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

[Robie Basak]

> I don't think the suggestion that CPC change that general guidance
because grub.d is less well known that other `.d` directories is a
strong rationale.

While .d directories are a common pattern and CPC should use them where
these are provided, that fundamentally has to be considered on a
package-by-package basis. You must adapt to the packaging provided and
cannot simply expect or demand to use .d where that causes issues and
refuse to budge otherwise.

In the general case, .d directories would override things that are
default because the underlying implementation makes them the default. In
contrast, using .d directories to override what is specified in other
configuration files in /etc by default makes for confusion and should be
avoided. We've already seen that with sshd_config.d/, where file
ordering collides with configuration syntax rules making some .d-based
overrides impossible (that case will hopefully be resolved by moving
more things into .d over time).

In this case, grub packaging provides a default in /etc/default/grub
leads to confusing duplication when overridden by .d, and I can't see
that changing.

In this case, the precise mechanism we need looks like it's already
provided using a well-established debconf mechanism, and if not, appears
to be able to be trivially extended to do so. That would resolve this
bug and all user confusion. Let's not use dogmatic adherence to one
specific pattern to excuse fixing the issue with the appropriate
mechanism.

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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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