First round of MQTT fixes (#3360)

* mqtt: remove concept of connection timeout

Just rely on the network stack to tell us when things have gone south.

* mqtt: remove write-only mqtt_state.port field

* mqtt: drop useless conditional

* mqtt: decouple message sent flag from timer

* mqtt: reconnect callback does not need to hang up

The network stack has certainly done that for us at this point.
Similarly, since we're about to call mqtt_socket_disconnected, don't
bother unregistering the timer here, either.

* mqtt: don't tick once per second

Set the timer for the duration of the wait and cancel it on the other side.

* mqtt: defer message queue destruction to _disconnect

We're going to want to publish a disconnect message for real, so doing
this in _close does no one any favors

* mqtt: miscellaneous cleanups

No functional change intended

* mqtt: close() should send disconnect message for real

This means waiting for _sent() to fire again before telling the network
stack to disconnect.

* mqtt: tidy connect and dns

- Push the self-ref to after all allocations and error returns

- Don't try to extract IPv4 from the domain string ourselves, let the
  resolver, since it can

- Don't try to connect to localhost.  That can't possibly work.

* mqtt: common up some callback invocations

* mqtt: don't retransmit messages on timeout

There's no point in retransmitting messages on timeout; the network
stack will be trying to do it for us anyway.

* mqtt: remove unnecessary NULL udata checks

* mqtt: hold strings in Lua, not C

Eliminates a host of C-side allocations.

While here, move the rest of the mqtt_connect_info structure out to its
own thing, and pack some flags using a bitfield.

* mqtt: mqtt_socket_on use lua_checkoption

* mqtt: slightly augment debug messages

These changes have made some debugging ever so slightly easier.

app/mqtt/mqtt_msg.h

@@ -99,16 +99,15 @@ typedef struct mqtt_message_buffer
 
 typedef struct mqtt_connect_info
 {
-  char* client_id;
-  char* username;
-  char* password;
-  char* will_topic;
-  char* will_message;
+  const char* client_id;
+  const char* username;
+  const char* password;
+  const char* will_topic;
+  const char* will_message;
   int keepalive;
   int will_qos;
   int will_retain;
   int clean_session;
-  uint16_t max_message_length;
 
 } mqtt_connect_info_t;
 

app/mqtt/msg_queue.c

@@ -18,6 +18,8 @@ msg_queue_t *msg_enqueue(msg_queue_t **head, mqtt_message_t *msg, uint16_t msg_i
     return NULL;
   }
 
+  node->sent = 0;
+
   node->msg.data = (uint8_t *)calloc(1,msg->length);
   if(!node->msg.data){
     NODE_DBG("not enough memory\n");

app/mqtt/msg_queue.h

@@ -13,6 +13,8 @@ typedef struct msg_queue_t {
   uint16_t msg_id;
   int msg_type;
   int publish_qos;
+
+  bool sent;
 } msg_queue_t;
 
 msg_queue_t * msg_enqueue(msg_queue_t **head, mqtt_message_t *msg, uint16_t msg_id, int msg_type, int publish_qos);

docs/modules/mqtt.md

@@ -64,8 +64,6 @@ m = mqtt.Client("clientid", 120, "user", "password")
 -- to topic "/lwt" if client don't send keepalive packet
 m:lwt("/lwt", "offline", 0, 0)
 
-m:on("connect", function(client) print ("connected") end)
-m:on("connfail", function(client, reason) print ("connection failed", reason) end)
 m:on("offline", function(client) print ("offline") end)
 
 -- on publish message receive event
@@ -96,11 +94,11 @@ m:connect("192.168.11.118", 1883, false, function(client)
   client:publish("/topic", "hello", 0, 0, function(client) print("sent") end)
 end,
 function(client, reason)
-  print("failed reason: " .. reason)
+  print("Connection failed reason: " .. reason)
 end)
 
-m:close();
--- you can call m:connect again
+m:close()
+-- you can call m:connect again after the offline callback fires
 ```
 
 # MQTT Client
@@ -108,7 +106,11 @@ m:close();
 
 ## mqtt.client:close()
 
-Closes connection to the broker.
+Schedules a clean teardown of the connection.
+
+MQTT requires clients to actively signal a desire to disconnect to the server
+to avoid sending their LWT.  Thus, the Client is not immediately reusable
+after this call, but only after the "offline" callback has fired.
 
 #### Syntax
 `mqtt:close()`
@@ -117,7 +119,7 @@ Closes connection to the broker.
 none
 
 #### Returns
-`true` on success, `false` otherwise
+`nil`
 
 ## mqtt.client:connect()