[ruby-core:115874] [Ruby master Bug#20078] StringIO cannot be moved between Ractors

Issue #20078 has been updated by forthoney (Seong-Heon Jung).


luke-gru (Luke Gruber) wrote in #note-3:
> I think this is due to not currently being able to move any T_DATA objects between ractors. StringIO is defined in a C extension and is a T_DATA object internally. See https://github.com/ruby/ruby/blob/master/ractor.c#L3417C1-L3417C13. This isn't a fundamental limitation (I don't think?), it's just it hasn't been implemented yet. But certain errors could happen, for example if the C extension allows you to move an object to another Ractor, but then uses this same object internally in the original Ractor, maybe it could cause errors.
> 
> The documentation should mention this, without going into too much detail. Something like:
> Currently, most types that are implemented in C extensions cannot be moved.

I agree. The documentation does mention T_DATA, but does not really mention the implications of being unable to move T_DATA, which is that "most types that are implemented in C extensions cannot be moved".

Additionally, I am wondering if it makes sense to make a `Ractor.movable?` module function similar to `Ractor.shareable?` This would greatly save developer time since they can check if a object is movable in IRB rather than having to dig through the source code.

----------------------------------------
Bug #20078: StringIO cannot be moved between Ractors
https://bugs.ruby-lang.org/issues/20078#change-105831

* Author: forthoney (Seong-Heon Jung)
* Status: Open
* Priority: Normal
* ruby -v: 3.2
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
The following code will raise an unusual Ractor error.
``` ruby
require 'stringio'
r = Ractor.new { loop { Ractor.receive } }
Ractor.shareable?(StringIO.new) #=> false
r.send(StringIO.new) # passing it via copy works
r.send(StringIO.new, move: true) # <internal:ractor>:587:in `send': can not move StringIO object. (Ractor::Error)
```
I'm not 100% sure but I believe that this probably not the intended behavior considering
* The error raised is a generic `Ractor::Error` rather than something specific like Ractor::MovedError or Ractor::IsolationError
* It can be copied
* [No documentation](https://docs.ruby-lang.org/en/master/ractor_md.html) exists for this error
* I have yet to seen this happen on any other class instance
* Typo (can not -> cannot)



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