From: marcandre-ruby-core@... Date: 2020-10-26T05:25:01+00:00 Subject: [ruby-core:100552] [Ruby master Feature#17284] Shareable Proc Issue #17284 has been updated by marcandre (Marc-Andre Lafortune). ko1 (Koichi Sasada) wrote in #note-4: > Thank you for your reply. Here's a longer reply. I would like a method to make a Proc independent of the binding it was created in. I'm thinking of `Proc#detach` that would make a snapshot of the values needed (shallow copy): ```ruby x = 1 a = ary = [] pr = Proc.new { ary << x ; x += 1 }.detach x = ary = nil # no effect on `p`, as binding is detached pr.call # => 2 pr.call # => 2, same since always starts from snapshot a # => [1, 1], since snapshot is shallow copy binding.local_variables # => [:x, :a, :ary, :pr] pr.binding.local_variables # => [] # always empty; value are passed like arguments pr.binding.snapshot # => { ary: [], x: 1 } # not necessary, but at least for illustration pr.binding.snapshot.frozen # => true ``` This is *always* what I want when I call `define_method` and I have to [jump through hoops](https://github.com/rubocop-hq/rubocop-ast/blob/master/lib/rubocop/ast/node_pattern/method_definer.rb#L43-L48) to make sure I don't capture another value by mistake... ```ruby def foo(text) text.each_line do |line| if special_line?(line) foo, bar = parse_line(line) define_method(foo) { puts bar } end end end # oops, `text` might *never* be garbage collected, and last `line` will not be either :-( ``` I see `Ractor.make_shareable(block)` as equivalent to `detach` + `make_shareable` on the values of the snapshot. This would make it easy to check if a block accesses non-shareable outer variables: ```ruby n = 42 ary = [] Ractor.shareable?(Proc.new { do_something }) # => false, has binding Ractor.shareable?(Proc.new { do_something(v) }.detach) # => true, snapshot shareable Ractor.shareable?(Proc.new { do_something(ary) }.detach) # => false, because `ary` not shareable ary.freeze Ractor.shareable?(Proc.new { do_something(ary) }.detach) # => true because `ary` is shareable ary2 = [] p = Ractor.make_shareable(Proc.new { do_something(ary2) }) ary2.frozen # => true Ractor.shareable?(p) # => true ``` > Do you think `Ractor.new()` can call (c) instead of current (a)? > In other words, can we accept (1) and (2) in the following example? > > ``` > b = a = [] > Ractor.new do > p a #=> (1) shows [] even if a is replaced with :sym > end > > a = :sym > > # (2) frozen error because an array is sharable (deep frozen) > b << 1 # frozen error > ``` I think it could definitely call `detach` above, so 1) yes. 2) is trickier/riskier. I think there are better solutions. Maybe a better way to resolve 2 is simply that: ```ruby b = a = [] Ractor.new do p a end # equivalent to: b = a = [] Ractor.new(a) do |a| p a end a = :sym # no effect b << 1 # no effect, array was deeply copied ``` #17286 would allow for: ```ruby b = a = [] Ractor.new(move: true) do p a end a = :sym # no effect b << 1 # Ractor::MovedError ``` I hope I'm not missing something obvious, it's getting late here :-) ---------------------------------------- Feature #17284: Shareable Proc https://bugs.ruby-lang.org/issues/17284#change-88180 * Author: ko1 (Koichi Sasada) * Status: Open * Priority: Normal ---------------------------------------- For some reasons, we need to provide a way to make sharable Proc between ractors. * (1) A block for the `Ractor.new`. * (2) Send a proc between ractors. * (3) A block for global callback methods: `define_method` ([Bug #17159]), `TracePoint`, ... For (1), we use `Proc#isolate` (`isolate` is temporary name here) which prohibit to access outer variables. ```ruby a = 1 Proc.new{ p a }.isolate # => can not isolate a Proc because it accesses outer variables (a). # error on `isolate` method call ``` There are no states to share, so it is okay. For (2), `Proc#isolate` is one option because we can send parameters with an argument `call`. But it should be a bit long. ```ruby i, j, k = nil pr = Proc.new do |i, j, k| p i, j, k end.isolate r = Ractor.new do |task, param| task.call(*param) end r.send([pr, [i, j, k]]) ``` For (3), maybe we need to make more flexible Proc which can *read* outer block parameter on that snapshot (discussed in #17159). Now, I named it with `freeze`, because it seems frozen Proc. ```ruby a = 1 # try to read, and returns old value (snapshot at `freeze`) pr = Proc.new{ p a #=> 1 } pr = pr.freeze pr.call a = 2 pr.call #=> 1 # try to write, and it is not allowed pr2 = Proc.new{ a = 1 } pr2 = pr.freeze #=> can not freeze a Proc because it accesses outer variables (a). (ArgumentError) ``` To share the "frozen" Proc between ractors, outer values should be (deep) frozen. It means readable values (in above case, `a`) should be shareable. Now we named it `Proc#shareable!` ```ruby a = [1, [2, 3]] pr = Proc.new{ p a.frozen? #=> true }.shareable! a[0] = 0 #=> frozen error ``` This ticket has three different variant of mutability and shareability for Proc. | | outer lvar | shareable | freeze/making shareable other objects |---------------|---------------|------------|------------------------------------------ |a. isolate | N/A | Yes | No |b. freeze | allow to read | No | No |c. shareable! | allow to read | Yes | Yes I want to introduce functionality of `shareable!`, but not sure the Ruby-level API. I think (b) `freeze` for this semantics is good name because it only allows to read-only local variables. However, it is not enough to make a sharable Proc because read objects from the Proc should be also sharable. Making `freeze` with (c) `shareable!` functionality is one idea, but I think `freeze` should not deep-freezing because it is very surprising that read objects become the sharable (== frozen) for usual Ruby users. Maybe `Ractor.make_sharable(pr)` makes `pr` sharable is no surprise because it is good declaration the `pr` should be shareable, even if the read objects from `pr` become shareable (== frozen). Removing (a) `isolate` and using (c) `shareable!` at `Ractor.new(&b)` is one idea, but I think it is surprising that they can access outer local variables, but the they can not access newly assigned variables as usual blocks. ``` a = 1 Ractor.new do p a # only 1 end a = 2 ``` (a) `isolate` does not have such issue because all outer lvars accesses are not allowed == easy to understand, easy to debug. In practice, accessing outer variables with multi-ractor program is very useful because we need to declare same local variables if we want to access them from different ractors. The following example is from [Feature #17261]: ```ruby tv1 = Thread::TVar.new(0) tv2 = Thread::TVar.new(0) r1 = Ractor.new tv1, tv2 do |tv1, tv2| # <-- here loop do Thread.atomically do v1, v2 = tv1.value, tv2.value raise if v1 != v2 end end end ``` With (c) `shareable!` semantics, it is easier to write: ```ruby tv1 = Thread::TVar.new(0) tv2 = Thread::TVar.new(0) r1 = Ractor.new do loop do Thread.atomically do v1, v2 = tv1.value, tv2.value raise if v1 != v2 end end end ``` Above example is also enable to make more simple: ```ruby i, j, k = nil pr = Proc.new do p i, j, k end r = Ractor.new do |task| task.call end r.send(pr) ``` However, using this semantics (`shareable!`) can freeze extra-variables in accidents: ```ruby a = [1, 2, 3] Ractor.new do do_something if a.length > 0 end a << 4 # raises FrozenError ``` It is clear that there is a syntax or method to apply `shareable!` functionality. ```ruby a = [1, 2, 3] Ractor.new &(Ractor.make_shareable(Proc.new{ a.length ... }) ``` It can be used with `define_method` which can invoke from ractors: ```ruby define_method(name, Ractor.make_shareable(Proc.new{ ... }))` ``` But it is too long. There are implementations for (a), (b) and (c), but the API is not fixed, so there is no PR now. I'm thinking to introduce (c)'s feature in `Ractor.make_sharaeble(pr)`. To use with `define_method`, maybe it should be more friendly. Ideally, new syntax is great. There is no conclusion, and your comments are welcome. Thanks, Koichi -- https://bugs.ruby-lang.org/ Unsubscribe: