Untangling the mess that gui startup has become

[Daniel van Vugt]

Hi Phillip,

If "the gnome-shell greeter does not appear to give you any choice in 
what login session you get" then that means the 'eglnative' back-end 
(Wayland) crashed on start-up so the Wayland login option gets silently 
removed.

If you wrote that 3 weeks ago and are using 18.10 then I think it may 
have been this bug:

   https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1786883

So just update your system and that will be fixed.

If it's not fixed and you still don't get any Wayland login option from 
gdm3 then please log a bug about that by running:

   ubuntu-bug gdm3

- Daniel


On 12/09/18 03:33, Phillip Susi wrote:
> Resending this after subscribing since after waiting for 3 weeks, no
> moderator has released it.
> 
> Things used to be fairly simple.  You ran startx, and it followed
> ~/.xinitrc which directed it to pick a display number and run XFree86 on
> that display number, configure it to use an XAUTHORITY file, set the
> environment variables to point to that display and XAUTHORITY file, then
> run your window manager and shell ( which were actually separate things ).
> 
> Fast forward 25 years and now it looks like you have a display manager (
> gdm3 or lightdm ).  It looks like gdm3 looks in /etc/gdm3/custom.conf,
> where you can tweak things but by default, it doesn't say much and gdm3
> defaults to running gdm-x-session, which has no man page.  It looks like
> it is a clone of gnome-session ( why not just use gnome-session? ) It
> apparently sets up XAUTHORITY and runs Xorg and tells it which display
> and XAUTHORITY file to use.  At this point under lightdm I think there
> were some config files that direct it to run unity-greeter to prompt you
> to login, but it looks like gnome-shell is being run on vt1 and it must
> have a built in greeter function, in addition to being a shell and
> window manager.  After logging in, gdm3 appears to fork
> gdm-session-worker ( no man page ) on the next unused vt.  It runs
> gnome-session-binary ( whose man page appears to be just gnome-session
> ). It is directed to set up a specific type of session chosen at the
> greeter or defaulted, and how to setup these sessions is defined in
> files in /usr/share/gnome-session/sessions.  These files seem to list a
> bunch of DBUS names so unlike other kinds of session files used by other
> display managers that just tell them what programs to launch,
> gnome-session must somehow punt to DBUS to launch them.  Though oddly,
> it appears that gnome-shell is forked from gdm-session-worker rather
> than the dbus daemon.  That is then both your window manager and shell.
> 
> Now for some reason, the gnome-shell greeter does not appear to give you
> any choice in what login session you get ( why? ), but lightdm does, so
> there you can choose to log in using a wayland session, and it goes
> through an entirely different set of config files that end up directing
> it to just run gnome-shell, which now besides being a shell and window
> manager, is also a wayland compositor, replacing Xorg.  gnome-shell
> appears to automatically fork Xwayland, and forgets to setup an
> XAUTHORITY for it.
> 
> A few months ago gdm did give you a choice to log in with wayland and
> iirc from running it and looking at its code, it decided to run
> gnome-shell and Xwayland.  I guess gnome-shell notices if Xwayland was
> already running and wouldn't fork one itself if so.
> 
> Do I have that about right and can anyone fill in the missing pieces?
> 
> 
> 
>