Fallback to alt nick

If the nickname we want is taken, fallback to another one by
appending underscores. Use MONITOR to figure out when we can request
our desired nick again.

Closes: https://todo.sr.ht/~emersion/soju/35

upstream.go

@@ -774,6 +774,8 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			}
 		}
 
+		uc.updateMonitor()
+
 		uc.forEachDownstream(func(dc *downstreamConn) {
 			if dc.network == nil {
 				return
@@ -876,6 +878,7 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			uc.forEachDownstream(func(dc *downstreamConn) {
 				dc.updateNick()
 			})
+			uc.updateMonitor()
 		}
 	case "SETNAME":
 		if msg.Prefix == nil {
@@ -1522,6 +1525,27 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			uc.monitored.SetValue(prefix.Name, online)
 		}
 
+		// Check if the nick we want is now free
+		wantNick := GetNick(&uc.user.User, &uc.network.Network)
+		wantNickCM := uc.network.casemap(wantNick)
+		if !online && uc.nickCM != wantNickCM {
+			found := false
+			for _, target := range targets {
+				prefix := irc.ParsePrefix(target)
+				if uc.network.casemap(prefix.Name) == wantNickCM {
+					found = true
+					break
+				}
+			}
+			if found {
+				uc.logger.Printf("desired nick %q is now available", wantNick)
+				uc.SendMessage(&irc.Message{
+					Command: "NICK",
+					Params:  []string{wantNick},
+				})
+			}
+		}
+
 		uc.forEachDownstream(func(dc *downstreamConn) {
 			for _, target := range targets {
 				prefix := irc.ParsePrefix(target)
@@ -1687,7 +1711,22 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			return err
 		}
 		return fmt.Errorf("fatal server error: %v", text)
-	case irc.ERR_PASSWDMISMATCH, irc.ERR_ERRONEUSNICKNAME, irc.ERR_NICKNAMEINUSE, irc.ERR_NICKCOLLISION, irc.ERR_UNAVAILRESOURCE, irc.ERR_NOPERMFORHOST, irc.ERR_YOUREBANNEDCREEP:
+	case irc.ERR_NICKNAMEINUSE:
+		// At this point, we haven't received ISUPPORT so we don't know the
+		// maximum nickname length or whether the server supports MONITOR. Many
+		// servers have NICKLEN=30 so let's just use that.
+		if !uc.registered && len(uc.nick)+1 < 30 {
+			uc.nick = uc.nick + "_"
+			uc.nickCM = uc.network.casemap(uc.nick)
+			uc.logger.Printf("desired nick is not available, falling back to %q", uc.nick)
+			uc.SendMessage(&irc.Message{
+				Command: "NICK",
+				Params:  []string{uc.nick},
+			})
+			return nil
+		}
+		fallthrough
+	case irc.ERR_PASSWDMISMATCH, irc.ERR_ERRONEUSNICKNAME, irc.ERR_NICKCOLLISION, irc.ERR_UNAVAILRESOURCE, irc.ERR_NOPERMFORHOST, irc.ERR_YOUREBANNEDCREEP:
 		if !uc.registered {
 			return registrationError{msg}
 		}
@@ -2087,14 +2126,21 @@ func (uc *upstreamConn) updateMonitor() {
 			if !uc.monitored.Has(targetCM) {
 				if _, ok := add[targetCM]; !ok {
 					addList = append(addList, targetCM)
+					add[targetCM] = struct{}{}
 				}
-				add[targetCM] = struct{}{}
 			} else {
 				seen[targetCM] = struct{}{}
 			}
 		}
 	})
 
+	wantNick := GetNick(&uc.user.User, &uc.network.Network)
+	wantNickCM := uc.network.casemap(wantNick)
+	if _, ok := add[wantNickCM]; !ok && !uc.monitored.Has(wantNick) && !uc.isOurNick(wantNick) {
+		addList = append(addList, wantNickCM)
+		add[wantNickCM] = struct{}{}
+	}
+
 	removeAll := true
 	var removeList []string
 	for targetCM, entry := range uc.monitored.innerMap {