[ruby-core:105071] [Ruby master Feature#18004] Add Async to the stdlib

Issue #18004 has been updated by larskanis (Lars Kanis).


Adding the core Async gem to stdlib requires to add 4 more dependent gems. Async is a big library with a lot of supplemental gems. It makes things more complicated if some gems are in stdlib and some are not. Also Async is made to be usable on rubies before 3.0, whereas ruby-3.x has `Fiber.schedule` as a builtin mechanism for starting async tasks.

The problem is, that `Fiber.schedule` is not usable without adding a complete scheduler. This makes `Fiber.schedule` a no-go in smaller scripts. So IMHO the thing that is missing in ruby-3.x is a simple scheduler. Otherwise the scheduler feature is rather incomplete.

My proposal is to promote [test/fiber/scheduler.rb](https://github.com/ruby/ruby/blob/master/test/fiber/scheduler.rb) to the stdlib (probably as a gem). With around 250 lines it is small enough to be considered a simple reference scheduler, but is too big to be copied into each and every script. Would you consider such a pull request?

Background: I'm going to make ruby-pg fully compatible to Fiber.scheduler [here](https://github.com/ged/ruby-pg/pull/397). For testing a simple scheduler is required, so I copied [test/fiber/scheduler.rb](https://github.com/ruby/ruby/blob/master/test/fiber/scheduler.rb), but it doesn't receive fixes like [this](https://github.com/ruby/ruby/pull/4777) when copied.


----------------------------------------
Feature #18004: Add Async to the stdlib
https://bugs.ruby-lang.org/issues/18004#change-93478

* Author: shan (Shannon Skipper)
* Status: Open
* Priority: Normal
----------------------------------------
Adding Async to the stdlib would signal a clear concurrency story for Ruby 3 to compliment Ractor-based parallelism. I don't know how ioquatix feels about adding Async to stdlib, but I wanted to propose it since we keep getting questions about concurrent I/O with Ruby 3 in the community.

Ractors get a fair amount of attention on the #ruby IRC channels and Ruby Discord. When Ractors are discussed, question around concurrent I/O in Ruby 3 often follow. Folk don't seem to be aware of Async, so we often cite the Ruby 3 release notes Async Net::HTTP example shown below.
``` ruby
require 'async'
require 'net/http'
require 'uri'

Async do
  ["ruby", "rails", "async"].each do |topic|
    Async do
      Net::HTTP.get(URI "https://www.google.com/search?q=#{topic}")
    end
  end
end
```
The main downside I see for this proposal is the bloat from Async's several gem dependencies. For what it's worth, nio4r has been a staple for a long time and is also the only dependency of Puma.
```
Async is a composable asynchronous I/O framework for Ruby based on nio4r and timers.
```
Async is just so useful it would be awesome to add to the stdlib. It fills and important gap for concurrent I/O with Ruby 3 and would be exciting to see included in a future release.

See https://github.com/socketry/async#readme



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