I admit I didn't spend much time investigating this issue (I need a
working Sid box), but from past experience, I guess it comes from a new
systemd-shim incompatibility introduced by some change in systemd-logind
(or GDM, but IMHO it's less unlikely than systemd).
The symptoms are identical to #801749 (but since people there mention
journalctl, I guess they were not using SysV init, so it can't be a
duplicate of this bug): GDM doesn't start (the screen keeps displaying
the console, with the cursor blinking frenetically), and syslog shows
lots and lots of "gdm3: Child process XYZ was already dead." and "gdm3:
Unable to kill session worker process".
lightdm (under SysV init) does start normally.
Switching the init system to systemd gets rid of the problem, and allows
GDM to start normally (this is why I'm filing this report against
Please note that I didn't try to downgrade systemd or GDM to find the
exact culprit (or the version introducing the change), I made an
educated guess and switched the init system directly, which worked. I
have a lot of stuff to do on my Sid box ATM and I can't afford the time.
Please also note that I don't reboot this box very often, so the
incompatibility may have been introduced by a systemd update a long time
ago, not necessarily the latest one.