[ruby-core:122602] [Ruby Bug#21451] Ractor.make_shareable(->{}, copy: true) raises unhelpful error

25 Jun 2025

      Issue #21451 has been updated by tenderlovemaking (Aaron Patterson).

Eregon (Benoit Daloze) wrote in #note-1:
...
This doesn't really explain from a user POV why it can't copy the Proc though, after all `->{}.dup` works fine.
"No allocator" is an internal thing `Ractor.make_shareable` could work around, is there a more fundamental reason why it shouldn't work?
Maybe the correct fix here is to actually support `Ractor.make_shareable(->{}, copy: true)`, as mentioned in #21039?
AFAIK, procs fundamentally can't be shared because their environment is mutable. Even if we copy the proc, its environment is still mutable so I'm not sure if `make_shareable` should ever work on them. OTOH, if you send the proc to a Ractor I could see it getting copied at that boundary as only the receiving Ractor gets the proc.

That said, I think I should have been more clear in this ticket description.  IMO the problem isn't with Procs in particular, it's any object that can't be copied via `make_shareable`.  The error message is not helpful.

Here is an example that doesn't use a Proc:

```ruby
obj = "".freeze
begin
  obj.bar
rescue => err
end

p err
Ractor.make_shareable err, copy: true
```

The exception is a `TypeError` rather than something more helpful:

```
...
ruby test.rb
#<NoMethodError: undefined method 'bar' for an instance of String>
<internal:ractor>:828:in 'Ractor.make_shareable': allocator undefined for RubyVM::InstructionSequence (TypeError)
   from test.rb:8:in '<main>'

I think we should just accept that `rb_obj_clone` can possibly raise an exception, and we should re-raise with a more helpful exception.  The current behavior is even more unhelpful when you consider that the non-copiable object may be part of a larger object graph.

Consider this code:

```ruby
def make_lambda
  lambda { }
end

def make_lambda2
  lambda { }
end

hash = { key: make_lambda }

Ractor.make_shareable hash, copy: true

With the current version of Ruby, the exception is like this:

```
...
ruby test.rb
<internal:ractor>:828:in 'Ractor.make_shareable': allocator undefined for Proc (TypeError)
   from test.rb:11:in '<main>'

Fortunately the test program is short, but if this hash came from a distant place in the application, how would we know _which_ Proc is the problem?  Currently we get this type of error when trying to make a Rails application shareable. But the only way I can find the specific lambdas causing problems is by hacking Ruby.  At least with the patch I've provided, I can see from the error message more info about the troublesome object.

----------------------------------------
Bug #21451: Ractor.make_shareable(->{}, copy: true) raises unhelpful error
https://bugs.ruby-lang.org/issues/21451#change-113840

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Assignee: ractor
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------

...
ruby -e'Ractor.make_shareable(->{}, copy:true)'
<internal:ractor>:828:in 'Ractor.make_shareable': allocator undefined for Proc (TypeError)
   from -e:1:in '<main>'

This error isn't very helpful and I think we can improve it.  The exception happens when we call `rb_obj_clone` on the lambda.

I've made a patch to improve the error message so it's like this:

...
./miniruby -e'Ractor.make_shareable(->{}, copy:true)'
-e:1:in 'Ractor.make_shareable': cannot copy #<Proc:0x000000011f311a80 -e:1 (lambda)> (Ractor::IsolationError)
   from -e:1:in '<main>'
-e:1:in 'Ractor.make_shareable': allocator undefined for Proc (TypeError)
   from -e:1:in '<main>'

The patch is here: https://github.com/ruby/ruby/pull/13703

-- 
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