objects from being garbage collected. JRuby currently only supports _id2ref via a flag, since the additional overhead of weakly tracking every requested object_id is extremely high. An alternative for MRI would be to implement _id2ref as a heap scan, as it is implemented in Rubinius. This would make it entirely unpractical due to the cost of scanning the heap for every ID lookup.

I propose that both methods should immediately be deprecated for removal in Ruby 3.0.

* They do not do what people expect.
* They cannot reliably do what they claim to do.
* They eventually lead to difficult-to-diagnose bugs in every possible use case.

Put simply, both methods have always been broken in MRI and making them unbroken would render them useless.



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