[systemd-devel] What exactly is multi-seat? -- questions about logind

[Pekka Paalanen]

On Tue, 26 Jan 2021 01:43:43 -0800
Kian Kasad <kian at kasad.com> wrote:

> Hi all,
> 
> After reading the documentation on logind and multi-seat (specifically
> sd-login(3) and "Multi-Seat on Linux"), I still have some questions.
> 
> First of all, what exactly is multi-seat? Does it just mean allowing
> multiple sessions to be running at once, like for multiple users to be
> logged into the same desktop, even though only one will be in use at a
> time?
> 
> Second, why is logind needed for this? Is it not possible to do without
> logind? I've run Xorg perfectly fine on systems without logind/systemd,
> so is logind only needed for multiple sessions at once? Is it just to
> handle the graphical device(s)?
> 
> Third, is multi-seat possible at all without logind? 
> 
> Most of these questions arose because my main OS, Artix Linux, requires
> logind (in the form of elogind) for Xorg, but I know that Xorg runs just
> fine on Alpine Linux, which does not use logind at all.

Hi,

I've heard good words of https://sr.ht/~kennylevinsen/seatd/ although I
haven't tried to use it myself. One more way to handle multiseat - to
bind them all.

IOW, there are many ways to achieve coordination between multiple
sessions and one or multiple physical seats, like the Linux virtual
terminal system that requires root permissions, ioctl and signal magic
to let things cooperate (and if things do not explicitly cooperate
correctly, it fails horribly) and does not support multiseat. But if
you want your program to not need root permissions, have a graceful
failure if it crashes, not accidentally hijack the seat by mistake, or
just support multiseat, you kind of need to have a system daemon
orchestrating things.

Logind is one of those possible system daemons. It tells your program
when it is active. It grants access to graphics and input devices
without your program needing root permissions. It also takes graphics
and input devices away from your program when deactivated, so you can't
e.g. spy on other programs' input. It takes care of the arcane magic of
setting up the VT and tty and restoring them. And more.

"Your program" here is Xorg, for instance. Or one of the Wayland
compositors.


Thanks,
pq
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