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Issue with MQTT + altcp_tls – LwIP heap and TCP_SEG are not cleared after an unhandled error.

Oleg_Sokolov
Associate

‎2025-07-18 2:02 AM - edited ‎2025-07-18 4:30 AM

Hello.
I've been struggling for several weeks to fix an issue when sending data over MQTT using LwIP on an STM32F407 microcontroller.
For periodic message publishing, I'm using the following code:

void MqttPublishTask(void const * argument)
{
	err_t err;
	while(1)
	{
        xEventGroupWaitBits(mqttSendEventGroup,
                            pdFALSE,
                            pdTRUE,
                            portMAX_DELAY);

		if(mqtt_client_is_connected(&client))
		{
			cJSON *root = cJSON_CreateObject();

			for(uint8_t i = 0; i < (nodeID - 64); i++)
			{
				cJSON *tap_metrics = cJSON_CreateObject();
				cJSON_AddNumberToObject(tap_metrics, "ER", taps[i].alarmCode);
				cJSON_AddNumberToObject(tap_metrics, "C", taps[i].coldWater);
				cJSON_AddNumberToObject(tap_metrics, "H", taps[i].hotWater);
				cJSON_AddNumberToObject(tap_metrics, "E", taps[i].totalEnergy);
				cJSON_AddNumberToObject(tap_metrics, "T", taps[i].waterFlowTime);
				cJSON_AddNumberToObject(tap_metrics, "N", taps[i].numberOfUses);

				snprintf(serialString, sizeof(serialString), "%lu", taps[i].serial);
				cJSON_AddItemToObject(root, serialString, tap_metrics);
			}

			char *pub_payload = cJSON_PrintUnformatted(root);
			uint8_t qos = 0;
			uint8_t retain = 0;

			snprintf(serialString, sizeof(serialString), "%lu", OD_PERSIST_COMM.x1018_identity.serialNumber);

			err = mqtt_publish(&client, serialString, pub_payload, strlen(pub_payload), qos, retain, mqtt_pub_request_cb, 0);
			if(err != ERR_OK)
			{
			  printf("Publish err: %d\n", err);
			}

			cJSON_Delete(root);
			free(pub_payload);
		}
		osDelay(30000);
	}
}

To establish and check the connection with the broker and also for debugging purposes, I’m using the following task:

void MqttConnectTask(void const *argument)
{
	mbedtls_threading_set_alt( cmsis_os_mutex_init,
    							cmsis_os_mutex_free,
								cmsis_os_mutex_lock,
								cmsis_os_mutex_unlock );

    if (tls_config == NULL)
    {
        tls_config = altcp_tls_create_config_client(
            (const unsigned char *)mbedtls_root_certificate,
            sizeof(mbedtls_root_certificate));

        if (tls_config == NULL)
        {
            printf("Failed to create TLS config\n");
            vTaskSuspend(NULL);
        }
    }
	ip_addr_t mqtt_server_ip;

    while (netconn_gethostbyname("my.mqtt.broker.es", &mqtt_server_ip) != ERR_OK)
    {
        printf("MQTT Server DNS resolve failed\n");
        osDelay(5000);
    }

    memset(&ci, 0, sizeof(ci));
    snprintf(serialString, sizeof(serialString), "%ldc", OD_PERSIST_COMM.x1018_identity.serialNumber);
    ci.client_id  = serialString;
    ci.client_user = "username";
    ci.client_pass = "password";
    ci.keep_alive = 14;
    ci.tls_config = tls_config;

    while (1)
    {
    	if (!mqtt_client_is_connected(&client) && netif_is_link_up(&gnetif))
    	{
    		xEventGroupClearBits(mqttSendEventGroup, MQTT_SEND_EVENT_RUN_BIT);
	        mqtt_disconnect(&client);

    		if (client.conn != NULL)
    	    {
    	        altcp_abort(client.conn);
    	        client.conn = NULL;
    	    }
    	    memset(&client, 0, sizeof(client));
    	    osDelay (100);

    	    printf("Attempting MQTT connect...\n");

    	    err_t err = mqtt_client_connect(&client, &mqtt_server_ip, 8883, mqtt_connection_cb, 0, &ci);

    	    if (err != ERR_OK)
    	    {
    	        printf("mqtt_connect return %d\n", err);

    	        if (client.conn != NULL)
    	        {
    	        	altcp_abort(client.conn);
    	        	client.conn = NULL;
    	        }
    	        memset(&client, 0, sizeof(client));
    	    }
    	}
    	else if (!netif_is_link_up(&gnetif))
    	{
    		xEventGroupClearBits(mqttSendEventGroup, MQTT_SEND_EVENT_RUN_BIT);
	        mqtt_disconnect(&client);

    		if (client.conn != NULL)
    	    {
    	        altcp_abort(client.conn);
    	        client.conn = NULL;
    	    }
    	    memset(&client, 0, sizeof(client));
    	    osDelay (100);
    	}
    	else
    	{
    		xEventGroupSetBits(mqttSendEventGroup, MQTT_SEND_EVENT_RUN_BIT);
    	}

