shell: mqtt: use topic levels

Change topic from <device_id>_rx (and tx) to <device_id>/sh/rx.

This allows use of wildcards. E.g. subscribe to all devices "+/sh/tx".

Level "sh" is added to the topic to make it less generic and prevent
potential clashes with other topics.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>

Author: Jeppe Odgaard

doc/releases/migration-guide-4.3.rst

@@ -41,6 +41,13 @@ Networking
 Other subsystems
 ****************
 
+Shell
+=====
+
+* The MQTT topics related to  :kconfig:option:`SHELL_BACKEND_MQTT`  have been renamed. Renamed
+  ``<device_id>_rx`` to ``<device_id>/sh/rx`` and ``<device_id>_tx`` to ``<device_id>/sh/rx``.
+  (:github:`92677`).
+
 Modules
 *******
 

include/zephyr/shell/shell_mqtt.h

@@ -25,7 +25,7 @@ extern "C" {
 #define SH_MQTT_BUFFER_SIZE 64
 #define DEVICE_ID_BIN_MAX_SIZE 3
 #define DEVICE_ID_HEX_MAX_SIZE ((DEVICE_ID_BIN_MAX_SIZE * 2) + 1)
-#define SH_MQTT_TOPIC_MAX_SIZE DEVICE_ID_HEX_MAX_SIZE + 3
+#define SH_MQTT_TOPIC_MAX_SIZE DEVICE_ID_HEX_MAX_SIZE + 6 /* /sh/rx */
 
 extern const struct shell_transport_api shell_mqtt_transport_api;
 

subsys/shell/backends/shell_mqtt.c

@@ -657,8 +657,8 @@ static int init(const struct shell_transport *transport, const void *config,
 
 	LOG_DBG("Client ID is %s", sh->device_id);
 
-	(void)snprintf(sh->pub_topic, SH_MQTT_TOPIC_MAX_SIZE, "%s_tx", sh->device_id);
-	(void)snprintf(sh->sub_topic, SH_MQTT_TOPIC_MAX_SIZE, "%s_rx", sh->device_id);
+	(void)snprintf(sh->pub_topic, SH_MQTT_TOPIC_MAX_SIZE, "%s/sh/tx", sh->device_id);
+	(void)snprintf(sh->sub_topic, SH_MQTT_TOPIC_MAX_SIZE, "%s/sh/rx", sh->device_id);
 
 	ring_buf_init(&sh->rx_rb, RX_RB_SIZE, sh->rx_rb_buf);