samples: net: update mqtt publisher

Added MQTT logging backend to publisher sample

Signed-off-by: Arif Balik <arifbalik@outlook.com>

Author: arifbalik

samples/net/mqtt_publisher/README.rst

@@ -221,6 +221,27 @@ broker or uses a different port number, modify the following values:
 	#define SOCKS5_PROXY_ADDR    SERVER_ADDR
 	#define SOCKS5_PROXY_PORT    1080
 
+MQTT logging backend
+====================
+
+The sample can be configured to use an MQTT logging backend, which allows log
+messages from the application to be published to an MQTT broker. This feature
+uses the same MQTT client instance as the main sample application.
+
+The MQTT logging backend uses the :c:func:`log_backend_mqtt_client_set` API to
+register an MQTT client for publishing log messages. The backend only uses the
+client's :c:func:`mqtt_publish` function and does not manage the client's
+connection lifecycle - this remains the application's responsibility.
+
+To enable the MQTT logging backend in the sample, build it with
+``-DEXTRA_CONF_FILE=overlay-log-backend-mqtt.conf`` parameter.
+
+Key configuration options available:
+
+- :kconfig:option:`CONFIG_LOG_BACKEND_MQTT_TOPIC_DEFAULT`: Topic for publishing logs (default: "zephyr/logs")
+- :kconfig:option:`CONFIG_LOG_BACKEND_MQTT_QOS`: QoS level for log messages (default: 0)
+- :kconfig:option:`CONFIG_LOG_BACKEND_MQTT_RETAIN`: Whether to retain log messages (default: disabled)
+- :kconfig:option:`CONFIG_LOG_BACKEND_MQTT_MAX_MSG_SIZE`: Maximum log message size (default: 256 bytes)
 
 Running on cc3220sf_launchxl
 ============================

samples/net/mqtt_publisher/overlay-log-backend-mqtt.conf

@@ -0,0 +1,5 @@
+CONFIG_LOG_BACKEND_MQTT=y
+CONFIG_LOG_BACKEND_MQTT_QOS=0
+CONFIG_LOG_BACKEND_MQTT_RETAIN=n
+CONFIG_LOG_BACKEND_MQTT_MAX_MSG_SIZE=1024
+CONFIG_LOG_MODE_DEFERRED=y

samples/net/mqtt_publisher/src/main.c

@@ -11,6 +11,9 @@ LOG_MODULE_REGISTER(net_mqtt_publisher_sample, LOG_LEVEL_DBG);
 #include <zephyr/net/socket.h>
 #include <zephyr/net/mqtt.h>
 #include <zephyr/random/random.h>
+#if defined(CONFIG_LOG_BACKEND_MQTT)
+#include <zephyr/logging/log_backend_mqtt.h>
+#endif
 
 #include <string.h>
 #include <errno.h>
@@ -172,6 +175,10 @@ void mqtt_evt_handler(struct mqtt_client *const client,
 		}
 #endif
 
+#if defined(CONFIG_LOG_BACKEND_MQTT)
+		log_backend_mqtt_client_set(client);
+#endif
+
 		break;
 
 	case MQTT_EVT_DISCONNECT:
@@ -180,6 +187,10 @@ void mqtt_evt_handler(struct mqtt_client *const client,
 		connected = false;
 		clear_fds();
 
+#if defined(CONFIG_LOG_BACKEND_MQTT)
+		log_backend_mqtt_client_set(NULL);
+#endif
+
 		break;
 
 	case MQTT_EVT_PUBACK: