[Bug 1876506] Re: eoan to focal upgrade hangs when lvm snapshot is present

[David Wittman]

@juliank well, it's full now but here's what my snapshot (pre-20.04)
looks like in `lvs`:

```
  LV        VG        Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert                                                
  pre-20.04 vgubuntu  swi-I-s---  20.00g      root   100.00                                 
  root      vgubuntu  owi-aos--- 400.00g                                                    
  swap_1    vgubuntu  -wi-ao---- 976.00m  
```

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https://bugs.launchpad.net/bugs/1876506

Title:
  eoan to focal upgrade hangs when lvm snapshot is present

Status in ubuntu-release-upgrader package in Ubuntu:
  New
Status in ubuntu-release-upgrader source package in Focal:
  New

Bug description:
  When upgrading from 19.10 (w/ LVM + encypted disks) to 20.04, the
  installer hangs when trying to probe an LVM snapshot volume created
  before the upgrade.

  Steps to reproduce:

  lvcreate -s -L 20G -n preupgrade /dev/vgubuntu/root 
  do-release-upgrade -d

  The installer will hang after the following message:

  /usr/sbin/grub-probe: error: unknown filesystem.
  Found Ubuntu 19.10 (19.10) on /dev/mapper/vgubuntu-preupgrade
  done

  Resolution:

  Killing the grub-mount process which is probing the LVM snapshot seems
  to unhang the installer and allow the upgrade to complete without
  issue.

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