Maintain MQTT message order during processing

Previously the message handler spawned a goroutine for each message and
hoped for the best, but obviously it didn't work. Z2M often emitted the
current (old) state first, before sending out the updated state. Without
ordering, the old state might get processed later and overwrote the new,
correct state.

Message handling is now serialized, but decoupled, using a buffered
channel and a separate goroutine for processing.

bridge.go

@@ -79,10 +79,17 @@ type Bridge struct {
 	pendingUpdates sync.Map // queued updates before HAP init
 
 	mqttClient   mqtt.Client
+	mqttMessages chan *mqttMessage
 
 	updateListeners sync.Map
 }
 
+type mqttMessage struct {
+	Tstamp  time.Time
+	Topic   string
+	Payload []byte
+}
+
 type BridgeDevice struct {
 	Device    *Device
 	Accessory *accessory.A
@@ -98,6 +105,8 @@ func NewBridge(ctx context.Context, storeDir string) *Bridge {
 
 		hapInitCh: make(chan struct{}),
 		devices:   make(map[string]*BridgeDevice),
+
+		mqttMessages: make(chan *mqttMessage, 100),
 	}
 
 	br.bridgeAcc = accessory.NewBridge(accessory.Info{
@@ -259,6 +268,16 @@ func (br *Bridge) ConnectMQTT() error {
 		return tok.Error()
 	}
 
+	// start MQTT message processor
+	go func() {
+		// only start processing after init
+		br.WaitConfigured()
+
+		for m := range br.mqttMessages {
+			br.UpdateAccessoryState(m.Topic, m.Payload, m.Tstamp)
+		}
+	}()
+
 	return nil
 }
 
@@ -383,8 +402,8 @@ func (br *Bridge) handleMqttMessage(_ mqtt.Client, msg mqtt.Message) {
 		}
 	}
 
-	// spawn a goroutine to handle message, since mutex might block
+	isBridgeTopic := strings.HasPrefix(topic, "bridge/")
-	go func() {
+
 	if br.DebugMode && topic != "bridge/logging" {
 		log.Printf("received MQTT %s: %s", topic, payload)
 	}
@@ -392,13 +411,19 @@ func (br *Bridge) handleMqttMessage(_ mqtt.Client, msg mqtt.Message) {
 	br.hapInitMutex.RLock()
 	defer br.hapInitMutex.RUnlock()
 
-		isBridgeTopic := strings.HasPrefix(topic, "bridge/")
-
 	// check if HAP bridge device has been initialized
 	if !br.hapInitDone {
-			if isBridgeTopic {
 		// need to look out for bridge/devices for intiial setup
 		if topic == "bridge/devices" {
+			go func() {
+				br.hapInitMutex.Lock()
+				defer br.hapInitMutex.Unlock()
+
+				// re-verify that init has not been done
+				if br.hapInitDone {
+					return
+				}
+
 				err := br.AddDevicesFromJSON(payload)
 				if err != nil {
 					log.Printf("unable to add devices from JSON: %v", err)
@@ -410,13 +435,6 @@ func (br *Bridge) handleMqttMessage(_ mqtt.Client, msg mqtt.Message) {
 					log.Printf("cannot load z2m state: %s", err)
 				}
 
-					// "upgrade" to write lock for modifications
-					br.hapInitMutex.RUnlock()     // unlock now, relock again later
-					defer br.hapInitMutex.RLock() // ... before defer kicks in
-
-					br.hapInitMutex.Lock()
-					defer br.hapInitMutex.Unlock()
-
 				// dequeue and apply state updates
 				log.Print("applying deferred updates...")
 				br.pendingUpdates.Range(func(k, v any) bool {
@@ -428,17 +446,21 @@ func (br *Bridge) handleMqttMessage(_ mqtt.Client, msg mqtt.Message) {
 				br.hapInitDone = true
 
 				close(br.hapInitCh) // signal to waiting threads
-				}
+			}()
-			} else {
+		} else if !isBridgeTopic {
 			// queue other messages for state updating after setup
 			// only keep latest message for each device
 			log.Printf("queueing updates for %s", topic)
 			br.pendingUpdates.Store(topic, payload)
 		}
 	} else if !isBridgeTopic {
-			br.UpdateAccessoryState(topic, payload, recvTime)
+		// queue up MQTT messages
+		select {
+		case br.mqttMessages <- &mqttMessage{recvTime, topic, payload}:
+		default:
+			log.Printf("cannot queue MQTT message %s: %s", topic, payload)
+		}
 	}
-	}()
 }
 
 // Gets a list of all added accessories
@@ -597,8 +619,7 @@ wait:
 		}
 	}
 
-	//br.updateListeners.CompareAndDelete(key, ch)	// needs go 1.20
+	br.updateListeners.CompareAndDelete(key, ch)
-	br.updateListeners.Delete(key)
 
 	if !updated {
 		err = ErrUpdateTimeout
@@ -704,7 +725,7 @@ func (br *Bridge) UpdateAccessoryState(devName string, payload []byte, tstamp ti
 			updateKey := makeUpdateKey(dev.Device, mapping)
 			if ch, waiting := br.updateListeners.Load(updateKey); waiting {
 				if BRIDGE_DEVMODE {
-					log.Printf("sending new value for %q via chan", updateKey)
+					log.Printf("sending new value %+v for %q via chan", newVal, updateKey)
 				}
 				select {
 				case ch.(chan any) <- newVal: