Preserve generation inputs

[copumpkin]

When I do nixos-rebuild switch, it creates a new generation of the system, and lets me roll back to an old generation. That's great, but if I have a long list of generations, it would be really nice to be able to ask nix "what changed between generations 4 and 5?" and get a meaningful diff of the configuration.nix and its dependencies that fed into it.

I don't think configuration.nix is currently getting "snapshotted" with nixos-rebuild calls, and @edolstra says it could be pretty expensive to preserve all inputs, but perhaps there's a nice lightweight way that this general goal could be accomplished without too much pain? Leaving this ticket here for ideas/discussion.

[zimbatm]

Another use-case that's better explained by a story:

One time I was configuring a new machine and /etc/nixos/* would change quite a lot. At one point I rebooted and noticed that the X last generation wouldn't work. I found a derivation that worked but couldn't remember what had changed in /etc/nixos/* so I was stuck with a broken configuration.nix and wasn't able to restore the one from the working generation.

After that I started checking out every change into git but it would have been nice to have a backup. A derivation from /etc/nixos would probably be enough.

[magnetophon]

nixos-scripts by @matthiasbeyer solves this partly, but I too feel this should be in NixOS.

[copumpkin]

@fkz perhaps I should just push the current PR to completion? I like the sound of your less disruptive version of it but it'll probably be easier to finish the current one than rewrite it in a new way. Then we can iterate and do it more nicely later?

[copumpkin]

This PR, for context: NixOS/nix#709

[Profpatsch]

(triage) any progress?

[DerSaidin]

I want this feature too!

Looking around a bit, I guessed this would be implemented in one of the files touched by this commit, but I don't yet understand how this works well enough to get further.

[stale]

Thank you for your contributions.

This has been automatically marked as stale because it has had no activity for 180 days.

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[DerSaidin]

Still important to me.

[stale]

I marked this as stale due to inactivity. → More info