Issue #21039 has been updated by Eregon (Benoit Daloze). jhawthorn (John Hawthorn) wrote in #note-27:
I really do not believe it is a departure from the existing semantics.
I think it is in many ways. It seems other committers see the problem as well. Nothing in Ruby gives these semantics currently, to shallow-copy the environment of a block (just that proves it's a departure from the existing semantics, it's behavior that is impossible without the currently-broken `Ractor.make_shareable(Proc)`). And I think for good reasons, I see it as breaking lexical scoping, and something as simple as `a = []; get("/") { a }; post("/add") { a << it }` can no longer rely on the `a` inside referencing the `a` outside, even though it must because it's defined there (in the outer scope). I would be fine with a lexical block and a clearly-named method (`Ractor.shareable_proc { }` or `Proc.isolated { }`, etc), because that would be a very good hint about new special semantics, and that block would be guaranteed to only be used with those semantics and not a mix. But altering the semantics of existing blocks, only in some conditions (e.g. when using `Ractor.make_shareable`) would I believe be a very big language design mistake. ---------------------------------------- Feature #21039: Ractor.make_shareable breaks block semantics (seeing updated captured variables) of existing blocks https://bugs.ruby-lang.org/issues/21039#change-114353 * Author: Eregon (Benoit Daloze) * Status: Closed * Assignee: ko1 (Koichi Sasada) ---------------------------------------- ```ruby def make_counter count = 0 nil.instance_exec do [-> { count }, -> { count += 1 }] end end get, increment = make_counter reader = Thread.new { sleep 0.01 loop do p get.call sleep 0.1 end } writer = Thread.new { loop do increment.call sleep 0.1 end } ractor_thread = Thread.new { sleep 1 Ractor.make_shareable(get) } sleep 2 ``` This prints: ``` 1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 10 10 ``` But it should print 1..20, and indeed it does when commenting out the `Ractor.make_shareable(get)`. This shows a given block/Proc instance is concurrently broken by `Ractor.make_shareable`, IOW Ractor is breaking fundamental Ruby semantics of blocks and their captured/outer variables or "environment". It's expected that `Ractor.make_shareable` can `freeze` objects and that may cause some FrozenError, but here it's not a FrozenError, it's wrong/stale values being read. I think what should happen instead is that `Ractor.make_shareable` should create a new Proc and mutate that. However, if the Proc is inside some other object and not just directly the argument, that wouldn't work (like `Ractor.make_shareable([get])`). So I think one fix would to be to only accept Procs for `Ractor.make_shareable(obj, copy: true)`. FWIW that currently doesn't allow Procs, it gives `<internal:ractor>:828:in 'Ractor.make_shareable': allocator undefined for Proc (TypeError)`. It makes sense to use `copy` here since `make_shareable` effectively takes a copy/snapshot of the Proc's environment. I think the only other way, and I think it would be a far better way would be to not support making Procs shareable with `Ractor.make_shareable`. Instead it could be some new method like `isolated { ... }` or `Proc.isolated { ... }` or `Proc.snapshot_outer_variables { ... }` or so, only accepting a literal block (to avoid mutating/breaking an existing block), and that would snapshot outer variables (or require no outer variables like Ractor.new's block, or maybe even do `Ractor.make_shareable(copy: true)` on outer variables) and possibly also set `self` since that's anyway needed. That would make such blocks with different semantics explicit, which would fix the problem of breaking the intention of who wrote that block and whoever read that code, expecting normal Ruby block semantics, which includes seeing updated outer variables. Related: #21033 https://bugs.ruby-lang.org/issues/18243#note-5 Extracted from https://bugs.ruby-lang.org/issues/21033#note-14 -- https://bugs.ruby-lang.org/