Running multiple Wayland window managers in different ttys

[Matt Hoosier]

Hi Deepak,

On Fri, Oct 20, 2017 at 11:50 AM, Deepak Jois <deepak.jois at gmail.com> wrote:
> Hi
>
> I have read the Wayland docs and skimmed through the API reference,
> and I am trying to understand some concepts better. The questions
> below are probably a bit naive, but I appreciate any
> suggestions/explanations.
>
> To illustrate my situation better, let me start with an actual
> scenario I tried to make work. I am trying to run Gnome and another
> Wayland-based window manager (Sway or Weston) in two different ttys as
> the same user.
>
> I have Gnome running under Wayland on tty2. The Gnome session is
> running some app like gnome-terminal.
>
> I do the following:
>
> * Press Ctrl-Alt-F3 (switching to tty3, the lowest free tty)
> * Login as the same user running the Gnome session
> * Launch sway
> * Launch urxvt and try to launch gnome-terminal.
>
> Now I have two issues I am trying to understand better:
>
> 1. When I check the WAYLAND_DISPLAY variable in the sway session, it
> shows 'wayland-0', which is the same as the one show in the Gnome
> session. Why is it the same? Does Wayland have a concept of multiple
> servers/sessions per user? Can I run the Gnome session and the Sway
> session separately?

Although you're right that the goal here is to bind the different
Wayland sessions' values for WAYLAND_DISPLAY to unique virtual
terminals, that won't happen automatically just because you launched
them from separate tty's.

The trick here is to inform each compositor that it should be
listening on a different Wayland socket name. For example, if you use
Weston:

    $ weston --socket=wayland-1

This will cause Weston to establish its listening socket under the
name /run/user/<uid>/wayland-1 rather than the default
/run/user/<uid>/wayland-0. I think you're running into collisions
where each of your compositor instances is mislead into believing that
it successfully managed to appropriate /run/user/<uid>/wayland-0 for
itself.

Also (at least for Weston), the value of the '--socket' parameter is
used to automatically compute and set the value of the
$WAYLAND_DISPLAY environment variable which is then passed along to
all children of the compositor. Presumably for you, your desktop
environment (e.g., Weston's desktop-shell) is the starting point from
which you're launching other graphical programs. They will all in turn
inherit this correct value of $WAYLAND_DISPLAY too.

>
> 2. I tried explicitly setting the WAYLAND_DISPLAY to wayland-1. It has
> no effect on Sway. The WAYLAND_DISPLAY variable is reset inside the
> session to wayland-0. But when I try to launch Weston, it errors out
> (log below). Is this expected, or is this a bug?

The way to get Weston to pass the right value of WAYLAND_DISPLAY to
its spawn is to use --socket=.

It's more difficult to invoke the Wayland gnome-session directly. I'm
not sure how you would micromanage its selection of the server socket
name.

-Matt