Null topic_prefix disables MQTT publishing/subscription unless topic is explicitly configured (#5644)

esphome/components/mqtt/__init__.py

@@ -133,33 +133,47 @@ def validate_config(value):
     # Populate default fields
     out = value.copy()
     topic_prefix = value[CONF_TOPIC_PREFIX]
+    # If the topic prefix is not null and these messages are not configured, then set them to the default
+    # If the topic prefix is null and these messages are not configured, then set them to null
     if CONF_BIRTH_MESSAGE not in value:
+        if topic_prefix != "":
             out[CONF_BIRTH_MESSAGE] = {
                 CONF_TOPIC: f"{topic_prefix}/status",
                 CONF_PAYLOAD: "online",
                 CONF_QOS: 0,
                 CONF_RETAIN: True,
             }
+        else:
+            out[CONF_BIRTH_MESSAGE] = {}
     if CONF_WILL_MESSAGE not in value:
+        if topic_prefix != "":
             out[CONF_WILL_MESSAGE] = {
                 CONF_TOPIC: f"{topic_prefix}/status",
                 CONF_PAYLOAD: "offline",
                 CONF_QOS: 0,
                 CONF_RETAIN: True,
             }
+        else:
+            out[CONF_WILL_MESSAGE] = {}
     if CONF_SHUTDOWN_MESSAGE not in value:
+        if topic_prefix != "":
             out[CONF_SHUTDOWN_MESSAGE] = {
                 CONF_TOPIC: f"{topic_prefix}/status",
                 CONF_PAYLOAD: "offline",
                 CONF_QOS: 0,
                 CONF_RETAIN: True,
             }
+        else:
+            out[CONF_SHUTDOWN_MESSAGE] = {}
     if CONF_LOG_TOPIC not in value:
+        if topic_prefix != "":
             out[CONF_LOG_TOPIC] = {
                 CONF_TOPIC: f"{topic_prefix}/debug",
                 CONF_QOS: 0,
                 CONF_RETAIN: True,
             }
+        else:
+            out[CONF_LOG_TOPIC] = {}
     return out
 
 

esphome/components/mqtt/mqtt_component.cpp

@@ -23,8 +23,13 @@ std::string MQTTComponent::get_discovery_topic_(const MQTTDiscoveryInfo &discove
 }
 
 std::string MQTTComponent::get_default_topic_for_(const std::string &suffix) const {
-  return global_mqtt_client->get_topic_prefix() + "/" + this->component_type() + "/" + this->get_default_object_id_() +
-         "/" + suffix;
+  const std::string &topic_prefix = global_mqtt_client->get_topic_prefix();
+  if (topic_prefix.empty()) {
+    // If the topic_prefix is null, the default topic should be null
+    return "";
+  }
+
+  return topic_prefix + "/" + this->component_type() + "/" + this->get_default_object_id_() + "/" + suffix;
 }
 
 std::string MQTTComponent::get_state_topic_() const {
@@ -245,17 +250,25 @@ std::string MQTTComponent::friendly_name() const { return this->get_entity()->ge
 std::string MQTTComponent::get_icon() const { return this->get_entity()->get_icon(); }
 bool MQTTComponent::is_disabled_by_default() const { return this->get_entity()->is_disabled_by_default(); }
 bool MQTTComponent::is_internal() {
-  if ((this->get_state_topic_().empty()) || (this->get_command_topic_().empty())) {
-    // If both state_topic and command_topic are empty, then the entity is internal to mqtt
+  if (this->has_custom_state_topic_) {
+    // If the custom state_topic is null, return true as it is internal and should not publish
+    // else, return false, as it is explicitly set to a topic, so it is not internal and should publish
+    return this->get_state_topic_().empty();
+  }
+
+  if (this->has_custom_command_topic_) {
+    // If the custom command_topic is null, return true as it is internal and should not publish
+    // else, return false, as it is explicitly set to a topic, so it is not internal and should publish
+    return this->get_command_topic_().empty();
+  }
+
+  // No custom topics have been set
+  if (this->get_default_topic_for_("").empty()) {
+    // If the default topic prefix is null, then the component, by default, is internal and should not publish
     return true;
   }
 
-  if (this->has_custom_state_topic_ || this->has_custom_command_topic_) {
-    // If a custom state_topic or command_topic is set, then the entity is not internal to mqtt
-    return false;
-  }
-
-  // Use ESPHome's entity internal state
+  // Use ESPHome's component internal state if topic_prefix is not null with no custom state_topic or command_topic
   return this->get_entity()->is_internal();
 }
 

tests/test4.yaml

@@ -24,6 +24,13 @@ ethernet:
 network:
   enable_ipv6: true
 
+mqtt:
+  broker: test.mosquitto.org
+  port: 1883
+  discovery: true
+  discovery_prefix: homeassistant
+  topic_prefix:
+
 api:
 
 i2c:
@@ -281,6 +288,7 @@ sensor:
     id: a01nyub_sensor
     name: "a01nyub Distance"
     uart_id: uart9600
+    state_topic: "esphome/sensor/a01nyub_sensor/state"
 
 #
 # platform sensor.apds9960 requires component apds9960
@@ -764,7 +772,6 @@ speaker:
     i2s_dout_pin: GPIO25
     mode: mono
 
-
 voice_assistant:
   microphone: mic_id_external
   speaker: speaker_id