factorio: rudimentary mod support for factorio's nixos module

nixos/modules/services/games/factorio.nix

···
 
 let
   cfg = config.services.factorio;
+  factorio = pkgs.factorio-headless;
   name = "Factorio";
   stateDir = "/var/lib/factorio";
+  mkSavePath = name: "${stateDir}/saves/${name}.zip";
   configFile = pkgs.writeText "factorio.conf" ''
     use-system-read-write-data-directories=true
     [path]
-    read-data=${pkgs.factorio-headless}/share/factorio/data
+    read-data=${factorio}/share/factorio/data
     write-data=${stateDir}
   '';
+  modDir = pkgs.factorio-mkModDirDrv cfg.mods;
 in
 {
   options = {
···
         description = ''
           The name of the savegame that will be used by the server.
 
-          When not present in ${stateDir}/saves, it will be generated before starting the service.
+          When not present in ${stateDir}/saves, a new map with default
+          settings will be generated before starting the service.
         '';
       };
       # TODO Add more individual settings as nixos-options?
···
           customizations.
         '';
       };
+      mods = mkOption {
+        type = types.listOf types.package;
+        default = [];
+        description = ''
+          Mods the server should install and activate.
+
+          The derivations in this list must "build" the mod by simply copying
+          the .zip, named correctly, into the output directory. Eventually,
+          there will be a way to pull in the most up-to-date list of
+          derivations via nixos-channel. Until then, this is for experts only.
+        '';
+      };
+      autosave-interval = mkOption {
+        type = types.nullOr types.int;
+        default = null;
+        example = 2;
+        description = ''
+          The time, in minutes, between autosaves.
+        '';
+      };
     };
   };
 
···
       wantedBy      = [ "multi-user.target" ];
       after         = [ "network.target" ];
 
-      preStart = ''
-          test -e ${stateDir}/saves/${cfg.saveName}.zip || \
-            ${pkgs.factorio-headless}/bin/factorio         \
-              --config=${cfg.configFile}                   \
-              --create=${stateDir}/saves/${cfg.saveName}.zip
-      '';
+      preStart = toString [
+        "test -e ${stateDir}/saves/${cfg.saveName}.zip"
+        "||"
+        "${factorio}/bin/factorio"
+          "--config=${cfg.configFile}"
+          "--create=${mkSavePath cfg.saveName}"
+          (optionalString (cfg.mods != []) "--mod-directory=${modDir}")
+      ];
 
       serviceConfig = {
         User = "factorio";
···
         PrivateTmp = true;
         UMask = "0007";
         ExecStart = toString [
-          "${pkgs.factorio-headless}/bin/factorio"
+          "${factorio}/bin/factorio"
           "--config=${cfg.configFile}"
           "--port=${toString cfg.port}"
-          "--start-server=${stateDir}/saves/${cfg.saveName}.zip"
+          "--start-server=${mkSavePath cfg.saveName}"
+          (optionalString (cfg.mods != []) "--mod-directory=${modDir}")
+          (optionalString (cfg.autosave-interval != null) "--autosave-interval ${toString cfg.autosave-interval}")
         ];
       };
     };

pkgs/games/factorio/default.nix

···
 { stdenv, callPackage, fetchurl, makeWrapper
 , alsaLib, libX11, libXcursor, libXinerama, libXrandr, libXi, mesa_noglu
+, factorio-utils
 , releaseType
+, mods ? []
 , username ? "" , password ? ""
 }:
 
···
     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
···
         $out/bin/factorio
     '';
 
-    preferLocalBuild = true;
-
     meta = {
       description = "A game in which you build and maintain factories";
       longDescription = ''
···
       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}

pkgs/games/factorio/utils.nix

···
+# This file provides a top-level function that will be used by both nixpkgs and nixos
+# to generate mod directories for use at runtime by factorio.
+{ stdenv }:
+with stdenv.lib;
+{
+  mkModDirDrv = mods: # a list of mod derivations
+    let
+      recursiveDeps = modDrv: [modDrv] ++ optionals (modDrv.deps == []) (map recursiveDeps modDrv.deps);
+      modDrvs = unique (flatten (map recursiveDeps mods));
+    in
+    stdenv.mkDerivation {
+      name = "factorio-mod-directory";
+
+      preferLocalBuild = true;
+      buildCommand = ''
+        mkdir -p $out
+        for modDrv in ${toString modDrvs}; do
+          # NB: there will only ever be a single zip file in each mod derivation's output dir
+          ln -s $modDrv/*.zip $out
+        done
+      '';
+    };
+
+    modDrv = { allRecommendedMods, allOptionalMods }:
+      { src
+      , name ? null
+      , deps ? []
+      , optionalDeps ? []
+      , recommendedDeps ? []
+      }: stdenv.mkDerivation {
+
+        inherit src;
+
+        # Use the name of the zip, but endstrip ".zip" and possibly the querystring that gets left in by fetchurl
+        name = replaceStrings ["_"] ["-"] (if name != null then name else removeSuffix ".zip" (head (splitString "?" src.name)));
+
+        deps = deps ++ optionals allOptionalMods optionalDeps
+                    ++ optionals allRecommendedMods recommendedDeps;
+
+        preferLocalBuild = true;
+        buildCommand = ''
+          mkdir -p $out
+          srcBase=$(basename $src)
+          srcBase=''${srcBase#*-}  # strip nix hash
+          srcBase=''${srcBase%\?*} # strip querystring leftover from fetchurl
+          cp $src $out/$srcBase
+        '';
+      };
+}

pkgs/top-level/all-packages.nix

···
 
   factorio-headless = callPackage ../games/factorio { releaseType = "headless"; };
 
+  factorio-utils = callPackage ../games/factorio/utils.nix { };
+
   fairymax = callPackage ../games/fairymax {};
 
   fish-fillets-ng = callPackage ../games/fish-fillets-ng {};