MQTT client for lwIP

Author: Erik Andersson

Details of the MQTT protocol can be found at:
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html 

-----------------------------------------------------------------
1. Initial steps, reserve memory and make connection to server:

1.1: Provide storage

Static allocation:
  mqtt_client_t static_client;
  example_do_connect(&static_client);

Dynamic allocation:
  mqtt_client_t *client = mqtt_client_new();
  if(client != NULL) {
    example_do_connect(&client);
  }
  
1.2: Establish Connection with server

void example_do_connect(mqtt_client_t *client)
{
  struct mqtt_connect_client_info_t ci;
  err_t err;
  
  /* Setup an empty client info structure */
  memset(&ci, 0, sizeof(ci));
  
  /* Minimal amount of information required is client identifier, so set it here */ 
  ci.client_id = "lwip_test";
  
  /* Initiate client and connect to server, if this fails immediately an error code is returned
     otherwise mqtt_connection_cb will be called with connection result after attempting 
     to establish a connection with the server. 
     For now MQTT version 3.1.1 is always used */
  
  err = mqtt_client_connect(client, ip_addr, MQTT_PORT, mqtt_connection_cb, 0, &ci);
  
  /* For now just print the result code if something goes wrong */
  if(err != ERR_OK) {
    printf("mqtt_connect return %d\n", err);
  }
}

Connection to server can also be probed by calling mqtt_client_is_connected(client) 

-----------------------------------------------------------------
2. Implementing the connection status callback


static void mqtt_connection_cb(mqtt_client_t *client, void *arg, mqtt_connection_status_t status)
{
  err_t err;
  if(status == MQTT_CONNECT_ACCEPTED) {
    printf("mqtt_connection_cb: Successfully connected\n");
    
    /* Setup callback for incoming publish requests */
    mqtt_set_inpub_callback(client, mqtt_incoming_publish_cb, mqtt_incoming_data_cb, arg);
    
    /* Subscribe to a topic named "subtopic" with QoS level 1, call mqtt_sub_request_cb with result */ 
    err = mqtt_subscribe(client, "subtopic", 1, mqtt_sub_request_cb, arg);

    if(err != ERR_OK) {
      printf("mqtt_subscribe return: %d\n", err);
    }
  } else {
    printf("mqtt_connection_cb: Disconnected, reason: %d\n", status);
    
    /* Its more nice to be connected, so try to reconnect */
    example_do_connect(client);
  }  
}

static void mqtt_sub_request_cb(void *arg, err_t result)
{
  /* Just print the result code here for simplicity, 
     normal behaviour would be to take some action if subscribe fails like 
     notifying user, retry subscribe or disconnect from server */
  printf("Subscribe result: %d\n", result);
}

-----------------------------------------------------------------
3. Implementing callbacks for incoming publish and data

/* The idea is to demultiplex topic and create some reference to be used in data callbacks
   Example here uses a global variable, better would be to use a member in arg
   If RAM and CPU budget allows it, the easiest implementation might be to just take a copy of
   the topic string and use it in mqtt_incoming_data_cb
*/
static int inpub_id;
static void mqtt_incoming_publish_cb(void *arg, const char *topic, u32_t tot_len)
{
  printf("Incoming publish at topic %s with total length %u\n", topic, (unsigned int)tot_len);

  /* Decode topic string into a user defined reference */
  if(strcmp(topic, "print_payload") == 0) {
    inpub_id = 0;
  } else if(topic[0] == 'A') {
    /* All topics starting with 'A' might be handled at the same way */
    inpub_id = 1;
  } else {
    /* For all other topics */
    inpub_id = 2;
  }
}

static void mqtt_incoming_data_cb(void *arg, const u8_t *data, u16_t len, u8_t flags)
{
  printf("Incoming publish payload with length %d, flags %u\n", len, (unsigned int)flags);

  if(flags & MQTT_DATA_FLAG_LAST) {
    /* Last fragment of payload received (or whole part if payload fits receive buffer
       See MQTT_VAR_HEADER_BUFFER_LEN)  */

    /* Call function or do action depending on reference, in this case inpub_id */
    if(inpub_id == 0) {
      /* Don't trust the publisher, check zero termination */
      if(data[len-1] == 0) {
        printf("mqtt_incoming_data_cb: %s\n", (const char *)data);
      }
    } else if(inpub_id == 1) {
      /* Call an 'A' function... */
    } else {
      printf("mqtt_incoming_data_cb: Ignoring payload...\n");
    }
  } else {
    /* Handle fragmented payload, store in buffer, write to file or whatever */
  }
}

-----------------------------------------------------------------
4. Using outgoing publish


void example_publish(mqtt_client_t *client, void *arg)
{
  const char *pub_payload= "PubSubHubLubJub";
  err_t err;
  u8_t qos = 2; /* 0 1 or 2, see MQTT specification */
  u8_t retain = 0; /* No don't retain such crappy payload... */
  err = mqtt_publish(client, "pub_topic", pub_payload, strlen(pub_payload), qos, retain, mqtt_pub_request_cb, arg);
  if(err != ERR_OK) {
    printf("Publish err: %d\n", err);
  }
}

/* Called when publish is complete either with success or failure */
static void mqtt_pub_request_cb(void *arg, err_t result)
{
  if(result != ERR_OK) {
    printf("Publish result: %d\n", result);
  }
}

-----------------------------------------------------------------
5. Disconnecting

Simply call mqtt_disconnect(client)
