[ruby-core:102368] [Ruby master Feature#17592] Ractor should allowing reading shareable class instance variables

[marcandre-ruby-core@marc-andre.ca]

Issue #17592 has been updated by marcandre (Marc-Andre Lafortune).


Dan0042 (Daniel DeLorme) wrote in #note-9:
> That makes sense, but I'm curious about the implementation. I believe this requires synchronization of every access to a class instance variable?

I'm not sure, but I think not. From non-main Ractor, grab the current value of the class instance variable. I think that can be made safe, e.g. if the ivar storage data uses immutable data structures.

Before returning the value, verify that it is shareable.

Objects have a "shareable" bit, but that might not be set for some shareable objects, so testing if an object is shareable might "modify" it by setting this "shareable" bit (see `rb_ractor_shareable_p_continue`) but I believe that this can be done simultaneously by different Ractors without issue as it is idempotent and happens only on immutable objects. Of course, ko1 will know if I'm missing something 😅

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Feature #17592: Ractor should allowing reading shareable class instance variables
https://bugs.ruby-lang.org/issues/17592#change-90229

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
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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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