System won't boot after failed upgrade from 23.10 to 24.05

[Walt Mankowski]

Hi.

I tried to upgrade from 23.10 to 24.04 today and it failed. It ran out
of disk space on /usr midway through the install, leaving things in a
weird state. I was able to run `apt upgrade` and install a bunch of
them, but several 100 more were still in a state where they were being
held. I tried doing another `apt do-release-upgrade`, but by that
point it thought it was already on 24.04 and so it returned
immediately.

At this point I tried rebooting to see if that would clear things up,
That was a mistake. Not it's not booting up. If gets pretty far into
the boot process then fails. The messages scroll off the screen and I
can't see all of them.. The last one says

Failed to start svscan.service - Daemontools service scanner.
See 'systemctl status svscan.service' for details.

Above that I see that dbus.service failed to start. And above that I
see a bunch of failed dependencies for half a dozen SSSD responder
sockets. I'm not able to scroll up beyond that.

I do nightly backups so with any luck I haven't lost any data. Is
there any way I can get back in there in single user mode and have it
finish the upgrade to 24.04?

Walt