#lang racket/base

(require racket/tcp)

(define (get-unused-tcp-port-number (hostname "localhost"))
  (let ((listener (tcp-listen 0 1 #f hostname)))
    (let-values (((local-hostname local-port remote-hostname remote-port)
                  (tcp-addresses listener #t)))
      (tcp-close listener)
      local-port)))

(let ((port (get-unused-tcp-port-number)))
  (printf "Port ~S\n" port)
  (let ((listener (tcp-listen port)))
    (display "YES!\n")))