Issue #22216 has been updated by ko1 (Koichi Sasada). I agree that the current behavior is a Ractor bug. In fact, this is one of the blockers preventing us from removing the "experimental" warning from Ractor. These variables should be thread-local. The main implementation challenge is how to manage the lifetimes of Procs and Threads. I use the term “svar” (short for “special variables”), following CRuby's implementation terminology. (1) If we manage per-thread svars in each Proc or binding, we need a dictionary (or a list) for each Proc: ```ruby $~ = ... # Conceptually compiled into: binding.svar[current_thread].$~ = ... ``` (2) If we manage per-thread svars in each Thread, we need a dictionary for each Thread: ```ruby $~ = ... # Conceptually compiled into: Thread.current.svar[binding_or_proc].$~ = ... ``` In either case, we need to manage the lifetimes of the dictionary keys and values, which seems complicated. I am also not sure which approach would allow for a more efficient implementation. If we assume that this is a rare case, option (1) may be acceptable, and we could leave stale dictionary keys behind when threads terminate. ---------------------------------------- Bug #22216: Regexp backref and IO lastline are incompatible with Ractor https://bugs.ruby-lang.org/issues/22216#change-118273 * Author: headius (Charles Nutter) * Status: Open * Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN ---------------------------------------- ## Problem Several Regexp-matching methods currently write (and sometimes read) the implicit "backref" `$~` variable in the local frame (and related variables like `$'`). Several IO methods read or write the "last line" `$_` variable in the same way. In both cases, the result is a mutable object, which makes these variables already problematic for ractors. Making matters worse, the frame might be shared if a proc is captured and used across threads or ractors, and there's no static way to inspect a piece of code to know if it expects to read or write these variables. Where procs can be rejected by a proc for accessing captured state, there's no such check possible for these variables. All of these facts make the backref and lastline variables fundamentally incompatible with Ractor. ## Possible remedies A wholesale removal of these variables would solve the problem, but there's a lot of code that depends on them... much of that code without even realizing it, since they might not access the variables directly. In some cases, the dependencies are internal and part of the behavior of core methods. Hard errors when using methods that read or write these variables would avoid introducing threading problems into a Ractor, but would also break a large number of commonly-used methods. There have been experiments to make these variables both frame and thread-local, but they have never been made standard. Updates to backref and lastline are visible across threads and already can lead to concurrency issues even on CRuby. Deprecating the implicit behavior and making it opt-in (or opt-out?), perhaps with keyword arguments or file pragmas, might be a halfway measure. It would probably not be an easy transition. I don't know the right path forward, but I believe this issue needs to be discussed. ## JRuby perspective We continue to mimic CRuby behavior, which has led to our users occasionally running into issues when a proc accesses these variables across threads. Our recommendation: "don't do that". We also have had our frustrations optimizing around these variables, since they implicitly require access across calls. Because we cannot statically detect when they will be used, we essentially treat all method names that might *potentially* access them as deopt triggers. It's not ideal. I'd like to hear ideas for how to make these variables less "magic", less implicit and easier to deal with across calls (and across threads/ractors). -- https://bugs.ruby-lang.org/