loginctl lock-session isn't working as services.xserver.xautolock.locker

[misuzu]

I'm trying to setup xautolock with slimlock as locker. Using slimlock directly as services.xserver.xautolock.locker does not preserve SLIM_CFGFILE and SLIM_THEMESDIR environment variables so slimlock uses default slim theme.
There is workaround to start slimlock via xss-lock triggered by loginctl lock-session but here is what i get:

Aug 23 16:26:08 localhost xautolock[27959]: Failed to issue method call: Caller does not belong to any known session

Seems that loginctl lock-session wants $XDG_SESSION_ID (systemd/systemd#6032) but xautolock process does not have it.

Adding $XDG_SESSION_ID here may be a fix for this.

nixpkgs/nixos/modules/services/x11/display-managers/default.nix

Line 79 in c6a082b

${config.systemd.package}/bin/systemctl --user import-environment DISPLAY XAUTHORITY DBUS_SESSION_BUS_ADDRESS

Here is what home-manager does: https://github.com/rycee/home-manager/blob/eb1b86a5ec7baf1a1ce2c277d568a8751c24a7ee/modules/xsession.nix#L80

Maybe adding some kind of importedVariables option is even better.

My xserver config:

{ config, pkgs, ... }:

{
  services.xserver = {
    enable = true;
    autorun = true;

    displayManager = {
      slim = {
        enable = true;
        theme = pkgs.fetchurl {
          url = "https://maxwell.ydns.eu/git/rnhmjoj/nix-slim/archive/0.3.0.tar.gz";
          sha256 = "1m0l1bmagxq4i8db51ldgxwpyhrqh9fnm4szqcz871r8q3fqgl3k";
        };
      };
    };

    xautolock = {
      enable = true;
      locker = ''${pkgs.systemd}/bin/loginctl lock-session ''${XDG_SESSION_ID}'';
      time = 1;
    };

    windowManager.dwm.enable = true;

    desktopManager = {
      default = "none";
      xterm.enable = false;
    };
  };
  environment.systemPackages = with pkgs; [
    xss-lock
  ];
}

To Reproduce
Steps to reproduce the behavior:

1. Use config from above
2. Login to your account
3. Start xss-lock: xss-lock -- slimlock &
4. Run loginctl lock-session to make sure xss-lock is working
5. Login to your account again
6. Wait one minute and check logs: journalctl --user -u xautolock

Expected behavior
slimlock starts with correct theme

Metadata

% nix run nixpkgs.nix-info -c nix-info -m
 - system: `"x86_64-linux"`
 - host os: `Linux 5.2.9, NixOS, 19.09pre190139.54c76638733 (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2.2`
 - channels(root): `"nixos-19.09pre190139.54c76638733, home-manager"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
- services.xserver.xautolock.locker
# a list of nixos modules affected by the problem
module:
- services.xserver.displayManager
- services.xserver.xautolock

[misuzu]

services.xserver.displayManager.sessionCommands = pkgs.lib.mkAfter ''
  ${pkgs.xss-lock}/bin/xss-lock -- ${pkgs.slim}/bin/slimlock &
  ${pkgs.xorg.xset}/bin/xset s 300
'';

This did the trick.