consider priorities only for downloadable pkgs in resolver

A package which can't be downloaded anymore is very likely dropped from
a release and can therefore no longer be 'standard' (or similar). We
therefore do not grant points for them anymore and demote them to
prio:extra instead which helps other packages breaking them away even if
they have a lower priority.

The testcase was initially created by Michael Vogt and just amended.

apt-pkg/algorithms.cc

@@ -445,19 +445,22 @@ void pkgProblemResolver::MakeScores()
 	  || (I->Flags & pkgCache::Flag::Important) == pkgCache::Flag::Important)
 	 Score += PrioEssentials;
 
-      // We transform the priority
-      if (Cache[I].InstVerIter(Cache)->Priority <= 5)
-	 Score += PrioMap[Cache[I].InstVerIter(Cache)->Priority];
-      
+      pkgCache::VerIterator const InstVer = Cache[I].InstVerIter(Cache);
+      // We apply priorities only to downloadable packages, all others are prio:extra
+      // as an obsolete prio:standard package can't be that standard anymore…
+      if (InstVer->Priority <= pkgCache::State::Extra && InstVer.Downloadable() == true)
+	 Score += PrioMap[InstVer->Priority];
+      else
+	 Score += PrioMap[pkgCache::State::Extra];
+
       /* This helps to fix oddball problems with conflicting packages
-         on the same level. We enhance the score of installed packages 
-	 if those are not obsolete
-      */
+	 on the same level. We enhance the score of installed packages
+	 if those are not obsolete */
       if (I->CurrentVer != 0 && Cache[I].CandidateVer != 0 && Cache[I].CandidateVerIter(Cache).Downloadable())
 	 Score += PrioInstalledAndNotObsolete;
 
       // propagate score points along dependencies
-      for (pkgCache::DepIterator D = Cache[I].InstVerIter(Cache).DependsList(); D.end() == false; ++D)
+      for (pkgCache::DepIterator D = InstVer.DependsList(); D.end() == false; ++D)
       {
 	 if (DepMap[D->Type] == 0)
 	    continue;

test/integration/test-ubuntu-bug-1304403-obsolete-priority-standard

@@ -0,0 +1,48 @@
+#!/bin/sh
+set -e
+
+TESTDIR=$(readlink -f $(dirname $0))
+. $TESTDIR/framework
+
+setupenvironment
+configarchitecture 'i386'
+
+# Regression test for LP: #1304403
+#
+# The issue here is that libkadm5srv-mit8 (priority standard) is replaced
+# by a new libkadm5srv-mit9 and libkbd5-7 breaks on the old -mit8 package.
+# The -mit8 package is no longer downloadable (and hence not upgradeable)
+
+# normal upradable pkg
+#  (libkdb5-7 that breaks on libkadm5srv-mit8 (<< 1.11+dfsg~)
+insertinstalledpackage 'upgradable' 'all' '1.0' '' 'extra'
+insertpackage 'unstable' 'upgradable' 'all' '2.0' 'Breaks: not-downloadable (<< 1.1)' 'optional'
+
+# no longer downloadable pkg (libkadm5srv-mit8, replaced by libkadm5srv-mit9)
+# but priority standard pushes it higher
+insertinstalledpackage 'not-downloadable' 'all' '1.0' '' 'standard'
+
+setupaptarchive
+
+# discourage keeping obsolete high-priority packages …
+testequal 'Reading package lists...
+Building dependency tree...
+The following packages will be REMOVED:
+  not-downloadable
+The following packages will be upgraded:
+  upgradable
+1 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
+Remv not-downloadable [1.0]
+Inst upgradable [1.0] (2.0 unstable [all])
+Conf upgradable (2.0 unstable [all])' aptget -s dist-upgrade
+
+# … but if it has dependencies we want to keep it as usual
+for i in $(seq 1 10); do
+insertinstalledpackage "depender$i" 'all' '1.0' 'Depends: not-downloadable'
+done
+
+testequal 'Reading package lists...
+Building dependency tree...
+The following packages have been kept back:
+  upgradable
+0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.' aptget -s dist-upgrade