[Mageia-dev] How broken are RPM dependencies allowed to be?

[Pascal Terjan]

On Wed, Dec 14, 2011 at 09:14, Dan Fandrich <dan at coneharvesters.com> wrote:
> On Wed, Dec 14, 2011 at 09:49:15AM +0200, Buchan Milne wrote:
>> This is unsupported. Maybe you should instead contribute documentation that
>> makes this more explicitly obvious, but it is a well-known rule in Mandriva and
>> Mageia (and usually applies to other distros as well).
>
> I can understand that my particular case is unsupported, but I described
> a different, supported, scenario that would also fail due to this problem.
> To reiterate, a distribution upgrade from 1 to 2 (once it's finalized)
> could involve urpmi first upgrading the perl-dependent package but avoid
> installing the new perl itself until the end of the upgrade, which could be
> hours or (if interrupted) days later.

During an upgrade urpmi starts by updating what it uses (perl, rpm,
few other things, itself) and then restarts.

> During the entirety of that time,
> that package would be unusable. If that package happened to be a key CGI
> script for a web site, the entire site would be down for that entire time.

That would not be prevented. The result would be that you need to
install thousands of packages in the same transaction as they are all
required by each other, and nothing would prevent your CGI from being
at the end of the transaction which will happen hours or days later.

>> If this weren't the case, there wouldn't be a need for backports ...
>
> Backports are nice in that they are leaf packages that don't generally
> require a ton of newer libraries be installed as well. Installing a
> single package of any complexity from a newer distribution often results
> in a cascading series of new packages to resolve all the dependencies.
> But it's often expeditious to upgrade simpler packages in that way in
> cases when the system can't completely upgraded right away.
>
> It's possible to handle that kind of case reliably, but I understand that
> it would be more work to get the dependencies just right. Many library
> authors put plenty of effort into maintaining binary compatibility across
> releases just so this sort of thing is possible. But even if this isn't an
> officially-supported mode of operation, problems like the one I described
> above can still result in broken systems if the dependencies aren't
> correctly described.
>
>> Installing packages individually from one release on another release is not
>> supported. Either upgrade the entire distro first, or stick to packages from
>> the version you are on. However 'upgrade from release to Cauldron', when done
>> correctly, should usually work as expected.
>
> Yes, "usually". Is Mageia the operating system that works reliably 95% of the
> time?
>
>> But, in supported use cases, urpmi *does* ensure that all the pieces to keep
>> urpmi are upgraded in one transaction.
>
> But only if the dependencies are set correctly. And my original bug report on
> that has just now been closed as WONTFIX.
>
>> Supporting the use case of installing any random package from a different
>> release will take more effort than just adding and maintaining a version on one
>> perl-base dependency.
>
> Yes, it will, but it can be automated to a certain extent. There just has to be
> a will to make sure that even the corner cases work.
>
>>>> Dan