Earlier quoted context omitted.
A fiber doesn't have a dedicated execution context, so it would be just as blocking.
Is there really no other way than creating a whole thread for this?
But that way your sacrificing integration into your system's nsswitch which may want to do something completely different with your requests.
You could also query over dbus which can be async https://www.freedesktop.org/software/systemd/man/latest/org.... (if you can depend on systemd)