[ruby-core:102371] [Ruby master Feature#17592] Ractor should allowing reading shareable class instance variables

Issue #17592 has been updated by Eregon (Benoit Daloze).


To be more precise, and looking at the source, currently there is no synchronization for module ivars, only a check that only the main Ractor can access them:
https://github.com/ruby/ruby/blob/5803ac1c734568837d2010bd38f122ba24cbae2b/variable.c#L906-L913

The fast path for the "ivar set" bytecode already doesn't handle T_MODULE/T_CLASS:
https://github.com/ruby/ruby/blob/5803ac1c734568837d2010bd38f122ba24cbae2b/vm_insnhelper.c#L1250-L1253

And it ends up here which has the Module-specific logic:
https://github.com/ruby/ruby/blob/5803ac1c734568837d2010bd38f122ba24cbae2b/variable.c#L1480-L1484

Other state like constants and methods already have synchronization, e.g.,
https://github.com/ruby/ruby/blob/5803ac1c734568837d2010bd38f122ba24cbae2b/variable.c#L3621-L3625

----------------------------------------
Feature #17592: Ractor should allowing reading shareable class instance variables
https://bugs.ruby-lang.org/issues/17592#change-90231

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
----------------------------------------
It would be very helpful if Ractor was allowing reading class instance variables from non-main Ractor.


Currently is raises an IsolationError:

```ruby
module Foo
  singleton_class.attr_accessor :config
  Foo.config = {example: 42}.freeze
end

Ractor.new { p Foo.config } # => IsolationError
```

This limitation makes it challenging to have an efficient way to store general configs, i.e. global data that mutated a few times when resources get loaded but it immutable afterwards, and needs to be read all the time.

Currently the only way to do this is to use a constant and use `remove_const` + `const_set` (which can not be made atomic easily).

I think that allowing reading only may be the best solution to avoid any race condition, e.g. two different Ractors that call `@counter += 1`.

The only 3 scenarios I see here are:
0) declare the constant hack the official way to store config-style data
1) allow reading of instance variables for shareable objects (as long as the data is shareable)
2) allow read-write

I prefer 1)



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