Pass Async::Semaphore to replace Async::Idler, in order to control the exact concurrency limit.

[shaokun]

Async::Job::Processor::Redis::Server#initialize(...parent: nil), by default, the parent is nil, hence it's set to Async::Idler.new.
But I want to be able to control the exact number of concurrency, so I am trying to pass Async::Semaphore to parent.

I do it in this way:

# config/initializers/async_job.rb

Rails.application.configure do
  # Create a queue for the "default" backend:
  config.async_job.define_queue "default" do
    dequeue Async::Job::Processor::Redis, parent: Async::Semaphore.new(10)
  end
end

It's passed to the parent parameter, but the Server still dequeue as many jobs as possible without taking the concurrency limit into account.
From code:

def start!
  return false if @task
  
  @task = true
  
  @parent.async(transient: true, annotation: self.class.name) do |task|
    @task = task
    
    while true
      self.dequeue(task)
    end
  ensure
    @task = nil
  end
end

It looks like that the infinite loop is within the @parent.async, hence the concurrency limit doesn't work at all.
Should it be?

while true
  @parent.async(transient: true, annotation: self.class.name) do |task| do
    self.dequeue
  end
end