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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 19.03.19 11:10, Baurzhan Ismagulov wrote:
> 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."
> 

Good to know. We will have to educate people coming from OE about this 
non-intuitive deviation from how things work over there.

> 
>> 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?

I don't see that we should add a redundant, counter-intuitive option. The user 
already clearly expressed that packages of choice should be installed. This is 
what whole Isar is about: customization. Moreover, the user will get exactly 
that by doing a clean rebuild.

So, please merge.

Jan

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