[Bug 2091657] Re: systemd-localed requires read-only-etc hacks for /etc/vconsole.conf and /etc/locale.conf

[James Henstridge]

I think the root cause here is this bind mount:

https://github.com/canonical/core-
base/blob/82c8fadb4cf77df5f771220d27c40dc011675f23/static/etc/system-
image/writable-paths#L32

When systemd-localed tries to update the configuration file, it writes
the new content to a temporary file and then renames it over the top of
the destination. Trying to rename over the bind mount fails.

In core24-desktop, we'd removed the bind mount and instead set it up as
a symlink to /etc/writable/locale.conf:

https://github.com/canonical/core-base-
desktop/blob/2149717d911493e26b3519a51798ae6a287189a7/hooks/009-locale-
archive.chroot#L28-L30

We did similar for the vconsole.conf file used for keymap settings.

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https://bugs.launchpad.net/bugs/2091657

Title:
  systemd-localed requires read-only-etc hacks for /etc/vconsole.conf
  and /etc/locale.conf

Status in systemd package in Ubuntu:
  Invalid
Status in systemd source package in Noble:
  Triaged

Bug description:
  [Impact]

  Ubuntu Core has a read-only /etc, but a writable partition in
  /etc/writable. In some versions of Ubuntu, the systemd package ships a
  patch to follow symlinks from /etc/ -> /etc/writable, e.g. for files
  like /etc/locale.conf.

  In Core24, this patch needs to be expanded for /etc/locale.conf and
  /etc/vconsole.conf in systemd-localed.

  [Test Plan]

  TODO

  [Where problems could occur]

  This hack is designed to be a no-op in non-Ubuntu Core cases. I.e.,
  when /etc/writable does not exist or does not have a link for the
  given path in etc, the original path is used.

  As is evident by the ever-expanding nature of this patch, this is not
  a complete solution. It's possible that more cases will be found where
  this hack needs to be applied.

  [Other information]

  This patch is not carried in supported releases newer than Noble.
  While this patch is far from ideal, expanding this hack has become the
  go-to for existing stable releases.

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