fix(mqtt): address code-review feedback on Mqtt5ConnectionPropertyConfig

- Replace `unwrap_or(0/false)` ladder with conditional assignment from
  `Default::default()`. `None` no longer silently becomes `0`/`false` at
  the C boundary; unset fields stay at the C struct's zero default,
  which the ESP-IDF encoder skips so the broker applies the MQTT 5
  protocol default. Avoids surprises on fields whose protocol default
  differs from zero (e.g. Request Problem Information defaults to 1)
- Log an explanatory `error!` line before returning `ESP_ERR_INVALID_ARG`
  when `mqtt5_connection_property` is set without `protocol_version =
  Some(V5)`, so the misconfiguration is discoverable without grepping
  ESP-IDF error codes
- Restore minimal per-field doc comments giving units and the MQTT 5
  spec section, plus a struct-level note that fields may be added in
  minor releases (callers should use `..Default::default()`)
- Enable `CONFIG_MQTT_PROTOCOL_5=y` in CI sdkconfig.defaults so the new
  `#[cfg(esp_idf_mqtt_protocol_5)]` paths are compile-checked
- Add a CONNECT-properties usage snippet to the CHANGELOG entry

Author: daniil4udo

.github/configs/sdkconfig.defaults

@@ -42,6 +42,9 @@ CONFIG_BT_SPP_ENABLED=y
 # Support for TLS with a pre-shared key.
 #CONFIG_ESP_TLS_PSK_VERIFICATION=y
 
+# Compile-check the MQTT 5.0 code paths in CI.
+CONFIG_MQTT_PROTOCOL_5=y
+
 CONFIG_LWIP_PPP_SUPPORT=y
 #CONFIG_LWIP_SLIP_SUPPORT=y
 

CHANGELOG.md

@@ -29,7 +29,17 @@ remote_component = { name = "espressif/lan87xx", version = "1.*" }
 - MQTT: MQTT 5.0 CONNECT properties can now be set on `EspMqttClient` without deadlocking on `MQTT_API_LOCK`; they are applied inside the library between `esp_mqtt_client_init` and `esp_mqtt_client_start` via the new [`MqttClientConfiguration::mqtt5_connection_property`] field.
 
 ### Added
-- MQTT: `MqttProtocolVersion::V5` variant and [`Mqtt5ConnectionPropertyConfig`] struct exposing MQTT 5.0 CONNECT properties (session/will/message expiry intervals, receive/packet/topic-alias maxima, request-response/problem info, payload format indicator). Gated on `CONFIG_MQTT_PROTOCOL_5=y`.
+- MQTT: `MqttProtocolVersion::V5` variant and [`Mqtt5ConnectionPropertyConfig`] struct exposing MQTT 5.0 CONNECT properties (session/will/message expiry intervals, receive/packet/topic-alias maxima, request-response/problem info, payload format indicator). Gated on `CONFIG_MQTT_PROTOCOL_5=y`. To negotiate MQTT 5 and set properties:
+  ```rust
+  MqttClientConfiguration {
+      protocol_version: Some(MqttProtocolVersion::V5),
+      mqtt5_connection_property: Some(Mqtt5ConnectionPropertyConfig {
+          session_expiry_interval: Some(60),
+          ..Default::default()
+      }),
+      ..Default::default()
+  }
+  ```
 - Compatibility with ESP-IDF V6.0, and some pre-release 6.0.x.
 - Added support for the Generic Ethernet PHY driver: particularly useful on ESP-IDF 6.0+ as it is built-in.
 

src/mqtt/client.rs

@@ -7,6 +7,7 @@ extern crate alloc;
 use alloc::boxed::Box;
 use alloc::sync::Arc;
 
+use ::log::error;
 use embedded_svc::mqtt::client::{asynch, Client, Connection, Enqueue, ErrorType, Publish};
 
 use crate::private::unblocker::Unblocker;
@@ -46,17 +47,27 @@ impl From<MqttProtocolVersion> for esp_mqtt_protocol_ver_t {
 }
 
 /// MQTT 5.0 CONNECT properties. Requires `protocol_version: Some(MqttProtocolVersion::V5)`.
+/// `None` on a field leaves the property unset and the broker applies the MQTT 5 default.
 #[cfg(esp_idf_mqtt_protocol_5)]
 #[derive(Debug, Clone, Default)]
 pub struct Mqtt5ConnectionPropertyConfig {
+    /// Session Expiry Interval, seconds (MQTT5 §3.1.2.11.2).
     pub session_expiry_interval: Option<u32>,
+    /// Will Delay Interval, seconds (MQTT5 §3.1.3.2.2).
     pub will_delay_interval: Option<u32>,
+    /// Receive Maximum, max concurrent inbound QoS>0 PUBLISHes (MQTT5 §3.1.2.11.3).
     pub receive_maximum: Option<u16>,
+    /// Maximum Packet Size the client accepts, bytes (MQTT5 §3.1.2.11.4).
     pub maximum_packet_size: Option<u32>,
+    /// Topic Alias Maximum the broker may use (MQTT5 §3.1.2.11.5).
     pub topic_alias_maximum: Option<u16>,
+    /// Request Response Information (MQTT5 §3.1.2.11.6). C field: `request_resp_info`.
     pub request_response_info: Option<bool>,
+    /// Request Problem Information (MQTT5 §3.1.2.11.7). Protocol default is `true`.
     pub request_problem_info: Option<bool>,
+    /// Will Message Expiry Interval, seconds (MQTT5 §3.3.2.3.3).
     pub message_expiry_interval: Option<u32>,
+    /// Will Payload Format Indicator: UTF-8 (`true`) or bytes (`false`) (MQTT5 §3.3.2.3.2).
     pub payload_format_indicator: Option<bool>,
 }
 
@@ -540,20 +551,39 @@ impl<'a> EspMqttClient<'a> {
         #[cfg(esp_idf_mqtt_protocol_5)]
         if let Some(props) = conf.mqtt5_connection_property.as_ref() {
             if conf.protocol_version != Some(MqttProtocolVersion::V5) {
+                error!(
+                    "mqtt5_connection_property requires protocol_version = Some(MqttProtocolVersion::V5)"
+                );
                 return Err(EspError::from_infallible::<ESP_ERR_INVALID_ARG>());
             }
-            let c_props = esp_mqtt5_connection_property_config_t {
-                session_expiry_interval: props.session_expiry_interval.unwrap_or(0),
-                will_delay_interval: props.will_delay_interval.unwrap_or(0),
-                receive_maximum: props.receive_maximum.unwrap_or(0),
-                maximum_packet_size: props.maximum_packet_size.unwrap_or(0),
-                topic_alias_maximum: props.topic_alias_maximum.unwrap_or(0),
-                request_resp_info: props.request_response_info.unwrap_or(false),
-                request_problem_info: props.request_problem_info.unwrap_or(false),
-                message_expiry_interval: props.message_expiry_interval.unwrap_or(0),
-                payload_format_indicator: props.payload_format_indicator.unwrap_or(false),
-                ..Default::default()
-            };
+            let mut c_props = esp_mqtt5_connection_property_config_t::default();
+            if let Some(v) = props.session_expiry_interval {
+                c_props.session_expiry_interval = v;
+            }
+            if let Some(v) = props.will_delay_interval {
+                c_props.will_delay_interval = v;
+            }
+            if let Some(v) = props.receive_maximum {
+                c_props.receive_maximum = v;
+            }
+            if let Some(v) = props.maximum_packet_size {
+                c_props.maximum_packet_size = v;
+            }
+            if let Some(v) = props.topic_alias_maximum {
+                c_props.topic_alias_maximum = v;
+            }
+            if let Some(v) = props.request_response_info {
+                c_props.request_resp_info = v;
+            }
+            if let Some(v) = props.request_problem_info {
+                c_props.request_problem_info = v;
+            }
+            if let Some(v) = props.message_expiry_interval {
+                c_props.message_expiry_interval = v;
+            }
+            if let Some(v) = props.payload_format_indicator {
+                c_props.payload_format_indicator = v;
+            }
             esp!(unsafe { esp_mqtt5_client_set_connect_property(client.raw_client, &c_props) })?;
         }