Let users force display number to match seat number

[iphands]

I would like to have seat0 always have :0 and seat1 have :1.
Right now it seems if I try and force this using any/all of:

• xserver-command=/usr/bin/X :0
• xserver-display-number=0

I end up with a situation like:

$ ps aux | fgrep X
root       21768  4.6  0.0 6652828 78060 ?       S<l  23:26   0:00 /usr/bin/X :1 :0 -layout seat1 -seat seat1 -auth /var/run/lightdm/root/:0 -nolisten tcp
root       21770 12.5  0.0 25396784 64132 tty7   S<sl+ 23:26   0:01 /usr/bin/X :0 :1 -layout seat0 -seat seat0 -auth /var/run/lightdm/root/:1 -nolisten tcp vt7 -novtswitch

I think this is due to how the code appends display_number no matter what.
https://github.com/canonical/lightdm/blob/main/src/x-server-local.c#L480

And that number is selected via:
https://github.com/canonical/lightdm/blob/main/src/x-server-local.c#L157

Maybe we could support a forced_display_number and fail if its unavailable?
Or even attempt to line up with the seat number?
Or attempt to line up with a desired_display_number and fall back to next available if that does not work?

[iphands]

This was being looked for here: https://unix.stackexchange.com/questions/721870/force-seatn-to-display-n

[iphands]

Oh I even tried this after reading the code:

[Seat:seat0]
xserver-command=/usr/bin/X :0
xserver-layout=seat0
xserver-display-number=0
minimum-display-number=0

[Seat:seat1]
xserver-command=/usr/bin/X :10
xserver-layout=seat1
xserver-display-number=10
minimum-display-number=10

But I guess it doesnt work (same X :0 :1 and X :1 :0 behavior) because minimum-display-number is not valid for an individual seat?

Its a global conf only option or something?