module Syntropy::SideRun

def call(machine, &block)
  setup if !@queue
  # TODO: share mailboxes, acquire them with e.g. with_mailbox { |mbox| ... }
  mailbox = Thread.current[:fiber_mailbox] ||= UM::Queue.new
  machine.push(@queue, [mailbox, block])
  result = machine.shift(mailbox)
  result.is_a?(Exception) ? (raise result) : result
end

def handle_request(machine, queue)
  response_mailbox, closure = machine.shift(queue)
  result = closure.call
  machine.push(response_mailbox, result)
rescue Exception => e
  machine.push(response_mailbox, e)
end

def setup
  @queue = UM::Queue.new
  count = (Etc.nprocessors - 1).clamp(2..6)
  @workers = count.times.map {
    Thread.new { side_run_worker(@queue) }
  }
end

def side_run_worker(queue)
  machine = UM.new
  loop { handle_request(machine, queue) }
rescue UM::Terminate
  # # We can also add a timeout here
  # t0 = Time.now
  # while !queue.empty? && (Time.now - t0) < 10
  #   handle_request(machine, queue)
  # end
end