Bug#112811: apt: more useful handling of cases where dependent packages are not downloadable

[Zygo Blaxell <zblaxell@genki.hungrycats.org>]

Package: apt
Version: 0.5.3
Severity: wishlist
File: /usr/bin/apt-get

First, some wishlist suggestions, then the situations that call for them.
IMHO, the wishlist suggestions are listed in decreasing order of
development-effort-required to end-user-benefit ratio:

1.  Rephrase the message "but $OFFENDING_VERSION is to be installed"
in the case where $OFFENDING_VERSION is _already_ installed, and
circumstances (apt-get limitations, version-specific dependencies and
unavailable packages, held packages, to name a few) prevent the offending
package from being downgraded, upgraded, or removed.

This happens if two deb sources in sources.list release newer versions of
packages without monotonically increasing version strings, which in
turn happens when you're testing packages in someone's unofficial
apt archive, and the packages later become official Debian packages.

Some suggested improved diagnostic messages:

	$OFFENDING_VERSION is already installed

	$OFFENDING_VERSION is installed and cannot be upgraded or removed
	without breaking a lot of other dependencies

	$OFFENDING_VERSION may need to be removed/purged and reinstalled

2.  An option which, similar to --reinstall, tells apt-get to perform
the downgrades or remove/purge-then-installs as required to satisfy
dependencies above.

3.  A "dist-downgrade" command, which removes or downgrades any packages
installed on the system that can no longer be downloaded.

Background:

I've seen this kind of thing way too often:

	genki:~# apt-get --reinstall install galeon
	Reading Package Lists... Done
	Building Dependency Tree... Done
	Some packages could not be installed. This may mean that you have
	requested an impossible situation or if you are using the unstable
	distribution that some required packages have not yet been created
	or been moved out of Incoming.

	Since you only requested a single operation it is extremely likely that
	the package is simply not installable and a bug report against
	that package should be filed.
	The following information may help to resolve the situation:

	Sorry, but the following packages have unmet dependencies:
	  galeon: Conflicts: mozilla-browser (>= 2:0.9.4) but 2:0.9.4-1 is to be install
	ed
	E: Sorry, broken packages

The actual preconditions are:  

1.  Galeon and various mozilla packages are already installed on the
system, and they are all dependent on exactly version 2:0.9.4-1 of
mozilla-.* and libnspr4.*.

2.  The deb source where these packages were downloaded from has
disappeared.

3.  An earlier version of mozilla.*, and libnspr4.* appeared in the
Debian unstable deb source.  A version of Galeon which depends on these
packages and conflicts with the later versions also appears in unstable.

As a result, apt-get can't install new galeon because it can't figure
out what to do about the old (new) mozilla stuff.

This is not an isolated incident, or at least it is one of many isolated
incidents--to cite an example using the official Debian archive, it also
happened during the great renaming of woody from "unstable" to "testing".
It is now almost a year later and I'm _still_ finding packages on my
systems that can't be upgraded because of package interdependencies.

-- System Information
Debian Release: testing/unstable
Architecture: i386
Kernel: Linux genki 2.2.19-zb-586-smp #1 SMP Wed Mar 28 00:02:08 EST 2001 i686
Locale: LANG=C, LC_CTYPE=C

Versions of packages apt depends on:
ii  libc6                  2.2.4-1           GNU C Library: Shared libraries an
ii  libstdc++2.10-glibc2.2 1:2.95.4-0.010902 The GNU stdc++ library