Socket error: POLLHUP, Connection was closed by the AWS IoT broker

[Nikil Rao]

Hi, 

I'm trying to periodically send sensor data to the AWS server and followed the documentation here to connect and send data to the server. I use these configurations - 

CONFIG_LTE_PSM_REQ_RPTAU="11000001" (320 hours)

CONFIG_LTE_PSM_REQ_RAT="00000001" (2 seconds)

CONFIG_MQTT_KEEPALIVE=1200 (20 min)

I have programmed the device to send data (QOS 0), poll the socket for 2 seconds and then sleeps for a specified interval of time (sleep_time). I see that if I have the sleep time anywhere between 1 to 328 seconds, the device works without any problem. If I increase the sleep time to say 358 seconds, the first message is sent properly but the second message receives a Socket error: POLLHUP, Connection was closed by the AWS IoT broker message. I've played with the CONFIG_MQTT_KEEPALIVE value but I cannot increase the data send interval above 328 seconds. Do you have any ideas how I could debug this? Thanks in advance!

Nikil

[Nikil Rao]

I debugged this further and it actually seems to be caused by a POLLHUP error. When I poll the socket after sending a message, I get a revents value of 0x11 on the file descriptor. This means that the POLLIN and POLLHUP flags were set. Since the POLLIN flag was set, I call the aws_iot_input function which finally leads to the recv function in mqtt_client_tls_read to return zero and I get disconnected from AWS.