Can't log in after Stretch to Buster upgrade

[cgibbs@surfnaked.ca]

The scariest part of any system upgrade is that first re-boot.
Up until then, you're still running - but after that re-boot,
maybe the machine will come up, and maybe it won't.
Mine won't, and now I have to figure out how to fix it.

I found instructions on the web for upgrading Stretch to Buster,
and tried it first on my laptop, which doesn't contain that much
important data.  It went well, so I applied the same process to
my main desktop machine: apply all updates to get Stretch right
up to date, change /etc/apt/sources.list to point to the Buster
repositories, then do apt-get upgrade and dist-upgrade.

I divide my hard drive into three partitions:
   / - 10GB (increased to 20, see below)
   swap - memory size
   /home - remainder of disk
I upgraded another machine about six months ago, and ran
out of disk space in the process, which I was reminded of when
it happened again with my laptop.  It seems that although 10GB
used to be enough for /, this is no longer the case for Buster.
I have several games and a flight simulator installed, and /var
seems to contain a lot of stuff.  I booted gparted from a CD,
shaved 10GB off /home, moved the swap partition, and gave
the extra 10GB to /, and successfully completed the upgrade.

Being concerned about high disk usage, I came across a note that
suggested removing obsolete packages with "aptitude purge ?obsolete".
This is the one difference between my laptop and desktop upgrades.
I hadn't tried it on my laptop, which upgraded successfully, but I did
try it on my desktop machine, and I suspect it might have removed
too much.

The Buster upgrade seemed to work OK.  I re-booted and got to my
xfce login screen.  But when I entered my user ID and password,
the screen blanked for a second or so, then came back to a blank
login screen.  It's not a password problem; entering an incorrect
password gets the expected "password incorrect" prompt.  I can
attempt to log in either as my normal ID or as root, but the result
is the same.  Thus I can't access any logs to find out what's amiss,
and the boot messages scroll by too fast to read.  (I think I saw
several copies of a message in red that says something like
"Failed to start OpenBSD Secure Shell server".)  I can ping the
machine from elsewhere but can't ssh into it ("Connection refused").
This happens even if I try to boot the old kernel.

I can boot into rescue mode, and poke around that way.  It seems
that my files and settings are intact - at least enough of them are
there that I'd rather not wipe the machine and start over.  (I took
backups of /etc and /usr as well as /home before starting.)

Can Humpty Dumpty be put back together again?  If so, where
do I start?

--
cgibbs@surfnaked.ca (Charlie Gibbs)
If your nose runs and your feet smell, you're built umop-apisdn.