Issue #19535 has been updated by Eregon (Benoit Daloze). I chatted with @tenderlovemaking. I think it's best to make the too-complex-shape objects use an ordered hash. TruffleRuby doesn't have a too-complex shape, so that case is not a concern for TruffleRuby (currently at least). But indeed with Shapes it's only natural that `Kernel#instance_variables` returns in the order the Shape has, which is the order the ivar was set on that object and not per class. Having to maintain that on the class just to have the previous per-class ordering seems a significant overhead and complication, so I don't consider that a real solution. Notice that if we would always sort ivars in a Shape in an attempt to deduplicate shapes (I think PyPy does), that would affect the ordering which would then be alphabetical or so. But currently neither TruffleRuby nor CRuby do that, and doing that also makes writing ivars more complex (need to move values around). It's anyway good if user code sets variables in the same order for various instances, it's more predictable for everyone. BTW it's probably quite a bad idea to capture `self` in `initialize` in a `Hash` and define `#hash` and not just the default identity hash, that would break with random ordering too. It's also not thread-safe typically (leaks `self` before it's fully initialized). ---------------------------------------- Misc #19535: Instance variables order is unpredictable on objects with `OBJ_TOO_COMPLEX_SHAPE_ID` https://bugs.ruby-lang.org/issues/19535#change-102448 * Author: byroot (Jean Boussier) * Status: Open * Priority: Normal ---------------------------------------- ### Context I've been helping the Mastodon folks in investigating a weird Marshal deserialization bug they randomly experience since they upgraded to Ruby 3.2: https://github.com/mastodon/mastodon/issues/23644 Ultimately the bug comes from a circular dependency issues in the object graph that is serialized when one call `Marshal.dump` on an `ActiveRecord::Base` object. A simplified reproduction to better explain the problem is: ```ruby class Status def normal_order @attributes = { id: 42 } @relations = { self => 1 } self end def inverse_order @relations = nil @attributes = { id: 42 } @relations = { self => 1 } self end def hash @attributes.fetch(:id) end end s = Marshal.load(Marshal.dump(Status.new.normal_order)) s = Marshal.load(Marshal.dump(Status.new.inverse_order)) ``` In short, that `Status` object is both the top level object, and is referenced as a key in a hash, in that same payload. It also defined a custom `#hash` method, that requires some other attribute to be set. It all "works" as long as `@attributes` is dumped before `@relations`. ### Problem The above micro-reproduction uses two different shapes to demonstrate the ordering issues, but in both case the ordering is predictable. However if you generate too many shapes from a single class, it will be marked as `TOO_COMPLEX` and future instance will have their instance variables backed by an `id_table`, which is unordered, and will cause a similar issue. I definitely consider this a bug on the Rails side, and I will do what I can so that Rails doesn't depend on that implicit ordering. However it's unlikely we'll be able to fix older version, and other users may run into this issue when upgrading to Ruby 3.2, so I think it may be worth to try to preserve some sort of predicable ordering, at least for a few more versions. Additionally, debugging it was made particularly difficult, because it would work fine initially, and then break after enough shapes had been generated. Generally speaking I think such semi-predictable behavior is much worse than a fully random behavior (similar to how Go randomize keys order in their maps). ### Historical behavior On Ruby 3.1 and older, the instance variables ordering was defined by the order in which each ivar appeared for the very first time: ```ruby class Foo def set @a = 1 @b = 2 @c = 3 self end def inverse_order @c = 3 @b = 2 @a = 1 self end end p Foo.new.set.instance_variables # => [:@a, :@b, :@c] p Foo.new.inverse_order.instance_variables # => [:@a, :@b, :@c] ``` This means that the order could be different from once execution of the program to another, but would remain stable inside a single process. On 3.2, it's now defined by the order in which each ivar appeared in that specific object instance: ```ruby [:@a, :@b, :@c] [:@c, :@b, :@a] ``` Except, if the object is backed by an `id_table`, in which case it's fully unpredictable. ### Possible changes I discussed this with @tenderlovemaking, and he suggested we could change the `id_table` for an `st_table` so that the ordering could be predictable again, and would behave like objects with a non-complex shape. Another possibility would be to preserve the observable behavior of 3.1 and older. Or of course we could clearly specify that the ordering is random, but if so I think it would be wise to make it always random so that this class of bugs has a much higher chance to be caught early in testing rather than in production. cc @Eregon as I presume this has implications on TruffleRuby as well. -- https://bugs.ruby-lang.org/