[Bug 1837300] Re: /usr/bin/locale-check is missing from bionic LTS versions

[Steve Langasek]

Downgrading is not supported.

Downgrading with the result that files / bugfixes present in the
-updates pocket are not installed because you only installed from the
release pocket is not a bug in Ubuntu.  Any changes we make to a stable
release after GA land only in the -updates pocket, never in the release
pocket.  So there is nothing we would change here to make this problem
go away.

The reason you saw the error is that /etc/profile.d/01-locale-fix.sh and
/usr/bin/locale-check were both introduced to bionic after release, so
are only in bionic-updates.  However, /usr/bin/locale-check is a regular
packaged file, and /etc/profile.d/01-locale-fix.sh is a conffile; which
means that on downgrade, /usr/bin/locale-check is removed because it's
not part of the older package, but /etc/profile.d/01-locale-fix.sh is
not removed because conffiles are only removed with additional handling
via the maintainer scripts, and there is no handling that would remove
this conffile as part of a downgrade.

Almost no conffiles are correctly removed on downgrade to a version of
the package earlier than the version which introduced the conffile.

This is why downgrades are not supported.

** Changed in: base-files (Ubuntu)
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1837300

Title:
  /usr/bin/locale-check is missing from bionic LTS versions

Status in base-files package in Ubuntu:
  Invalid

Bug description:
  After downgrading from 19.04 to 18.04 (which involved a bunch of `apt
  remove --purge` and manual dependency conflict resolution) I had the
  following error on logging into my machine:

  ```
  bash: /usr/bin/locale-check: No such file or directory
  ```

  On investigation, it seems that this file is supposed to be provided
  by `base-files`, but it is not present in the two current LTS versions
  (10.1ubuntu2 and 10.2ubuntu2.2).

  ```
  tl at hotbox:~$ apt download base-files=10.1ubuntu2
  Get:1 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 base-files amd64 10.1ubuntu2 [58.2 kB]
  Fetched 58.2 kB in 0s (1,000 kB/s)
  tl at hotbox:~$ apt download base-files=10.1ubuntu2.2
  Get:1 http://security.ubuntu.com/ubuntu bionic-security/main amd64 base-files amd64 10.1ubuntu2.2 [58.2 kB]
  Fetched 58.2 kB in 0s (189 kB/s)
  tl at hotbox:~$ dpkg -c base-files_10.1ubuntu2_amd64.deb | rg "/usr/bin"
  drwxr-xr-x root/root         0 2018-04-24 04:34 ./usr/bin/
  tl at hotbox:~$ dpkg -c base-files_10.1ubuntu2.2_amd64.deb | rg "/usr/bin"
  drwxr-xr-x root/root         0 2018-08-20 09:44 ./usr/bin/
  ```

  It is present in `bionic-updates` which is currently `10.1ubuntu2.5`,
  however.

  The reason I think this happened to this system in particular is that
  as part of the dist-downgrade process I added the following to
  `/etc/apt/preferences`:

  ```
  Package: *
  Pin: release a=bionic
  Pin-Priority: 1001
  ```

  This appears to have prevented me from getting `bionic-updates`.
  Removing this file allows apt to install `base-files` from `bionic-
  updates` and resolved the issue for me.

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