work.rb

require 'set'

LOCK = Mutex.new

num = Integer(ARGV.shift || 4)
processes = Set.new

Signal.trap "INT" do |s|
  $stderr.puts "Shutdown"

  LOCK.synchronize do
    num = 0 # prevent further

    processes.each do |pid|
      Process.kill "TERM", pid rescue nil
    end
  end

  Process.waitall # optional

  exit 0
end

catch :exit do
  loop do
    LOCK.synchronize do
      throw :exit if num == 0

      while processes.size < num
        child = fork do
          Signal.trap "INT", "EXIT"
          5.times {|i| puts "Working in #$$ step #{i}"; sleep(1+ rand(3)) }
        end

        processes.add child
      end
    end

    pid = Process.wait

    LOCK.synchronize do
      processes.delete pid
    end
  end
end