Nix: Tell user why a given path is live when it can't be deleted (continued from #171)

[nh2]

Continuation of #171 (due to https://github.com/NixOS/nix/issues/171#issuecomment-359180537).

CC subscribers from over there: @shlevy @jb55 @qknight @nh2 @edolstra

Original problem:

---

$ nix-store --delete  /nix/store/ixmvap0270115iwkz6fljjccghvnf928-nixos-13.10pre34775.a63b4dc
finding garbage collector roots...
error: cannot delete path `/nix/store/ixmvap0270115iwkz6fljjccghvnf928-nixos-13.10pre34775.a63b4dc' since it is still alive
0 store paths deleted, 0.00 MiB freed

And

$ nix-store -q --roots  /nix/store/ixmvap0270115iwkz6fljjccghvnf928-nixos-13.10pre34775.a63b4dc
/nix/var/nix/profiles/system-53-link`

Would be nice if instead the first command output:

$ nix-store --delete  /nix/store/ixmvap0270115iwkz6fljjccghvnf928-nixos-13.10pre34775.a63b4dc
finding garbage collector roots...
error: cannot delete path `/nix/store/ixmvap0270115iwkz6fljjccghvnf928-nixos-13.10pre34775.a63b4dc' since it is still alive (reachable from /nix/var/nix/profiles/system-53-link)
0 store paths deleted, 0.00 MiB freed

[NinjaTrappeur]

Hello,

I am looking for an excuse to dive in the nix-store garbage collector code. I'm fixing this one if it's ok with @shlevy :)

[bobvanderlinden]

@NinjaTrappeur looks like you already got steps in the right direction. What is the current status? Did you run into a problem? Can I help get this merged into Nix?

[NinjaTrappeur]

Hey @bobvanderlinden ,

So, I made a UI proposal there, and tested it a bit in order to spot obvious regressions.

I am waiting for some feedback. Both for the feature by itself as well as for the code.

[fmnxl]

For me it was because it was still used by direnv (with use nix), and I imported the package in my shell.nix. I forget sometimes that direnv is on the background.

Just sharing my finding. Hope it helps someone.