preseeded installation fails on critical question: partman/unmount_active DISKS /dev/vda

[Jean-Baptiste Lallement]

Preseed installation of xenial server daily (20160112) fails on the following question:

Jan 12 07:25:15 debconf: --> SUBST partman/unmount_active DISKS /dev/vda
Jan 12 07:25:15 debconf: Adding [DISKS] -> [/dev/vda]
Jan 12 07:25:15 debconf: <-- 0
Jan 12 07:25:15 debconf: --> FSET partman/unmount_active seen false
Jan 12 07:25:15 debconf: <-- 0 false
Jan 12 07:25:15 debconf: --> INPUT critical partman/unmount_active
Jan 12 07:25:15 debconf: <-- 0 question will be asked

full log attached.

Related bugs:
 * bug 524674: apt-check hangs, preventing login via SSH
 * bug 1527710: apt-get update triggers asynchronous task
 * bug 1533243: preseeded installation fails on critical question: partman/unmount_active DISKS /dev/vda
 * bug 1355845: partman/unmount_active is not preseedable

[Scott Moser (smoser)]

This was discovered via automated tests.
The last pass was at http://d-jenkins.ubuntu-ci:8080/job/xenial-server-i386-smoke-default/21/

[Scott Moser (smoser)]

The workaround fix was added/merged at https://code.launchpad.net/~canonical-platform-qa/ubuntu-test-cases/server-preseed/+merge/282346