Patches fixing this bug for branches git master and git STABLE-2_1_x are now available in patch #10122.

I can confirm Ondrej's code change works, but we would suggest adding the code to mqtt_disconnect() (instead of mqtt_close()):

diff --git a/Firmware/lwip-STABLE-2_1_2_RELEASE/src/apps/mqtt/mqtt.c b/Firmware/lwip-STABLE-2_1_2_RELEASE/src/apps/mqtt/mqtt.c
index 269f4a4..ce640b8 100644
--- a/Firmware/lwip-STABLE-2_1_2_RELEASE/src/apps/mqtt/mqtt.c
+++ b/Firmware/lwip-STABLE-2_1_2_RELEASE/src/apps/mqtt/mqtt.c
@@ -1440,6 +1440,10 @@ mqtt_disconnect(mqtt_client_t *client)
   LWIP_ASSERT("mqtt_disconnect: client != NULL", client);
   /* If connection in not already closed */
   if (client->conn_state != TCP_DISCONNECTED) {
+      if (mqtt_output_check_space(&client->output, 0) != 0) {
+          mqtt_output_append_fixed_header(&client->output, MQTT_MSG_TYPE_DISCONNECT, 0, 0, 0, 0);
+          mqtt_output_send(&client->output, client->conn);
+      }
     /* Set conn_state before calling mqtt_close to prevent callback from being called */
     client->conn_state = TCP_DISCONNECTED;
     mqtt_close(client, (mqtt_connection_status_t)0);

I think it would be important to include this in v2.1.3, as this is fixing an important deviation from the MQTT spec.

Hi,

I use LwIP on STM32 MCU. I have a working combination of LwIP + MbedTLS + MQTT. I found a possible faulty behavior in the function:

static void mqtt_close(mqtt_client_t *client, mqtt_connection_status_t reason)

According to the MQTT standard, a client should send the DISCONNECT (ID = 14) packet before closing the connection. This function terminates the connection directly without sending this packet. In my case, I connect to the Amazon AWS service, and this behavior led to this error:

1. I set Last Will Testament (LWT) before I call mqtt_connect()
2. I open the connection using mqtt_connect()
3. When the connection is lost (e.g. I unplug the ethernet cable from my device) - LWT is correctly published to AWS
4. But when I close the connection from code (calling mqtt_disconnect(), LWT should not be published
    - according to the MQTT specs http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718090 section 3.14.4
    - this is caused because of the missing DISCONNECT packet
5. When I added this code to the begging of the mqtt_close() function, it works OK:

   /* If we have memory -> send disconnect packet */
   if (mqtt_output_check_space(&client->output, 0) != 0) {

     /* Append fixed header for disconnect packet */
     mqtt_output_append_fixed_header(&client->output, MQTT_MSG_TYPE_DISCONNECT, 0, 0, 0, 0);

     /* Send it */
     mqtt_output_send(&client->output, client->conn);
   }

Can you please verify this behavior and possible solution?
Thank you very much.
Ondrej