upstream: regain desired nick when MONITOR is missing

We already have logic to regain our desired nick when the upstream
server supports MONITOR. However some networks (e.g. OFTC, Rizon)
don't support MONITOR. Also try to regain our desired nick in that
case, by periodically sending NICK commands.

Closes: https://todo.sr.ht/~emersion/soju/197

upstream.go

@@ -153,6 +153,10 @@ type upstreamConn struct {
 	// been sent yet.
 	pendingCmds map[string][]pendingUpstreamCommand
 
+	pendingRegainNick string
+	regainNickTimer   *time.Timer
+	regainNickBackoff *backoffer
+
 	gotMotd bool
 }
 
@@ -869,6 +873,13 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			// Ignore the initial MOTD upon connection, but forward
 			// subsequent MOTD messages downstream
 			uc.gotMotd = true
+
+			// If the server doesn't support MONITOR, periodically try to
+			// regain our desired nick
+			if _, ok := uc.isupport["MONITOR"]; !ok {
+				uc.startRegainNickTimer()
+			}
+
 			return nil
 		}
 
@@ -924,6 +935,11 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			uc.logger.Printf("changed nick from %q to %q", uc.nick, newNick)
 			me = true
 			uc.nick = newNick
+
+			if uc.network.equalCasemap(uc.pendingRegainNick, newNick) {
+				uc.pendingRegainNick = ""
+				uc.stopRegainNickTimer()
+			}
 		}
 
 		uc.channels.ForEach(func(ch *upstreamChannel) {
@@ -1773,6 +1789,18 @@ func (uc *upstreamConn) handleMessage(ctx context.Context, msg *irc.Message) err
 			})
 			return nil
 		}
+
+		var failedNick string
+		if err := parseMessageParams(msg, nil, &failedNick); err != nil {
+			return err
+		}
+		if uc.network.equalCasemap(uc.pendingRegainNick, failedNick) {
+			// This message comes from our own logic to try to regain our
+			// desired nick, don't relay to downstream connections
+			uc.pendingRegainNick = ""
+			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 {
@@ -2238,3 +2266,67 @@ func (uc *upstreamConn) updateMonitor() {
 		uc.monitored.Del(target)
 	}
 }
+
+func (uc *upstreamConn) stopRegainNickTimer() {
+	if uc.regainNickTimer != nil {
+		uc.regainNickTimer.Stop()
+		// Maybe we're racing with the timer goroutine, so maybe we'll receive
+		// an eventTryRegainNick later on, but tryRegainNick handles that case
+	}
+	uc.regainNickTimer = nil
+	uc.regainNickBackoff = nil
+}
+
+func (uc *upstreamConn) startRegainNickTimer() {
+	if uc.regainNickBackoff != nil || uc.regainNickTimer != nil {
+		panic("startRegainNickTimer called twice")
+	}
+
+	wantNick := database.GetNick(&uc.user.User, &uc.network.Network)
+	if uc.isOurNick(wantNick) {
+		return
+	}
+
+	const (
+		min    = 15 * time.Second
+		max    = 10 * time.Minute
+		jitter = 10 * time.Second
+	)
+	uc.regainNickBackoff = newBackoffer(min, max, jitter)
+	uc.regainNickTimer = time.AfterFunc(uc.regainNickBackoff.Next(), func() {
+		e := eventTryRegainNick{uc: uc, nick: wantNick}
+		select {
+		case uc.network.user.events <- e:
+			// ok
+		default:
+			uc.logger.Printf("skipping nick regain attempt: event queue is full")
+		}
+	})
+}
+
+func (uc *upstreamConn) tryRegainNick(nick string) {
+	ctx := context.TODO()
+
+	if uc.regainNickTimer == nil {
+		return
+	}
+
+	// Maybe the user has updated their desired nick
+	wantNick := database.GetNick(&uc.user.User, &uc.network.Network)
+	if wantNick != nick || uc.isOurNick(wantNick) {
+		uc.stopRegainNickTimer()
+		return
+	}
+
+	uc.regainNickTimer.Reset(uc.regainNickBackoff.Next())
+
+	if uc.pendingRegainNick != "" {
+		return
+	}
+
+	uc.SendMessage(ctx, &irc.Message{
+		Command: "NICK",
+		Params:  []string{wantNick},
+	})
+	uc.pendingRegainNick = wantNick
+}

user.go

@@ -75,6 +75,11 @@ type eventUserUpdate struct {
 	done     chan error
 }
 
+type eventTryRegainNick struct {
+	uc   *upstreamConn
+	nick string
+}
+
 type deliveredClientMap map[string]string // client name -> msg ID
 
 type deliveredStore struct {
@@ -755,6 +760,8 @@ func (u *user) run() {
 					dc.Close()
 				}
 			}
+		case eventTryRegainNick:
+			e.uc.tryRegainNick(e.nick)
 		case eventStop:
 			for _, dc := range u.downstreamConns {
 				dc.Close()
@@ -776,6 +783,7 @@ func (u *user) run() {
 func (u *user) handleUpstreamDisconnected(uc *upstreamConn) {
 	uc.network.conn = nil
 
+	uc.stopRegainNickTimer()
 	uc.abortPendingCommands()
 
 	uc.channels.ForEach(func(uch *upstreamChannel) {