remotely connect to existing x session without unlocking local display (or while another user is logged in locally)

[Bruno Gravato]

Hi,

First of all I'm sorry if I'm posting this in the wrong place (if so please
point me in the right direction), but I've been struggling with this for a
long time and I've searched the web extensively but I haven't found a
working solution...


*Scenario:*

I have one workstation running Debian 10, installed DE are MATE and XFCE
with lightdm as display manager.
Two users (lets say mary and john) use this workstation locally, each one
has it's own login.
Both have their local xsessions running with a few open windows. When one
walks away (s)he locks his session and the other can login and resume their
running session.


*Problem to be solved:*

Sometimes mary is using the workstation and john wants to remotely have
access to its running session and all the windows running there and resume
it's work *while mary is logged in locally*.

Or even if no one is locally logged, security-wise it would still be nice
not to have the display unlocked locally giving access to any bystander.


*What I've tried:*

I've tried multiple servers: x11vnc, x2go (with "connect to local desktop"
on the client) and xpra (using shadow feature). With all of them I get the
same result: I can login remotely and access the running session only if
it's unlocked on the local display, otherwise all I get is a black screen.
I can unlock it remotely, that's not the issue, but if the other user is
logged in locally, I can't unlock my session on the local display unless I
lock out the other user.


*Partially working workarounds:*

I can start a new remote session with x2go or xpra and detach it and
reattach to it, so one possible workaround I found is to always use a
remote session inside x2go client or xpra even when logged in locally at
the workstation.
This of course has some limitations and glitches... For example hardware
video acceleration usually not available or with limitations. I'm ok with
not having VA or any glitches when accessing remotely, I expect that, but
locally I'd rather not have them.


*Possible solutions I haven't tried:*

Try other servers such as xrdp, but I suspect the result would be the same.

Run two x sessions locally on different ttys. Not sure how easy would be to
pull this one out or how well this will work with lightdm (or any other
display manager) and locking the screen locally.
I can use I different display manager other than lightdm if that helps, but
I'd prefer to stick to XFCE or MATE as the DE if possible.


*What else can I try?*

Is what I'm trying to achieve possible in any way?
Which software and approach should I use?
Any tips are most welcome.


Thank you very much for your time reading this,
Bruno
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