# debug session #1

If I really want to put out what I do I think I should not only be including my good project showcases but also what I learn along the way. So I will be adding a tag for debug sessions.

For today we have an issue I ran into while working on writing an introduction to flakes. I have both a desktop and a laptop running NixOS and I tend to do work on both and silly me thought nix and flakes were supposed to just work, what happened to ending the “it works on my machine” syndrome, both machines are literally mine and got imaged with the same nix os iso, why did I run into issues?!?

I expected to be able to run nix develop and be able to use the same flake file with literally 3 pkgs on both my laptop and desktop. Why am I having to debug, is this just some massive skill issue on my side (edit: kinda)?

For context I wrote a simple flake for this site on my desktop and it seemed to work with no issues. I go to my laptop to pull the changes to the repo and I get the following error:

error:
       … while evaluating the attribute 'legacyPackages.x86_64-linux'
         at /nix/store/3p306srz83h9z9v0ma9xcxb8y8cdxkxj-source/lib/attrsets.nix:977:47:
          976|   */
          977|   nameValuePair = name: value: { inherit name value; };
             |                                               ^
          978|

       … while evaluating a branch condition
         at /nix/store/3p306srz83h9z9v0ma9xcxb8y8cdxkxj-source/pkgs/stdenv/booter.nix:107:9:
          106|       thisStage =
          107|         if args.__raw or false then
             |         ^
          108|           args'

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: error parsing derivation '/nix/store/rr4cpsx1w84h2l1h71ch8spcp221kzkr-m4-1.4.19.tar.bz2.drv': error: exp
ected string 'D'

I thought it was related to the channel I was using for nixpks so I changed it to the unstable channel, ran nix develop one more time and then I got this:

error: builder for '/nix/store/h414vl12bmgrf03asyslmw4yx0dx242w-nix-shell-env.drv' failed to produce output path for output 'out' at '/nix/store/h414vl12bmgrf03asyslmw4yx0dx242w-nix-shell-env.drv.chroot/root/nix/store/kxwmaaskhx4cb5l3jvc1yh8njdwq865b-nix-shell-env'

After a lot of research I found that a possible cause of this is a corrupted nix store. I read a bit more about the nix store and the nix-store cmd and it turns out there is a way to repair it, great! … Wait why did this sudo nix-store --verify --check-contents --repair not work. Why does it say that the hashes not match and that it cant repair it.

Ok fine back to research. This is when I found this github issue. I went through it and concluded that my nix store most likely got corrupted during a nix os rebuild and then my laptop “died” midway through since the battery is pretty bad and it will turn off even if it says 30% battery remaining. I don’t remember that happening but I also do not use it all the time so it might’ve been a while ago.

Anyways after reading through the issue comments I found a comment by @n8henrie. And that did the trick. I did have to change it a bit since my config was a bit weird and it would not take the nix store command so I just used the old nix-store command.

Ultimately this is what fixed my issues:

nix-store --query --referrers-closure \
    $(find /nix/store -maxdepth 1 -type f -name '*.drv' -size 0) |
    xargs sudo nix-store --delete --ignore-liveness
sudo nix-store --gc
sudo nix-store --verify --check-contents --repair
sudo nixos-rebuild switch

Finally, I rebooted for good measure, should’ve tried to at least get a package with nix-shell to see if that had worked. But I assumed that since the nix store repair did not output any errors I was confident enough that it had worked.

End of debug session #1.