From: "ioquatix (Samuel Williams)" Date: 2022-10-17T10:48:52+00:00 Subject: [ruby-core:110359] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state. Issue #19062 has been updated by ioquatix (Samuel Williams). > It would be a semantic bug if adding a new local in a child fiber affects a parent fiber as @byroot (Jean Boussier) said. I don't agree this is a semantic bug. Ruby doesn't have any existing data structures which behave like this, and Ruby is fundamentally a mutable language. > The question is should we deep copy / dup the values too? No, I think this is a bad idea, not only is it expensive, it's not a well defined operation in Ruby, and prevents lots of useful sharing, like connection pools. > I'm not sure what's the advantage of this proposal vs #19058. In both cases it's O(locals) per new fiber if using a hash. No, it's not. This is O(1) per fiber creation, and O(hash dup) per thread creation. So it's more efficient. > Isn't that always a bad idea? Sharing e.g. a DB connection between threads would be unsafe. No, it' snot bad to share a DB connection between threads, if the DB connection itself is thread safe, it's totally fine. > If we allow inheriting across threads then at least it should be very explicit. I don't agree with this, the entire point of this model is to have implicit sharing of important things, e.g. `request_id`, `connection_pool`, `trace_id`, etc. Being explicit is more likely to ensure that these fields are not propagated correctly. ---------------------------------------- Bug #19062: Introduce `Fiber#locals` for shared inheritable state. https://bugs.ruby-lang.org/issues/19062#change-99652 * 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: