luamqtt - MQTT client written in pure lua

I currently have the problem that both update functions (loop and sync) block the process until a message is received.
With the system I want to implement this library in I don't have the possibility to create a thread or have this run externally somehow. So the whole program will stop running until messages come in. I do however have the possibility to continually call a function. I thought I could use the run_sync methode as from the description, but that also waits for an incoming message.
So I would need something more in line of a function that checks if a message was reveiced, if not it just moves along. Is there a straightforward way of calling such a function?