Bug#878905: debian-policy: Document installability recommendations for dependency alternatives

[Jonathan Nieder <jrnieder@gmail.com>]

Hi,

Julian Andres Klode wrote:

> APT's solver is greedy and sometimes has a hard time to recover from paths that
> don't work out in the end. We see this with opencv failing to build on !linux-any
> because:
>
> (1) dconf-service depends default-dbus-session-bus | dbus-session-bus
> (2) default-dbus-session-bus is provided by an Architecture: all package, but
>     depends on systemd
>
> APT refuses to install that.
>
> I think it makes sense to amend section 7.1 with the following information:

I agree with this goal.

>     Packages on the left hand side of a pipe symbol should either be installable
>     or should not exist in the given situation (for example, because it is linux-only
>     and the package only exists on non-Linux platform).
>
> This would help reduce hard to solve situations for greedy algorithms.

I'm wondering how a packager would go about fulfilling this recommendation.
Should they audit their dependencies (and dependencies' dependencies, etc) for
installability?  Is there a reliable process they can follow for this?

This is made especially difficult because since policy 4.0.1.0 we are not able
to rely on 'priority: optional' packages being installable any more.

Without such advice, I don't think this makes sense to add as a normative change
to policy (or in other words a policy "should").  An informative note would
still be useful, though.

Thanks,
Jonathan