[ruby-core:122606] [Ruby Bug#21451] Ractor.make_shareable(->{}, copy: true) raises unhelpful error

Issue #21451 has been updated by Eregon (Benoit Daloze).


tenderlovemaking (Aaron Patterson) wrote in #note-2:
> AFAIK, procs fundamentally can't be shared because their environment is mutable.

Currently `Ractor.make_shareable(proc)` makes a shallow copy of the environment, inplace. I believe that's wrong for Ruby semantics because it "breaks" an existing Proc, that's the subject of #21039.
But with `copy: true` it would at least not affect existing references to the Proc, which seems significantly better, and also it makes it clearer it takes a shallow copy of the Proc environment.

tenderlovemaking (Aaron Patterson) wrote in #note-2:
> 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.

I see, makes sense, agreed it's a good way.
I would suggest a different message and error class to improve clarity though:
```
-e:1:in 'Ractor.make_shareable': cannot copy #<Proc:0x000000011f311a80 -e:1 (lambda)> (Ractor::IsolationError)
```
=>
```
-e:1:in 'Ractor.make_shareable': cannot make #<Proc:0x000000011f311a80 -e:1 (lambda)> shareable with copy (ArgumentError)
OR
-e:1:in 'Ractor.make_shareable': cannot make shareable with copy: #<Proc:0x000000011f311a80 -e:1 (lambda)> (ArgumentError)
```

`Ractor::IsolationError` seems a bit strange, so I think simply `ArgumentError`, or maybe something like `Ractor::CopyError` would be better.

----------------------------------------
Bug #21451: Ractor.make_shareable(->{}, copy: true) raises unhelpful error
https://bugs.ruby-lang.org/issues/21451#change-113844

* 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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