nixos/gnome3: launch gnome-shell wayland with RT scheduling

We create a wrapper which launches gnome-shell with the correct environment and
cap_sys_nice.

We can then override gnome-shell-wayland.service to use this wrapper.

NOTE: We need to force clear the environment, because the defaults aren't good
for user services. That should probably be fixed.

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -269,6 +269,26 @@ in
         source-sans-pro
       ];
 
+      ## Enable soft realtime scheduling, only supported on wayland ##
+
+      security.wrappers.".gnome-shell-wrapped" = {
+        source = "${pkgs.gnome3.gnome-shell}/bin/.gnome-shell-wrapped";
+        capabilities = "cap_sys_nice=ep";
+      };
+
+      systemd.user.services.gnome-shell-wayland = let
+        gnomeShellRT = with pkgs.gnome3; pkgs.runCommand "gnome-shell-rt" {} ''
+          mkdir -p $out/bin/
+          cp ${gnome-shell}/bin/gnome-shell $out/bin
+          sed -i "s@${gnome-shell}/bin/@${config.security.wrapperDir}/@" $out/bin/gnome-shell
+        '';
+      in {
+        # Note we need to clear ExecStart before overriding it
+        serviceConfig.ExecStart = ["" "${gnomeShellRT}/bin/gnome-shell"];
+        # Do not use the default environment, it provides a broken PATH
+        environment = mkForce {};
+      };
+
       # Adapt from https://gitlab.gnome.org/GNOME/gnome-build-meta/blob/gnome-3-32/elements/core/meta-gnome-core-shell.bst
       environment.systemPackages = with pkgs.gnome3; [
         adwaita-icon-theme