nixos/xsession: use graphical systemd user target

While systemd suggests using the pre-defined graphical-session user
target, I found that this interface is difficult to use. Additionally,
no other major distribution, even in their unstable versions, currently
use this mechanism.

The window or desktop manager is supposed to run in a systemd user service
which activates graphical-session.target and the user services that are
binding to this target. The issue is that we can't elegantly pass the
xsession environment to the window manager session, in particular
whereas the PassEnvironment option does work for DISPLAY, it for some
mysterious reason won't for PATH.

This commit implements a new graphical user target that works just like
default.target. Services which should be run in a graphical session just
need to declare wantedBy graphical.target. The graphical target will be
activated in the xsession before executing the window or display manager.

Fixes #17858.

10 files changed, 35 insertions, 25 deletions

diff --git a/nixos/modules/config/pulseaudio.nix b/nixos/modules/config/pulseaudio.nix
index bf66994b502..bd80c811348 100644
--- a/nixos/modules/config/pulseaudio.nix
+++ b/nixos/modules/config/pulseaudio.nix
@@ -240,11 +240,14 @@ in {
       };
       systemd.user = {
         services.pulseaudio = {
+          restartIfChanged = true;
           serviceConfig = {
             RestartSec = "500ms";
+            PassEnvironment = "DISPLAY";
           };
-          environment = { DISPLAY = ":${toString config.services.xserver.display}"; };
-          restartIfChanged = true;
+        };
+        sockets.pulseaudio = {
+          wantedBy = [ "sockets.target" ];
         };
       };
     })
diff --git a/nixos/modules/programs/ssh.nix b/nixos/modules/programs/ssh.nix
index a00fc0dfd19..4faef2c609b 100644
--- a/nixos/modules/programs/ssh.nix
+++ b/nixos/modules/programs/ssh.nix
@@ -199,9 +199,8 @@ in
     environment.etc."ssh/ssh_known_hosts".text = knownHostsText;
 
     # FIXME: this should really be socket-activated for über-awesomeness.
-    systemd.user.services.ssh-agent =
-      { enable = cfg.startAgent;
-        description = "SSH Agent";
+    systemd.user.services.ssh-agent = mkIf cfg.startAgent
+      { description = "SSH Agent";
         wantedBy = [ "default.target" ];
         serviceConfig =
           { ExecStartPre = "${pkgs.coreutils}/bin/rm -f %t/ssh-agent";
diff --git a/nixos/modules/services/monitoring/arbtt.nix b/nixos/modules/services/monitoring/arbtt.nix
index a8d5e3b7fa0..b41a3c7b501 100644
--- a/nixos/modules/services/monitoring/arbtt.nix
+++ b/nixos/modules/services/monitoring/arbtt.nix
@@ -48,7 +48,8 @@ in {
   config = mkIf cfg.enable {
     systemd.user.services.arbtt = {
       description = "arbtt statistics capture service";
-      wantedBy = [ "default.target" ];
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
 
       serviceConfig = {
         Type = "simple";
diff --git a/nixos/modules/services/x11/compton.nix b/nixos/modules/services/x11/compton.nix
index d75d24830f8..56bc66b7179 100644
--- a/nixos/modules/services/x11/compton.nix
+++ b/nixos/modules/services/x11/compton.nix
@@ -208,13 +208,13 @@ in {
   config = mkIf cfg.enable {
     systemd.user.services.compton = {
       description = "Compton composite manager";
-      wantedBy = [ "default.target" ];
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/compton --config ${configFile}";
         RestartSec = 3;
         Restart = "always";
       };
-      environment.DISPLAY = ":0";
     };
 
     environment.systemPackages = [ cfg.package ];
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index cf6efb7dae7..58773685ec1 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -122,6 +122,9 @@ let
           source ~/.xprofile
       fi
 
+      # Start systemd user services for graphical sessions
+      ${config.systemd.package}/bin/systemctl --user start graphical-session.target
+
       # Allow the user to setup a custom session type.
       if test -x ~/.xsession; then
           exec ~/.xsession
@@ -164,6 +167,9 @@ let
       ''}
 
       test -n "$waitPID" && wait "$waitPID"
+
+      ${config.systemd.package}/bin/systemctl --user stop graphical-session.target
+
       exit 0
     '';
 
