Unhelpful error/Parser error during installation.

[jamen]

Platform

• Linux: NixOS
• macOS
• WSL

Additional information

This was my first experience trying to install NixOS (with the nixos-minimal-24.05.5993.cd3e8833d706-x86_64-linux.iso image.)

I was going for something pretty simple, using the default config, with a few things added for ZFS. I was following the Manual installation guide and the ZFS article on the NixOS Wiki.

These are the configs I ended up with:

• configuration.nix: https://gist.github.com/jamen/a12a291368ecd7839f1fd0b1d44f2ce5#file-configuration-nix
• hardware-configuration.nix: https://gist.github.com/jamen/cb539e263d478675fd9ed122a7d0965d#file-hardware-configuration-nix

As you can see, I had accidently duplicated some of the settings between them, which was fine, until these lines:

  swapDevices = [ "/dev/disk/by-label/swap" ];

  swapDevices =
    [ { device = "/dev/disk/by-uuid/b999f1fd-5d14-4a02-af87-6c83da6dbab2"; }
    ];

This gave me an unhelpful error when it came time to nixos-install:

[root@nixos:~]# nixos-install
copying channel...
building the configuration in /mnt/etc/nixos/configuration.nix...
fatal flex scanner internal error--end of buffer missed

The issue was resolved by properly defining swapDevices (or just deferring to the hardware-configuration.nix definitions, which wasn't clear to me at the time).

I figured its still worth reporting this issue because the config errors were more helpful up until that, and it doesn't seem intended because I found no information about this error (as it relates to Nix). It took me about half an hour to figure out.

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[roberth]

Surprisingly, /dev/disk/by-label/swap was attempted to be loaded as a NixOS module.
This could be prevented in any of multiple places.

Nix

Nix tried to read from a device. Flakes/pure mode will generally prevent this, but nix-build and nix --impure do not.

• Do not read from device files

Bonus improvement, as pure mode only reports /dev:

• When rejecting a file in pure mode, report the whole path (in addition to the rejected path)

Module System

A submodule will load string-typed definitions as modules. This is quite surprising.

The excuse is that a string is a valid module in imports, and a submodule is just a module evaluation.

This would be a breaking change. I think all we could do for now is add another flag to submoduleWith.

NixOS

Submodules that are prone to this could use a type like coercedTo str f (submodule m) to catch errors like this.