From: "ioquatix (Samuel Williams)" Date: 2022-10-17T11:12:34+00:00 Subject: [ruby-core:110361] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state. Issue #19062 has been updated by ioquatix (Samuel Williams). ActiveRecord connections are thread safe, and it makes total sense to me that an enumerator might like to run in the same transaction as its surrounding context. > I believe having an "inheritance mechanism" makes perfect sense Yes, it can do, but it's also costly. > but also having a store that isn't inherited does too. In practice, as shown with lazy enumerators that internally create fibers, I would argue that it doesn't actually make a huge amount of sense in comparison to dynamically scoped variables (which is roughly what this proposal implements at the fiber level, rather than stack frame level which would be prohibitively complex). ---------------------------------------- Bug #19062: Introduce `Fiber#locals` for shared inheritable state. https://bugs.ruby-lang.org/issues/19062#change-99654 * 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: