refactor: modularize topology config

topology/default.nix

@@ -1,2 +1,12 @@
-{renderer ? "d2", ...} @ attrs:
-import ./renderers/${renderer} attrs
+inputs: {
+  pkgs,
+  modules ? [],
+}:
+inputs.nixpkgs.lib.evalModules {
+  prefix = ["topology"];
+  modules = [./modules] ++ modules;
+  specialArgs = {
+    modulesPath = builtins.toString ./modules;
+    inherit pkgs;
+  };
+}

topology/modules/default.nix

@@ -0,0 +1,49 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  inherit
+    (lib)
+    attrNames
+    filterAttrs
+    literalExpression
+    mkDefault
+    mkIf
+    mkOption
+    types
+    ;
+
+  availableRenderers = attrNames (filterAttrs (_: v: v == "directory") (builtins.readDir ./renderers));
+in {
+  imports = map (x: ./renderers/${x}) (attrNames (builtins.readDir ./renderers));
+
+  options = {
+    nixosConfigurations = mkOption {
+      description = ''
+        The list of nixos configurations to process for topology rendering.
+        All of these must include the relevant nixos topology module.
+      '';
+      type = types.unspecified;
+    };
+
+    renderer = mkOption {
+      description = "Which renderer to use for the default output. Availble options: ${toString availableRenderers}";
+      type = types.nullOr (types.enum availableRenderers);
+      default = "d2";
+    };
+
+    output = mkOption {
+      description = "The derivation containing the rendered output";
+      type = types.path;
+      readOnly = true;
+      defaultText = literalExpression ''config.renderers.${config.renderer}.output'';
+    };
+  };
+
+  config = {
+    output =
+      mkIf (config.renderer != null)
+      (mkDefault config.renderers.${config.renderer}.output);
+  };
+}

topology/modules/renderers/d2/default.nix

@@ -0,0 +1,28 @@
+{
+  lib,
+  config,
+  pkgs,
+  ...
+}: let
+  inherit
+    (lib)
+    mkOption
+    types
+    ;
+in {
+  options.renderers.d2 = {
+    output = mkOption {
+      description = "The derivation containing the rendered output";
+      type = types.path;
+      readOnly = true;
+    };
+  };
+
+  config.renderers.d2.output = pkgs.runCommand "build-d2-topology" {} ''
+    mkdir -p $out
+    cp ${import ./network.nix {
+      inherit pkgs;
+      inherit (config) nixosConfigurations;
+    }} $out/network.d2
+  '';
+}

topology/modules/renderers/d2/network.nix

@@ -76,6 +76,6 @@
 
   d2ForNodes = mapAttrs' (_: node: nameValuePair node.config.topology.id (toD2 node)) nodesById;
 in
-  pkgs.writeText "topology.d2" ''
+  pkgs.writeText "network.d2" ''
     ${concatLines (map (x: d2ForNodes.${x}) (attrNames rootNodes))}
   ''

topology/renderers/d2/default.nix

@@ -1,5 +0,0 @@
-{pkgs, ...} @ attrs:
-pkgs.runCommand "build-d2-topology" {} ''
-  mkdir -p $out
-  cp ${import ./network.nix attrs} $out/network.d2
-''