You are providing a bunch of patches that deactivate fetch-engine.sh. Why not just replace the file itself to a script that does no-op or a symlink to ${coreutils}/bin/true?

Good suggestion, sometimes you become blind to obvious solutions like these. A symlink was not allowed, but makeWrapper ${coreutils}/bin/true fetch-engine.sh works great.

There seem to be some mismatched descriptions.

In nixpkgs a more idiomatic approach would be to introduce a function like buildOpenRAMod (come up with a better name) and then to have attribute set:

{
  inherit buildOpenRAMod;

  redalert2 = buildOpenRAMod {
    version = "123";
    engine = {
      # ...
    };
    # ...
  };

  # ...
}

Then buildOpenRAMod can be reused by external mods.

This can already be done by just importing pkgs/games/openra-mods/default.nix, which I would argue is just as common practice, but I agree a function is prettier.

Great wrapper. Probably doesn't belong here, though.

Where else would it go?

inherit (mod) description;

Can this replace openra?

I considered it, but it will complicate the generic mod package definition, because it is structured around having a mod with an engine. I can try to make the mod optional and adding a bunch of conditionals, but if that becomes to unwieldy, I will keep it as is.

Tried it, but it would complicate matters too much, better to keep it packaged separately.

I don't see why you need to do concatStringsSep in nix antiquotation here.

Need to? No. Want to? Yes, this way I can clearly see the list of files that need to be copied over.

Did you really check that every mod is gpl3?

Yes mate,

ca: https://github.com/Inq8/CAmod/blob/master/COPYING.
d2: https://github.com/drogoganor/DarkReign/blob/master/COPYING.
gen: https://github.com/MustaphaTR/Generals-Alpha/blob/master/COPYING.
kknd: https://github.com/IceReaper/KKnD/blob/master/COPYING.
ra2: https://github.com/OpenRA/ra2/blob/master/LICENSE.
raclassic: https://github.com/OpenRA/raclassic/blob/master/COPYING.
rv: https://github.com/MustaphaTR/Romanovs-Vengeance/blob/master/LICENSE.
sp: https://github.com/ABrandau/OpenRAModSDK/blob/master/COPYING.
ura: https://github.com/RAunplugged/uRA/blob/master/COPYING.
yr: https://github.com/cookgreen/yr/blob/master/LICENSE.

Can use

install -Dm444 README.md $out/share/doc/packages/openra-${mod.name}/README.md

it will even create the directory for you

is it documented somewhere what files are needed?

I based it on https://build.opensuse.org/package/view_file/home:fusion809/openra-ura/PKGBUILD?expand=1, but it in turn is based on what is installed by the engine. Maybe I can leverage the Makefile that does the installation https://github.com/OpenRA/OpenRA/blob/bleed/Makefile to not having to have a hardcoded list anymore.

Nice, leveraging the Makefile of the engine, this code is no longer needed.

Do we have a way to keep this file up to date?