From: "byroot (Jean Boussier)" Date: 2022-10-17T08:42:28+00:00 Subject: [ruby-core:110354] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state. Issue #19062 has been updated by byroot (Jean Boussier). > I think of that more of a design choice rather than an outright issue. Of course. But it is generally considered good design to help avoid shared mutable state. But again, I can live with the current proposal. > Can you explain in more detail what you mean? Well same than for your Fiber example, but with threads. You may want to use threads to process a unit of work concurrently: ``` Server Process | |- Request Thread 1 | |- Sub Thread 1 (fetch and compute user info) | \- Sub Thread 2 (fetch and compute product info) | \- Request Thread 2 |- Sub Thread 1 (fetch and compute user info) \- Sub Thread 2 (fetch and compute product info) ``` When you do this today, you end up with the similar challenge you describe for fibers, as the newly spawned thread starts with an empty local store, and may lose some of the unit of work context that was stored there (typically `ActiveSupport::CurrentAttributes`). > `Fiber#locals` This makes me think that `locals` may not be the best name, since (to me at least) "locals" suggest that it isn't shared with anyone else. I think naming it `inheritable-something` instead would make sense. As a library or application author, I could then pick and chose between local and inherited storage depending on my use case. ---------------------------------------- Bug #19062: Introduce `Fiber#locals` for shared inheritable state. https://bugs.ruby-lang.org/issues/19062#change-99647 * 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: