Use random available port number

auth/oauth.go

@@ -1,23 +1,28 @@
 package main
 
 import (
-	"context"
 	"fmt"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
 	"os/exec"
-	"time"
 )
 
 func main() {
-	port := 3999    // TODO: find available port number
 	state := "TODO" // replace with random unguessable value
 
 	clientID := os.Getenv("GH_OAUTH_CLIENT_ID")
 	clientSecret := os.Getenv("GH_OAUTH_CLIENT_SECRET")
 
+	code := ""
+	listener, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		panic(err)
+	}
+	port := listener.Addr().(*net.TCPAddr).Port
+
 	q := url.Values{}
 	q.Set("client_id", clientID)
 	q.Set("redirect_uri", fmt.Sprintf("http://localhost:%d", port))
@@ -25,24 +30,19 @@ func main() {
 	q.Set("state", state)
 
 	cmd := exec.Command("open", fmt.Sprintf("https://github.com/login/oauth/authorize?%s", q.Encode()))
-	err := cmd.Run()
+	err = cmd.Run()
 	if err != nil {
 		panic(err)
 	}
 
-	code := ""
-	srv := &http.Server{Addr: fmt.Sprintf("localhost:%d", port)}
-	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+	http.Serve(listener, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		rq := r.URL.Query()
 		code = rq.Get("code")
 		// TODO: rq.Get("state")
 		w.Header().Add("content-type", "text/html")
 		fmt.Fprintf(w, "<p>You have authenticated <strong>GitHub CLI</strong>. You may now close this page.</p>")
-		time.AfterFunc(10*time.Millisecond, func() {
-			srv.Shutdown(context.TODO())
-		})
-	})
-	srv.ListenAndServe()
+		defer listener.Close()
+	}))
 
 	resp, err := http.PostForm("https://github.com/login/oauth/access_token",
 		url.Values{