nixos(steam): add gamescope session option

nixos/modules/programs/gamescope.nix

@@ -36,7 +36,8 @@ in
       type = types.bool;
       default = false;
       description = mdDoc ''
-        Add cap_sys_nice capability to the GameScope binary.
+        Add cap_sys_nice capability to the GameScope
+        binary so that it may renice itself.
       '';
     };
 

nixos/modules/programs/steam.nix

@@ -4,6 +4,24 @@ with lib;
 
 let
   cfg = config.programs.steam;
+  gamescopeCfg = config.programs.gamescope;
+
+  steam-gamescope = let
+    exports = builtins.attrValues (builtins.mapAttrs (n: v: "export ${n}=${v}") cfg.gamescopeSession.env);
+  in
+    pkgs.writeShellScriptBin "steam-gamescope" ''
+      ${builtins.concatStringsSep "\n" exports}
+      gamescope --steam ${toString cfg.gamescopeSession.args} -- steam -tenfoot -pipewire-dmabuf
+    '';
+
+  gamescopeSessionFile =
+    (pkgs.writeTextDir "share/wayland-sessions/steam.desktop" ''
+      [Desktop Entry]
+      Name=Steam
+      Comment=A digital distribution platform
+      Exec=${steam-gamescope}/bin/steam-gamescope
+      Type=Application
+    '').overrideAttrs (_: { passthru.providedSessions = [ "steam" ]; });
 in {
   options.programs.steam = {
     enable = mkEnableOption (lib.mdDoc "steam");
@@ -32,6 +50,12 @@ in {
             then [ package ] ++ extraPackages
             else [ package32 ] ++ extraPackages32;
         in prevLibs ++ additionalLibs;
+      } // optionalAttrs (cfg.gamescopeSession.enable && gamescopeCfg.capSysNice)
+      {
+        buildFHSEnv = pkgs.buildFHSEnv.override {
+          # use the setuid wrapped bubblewrap
+          bubblewrap = "${config.security.wrapperDir}/..";
+        };
       });
       description = lib.mdDoc ''
         The Steam package to use. Additional libraries are added from the system
@@ -57,6 +81,30 @@ in {
         Open ports in the firewall for Source Dedicated Server.
       '';
     };
+
+    gamescopeSession = mkOption {
+      description = mdDoc "Run a GameScope driven Steam session from your display-manager";
+      type = types.submodule {
+        options = {
+          enable = mkEnableOption (mdDoc "GameScope Session");
+          args = mkOption {
+            type = types.listOf types.string;
+            default = [ ];
+            description = mdDoc ''
+              Arguments to be passed to GameScope for the session.
+            '';
+          };
+
+          env = mkOption {
+            type = types.attrsOf types.string;
+            default = { };
+            description = mdDoc ''
+              Environmental variables to be passed to GameScope for the session.
+            '';
+          };
+        };
+      };
+    };
   };
 
   config = mkIf cfg.enable {
@@ -66,6 +114,19 @@ in {
       driSupport32Bit = true;
     };
 
+    security.wrappers = mkIf (cfg.gamescopeSession.enable && gamescopeCfg.capSysNice) {
+      # needed or steam fails
+      bwrap = {
+        owner = "root";
+        group = "root";
+        source = "${pkgs.bubblewrap}/bin/bwrap";
+        setuid = true;
+      };
+    };
+
+    programs.gamescope.enable = mkDefault cfg.gamescopeSession.enable;
+    services.xserver.displayManager.sessionPackages = mkIf cfg.gamescopeSession.enable [ gamescopeSessionFile ];
+
     # optionally enable 32bit pulseaudio support if pulseaudio is enabled
     hardware.pulseaudio.support32Bit = config.hardware.pulseaudio.enable;