Issue #20182 has been updated by Eregon (Benoit Daloze).
or are users supposed to avoid mutating the array in the middle of its loop?
Not supporting to mutate the array while looping would be a pretty big incompatibility as written in https://github.com/ruby/ruby/pull/6687#discussion_r1019493851. IIRC matz would prefer this to be avoided/undefined, but the reality is such code is used quite a bit and would break. And in some sense even Array#map! mutates the array while looping over it. Regarding atomicity on its own though, it seems Rubyists generally expect Array & Hash to be "thread-safe", from my experience of seeing many issues when Array or Hash are not thread-safe (and trying to report/fix most of them). (incidentally this is also the concurrency guarantees that Python has) For instance currently even RubyGems [relies on thread-safe Hash](https://github.com/oracle/truffleruby/issues/3112) (and probably Array) too. A `nil` here would be an out-of-thin-air value type of race, which seems quite low-level for Ruby. IOW I think it would be unexpected by most people that Array#each can yield `nil` when there never were any `nil` element added to it. As discussed on the PR I think this can be solved by using a Primitive which does both things atomically. Or maybe by YJIT intrinsifying the whole Array#each (with side exit for the no block case). OTOH I think moving more of the core library to Ruby is definitely good/welcome. But it's hard for fundamental things like Array#each (unlike say Array#map which can be done more easily). ---------------------------------------- Feature #20182: Rewrite Array#each in Ruby https://bugs.ruby-lang.org/issues/20182#change-106235 * Author: k0kubun (Takashi Kokubun) * Status: Open * Priority: Normal ---------------------------------------- ## Proposal Rewrite Array#each in Ruby https://github.com/ruby/ruby/pull/6687. ```rb class Array def each unless block_given? return to_enum(:each) { self.length } end i = 0 while i < self.length yield self[i] i = i.succ end self end end ``` ## Purpose Make it possible for YJIT to optimize ISEQs across `Array#each`. ## Background Whether JIT-compiled or not, calling Ruby from C is more expensive than calling Ruby from Ruby. It also prevents YJIT from making cross-ISEQ optimizations. This is problematic especially for loop methods written in C like `Array#each` since the overhead is repeated at every iteration. ## Discussions There are a couple of things I'd like to discuss in this ticket: 1. @Eregon has pointed out that there's a race condition in the above implementation. `self[i]` would yield `nil` if the element was removed by another thread or TracePoint after `i < self.length`. Is it `Array#each`'s responsibility to atomically operate on elements, or are users supposed to avoid mutating the array in the middle of its loop? 2. If `Integer#<`, `Integer#length`, `Integer#succ`, or `Array#[]` is overridden in an incompatible way, the Ruby implementation may not work correctly. May I assume it's acceptable? -- https://bugs.ruby-lang.org/