p2p: Disable port mapping task with --nat extip (#3612)

If --nat extip:1.2.3.4 option is specified, the port mapping logic
(AddMapping/DeleteMapping) does nothing.
This optimization avoids running a goroutine for doing nothing.

cmd/bootnode/main.go

@@ -111,7 +111,7 @@ func main() {
 
 	realaddr := conn.LocalAddr().(*net.UDPAddr)
 	if natm != nil {
-		if !realaddr.IP.IsLoopback() {
+		if !realaddr.IP.IsLoopback() && natm.SupportsMapping() {
 			go nat.Map(natm, nil, "udp", realaddr.Port, realaddr.Port, "ethereum discovery")
 		}
 		if ext, err := natm.ExternalIP(); err == nil {

p2p/nat/nat.go

@@ -41,6 +41,7 @@ type Interface interface {
 	// the gateway when its lifetime ends.
 	AddMapping(protocol string, extport, intport int, name string, lifetime time.Duration) error
 	DeleteMapping(protocol string, extport, intport int) error
+	SupportsMapping() bool
 
 	// This method should return the external (Internet-facing)
 	// address of the gateway device.
@@ -98,6 +99,10 @@ const (
 // Map adds a port mapping on m and keeps it alive until c is closed.
 // This function is typically invoked in its own goroutine.
 func Map(m Interface, c <-chan struct{}, protocol string, extport, intport int, name string) {
+	if !m.SupportsMapping() {
+		panic("Port mapping is not supported")
+	}
+
 	logger := log.New("proto", protocol, "extport", extport, "intport", intport, "interface", m)
 	refresh := time.NewTimer(mapTimeout)
 	defer func() {
@@ -138,6 +143,7 @@ func (n ExtIP) String() string              { return fmt.Sprintf("ExtIP(%v)", ne
 
 func (ExtIP) AddMapping(string, int, int, string, time.Duration) error { return nil }
 func (ExtIP) DeleteMapping(string, int, int) error                     { return nil }
+func (ExtIP) SupportsMapping() bool                                    { return false }
 
 // Any returns a port mapper that tries to discover any supported
 // mechanism on the local network.
@@ -208,6 +214,10 @@ func (n *autodisc) DeleteMapping(protocol string, extport, intport int) error {
 	return n.found.DeleteMapping(protocol, extport, intport)
 }
 
+func (n *autodisc) SupportsMapping() bool {
+	return true
+}
+
 func (n *autodisc) ExternalIP() (net.IP, error) {
 	if err := n.wait(); err != nil {
 		return nil, err

p2p/nat/natpmp.go

@@ -63,6 +63,10 @@ func (n *pmp) DeleteMapping(protocol string, extport, intport int) (err error) {
 	return err
 }
 
+func (n *pmp) SupportsMapping() bool {
+	return true
+}
+
 func discoverPMP() Interface {
 	// run external address lookups on all potential gateways
 	gws := potentialGateways()

p2p/nat/natupnp.go

@@ -123,6 +123,10 @@ func (n *upnp) DeleteMapping(protocol string, extport, intport int) error {
 	})
 }
 
+func (n *upnp) SupportsMapping() bool {
+	return true
+}
+
 func (n *upnp) String() string {
 	return "UPNP " + n.service
 }

p2p/server.go

@@ -581,7 +581,7 @@ func (srv *Server) setupDiscovery(ctx context.Context) error {
 	realaddr := conn.LocalAddr().(*net.UDPAddr)
 	srv.log.Trace("UDP listener up", "addr", realaddr)
 	if srv.NAT != nil {
-		if !realaddr.IP.IsLoopback() {
+		if !realaddr.IP.IsLoopback() && srv.NAT.SupportsMapping() {
 			srv.loopWG.Add(1)
 			go func() {
 				defer debug.LogPanic()
@@ -693,7 +693,7 @@ func (srv *Server) setupListening() error {
 	// Update the local node record and map the TCP listening port if NAT is configured.
 	if tcp, ok := listener.Addr().(*net.TCPAddr); ok {
 		srv.localnode.Set(enr.TCP(tcp.Port))
-		if !tcp.IP.IsLoopback() && srv.NAT != nil {
+		if !tcp.IP.IsLoopback() && (srv.NAT != nil) && srv.NAT.SupportsMapping() {
 			srv.loopWG.Add(1)
 			go func() {
 				defer debug.LogPanic()