a.rb

# $ ruby a.rb
# $ nc localhost 8899
# $ pkill -TERM -f 'ruby this.rb'

require 'monitor'
require 'socket'

@monitor = Monitor.new
@threads = {}

Signal.trap("TERM") do
  puts "trapped SIGTERM"
  @monitor.synchronize do
    puts "safe termination..."
    @threads.values.each(&:join)
    puts "...done"
    exit
  end
end

server = TCPServer.new(8899)
loop do
  @monitor.synchronize do
    th = Thread.new(server.accept) do |client|
      client.puts "Hell"
      sleep 20
      client.puts "World..."
      client.close
      @threads.delete(Thread.current)
    end
    @threads[th] = th
  end
end