flake.nix

@@ -21,7 +21,7 @@
     };
   };
 
-  outputs = { nixpkgs, nixos-hardware, flake-utils, tlaternet-webserver
+  outputs = { self, nixpkgs, nixos-hardware, flake-utils, tlaternet-webserver
     , tlaternet-templates, ... }@inputs:
     let
       overlays = [
@@ -30,14 +30,18 @@
             tlaternet-webserver.legacyPackages.${prev.system}.packages;
           tlaternet-templates =
             tlaternet-templates.legacyPackages.${prev.system}.packages;
-          local = import ./pkgs { pkgs = prev; };
+          local = import ./pkgs {
+            pkgs = prev;
+            local-lib = self.lib.${prev.system};
+          };
         })
       ];
 
     in {
       nixosConfigurations = {
-        tlaternet = nixpkgs.lib.nixosSystem {
-          system = "x86_64-linux";
+        tlaternet = let system = "x86_64-linux";
+        in nixpkgs.lib.nixosSystem {
+          inherit system;
 
           modules = [
             ({ modulesPath, ... }: {
@@ -52,8 +56,9 @@
           ];
         };
 
-        vm = nixpkgs.lib.nixosSystem {
-          system = "x86_64-linux";
+        vm = let system = "x86_64-linux";
+        in nixpkgs.lib.nixosSystem {
+          inherit system;
 
           modules = [
             ({ modulesPath, ... }: {
@@ -88,6 +93,14 @@
             '';
           };
 
-        packages = import ./pkgs { inherit pkgs; };
+        packages = import ./pkgs {
+          inherit pkgs;
+          local-lib = self.lib.${system};
+        };
+
+        lib = import ./lib {
+          inherit pkgs inputs;
+          lib = nixpkgs.lib;
+        };
       });
 }

lib/default.nix

@@ -0,0 +1,8 @@
+{ inputs, lib, pkgs, ... }:
+
+let
+  inherit (lib) makeExtensible foldr attrValues;
+  tlater-lib = makeExtensible (self:
+    let callLibs = file: import file { inherit self lib pkgs inputs; };
+    in { minecraft = callLibs ./minecraft.nix; });
+in tlater-lib.extend (self: super: foldr (a: b: a // b) { } (attrValues super))

lib/minecraft.nix

@@ -0,0 +1,30 @@
+{ lib, pkgs, ... }:
+
+{
+  # Make a modpack given its mod inputs.
+  #
+  # Mods should be attrsets in this format:
+  #   {
+  #     project = "";
+  #     id = "";
+  #     filename = "";
+  #     sha256 = "";
+  #   }
+  #
+  # This may be nice to read from a json ;)
+  #
+  mkModpack = { name, version, mods }:
+    pkgs.stdenv.mkDerivation {
+      inherit name version;
+      srcs = map pkgs.local.fetchFromCurseForge mods;
+
+      sourceRoot = "src/";
+      preUnpack = "mkdir -p src/";
+      unpackCmd = "cp $curSrc src/";
+
+      installPhase = ''
+        mkdir -p $out/mods/
+        cp * $out/mods
+      '';
+    };
+}

pkgs/build-support/fetchFromCurseForge.nix

@@ -0,0 +1,23 @@
+{ lib, fetchurl }:
+
+with builtins;
+
+{ project, id, filename, ... }@args:
+
+let
+  # I think this is supposed to be some weak automation
+  # protection. This split "id" is simply part of the download URL.
+  #
+  # Note that if it's zero-prefixed we need to remove the zeroes. It's
+  # really an odd one...
+  a = head (match "0*([[:digit:]]+)" (substring 0 4 id));
+  b = head (match "0*([[:digit:]]+)" (substring 4 7 id));
+
+  url = "https://media.forgecdn.net/files/${a}/${b}/${filename}";
+  otherArgs = removeAttrs args [ "project" "id" "filename" ];
+
+in fetchurl (otherArgs // {
+  inherit url;
+  # Rename files to avoid names incompatible with the nix store
+  name = "${project}.jar";
+})

pkgs/default.nix

@@ -1,6 +1,12 @@
-{ pkgs, ... }:
+{ pkgs, local-lib, ... }:
 
-{
+let
+  inherit (pkgs.lib) callPackageWith;
+  callPackage = callPackageWith (pkgs // { inherit local-lib; });
+in {
   # Forge
-  forge-server = pkgs.callPackage ./minecraft/forge-server.nix { };
+  forge-server = callPackage ./minecraft/forge-server.nix { };
+
+  # Build support
+  fetchFromCurseForge = callPackage ./build-support/fetchFromCurseForge.nix { };
 }

pkgs/minecraft/forge-server.nix

@@ -1,4 +1,5 @@
-{ lib, stdenv, fetchurl, jre_headless, runtimeShell }:
+{ lib, stdenv, fetchurl, busybox, coreutils, jre_headless, runtimeShell
+, mods ? null, config ? null, defaultconfigs ? null }:
 
 let
   name = "forge-server";
@@ -45,6 +46,35 @@ in stdenv.mkDerivation rec {
     #!${runtimeShell}
     set -eu
 
+    # Delete any previous mods directory so that it can be updated
+    ${busybox}/bin/rm -fr mods
+
+    ${lib.optionalString (mods != null) ''
+      # Copy the specified mods into the directory. Note that, sadly,
+      # forge doesn't support symlinks here.
+      ${busybox}/bin/mkdir -p mods
+      ${busybox}/bin/cp -r '${mods}/mods/'*.jar mods
+    ''}
+
+    # Delete any previous config directories so that they can be updated
+    ${busybox}/bin/rm -fr config defaultconfigs
+
+    ${lib.optionalString (config != null) ''
+      # Copy the specified configs into the directory. Forge (and
+      # mods) will try to write here, so we cannot symlink.
+      ${busybox}/bin/mkdir -p config
+      ${busybox}/bin/cp -r '${config}'/* config
+      ${busybox}/bin/chmod -R u+w config
+    ''}
+
+    ${lib.optionalString (defaultconfigs != null) ''
+      # Copy the specified configs into the directory. Forge (and
+      # mods) will try to write here, so we cannot symlink.
+      ${busybox}/bin/mkdir -p defaultconfigs
+      ${busybox}/bin/cp -r '${defaultconfigs}'/* defaultconfigs
+      ${busybox}/bin/chmod -R u+w defaultconfigs
+    ''}
+
     exec ${jre_headless}/bin/java \$@ -jar $out'/lib/forge/forge-${version}.jar' nogui
     EOF