Disconnect before reconnecting

If a reconnect is triggered on a built-up connection the current
espconn instance is replaced without waiting for the invocation of
its disconnection callback.
As a result the disconnection callback gets invoked when the new
espconn instance has already been set, falsely terminating the newly
created connection.

Fix this by disconnecting the current connection cleanly before trying
to reestablish the connection. This change also prevents reconnections
via MQTT_Connect. Instead users should disconnect the current
connection via MQTT_Disconnect before invoking MQTT_Connect for an
existing client.

mqtt/include/mqtt.h

@@ -70,6 +70,7 @@ typedef enum {
 	DNS_RESOLVE,
 	TCP_DISCONNECTING,
 	TCP_DISCONNECTED,
+	TCP_RECONNECT_DISCONNECTING,
 	TCP_RECONNECT_REQ,
 	TCP_RECONNECT,
 	TCP_CONNECTING,

mqtt/mqtt.c

@@ -144,7 +144,8 @@ mqtt_send_keepalive(MQTT_Client *client)
 		system_os_post(MQTT_TASK_PRIO, 0, (os_param_t)client);
 	} 
 	else {
-		client->connState = TCP_RECONNECT_REQ;
+		client->connState = TCP_RECONNECT_DISCONNECTING;
+		system_os_post(MQTT_TASK_PRIO, 0, (os_param_t)client);
 	}
 }
 
@@ -593,12 +594,14 @@ MQTT_Task(os_event_t *e)
 	case TCP_RECONNECT_REQ:
 		break;
 	case TCP_RECONNECT:
+		mqtt_tcpclient_delete(client);
 		MQTT_Connect(client);
 		INFO("TCP: Reconnect to: %s:%d\r\n", client->host, client->port);
 		client->connState = TCP_CONNECTING;
 		break;
 	case MQTT_DELETING:
 	case TCP_DISCONNECTING:
+	case TCP_RECONNECT_DISCONNECTING:
 		if (client->security) {
 #ifdef MQTT_SSL_ENABLE
 			espconn_secure_connect(client->pCon);
@@ -748,7 +751,11 @@ MQTT_InitLWT(MQTT_Client *mqttClient, uint8_t* will_topic, uint8_t* will_msg, ui
 void ICACHE_FLASH_ATTR
 MQTT_Connect(MQTT_Client *mqttClient)
 {
-	MQTT_Disconnect(mqttClient);
+	// Do not connect if this client is already connected otherwise the
+	// two espconn connections may interfere causing unexpected behaviour.
+	if (mqttClient->pCon) {
+		return;
+	}
 	mqttClient->pCon = (struct espconn *)os_zalloc(sizeof(struct espconn));
 	mqttClient->pCon->type = ESPCONN_TCP;
 	mqttClient->pCon->state = ESPCONN_NONE;