Fix bindings.mqtt deadlock on Disconnect()

Update bindings.mqtt component to avoid deadlocking on Close() when the
app-provided message handler is blocked.

- Run the app-provided message handler in a separate goroutine so that
  the wrapper handler function it passes to the mqtt library can be
  canceled.
- Cancel the read context to terminate any in flight message handling
  before calling Disconnect() in Close()

bindings/mqtt/mqtt.go

@@ -196,6 +196,32 @@ func (m *MQTT) Invoke(req *bindings.InvokeRequest) (*bindings.InvokeResponse, er
 	return nil, nil
 }
 
+func (m *MQTT) handleMessage(handler func(*bindings.ReadResponse) ([]byte, error), mqttMsg mqtt.Message) error {
+	msg := bindings.ReadResponse{Data: mqttMsg.Payload()}
+
+	// paho.mqtt.golang requires that handlers never block or it can deadlock on client.Disconnect.
+	// To ensure that the Dapr runtime does not hang on teardown on of the component, run the app's
+	// handling code in a goroutine so that this handler function is always cancellable on Close().
+	ch := make(chan error)
+	go func(m *bindings.ReadResponse) {
+		defer close(ch)
+		_, err := handler(m)
+		ch <- err
+	}(&msg)
+
+	select {
+	case handlerErr := <-ch:
+		if handlerErr != nil {
+			return handlerErr
+		}
+		mqttMsg.Ack()
+		return nil
+	case <-m.ctx.Done():
+		m.logger.Infof("Read context cancelled: %v", m.ctx.Err())
+		return m.ctx.Err()
+	}
+}
+
 func (m *MQTT) Read(handler func(*bindings.ReadResponse) ([]byte, error)) error {
 	sigterm := make(chan os.Signal, 1)
 	signal.Notify(sigterm, os.Interrupt, syscall.SIGTERM)
@@ -217,21 +243,14 @@ func (m *MQTT) Read(handler func(*bindings.ReadResponse) ([]byte, error)) error
 
 	m.logger.Debugf("mqtt subscribing to topic %s", m.metadata.topic)
 	token := m.consumer.Subscribe(m.metadata.topic, m.metadata.qos, func(client mqtt.Client, mqttMsg mqtt.Message) {
-		msg := bindings.ReadResponse{Data: mqttMsg.Payload()}
-
 		b := m.backOff
 		if m.metadata.backOffMaxRetries >= 0 {
 			b = backoff.WithMaxRetries(m.backOff, uint64(m.metadata.backOffMaxRetries))
 		}
+
 		if err := retry.NotifyRecover(func() error {
 			m.logger.Debugf("Processing MQTT message %s/%d", mqttMsg.Topic(), mqttMsg.MessageID())
-			if _, err := handler(&msg); err != nil {
-				return err
-			}
-
-			mqttMsg.Ack()
-
-			return nil
+			return m.handleMessage(handler, mqttMsg)
 		}, b, func(err error, d time.Duration) {
 			m.logger.Errorf("Error processing MQTT message: %s/%d. Retrying...", mqttMsg.Topic(), mqttMsg.MessageID())
 		}, func() {
@@ -313,6 +332,9 @@ func (m *MQTT) createClientOptions(uri *url.URL, clientID string) *mqtt.ClientOp
 }
 
 func (m *MQTT) Close() error {
+	// Cancel any read callback handlers before Disconnect to prevent deadlocks.
+	m.cancel()
+
 	if m.consumer != nil {
 		m.consumer.Disconnect(1)
 	}