freeport.scala

// intersection of macOS, Linux and Windows ephemeral ports
val EphemeralPortRange: Range.Inclusive =
  49152 to 60999

val freePort: IO[IOException, Int] = {
  def ensureAvailable(int: Int): IO[IOException, Unit] =
    ZIO.attemptBlockingIO {
      ServerSocketFactory
        .getDefault
        .createServerSocket(int, 1 /* backlog */ , InetAddress.getByName("localhost"))
        .close()
    }

  val available = for {
    port <- Random.nextIntBetween(EphemeralPortRange.start, EphemeralPortRange.end)
    _    <- ensureAvailable(port)
  } yield port

  available.retry(Schedule.spaced(10.millis).upTo(1.second))
}