unicorn.rb - safe reload

children = 40 # as desired
min_children = 2 # as desired

after_fork do |server, worker|
  # on re-deploys, want new workers to start up one at a time first, check if new master was re-deployed
  if ENV['UNICORN_FD']
    # second, check if still in a re-deploy
    old_pid = "#{server.config[:pid]}.oldbin"
    if File.exists?(old_pid) && server.pid != old_pid
      # kill an old worker|master only when a couple new workers are fully running
      if worker.nr >= min_children
        begin
          # kill an old worker if this is not the last new worker
          # kill the old master if this is the last worker
          Process.kill(worker.nr >= (children - 1) ? :QUIT : :TTOU, File.read(old_pid).to_i)
        rescue Errno::ENOENT, Errno::ESRCH
          # someone else did our job for us
        end
      end

      # start a new worker if this is not the last new worker
      if worker.nr < (children - 1)
        # first new worker must sleep to let new master finish new worker forking and install signal handlers
        if worker.nr == 0 then sleep 5 end

        # signal for a new worker
        new_pid = "#{server.config[:pid]}"
        Process.kill(:TTIN, File.read(new_pid).to_i)
      end
    end
  end
end