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

[lunix]

I apologize if this is not relevant to this blocking bug.  I am trying to help a friend.
Machine is dual boot Windows 10 and Ubuntu 18.04  Has Extended Partition where Ubuntu went into Parition 5.  Machine worked fine with 18.04.  Tried upgrade offered to Ubuntu 19,04 which failed and won't no longer boots to Ubuntu selection from GRUB but Windows still boots.  Tried a fresh install of 20.04 which resulted in:

error: file '/boot/grub/i386-pc/normal.mod' not found.
Entering rescue mode...

So won't boot either OS now.

I am hopeful to do a fresh install of Ubuntu 18.04-5 next in hopes that
the machine will work again.  Attached is the Disk partitioning table if
this is of interest.

** Attachment added: "Partioin info."
   https://bugs.launchpad.net/ubuntu/+source/os-prober/+bug/1876506/+attachment/5402221/+files/mort-disk.png

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

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

Status in os-prober package in Ubuntu:
  Confirmed
Status in os-prober source package in Focal:
  Confirmed

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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