ncrev.go

// ncrev is essentially "nc -l NNNN", i.e. netcat listening on a port and
// directing the traffic to stdin/stdout, but it first checks that nothing is
// already listening on port NNNN.
package main

import (
	"flag"
	"io"
	"log"
	"net"
	"os"
	"sync"
)

var port = flag.String("port", "", `The port to listen on, e.g. ":4000"`)

func main() {
	flag.Parse()
	if *port == "" {
		log.Fatal("Port is required.")
	}

	// First start a connection to the local port. If there is already a
	// listener there then we should fail out.
	conn, err := net.Dial("tcp", *port)
	if err == nil {
		conn.Close()
		log.Fatalf("Found a listener at port %q", *port)
	}

	// Now listen for a connection.
	ln, err := net.Listen("tcp", *port)
	if err != nil {
		log.Fatalf("Failed to listen on port %q: %s", *port, err)
	}
	conn, err = ln.Accept()
	if err != nil {
		log.Fatalf("Failed to accept connection on port %q: %s", *port, err)
	}
	defer conn.Close()

	var wg sync.WaitGroup
	wg.Add(1) // We will exit once one of the streams is closed.

	go func() {
		defer wg.Done()
		if _, err := io.Copy(conn, os.Stdin); err != nil {
			log.Fatalf("Failed while streaming stdin to port %q: %s", *port, err)
		}
	}()

	go func() {
		defer wg.Done()
		if _, err := io.Copy(os.Stdout, conn); err != nil {
			log.Fatalf("Failed while streaming stdout from port %q: %s", *port, err)
		}
	}()

	wg.Wait()
}