Re: [PATCH] buildchroot: Allow downgrades when installing packages

[Baurzhan Ismagulov <ibr@radix50.net>]

On Mon, Feb 25, 2019 at 12:08:23PM +0100, Jan Kiszka wrote:
> Removing -rcX from a version string is lexically a downgrade, logically not.
> I'm not that sure if you can compensate this.

Debian uses tilde for that, e.g. 1.0~rc1-1+deb9u2. This is addressed in
https://www.debian.org/doc/debian-policy/ch-controlfields.html#version:

  "First the initial part of each string consisting entirely of non-digit
  characters is determined. These two parts (one of which may be empty) are
  compared lexically. If a difference is found it is returned. The lexical
  comparison is a comparison of ASCII values modified so that all the letters
  sort earlier than all the non-letters and so that a tilde sorts before
  anything, even the end of a part. For example, the following parts are in
  sorted order from earliest to latest: ~~, ~~a, ~, the empty part, a. [7]

  [7] One common use of ~ is for upstream pre-releases. For example,
      1.0~beta1~svn1245 sorts earlier than 1.0~beta1, which sorts earlier than
      1.0."


> Anyway, a real downgrade is a valid development scenario as well for our
> isar-apt.

Downgrade is supported by the packaging toolchain (dpkg / apt), but not by the
distribution (packages themselves, notably postinst, etc.).
https://www.debian.org/doc/manuals/debian-reference/ch02.en.html#_emergency_downgrading
says:

  "Downgrading is not officially supported by the Debian by design. It should
  be done only as a part of emergency recovery process. Despite of this
  situation, it is known to work well in many incidents. For critical systems,
  you should backup all important data on the system after the recovery
  operation and re-install the new system from the scratch.

  You may be lucky to downgrade from newer archive to older archive to recover
  from broken system upgrade by manipulating candidate version..."

So I'd be reluctant to do that by default. What about extracting the apt
options into a config file, where the user may add his options?


With kind regards,
Baurzhan.