I run sid and am usually able to resolve the problems by myself.
The fact is that during some upgrades X gets killed unexpectedly,
without any warning, and on the console I see that some services
do not start. I usually reboot but it's annoying; a couple of kills ago
I tried to manually restart the failed services, and it's very time
consuming.
I still don't know *what upgrade(s)* kill(s) X. I have suspected
logind, libpam and udev, but I don't know for sure. I'll try udevadm
monitor as you suggested.
Hence I asked if anybody else experienced the same behaviour,
and implicitly what I could do to prevent those X kills.