nixos/sway: add enableRealtime option

This option wraps the `sway` binary to allow it to request realtime
scheduling (SCHED_RR). This allow for possible lower latency, specially
when the system is under high load.

Available since Sway 1.8. See:
- https://github.com/swaywm/sway/pull/6994
- https://github.com/swaywm/sway/releases/tag/1.8

nixos/modules/programs/wayland/sway.nix

@@ -42,6 +42,11 @@ in {
       <https://github.com/swaywm/sway/wiki> and
       "man 5 sway" for more information'');
 
+    enableRealtime = mkEnableOption (lib.mdDoc ''
+      add CAP_SYS_NICE capability on `sway` binary for realtime scheduling
+      privileges. This may improve latency and reduce stuttering, specially in
+      high load scenarios'') // { default = true; };
+
     package = mkOption {
       type = with types; nullOr package;
       default = defaultSwayPackage;
@@ -149,6 +154,14 @@ in {
             "sway/config".source = mkOptionDefault "${cfg.package}/etc/sway/config";
           };
         };
+        security.wrappers = mkIf (cfg.enableRealtime && cfg.package != null) {
+          sway = {
+            owner = "root";
+            group = "root";
+            source = "${cfg.package}/bin/sway";
+            capabilities = "cap_sys_nice+ep";
+          };
+        };
         # To make a Sway session available if a display manager like SDDM is enabled:
         services.xserver.displayManager.sessionPackages = optionals (cfg.package != null) [ cfg.package ]; }
       (import ./wayland-session.nix { inherit lib pkgs; })