nixos-unstable evaluation broken on Hydra (error: [json.exception.type_error.302] type must be string, but is null)

[andersk]

https://hydra.nixos.org/jobset/nixos/trunk-combined#tabs-errors

error: [json.exception.type_error.302] type must be string, but is null

This is the same meaningless error message that’s bitten us several times before. An attempt was made to replace it with a useful error message in NixOS/hydra#825, but that was reverted in NixOS/hydra#1047. It looks like there’s another attempt pending in NixOS/hydra#1025.

I was able to locally reproduce the error with nix-shell -p hydra-unstable --run 'hydra-eval-jobs -I . nixos/release-combined.nix --verbose', and spent a day bisecting it to commit d76a91b “source-sans-pro: 3.006 -> 3.046” (#142434). Probably related: #142864 “nixos/gnome: remove alias reference to source-sans-pro”.

Cc @marsam @SuperSandro2000 @jonringer

[andersk]

Confirmed that patching hydra-eval-jobs with NixOS/hydra#1025 would have revealed the real error much more quickly.

nixos.tests.gnome-xorg.x86_64-linux: error: undefined variable 'source-sans-pro'

       at /home/anders/nixpkgs/nixos/modules/services/x11/desktop-managers/gnome.nix:456:9:

          455|         source-code-pro # Default monospace font in 3.32
          456|         source-sans-pro
             |         ^
          457|       ];
nixos.tests.gnome.x86_64-linux: error: undefined variable 'source-sans-pro'

       at /home/anders/nixpkgs/nixos/modules/services/x11/desktop-managers/gnome.nix:456:9:

          455|         source-code-pro # Default monospace font in 3.32
          456|         source-sans-pro
             |         ^
          457|       ];

[r-burns]

Interesting - if I understand correctly, this eval failure is in a nixos module, which snuck by ofborg (it only checks packages). Also nixpkgs-unstable is chugging along happily, as the issue only affects nixos-unstable.