[Bug 2060695] Re: 24.04 grub-pc cannot upgrade on mirrored software RAID root disk

[Chad Smith]

@Mate thanks for the patch suggestion and triage work here.

While this approach will work for live-server/live-desktop and or Ubuntu
cloudimage which have downstream grub2 deb fixes for handling the 'new'
grub-pc/cloud_style_intialization debconf setting, are we missing other
use-cases that will fall over if cloud-init never attempts to detect the
proper boot device on image launch?

Does this issue also affect debian downstreams too which may not set
grub-pc/cloud_style_initialization? -  I'm not seeing the comparable
changes in upstream debug grub2 that'd take care of properly determining
the boot device based on the debconf grub-pc/cloud_style_installation
boolean.


This bug feels like it is reminiscent of LP: 1993503  which affects live server(subiquity/curtin based) installs which are perfoming that disk setup in the first place.

I'm concerned about complete removal of cloud-init's grub_dpkg module as a solution because it's a big hammer. 
Without cloud-init's cc_grub_dpkg module, cloud-init may not find the right boot devices if grub2 doesn't support the grub-pc/cloud_style_installation boolean or if subiquity wasn't involved in the initial disk setup.

An additional concern is image creation tools like packer currently rely
on cloud-init's behavior to detect and correct debconf grub-
pc/install_devices values in 'first boot' scenerios to ensure the boot
device is found https://bugs.launchpad.net/cloud-
init/+bug/1993503/comments/6. Dropping the module completely from
cloud.cfg prevents any workaround in user-data or vendor-data to re-
enable this module for some unstatisfied corner-cases.


If we were to change anything in cloud-init's behavior of related to grub2, maybe we limit this changeset to setting the default of the "grub_dpkg" config module[1] to "enabled: false" so it makes no changes by default. This would still permit users or platforms the ability to provide "grub_dpkg: {enabled: true}" in either #cloud-config userdata or cloud vendor-data if the default behavior was insufficient.

[1] https://github.com/canonical/cloud-
init/blob/24.1/cloudinit/config/cc_grub_dpkg.py#L148

>From live installer(subiquity/curtin) derived images which are
performing disk setup, it would be possible for those images to provide
/etc/cloud/cloud.cfg.d/ configuration snippet to disable cloud-init's
cc_grub_dpkg configuration module which would prevent this specific
issue in the first place as cloud-init would not attempt to use grub
probe to provide debconf selections to grub-pc in when it encourters
this config.

grub_dpkg:
  enabled: false

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https://bugs.launchpad.net/bugs/2060695

Title:
  24.04 grub-pc cannot upgrade on mirrored software RAID root disk

Status in subiquity:
  Triaged
Status in cloud-init package in Ubuntu:
  New
Status in grub2 package in Ubuntu:
  Invalid
Status in subiquity package in Ubuntu:
  Triaged

Bug description:
  I am testing the 24.04 pre-beta in a libvirt virtual machine with two
  /dev/vd* disks set up as a single mirrored software RAID device,
  /dev/md0, that is used for the root filesystem. Since this is a
  libvirt install, it is using BIOS booting, not UEFI (maybe someday
  libvirt will support snapshots of UEFI based VMs). When I attempt to
  install Ubuntu updates, the grub-pc install fails with:

  grub-pc: Running grub-install ...
  Installing for i386-pc platform.
  grub-install: warning: File system `ext2' doesn't support embedding.
  grub-install: warning: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
  grub-install: error: diskfilter writes are not supported.
    grub-install failure for /dev/md0
  You must correct your GRUB install devices before proceeding:

    DEBIAN_FRONTEND=dialog dpkg --configure grub-pc
    dpkg --configure -a
  dpkg: error processing package grub-pc (--configure):
   installed grub-pc package post-installation script subprocess returned error exit status 1

  'debconf-show' reports (changed) settings as:
  * grub-efi/cloud_style_installation: false
  * grub-pc/install_devices: /dev/disk/by-id/md-name-ubuntu-server:0
  * grub-pc/install_devices_empty: false

  The same mirrored root filesystem configuration works on 22.04 LTS.

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