[Bug 1891721] Re: grub-multi-install attempts to mount a non-existing device after migrating to another system

[Dawid Wróbel]

@Seth, I tried dpkg-reconfigure through and through, it didn't help.

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Title:
  grub-multi-install attempts to mount a non-existing device after
  migrating to another system

Status in grub2 package in Ubuntu:
  Confirmed

Bug description:
  I recently moved my Ubuntu installation from Parallels to Virtualbox,
  which naturally resulted in updated disks' UUIDs. Normally that should
  be a problem as Ubuntu should autodetect such change and act
  accordingly.

  However, upon upgrading grub-efi-amd64-signed, dpkg kept failing with:

  Setting up grub-efi-amd64-signed (1.142.4+2.04-1ubuntu26.2) ...
  mount: /var/lib/grub/esp: special device /dev/disk/by-id/ata-Ubuntu_Linux-1_SSD_SF74YQ6DP1KMD07F8JED-part1 do
  es not exist.

  The Parallel's disk ID that it attempts to mount indeed no longer
  exists. The EFI partition is correctly mounted at /boot/efi, since the
  /etc/fstab uses partition' UUID to address it and that didn't change
  during the migration, so there's no reason why the script should be
  failing.

  The debconf shows the following:

  dawidw at ubuntu:~$ sudo debconf-get-selections | grep grub-efi
  grub-efi-amd64	grub2/kfreebsd_cmdline	string
  grub-efi-amd64	grub-efi/install_devices	multiselect	/dev/disk/by-id/ata-Ubuntu_Linux-1_SSD_SF74YQ6DP1KMD07F8JED-part1, /dev/disk/by-id/ata-VBOX_HARDDISK_VB1-1a2b3c4d-part1
  grub-efi-amd64	grub2/force_efi_extra_removable	boolean	false
  grub-efi-amd64	grub2/linux_cmdline_default	string	quiet splash i8042.nopnp
  grub-efi-amd64	grub2/update_nvram	boolean	true
  grub-efi-amd64	grub2/linux_cmdline	string
  grub-efi-amd64	grub2/unsigned_kernels	note
  grub-efi-amd64	grub2/no_efi_extra_removable	boolean	true
  grub-efi-amd64	grub2/device_map_regenerated	note
  grub-efi-amd64	grub-efi/install_devices_disks_changed	multiselect	/dev/disk/by-id/ata-Ubuntu_Linux-1_SSD_SF74YQ6DP1KMD07F8JED-part1, /dev/disk/by-id/ata-VBOX_HARDDISK_VB1-1a2b3c4d-part1
  grub-efi-amd64	grub-efi/install_devices_empty	boolean	false
  grub-efi-amd64	grub-efi/install_devices_failed	boolean	false
  grub-efi-amd64	grub2/kfreebsd_cmdline_default	string	quiet splash

  Note that it recored the new, correct "/dev/disk/by-id/ata-
  VBOX_HARDDISK_VB1-1a2b3c4d-part1" UUID of the disk that contains the
  EFI partition.

  I debugged it further and can see that the failure happens in /usr/lib/grub/grub-multi-install (which is part of grub-common 2.04-1ubuntu26.2 package):
  1) in "# We either migrate /boot/efi over, or we check if we have invalid devices" section, it correctly detects that the Parallels device does not exist ("if [ ! -e "${device%,}" ]", setting the valid=0
  2) given that valid=0, my understanding is that at this point the "install_devices_disks_changed" prompt should be triggered, but it is not.
  3) the "$RET" variable still contains both of the disks from grub-efi/install_devices
  4) in "# Run the installer" section, the script iterates over each of the disks, attempting to mount ("mount $real_device $mntpoint) the non-existing one, which results in the aforementioned script failure.

  My understanding here is that there are two problems:
  1) "install_devices_disks_changed" prompt was not shown
  2) there's no check for whether the device exists at all before the script proceeds with mounting and grub-installation attempt

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