Unable to override `mpv-unwrapped` within a custom module

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1

To make mpv handle jxls properly, I used this:

environment.systemPackages = [
    (pkgs.mpv.override { mpv-unwrapped = pkgs.mpv-unwrapped.override { ffmpeg = pkgs.ffmpeg-full; }; })
];

And it works perfectly, I’m able to for example take screenshots in jxl.

However, when I wanted to add this to my custom module for mpv, like so:

config = lib.mkIf cfg.enable {
  environment.systemPackages = [
    (cfg.package.override {
      extraMakeWrapperArgs = [
        "--add-flags"
        "--config-dir=${
          pkgs.symlinkJoin {
            inherit name;
            paths = [
              (pkgs.writeTextDir "${name}.conf" cfg.extraConfig)
              (pkgs.writeTextDir "input.conf" cfg.extraInputConf)
            ];
          }
        }"
      ];
      mpv-unwrapped = lib.mkIf cfg.useFullFFmpeg (
        pkgs.mpv-unwrapped.override {
          ffmpeg = pkgs.ffmpeg-full;
        }
      );
    })
  ];
};

It causes an evaluation error:

error:
       … while evaluating an expression to select 'drvPath' on it
         at «internal»:1:552:
       … while evaluating strict
         at «internal»:1:552:
       (stack trace truncated; use '--show-trace' to show the full trace)

       error: attribute 'meta' missing
       at /nix/store/dx2qikyb4dyb6hbdfywbmsyla0z5a1h3-source/pkgs/by-name/mp/mpv/package.nix:146:28:
          145|   meta = {
          146|     inherit (mpv-unwrapped.meta)
             |                            ^
          147|       homepage

       note: trace involved the following derivations:
       derivation 'nixos-system-7730U-26.05.20260217.0182a36'
       derivation 'etc'
       derivation 'dbus-1'
       derivation 'system-path'
Command 'nix --extra-experimental-features 'nix-command flakes' build --print-out-paths '/etc/nixos#nixosConfigurations."7730U".config.system.build.toplevel' --no-link --log-format multiline-with-logs' returned non-zero exit status 1.

From my understanding of how override works, my code should not be affecting the meta attribute of mpv-unwrapped; and it should be almost entirely identical in effect to the first snippet, which did work.

What’s going on here?

2

You cannot use mkIf inside a .override argument. That’s only for module system options.

You’ve replaced mpv-unwrapped with something that isn’t even a derivation, it’s the tag that mkIf leaves behind for the module system to interpret.

1 Like
3

Use an if-else statement here, not mkIf, since it’s not a module option. Or this could work:

cfg.package.override (
  { ... } //
  lib.optionalAttrs cfg.useFullFFmpeg { ... }
)
1 Like
4

Thank you both! This makes a lot of sense.