flake.nix: add images flake output

configs/flake-parts/nixos.nix

@@ -10,6 +10,8 @@
 }:
 
 let
+  inherit (import ../../lib/extras/flake-helpers.nix { inherit lib inputs; }) mkHost mkImage listImagesWithSystems;
+
   nixosConfigs = import ../../setups/nixos.nix { inherit lib inputs; };
 
   # A function that generates a NixOS module setting up the baseline
@@ -138,9 +140,6 @@ in
     # A list of NixOS configurations from the `./configs/nixos` folder starting
     # from project root. It also has some sensible default configurations.
     nixosConfigurations =
-      let
-        inherit (import ../../lib/extras/flake-helpers.nix { inherit lib inputs; }) mkHost listImagesWithSystems;
-      in
       lib.mapAttrs
         (user: metadata:
           mkHost {
@@ -174,6 +173,39 @@ in
         inputs.self.nixosConfigurations;
   };
 
+  perSystem = { system, lib, ... }: {
+    # This contains images that are meant to be built and distributed
+    # somewhere else including those NixOS configurations that are built as
+    # an ISO.
+    images =
+      let
+        validImages = lib.filterAttrs
+          (host: metadata:
+             metadata.format != null && (lib.elem system metadata.systems))
+          nixosConfigs;
+      in
+      lib.mapAttrs'
+        (host: metadata:
+          let
+            name = metadata.hostname or host;
+            nixpkgs-channel = metadata.nixpkgs-channel or "nixpkgs";
+          in
+          lib.nameValuePair name (mkImage {
+            inherit (metadata) format;
+            inherit nixpkgs-channel;
+            extraModules = [
+              (hostSpecificModule host metadata)
+
+              # Forcing the host platform set by the host (if there's any).
+              # Ideally, there shouldn't be.
+              ({ lib, ... }: {
+                nixpkgs.hostPlatform = lib.mkForce system;
+              })
+            ];
+          }))
+        validImages;
+  };
+
   _module.args = {
     inherit defaultNixOSConfig nixosConfigs;
   };

flake.nix

@@ -92,6 +92,7 @@
       ];
 
       imports = [
+        ./modules/flake-parts
         ./configs/flake-parts
       ];
     };

modules/flake-parts/default.nix

@@ -0,0 +1,11 @@
+# Unlike other custom modules such as from NixOS and home-manager, all
+# flake-part modules are considered internal so there's no need for an internal
+# flag. We can just import these directly. Nobody should be using this except
+# this project (and also my other projects).
+{ lib, ... }:
+
+{
+  imports = [
+    ./images.nix
+  ];
+}

modules/flake-parts/images.nix

@@ -0,0 +1,53 @@
+# A custom flake-parts module to configure my NixOS images generated with
+# nixos-generators. For more details, see the "Declarative hosts management"
+# section from the documentation.
+{ config, lib, flake-parts-lib, ... }:
+
+let
+  inherit (flake-parts-lib) mkSubmoduleOptions mkPerSystemOption;
+in
+{
+  options = {
+    flake = mkSubmoduleOptions {
+      images = lib.mkOption {
+        type = with lib.types; lazyAttrsOf (attrsOf package);
+        default = { };
+        description = ''
+          An attribute set of per-system NixOS configurations built as an image
+          output supported by
+          [nixos-generators](https://github.com/nix-community/nixos-generators).
+          This is exclusively used for foodogsquared's NixOS setup.
+        '';
+      };
+    };
+
+    perSystem = mkPerSystemOption {
+      options = {
+        images = lib.mkOption {
+          type = with lib.types; attrsOf package;
+          default = {};
+          description = ''
+            An attribute set of NixOS configurations built as an image output
+            supported by
+            [nixos-generators](https://github.com/nix-community/nixos-generators).
+          '';
+        };
+      };
+    };
+  };
+
+  config = {
+    flake.images =
+      lib.mapAttrs
+        (k: v: v.images)
+        (lib.filterAttrs
+          (k: v: v.images != {})
+          config.allSystems
+        );
+
+    perInput = system: flake:
+      lib.optionalAttrs (flake ? images.${system}) {
+        images = flake.images.${system};
+      };
+  };
+}