@@ -325,6 +331,13 @@ in
 
   config = {
     services.xserver.displayManager.xserverBin = "${xorg.xorgserver.out}/bin/X";
+
+    systemd.user.targets.graphical-session = {
+      unitConfig = {
+        RefuseManualStart = false;
+        StopWhenUnneeded = false;
+      };
+    };
   };
 
   imports = [
diff --git a/nixos/modules/services/x11/redshift.nix b/nixos/modules/services/x11/redshift.nix
index eb5dfdf9584..992709ed000 100644
--- a/nixos/modules/services/x11/redshift.nix
+++ b/nixos/modules/services/x11/redshift.nix
@@ -95,7 +95,8 @@ in {
   config = mkIf cfg.enable {
     systemd.user.services.redshift = {
       description = "Redshift colour temperature adjuster";
-      wantedBy = [ "default.target" ];
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
       serviceConfig = {
         ExecStart = ''
           ${cfg.package}/bin/redshift \
@@ -107,12 +108,6 @@ in {
         RestartSec = 3;
         Restart = "always";
       };
-      environment = {
-        DISPLAY = ":${toString (
-          let display = config.services.xserver.display;
-          in if display != null then display else 0
-        )}";
-      };
     };
   };
 
diff --git a/nixos/modules/services/x11/unclutter-xfixes.nix b/nixos/modules/services/x11/unclutter-xfixes.nix
index b94dfb1a26a..71262431b68 100644
--- a/nixos/modules/services/x11/unclutter-xfixes.nix
+++ b/nixos/modules/services/x11/unclutter-xfixes.nix
@@ -43,7 +43,8 @@ in {
   config = mkIf cfg.enable {
     systemd.user.services.unclutter-xfixes = {
       description = "unclutter-xfixes";
-      wantedBy = [ "graphical.target" ];
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
       serviceConfig.ExecStart = ''
         ${cfg.package}/bin/unclutter \
           --timeout ${toString cfg.timeout} \
diff --git a/nixos/modules/services/x11/unclutter.nix b/nixos/modules/services/x11/unclutter.nix
index a22e5ac2c95..5f16a680050 100644
--- a/nixos/modules/services/x11/unclutter.nix
+++ b/nixos/modules/services/x11/unclutter.nix
@@ -56,19 +56,17 @@ in {
   config = mkIf cfg.enable {
     systemd.user.services.unclutter = {
       description = "unclutter";
-      wantedBy = [ "default.target" ];
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
       serviceConfig.ExecStart = ''
         ${cfg.package}/bin/unclutter \
           -idle ${toString cfg.timeout} \
-          -display :${toString (
-             let display = config.services.xserver.display;
-             in if display != null then display else 0
-          )} \
           -jitter ${toString (cfg.threeshold - 1)} \
           ${optionalString cfg.keystroke "-keystroke"} \
           ${concatMapStrings (x: " -"+x) cfg.extraOptions} \
           -not ${concatStringsSep " " cfg.excluded} \
       '';
+      serviceConfig.PassEnvironment = "DISPLAY";
       serviceConfig.RestartSec = 3;
       serviceConfig.Restart = "always";
     };
diff --git a/nixos/modules/services/x11/urxvtd.nix b/nixos/modules/services/x11/urxvtd.nix
index 57ad93f2017..f2ce089ce19 100644
--- a/nixos/modules/services/x11/urxvtd.nix
+++ b/nixos/modules/services/x11/urxvtd.nix
@@ -21,9 +21,8 @@ in {
     systemd.user = {
       sockets.urxvtd = {
         description = "socket for urxvtd, the urxvt terminal daemon";
-        after = [ "graphical.target" ];
-        wants = [ "graphical.target" ];
-        wantedBy = [ "sockets.target" ];
+        wantedBy = [ "graphical-session.target" ];
+        partOf = [ "graphical-session.target" ];
         socketConfig = {
           ListenStream = "%t/urxvtd-socket";
         };
diff --git a/nixos/modules/services/x11/xbanish.nix b/nixos/modules/services/x11/xbanish.nix
index e1e3cbc8e44..b95fac68f16 100644
--- a/nixos/modules/services/x11/xbanish.nix
+++ b/nixos/modules/services/x11/xbanish.nix
@@ -20,7 +20,8 @@ in {
   config = mkIf cfg.enable {
     systemd.user.services.xbanish = {
       description = "xbanish hides the mouse pointer";
-      wantedBy = [ "default.target" ];
+      wantedBy = [ "graphical-session.target" ];
+      partOf = [ "graphical-session.target" ];
       serviceConfig.ExecStart = ''
         ${pkgs.xbanish}/bin/xbanish ${cfg.arguments}
       '';