Comment 12 for bug 1670291

Seems like there's two issues here which I'll address in separate PRs to the landscape-client:

First is the issue that the landscape-client fails to report the completion (or failure) of the release upgrade process. This turns out to be due to a delayed import in the version of twisted distributed with trusty. Because the import (of twisted.internet.unix) is delayed, by the time we *do* import it (implicitly by calling another twisted method), the twisted version on disk has been replaced with the xenial version and things (unsurprisingly) break. The trivial fix is to pre-emptively import the module prior to starting the release upgrade; I'll push a PR for this in a moment.

The second issue is that landscape-client fails to reboot (or shutdown) the machine immediately following the release upgrade procedure. This is due to the fact landscape-client currently relies upon /sbin/shutdown to handle reboot or shutdown requests. This utility operates happily in its upstart variant (in trusty), and systemd variant (in xenial and beyond), but fails in the post-trusty-to-xenial upgrade environment when the system on disk is effectively xenial (so /sbin/shutdown is a link to systemctl), but the running environment is still partially trusty. It transpires that systemctl (without the double --force --force option) requires both systemd and dbus to be operational in order to successfully (and cleanly) shutdown or restart.

The fix for this is a little more complex as the landscape-client relies upon the scheduling function of /sbin/shutdown to ensure it can request a shutdown and still report back to the server before the machine actually goes down. I'm still investigating the best method which will work reliably in all potential environments.