xdg: add `XDG_*_HOME` variables to `systemd.user.sessionVariables` (#2790)

Make sure those variables are set in systemd user services.
This mirrors the situation for `XDG_*_DIRS` variables.

modules/misc/xdg.nix

@@ -80,18 +80,22 @@ in {
   };
 
   config = mkMerge [
-    (mkIf cfg.enable {
-      xdg.cacheHome = mkDefault defaultCacheHome;
-      xdg.configHome = mkDefault defaultConfigHome;
-      xdg.dataHome = mkDefault defaultDataHome;
-      xdg.stateHome = mkDefault defaultStateHome;
-
-      home.sessionVariables = {
+    (let
+      variables = {
         XDG_CACHE_HOME = cfg.cacheHome;
         XDG_CONFIG_HOME = cfg.configHome;
         XDG_DATA_HOME = cfg.dataHome;
         XDG_STATE_HOME = cfg.stateHome;
       };
+    in mkIf cfg.enable {
+      xdg.cacheHome = mkDefault defaultCacheHome;
+      xdg.configHome = mkDefault defaultConfigHome;
+      xdg.dataHome = mkDefault defaultDataHome;
+      xdg.stateHome = mkDefault defaultStateHome;
+
+      home.sessionVariables = variables;
+      systemd.user.sessionVariables =
+        mkIf pkgs.stdenv.hostPlatform.isLinux variables;
     })
 
     # Legacy non-deterministic setup.

tests/modules/misc/xdg/system-dirs.nix

@@ -11,8 +11,12 @@
       assertFileContent $envFile ${
         pkgs.writeText "expected" ''
           LOCALE_ARCHIVE_2_27=${pkgs.glibcLocales}/lib/locale/locale-archive
+          XDG_CACHE_HOME=/home/hm-user/.cache
           XDG_CONFIG_DIRS=/etc/xdg:/foo/bar''${XDG_CONFIG_DIRS:+:$XDG_CONFIG_DIRS}
+          XDG_CONFIG_HOME=/home/hm-user/.config
           XDG_DATA_DIRS=/usr/local/share:/usr/share:/baz/quux''${XDG_DATA_DIRS:+:$XDG_DATA_DIRS}
+          XDG_DATA_HOME=/home/hm-user/.local/share
+          XDG_STATE_HOME=/home/hm-user/.local/state
         ''
       }
 

tests/modules/systemd/session-variables-expected.conf

@@ -1,3 +0,0 @@
-LOCALE_ARCHIVE_2_27=@glibcLocales@/lib/locale/locale-archive
-V_int=1
-V_str=2

tests/modules/systemd/session-variables.nix

@@ -15,6 +15,10 @@
           LOCALE_ARCHIVE_2_27=${pkgs.glibcLocales}/lib/locale/locale-archive
           V_int=1
           V_str=2
+          XDG_CACHE_HOME=/home/hm-user/.cache
+          XDG_CONFIG_HOME=/home/hm-user/.config
+          XDG_DATA_HOME=/home/hm-user/.local/share
+          XDG_STATE_HOME=/home/hm-user/.local/state
         ''
       }
     '';