[core] Add make_name_with_suffix helper to optimize string concatenation (#11176)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

esphome/components/esp32_ble/ble.cpp

@@ -213,8 +213,11 @@ bool ESP32BLE::ble_setup_() {
   if (this->name_.has_value()) {
     name = this->name_.value();
     if (App.is_name_add_mac_suffix_enabled()) {
-      name += "-";
-      name += get_mac_address().substr(6);
+      // MAC address suffix length (last 6 characters of 12-char MAC address string)
+      constexpr size_t mac_address_suffix_len = 6;
+      const std::string mac_addr = get_mac_address();
+      const char *mac_suffix_ptr = mac_addr.c_str() + mac_address_suffix_len;
+      name = make_name_with_suffix(name, '-', mac_suffix_ptr, mac_address_suffix_len);
     }
   } else {
     name = App.get_name();

esphome/components/ethernet/ethernet_component.cpp

@@ -691,7 +691,9 @@ void EthernetComponent::set_manual_ip(const ManualIP &manual_ip) { this->manual_
 
 std::string EthernetComponent::get_use_address() const {
   if (this->use_address_.empty()) {
-    return App.get_name() + ".local";
+    // ".local" suffix length for mDNS hostnames
+    constexpr size_t mdns_local_suffix_len = 5;
+    return make_name_with_suffix(App.get_name(), '.', "local", mdns_local_suffix_len);
   }
   return this->use_address_;
 }

esphome/components/mqtt/mqtt_client.cpp

@@ -29,7 +29,8 @@ static const char *const TAG = "mqtt";
 
 MQTTClientComponent::MQTTClientComponent() {
   global_mqtt_client = this;
-  this->credentials_.client_id = App.get_name() + "-" + get_mac_address();
+  const std::string mac_addr = get_mac_address();
+  this->credentials_.client_id = make_name_with_suffix(App.get_name(), '-', mac_addr.c_str(), mac_addr.size());
 }
 
 // Connection

esphome/components/wifi/wifi_component.cpp

@@ -267,7 +267,9 @@ network::IPAddress WiFiComponent::get_dns_address(int num) {
 }
 std::string WiFiComponent::get_use_address() const {
   if (this->use_address_.empty()) {
-    return App.get_name() + ".local";
+    // ".local" suffix length for mDNS hostnames
+    constexpr size_t mdns_local_suffix_len = 5;
+    return make_name_with_suffix(App.get_name(), '.', "local", mdns_local_suffix_len);
   }
   return this->use_address_;
 }

esphome/config_validation.py

@@ -1195,6 +1195,13 @@ def validate_bytes(value):
 
 
 def hostname(value):
+    """Validate that the value is a valid hostname.
+
+    Maximum length is 63 characters per RFC 1035.
+
+    Note: If this limit is changed, update MAX_NAME_WITH_SUFFIX_SIZE in
+    esphome/core/helpers.cpp to accommodate the new maximum length.
+    """
     value = string(value)
     if re.match(r"^[a-z0-9-]{1,63}$", value, re.IGNORECASE) is not None:
         return value

esphome/core/application.h

@@ -102,9 +102,15 @@ class Application {
     arch_init();
     this->name_add_mac_suffix_ = name_add_mac_suffix;
     if (name_add_mac_suffix) {
-      const std::string mac_suffix = get_mac_address().substr(6);
-      this->name_ = name + "-" + mac_suffix;
-      this->friendly_name_ = friendly_name.empty() ? "" : friendly_name + " " + mac_suffix;
+      // MAC address suffix length (last 6 characters of 12-char MAC address string)
+      constexpr size_t mac_address_suffix_len = 6;
+      const std::string mac_addr = get_mac_address();
+      // Use pointer + offset to avoid substr() allocation
+      const char *mac_suffix_ptr = mac_addr.c_str() + mac_address_suffix_len;
+      this->name_ = make_name_with_suffix(name, '-', mac_suffix_ptr, mac_address_suffix_len);
+      if (!friendly_name.empty()) {
+        this->friendly_name_ = make_name_with_suffix(friendly_name, ' ', mac_suffix_ptr, mac_address_suffix_len);
+      }
     } else {
       this->name_ = name;
       this->friendly_name_ = friendly_name;

esphome/core/config.py

@@ -200,7 +200,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Required(CONF_NAME): cv.valid_name,
-            cv.Optional(CONF_FRIENDLY_NAME, ""): cv.string,
+            cv.Optional(CONF_FRIENDLY_NAME, ""): cv.All(cv.string, cv.Length(max=120)),
             cv.Optional(CONF_AREA): validate_area_config,
             cv.Optional(CONF_COMMENT): cv.string,
             cv.Required(CONF_BUILD_PATH): cv.string,

esphome/core/helpers.cpp

@@ -235,6 +235,30 @@ std::string str_sprintf(const char *fmt, ...) {
   return str;
 }
 
+// Maximum size for name with suffix: 120 (max friendly name) + 1 (separator) + 6 (MAC suffix) + 1 (null term)
+static constexpr size_t MAX_NAME_WITH_SUFFIX_SIZE = 128;
+
+std::string make_name_with_suffix(const std::string &name, char sep, const char *suffix_ptr, size_t suffix_len) {
+  char buffer[MAX_NAME_WITH_SUFFIX_SIZE];
+  size_t name_len = name.size();
+  size_t total_len = name_len + 1 + suffix_len;
+
+  // Silently truncate if needed: prioritize keeping the full suffix
+  if (total_len >= MAX_NAME_WITH_SUFFIX_SIZE) {
+    // NOTE: This calculation could underflow if suffix_len >= MAX_NAME_WITH_SUFFIX_SIZE - 2,
+    // but this is safe because this helper is only called with small suffixes:
+    // MAC suffixes (6-12 bytes), ".local" (5 bytes), etc.
+    name_len = MAX_NAME_WITH_SUFFIX_SIZE - suffix_len - 2;  // -2 for separator and null terminator
+    total_len = name_len + 1 + suffix_len;
+  }
+
+  memcpy(buffer, name.c_str(), name_len);
+  buffer[name_len] = sep;
+  memcpy(buffer + name_len + 1, suffix_ptr, suffix_len);
+  buffer[total_len] = '\0';
+  return std::string(buffer, total_len);
+}
+
 // Parsing & formatting
 
 size_t parse_hex(const char *str, size_t length, uint8_t *data, size_t count) {

esphome/core/helpers.h

@@ -354,6 +354,16 @@ std::string __attribute__((format(printf, 1, 3))) str_snprintf(const char *fmt,
 /// sprintf-like function returning std::string.
 std::string __attribute__((format(printf, 1, 2))) str_sprintf(const char *fmt, ...);
 
+/// Concatenate a name with a separator and suffix using an efficient stack-based approach.
+/// This avoids multiple heap allocations during string construction.
+/// Maximum name length supported is 120 characters for friendly names.
+/// @param name The base name string
+/// @param sep The separator character (e.g., '-', ' ', or '.')
+/// @param suffix_ptr Pointer to the suffix characters
+/// @param suffix_len Length of the suffix
+/// @return The concatenated string: name + sep + suffix
+std::string make_name_with_suffix(const std::string &name, char sep, const char *suffix_ptr, size_t suffix_len);
+
 ///@}
 
 /// @name Parsing & formatting