What triggered this was my attempt to use micropython-mqtt from the Unix port, in order to run some tests with my code before moving this to an actual board.

The module uses a few things that are not implemented in the Unix port, and probably needs to be ported itself.

One of the issues, however, is machine.unique_id(), which seems... basic enough for the Unix port to have it.

One strategy could be to use gethostid(), which is deprecated in most Unix systems, as it's a 32-bit integer (not very unique). Another one could be to use /etc/machine-id, a UUID, which exists in -at least- modern Linux systems.

A search for /etc/machine-id yields:
https://codesearch.debian.net/search?q=%2Fetc%2Fmachine-id&literal=1
Most seem to be falling back to /var/lib/dbus/machine-id.