Added dpkg -l to the buildbot's upgrade/upgrade2 to find out what it conflicts with. It turns out we have MySQL 5.6 pre-installed on vivid upgrade images.
I suppose if we really want upgrade from 5.6 to work, something similar to MDEV-5977 should be done in regard to 5.6.

link for 10.0: http://buildbot.askmonty.org/buildbot/builders/kvm-deb-vivid-amd64/builds/146/steps/test_1/logs/stdio

For the record, I solved these issues a few days ago in my repo https://github.com/ottok/mariadb/tree/ok-debpkg-10.1 but I cannot open a pull request as the buildbot column is not all green yet. due to test failures that most likely are inherited from failing master 10.1 branch, though I am not sure.

otto, should we disable these tests on 10.0, or is there a plan to backport the fix there?

elenst Fix available at https://github.com/MariaDB/server/pull/124 but the test is not fully green yet because mariadb-10.0 from mariadb.org does not seem compatible out-of-the-box with mysql-5.6 systemd stuff in Ubuntu. Accepting that PR would though get us a step forward and it is backwards safe, should not break anything for anything else.

Re-opening for backporting to 10.0, see the pull request above.