add:port: new port packet to get free port

port/get.go

@@ -0,0 +1,28 @@
+package port
+
+import (
+	"errors"
+	"net"
+)
+
+var ErrGetTCP = errors.New("get tcp address")
+
+func GetFree() (port int, err error) {
+	var a *net.TCPAddr
+
+	if a, err = net.ResolveTCPAddr("tcp", "localhost:0"); err == nil {
+		var l *net.TCPListener
+		if l, err = net.ListenTCP("tcp", a); err == nil {
+			defer l.Close()
+
+			addr, ok := l.Addr().(*net.TCPAddr)
+			if !ok {
+				return 0, ErrGetTCP
+			}
+
+			return addr.Port, nil
+		}
+	}
+
+	return
+}