Calls to MQTT Library Hang Indefinitely

[ethanbarry]

I was going to open an issue, but the bug report template suggests opening a discussion first.

I have an application derived from the MQTT publisher example, and I'm using an nRF9160 cellular modem with the following config options:

CONFIG_MODEM_SOCKET=y
CONFIG_NET_SOCKETS_OFFLOAD=y

I reported this at Nordic's site, and they suggested asking here.

The problem is that occasionally a call inside the MQTT library will block indefinitely, and control never returns to my application. This is usually zsock_sendmsg(), called from mqtt_client_tls_write_msg(), but it can also be one of the other zsock_*() functions (I added log statements to check this).

If a socket does not receive a close signal, or encounters some similar issue, it would remain in a waiting state indefinitely according to the BSD sockets spec. However, this will never return control to the application, and in an environment where the network is guaranteed to be unreliable, this feels like an issue.

How can I avoid or reconfigure this?