Re: Pining for Qt 5.4
On Saturday 02 May 2015 16:36:30 Christian Seiler wrote:
> On 05/02/2015 05:06 PM, Lisi Reisz wrote:
> > Does the "-t wheezy-backports" format only work with backports?
>
> No, it works here as well. From the manpage of apt-get:
> > This option controls the default input to the policy engine; it
> > creates a default pin at priority 990 using the specified release
> > string.
>
> This means that ALL packages from that suit will get 990 priority (which
> is really high, see the apt_preferences(5) manpage). This has the
> consequence that if you do that for experimental, ALL dependencies of a
> package will then also come from experimental (because -t affects the
> WHOLE apt-get run, not just a single package). For backports it's
> generally OK, but for other things such as running testing with 1 or 2
> packages from experimental, it can be very problematic.
>
> To give you an example of the consequences:
>
> - package libA is version 1 in stable, version 2~bpo in
> stable-backports.
> - package libD is version 10 in stable and version 11~bpo in
> stable-backports
> - package B is version 23 in stable and version 24~bpo in
> stable-backports
> - both versions require libA in any version >= 1
> - version 24~bpo additionally requires libD >= 11~bpo
> (version 23 didn't require libD at all)
> - package C is version 42 in stable and version 50~bpo in
> stable-backports
> - version 42 requires libA with version >= 1
> - version 50~bpo requires libA with version >= 2~bpo
>
> If you now just want to install package B from backports (but are
> uninterested in C), and neither of the four packages mentioned is
> installed yet, you can do that in two ways:
>
> 1. pin D from backports to priority 500, then try to install it, APT
> will complain about libD not being installed in the correct version,
> you then pin libD from backports to priority 500 and then you can
> install the package
>
> => libD and B will be from backports
> => libA is still from stable, because B doesn't require a newer
> version
>
> 2. apt-get install -t stable-backports B
>
> => both libA and libD will also be taken from backports
>
> Now with backports this is typically fine (because the aforementioned
> case is also very rare when it comes to backports, and even if it does
> happen, libraries in backports are typically not that prone to causing
> problems), but for experimental - or worse yet, other repositories, this
> can be something you really, really don't want.
>
> (Doesn't have to be libraries btw., can be any dependency.)
>
> Therefore: use apt-get install -t with great care. It's probably fine
> for backports, but don't use it for anything else unless you REALLY know
> what you are doing.
Thanks again, Christian. That is really helpful and lucid. I have always had
trouble getting my head round pinning, for some reason. Not the concepts,
but actually doing it. Thanks to you today, light has dawned. I'm very
grateful.
Lisi
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