use calculate-id as a regular package to generate /etc/passwd

[proger]

Currently calculate-id generates uids at evaluation time.
I'm proposing to make calculate-id as a regular package that generates contents of /etc/passwd that can be plugged in to NixOS — this way we can define lists of users in a usual way (e.g. readDir over public keys directory), but have IDs generated for us and collisions can be detected by looking at the output file, instead of threading those computations around. (in a nutshell, move calculate-id from eval-support to become a regular package you can use with stdenv.mkDerivation).

This will also simplify evaluation to use simple Nix instantiate+build model without nix-exec-style native plugins (thus this proposal conflicts with #33).

[proger]

I've seen that that's what roughly ipsec-wrapper does but it still can be done without using any of Nix extensions like import-from-derivation or custom evaluation engines like nix-exec + native plugins.

[shlevy]

The problem with this is that NixOS currently requires the uids to be available at eval time. That's not a fundamental restriction if we're fine with setting up passwd some other way, but I don't really see an easy way to do that without changing a chunk of upstream code.

That being said, I'm working on a presentation that will hopefully sell you on nix-exec 😁

[proger]

Why does it need uids at eval time?

[shlevy]

uids are managed in users.extraUsers.<foo>.uid, which is then interpolated in the script that builds /etc/passwd upstream. There's no hook to build passwd yourself.