Stop the streamer without sending on closed queues (closes #34)

Stopping the daemon while the RIS Live feed was flowing could panic with "send on closed channel" and skip the rest of the shutdown. Stop cancels the stream and closes the handler queues under the streamer's write lock, but the read loop checked for a stop only before parsing each line. It now checks again under the read lock it already takes just before handing a message to the queues, so it never sends to a closed queue. Stop also clears its cancel function and returns early when there is none, so a second call no longer closes the queues again. A test forces both cases.

Behaviour change: Stop before Start now does nothing.

Model: opus-5-5

internal/streamer/streamer.go

@@ -210,9 +210,14 @@ func (s *Streamer) Start() error {
 // the connection status in metrics. This method is safe to call multiple times.
 func (s *Streamer) Stop() {
 	s.mu.Lock()
-	if s.cancel != nil {
-		s.cancel()
+	if s.cancel == nil {
+		// Not started, or already stopped: closing the queues again would panic.
+		s.mu.Unlock()
+
+		return
 	}
+	s.cancel()
+	s.cancel = nil
 	// Close all handler queues to signal workers to stop
 	for _, info := range s.handlers {
 		close(info.queue)
@@ -660,8 +665,15 @@ func (s *Streamer) stream(ctx context.Context) error {
 			continue
 		}
 
-		// Dispatch to interested handlers
+		// Dispatch to interested handlers. Stop cancels ctx and closes the
+		// queues under the write lock, so if ctx is cancelled here, under the
+		// read lock, the queues are closed and must not be sent to.
 		s.mu.RLock()
+		if ctx.Err() != nil {
+			s.mu.RUnlock()
+
+			return ctx.Err()
+		}
 		for _, info := range s.handlers {
 			if !info.handler.WantsMessage(msg.Type) {
 				continue