Spring background process solution (https://github.com/pry/pry/issues/1290#issuecomment-56624349 and https://github.com/rails/spring/pull/514)

client.rb

require 'socket'
FORWARDED_SIGNALS = %w(INT QUIT USR1 USR2 INFO WINCH) & Signal.list.keys

receiver = UNIXSocket.new('whatevz.sock')
pid = receiver.gets.to_i
receiver.send_io $stdin
receiver.send_io $stdout
$stdin.close
$stdout.close


FORWARDED_SIGNALS.each do |signal|
  trap(signal) { Process.kill signal, pid }
end

# fg/bg applies to both the server and ourselves
orig_tstp_handler = orig_cont_handler = nil

tstp_handler = lambda do |*|
  Process.kill 'TSTP', pid
  trap 'TSTP', orig_tstp_handler
  Process.kill 'TSTP', Process.pid
end

cont_handler = lambda do |*|
  trap 'CONT', orig_cont_handler
  Process.kill 'CONT', Process.pid
  trap 'CONT', cont_handler
  Process.kill 'CONT', pid
  trap 'TSTP', tstp_handler
end

orig_tstp_handler = trap 'TSTP', tstp_handler
orig_cont_handler = trap 'CONT', cont_handler

loop do
  break if receiver.closed?
  break if IO.select [receiver], [], [], 1
end

server.rb

require 'socket'

# This sockets listens for local connections.
name = 'whatevz.sock'
File.delete name if File.exist? name
at_exit { File.delete name if File.exist? name }

listener = UNIXServer.new name
at_exit { listener.close unless listener.closed? }

client = listener.accept
at_exit { client.close unless client.closed? }
at_exit { client.puts "fin" unless client.closed? }

client.puts Process.pid

def log(type, message)
  puts "#{type}: #{message.inspect}"
end

read = client.recv_io
log :reader, read

write = client.recv_io
log :writer, write

loop do
  message = read.gets
  log :message, message
  write.puts message
end