cmd/devp2p: support dual-stack discovery listener (#35220)

`devp2p discv4 listen` / `discv5 listen` is the supported replacement
for the removed bootnode tool, but it bound IPv4-only and `-extaddr`
took a single address, so it couldn't run a dual-stack bootnode.

This binds the listener dual-stack (falling back to IPv4-only where IPv6
is unavailable) and lets `-extaddr` take a comma-separated IPv4/IPv6
pair. A single node can then advertise both `ip` and `ip6` in its ENR
over one UDP port:

```
devp2p discv4 listen --nodekey <key> --addr [::]:30301 \
  --extaddr 203.0.113.10:30301,[2001:db8::1]:30301
```

The fallback IP is only derived from the listener when no `-extaddr` is
given, so a v4- or v6-only `-extaddr` no longer leaks a loopback entry.
All addresses must share one UDP port (single socket).

cmd/devp2p/discv4cmd.go

@@ -126,7 +126,7 @@ var (
 	}
 	extAddrFlag = &cli.StringFlag{
 		Name:  "extaddr",
-		Usage: "UDP endpoint announced in ENR. You can provide a bare IP address or IP:port as the value of this flag.",
+		Usage: "UDP endpoint announced in ENR. You can provide a bare IP address or IP:port as the value of this flag. Provide a comma-separated pair to announce both an IPv4 and an IPv6 endpoint.",
 	}
 	crawlTimeoutFlag = &cli.DurationFlag{
 		Name:  "timeout",
@@ -344,36 +344,60 @@ func parseExtAddr(spec string) (ip net.IP, port int, ok bool) {
 
 func listen(ctx *cli.Context, ln *enode.LocalNode) *net.UDPConn {
 	addr := ctx.String(listenAddrFlag.Name)
+	extAddr := ctx.String(extAddrFlag.Name)
+	var (
+		socket net.PacketConn
+		err    error
+	)
 	if addr == "" {
-		addr = "0.0.0.0:0"
+		// Dual-stack socket, falling back to IPv4-only where IPv6 is unavailable.
+		if socket, err = net.ListenPacket("udp", "[::]:0"); err != nil {
+			socket, err = net.ListenPacket("udp", "0.0.0.0:0")
+		}
+	} else {
+		socket, err = net.ListenPacket("udp", addr)
 	}
-	socket, err := net.ListenPacket("udp4", addr)
 	if err != nil {
 		exit(err)
 	}
 
-	// Configure UDP endpoint in ENR from listener address.
+	// Configure the ENR endpoint from the listener address, but only without an
+	// explicit -extaddr: otherwise we'd announce a fallback IP for an address
+	// family the user didn't specify (e.g. loopback IPv4 on an IPv6-only node).
 	usocket := socket.(*net.UDPConn)
 	uaddr := socket.LocalAddr().(*net.UDPAddr)
-	if uaddr.IP.IsUnspecified() {
-		ln.SetFallbackIP(net.IP{127, 0, 0, 1})
-	} else {
-		ln.SetFallbackIP(uaddr.IP)
+	if extAddr == "" {
+		if uaddr.IP.IsUnspecified() {
+			ln.SetFallbackIP(net.IP{127, 0, 0, 1})
+		} else {
+			ln.SetFallbackIP(uaddr.IP)
+		}
 	}
 	ln.SetFallbackUDP(uaddr.Port)
 
-	// If an ENR endpoint is set explicitly on the command-line, override
-	// the information from the listening address. Note this is careful not
-	// to set the UDP port if the external address doesn't have it.
-	extAddr := ctx.String(extAddrFlag.Name)
+	// Override with explicit -extaddr address(es). A static IP is set per family,
+	// and all specs share one UDP port because the node has a single socket.
 	if extAddr != "" {
-		ip, port, ok := parseExtAddr(extAddr)
-		if !ok {
-			exit(fmt.Errorf("-%s: invalid external address %q", extAddrFlag.Name, extAddr))
+		var extPort int
+		for spec := range strings.SplitSeq(extAddr, ",") {
+			spec = strings.TrimSpace(spec)
+			if spec == "" {
+				continue
+			}
+			ip, port, ok := parseExtAddr(spec)
+			if !ok {
+				exit(fmt.Errorf("-%s: invalid external address %q", extAddrFlag.Name, spec))
+			}
+			ln.SetStaticIP(ip)
+			if port != 0 {
+				if extPort != 0 && port != extPort {
+					exit(fmt.Errorf("-%s: all addresses must announce the same UDP port, got %d and %d", extAddrFlag.Name, extPort, port))
+				}
+				extPort = port
+			}
 		}
-		ln.SetStaticIP(ip)
-		if port != 0 {
-			ln.SetFallbackUDP(port)
+		if extPort != 0 {
+			ln.SetFallbackUDP(extPort)
 		}
 	}