From: "byroot (Jean Boussier)" Date: 2022-10-17T20:05:18+00:00 Subject: [ruby-core:110376] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state. Issue #19062 has been updated by byroot (Jean Boussier). > Here is a short example of what I want: That example makes very little sense to me: - How is this a "local" if it's accessible by other fibers? - I could understand inheriting (a copy of) the locals of the parent fibers, but not sharing these "locals". - Even if the "locals hash" is copied when a new fiber is created, you can still have the behavior your want by storing a mutable object. ```ruby Counter = Struct.new(:count) Fiber[:counter] = Count.new(0) enumerator = Enumerator.new do |y| 10.times do |i| Fiber[:counter].count += 1 y << i end end enumerator.next p Fiber[:counter].count # 1 enumerator.to_a p Fiber[:counter].count # 11 ``` I think what you currently propose as the same type of semantic issue than Ruby's class variables, where the variable is shared with all the descendants. ---------------------------------------- Bug #19062: Introduce `Fiber#locals` for shared inheritable state. https://bugs.ruby-lang.org/issues/19062#change-99669 * Author: ioquatix (Samuel Williams) * Status: Open * Priority: Normal * Assignee: ioquatix (Samuel Williams) * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN ---------------------------------------- After exploring , I felt uncomfortable about the performance of copying lots of inheritable attributes. Please review that issue for the background and summary of the problem. ## Proposal Introduce `Fiber#locals` which is a hash table of local attributes which are inherited by child fibers. ```ruby Fiber.current.locals[:x] = 10 Fiber.new do pp Fiber.current.locals[:x] # => 10 end ``` It's possible to reset `Fiber.current.locals`, e.g. ```ruby def accept_connection(peer) Fiber.new(locals: nil) do # This causes a new hash table to be allocated. # Generate a new request id for all fibers nested in this one: Fiber[:request_id] = SecureRandom.hex(32) @app.call(env) end.resume end ``` A high level overview of the proposed changes: ```ruby class Fiber def initialize(..., locals: Fiber.current.locals) @locals = locals || Hash.new end attr_accessor :locals def self.[] key self.current.locals[key] end def self.[]= key, value self.current.locals[key] = value end end ``` See the pull request for the full proposed implementation. ## Expected Usage Currently, a lot of libraries use `Thread.current[:x]` which is unexpectedly "fiber local". A common bug shows up when lazy enumerators are used, because it may create an internal fiber. Because `locals` are inherited, code which uses `Fiber[:x]` will not suffer from this problem. Any program that uses true thread locals for per-request state, can adopt the proposed `Fiber#locals` and get similar behaviour, without breaking on per-fiber servers like Falcon, because Falcon can "reset" `Fiber.current.locals` for each request fiber, while servers like Puma won't have to do that and will retain thread-local behaviour. Libraries like ActiveRecord can adopt `Fiber#locals` to avoid the need for users to opt into different "IsolatedExecutionState" models, since it can be transparently handled by the web server (see for more details). We hope by introducing `Fiber#locals`, we can avoid all the confusion and bugs of the past designs. -- https://bugs.ruby-lang.org/ Unsubscribe: