Update downstream nicks in single-server mode and after NICK

Previously, the downstream nick was never changed, even when the
downstream sent a NICK message or was in single-server mode with a
different nick.

This adds support for updating the downstream nick in the following
cases:
- when a downstream sends NICK
- additionally, in single-server mode:
  - when a downstream connects and its single network is connected
  - when an upstream connects
  - when an upstream sends NICK

downstream.go

@@ -620,6 +620,17 @@ func (dc *downstreamConn) updateSupportedCaps() {
 	}
 }
 
+func (dc *downstreamConn) updateNick() {
+	if uc := dc.upstream(); uc != nil && uc.nick != dc.nick {
+		dc.SendMessage(&irc.Message{
+			Prefix:  dc.prefix(),
+			Command: "NICK",
+			Params:  []string{uc.nick},
+		})
+		dc.nick = uc.nick
+	}
+}
+
 func sanityCheckServer(addr string) error {
 	dialer := net.Dialer{Timeout: 30 * time.Second}
 	conn, err := tls.DialWithDialer(&dialer, "tcp", addr, nil)
@@ -777,6 +788,8 @@ func (dc *downstreamConn) welcome() error {
 		Params:  []string{dc.nick, "No MOTD"},
 	})
 
+	dc.updateNick()
+
 	dc.forEachUpstream(func(uc *upstreamConn) {
 		for _, ch := range uc.channels {
 			if !ch.complete {
@@ -935,6 +948,15 @@ func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
 		dc.forEachUpstream(func(uc *upstreamConn) {
 			uc.SendMessage(msg)
 		})
+
+		if dc.upstream() == nil && dc.nick != nick {
+			dc.SendMessage(&irc.Message{
+				Prefix:  dc.prefix(),
+				Command: "NICK",
+				Params:  []string{nick},
+			})
+			dc.nick = nick
+		}
 	case "JOIN":
 		var namesStr string
 		if err := parseMessageParams(msg, &namesStr); err != nil {

upstream.go

@@ -656,6 +656,10 @@ func (uc *upstreamConn) handleMessage(msg *irc.Message) error {
 			uc.forEachDownstream(func(dc *downstreamConn) {
 				dc.SendMessage(dc.marshalMessage(msg, uc.network))
 			})
+		} else {
+			uc.forEachDownstream(func(dc *downstreamConn) {
+				dc.updateNick()
+			})
 		}
 	case "JOIN":
 		if msg.Prefix == nil {

user.go

@@ -298,6 +298,8 @@ func (u *user) run() {
 			uc.forEachDownstream(func(dc *downstreamConn) {
 				dc.updateSupportedCaps()
 				sendServiceNOTICE(dc, fmt.Sprintf("connected to %s", uc.network.GetName()))
+
+				dc.updateNick()
 			})
 			uc.network.lastError = nil
 		case eventUpstreamDisconnected: