Add Async::Waiter

[zhulik]

WIP

As discussed here I introduce a new class called Waiter which can be a drop-in replacement Barrier. The only difference is that it can await for first N tasks.

Once and if the approach is approved I'll add more specific tests

Questions:

• Better name?
• Replace Barrier?
• How to get rid of warnings in logs when a task raises an exception? See example:

Async do |task|
  waiter = Async::Waiter.new
  waiter.async do
    task.sleep(1)
    raise StandardError, "Error!"
  end
  waiter.async do
    task.sleep(2)
    2
  end
  waiter.async do
    task.sleep(3)
    3
  end
  waiter.async do
    task.sleep(3)
    raise StandardError, "Error!"
  end

  done, pending = waiter.wait_for(2)

  done.each do |task|
    Console.logger.info(self, "Done result: #{task.wait}")
  rescue StandardError => e
    Console.logger.info(self, e)
  end

  pending.each do |task|
    Console.logger.info(self, "Pending result: #{task.wait}")
  rescue StandardError => e
    Console.logger.info(self, e)
  end
  Console.logger.info(self, "Done")
end

Even though the tasks with raises have eventually been awaited, I still see warnings in the logs:

  0.0s     warn: Async::Task [oid=0x8c] [ec=0xa0] [pid=453505] [2022-07-30 23:31:16 +0200]
               | Task may have ended with unhandled exception.
               |   StandardError: Error!
               |   → ./waiter.rb:56 in `block (2 levels) in <main>'
               |     ./waiter.rb:20 in `block in async'
               |     /home/zhulik/.asdf/installs/ruby/3.1.2/lib/ruby/gems/3.1.0/gems/async-2.0.3/lib/async/task.rb:255 in `block in schedule'
  1.0s     info: Object [oid=0xb4] [ec=0xc8] [pid=453505] [2022-07-30 23:31:17 +0200]
               |   StandardError: Error!
               |   → ./waiter.rb:56 in `block (2 levels) in <main>'
               |     ./waiter.rb:20 in `block in async'
               |     /home/zhulik/.asdf/installs/ruby/3.1.2/lib/ruby/gems/3.1.0/gems/async-2.0.3/lib/async/task.rb:255 in `block in schedule'
  1.0s     info: Object [oid=0xb4] [ec=0xc8] [pid=453505] [2022-07-30 23:31:17 +0200]
               | Done result: 2
  2.0s     info: Object [oid=0xb4] [ec=0xc8] [pid=453505] [2022-07-30 23:31:18 +0200]
               | Pending result: 3
  2.0s     info: Object [oid=0xb4] [ec=0xc8] [pid=453505] [2022-07-30 23:31:18 +0200]
               |   StandardError: Error!
               |   → ./waiter.rb:68 in `block (2 levels) in <main>'
               |     ./waiter.rb:20 in `block in async'
               |     /home/zhulik/.asdf/installs/ruby/3.1.2/lib/ruby/gems/3.1.0/gems/async-2.0.3/lib/async/task.rb:255 in `block in schedule'
  2.0s     info: Object [oid=0xb4] [ec=0xc8] [pid=453505] [2022-07-30 23:31:18 +0200]
               | Done

Note: I'm leaving on vacation on August 2, the day after tomorrow and won't be able to code till August 9.

Types of Changes

• New feature.

Testing

• [ ] I added tests for my changes.
• [x] I tested my changes locally.
• [ ] I tested my changes in staging.
• [ ] I tested my changes in production.

[ioquatix]

This looks good to me - I like using a sub-class since you need to opt into it - there is extra overhead. It will require people know ahead of time they want to wait on a subset of items. Is there any other design we should consider?

I wonder if it's worth looking at how other implementations work and how they handle this particular overhead.

[bruno-]

From #62:

The idea of a barrier is that it's stateful, you can call wait(2) several times until it's exhausted.

Does the current implementation enable this feature? Since @done never changes I think #wait(2) will always return the same result, no?

[zhulik]

@bruno- Waiter passes all tests written for Barrier. At least for the case when all task are being awaited. For the case when N tasks are being awainted I decided that it's better when all coroutines waiting for a waiter get same N first results

[ioquatix]

Closing in favour of #189 as discussed.