We need to think about system user lookup for a bit.
Currently we’re doing it two ways: for some operations we scan /home, and for others we use os/user‘s Lookup function (or Current which calls that). There being two ways to answer essentially the same questions can result, in some situations, in things getting out of sync (think: old users’ homes left over in /home, or some user homes being in /home2, or local users but NFS homes, or network users with local homes, or…).
Both of these implementations have the problem that “list all users” is not a question we can ask, as they could lock up the system for a long time (I’ve worked on systems where a full user listing can take hours; although pathological, it points at the problem).
os/user uses cgo, which we try to avoid, and there’s a PR up which vendors the non-cgo implementation from 1.9 (which I expect will get tweaked to support extrausers, otherwise it breaks on core).
If just looking up users (and groups) in files is enough for snapd, we should own it and switch everything to that (and we get to ask for all users! some UX gets a lot nicer). If not, we should own it and fix things.
What do we want to do?
Basically the question is: do we care about, and support, the weird and wonderful cases a system can get into via NSS and networked homes? If yes, we cannot use non-cgo implementations of these lookups, we cannot use reading /home/*/snap to get a listing of snap user dirs, and we can’t ask for all the users in a system.
If we don’t care nor support generic NSS and only support files, we can use a non-cgo implementation of the lookup funcs, and as long as we don’t list /home we shouldn’t need to worry about it being networked.
