Add keepCurrentBootedConfiguration option

[jbgi]

to always keep booted system configuration in boot menu.

it is mostly a prof of concept (this is my first time writing some rust code) so probably need some work.

This is a fix/workaround for NixOS/nixpkgs#24158 / NixOS/nix#1095 (only when using lanzaboote of course).

[blitz]

This is a super useful feature and should probably be the default.

@nikstur Do you have bandwidth to do a review here?

[Majiir]

Why should this be a feature of Lanzaboote or any other boot loader? Couldn't this be a feature of nix-env --delete-generations/nix profile wipe-history instead? That way, garbage collection is handled robustly (relying on gcroots to profile generations, not just /run/booted-system which changes on next boot) and boot loaders don't have to be concerned with how generations are managed.

[jbgi]

Probably a solution built in NixOS would be ideal, following comment from a lix developer on this matter: https://git.lix.systems/lix-project/lix/issues/723#issuecomment-9321

[nlewo]

I also think this should not be done by lanzaboote, especially since this could lead to non bootable entry. For instance:

1. I boot on /nix/var/nix/profiles/generation-1
2. I switch to /nix/var/nix/profiles/generation-2
3. I reboot: /run/booted-system is no longer a link to /nix/var/nix/profiles/generation-1
4. I garbage collect /nix/var/nix/profiles/generation-1
5. I reboot: it fails becasue /nix/var/nix/profiles/generation-1 doesn't exist anymore

With this option, lanzaboote would propose to boot on /nix/var/nix/profiles/generation-1 while this generation has been garbage collected...

So, i also think preserving the current booted closure should be handle by deployment tools (nixos-rebuild/nix-env, comin (author here), ...).

[jbgi]

@nlewo The issue of non-bootable entries is not introduced by this patch though. In your scenario, given a configurationLimit >= 2, Generation-1 ends-up being a non-bootable entry, with or without this patch. This patch would just add another entry (in last position, so never the default), namely Generation-SAFE equivalent to Generation-1.
Also your last point (5.) only happen if you chose a non default entry (Generation-1 or Generation-SAFE), just rebooting would boot Generation-2, right?