lightdm can't start a GNOME wayland session

[worldofpeace]

Issue description

Mostly opening for investigation, someone in IRC mentioned that they couldn't
get a wayland session to start when using lightdm, as they noticed this when $XDG_SESSION_TYPE == x11.

I was then able to reproduce this within a VM.

Steps to reproduce

1. Enable GNOME3 and use lightdm as a display-manager
2. Try to start the GNOME session
3. Realize that you're running the gnome-xorg session

---

cc @hedning @jtojnar

[hedning]

Yeah, I didn't turn it on in the gnome wayland pr as lightdm didn't handle the duplicate gnome.desktop sesssion names:

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

Line 48 in f3a676a

sessions-directory = ${dmcfg.session.desktops}/share/xsessions

#53843 turns it on (note this pr needs to be updated for pantheon).

[worldofpeace]

Yeah, I didn't turn it on in the gnome wayland pr as lightdm didn't handle the duplicate gnome.desktop sesssion names

Thanks for the heads up

#53843 turns it on (note this pr needs to be updated for pantheon).

I'll push some fixup commits to that branch so it can work for pantheon

[hedning]

Cheers :)

According to upstream it's up to the specific greeter to handle duplicates: canonical/lightdm#16

[lePerdu]

This isn't specific to LightDM: GDM does the same thing for me. I don't know how all of the XSession stuff works, but I looked at the .desktop files in /nix/store/...gnome-session... (xsessions/gnome.desktop, xsessions/gnome-xorg.desktop, and wayland-sessions/gnome.desktop) and found them identical except for the "Name" field. Am I missing something, or should the "Exec" fields (and possibly others) be different for the XOrg and Wayland sessions?

EDIT: The corresponding files are the same in Fedora (which runs the Wayland session), so I guess that isn't the problem (GDM still runs the X session for both in NixOS, however)

[hedning]

Yeah, there's some behind the scenes stuff being done depending on where the desktop files reside (ie. wayland-sesisons/ vs. xsessions/)

GDM still runs the X session for both in NixOS

Works here. nixos-18.09 doesn't have gnome wayland support, that might be what you're seeing. Otherwise I would double check that services.xserver.displayManager.gdm.wayland is true (should be by default). There might be some hardware thing too (eg. nvidia card with the proprietary drivers, don't think we support that yet).

[lePerdu]

I'm on the nixos-unstable channel and can start the GNOME wayland session manually with

env XDG_SESSION_TYPE=wayland dbus-run-session /nix/store/...gnome-session-3.28.1/bin/gnome-session

[worldofpeace]

LightDM can now start wayland sessions, it needed a patch upstream that I provided

• 2cb421a

However it is broken with GNOME still. Haven't had time to debug it.

[hedning]

I'm guessing lightdm finds the xsession first (since there's a gnome.desktop in both). Should probably work to just add the wayland session dir before the xsession one. Checking now.

[worldofpeace]

Oh, I forgot to mention how when I tested, I worked around the duplicate session name bug (which plasma5 actually compensates for in sddm, gnome could do similar).
I removed the xsessions from gnome-session in testing to be sure.

[hedning]

Ahh, right, using the wayland dir first worked, in the sense that it tries to launch a wayland session. But something is indeed broken as it won't start the session.