From: "ioquatix (Samuel Williams)" Date: 2022-10-18T10:15:04+00:00 Subject: [ruby-core:110389] [Ruby master Bug#19062] Introduce `Fiber#locals` for shared inheritable state. Issue #19062 has been updated by ioquatix (Samuel Williams). Thanks for your examples. I agree there are different ways to solve this problem. The key model I'm trying to introduce is a shared state "per request". It's actually similar to in a lot of ways. I'm concerned about the memory and performance cost of calling dup per fiber. I'm also concerned about hidden fibers (and threads) causing the visible behaviour to change unexpectedly. It also prevents shared optimisations which is something I want to use for my connection pool code. ```ruby def connection_pool Fiber[:connection_pool] ||= ConnectionPool.new end ``` Scenario 1: ```ruby connection_pool.query(...) Fiber.new do # Reuse connection pool: connection_pool.query(...) end.resume ``` Scenario 2: ```ruby Fiber.new do connection_pool.query(...) end.resume connection_pool.query # oh no, it will create a 2nd connection pool. ``` It also stands to reason, that even if we followed your suggestion, simply having `Fiber.curent.locals[:shared] = {}` at the very root reintroduces the same behaviour as I'm proposing, so it's unavoidable. I feel that the current PR is pragmatic, small, and efficient, and solves a fairly fundamental problem. I understand the semantic value in always calling dup, but as of right now, I'm not convinced that semantic model outweighs the practical issues listed above. I can be convinced of course! But especially the performance cost concerns me a lot, because Async encourages lots of short lived fibers. ---------------------------------------- Bug #19062: Introduce `Fiber#locals` for shared inheritable state. https://bugs.ruby-lang.org/issues/19062#change-99695 * 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: