nixos/systemd: merge unit options as lists when at least one value is a list

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -348,6 +348,11 @@ The pre-existing [services.ankisyncd](#opt-services.ankisyncd.enable) has been m
 
 - The `mpich` package expression now requires `withPm` to be a list, e.g. `"hydra:gforker"` becomes `[ "hydra" "gforker" ]`.
 
+- When merging systemd unit options (of type `unitOption`),
+  if at least one definition is a list, all those which aren't are now lifted into a list,
+  making it possible to accumulate definitions without resorting to `mkForce`,
+  hence to retain the definitions not anticipating that need.
+
 - YouTrack is bumped to 2023.3. The update is not performed automatically, it requires manual interaction. See the YouTrack section in the manual for details.
 
 - QtMultimedia has changed its default backend to `QT_MEDIA_BACKEND=ffmpeg` (previously `gstreamer` on Linux or `darwin` on MacOS).

nixos/lib/systemd-unit-options.nix

@@ -21,14 +21,8 @@ in rec {
       let
         defs' = filterOverrides defs;
       in
-        if isList (head defs').value
-        then concatMap (def:
-          if builtins.typeOf def.value == "list"
-          then def.value
-          else
-            throw "The definitions for systemd unit options should be either all lists, representing repeatable options, or all non-lists, but for the option ${showOption loc}, the definitions are a mix of list and non-list ${lib.options.showDefs defs'}"
-        ) defs'
-
+        if any (def: isList def.value) defs'
+        then concatMap (def: toList def.value) defs'
         else mergeEqualOption loc defs';
   };
 

pkgs/test/default.nix

@@ -174,4 +174,6 @@ with pkgs;
   nixpkgs-check-by-name = callPackage ./nixpkgs-check-by-name { };
 
   auto-patchelf-hook = callPackage ./auto-patchelf-hook { };
+
+  systemd = callPackage ./systemd { };
 }

pkgs/test/systemd/default.nix

@@ -0,0 +1,5 @@
+{ lib, callPackage }:
+
+lib.recurseIntoAttrs {
+  nixos = callPackage ./nixos { };
+}

pkgs/test/systemd/nixos/default.nix

@@ -0,0 +1,37 @@
+{ pkgs, lib, stdenv, ... }:
+
+lib.runTests {
+  # Merging two non-list definitions must still result in an error
+  # about a conflicting definition.
+  test-unitOption-merging-non-lists-conflict =
+    let nixos = pkgs.nixos {
+        system.stateVersion = lib.trivial.release;
+        systemd.services.systemd-test-nixos = {
+          serviceConfig = lib.mkMerge [
+            { StateDirectory = "foo"; }
+            { StateDirectory = "bar"; }
+          ];
+        };
+      };
+    in {
+    expr = (builtins.tryEval (nixos.config.systemd.services.systemd-test-nixos.serviceConfig.StateDirectory)).success;
+    expected = false;
+  };
+
+  # Merging must lift non-list definitions to a list
+  # if at least one of them is a list.
+  test-unitOption-merging-list-non-list-append =
+    let nixos = pkgs.nixos {
+        system.stateVersion = lib.trivial.release;
+        systemd.services.systemd-test-nixos = {
+          serviceConfig = lib.mkMerge [
+            { StateDirectory = "foo"; }
+            { StateDirectory = ["bar"]; }
+          ];
+        };
+      };
+    in {
+    expr = nixos.config.systemd.services.systemd-test-nixos.serviceConfig.StateDirectory;
+    expected = [ "foo" "bar" ];
+  };
+}