nixos/systemd/userdbd: add method to enable service

This is recommended to enable in conjunction with systemd-homed.

nixos/modules/module-list.nix

···
   ./system/boot/systemd/shutdown.nix
   ./system/boot/systemd/tmpfiles.nix
   ./system/boot/systemd/user.nix
+  ./system/boot/systemd/userdbd.nix
   ./system/boot/timesyncd.nix
   ./system/boot/tmp.nix
   ./system/boot/uvesafb.nix

nixos/modules/system/boot/systemd/userdbd.nix

···
+{ config, lib, ... }:
+
+let
+  cfg = config.services.userdbd;
+in
+{
+  options.services.userdbd.enable = lib.mkEnableOption (lib.mdDoc ''
+    Enables the systemd JSON user/group record lookup service
+  '');
+  config = lib.mkIf cfg.enable {
+    systemd.additionalUpstreamSystemUnits = [
+      "systemd-userdbd.socket"
+      "systemd-userdbd.service"
+    ];
+
+    systemd.sockets.systemd-userdbd.wantedBy = [ "sockets.target" ];
+  };
+}

nixos/tests/all-tests.nix

···
   systemd-shutdown = handleTest ./systemd-shutdown.nix {};
   systemd-timesyncd = handleTest ./systemd-timesyncd.nix {};
   systemd-misc = handleTest ./systemd-misc.nix {};
+  systemd-userdbd = handleTest ./systemd-userdbd.nix {};
   tandoor-recipes = handleTest ./tandoor-recipes.nix {};
   taskserver = handleTest ./taskserver.nix {};
   tayga = handleTest ./tayga.nix {};

nixos/tests/systemd-userdbd.nix

···
+import ./make-test-python.nix ({ pkgs, lib, ... }: {
+  name = "systemd-userdbd";
+  nodes.machine = { config, pkgs, ... }: {
+    services.userdbd.enable = true;
+
+    users.users.test-user-nss = {
+      isNormalUser = true;
+    };
+
+    environment.etc."userdb/test-user-dropin.user".text = builtins.toJSON {
+      userName = "test-user-dropin";
+    };
+
+    environment.systemPackages = with pkgs; [ libvarlink ];
+  };
+  testScript = ''
+    import json
+    from shlex import quote
+
+    def getUserRecord(name):
+      Interface = "unix:/run/systemd/userdb/io.systemd.Multiplexer/io.systemd.UserDatabase"
+      payload = json.dumps({
+        "service": "io.systemd.Multiplexer",
+        "userName": name
+      })
+      return json.loads(machine.succeed(f"varlink call {Interface}.GetUserRecord {quote(payload)}"))
+
+    machine.wait_for_unit("systemd-userdbd.socket")
+    getUserRecord("test-user-nss")
+    getUserRecord("test-user-dropin")
+  '';
+})