Bug#282278: apt mixes essential flag from all sources

[Jonathan Nieder <jrnieder@gmail.com>]

Vincent Lefevre wrote:

> So, once shellutils had moved to a dummy package, the coreutils
> version with "Essential: yes" was necessarily installed. And
> there was no risk to remove shellutils concerning essential
> packages

Yes, I know that.  But how could apt know?

Imagine that we were not just shifting around names, but actually
successfully eliminated some functionality from the essential set.
For example, let's say in wheezy+20, that perl-base is no longer
essential.  Then:

> The warning was triggered here just because (according to what was
> said) I had something like:
>
> deb ... stable main contrib non-free
>
> in my /etc/apt/sources.list file, and that the shellutils version
> from Debian/stable had "Essential: yes". But what is wrong is that
> the version that was currently installed (and was requested to be
> removed) was from testing or unstable, and it was not essential.

I upgrade perl-base to wheezy+20, but I still have packages from
wheezy+19, whose maintainer scripts (e.g., preinst) use perl without
depending on it.  Then apt _must_ prevent me from casually removing
perl-base.

Ideas for rewording the error message to make its purpose clearer
would be very welcome.