How can I get lightdm to start xmonad?

Question

I've been running nixos with xmonad and using startx to start the xserver. However I decided I would like to have a greeter.

I first tried sddm by disabling startx and enabling sddm

services.xserver {
  displayManager {
    startx.enable = false;
    sddm.enable = true;
  };
};

I rebuilt nixos (nixos-rebuild switch) and the sddm appeared to work fine but when I entered my password correctly nothing happened. So being lazy and not commited to sddm, I decided rather than try and fix the issue I would just switch to another display manager, so I switched to lightdm

services.xserver {
  displayManager {
    startx.enable = false;
    lightdm.enable = true;
  };
};

And when I rebuilt a similar thing happened, lightdm starts fine but when I tried to log in it informs me "failed to login". So I checked the logs in /var/log/lightdm/lightdm.log.

The log is fairly long so here is a selection after I logged on

[+15.51s] DEBUG: Authenticate result for user ww: Success
[+15.51s] DEBUG: User ww authorized
[+15.51s] DEBUG: Greeter requests default session
[+15.56s] DEBUG: Seat seat0: Failed to find session configuration default
[+15.56s] DEBUG: Seat seat0: Can't find session 'default'
[+31.69s] DEBUG: Seat seat0 changes active session to 
[+68.26s] DEBUG: Seat seat0 changes active session to 2
[+701.81s] DEBUG: Got signal 15 from process 1
[+701.81s] DEBUG: Caught Terminated signal, shutting down
[+701.81s] DEBUG: Stopping display manager
[+701.81s] DEBUG: Seat seat0: Stopping
[+701.81s] DEBUG: Seat seat0: Stopping display server
[+701.81s] DEBUG: Sending signal 15 to process 997
[+701.81s] DEBUG: Seat seat0: Stopping session
[+701.81s] DEBUG: Terminating login1 session c1
[+701.87s] DEBUG: Session pid=1043: Sending SIGTERM
[+701.87s] DEBUG: Seat seat0: Stopping session
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe
[+701.87s] WARNING: Session pid=1061: Error writing to session: Broken pipe

So it looks like it cant' find the "default session". And after looking around a bit I create /usr/share/sessions/xmonad.desktop with the contents:

[Desktop Entry]
Encoding=UTF-8
Name=xmonad
Comment=This session starts xmonad
Exec=/home/ww/.local/bin/xmonad
Type=Application

Based on the xmonad faq. To reference this file I also create /usr/share/lightdm/lightdm.conf.d with the contents:

[SeatDefaults]
user-session=xmonad

Based on the lightdm page from the Ubuntu wiki.

However on reboot this does not solve the problem or meaningfully change the error logs.

How can I bridge the gap so that lightdm will start xmonad properly? I feel that I have a conceptual failing in understanding what is currently going on and what should be going on.

Answer 1

I am using XMonad with LightDM. Here is the relevant bit of my configuration.nix:

  services.xserver = {
    enable = true;
    windowManager = {
      default = "xmonad";
      xmonad = {
        enable = true;
        enableContribAndExtras = true;
      };
    };
    # On nixos-unstable I needed to add this deprecated setting (not sure if still needed)
    desktopManager.default = "none";
  };

I also have symlinked my .xsession to .xinitrc so my sessions started via {X,Light,G}DM would start the same programs.

ln -s .xinitrc .xsession