[Bug 1838525] Re: LVM setup fails to install grub on virtio storage

[Rafael David Tinoco]

## SUMMARY OF THE ISSUE ##

After reading the grub-installer script, and following execution flow:

...

if [ "$found" = "0" ] && type lvdisplay >/dev/null 2>&1 && \
     (lvdisplay "$disc_offered" | grep -q 'LV Name' 2>/dev/null || \
      [ -e "$(dirname "$disc_offered")/control" ]); then
	# Don't set frdev/frdisk here, otherwise you'll end up in different
	# code paths below ...
	frtype=lvm
fi

...

default_bootdev_os="$($chroot $ROOT grub-mkdevicemap --no-floppy -m - | head -n1 | cut -f2)"
if [ "$default_bootdev_os" ]; then
	default_bootdev="$($chroot $ROOT readlink -f "$default_bootdev_os")"
	if db_get grub-installer/bootdev && [ "$RET" = '(hd0)' ]; then
		db_set grub-installer/bootdev "$default_bootdev"
	fi
else
	default_bootdev="(hd0)"
fi

DEBUG:

default_bootdev_os=/dev/disk/by-id/lvm-pv-uuid-IKvzNq-IRE5-arLC-hSS1-jDU2-ZSKA-90lATY
and bootdev wasn't set to anything because it isn't '(hd0)'.

...

elif [ "$(device_to_disk "$cdsrc")" = "$default_bootdev" ] || \
   ([ -n "$hdsrc" ] && [ "$(device_to_disk "$hdsrc")" = "$default_bootdev" ]) || \
   ([ "$default_bootdev" = '(hd0)' ] && \
    (([ -n "$cdfs" ] && [ "$cdfs" != "iso9660" ]) || \
     [ "$hybrid" = true ])) || \
   ([ "$default_bootdev" != '(hd0)' ] && \
    ! partmap "$default_bootdev" >/dev/null && \
    ! grub_probe -t fs -d "$default_bootdev" >/dev/null); then
	db_fget grub-installer/bootdev seen
	if [ "$RET" != true ]; then
		bootfs=$(findfs /boot)
		[ "$bootfs" ] || bootfs="$(findfs /)"
		disk=$(device_to_disk "$bootfs")
		db_set grub-installer/bootdev "$disk"
		state=2
	fi
fi

DEBUG:

bootdev=/dev/mapper after disk=$(device_to_disk bootfs)
since bootfs is bootfs=/dev/mapper/vgubuntu-root
and device_to_disk will remove last part from /dev/mapper/XXX.

bootfs is /dev/mapper/vgubuntu-root BECAUSE:

rootfs=$(findfs /)
bootfs=$(findfs /boot
[ -n "$bootfs" ] || bootfs="$rootfs"

so, device_to_disk() has cut "vgubuntu-root" from the string and that is
how bootdev has become "/dev/mapper" only.

----

Testing with a SCSI disk, I got the exact same DEBUG output, BUT ...

I was able to see that the INITIAL value for:

grub-installer/bootdev WAS "/dev/sda"

but, after purging it with "debconf-set grub-installer/bootdev ''", I
could NOT repopulate db variable again by executing only "grub-installer
/target", DIFFERENTLY than when executing  "grub-installer /target" in a
/dev/vda based LVM, when it ALWAYS gets populated like described above
(with /dev/mapper).

TODO:

discover why grub-installer/bootdev isn't populated back when re-
executing "grub-installer /target" after purging the original (right
after installation) value. Whenever bootdev defaults to
/dev/mapper/XXXX, bootdev should be replaced with /dev/sdXXX or
/dev/vdXXX, one of the LVM PV disks of under laying volume group (to
confirm).

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https://bugs.launchpad.net/bugs/1838525

Title:
  LVM setup fails to install grub on virtio storage

Status in debian-installer package in Ubuntu:
  New
Status in grub-installer package in Ubuntu:
  Triaged
Status in debian-installer source package in Eoan:
  New
Status in grub-installer source package in Eoan:
  Triaged
Status in debian-installer package in Debian:
  New

Bug description:
  The Eoan debian-installer ISO fails to install GRUB on LVM installs
  with virtio storage, as it runs grub-install with /dev/mapper as a
  target (a directory), even if instructed to target a device.

  The following steps to reproduce have been prepared running the
  20190730 build, but this has been broken since about June 18, 2019.
  Steps to reproduce:

  $ md5sum eoan-server-amd64.iso
  f591e30485e5f0b5117f6c116e538c42  eoan-server-amd64.iso
  $ qemu-img create -f raw disk1.img 8G
  Formatting 'disk1.img', fmt=raw size=8589934592
  $ kvm -m 1024 -boot d -cdrom eoan-server-amd64.iso -drive file=disk1.img,if=virtio

  Proceed with all the defaults. In the "Partition disk" step select
  "Guided - use entire disk and set up LVM". Go ahead accepting the
  defaults. At the "Install the GRUB boot loader" step select "/dev/vda"
  as the target device. The installer will actually run `grub-install
  --force /dev/mapper` and fail after a while. The wrong command is
  visible both in the d-i screen and by running `ps` on a different
  console.

  Full installer syslog: http://paste.ubuntu.com/p/qtZy86dTp6/

  It's interesting how this doesn't happen when not using virtio. If
  from the commands above the "if=virtio" option is dropped then
  everything works as expected. In this case the target block device is
  called /dev/sda instead of /dev/vda.

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