preseeding for partman/installation_medium_mounted fails because 'seen' is forced to false

[John Morrissey]

partman/init.d/30parted always forces 'seen' to 'false' for partman/installation_medium_mounted. This prevents it from being preseeded, as Colin suggests in https://bugs.launchpad.net/ubuntu/+source/partman-base/+bug/344976.

--
  if [ "$part_warn" ]; then
   db_subst partman/installation_medium_mounted PARTITION "$part_warn"
   db_fset partman/installation_medium_mounted seen false
   db_capb align
   db_input high partman/installation_medium_mounted || true
   db_go || true
   db_capb backup align
  fi
--

https://bugs.launchpad.net/ubuntu/+source/partman-base/+bug/1355845 is the same problem, but for partman/unmount_active.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Neil Roza (realtime-neil)]

I can demonstrate that

    https://cdimage.ubuntu.com/ubuntu-legacy-server/releases/focal/release/ubuntu-20.04.1-legacy-server-amd64.iso

is affected. The installer prompts with

> Your installation medium is on /dev/sdb. You will not be able to create,
> delete, or resize partitions on this disk, but you may be able to install
> to existing partitions there.

If I preseed

    d-i partman/filter_mounted boolean false

then the prompt is silenced.