ESP32-C3在MQTT访问时出现“Last error code reported from esp-tls: 0x8001”和问题的分析（1）

笔者最近在调试MQTT时，遇到了不少错误。之前笔者遇到了“transport_base: Poll timeout or error”问题，这才写了“ESP32-C3在MQTT访问时出现“transport_base: Poll timeout or error”问题的分析”系列文章。但最近笔者又遇到了另一些错误：“Last error code reported from esp-tls: 0x8001”和“Last error code reported from esp-tls: 0x8006”。本文就来针对于新出的这几个错误进行原因查找和问题定位。

弯弓射箭照直镚，先来直接找出“Last error code reported from esp-tls: 0x8001”打印的具体位置。在ESP32-IDF源码工程下搜索，定位到是在main.c的mqtt_event_handler函数即MQTT事件回调函数中：

/*
 * @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=%" PRIi32, base, event_id);
    //esp_mqtt_event_handle_t event = event_data;
    //esp_mqtt_client_handle_t client = event->client;
    //int msg_id;
    event = event_data;
    client = event->client;

    switch ((esp_mqtt_event_id_t)event_id) {
    case MQTT_EVENT_CONNECTED:
        ESP_LOGI(TAG, "MQTT_EVENT_CONNECTED");
       
        msg_id = esp_mqtt_client_subscribe(client, DOWNSTREAM_TOPIC, 1);
        ESP_LOGI(TAG, "sent subscribe 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, "sensor", "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);
        if (strncmp(event->data, "send binary please", event->data_len) == 0) {
            ESP_LOGI(TAG, "Sending the binary");
            send_binary(client);
        }

        cJSON *pJsonRoot = cJSON_Parse(event->data);
        if (pJsonRoot == NULL)
        {
            printf("Error before: [%s]\n", cJSON_GetErrorPtr());
            break;
        }

        //msg_id = esp_mqtt_client_publish(client, "sensor", "data3", 0, 0, 0);

        cJSON_Delete(pJsonRoot);
        break;
    case MQTT_EVENT_ERROR:
        ESP_LOGI(TAG, "MQTT_EVENT_ERROR");
        if (event->error_handle->error_type == MQTT_ERROR_TYPE_TCP_TRANSPORT) {
            ESP_LOGI(TAG, "Last error code reported from esp-tls: 0x%x", event->error_handle->esp_tls_last_esp_err);
            ESP_LOGI(TAG, "Last tls stack error number: 0x%x", event->error_handle->esp_tls_stack_err);
            ESP_LOGI(TAG, "Last captured errno : %d (%s)",  event->error_handle->esp_transport_sock_errno,
                     strerror(event->error_handle->esp_transport_sock_errno));
        } else if (event->error_handle->error_type == MQTT_ERROR_TYPE_CONNECTION_REFUSED) {
            ESP_LOGI(TAG, "Connection refused error: 0x%x", event->error_handle->connect_return_code);
        } else {
            ESP_LOGW(TAG, "Unknown error type: 0x%x", event->error_handle->error_type);
        }
        break;
    default:
        ESP_LOGI(TAG, "Other event id:%d", event->event_id);
        break;
    }
}

重点关注以下代码片段：

    case MQTT_EVENT_ERROR:
        ESP_LOGI(TAG, "MQTT_EVENT_ERROR");
        if (event->error_handle->error_type == MQTT_ERROR_TYPE_TCP_TRANSPORT) {
            ESP_LOGI(TAG, "Last error code reported from esp-tls: 0x%x", event->error_handle->esp_tls_last_esp_err);
            ESP_LOGI(TAG, "Last tls stack error number: 0x%x", event->error_handle->esp_tls_stack_err);
            ESP_LOGI(TAG, "Last captured errno : %d (%s)",  event->error_handle->esp_transport_sock_errno,
                     strerror(event->error_handle->esp_transport_sock_errno));
        } else if (event->error_handle->error_type == MQTT_ERROR_TYPE_CONNECTION_REFUSED) {
            ESP_LOGI(TAG, "Connection refused error: 0x%x", event->error_handle->connect_return_code);
        } else {
            ESP_LOGW(TAG, "Unknown error type: 0x%x", event->error_handle->error_type);
        }
        break;

这部分就对应上报出错信息中的：

这个0x8001具体代表的是什么意思呢？其定义是在components\components\esp_common\src\esp_err_to_name.c中，相关代码如下：

#   ifdef      ESP_ERR_ESP_TLS_CANNOT_RESOLVE_HOSTNAME
    ERR_TBL_IT(ESP_ERR_ESP_TLS_CANNOT_RESOLVE_HOSTNAME),        /* 32769 0x8001 Error if hostname couldn't be resolved
                                                                                upon tls connection */
#   endif

由定义可知，0x8001即32769代表的是tls连接时无法解析主机名。

那么为什么会引发这个错误？就需要进一步跟进了。