[ruby-core:101579] [Ruby master Feature#17414] Ractor should allow access to shareable attributes for Modules/Classes

Issue #17414 has been updated by Eregon (Benoit Daloze).


Sounds a bit to me like a case where Thread makes more sense than Ractor (and they are far more compatible with existing code).

Wanting to mutate global state is explicitly against the principle of (recent) actor models, isn't it?
(even if only the main Ractor can write, it's global state).

Unfortunately, an approach like e.g. in Erlang to have extra actors for representing global state doesn't work well, since each Ractor needs its own native thread.

----------------------------------------
Feature #17414: Ractor should allow access to shareable attributes for Modules/Classes
https://bugs.ruby-lang.org/issues/17414#change-89364

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
----------------------------------------
Current situation is *very* limiting.

Use-case: global config.

Example: [yaml has a global config](https://github.com/ruby/psych/blob/master/lib/psych.rb#L637-L640) and it's not clear to me how to make that Ractor-aware (nicely).

It is possible to have the same effect but in ugly ways:

```ruby
# Using instance variables of Module not allowed:

module Config
  class << self
    attr_accessor :conf
  end
  self.conf = 42
end

Ractor.new { Config.conf = 66 }.take # => can not access instance variables from non-main Ractors
Ractor.new { puts Config.conf }.take # => can not access instance variables from non-main Ractors

# Same functionality using constants allowed:
module Config
  class << self
    def conf
      CONF
    end

    def conf=(new_conf)
      remove_const(:CONF)
      const_set(:CONF, new_conf)
    end
  end
  CONF = 42
end

Ractor.new { Config.conf = 66 }.take # => ok
Ractor.new { puts Config.conf }.take # => 66

# Same functionality using methods allowed:
module Config
  class << self
    def conf
      42
    end

    def conf=(new_conf)
      singleton_class.undef_method(:conf)
      define_singleton_method(:conf, &Ractor.make_shareable(Proc.new { new_conf }))
    end
  end
end

Ractor.new { Config.conf = 66 }.take # => ok
Ractor.new { puts Config.conf }.take # => 66
```

The priority would be to allow reading these instance variables if they are shareable. Ideally writing would also be allowed, but limiting that to main ractor is less probablematic than with reading.



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