Free random TCP port listener

freeportln.go

package main

import (
	"fmt"
	"net"
)

func freeportln(s string) *net.TCPListener {
	addr, err := net.ResolveTCPAddr("tcp", s)
	if err != nil {
		return nil
	}

	ln, err := net.ListenTCP("tcp", addr)
	if err != nil {
		return nil
	}

	return ln
}

func main() {
	for i, ln := 0, freeportln(":0"); i < 3; i++ {
		if ln == nil {
			continue
		}
		defer ln.Close()

		fmt.Println("random port", ln.Addr().(*net.TCPAddr).Port)
		return
	}

	panic("attempts exceeded")
}