Better handling of when the specified port is already in use

commands/server.go

@@ -15,6 +15,7 @@ package commands
 
 import (
 	"fmt"
+	"net"
 	"net/http"
 	"os"
 	"strconv"
@@ -56,6 +57,19 @@ func server(cmd *cobra.Command, args []string) {
 		BaseUrl = "http://" + BaseUrl
 	}
 
+	l, err := net.Listen("tcp", ":"+strconv.Itoa(serverPort))
+	if err == nil {
+		l.Close()
+	} else {
+		jww.ERROR.Println("port", serverPort, "already in use, attempting to use an available port")
+		sp, err := helpers.FindAvailablePort()
+		if err != nil {
+			jww.ERROR.Println("Unable to find alternative port to use")
+			jww.ERROR.Fatalln(err)
+		}
+		serverPort = sp.Port
+	}
+
 	if serverAppend {
 		viper.Set("BaseUrl", strings.TrimSuffix(BaseUrl, "/")+":"+strconv.Itoa(serverPort))
 	} else {

helpers/general.go

@@ -15,6 +15,8 @@ package helpers
 
 import (
 	"bytes"
+	"fmt"
+	"net"
 	"strings"
 )
 
@@ -49,3 +51,16 @@ func StripHTML(s string) string {
 	}
 	return output
 }
+
+func FindAvailablePort() (*net.TCPAddr, error) {
+	l, err := net.Listen("tcp", ":0")
+	if err == nil {
+		defer l.Close()
+		addr := l.Addr()
+		if a, ok := addr.(*net.TCPAddr); ok {
+			return a, nil
+		}
+		return nil, fmt.Errorf("Unable to obtain a valid tcp port. %v", addr)
+	}
+	return nil, err
+}