factorio: rudimentary mod support for factorio's nixos module

pkgs/games/factorio/default.nix

@@ -1,6 +1,8 @@
 { stdenv, callPackage, fetchurl, makeWrapper
 , alsaLib, libX11, libXcursor, libXinerama, libXrandr, libXi, mesa_noglu
+, factorio-utils
 , releaseType
+, mods ? []
 , username ? "" , password ? ""
 }:
 
@@ -54,14 +56,16 @@ let
     fi
   '';
 
+  modDir = factorio-utils.mkModDirDrv mods;
+
   base = {
     name = "factorio-${releaseType}-${version}";
 
     src = fetch.${arch.inTar}.${releaseType};
 
+    preferLocalBuild = true;
     dontBuild = true;
 
-    # TODO detangle headless/normal mode wrapping, libs, etc.  test all urls 32/64/headless/gfx
     installPhase = ''
       mkdir -p $out/{bin,share/factorio}
       cp -a data $out/share/factorio
@@ -71,8 +75,6 @@ let
         $out/bin/factorio
     '';
 
-    preferLocalBuild = true;
-
     meta = {
       description = "A game in which you build and maintain factories";
       longDescription = ''
@@ -112,7 +114,23 @@ let
       wrapProgram $out/bin/factorio                                \
         --prefix LD_LIBRARY_PATH : /run/opengl-driver/lib:$libPath \
         --run "$out/share/factorio/update-config.sh"               \
-        --add-flags "-c \$HOME/.factorio/config.cfg"
+        --add-flags "-c \$HOME/.factorio/config.cfg ${optionalString (mods != []) "--mod-directory=${modDir}"}"
+
+        # TODO Currently, every time a mod is changed/added/removed using the
+        # modlist, a new derivation will take up the entire footprint of the
+        # client. The only way to avoid this is to remove the mods arg from the
+        # package function. The modsDir derivation will have to be built
+        # separately and have the user specify it in the .factorio config or
+        # right along side it using a symlink into the store I think i will
+        # just remove mods for the client derivation entirely. this is much
+        # cleaner and more useful for headless mode.
+
+        # TODO: trying to toggle off a mod will result in read-only-fs-error.
+        # not much we can do about that except warn the user somewhere. In
+        # fact, no exit will be clean, since this error will happen on close
+        # regardless. just prints an ugly stacktrace but seems to be otherwise
+        # harmless, unless maybe the user forgets and tries to use the mod
+        # manager.
 
       install -m0644 <(cat << EOF
       ${configBaseCfg}