[mqtt] Reduce heap allocations in topic string building (#13072)

esphome/components/mqtt/__init__.py

@@ -77,6 +77,13 @@ CONF_DISCOVER_IP = "discover_ip"
 CONF_IDF_SEND_ASYNC = "idf_send_async"
 CONF_WAIT_FOR_CONNECTION = "wait_for_connection"
 
+# Max lengths for stack-based topic building.
+# These values are used in cv.Length() validators below to ensure the C++ code
+# in mqtt_component.cpp can safely use fixed-size stack buffers without overflow.
+# If you change these, update the corresponding constants in mqtt_component.cpp.
+TOPIC_PREFIX_MAX_LEN = 64  # Default is device name, typically short
+DISCOVERY_PREFIX_MAX_LEN = 64  # Default is "homeassistant" (13 chars)
+
 
 def validate_message_just_topic(value):
     value = cv.publish_topic(value)
@@ -253,9 +260,9 @@ CONFIG_SCHEMA = cv.All(
             ),
             cv.Optional(CONF_DISCOVERY_RETAIN, default=True): cv.boolean,
             cv.Optional(CONF_DISCOVER_IP, default=True): cv.boolean,
-            cv.Optional(
-                CONF_DISCOVERY_PREFIX, default="homeassistant"
-            ): cv.publish_topic,
+            cv.Optional(CONF_DISCOVERY_PREFIX, default="homeassistant"): cv.All(
+                cv.publish_topic, cv.Length(max=DISCOVERY_PREFIX_MAX_LEN)
+            ),
             cv.Optional(CONF_DISCOVERY_UNIQUE_ID_GENERATOR, default="legacy"): cv.enum(
                 MQTT_DISCOVERY_UNIQUE_ID_GENERATOR_OPTIONS
             ),
@@ -266,7 +273,9 @@ CONFIG_SCHEMA = cv.All(
             cv.Optional(CONF_BIRTH_MESSAGE): MQTT_MESSAGE_SCHEMA,
             cv.Optional(CONF_WILL_MESSAGE): MQTT_MESSAGE_SCHEMA,
             cv.Optional(CONF_SHUTDOWN_MESSAGE): MQTT_MESSAGE_SCHEMA,
-            cv.Optional(CONF_TOPIC_PREFIX, default=lambda: CORE.name): cv.publish_topic,
+            cv.Optional(CONF_TOPIC_PREFIX, default=lambda: CORE.name): cv.All(
+                cv.publish_topic, cv.Length(max=TOPIC_PREFIX_MAX_LEN)
+            ),
             cv.Optional(CONF_LOG_TOPIC): cv.Any(
                 None,
                 MQTT_MESSAGE_BASE.extend(

esphome/components/mqtt/mqtt_alarm_control_panel.cpp

@@ -79,7 +79,7 @@ void MQTTAlarmControlPanelComponent::send_discovery(JsonObject root, mqtt::SendD
   root[MQTT_CODE_ARM_REQUIRED] = this->alarm_control_panel_->get_requires_code_to_arm();
 }
 
-std::string MQTTAlarmControlPanelComponent::component_type() const { return "alarm_control_panel"; }
+MQTT_COMPONENT_TYPE(MQTTAlarmControlPanelComponent, "alarm_control_panel")
 const EntityBase *MQTTAlarmControlPanelComponent::get_entity() const { return this->alarm_control_panel_; }
 
 bool MQTTAlarmControlPanelComponent::send_initial_state() { return this->publish_state(); }

esphome/components/mqtt/mqtt_alarm_control_panel.h

@@ -25,7 +25,7 @@ class MQTTAlarmControlPanelComponent : public mqtt::MQTTComponent {
   void dump_config() override;
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   alarm_control_panel::AlarmControlPanel *alarm_control_panel_;

esphome/components/mqtt/mqtt_binary_sensor.cpp

@@ -10,7 +10,7 @@ namespace esphome::mqtt {
 
 static const char *const TAG = "mqtt.binary_sensor";
 
