[Bug 1812752] Re: UX: changing GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub has no effect in cloud images

[Julian Andres Klode]

Robie, debconf is not a database and any setting in there needs to be
read to a configuration file and any changes in the configuration file
need to be filled back into debconf. The integration for default cmdline
with /etc/default/grub is very complex and already broke a bunch of
times.

Drop-in units are the standard mechanism to override this in many
projects. We specifically added the mechanism to allow overrides, and
that is used in various places to set grub defaults for images, OEM
kernels, and plenty of other things I can't possibly remember. It is the
preferred place to store such overrides.

Of course it's preferable to have overrides that only append items, I
don't recall if that works correctly, after all the files are parsed by
C code in grub-install I believe.

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https://bugs.launchpad.net/bugs/1812752

Title:
  UX: changing GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub has no
  effect in cloud images

Status in cloud-images:
  New
Status in grub2 package in Ubuntu:
  Confirmed

Bug description:
  I need to modify the kernel command line, such as to work around bug
  1573095. I edit GRUB_CMDLINE_LINUX_DEFAULT in /etc/default/grub and
  rerun update-grub but this has no effect.

  The reason is that this is redefined in
  /etc/default/grub.d/50-cloudimg-settings.cfg and that one overrides
  /etc/default/grub. I was misled by the comment in /etc/default/grub
  and information widely available on the Internet that editing
  /etc/default/grub is sufficient, and looked no further.

  I understand the reason for not modifying /etc/default/grub in cloud
  image builds (to avoid future conffile prompts) and I'm not suggesting
  that it should be modified. To fix this UX issue properly I think the
  grub package needs to provide a mechanism that allows cloud image
  builds to override the default without dropping in an overriding file
  (or for that override file to be the sole place that users need to
  look).

  This affects grub2 2.02-2ubuntu8.9 from Bionic cloud image build
  20190114 for example.

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