nixos: Add assertion for persisting UIDs/GIDs

[kuruczgy]

Fixes #178

[kuruczgy]

Gentle reminder @talyz. Do you in general like the idea of this PR? Do you think we could find someone to review this?

The corresponding issue I opened (#178) did get attention from a couple other people as well, so it's clear that I was not the only person who ran into this issue, and this check would be useful to have.

[talyz]

Looks really useful! Thanks! :) And sorry for the slow response!

[surfaceflinger]

It should be possible for this to be skipped by some option

9de98e0#commitcomment-145532610
9de98e0#commitcomment-145533686

[nakoo]

I feel this merge needs to be more aware. At least it needs to be documented.

[NickCao]

And we should check all possible parent directories, /, /var and /var/lib

[merrkry]

When parent directories like /var or /var/lib are persisted, impermanence will give false alarm and prevent rebuilding.

[accelbread]

There should be a way to turn this off.
On my system, I have my own users's uid and gid set, but for the rest of the users/groups, I have no dependency on what uid/gid they have since I dont persist any files from those users.

[kuruczgy]

Oops, yeah, didn't think of that.

At least we got some quick feedback on this now (haha), but @talyz we should probably revert this as it's clearly breaking people's setups.

I might have time in a few days to work on a fixup. From the comments above I gather that:

1. We should check all parent dirs of /var/lib/nixos, this is obvious, sorry I missed this.
2. There should still be a way to turn it off.

If anyone has additional ideas for how a future attempt at this could be improved, please leave a comment.

[talyz]

Yep, I agree with all of this. I've pushed a few changes to solve this now:

• changed the assertion to a warning, since it's not critical to most people
• include the parents directories in the check
• added an option to toggle warnings on and off (enableWarnings)