PoC part 2

default.nix

@@ -30,15 +30,22 @@
 		dynamicBefore = nixosBefore.config.dynamicism.finalSettings;
 
 		nixosAfter = evalNixos {
-			configuration = lib.mkMerge [
-				nixosBefore
-				(lib.setAttrByPath option (lib.mkOverride (-999) value))
-			];
+			configuration = { ... }: {
+				imports = [
+					./configuration.nix
+					(lib.setAttrByPath option (lib.mkOverride (-999) value))
+				];
+			};
 		};
 
-		dynamicAfter = nixosAfter.config.dynamicism.finalSettings;
+		withActivationScripts = evalNixos {
+			configuration = ({ ... }: {
+				imports = [ ./configuration.nix ];
+				config.environment.systemPackages = [ nixosAfter.config.dynamicism.for.gotosocial.activate ];
+			});
+		};
 	in {
-
+		inherit nixosBefore nixosAfter withActivationScripts;
 	};
 
 in dynix.overrideAttrs (final: prev: let
@@ -56,5 +63,9 @@ in lib.recursiveUpdate prev {
 		c = self.nixos;
 		nixos-vm = self.nixos.config.system.build.vm;
 		doChange = builtins.seq self.nixos.config.dynamicism doChange;
+		withVox = self.doChange {
+			option = lib.splitString "." "services.gotosocial.settings.application-name";
+			value = "Vox is an asshole";
+		};
 	};
 })

modules/dynamicism/default.nix

@@ -13,19 +13,43 @@ let
   cfg = config.dynamicism;
   opts = options.dynamicism;
   subOpts = lib.mapAttrs (_: metaAttr: metaAttr.configuration.options) options.dynamicism.for.valueMeta.attrs;
+  settingsFormat = pkgs.formats.yaml { };
+
+  concatFoldl = f: list: lib.foldl' (acc: value: acc ++ (f value)) [ ] list;
+  recUpdateFoldlAttrs = f: attrs: lib.foldlAttrs (acc: name: value: lib.recursiveUpdate acc (f name value)) { } attrs;
 
   finalSettingsFor = { ... }@submod: lib.foldl (acc: optPath: let
     next =
       assert lib.isList optPath;
       lib.setAttrByPath optPath (lib.getAttrFromPath optPath config);
   in lib.recursiveUpdate acc next) { } submod.source-options;
+
+  ourAssertions = lib.concatAttrValues {
+    unitsExist = concatFoldl (submod: let
+      next = lib.map (unit: assert lib.isString unit; {
+        assertion = config.systemd.units.${unit}.enable or false;
+        message = ''
+          '${showOption submod.systemd-services-updated.loc}' specified non-existent unit '${unit}'
+        '';
+      }) submod.systemd-services-updated.value;
+    in lib.optionals submod.enable.value next) (lib.attrValues subOpts);
+
+    optsExist = concatFoldl (submod: lib.optionals submod.enable.value (lib.map (optPath: {
+      assertion = lib.hasAttrByPath optPath options;
+      message = "'${showOption submod.source-options.loc}' specified non-existent option '${showOption optPath}'";
+    }) submod.source-options.value)) (lib.attrValues subOpts);
+  };
 in
 {
+  #
+  # Interface.
+  #
   options.dynamicism = {
     for = mkOption {
       type = t.attrsOf (t.submoduleWith {
         modules = [ ./submodule.nix ];
         shorthandOnlyDefinesConfig = false;
+        specialArgs = { inherit pkgs; };
       });
       default = { };
     };
@@ -40,39 +64,30 @@ in
     };
   };
 
-  config.assertions = let
-    unitsExist = lib.foldl' (acc: submod: let
-      next = lib.map (unit: assert lib.isString unit; {
-        assertion = config.systemd.units.${unit}.enable or false;
-        message = ''
-          '${showOption submod.systemd-services-updated.loc}' specified non-existent unit '${unit}'
-        '';
-      }) submod.systemd-services-updated.value;
-    in acc ++ lib.optionals submod.enable.value next) [ ] (lib.attrValues subOpts);
+  # Assertions.
+  config.assertions = ourAssertions;
 
-    optsExist = lib.foldl' (acc: submod: let
-      next = lib.map (optPath: {
-        assertion = lib.hasAttrByPath optPath options;
-        message = "'${showOption submod.source-options.loc}' specified non-existent option '${showOption optPath}'";
-      }) submod.source-options.value;
-    in acc ++ lib.optionals submod.enable.value next) [ ] (lib.attrValues subOpts);
-  in lib.concatLists [
-    unitsExist
-    optsExist
-  ];
+  #
+  # Generic implementation.
+  #
+  config.dynamicism.finalSettings = lib.asserts.checkAssertWarn ourAssertions [ ] (
+    recUpdateFoldlAttrs (name: { ... }@submod: finalSettingsFor submod) config.dynamicism.for
+  );
 
-  config.dynamicism.for = {
-    gotosocial = {
-      source-options = [
-        "services.gotosocial.setting"
-      ];
-      systemd-services-updated = [
-        "gotosocial.service"
-      ];
-    };
+  # Implementations.
+  config.dynamicism.for.gotosocial = let
+    cfg = config.dynamicism.for.gotosocial;
+  in {
+    source-options = [
+      "services.gotosocial.settings"
+    ];
+
+    configFile = settingsFormat.generate "gotosocial-override.yml" config.services.gotosocial.settings;
+
+    unitDropins."gotosocial.service" = pkgs.writeText "gotosocial-override.conf" ''
+      [Service]
+      ExecStart=
+      ExecStart=${lib.getExe' pkgs.gotosocial "gotosocial"} --config-path ${cfg.configFile} start
+    '';
   };
-
-  config.dynamicism.finalSettings = lib.foldlAttrs (acc: name: { ... }@submod: let
-    next = finalSettingsFor submod;
-  in lib.recursiveUpdate acc next) { } config.dynamicism.for;
 }

modules/dynamicism/submodule.nix

@@ -1,5 +1,6 @@
 {
   name,
+  pkgs,
   lib,
   config,
   ...
@@ -13,10 +14,26 @@ let
     mkEnableOption
     literalExpression
   ;
+  inherit (lib.types)
+    mkOptionType
+  ;
   t = lib.types;
 
   /** Either a list of strings, or a dotted string that will be split. */
   convenientAttrPath = t.coercedTo t.str (lib.splitString ".") (t.listOf t.str);
+
+  executablePathInStore = mkOptionType {
+    name = "exepath";
+    description = "executable path in the Nix store";
+    descriptionClass = "noun";
+    merge = lib.mergeEqualOption;
+    functor = lib.defaultFunctor "exepath";
+    check = x: if lib.isDerivation x then (
+      x.meta.mainProgram or null != null
+    ) else (
+      lib.pathInStore.check x
+    );
+  };
 in
 {
   options = {
@@ -36,12 +53,46 @@ in
         A list of systemd unit names (including the suffix, e.g. `.service`) that need to be updated.
       '';
       example = literalExpression ''
-        "gotosocial.service"
+        [ "gotosocial.service" ]
       '';
+
+      default = lib.attrNames config.unitDropins;
+    };
+
+    configFile = mkOption {
+      type = t.pathInStore;
+      internal = true;
+    };
+
+    unitDropins = mkOption {
+      type = t.attrsOf t.pathInStore;
+      internal = true;
+    };
+
+    activate = mkOption {
+      type = executablePathInStore;
+      internal = true;
     };
   };
 
   config = mkIf config.enable {
-
+    activate = pkgs.writeShellApplication {
+      name = "dynamicism-for-${name}-activate";
+      runtimeInputs = [ pkgs.systemd ];
+      text = let
+        doEdits = config.unitDropins
+          |> lib.mapAttrsToList (service: dropin: ''
+            cat "${dropin}" | systemctl edit "${service}" --runtime --stdin
+          '');
+        doReloads = config.unitDropins
+        |> lib.mapAttrsToList (service: _: ''
+          systemctl reload-or-restart "${service}"
+        '');
+      in [
+        doEdits
+        doReloads
+      ] |> lib.concatLists
+      |> lib.concatStringsSep "\n";
+    };
   };
 }