Bug #21649: Ractors can access non-shareable object through singleton classes

• Status: Open
• Assignee: ractor
• Target version: -
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[jhawthorn (John Hawthorn)]

If we send a non-shareable object into a Ractor, it needs to be copied.

❯ ruby -W0 -e 'Ractor.new(p(Object.new)) { |o| p(o) }.join'
#<Object:0x00000001240f8cf8>
#<Object:0x00000001240f8870>

However we're able to instead smuggle it through a singleton class to unsafely get a mutable reference to the same object in both Ractors.

❯ ruby -W0 -e 'Ractor.new(p(Object.new).singleton_class) { |o| p(o.attached_object) }.join'
#<Object:0x0000000120768c40>
#<Object:0x0000000120768c40>

I think we should probably make singleton classes copy the shareability of their attached object.