-std::string MQTTBinarySensorComponent::component_type() const { return "binary_sensor"; }
+MQTT_COMPONENT_TYPE(MQTTBinarySensorComponent, "binary_sensor")
 const EntityBase *MQTTBinarySensorComponent::get_entity() const { return this->binary_sensor_; }
 
 void MQTTBinarySensorComponent::setup() {

esphome/components/mqtt/mqtt_binary_sensor.h

@@ -29,7 +29,7 @@ class MQTTBinarySensorComponent : public mqtt::MQTTComponent {
   bool publish_state(bool state);
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   binary_sensor::BinarySensor *binary_sensor_;

esphome/components/mqtt/mqtt_button.cpp

@@ -39,7 +39,7 @@ void MQTTButtonComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryCon
   // NOLINTEND(clang-analyzer-cplusplus.NewDeleteLeaks)
 }
 
-std::string MQTTButtonComponent::component_type() const { return "button"; }
+MQTT_COMPONENT_TYPE(MQTTButtonComponent, "button")
 const EntityBase *MQTTButtonComponent::get_entity() const { return this->button_; }
 
 }  // namespace esphome::mqtt

esphome/components/mqtt/mqtt_button.h

@@ -26,7 +26,7 @@ class MQTTButtonComponent : public mqtt::MQTTComponent {
 
  protected:
   /// "button" component type.
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   button::Button *button_;

esphome/components/mqtt/mqtt_climate.cpp

@@ -254,7 +254,7 @@ void MQTTClimateComponent::setup() {
 }
 MQTTClimateComponent::MQTTClimateComponent(Climate *device) : device_(device) {}
 bool MQTTClimateComponent::send_initial_state() { return this->publish_state_(); }
-std::string MQTTClimateComponent::component_type() const { return "climate"; }
+MQTT_COMPONENT_TYPE(MQTTClimateComponent, "climate")
 const EntityBase *MQTTClimateComponent::get_entity() const { return this->device_; }
 
 bool MQTTClimateComponent::publish_state_() {

esphome/components/mqtt/mqtt_climate.h

@@ -15,7 +15,7 @@ class MQTTClimateComponent : public mqtt::MQTTComponent {
   MQTTClimateComponent(climate::Climate *device);
   void send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) override;
   bool send_initial_state() override;
-  std::string component_type() const override;
+  const char *component_type() const override;
   void setup() override;
 
   MQTT_COMPONENT_CUSTOM_TOPIC(current_temperature, state)

esphome/components/mqtt/mqtt_component.cpp

@@ -13,6 +13,34 @@ namespace esphome::mqtt {
 
 static const char *const TAG = "mqtt.component";
 
+// Helper functions for building topic strings on stack
+inline char *append_str(char *p, const char *s, size_t len) {
+  memcpy(p, s, len);
+  return p + len;
+}
+
+inline char *append_char(char *p, char c) {
+  *p = c;
+  return p + 1;
+}
+
+// Max lengths for stack-based topic building.
+// These limits are enforced at Python config validation time in mqtt/__init__.py
+// using cv.Length() validators for topic_prefix and discovery_prefix.
+// MQTT_COMPONENT_TYPE_MAX_LEN and MQTT_SUFFIX_MAX_LEN are defined in mqtt_component.h.
+// ESPHOME_DEVICE_NAME_MAX_LEN and OBJECT_ID_MAX_LEN are defined in entity_base.h.
+// This ensures the stack buffers below are always large enough.
+static constexpr size_t TOPIC_PREFIX_MAX_LEN = 64;      // Validated in Python: cv.Length(max=64)
+static constexpr size_t DISCOVERY_PREFIX_MAX_LEN = 64;  // Validated in Python: cv.Length(max=64)
+
+// Stack buffer sizes - safe because all inputs are length-validated at config time
+// Format: prefix + "/" + type + "/" + object_id + "/" + suffix + null
+static constexpr size_t DEFAULT_TOPIC_MAX_LEN =
+    TOPIC_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 1 + MQTT_SUFFIX_MAX_LEN + 1;
+// Format: prefix + "/" + type + "/" + name + "/" + object_id + "/config" + null
+static constexpr size_t DISCOVERY_TOPIC_MAX_LEN = DISCOVERY_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN + 1 +
+                                                  ESPHOME_DEVICE_NAME_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 7 + 1;
+
 void MQTTComponent::set_qos(uint8_t qos) { this->qos_ = qos; }
 
 void MQTTComponent::set_subscribe_qos(uint8_t qos) { this->subscribe_qos_ = qos; }
@@ -21,8 +49,23 @@ void MQTTComponent::set_retain(bool retain) { this->retain_ = retain; }
 
 std::string MQTTComponent::get_discovery_topic_(const MQTTDiscoveryInfo &discovery_info) const {
   std::string sanitized_name = str_sanitize(App.get_name());
-  return discovery_info.prefix + "/" + this->component_type() + "/" + sanitized_name + "/" +
-         this->get_default_object_id_() + "/config";
+  const char *comp_type = this->component_type();
+  char object_id_buf[OBJECT_ID_MAX_LEN];
+  StringRef object_id = this->get_default_object_id_to_(object_id_buf);
+
+  char buf[DISCOVERY_TOPIC_MAX_LEN];
+  char *p = buf;
+
+  p = append_str(p, discovery_info.prefix.data(), discovery_info.prefix.size());
+  p = append_char(p, '/');
+  p = append_str(p, comp_type, strlen(comp_type));
+  p = append_char(p, '/');
+  p = append_str(p, sanitized_name.data(), sanitized_name.size());
+  p = append_char(p, '/');
+  p = append_str(p, object_id.c_str(), object_id.size());
+  p = append_str(p, "/config", 7);
+
+  return std::string(buf, p - buf);
 }
 
 std::string MQTTComponent::get_default_topic_for_(const std::string &suffix) const {
@@ -32,7 +75,22 @@ std::string MQTTComponent::get_default_topic_for_(const std::string &suffix) con
     return "";
   }
 
-  return topic_prefix + "/" + this->component_type() + "/" + this->get_default_object_id_() + "/" + suffix;
+  const char *comp_type = this->component_type();
+  char object_id_buf[OBJECT_ID_MAX_LEN];
+  StringRef object_id = this->get_default_object_id_to_(object_id_buf);
+
+  char buf[DEFAULT_TOPIC_MAX_LEN];
+  char *p = buf;
+
+  p = append_str(p, topic_prefix.data(), topic_prefix.size());
+  p = append_char(p, '/');
+  p = append_str(p, comp_type, strlen(comp_type));
+  p = append_char(p, '/');
+  p = append_str(p, object_id.c_str(), object_id.size());
+  p = append_char(p, '/');
+  p = append_str(p, suffix.data(), suffix.size());
+
+  return std::string(buf, p - buf);
 }
 
 std::string MQTTComponent::get_state_topic_() const {
@@ -123,6 +181,8 @@ bool MQTTComponent::send_discovery_() {
         }
 
         const MQTTDiscoveryInfo &discovery_info = global_mqtt_client->get_discovery_info();
+        char object_id_buf[OBJECT_ID_MAX_LEN];
+        StringRef object_id = this->get_default_object_id_to_(object_id_buf);
         if (discovery_info.unique_id_generator == MQTT_MAC_ADDRESS_UNIQUE_ID_GENERATOR) {
           char friendly_name_hash[9];
           sprintf(friendly_name_hash, "%08" PRIx32, fnv1_hash(this->friendly_name_()));
@@ -131,12 +191,12 @@ bool MQTTComponent::send_discovery_() {
         } else {
           // default to almost-unique ID. It's a hack but the only way to get that
           // gorgeous device registry view.
-          root[MQTT_UNIQUE_ID] = "ESP" + this->component_type() + this->get_default_object_id_();
+          root[MQTT_UNIQUE_ID] = "ESP" + std::string(this->component_type()) + object_id.c_str();
         }
 
         const std::string &node_name = App.get_name();
         if (discovery_info.object_id_generator == MQTT_DEVICE_NAME_OBJECT_ID_GENERATOR)
-          root[MQTT_OBJECT_ID] = node_name + "_" + this->get_default_object_id_();
+          root[MQTT_OBJECT_ID] = node_name + "_" + object_id.c_str();
 
         const std::string &friendly_name_ref = App.get_friendly_name();
         const std::string &node_friendly_name = friendly_name_ref.empty() ? node_name : friendly_name_ref;
@@ -194,10 +254,6 @@ bool MQTTComponent::is_discovery_enabled() const {
   return this->discovery_enabled_ && global_mqtt_client->is_discovery_enabled();
 }
 
-std::string MQTTComponent::get_default_object_id_() const {
-  return str_sanitize(str_snake_case(this->friendly_name_()));
-}
-
 void MQTTComponent::subscribe(const std::string &topic, mqtt_callback_t callback, uint8_t qos) {
   global_mqtt_client->subscribe(topic, std::move(callback), qos);
 }
@@ -280,6 +336,9 @@ bool MQTTComponent::is_connected_() const { return global_mqtt_client->is_connec
 
 // Pull these properties from EntityBase if not overridden
 std::string MQTTComponent::friendly_name_() const { return this->get_entity()->get_name(); }
+StringRef MQTTComponent::get_default_object_id_to_(std::span<char, OBJECT_ID_MAX_LEN> buf) const {
+  return this->get_entity()->get_object_id_to(buf);
+}
 StringRef MQTTComponent::get_icon_ref_() const { return this->get_entity()->get_icon_ref(); }
 bool MQTTComponent::is_disabled_by_default_() const { return this->get_entity()->is_disabled_by_default(); }
 bool MQTTComponent::is_internal() {

esphome/components/mqtt/mqtt_component.h

@@ -19,6 +19,10 @@ struct SendDiscoveryConfig {
   bool command_topic{true};  ///< If the command topic should be included. Default to true.
 };
 
+// Max lengths for stack-based topic building (must match mqtt_component.cpp)
+static constexpr size_t MQTT_COMPONENT_TYPE_MAX_LEN = 20;
+static constexpr size_t MQTT_SUFFIX_MAX_LEN = 32;
+
 #define LOG_MQTT_COMPONENT(state_topic, command_topic) \
   if (state_topic) { \
     ESP_LOGCONFIG(TAG, "  State Topic: '%s'", this->get_state_topic_().c_str()); \
@@ -27,7 +31,18 @@ struct SendDiscoveryConfig {
     ESP_LOGCONFIG(TAG, "  Command Topic: '%s'", this->get_command_topic_().c_str()); \
   }
 
+// Macro to define component_type() with compile-time length verification
+// Usage: MQTT_COMPONENT_TYPE(MQTTSensorComponent, "sensor")
+#define MQTT_COMPONENT_TYPE(class_name, type_str) \
+  const char *class_name::component_type() const { return type_str; } \
+  static_assert(sizeof(type_str) - 1 <= MQTT_COMPONENT_TYPE_MAX_LEN, \
+                #class_name "::component_type() exceeds MQTT_COMPONENT_TYPE_MAX_LEN");
+
+// Macro to define custom topic getter/setter with compile-time suffix length verification
 #define MQTT_COMPONENT_CUSTOM_TOPIC_(name, type) \
+  static_assert(sizeof(#name "/" #type) - 1 <= MQTT_SUFFIX_MAX_LEN, \
+                "topic suffix " #name "/" #type " exceeds MQTT_SUFFIX_MAX_LEN"); \
+\
  protected: \
   std::string custom_##name##_##type##_topic_{}; \
 \
@@ -92,7 +107,7 @@ class MQTTComponent : public Component {
   void set_subscribe_qos(uint8_t qos);
 
   /// Override this method to return the component type (e.g. "light", "sensor", ...)
-  virtual std::string component_type() const = 0;
+  virtual const char *component_type() const = 0;
 
   /// Set a custom state topic. Set to "" for default behavior.
   void set_custom_state_topic(const char *custom_state_topic);
@@ -185,8 +200,8 @@ class MQTTComponent : public Component {
 
   // ========== INTERNAL METHODS ==========
   // (In most use cases you won't need these)
-  /// Generate the Home Assistant MQTT discovery object id by automatically transforming the friendly name.
-  std::string get_default_object_id_() const;
+  /// Get the object ID for this MQTT component, writing to the provided buffer.
+  StringRef get_default_object_id_to_(std::span<char, OBJECT_ID_MAX_LEN> buf) const;
 
   StringRef custom_state_topic_{};
   StringRef custom_command_topic_{};

esphome/components/mqtt/mqtt_cover.cpp

@@ -90,7 +90,7 @@ void MQTTCoverComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConf
   }
 }
 
-std::string MQTTCoverComponent::component_type() const { return "cover"; }
+MQTT_COMPONENT_TYPE(MQTTCoverComponent, "cover")
 const EntityBase *MQTTCoverComponent::get_entity() const { return this->cover_; }
 
 bool MQTTCoverComponent::send_initial_state() { return this->publish_state(); }

esphome/components/mqtt/mqtt_cover.h

@@ -29,7 +29,7 @@ class MQTTCoverComponent : public mqtt::MQTTComponent {
   void dump_config() override;
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   cover::Cover *cover_;

esphome/components/mqtt/mqtt_date.cpp

@@ -39,7 +39,7 @@ void MQTTDateComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true)
 }
 
-std::string MQTTDateComponent::component_type() const { return "date"; }
+MQTT_COMPONENT_TYPE(MQTTDateComponent, "date")
 const EntityBase *MQTTDateComponent::get_entity() const { return this->date_; }
 
 void MQTTDateComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {

esphome/components/mqtt/mqtt_date.h

@@ -31,7 +31,7 @@ class MQTTDateComponent : public mqtt::MQTTComponent {
   bool publish_state(uint16_t year, uint8_t month, uint8_t day);
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   datetime::DateEntity *date_;

esphome/components/mqtt/mqtt_datetime.cpp

@@ -50,7 +50,7 @@ void MQTTDateTimeComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true)
 }
 
-std::string MQTTDateTimeComponent::component_type() const { return "datetime"; }
+MQTT_COMPONENT_TYPE(MQTTDateTimeComponent, "datetime")
 const EntityBase *MQTTDateTimeComponent::get_entity() const { return this->datetime_; }
 
 void MQTTDateTimeComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {

esphome/components/mqtt/mqtt_datetime.h

@@ -31,7 +31,7 @@ class MQTTDateTimeComponent : public mqtt::MQTTComponent {
   bool publish_state(uint16_t year, uint8_t month, uint8_t day, uint8_t hour, uint8_t minute, uint8_t second);
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   datetime::DateTimeEntity *datetime_;

esphome/components/mqtt/mqtt_event.cpp

@@ -50,7 +50,7 @@ bool MQTTEventComponent::publish_event_(const std::string &event_type) {
   });
 }
 
-std::string MQTTEventComponent::component_type() const { return "event"; }
+MQTT_COMPONENT_TYPE(MQTTEventComponent, "event")
 const EntityBase *MQTTEventComponent::get_entity() const { return this->event_; }
 
 }  // namespace esphome::mqtt

esphome/components/mqtt/mqtt_event.h

@@ -25,7 +25,7 @@ class MQTTEventComponent : public mqtt::MQTTComponent {
 
  protected:
   bool publish_event_(const std::string &event_type);
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   event::Event *event_;

esphome/components/mqtt/mqtt_fan.cpp

@@ -15,7 +15,7 @@ using namespace esphome::fan;
 MQTTFanComponent::MQTTFanComponent(Fan *state) : state_(state) {}
 
 Fan *MQTTFanComponent::get_state() const { return this->state_; }
-std::string MQTTFanComponent::component_type() const { return "fan"; }
+MQTT_COMPONENT_TYPE(MQTTFanComponent, "fan")
 const EntityBase *MQTTFanComponent::get_entity() const { return this->state_; }
 
 void MQTTFanComponent::setup() {

esphome/components/mqtt/mqtt_fan.h

@@ -36,7 +36,7 @@ class MQTTFanComponent : public mqtt::MQTTComponent {
   bool send_initial_state() override;
   bool publish_state();
   /// 'fan' component type for discovery.
-  std::string component_type() const override;
+  const char *component_type() const override;
 
   fan::Fan *get_state() const;
 

esphome/components/mqtt/mqtt_light.cpp

@@ -14,7 +14,7 @@ static const char *const TAG = "mqtt.light";
 
 using namespace esphome::light;
 
-std::string MQTTJSONLightComponent::component_type() const { return "light"; }
+MQTT_COMPONENT_TYPE(MQTTJSONLightComponent, "light")
 const EntityBase *MQTTJSONLightComponent::get_entity() const { return this->state_; }
 
 void MQTTJSONLightComponent::setup() {

esphome/components/mqtt/mqtt_light.h

@@ -28,7 +28,7 @@ class MQTTJSONLightComponent : public mqtt::MQTTComponent, public light::LightRe
   void on_light_remote_values_update() override;
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   bool publish_state_();

esphome/components/mqtt/mqtt_lock.cpp

@@ -34,7 +34,7 @@ void MQTTLockComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true);
 }
 
-std::string MQTTLockComponent::component_type() const { return "lock"; }
+MQTT_COMPONENT_TYPE(MQTTLockComponent, "lock")
 const EntityBase *MQTTLockComponent::get_entity() const { return this->lock_; }
 void MQTTLockComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {
   // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks) false positive with ArduinoJson

esphome/components/mqtt/mqtt_lock.h

@@ -27,7 +27,7 @@ class MQTTLockComponent : public mqtt::MQTTComponent {
 
  protected:
   /// "lock" component type.
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   lock::Lock *lock_;

esphome/components/mqtt/mqtt_number.cpp

@@ -33,7 +33,7 @@ void MQTTNumberComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, false)
 }
 
-std::string MQTTNumberComponent::component_type() const { return "number"; }
+MQTT_COMPONENT_TYPE(MQTTNumberComponent, "number")
 const EntityBase *MQTTNumberComponent::get_entity() const { return this->number_; }
 
 void MQTTNumberComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {

esphome/components/mqtt/mqtt_number.h

@@ -32,7 +32,7 @@ class MQTTNumberComponent : public mqtt::MQTTComponent {
 
  protected:
   /// Override for MQTTComponent, returns "number".
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   number::Number *number_;

esphome/components/mqtt/mqtt_select.cpp

@@ -28,7 +28,7 @@ void MQTTSelectComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, false)
 }
 
-std::string MQTTSelectComponent::component_type() const { return "select"; }
+MQTT_COMPONENT_TYPE(MQTTSelectComponent, "select")
 const EntityBase *MQTTSelectComponent::get_entity() const { return this->select_; }
 
 void MQTTSelectComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {

esphome/components/mqtt/mqtt_select.h

@@ -32,7 +32,7 @@ class MQTTSelectComponent : public mqtt::MQTTComponent {
 
  protected:
   /// Override for MQTTComponent, returns "select".
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   select::Select *select_;

esphome/components/mqtt/mqtt_sensor.cpp

@@ -31,7 +31,7 @@ void MQTTSensorComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, false)
 }
 
-std::string MQTTSensorComponent::component_type() const { return "sensor"; }
+MQTT_COMPONENT_TYPE(MQTTSensorComponent, "sensor")
 const EntityBase *MQTTSensorComponent::get_entity() const { return this->sensor_; }
 
 uint32_t MQTTSensorComponent::get_expire_after() const {

esphome/components/mqtt/mqtt_sensor.h

@@ -43,7 +43,7 @@ class MQTTSensorComponent : public mqtt::MQTTComponent {
 
  protected:
   /// Override for MQTTComponent, returns "sensor".
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   sensor::Sensor *sensor_;

esphome/components/mqtt/mqtt_switch.cpp

@@ -41,7 +41,7 @@ void MQTTSwitchComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true);
 }
 
-std::string MQTTSwitchComponent::component_type() const { return "switch"; }
+MQTT_COMPONENT_TYPE(MQTTSwitchComponent, "switch")
 const EntityBase *MQTTSwitchComponent::get_entity() const { return this->switch_; }
 void MQTTSwitchComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {
   // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks) false positive with ArduinoJson

esphome/components/mqtt/mqtt_switch.h

@@ -27,7 +27,7 @@ class MQTTSwitchComponent : public mqtt::MQTTComponent {
 
  protected:
   /// "switch" component type.
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   switch_::Switch *switch_;

esphome/components/mqtt/mqtt_text.cpp

@@ -29,7 +29,7 @@ void MQTTTextComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true)
 }
 
-std::string MQTTTextComponent::component_type() const { return "text"; }
+MQTT_COMPONENT_TYPE(MQTTTextComponent, "text")
 const EntityBase *MQTTTextComponent::get_entity() const { return this->text_; }
 
 void MQTTTextComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {

esphome/components/mqtt/mqtt_text.h

@@ -32,7 +32,7 @@ class MQTTTextComponent : public mqtt::MQTTComponent {
 
  protected:
   /// Override for MQTTComponent, returns "text".
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   text::Text *text_;

esphome/components/mqtt/mqtt_text_sensor.cpp

@@ -39,7 +39,7 @@ bool MQTTTextSensor::send_initial_state() {
     return true;
   }
 }
-std::string MQTTTextSensor::component_type() const { return "sensor"; }
+MQTT_COMPONENT_TYPE(MQTTTextSensor, "sensor")
 const EntityBase *MQTTTextSensor::get_entity() const { return this->sensor_; }
 
 }  // namespace esphome::mqtt

esphome/components/mqtt/mqtt_text_sensor.h

@@ -25,7 +25,7 @@ class MQTTTextSensor : public mqtt::MQTTComponent {
   bool send_initial_state() override;
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   text_sensor::TextSensor *sensor_;

esphome/components/mqtt/mqtt_time.cpp

@@ -39,7 +39,7 @@ void MQTTTimeComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true)
 }
 
-std::string MQTTTimeComponent::component_type() const { return "time"; }
+MQTT_COMPONENT_TYPE(MQTTTimeComponent, "time")
 const EntityBase *MQTTTimeComponent::get_entity() const { return this->time_; }
 
 void MQTTTimeComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConfig &config) {

esphome/components/mqtt/mqtt_time.h

@@ -31,7 +31,7 @@ class MQTTTimeComponent : public mqtt::MQTTComponent {
   bool publish_state(uint8_t hour, uint8_t minute, uint8_t second);
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   datetime::TimeEntity *time_;

esphome/components/mqtt/mqtt_update.cpp

@@ -52,7 +52,7 @@ void MQTTUpdateComponent::dump_config() {
   LOG_MQTT_COMPONENT(true, true);
 }
 
-std::string MQTTUpdateComponent::component_type() const { return "update"; }
+MQTT_COMPONENT_TYPE(MQTTUpdateComponent, "update")
 const EntityBase *MQTTUpdateComponent::get_entity() const { return this->update_; }
 
 }  // namespace esphome::mqtt

esphome/components/mqtt/mqtt_update.h

@@ -27,7 +27,7 @@ class MQTTUpdateComponent : public mqtt::MQTTComponent {
 
  protected:
   /// "update" component type.
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   update::UpdateEntity *update_;

esphome/components/mqtt/mqtt_valve.cpp

@@ -65,7 +65,7 @@ void MQTTValveComponent::send_discovery(JsonObject root, mqtt::SendDiscoveryConf
   }
 }
 
-std::string MQTTValveComponent::component_type() const { return "valve"; }
+MQTT_COMPONENT_TYPE(MQTTValveComponent, "valve")
 const EntityBase *MQTTValveComponent::get_entity() const { return this->valve_; }
 
 bool MQTTValveComponent::send_initial_state() { return this->publish_state(); }

esphome/components/mqtt/mqtt_valve.h

@@ -27,7 +27,7 @@ class MQTTValveComponent : public mqtt::MQTTComponent {
   void dump_config() override;
 
  protected:
-  std::string component_type() const override;
+  const char *component_type() const override;
   const EntityBase *get_entity() const override;
 
   valve::Valve *valve_;

esphome/core/config.py

@@ -76,6 +76,7 @@ VALID_INCLUDE_EXTS = {".h", ".hpp", ".tcc", ".ino", ".cpp", ".c"}
 
 
 def validate_hostname(config):
+    # Keep in sync with ESPHOME_DEVICE_NAME_MAX_LEN in esphome/core/entity_base.h
     max_length = 31
     if config[CONF_NAME_ADD_MAC_SUFFIX]:
         max_length -= 7  # "-AABBCC" is appended when add mac suffix option is used
@@ -207,6 +208,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Required(CONF_NAME): cv.valid_name,
+            # Keep max=120 in sync with OBJECT_ID_MAX_LEN in esphome/core/entity_base.h
             cv.Optional(CONF_FRIENDLY_NAME, ""): cv.All(
                 cv.string_no_slash, cv.Length(max=120)
             ),

esphome/core/entity_base.h

@@ -13,7 +13,10 @@
 
 namespace esphome {
 
-// Maximum size for object_id buffer (friendly_name max ~120 + margin)
+// Maximum device name length - keep in sync with validate_hostname() in esphome/core/config.py
+static constexpr size_t ESPHOME_DEVICE_NAME_MAX_LEN = 31;
+
+// Maximum size for object_id buffer - keep in sync with friendly_name cv.Length(max=120) in esphome/core/config.py
 static constexpr size_t OBJECT_ID_MAX_LEN = 128;
 
 enum EntityCategory : uint8_t {