feat(examples): Add example of using mqtt client on linux

examples/mqtt/CMakeLists.txt

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+# The following four lines of boilerplate have to be in your project's CMakeLists
+# in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.16)
+
+# (Not part of the boilerplate)
+# This example uses an extra component for common functions such as Wi-Fi and Ethernet connection.
+set(EXTRA_COMPONENT_DIRS $ENV{IDF_PATH}/examples/common_components/protocol_examples_common)
+
+if(${IDF_TARGET} STREQUAL "linux")
+    list(APPEND EXTRA_COMPONENT_DIRS "../../common_components/linux_compat/freertos"
+            "$ENV{IDF_PATH}/examples/protocols/linux_stubs/esp_stubs")
+    set(COMPONENTS main nvs_flash)
+endif()
+
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(mqtt_tcp)

examples/mqtt/main/CMakeLists.txt

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+set(requires "")
+if(${IDF_TARGET} STREQUAL "linux")
+    list(APPEND requires esp_stubs esp-tls protocol_examples_common mqtt)
+endif()
+idf_component_register(SRCS "app_main.c"
+                    INCLUDE_DIRS "."
+                    REQUIRES ${requires})
+target_compile_options(${COMPONENT_LIB} PRIVATE "-Wno-format")

examples/mqtt/main/Kconfig.projbuild

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+menu "Example Configuration"
+
+    config BROKER_URL
+        string "Broker URL"
+        default "mqtt://mqtt.eclipseprojects.io"
+        help
+            URL of the broker to connect to
+
+    config BROKER_URL_FROM_STDIN
+        bool
+        default y if BROKER_URL = "FROM_STDIN"
+
+endmenu

