p2p chat (with NAT hole punching, I think)

p2p_chat.rb

require 'socket'
require 'io/wait' # Provides socket#ready?
require 'timeout'

if ARGV.length != 1
  abort 'Usage: ruby p2p_chat.rb their_ip_address'
end

my_port = 2000
their_host = ARGV[0]
their_port = 2000
socket = nil

# Try to connect twice then listen, and repeat.
# I *think* this achieves NAT hole punching, but I don't think I've confirmed
# that yet and there are likely better hole punching algorithms.
server = nil
until socket
  server.close if server
  2.times do
    unless socket
      puts 'punch'
      socket = Timeout::timeout(1, Timeout::Error) do
        begin
          port_available = false
          until port_available
            begin
              begin
                s = TCPSocket.new their_host, their_port, '0.0.0.0', 2000
              rescue Errno::ECONNREFUSED
              end
              port_available = true
            rescue Errno::EADDRINUSE
              sleep 1
            end
          end
          s
        rescue Timeout::Error
        end
      end
    end
  end
  unless socket
    puts 'listen'
    socket = Timeout::timeout(rand * 4 + 2, Timeout::Error) do
      begin
        port_available = false
        until port_available
          begin
            begin
              server = TCPServer.new my_port
              s = server.accept
            rescue Errno::ECONNREFUSED
            end
            port_available = true
          rescue Errno::EADDRINUSE
            sleep 1
          end
        end
        s
      rescue Timeout::Error
      end
    end
  end
end

Thread.new do
  loop do
    if socket.ready?
      line = socket.gets
      puts ' >> ' + line
      if line.chomp == 'quit'
        socket.close
        puts '== Peer quit =='
        exit
      end
    else
      sleep 1
    end
  end
end

puts "== Ready (type 'quit' to quit) =="

# I thought some kind of keep-alive loop would be needed to send messages to keep the connection through NAT open if neither peer sends a message for a while, but that doesn't seem to be necessary. Maybe TCP sends messages that keep the NAT open, or maybe I'm not actually testing with a NAT.

loop do
  input = $stdin.gets
  socket.puts input
  break if input.chomp == 'quit'
end

puts '== You quit =='

socket.close