Recent logind changes break GNOME

[AdrianVovk]

systemd version the issue has been seen with

256 git

Used distribution

GNOME OS git

Linux kernel version used

6.6.10

CPU architectures issue was seen on

x86_64

Component

systemd, systemd-logind

Expected behaviour you didn't see

Able to log into my GNOME session

Unexpected behaviour you saw

Couldn't log into my GNOME session. The moment gnome-shell starts, whatever was on screen freezes and remains frozen. The rest of the session runs just fine underneath the frozen display output

I bisected this to 4cb4e6c, and then further tracked the bug. Here's what's happening: as of the commit user@.service has started running in a logind session, and thus XDG_SESSION_ID is now set by pam_systemd for the manager. The manager propagates this to all the services it starts. But the problem is that XDG_SESSION_ID is set to the ID of the manager's session, not the actual user session! The manager's session, notably, does not have a seat or TTY set. So, once gnome-shell is started as a systemd service, it sees the XDG_SESSION_ID variable and decides to skip all of its special logic looking for the logind session. It then looks up the session's seat (again, for the wrong session!), finds none, and declares that it should run headlessly. Thus it makes no attempt to put anything on the screen, and the GPU driver leaves a frozen copy of whatever was shown last instead. Underneath, however, GNOME is still running in headless mode, which allows the rest of the session to startup correctly.

Steps to reproduce the problem

• Run a systemd version w/ 4cb4e6c
• Try to log into GNOME Shell

Additional program output to the terminal or log subsystem illustrating the issue

Feb 12 20:24:51 gnomeos gnome-shell[3694]: No seat assigned, running headlessly