Simplify unlocking dance in *Serve functions (#719)

server.go

@@ -367,10 +367,17 @@ func (srv *Server) spawnWorker(w *response) {
 	}
 }
 
+func unlockOnce(l sync.Locker) func() {
+	var once sync.Once
+	return func() { once.Do(l.Unlock) }
+}
+
 // ListenAndServe starts a nameserver on the configured address in *Server.
 func (srv *Server) ListenAndServe() error {
+	unlock := unlockOnce(&srv.lock)
 	srv.lock.Lock()
-	defer srv.lock.Unlock()
+	defer unlock()
+
 	if srv.started {
 		return &Error{err: "server already started"}
 	}
@@ -396,10 +403,8 @@ func (srv *Server) ListenAndServe() error {
 		}
 		srv.Listener = l
 		srv.started = true
-		srv.lock.Unlock()
-		err = srv.serveTCP(l)
-		srv.lock.Lock() // to satisfy the defer at the top
-		return err
+		unlock()
+		return srv.serveTCP(l)
 	case "tcp-tls", "tcp4-tls", "tcp6-tls":
 		network := "tcp"
 		if srv.Net == "tcp4-tls" {
@@ -414,10 +419,8 @@ func (srv *Server) ListenAndServe() error {
 		}
 		srv.Listener = l
 		srv.started = true
-		srv.lock.Unlock()
-		err = srv.serveTCP(l)
-		srv.lock.Lock() // to satisfy the defer at the top
-		return err
+		unlock()
+		return srv.serveTCP(l)
 	case "udp", "udp4", "udp6":
 		a, err := net.ResolveUDPAddr(srv.Net, addr)
 		if err != nil {
@@ -432,10 +435,8 @@ func (srv *Server) ListenAndServe() error {
 		}
 		srv.PacketConn = l
 		srv.started = true
-		srv.lock.Unlock()
-		err = srv.serveUDP(l)
-		srv.lock.Lock() // to satisfy the defer at the top
-		return err
+		unlock()
+		return srv.serveUDP(l)
 	}
 	return &Error{err: "bad network"}
 }
@@ -443,8 +444,10 @@ func (srv *Server) ListenAndServe() error {
 // ActivateAndServe starts a nameserver with the PacketConn or Listener
 // configured in *Server. Its main use is to start a server from systemd.
 func (srv *Server) ActivateAndServe() error {
+	unlock := unlockOnce(&srv.lock)
 	srv.lock.Lock()
-	defer srv.lock.Unlock()
+	defer unlock()
+
 	if srv.started {
 		return &Error{err: "server already started"}
 	}
@@ -464,18 +467,14 @@ func (srv *Server) ActivateAndServe() error {
 				return e
 			}
 			srv.started = true
-			srv.lock.Unlock()
-			e := srv.serveUDP(t)
-			srv.lock.Lock() // to satisfy the defer at the top
-			return e
+			unlock()
+			return srv.serveUDP(t)
 		}
 	}
 	if l != nil {
 		srv.started = true
-		srv.lock.Unlock()
-		e := srv.serveTCP(l)
-		srv.lock.Lock() // to satisfy the defer at the top
-		return e
+		unlock()
+		return srv.serveTCP(l)
 	}
 	return &Error{err: "bad listeners"}
 }