        // LwIP heap (MEM_SIZE)
        	    printf("LwIP heap: used: %u / %u bytes (max used: %u bytes)\n",
        	           lwip_stats.mem.used,
        	           lwip_stats.mem.avail,
        	           lwip_stats.mem.max);

        	    // FreeRTOS heap
        	    printf("FreeRTOS heap: current free: %u bytes, minimum ever free: %u bytes\n",
        	           xPortGetFreeHeapSize(),
        	           xPortGetMinimumEverFreeHeapSize());

        	    printf("LwIP pools (current / max / avail):\n");

        	    printf("  PBUF        : %u / %u / %u\n",
        	           lwip_stats.memp[MEMP_PBUF]->used,
        	           lwip_stats.memp[MEMP_PBUF]->max,
        	           lwip_stats.memp[MEMP_PBUF]->avail);

        	    printf("  TCP_PCB     : %u / %u / %u\n",
        	           lwip_stats.memp[MEMP_TCP_PCB]->used,
        	           lwip_stats.memp[MEMP_TCP_PCB]->max,
        	           lwip_stats.memp[MEMP_TCP_PCB]->avail);

        	    printf("  TCP_SEG     : %u / %u / %u\n",
        	           lwip_stats.memp[MEMP_TCP_SEG]->used,
        	           lwip_stats.memp[MEMP_TCP_SEG]->max,
        	           lwip_stats.memp[MEMP_TCP_SEG]->avail);

        	    printf("  ALTCP_PCB   : %u / %u / %u\n",
        	           lwip_stats.memp[MEMP_ALTCP_PCB]->used,
        	           lwip_stats.memp[MEMP_ALTCP_PCB]->max,
        	           lwip_stats.memp[MEMP_ALTCP_PCB]->avail);

                UBaseType_t stackLeft = uxTaskGetStackHighWaterMark(NULL);
                printf("Free stack space MqttConnectTask: %lu words (%lu bytes)\n", stackLeft, stackLeft * sizeof(StackType_t));


        osDelay(5000);
    }

}

 Everything works perfectly, but once every few hours the following appears in the log:

LwIP heap: used: 19708 / 36864 bytes (max used: 24452 bytes)
FreeRTOS heap: current free: 12672 bytes, minimum ever free: 10424 bytes
LwIP pools (current / max / avail):
  PBUF        : 0 / 0 / 16
  TCP_PCB     : 1 / 2 / 5
  TCP_SEG     : 0 / 3 / 16
  ALTCP_PCB   : 2 / 2 / 5
Free stack space MqttConnectTask: 652 words (2608 bytes)

Assertion "unhandled error" failed at line 1079 in ../Middlewares/Third_Party/LwIP/src/apps/altcp_tls/altcp_tls_mbedtls.c

LwIP heap: used: 22300 / 36864 bytes (max used: 24452 bytes)
FreeRTOS heap: current free: 12672 bytes, minimum ever free: 10424 bytes
LwIP pools (current / max / avail):
  PBUF        : 0 / 0 / 16
  TCP_PCB     : 1 / 2 / 5
  TCP_SEG     : 4 / 5 / 16
  ALTCP_PCB   : 2 / 2 / 5
Free stack space MqttConnectTask: 652 words (2608 bytes)

That is, before the message "Assertion "unhandled error" failed at line 1079" appears, there are 0 TCP_SEG in use and 19708 bytes used from the LwIP heap.
After the error, I see 2 TCP_SEG in use and 22300 bytes used from the heap — and this memory is never freed.

After 5, 10, or 20 hours, the same error happens again, and these values increase further. This continues until all available TCP_SEG or heap memory is exhausted, at which point the task simply hangs and stops the data transfer.

At line 1079 in altcp_tls_mbedtls.c, there's only this line:

 
LWIP_ASSERT("unhandled error", 0);
return ERR_VAL;

This error is not handled by LwIP in any way, so the memory is not freed, leading to a memory leak.

I tried disconnecting and re-establishing the connection, replacing the lines above with the following:

LWIP_DEBUGF(ALTCP_MBEDTLS_DEBUG, ("mbedtls_ssl_write failed: %d\n", ret));
if (conn) 
{ 
altcp_abort(conn); 
} 
return ERR_ABRT;
—but it had no effect.

Has anyone encountered a similar issue?
There are many reports about MQTT + mbedTLS hanging after some time, but I haven’t found any working solution yet.

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