nixos/greetd: Add greetd module (#118294)

* nixos/greetd: Add greetd module

* nixos/greetd: Configure 'greeter'-user

* nixos/greetd: Add default for restart-option

Co-authored-by: Jens Nolte <git@queezle.net>

nixos/modules/module-list.nix

@@ -358,6 +358,7 @@
   ./services/development/jupyter/default.nix
   ./services/development/jupyterhub/default.nix
   ./services/development/lorri.nix
+  ./services/display-managers/greetd.nix
   ./services/editors/emacs.nix
   ./services/editors/infinoted.nix
   ./services/games/factorio.nix

nixos/modules/services/display-managers/greetd.nix

@@ -0,0 +1,106 @@
+{ config, lib, pkgs, ... }:
+with lib;
+
+let
+  cfg = config.services.greetd;
+  tty = "tty${toString cfg.vt}";
+  settingsFormat = pkgs.formats.toml {};
+in
+{
+  options.services.greetd = {
+    enable = mkEnableOption "greetd";
+
+    package = mkOption {
+      type = types.package;
+      default = pkgs.greetd.greetd;
+      defaultText = "pkgs.greetd.greetd";
+      description = "The greetd package that should be used.";
+    };
+
+    settings = mkOption {
+      type = settingsFormat.type;
+      example = literalExample ''
+        {
+          default_session = {
+            command = "''${pkgs.greetd.greetd}/bin/agreety --cmd sway";
+          };
+        }
+      '';
+      description = ''
+        greetd configuration (<link xlink:href="https://man.sr.ht/~kennylevinsen/greetd/">documentation</link>)
+        as a Nix attribute set.
+      '';
+    };
+
+    vt = mkOption  {
+      type = types.int;
+      default = 1;
+      description = ''
+        The virtual console (tty) that greetd should use. This option also disables getty on that tty.
+      '';
+    };
+
+    restart = mkOption {
+      type = types.bool;
+      default = !(cfg.settings ? initial_session);
+      defaultText = "!(config.services.greetd.settings ? initial_session)";
+      description = ''
+        Wether to restart greetd when it terminates (e.g. on failure).
+        This is usually desirable so a user can always log in, but should be disabled when using 'settings.initial_session' (autologin),
+        because every greetd restart will trigger the autologin again.
+      '';
+    };
+  };
+  config = mkIf cfg.enable {
+
+    services.greetd.settings.terminal.vt = mkDefault cfg.vt;
+    services.greetd.settings.default_session = mkDefault "greeter";
+
+    security.pam.services.greetd = {
+      allowNullPassword = true;
+      startSession = true;
+    };
+
+    # This prevents nixos-rebuild from killing greetd by activating getty again
+    systemd.services."autovt@${tty}".enable = false;
+
+    systemd.services.greetd = {
+      unitConfig = {
+        Wants = [
+          "systemd-user-sessions.service"
+        ];
+        After = [
+          "systemd-user-sessions.service"
+          "plymouth-quit-wait.service"
+          "getty@${tty}.service"
+        ];
+        Conflicts = [
+          "getty@${tty}.service"
+        ];
+      };
+
+      serviceConfig = {
+        ExecStart = "${pkgs.greetd.greetd}/bin/greetd --config ${settingsFormat.generate "greetd.toml" cfg.settings}";
+
+        Restart = mkIf cfg.restart "always";
+
+        # Defaults from greetd upstream configuration
+        IgnoreSIGPIPE = false;
+        SendSIGHUP = true;
+        TimeoutStopSec = "30s";
+        KeyringMode = "shared";
+      };
+
+      # Don't kill a user session when using nixos-rebuild
+      restartIfChanged = false;
+
+      wantedBy = [ "graphical.target" ];
+    };
+
+    systemd.defaultUnit = "graphical.target";
+
+    users.users.greeter.isSystemUser = true;
+  };
+
+  meta.maintainers = with maintainers; [ queezle ];
+}