mqtt: add Subscribe method; server subscribes and logs incoming messages

internal/mqtt/mqtt.go

@@ -64,6 +64,24 @@ func (c *Client) Publish(topic string, payload []byte) error {
 	return token.Error()
 }
 
+// Subscribe subscribes to the given topic and calls the provided handler
+// for each incoming message.
+func (c *Client) Subscribe(topic string, handler func(topic string, payload []byte)) error {
+	if c == nil || c.client == nil {
+		return fmt.Errorf("mqtt client not connected")
+	}
+	mh := func(cli paho.Client, msg paho.Message) {
+		if handler != nil {
+			handler(msg.Topic(), msg.Payload())
+		}
+	}
+	token := c.client.Subscribe(topic, 0, mh)
+	if ok := token.WaitTimeout(5 * time.Second); !ok {
+		return fmt.Errorf("subscribe timeout")
+	}
+	return token.Error()
+}
+
 func (c *Client) Close() {
 	if c == nil || c.client == nil {
 		return