p2p, p2p/discover, p2p/discv5: implement UDP port sharing (#15200)

This commit affects p2p/discv5 "topic discovery" by running it on
the same UDP port where the old discovery works. This is realized
by giving an "unhandled" packet channel to the old v4 discovery
packet handler where all invalid packets are sent. These packets
are then processed by v5. v5 packets are always invalid when
interpreted by v4 and vice versa. This is ensured by adding one
to the first byte of the packet hash in v5 packets.

DiscoveryV5Bootnodes is also changed to point to new bootnodes
that are implementing the changed packet format with modified
hash. Existing and new v5 bootnodes are both running on different
ports ATM.

p2p/server.go

@@ -78,9 +78,6 @@ type Config struct {
 	// protocol should be started or not.
 	DiscoveryV5 bool `toml:",omitempty"`
 
-	// Listener address for the V5 discovery protocol UDP traffic.
-	DiscoveryV5Addr string `toml:",omitempty"`
-
 	// Name sets the node name of this server.
 	// Use common.MakeName to create a name that follows existing conventions.
 	Name string `toml:"-"`
@@ -354,6 +351,32 @@ func (srv *Server) Stop() {
 	srv.loopWG.Wait()
 }
 
+// sharedUDPConn implements a shared connection. Write sends messages to the underlying connection while read returns
+// messages that were found unprocessable and sent to the unhandled channel by the primary listener.
+type sharedUDPConn struct {
+	*net.UDPConn
+	unhandled chan discover.ReadPacket
+}
+
+// ReadFromUDP implements discv5.conn
+func (s *sharedUDPConn) ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error) {
+	packet, ok := <-s.unhandled
+	if !ok {
+		return 0, nil, fmt.Errorf("Connection was closed")
+	}
+	l := len(packet.Data)
+	if l > len(b) {
+		l = len(b)
+	}
+	copy(b[:l], packet.Data[:l])
+	return l, packet.Addr, nil
+}
+
+// Close implements discv5.conn
+func (s *sharedUDPConn) Close() error {
+	return nil
+}
+
 // Start starts running the server.
 // Servers can not be re-used after stopping.
 func (srv *Server) Start() (err error) {
@@ -388,9 +411,43 @@ func (srv *Server) Start() (err error) {
 	srv.peerOp = make(chan peerOpFunc)
 	srv.peerOpDone = make(chan struct{})
 
+	var (
+		conn      *net.UDPConn
+		sconn     *sharedUDPConn
+		realaddr  *net.UDPAddr
+		unhandled chan discover.ReadPacket
+	)
+
+	if !srv.NoDiscovery || srv.DiscoveryV5 {
+		addr, err := net.ResolveUDPAddr("udp", srv.ListenAddr)
+		if err != nil {
+			return err
+		}
+		conn, err = net.ListenUDP("udp", addr)
+		if err != nil {
+			return err
+		}
+
+		realaddr = conn.LocalAddr().(*net.UDPAddr)
+		if srv.NAT != nil {
+			if !realaddr.IP.IsLoopback() {
+				go nat.Map(srv.NAT, srv.quit, "udp", realaddr.Port, realaddr.Port, "ethereum discovery")
+			}
+			// TODO: react to external IP changes over time.
+			if ext, err := srv.NAT.ExternalIP(); err == nil {
+				realaddr = &net.UDPAddr{IP: ext, Port: realaddr.Port}
+			}
+		}
+	}
+
+	if !srv.NoDiscovery && srv.DiscoveryV5 {
+		unhandled = make(chan discover.ReadPacket, 100)
+		sconn = &sharedUDPConn{conn, unhandled}
+	}
+
 	// node table
 	if !srv.NoDiscovery {
-		ntab, err := discover.ListenUDP(srv.PrivateKey, srv.ListenAddr, srv.NAT, srv.NodeDatabase, srv.NetRestrict)
+		ntab, err := discover.ListenUDP(srv.PrivateKey, conn, realaddr, unhandled, srv.NodeDatabase, srv.NetRestrict)
 		if err != nil {
 			return err
 		}
@@ -401,7 +458,15 @@ func (srv *Server) Start() (err error) {
 	}
 
 	if srv.DiscoveryV5 {
-		ntab, err := discv5.ListenUDP(srv.PrivateKey, srv.DiscoveryV5Addr, srv.NAT, "", srv.NetRestrict) //srv.NodeDatabase)
+		var (
+			ntab *discv5.Network
+			err  error
+		)
+		if sconn != nil {
+			ntab, err = discv5.ListenUDP(srv.PrivateKey, sconn, realaddr, "", srv.NetRestrict) //srv.NodeDatabase)
+		} else {
+			ntab, err = discv5.ListenUDP(srv.PrivateKey, conn, realaddr, "", srv.NetRestrict) //srv.NodeDatabase)
+		}
 		if err != nil {
 			return err
 		}