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[mqtt] Use stack buffer for discovery topic to avoid heap allocation

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This commit is contained in:
J. Nick Koston
2026-02-06 20:48:18 +01:00
parent 86f91eed2f
commit b454cac1dc
2 changed files with 18 additions and 12 deletions
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21
esphome/components/mqtt/mqtt_component.cpp
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@@ -34,10 +34,7 @@ inline char *append_char(char *p, char c) {
// MQTT_COMPONENT_TYPE_MAX_LEN, MQTT_SUFFIX_MAX_LEN, and MQTT_DEFAULT_TOPIC_MAX_LEN are in mqtt_component.h. // MQTT_COMPONENT_TYPE_MAX_LEN, MQTT_SUFFIX_MAX_LEN, and MQTT_DEFAULT_TOPIC_MAX_LEN are in mqtt_component.h.
// ESPHOME_DEVICE_NAME_MAX_LEN and OBJECT_ID_MAX_LEN are defined in entity_base.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. // This ensures the stack buffers below are always large enough.
static constexpr size_t DISCOVERY_PREFIX_MAX_LEN = 64; // Validated in Python: cv.Length(max=64) // MQTT_DISCOVERY_PREFIX_MAX_LEN and MQTT_DISCOVERY_TOPIC_MAX_LEN are defined in mqtt_component.h
// 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;
// Function implementation of LOG_MQTT_COMPONENT macro to reduce code size // Function implementation of LOG_MQTT_COMPONENT macro to reduce code size
void log_mqtt_component(const char *tag, MQTTComponent *obj, bool state_topic, bool command_topic) { void log_mqtt_component(const char *tag, MQTTComponent *obj, bool state_topic, bool command_topic) {
@@ -54,15 +51,15 @@ void MQTTComponent::set_subscribe_qos(uint8_t qos) { this->subscribe_qos_ = qos;
void MQTTComponent::set_retain(bool retain) { this->retain_ = retain; } void MQTTComponent::set_retain(bool retain) { this->retain_ = retain; }
std::string MQTTComponent::get_discovery_topic_(const MQTTDiscoveryInfo &discovery_info) const { StringRef MQTTComponent::get_discovery_topic_to_(std::span<char, MQTT_DISCOVERY_TOPIC_MAX_LEN> buf,
const MQTTDiscoveryInfo &discovery_info) const {
char sanitized_name[ESPHOME_DEVICE_NAME_MAX_LEN + 1]; char sanitized_name[ESPHOME_DEVICE_NAME_MAX_LEN + 1];
str_sanitize_to(sanitized_name, App.get_name().c_str()); str_sanitize_to(sanitized_name, App.get_name().c_str());
const char *comp_type = this->component_type(); const char *comp_type = this->component_type();
char object_id_buf[OBJECT_ID_MAX_LEN]; char object_id_buf[OBJECT_ID_MAX_LEN];
StringRef object_id = this->get_default_object_id_to_(object_id_buf); StringRef object_id = this->get_default_object_id_to_(object_id_buf);
char buf[DISCOVERY_TOPIC_MAX_LEN]; char *p = buf.data();
char *p = buf;
p = append_str(p, discovery_info.prefix.data(), discovery_info.prefix.size()); p = append_str(p, discovery_info.prefix.data(), discovery_info.prefix.size());
p = append_char(p, '/'); p = append_char(p, '/');
@@ -72,8 +69,9 @@ std::string MQTTComponent::get_discovery_topic_(const MQTTDiscoveryInfo &discove
p = append_char(p, '/'); p = append_char(p, '/');
p = append_str(p, object_id.c_str(), object_id.size()); p = append_str(p, object_id.c_str(), object_id.size());
p = append_str(p, "/config", 7); p = append_str(p, "/config", 7);
*p = '\0';
return std::string(buf, p - buf); return StringRef(buf.data(), p - buf.data());
} }
StringRef MQTTComponent::get_default_topic_for_to_(std::span<char, MQTT_DEFAULT_TOPIC_MAX_LEN> buf, const char *suffix, StringRef MQTTComponent::get_default_topic_for_to_(std::span<char, MQTT_DEFAULT_TOPIC_MAX_LEN> buf, const char *suffix,
