From: "ioquatix (Samuel Williams)" Date: 2022-10-17T09:18:35+00:00 Subject: [ruby-core:110355] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state. Issue #19062 has been updated by ioquatix (Samuel Williams). > Well same than for your Fiber example, but with threads. I don't think we should introduce `Thread#locals` because then we end up with two interfaces for execution context state. Fiber locals are introducing a limited form of "dynamically scoped" variables (or extent local as the JEP calls them). The entire point of this proposal is to avoid having distinct interfaces for this state - so that we don't end up with code which is compatible with threads but not fibers or vice versa. If you want to propagate state between threads, it can be done using the current proposal and is fairly straight forward. When constructing the thread, we can copy from the current fiber locals, and then this copied (or assigned) into the root fiber locals. For threads, the shared mutable state is more of a problem, since without locking it would be unsafe to mutate. Using `dup` when copying into the thread would be one way, but it doesn't prevent nested objects from being thread unsafe. For very basic things, like `request_id` or connection pools (which in theory can be thread safe), it should be totally fine. I'll update the PR to show how this can work. ---------------------------------------- Bug #19062: Introduce `Fiber#locals` for shared inheritable state. https://bugs.ruby-lang.org/issues/19062#change-99648 * 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: