[mdns] Reduce RAM usage by eliminating MAC address heap allocation (#12073)

esphome/components/mdns/__init__.py

@@ -184,10 +184,8 @@ async def to_code(config):
 
     # Calculate compile-time dynamic TXT value count
     # Dynamic values are those that cannot be stored in flash at compile time
+    # Note: MAC address is now stored in a fixed char[13] buffer, not dynamic storage
     dynamic_txt_count = 0
-    if "api" in CORE.config:
-        # Always: get_mac_address()
-        dynamic_txt_count += 1
     # User-provided templatable TXT values (only lambdas, not static strings)
     dynamic_txt_count += sum(
         1
@@ -196,8 +194,10 @@ async def to_code(config):
         if cg.is_template(txt_value)
     )
 
-    # Ensure at least 1 to avoid zero-size array
-    cg.add_define("MDNS_DYNAMIC_TXT_COUNT", max(1, dynamic_txt_count))
+    # Only add define if we actually need dynamic storage
+    if dynamic_txt_count > 0:
+        cg.add_define("USE_MDNS_DYNAMIC_TXT")
+        cg.add_define("MDNS_DYNAMIC_TXT_COUNT", dynamic_txt_count)
 
     # Enable storage if verbose logging is enabled (for dump_config)
     if get_logger_level() in ("VERBOSE", "VERY_VERBOSE"):

esphome/components/mdns/mdns_component.cpp

@@ -35,7 +35,7 @@ MDNS_STATIC_CONST_CHAR(SERVICE_TCP, "_tcp");
 // Wrap build-time defines into flash storage
 MDNS_STATIC_CONST_CHAR(VALUE_VERSION, ESPHOME_VERSION);
 
-void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services) {
+void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services, char *mac_address_buf) {
   // IMPORTANT: The #ifdef blocks below must match COMPONENTS_WITH_MDNS_SERVICES
   // in mdns/__init__.py. If you add a new service here, update both locations.
 
@@ -86,7 +86,9 @@ void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUN
       txt_records.push_back({MDNS_STR(TXT_FRIENDLY_NAME), MDNS_STR(friendly_name.c_str())});
     }
     txt_records.push_back({MDNS_STR(TXT_VERSION), MDNS_STR(VALUE_VERSION)});
-    txt_records.push_back({MDNS_STR(TXT_MAC), MDNS_STR(this->add_dynamic_txt_value(get_mac_address()))});
+
+    // MAC address: passed from caller (either member buffer or stack buffer depending on USE_MDNS_STORE_SERVICES)
+    txt_records.push_back({MDNS_STR(TXT_MAC), MDNS_STR(mac_address_buf)});
 
 #ifdef USE_ESP8266
     MDNS_STATIC_CONST_CHAR(PLATFORM_ESP8266, "ESP8266");

esphome/components/mdns/mdns_component.h

@@ -60,22 +60,58 @@ class MDNSComponent : public Component {
 
   void on_shutdown() override;
 
+#ifdef USE_MDNS_DYNAMIC_TXT
   /// Add a dynamic TXT value and return pointer to it for use in MDNSTXTRecord
   const char *add_dynamic_txt_value(const std::string &value) {
     this->dynamic_txt_values_.push_back(value);
     return this->dynamic_txt_values_[this->dynamic_txt_values_.size() - 1].c_str();
   }
+#endif
 
-  /// Storage for runtime-generated TXT values (MAC address, user lambdas)
+ protected:
+  /// Helper to set up services and MAC buffers, then call platform-specific registration
+  using PlatformRegisterFn = void (*)(MDNSComponent *, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &);
+
+  void setup_buffers_and_register_(PlatformRegisterFn platform_register) {
+#ifdef USE_MDNS_STORE_SERVICES
+    auto &services = this->services_;
+#else
+    StaticVector<MDNSService, MDNS_SERVICE_COUNT> services_storage;
+    auto &services = services_storage;
+#endif
+
+#ifdef USE_API
+#ifdef USE_MDNS_STORE_SERVICES
+    get_mac_address_into_buffer(this->mac_address_);
+    char *mac_ptr = this->mac_address_;
+#else
+    char mac_address[MAC_ADDRESS_BUFFER_SIZE];
+    get_mac_address_into_buffer(mac_address);
+    char *mac_ptr = mac_address;
+#endif
+#else
+    char *mac_ptr = nullptr;
+#endif
+
+    this->compile_records_(services, mac_ptr);
+    platform_register(this, services);
+  }
+
+#ifdef USE_MDNS_DYNAMIC_TXT
+  /// Storage for runtime-generated TXT values from user lambdas
   /// Pre-sized at compile time via MDNS_DYNAMIC_TXT_COUNT to avoid heap allocations.
   /// Static/compile-time values (version, board, etc.) are stored directly in flash and don't use this.
   StaticVector<std::string, MDNS_DYNAMIC_TXT_COUNT> dynamic_txt_values_;
+#endif
 
- protected:
+#if defined(USE_API) && defined(USE_MDNS_STORE_SERVICES)
+  /// Fixed buffer for MAC address (only needed when services are stored)
+  char mac_address_[MAC_ADDRESS_BUFFER_SIZE];
+#endif
 #ifdef USE_MDNS_STORE_SERVICES
   StaticVector<MDNSService, MDNS_SERVICE_COUNT> services_{};
 #endif
-  void compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services);
+  void compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services, char *mac_address_buf);
 };
 
 }  // namespace esphome::mdns

esphome/components/mdns/mdns_esp32.cpp

@@ -11,19 +11,11 @@ namespace esphome::mdns {
 
 static const char *const TAG = "mdns";
 
-void MDNSComponent::setup() {
-#ifdef USE_MDNS_STORE_SERVICES
-  this->compile_records_(this->services_);
-  const auto &services = this->services_;
-#else
-  StaticVector<MDNSService, MDNS_SERVICE_COUNT> services;
-  this->compile_records_(services);
-#endif
-
+static void register_esp32(MDNSComponent *comp, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services) {
   esp_err_t err = mdns_init();
   if (err != ESP_OK) {
     ESP_LOGW(TAG, "Init failed: %s", esp_err_to_name(err));
-    this->mark_failed();
+    comp->mark_failed();
     return;
   }
 
@@ -50,6 +42,8 @@ void MDNSComponent::setup() {
   }
 }
 
+void MDNSComponent::setup() { this->setup_buffers_and_register_(register_esp32); }
+
 void MDNSComponent::on_shutdown() {
   mdns_free();
   delay(40);  // Allow the mdns packets announcing service removal to be sent

esphome/components/mdns/mdns_esp8266.cpp

@@ -11,15 +11,7 @@
 
 namespace esphome::mdns {
 
-void MDNSComponent::setup() {
-#ifdef USE_MDNS_STORE_SERVICES
-  this->compile_records_(this->services_);
-  const auto &services = this->services_;
-#else
-  StaticVector<MDNSService, MDNS_SERVICE_COUNT> services;
-  this->compile_records_(services);
-#endif
-
+static void register_esp8266(MDNSComponent *, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services) {
   MDNS.begin(App.get_name().c_str());
 
   for (const auto &service : services) {
@@ -44,6 +36,8 @@ void MDNSComponent::setup() {
   }
 }
 
+void MDNSComponent::setup() { this->setup_buffers_and_register_(register_esp8266); }
+
 void MDNSComponent::loop() { MDNS.update(); }
 
 void MDNSComponent::on_shutdown() {

esphome/components/mdns/mdns_host.cpp

@@ -9,6 +9,15 @@
 namespace esphome::mdns {
 
 void MDNSComponent::setup() {
+#ifdef USE_MDNS_STORE_SERVICES
+#ifdef USE_API
+  get_mac_address_into_buffer(this->mac_address_);
+  char *mac_ptr = this->mac_address_;
+#else
+  char *mac_ptr = nullptr;
+#endif
+  this->compile_records_(this->services_, mac_ptr);
+#endif
   // Host platform doesn't have actual mDNS implementation
 }
 

esphome/components/mdns/mdns_libretiny.cpp

@@ -11,15 +11,7 @@
 
 namespace esphome::mdns {
 
-void MDNSComponent::setup() {
-#ifdef USE_MDNS_STORE_SERVICES
-  this->compile_records_(this->services_);
-  const auto &services = this->services_;
-#else
-  StaticVector<MDNSService, MDNS_SERVICE_COUNT> services;
-  this->compile_records_(services);
-#endif
-
+static void register_libretiny(MDNSComponent *, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services) {
   MDNS.begin(App.get_name().c_str());
 
   for (const auto &service : services) {
@@ -43,6 +35,8 @@ void MDNSComponent::setup() {
   }
 }
 
+void MDNSComponent::setup() { this->setup_buffers_and_register_(register_libretiny); }
+
 void MDNSComponent::on_shutdown() {}
 
 }  // namespace esphome::mdns

esphome/components/mdns/mdns_rp2040.cpp

@@ -11,15 +11,7 @@
 
 namespace esphome::mdns {
 
-void MDNSComponent::setup() {
-#ifdef USE_MDNS_STORE_SERVICES
-  this->compile_records_(this->services_);
-  const auto &services = this->services_;
-#else
-  StaticVector<MDNSService, MDNS_SERVICE_COUNT> services;
-  this->compile_records_(services);
-#endif
-
+static void register_rp2040(MDNSComponent *, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services) {
   MDNS.begin(App.get_name().c_str());
 
   for (const auto &service : services) {
@@ -43,6 +35,8 @@ void MDNSComponent::setup() {
   }
 }
 
+void MDNSComponent::setup() { this->setup_buffers_and_register_(register_rp2040); }
+
 void MDNSComponent::loop() { MDNS.update(); }
 
 void MDNSComponent::on_shutdown() {

esphome/core/defines.h

@@ -90,7 +90,8 @@
 #define USE_MDNS
 #define USE_MDNS_STORE_SERVICES
 #define MDNS_SERVICE_COUNT 3
-#define MDNS_DYNAMIC_TXT_COUNT 3
+#define USE_MDNS_DYNAMIC_TXT
+#define MDNS_DYNAMIC_TXT_COUNT 2
 #define SNTP_SERVER_COUNT 3
 #define USE_MEDIA_PLAYER
 #define USE_NEXTION_TFT_UPLOAD

esphome/core/helpers.cpp

@@ -642,17 +642,17 @@ std::string get_mac_address() {
 }
 
 std::string get_mac_address_pretty() {
-  char buf[18];
+  char buf[MAC_ADDRESS_PRETTY_BUFFER_SIZE];
   return std::string(get_mac_address_pretty_into_buffer(buf));
 }
 
-void get_mac_address_into_buffer(std::span<char, 13> buf) {
+void get_mac_address_into_buffer(std::span<char, MAC_ADDRESS_BUFFER_SIZE> buf) {
   uint8_t mac[6];
   get_mac_address_raw(mac);
   format_mac_addr_lower_no_sep(mac, buf.data());
 }
 
-const char *get_mac_address_pretty_into_buffer(std::span<char, 18> buf) {
+const char *get_mac_address_pretty_into_buffer(std::span<char, MAC_ADDRESS_PRETTY_BUFFER_SIZE> buf) {
   uint8_t mac[6];
   get_mac_address_raw(mac);
   format_mac_addr_upper(mac, buf.data());

esphome/core/helpers.h

@@ -1056,6 +1056,12 @@ class HighFrequencyLoopRequester {
 /// Get the device MAC address as raw bytes, written into the provided byte array (6 bytes).
 void get_mac_address_raw(uint8_t *mac);  // NOLINT(readability-non-const-parameter)
 
+/// Buffer size for MAC address in lowercase hex notation (12 hex chars + null terminator)
+constexpr size_t MAC_ADDRESS_BUFFER_SIZE = 13;
+
+/// Buffer size for MAC address in colon-separated uppercase hex notation (17 chars + null terminator)
+constexpr size_t MAC_ADDRESS_PRETTY_BUFFER_SIZE = 18;
+
 /// Get the device MAC address as a string, in lowercase hex notation.
 std::string get_mac_address();
 
@@ -1063,13 +1069,14 @@ std::string get_mac_address();
 std::string get_mac_address_pretty();
 
 /// Get the device MAC address into the given buffer, in lowercase hex notation.
-/// Assumes buffer length is 13 (12 digits for hexadecimal representation followed by null terminator).
-void get_mac_address_into_buffer(std::span<char, 13> buf);
+/// Assumes buffer length is MAC_ADDRESS_BUFFER_SIZE (12 digits for hexadecimal representation followed by null
+/// terminator).
+void get_mac_address_into_buffer(std::span<char, MAC_ADDRESS_BUFFER_SIZE> buf);
 
 /// Get the device MAC address into the given buffer, in colon-separated uppercase hex notation.
-/// Buffer must be exactly 18 bytes (17 for "XX:XX:XX:XX:XX:XX" + null terminator).
+/// Buffer must be exactly MAC_ADDRESS_PRETTY_BUFFER_SIZE bytes (17 for "XX:XX:XX:XX:XX:XX" + null terminator).
 /// Returns pointer to the buffer for convenience.
-const char *get_mac_address_pretty_into_buffer(std::span<char, 18> buf);
+const char *get_mac_address_pretty_into_buffer(std::span<char, MAC_ADDRESS_PRETTY_BUFFER_SIZE> buf);
 
 #ifdef USE_ESP32
 /// Set the MAC address to use from the provided byte array (6 bytes).