feat(esp_modem): Add mqtt example in AT-only mode

Similar to pppos-client, but without PPP mode.
This uses standard mqtt client and a loopback service that forwards the
TCP traffic from localhost to the modem and vice-versa.
(next step is to create a dedicated tcp-transport layer for modem)

components/esp_modem/examples/modem_tcp_client/CMakeLists.txt

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+# The following lines of boilerplate have to be in your project's CMakeLists
+# in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.5)
+
+set(EXTRA_COMPONENT_DIRS "../..")
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(modem_tcp_client)
+
+

components/esp_modem/examples/modem_tcp_client/README.md

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+# Modem TCP client
+
+(See the README.md file in the upper level 'examples' directory for more information about examples.)
+
+## Overview
+This example demonstrates how to act as a MQTT client using modem's TCP commands (provided, the device supports "socket" related commands)
+
+### Supported IDF versions
+
+This example is only supported from `v4.1`, as this is the default dependency of `esp-modem` component. 

components/esp_modem/examples/modem_tcp_client/main/CMakeLists.txt

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+idf_component_register(SRCS "modem_client.cpp"
+                            "sock_dce.cpp"
+                            "sock_commands.cpp"
+                       INCLUDE_DIRS ".")
+target_compile_options(${COMPONENT_LIB} PRIVATE "-Wno-format")

components/esp_modem/examples/modem_tcp_client/main/Kconfig.projbuild

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+menu "Example Configuration"
+
+    choice EXAMPLE_MODEM_DEVICE
+        prompt "Choose supported modem device (DCE)"
+        default EXAMPLE_MODEM_DEVICE_BG96
+        help
+            Select modem device connected to the ESP DTE.
+        config EXAMPLE_MODEM_DEVICE_SIM800
+            bool "SIM800"
+            help
+                SIMCom SIM800L is a GSM/GPRS module.
+                It supports Quad-band 850/900/1800/1900MHz.
+        config EXAMPLE_MODEM_DEVICE_BG96
+            bool "BG96"
+            help
+                Quectel BG96 is a series of LTE Cat M1/Cat NB1/EGPRS module.
+        config EXAMPLE_MODEM_DEVICE_SIM7600
+            bool "SIM7600"
+            help
+                SIM7600 is Multi-Band LTE-TDD/LTE-FDD/HSPA+ and GSM/GPRS/EDGE module
+    endchoice
+
+    config EXAMPLE_MODEM_APN
+        string "Set MODEM APN"
+        default "internet"
+        help
+            Set APN (Access Point Name), a logical name to choose data network
+
+    config EXAMPLE_MODEM_PPP_AUTH_USERNAME
+        string "Set username for authentication"
+        default "espressif"
+        depends on !EXAMPLE_MODEM_PPP_AUTH_NONE
+        help
+            Set username for PPP Authentication.
+
+    config EXAMPLE_MODEM_PPP_AUTH_PASSWORD
+        string "Set password for authentication"
+        default "esp32"
+        depends on !EXAMPLE_MODEM_PPP_AUTH_NONE
+        help
+            Set password for PPP Authentication.
+
+    config EXAMPLE_MODEM_PPP_AUTH_NONE
+        bool "Skip PPP authentication"
+        default n
+        help
+            Set to true for the PPP client to skip authentication
+
+    config EXAMPLE_SEND_MSG
+        bool "Short message (SMS)"
+        default n
+        help
+            Select this, the modem will send a short message before power off.
+
+    if EXAMPLE_SEND_MSG
+        config EXAMPLE_SEND_MSG_PEER_PHONE_NUMBER
+            string "Peer Phone Number (with area code)"
+            default "+8610086"
+            help
+                Enter the peer phone number that you want to send message to.
+    endif
+
+    config EXAMPLE_NEED_SIM_PIN
+        bool "SIM PIN needed"
+        default n
+        help
+            Enable to set SIM PIN before starting the example
+
+    config EXAMPLE_SIM_PIN
+        string "Set SIM PIN"
+        default "1234"
+        depends on EXAMPLE_NEED_SIM_PIN
+        help
+            Pin to unlock the SIM
+
+    menu "UART Configuration"
+        config EXAMPLE_MODEM_UART_TX_PIN
+            int "TXD Pin Number"
+            default 25
+            range 0 31
+            help
+                Pin number of UART TX.
+
+        config EXAMPLE_MODEM_UART_RX_PIN
+            int "RXD Pin Number"
+            default 26
+            range 0 31
+            help
+                Pin number of UART RX.
+
+        config EXAMPLE_MODEM_UART_RTS_PIN
+            int "RTS Pin Number"
+            default 27
+            range 0 31
+            help
+                Pin number of UART RTS.
+
+        config EXAMPLE_MODEM_UART_CTS_PIN
+            int "CTS Pin Number"
+            default 23
+            range 0 31
+            help
+                Pin number of UART CTS.
+
+        config EXAMPLE_MODEM_UART_EVENT_TASK_STACK_SIZE
+            int "UART Event Task Stack Size"
+            range 2000 6000
+            default 2048
+            help
+                Stack size of UART event task.
+
+        config EXAMPLE_MODEM_UART_EVENT_TASK_PRIORITY
+            int "UART Event Task Priority"
+            range 3 22
+            default 5
+            help
+                Priority of UART event task.
+
+        config EXAMPLE_MODEM_UART_EVENT_QUEUE_SIZE
+            int "UART Event Queue Size"
+            range 10 40
+            default 30
+            help
+                Length of UART event queue.
+
+        config EXAMPLE_MODEM_UART_PATTERN_QUEUE_SIZE
+            int "UART Pattern Queue Size"
+            range 10 40
+            default 20
+            help
+                Length of UART pattern queue.
+
+        config EXAMPLE_MODEM_UART_TX_BUFFER_SIZE
+            int "UART TX Buffer Size"
+            range 256 2048
+            default 512
+            help
+                Buffer size of UART TX buffer.
+
+        config EXAMPLE_MODEM_UART_RX_BUFFER_SIZE
+            int "UART RX Buffer Size"
+            range 256 2048
+            default 1024
+            help
+                Buffer size of UART RX buffer.
+    endmenu
+
+endmenu

