Comment 1 for bug 1688508

Hi Christoph and thanks for your report and thereby help to make Ubuntu better!

The default value for this is PARALLEL_SHUTDOWN=10 so everybody would run into this issue.
I assume that there needs to be more to this than just "broken in general", so let us try to find what it is that makes this fail for you.

These scripts weren't touched a long time as they just used to work so far.
I wondered that for me "check_guests_shutdown" is on a different line (353) then.

That might just be a type or such, but to be sure could you check with verify if the package thinks the file is non default (after you remove your modification of course):
dpkg --verify libvirt-bin
I checked the md5 of the file in the version you referred to which is:
$ md5sum /usr/lib/libvirt/libvirt-guests.sh
611e4b35894329192f0313c1c2c639aa /usr/lib/libvirt/libvirt-guests.sh

Never the less I found the issue you are describing:
The assignment is:
444: on_shutdown=$(check_guests_shutdown "$uri" "$on_shutdown")
The report of the translated message it like:
361: eval_gettext "Failed to determine state of guest: \$guest. Not tracking it anymore."

While certainly broken and needing a fix this should at least still time out for you after the default of 2 minutes right?
You could lessen the timeout as the most convenient until a proper fix is there then.

Also the issue only occurs if function guest_is_on fails (so neither detected run, nor not running, but really failing). Eventually that executes:
$ virsh domname <uuid>
That should also fail in your case to trigger the issue - is there any obvious reason you'd know why that fails for you? The output of this should also be mixed into the result in your case, so maybe you find it there.

But while it is interesting to understand why this is triggering for you it is an issue none-the-less