Bug#451535: debian-installer: allow to 'reuse' encrypted volumes

[M Santala <markoadsl2@gmail.com>]

Package: partman-crypto
Followup-For: Bug #451535

Dear Maintainer,

   * What led up to the situation?

Trying to do a fresh Buster installation using "Expert install" on an
old computer with encrypted LVM volumes, with the aim of not
formatting the volumes. I was using ISO image
debian-10.0.0-amd64-DVD-1.iso on a MicroSD card.

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

The encrypted partition was not recognized initially at all, however,
the embedded LVM volumes were recognised after manually opening the LUKS
wrapper (along the lines in Message #90).

NB. A boot into Rescue mode recognizes and opens the encrypted volume
correctly.

   * What was the outcome of this action?
   
The installer does not recognize the existing filesystems on LVM
volumes (unlike the Rescue mode). Proceeding with installation results
in formatting of the selected volumes and data loss.

This may be mitigated by selecting only a root volume at installation
stage and manually reconfiguring the other filesystems later.

   * What outcome did you expect instead?

The existing filesystems should be recognized and formatting should be
optional, just as it is with pre-existing native disk volumes.
[Does this work with unencrypted LVM?]

This bug has persisted for a long time and it bites long-term Debian
users who are upgrading their systems, causing loss of time and
data. This makes upgrade a challenge and encourages to keep obsolete
systems in operation.


-marko

-- System Information (the affected computer):
Debian Release: Wheezy
Architecture: amd64 (x86_64)