components/esp_modem/examples/modem_tcp_client/main/modem_client.cpp

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+/* PPPoS Client Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include <string>
+#include "freertos/FreeRTOS.h"
+#include "freertos/event_groups.h"
+#include "esp_netif.h"
+#include "esp_netif_ppp.h"
+#include "mqtt_client.h"
+#include "esp_modem_config.h"
+#include "cxx_include/esp_modem_api.hpp"
+#include "sock_dce.hpp"
+#include "esp_log.h"
+
+#define BROKER_URL "mqtt://mqtt.eclipseprojects.io"
+
+static const char *TAG = "modem_client";
+static EventGroupHandle_t event_group = NULL;
+static const int CONNECT_BIT = BIT0;
+static const int GOT_DATA_BIT = BIT2;
+
+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 = (esp_mqtt_event_handle_t)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_subscribe(client, "/topic/esp-pppos", 0);
+        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, "/topic/esp-pppos", "esp32-pppos", 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);
+        xEventGroupSetBits(event_group, GOT_DATA_BIT);
+        break;
+    case MQTT_EVENT_ERROR:
+        ESP_LOGI(TAG, "MQTT_EVENT_ERROR");
+        break;
+    default:
+        ESP_LOGI(TAG, "MQTT other event id: %d", event->event_id);
+        break;
+    }
+}
+
+
+
+
+
+
+
+
+extern "C" void app_main(void)
+{
+
+    /* Init and register system/core components */
+    ESP_ERROR_CHECK(esp_netif_init());
+    ESP_ERROR_CHECK(esp_event_loop_create_default());
+
+    event_group = xEventGroupCreate();
+
+    /* Configure and create the UART DTE */
+    esp_modem_dte_config_t dte_config = ESP_MODEM_DTE_DEFAULT_CONFIG();
+    /* setup UART specific configuration based on kconfig options */
+    dte_config.uart_config.tx_io_num = CONFIG_EXAMPLE_MODEM_UART_TX_PIN;
+    dte_config.uart_config.rx_io_num = CONFIG_EXAMPLE_MODEM_UART_RX_PIN;
+    dte_config.uart_config.rts_io_num = CONFIG_EXAMPLE_MODEM_UART_RTS_PIN;
+    dte_config.uart_config.cts_io_num = CONFIG_EXAMPLE_MODEM_UART_CTS_PIN;
+    dte_config.uart_config.rx_buffer_size = CONFIG_EXAMPLE_MODEM_UART_RX_BUFFER_SIZE;
+    dte_config.uart_config.tx_buffer_size = CONFIG_EXAMPLE_MODEM_UART_TX_BUFFER_SIZE;
+    dte_config.uart_config.event_queue_size = CONFIG_EXAMPLE_MODEM_UART_EVENT_QUEUE_SIZE;
+    dte_config.task_stack_size = CONFIG_EXAMPLE_MODEM_UART_EVENT_TASK_STACK_SIZE*2;
+    dte_config.task_priority = CONFIG_EXAMPLE_MODEM_UART_EVENT_TASK_PRIORITY;
+    dte_config.dte_buffer_size = CONFIG_EXAMPLE_MODEM_UART_RX_BUFFER_SIZE / 2;
+
+    auto dte = esp_modem::create_uart_dte(&dte_config);
+    assert(dte);
+
+    /* Configure the DCE */
+    esp_modem_dce_config_t dce_config = ESP_MODEM_DCE_DEFAULT_CONFIG(CONFIG_EXAMPLE_MODEM_APN);
+
+    /* create the DCE and initialize network manually (using AT commands) */
+    auto dce = sock_dce::create(&dce_config, std::move(dte));
+    if (!dce->init_network()) {
+        ESP_LOGE(TAG,  "Failed to setup network");
+        return;
+    }
+
+    dce->init(1883);
+    esp_mqtt_client_config_t mqtt_config = {};
+    mqtt_config.broker.address.uri = "mqtt://127.0.0.1";
+    mqtt_config.session.message_retransmit_timeout = 10000;
+    esp_mqtt_client_handle_t mqtt_client = esp_mqtt_client_init(&mqtt_config);
+    esp_mqtt_client_register_event(mqtt_client, static_cast<esp_mqtt_event_id_t>(ESP_EVENT_ANY_ID), mqtt_event_handler, NULL);
+    esp_mqtt_client_start(mqtt_client);
+    if (!dce->start("test.mosquitto.org", 1883)) {
+        ESP_LOGE(TAG, "Failed to start DCE");
+        return;
+    }
+    while (1) {
+        while (dce->perform()) {
+            ESP_LOGD(TAG, "...performing");
+        }
+        ESP_LOGE(TAG, "Loop exit.. retrying");
+        // handle disconnections errors
+        if (!dce->init_network()) {
+            ESP_LOGE(TAG, "Failed to reinit network");
+            return;
+        }
+        if (!dce->start("test.mosquitto.org", 1883)) {
+            ESP_LOGI(TAG, "Network reinitialized, retrying");
+        }
+    }
+
+}