@@ -182,16 +180,19 @@ bool MQTTComponent::publish_json(const char *topic, const json::json_build_t &f)
bool MQTTComponent::send_discovery_() { bool MQTTComponent::send_discovery_() {
const MQTTDiscoveryInfo &discovery_info = global_mqtt_client->get_discovery_info(); const MQTTDiscoveryInfo &discovery_info = global_mqtt_client->get_discovery_info();
char discovery_topic_buf[MQTT_DISCOVERY_TOPIC_MAX_LEN];
StringRef discovery_topic = this->get_discovery_topic_to_(discovery_topic_buf, discovery_info);
if (discovery_info.clean) { if (discovery_info.clean) {
ESP_LOGV(TAG, "'%s': Cleaning discovery", this->friendly_name_().c_str()); ESP_LOGV(TAG, "'%s': Cleaning discovery", this->friendly_name_().c_str());
return global_mqtt_client->publish(this->get_discovery_topic_(discovery_info), "", 0, this->qos_, true); return global_mqtt_client->publish(discovery_topic.c_str(), "", 0, this->qos_, true);
} }
ESP_LOGV(TAG, "'%s': Sending discovery", this->friendly_name_().c_str()); ESP_LOGV(TAG, "'%s': Sending discovery", this->friendly_name_().c_str());
// NOLINTBEGIN(clang-analyzer-cplusplus.NewDeleteLeaks) false positive with ArduinoJson // NOLINTBEGIN(clang-analyzer-cplusplus.NewDeleteLeaks) false positive with ArduinoJson
return global_mqtt_client->publish_json( return global_mqtt_client->publish_json(
this->get_discovery_topic_(discovery_info), discovery_topic.c_str(),
[this](JsonObject root) { [this](JsonObject root) {
SendDiscoveryConfig config; SendDiscoveryConfig config;
config.state_topic = true; config.state_topic = true;

9
esphome/components/mqtt/mqtt_component.h
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@@ -32,6 +32,10 @@ static constexpr size_t MQTT_TOPIC_PREFIX_MAX_LEN = 64; // Validated in Python:
// Format: prefix + "/" + type + "/" + object_id + "/" + suffix + null // Format: prefix + "/" + type + "/" + object_id + "/" + suffix + null
static constexpr size_t MQTT_DEFAULT_TOPIC_MAX_LEN = static constexpr size_t MQTT_DEFAULT_TOPIC_MAX_LEN =
MQTT_TOPIC_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 1 + MQTT_SUFFIX_MAX_LEN + 1; MQTT_TOPIC_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 1 + MQTT_SUFFIX_MAX_LEN + 1;
static constexpr size_t MQTT_DISCOVERY_PREFIX_MAX_LEN = 64; // Validated in Python: cv.Length(max=64)
// Format: prefix + "/" + type + "/" + name + "/" + object_id + "/config" + null
static constexpr size_t MQTT_DISCOVERY_TOPIC_MAX_LEN = MQTT_DISCOVERY_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN +
1 + ESPHOME_DEVICE_NAME_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 7 + 1;
class MQTTComponent; // Forward declaration class MQTTComponent; // Forward declaration
void log_mqtt_component(const char *tag, MQTTComponent *obj, bool state_topic, bool command_topic); void log_mqtt_component(const char *tag, MQTTComponent *obj, bool state_topic, bool command_topic);
@@ -263,8 +267,9 @@ class MQTTComponent : public Component {
void subscribe_json(const std::string &topic, const mqtt_json_callback_t &callback, uint8_t qos = 0); void subscribe_json(const std::string &topic, const mqtt_json_callback_t &callback, uint8_t qos = 0);
protected: protected:
/// Helper method to get the discovery topic for this component. /// Helper method to get the discovery topic for this component into a buffer.
std::string get_discovery_topic_(const MQTTDiscoveryInfo &discovery_info) const; StringRef get_discovery_topic_to_(std::span<char, MQTT_DISCOVERY_TOPIC_MAX_LEN> buf,
const MQTTDiscoveryInfo &discovery_info) const;
/** Get this components state/command/... topic into a buffer. /** Get this components state/command/... topic into a buffer.
* *