cmd/discosrv: Respect the listen address scheme (fixes #3346)

If the listen address scheme is set to tcp4:// or tcp6://, it needs to be
made sure that the remote address matches this scheme before it is added to
the database.

This prevents invalid URIs like tcp4://<IPv6 address>:<port> or tcp6://<IPv4
address>:<port>.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3378

cmd/discosrv/querysrv.go

@@ -330,6 +330,16 @@ func (s *querysrv) handleAnnounce(ctx context.Context, remote net.IP, deviceID p
 
 		ip := net.ParseIP(host)
 		if host == "" || ip.IsUnspecified() {
+			// Do not use IPv6 remote address if requested scheme is tcp4
+			if uri.Scheme == "tcp4" && remote.To4() == nil {
+				continue
+			}
+
+			// Do not use IPv4 remote address if requested scheme is tcp6
+			if uri.Scheme == "tcp6" && remote.To4() != nil {
+				continue
+			}
+
 			host = remote.String()
 		}