error: The unique option `system.build.installBootLoader' is defined multiple times.

[SFrijters]

Describe the bug

After updating my system flake I get this error on rebuild:

building the system configuration...
error: The unique option `system.build.installBootLoader' is defined multiple times. Definition values:
       - In `/nix/store/if27n2zk0w5s62jk2bq32qm33r4xzyd0-source/nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix': <derivation /nix/store/137hq8hkiicgi8zkmbpvnmqwbgr0ds9c-systemd-boot.drv>
       - In `/nix/store/if27n2zk0w5s62jk2bq32qm33r4xzyd0-source/nixos/modules/system/boot/loader/grub/grub.nix': <derivation /nix/store/4nmjqwyazvpaq9qbly4gd25gbijxb0pz-install-grub.sh.drv>

The relevant section of my configuration.nix is (I think):

  # Use the systemd-boot EFI boot loader.
  boot = {
    loader = {
      timeout = 10;
      systemd-boot.enable = true;
      efi.canTouchEfiVariables = true;
      grub = {
        enable = true;
        device = "nodev";
        efiSupport = true;
        useOSProber = true;
      };
    };
   ...

I assume this has something to do with #155963 - what would be the proper way to resolve this? I think one of them would have won out over the other before, is there any way to know which one so I can keep the status quo?

Steps To Reproduce

Have the above snippet of configuration in configuration.nix, build the system with nixpkgs 80458a3.

Expected behavior

The system builds succesfully.

Notify maintainers

@roberth @ncfavier

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

 - system: `"x86_64-linux"`
 - host os: `Linux 5.16.0, NixOS, 22.05 (Quokka)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.6.0pre20211217_6e6e998`
 - channels(frijters): `""`
 - channels(root): `"nixos, nixos-unstable-20.09pre225264.683c68232e9"`
 - nixpkgs: `/etc/nixpkgs`

[roberth]

Do you use both bootloaders?

[ncfavier]

In this case grub was winning. Generally the "last" definition wins because of the way types.attrs.merge is implemented, and you can see the order from your error message.

[SFrijters]

I went back through the git history of my config and the boot block was effectively there from the very beginning, so I probably copied it from somewhere without full understanding. Since grub was apparently winning anyway I've removed the systemd-boot.enable = true; line and all seems to be well now. Thanks for the fast responses.

Regarding the error message: I don't think it's clear (without digging into the source) that the last entry was winning before. But that seems a moot point now anyway.

[ncfavier]

I never said it was clear!

[roberth]

#156503 should improve the error message.