examples/mqtt/main/app_main.c

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+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+#include "esp_system.h"
+#include "nvs_flash.h"
+#include "esp_event.h"
+#include "esp_netif.h"
+#include "protocol_examples_common.h"
+
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_system.h"
+
+#include "esp_log.h"
+#include "mqtt_client.h"
+
+static const char *TAG = "MQTT_EXAMPLE";
+
+
+static void log_error_if_nonzero(const char *message, int error_code)
+{
+    if (error_code != 0) {
+        ESP_LOGE(TAG, "Last error %s: 0x%x", message, error_code);
+    }
+}
+
+/*
+ * @brief Event handler registered to receive MQTT events
+ *
+ *  This function is called by the MQTT client event loop.
+ *
+ * @param handler_args user data registered to the event.
+ * @param base Event base for the handler(always MQTT Base in this example).
+ * @param event_id The id for the received event.
+ * @param event_data The data for the event, esp_mqtt_event_handle_t.
+ */
+static void mqtt_event_handler(void *handler_args, esp_event_base_t base, int32_t event_id, void *event_data)
+{
+    ESP_LOGD(TAG, "Event dispatched from event loop base=%s, event_id=%d", base, event_id);
+    esp_mqtt_event_handle_t event = event_data;
+    esp_mqtt_client_handle_t client = event->client;
+    int msg_id;
+    switch ((esp_mqtt_event_id_t)event_id) {
+    case MQTT_EVENT_CONNECTED:
+        ESP_LOGI(TAG, "MQTT_EVENT_CONNECTED");
+        msg_id = esp_mqtt_client_publish(client, "/topic/qos1", "data_3", 0, 1, 0);
+        ESP_LOGI(TAG, "sent publish successful, msg_id=%d", msg_id);
+
+        msg_id = esp_mqtt_client_subscribe(client, "/topic/qos0", 0);
+        ESP_LOGI(TAG, "sent subscribe successful, msg_id=%d", msg_id);
+
+        msg_id = esp_mqtt_client_subscribe(client, "/topic/qos1", 1);
+        ESP_LOGI(TAG, "sent subscribe successful, msg_id=%d", msg_id);
+
+        msg_id = esp_mqtt_client_unsubscribe(client, "/topic/qos1");
+        ESP_LOGI(TAG, "sent unsubscribe successful, msg_id=%d", msg_id);
+        break;
+    case MQTT_EVENT_DISCONNECTED:
+        ESP_LOGI(TAG, "MQTT_EVENT_DISCONNECTED");
+        break;
+
+    case MQTT_EVENT_SUBSCRIBED:
+        ESP_LOGI(TAG, "MQTT_EVENT_SUBSCRIBED, msg_id=%d", event->msg_id);
+        msg_id = esp_mqtt_client_publish(client, "/topic/qos0", "data", 0, 0, 0);
+        ESP_LOGI(TAG, "sent publish successful, msg_id=%d", msg_id);
+        break;
+    case MQTT_EVENT_UNSUBSCRIBED:
+        ESP_LOGI(TAG, "MQTT_EVENT_UNSUBSCRIBED, msg_id=%d", event->msg_id);
+        break;
+    case MQTT_EVENT_PUBLISHED:
+        ESP_LOGI(TAG, "MQTT_EVENT_PUBLISHED, msg_id=%d", event->msg_id);
+        break;
+    case MQTT_EVENT_DATA:
+        ESP_LOGI(TAG, "MQTT_EVENT_DATA");
+        printf("TOPIC=%.*s\r\n", event->topic_len, event->topic);
+        printf("DATA=%.*s\r\n", event->data_len, event->data);
+        break;
+    case MQTT_EVENT_ERROR:
+        ESP_LOGI(TAG, "MQTT_EVENT_ERROR");
+        if (event->error_handle->error_type == MQTT_ERROR_TYPE_TCP_TRANSPORT) {
+            log_error_if_nonzero("reported from esp-tls", event->error_handle->esp_tls_last_esp_err);
+            log_error_if_nonzero("reported from tls stack", event->error_handle->esp_tls_stack_err);
+            log_error_if_nonzero("captured as transport's socket errno",  event->error_handle->esp_transport_sock_errno);
+            ESP_LOGI(TAG, "Last errno string (%s)", strerror(event->error_handle->esp_transport_sock_errno));
+
+        }
+        break;
+    default:
+        ESP_LOGI(TAG, "Other event id:%d", event->event_id);
+        break;
+    }
+}
+
+static void mqtt_app_start(void)
+{
+    esp_mqtt_client_config_t mqtt_cfg = {
+        .broker.address.uri = CONFIG_BROKER_URL,
+    };
+
+    esp_mqtt_client_handle_t client = esp_mqtt_client_init(&mqtt_cfg);
+    /* The last argument may be used to pass data to the event handler, in this example mqtt_event_handler */
+    esp_mqtt_client_register_event(client, ESP_EVENT_ANY_ID, mqtt_event_handler, NULL);
+    esp_mqtt_client_start(client);
+}
+
+void app_main(void)
+{
+    ESP_LOGI(TAG, "[APP] Startup..");
+    ESP_LOGI(TAG, "[APP] Free memory: %d bytes", esp_get_free_heap_size());
+    ESP_LOGI(TAG, "[APP] IDF version: %s", esp_get_idf_version());
+
+    esp_log_level_set("*", ESP_LOG_INFO);
+    esp_log_level_set("mqtt_client", ESP_LOG_VERBOSE);
+    esp_log_level_set("MQTT_EXAMPLE", ESP_LOG_VERBOSE);
+    esp_log_level_set("TRANSPORT_BASE", ESP_LOG_VERBOSE);
+    esp_log_level_set("esp-tls", ESP_LOG_VERBOSE);
+    esp_log_level_set("TRANSPORT", ESP_LOG_VERBOSE);
+    esp_log_level_set("outbox", ESP_LOG_VERBOSE);
+
+    ESP_ERROR_CHECK(nvs_flash_init());
+    ESP_ERROR_CHECK(esp_netif_init());
+    ESP_ERROR_CHECK(esp_event_loop_create_default());
+
+    /* This helper function configures Wi-Fi or Ethernet, as selected in menuconfig.
+     * Read "Establishing Wi-Fi or Ethernet Connection" section in
+     * examples/protocols/README.md for more information about this function.
+     */
+    ESP_ERROR_CHECK(example_connect());
+
+    mqtt_app_start();
+    vTaskDelay(pdMS_TO_TICKS(100000));
+}

examples/mqtt/sdkconfig.defaults

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+CONFIG_IDF_TARGET="linux"
+# CONFIG_ESP_EVENT_POST_FROM_ISR is not set