Re: how to compare versions

[Goswin von Brederlow <goswin-v-b@web.de>]

"Wesley J. Landaker" <wjl@icecavern.net> writes:

> On Friday 05 February 2010 04:43:05 Hideki Yamane wrote:
>> > This is true in general, although it's perhaps worth noting that a rare
>> > pre- depends on a priority required package like debconf by a priority
>> > optional or extra package isn't likely to cause any trouble.
>> 
>>  Yes, it is true in general but I want to know the example for that :)
>>  For example, if I have foobar package and it says "Pre-Depends:
>>  debconf", what would happen?
>
> Short answer: nothing, it would work fine in practice.
>
> Long answer:
>
> Debconf would be required to be fully unpacked and configured before foobar 
> could even be unpacked. But, since debconf is priority "required", and is 
> already depended on by so many other packages that it's infeasible that it 
> won't already be completely installed, the pre-depends would be a no-op. In 
> the rare case that debconf wasn't already installed, it would simply be 
> unpacked and configured first, which might slow down the resolver (and hence 
> installation) but otherwise would be no problem.
>
> However, it is possible that you can dream up a bizarre corner case where 
> you are pre-depending on a specific version, your doing a big dist-upgrade, 
> the foobar package has a pre-depends and so do a bunch of other packages 
> that are intertwined in foobar's dependency graph, and the whole thing 
> explodes in a big unresolvable mess.
>
> The last paragraph is incredibly unlikely for just foobar pre-depending on 
> debconf that's you'd have to come up with some silly scenerio to show it 
> breaking, but if pre-depends were used all over the place on lots of 
> packages, that kind of scenerio could happen really quickly, which is why 
> they are generally to be avoided.

The graph doesn't even have to be that big. It just has to have a
depends/pre-depends/conflicts circle like:

debconf     Conflicts: whiptail (<< 0.51.4-11)
whiptail    Depends: libc6 (>= 2.2.5)
libc6       Conflicts: foo (<< 1.2-3)
foo         Pre-Depends: debconf (>= 1.5.28)

If you have foo 1.2-3 installed with an old libc6 (< 2.2.5) things blow
up and foo has to be removed temporary.

But since now that we have Breaks it is far less likely to have such a
Conflicts from a lib to one of its reverse dependencies. With Breaks foo
would only be deconfigured, then debconf can be upgraded and configured
and last foo is upgraded and configured.

>
> I think the general idea is:
>
>   1) Don't use pre-depends.
>   2) No, really, don't use pre-depends.
>   2) Don't use pre-depends unless it's the best technical solution.
>
> =)

What he says.

MfG
        Goswin