Stop the earlier poll loop on a second /connect (closes #228)

A second /connect started a new poll loop without stopping the first.
The first loop then polled with the new client too, so each message
could be shown twice. It also read stopPoll without a.mu while
cmdConnect wrote it.

cmdConnect now closes the earlier loop's channel in the same locked
step that replaces the client and the cursor, and hands the new
channel to the new loop. The loop checks its channel under a.mu
before each poll and again before it moves the cursor, so a stopped
loop never polls with the new client or moves the new cursor.

Model: opus-5-5

internal/cli/app.go

@@ -199,11 +199,21 @@ func (a *App) cmdConnect(serverURL string) {
 		return
 	}
 
+	// The poll loop of an earlier connection is stopped in the same
+	// step that replaces the client, so it never polls with the new
+	// one.
 	a.mu.Lock()
 	a.client = client
 	a.nick = resp.Nick
 	a.connected = true
 	a.lastQID = 0
+
+	if a.stopPoll != nil {
+		close(a.stopPoll)
+	}
+
+	stop := make(chan struct{})
+	a.stopPoll = stop
 	a.mu.Unlock()
 
 	a.ui.AddStatus(fmt.Sprintf(
@@ -212,9 +222,7 @@ func (a *App) cmdConnect(serverURL string) {
 	))
 	a.ui.SetStatus(resp.Nick, "", "connected")
 
-	a.stopPoll = make(chan struct{})
-
-	go a.pollLoop()
+	go a.pollLoop(stop)
 }
 
 func (a *App) cmdNick(nick string) {
@@ -702,16 +710,23 @@ func (a *App) cmdHelp() {
 	}
 }
 
-// pollLoop long-polls for messages in the background.
-func (a *App) pollLoop() {
+// pollLoop long-polls for messages in the background until stop is
+// closed. A later /connect closes stop under a.mu as it replaces the
+// client and the cursor, so the loop checks stop under a.mu too. A
+// stopped loop then neither polls with the new client nor moves the
+// new cursor.
+func (a *App) pollLoop(stop <-chan struct{}) {
 	for {
+		a.mu.Lock()
+
 		select {
-		case <-a.stopPoll:
+		case <-stop:
+			a.mu.Unlock()
+
 			return
 		default:
 		}
 
-		a.mu.Lock()
 		client := a.client
 		lastQID := a.lastQID
 		a.mu.Unlock()
@@ -729,12 +744,24 @@ func (a *App) pollLoop() {
 			continue
 		}
 
-		if result.LastID > 0 {
-			a.mu.Lock()
-			a.lastQID = result.LastID
+		// A poll sent before stop was closed answers for the earlier
+		// connection.
+		a.mu.Lock()
+
+		select {
+		case <-stop:
 			a.mu.Unlock()
+
+			return
+		default:
 		}
 
+		if result.LastID > 0 {
+			a.lastQID = result.LastID
+		}
+
+		a.mu.Unlock()
+
 		for i := range result.Messages {
 			a.handleServerMessage(&result.Messages[i])
 		}