Issue #21309 has been updated by osyoyu (Daisuke Aritomo).
At least for Queue it's not that simple, because it contains objects
Indeed, you're right about Queue. I overlooked that. Since Queue isn't really a concurrency primitive, I think it'd be fine remaining Ractor unshareable. On the other hand, I still believe that Mutex and ConditionVariable should be Ractor-shareable. Of course it'd be a great boost if Timeout becomes Ractor compatible, but in my view there's no harm in making Mutex shareable as well.
My impression is yes it's (sometimes very) hard to make existing Ruby code Ractor-compatible, and it's due to the Ractor/actor programming model.
I agree that there are fundamental constraints imposed by the actor model. At the same time, I think it's also true that existing code often can't run inside a Ractor simply because the necessary building blocks haven't been made Ractor compatible yet. ---------------------------------------- Feature #21309: Can Thread::Mutex be Ractor shareable? https://bugs.ruby-lang.org/issues/21309#change-112905 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- ## Background Keeping a `Mutex` object in a constant or a class instance variable is a common pattern seen in code with thread safety in mind. However, this kind of code does not play well with Ractors: ```ruby require 'thread' class C MUTEX = Mutex.new def self.foo MUTEX.synchronize { p 1 } end end Ractor.new { C.foo }.take ``` ``` t.rb:11: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues. #<Thread:0x000000011d80f368 run> terminated with exception (report_on_exception is true): t.rb:7:in 'C.foo': can not access non-shareable objects in constant C::MUTEX by non-main ractor. (Ractor::IsolationError) from t.rb:12:in 'block in <main>' <internal:ractor>:711:in 'Ractor#take': thrown by remote Ractor. (Ractor::RemoteError) from t.rb:13:in '<main>' t.rb:7:in 'C.foo': can not access non-shareable objects in constant C::MUTEX by non-main ractor. (Ractor::IsolationError) from t.rb:12:in 'block in <main>' ``` Many libraries follow this pattern. `Mutex` not being Ractor shareable is blocking these libraries from being used from inside Ractors. `Timeout` in stdlib in particular has large impact since it is required from many other gems by default, including `net/http`. https://github.com/ruby/timeout/blob/v0.4.3/lib/timeout.rb#L49-L50 https://github.com/lostisland/faraday/blob/v2.13.1/lib/faraday/middleware.rb... ## Proposal Make built-in concurrency primitives (Thread::Mutex, Thread::ConditionVariable and Thread::Queue) Ractor shareable. While this idea may not be strictly aligned with idea of the Ractor world (exchanging messages for controlling concurrency?), I have the feeling that too many code is blocked from running in Ractors because `Mutex` is not Ractor shareable. Allowing `Mutex`es to be shared would make a large portion of existing Ruby code Ractor-compatible, or at least make migration much easier. I believe that it won't be semantically incorrect, since they are concurrency primitives after all. One thing to consider that the current `Mutex` implementation is based on the GVL (I believe so). Migration to some other implementation e.g. pthread_mutex or CRITICAL_SECTION may be needed to make Mutex work well on Ractors. -- https://bugs.ruby-lang.org/