Prevent cross-architecture downgrades (A:amd64=2 to A:i386=1)
by default, as they are somewhat surprising and can also
cause issues.

We should special case that for foreign/allowed, but doing
this more generically should not hurt either, as 'same' has
implicit same-version dependencies and 'no' only allows a
single installed version in a group anyway.

apt-pkg/policy.cc

@@ -30,6 +30,7 @@
 #include <ctype.h>
 #include <stddef.h>
 #include <string.h>
+#include <algorithm>
 #include <string>
 #include <vector>
 #include <iostream>
@@ -230,20 +231,31 @@ pkgCache::VerIterator pkgPolicy::GetCandidateVer(pkgCache::PkgIterator const &Pk
 pkgCache::VerIterator pkgPolicy::GetCandidateVerNew(pkgCache::PkgIterator const &Pkg)
 {
    // TODO: Replace GetCandidateVer()
+   pkgCache::GrpIterator group;
    pkgCache::VerIterator cand;
-   pkgCache::VerIterator cur = Pkg.CurrentVer();
+   std::vector<pkgCache::VerIterator> curs;
    int candPriority = -1;
    pkgVersioningSystem *vs = Cache->VS;
 
+   // Find all installed versions, across all architectures
+   group = Pkg.Group();
+   for (auto p = group.PackageList(); p.end() == false; p = group.NextPkg(p)) {
+      auto cur = p.CurrentVer();
+      if (cur.end() == false)
+	 curs.push_back(cur);
+   }
+
    for (pkgCache::VerIterator ver = Pkg.VersionList(); ver.end() == false; ++ver) {
       int priority = GetPriority(ver, true);
 
       if (priority == 0 || priority <= candPriority)
 	 continue;
 
-      // TODO: Maybe optimize to not compare versions
-      if (!cur.end() && priority < 1000
-	  && (vs->CmpVersion(ver.VerStr(), cur.VerStr()) < 0))
+      // Check that we do not downgrade, even across architectures, unless
+      // priority is >= 1000.
+      if (!curs.empty() && priority < 1000 &&
+	  std::any_of(curs.begin(), curs.end(), [&vs, &ver](pkgCache::VerIterator const &cur) {
+	     return vs->CmpVersion(ver.VerStr(), cur.VerStr()) < 0; }))
 	 continue;
 
       candPriority = priority;

test/integration/test-bug-612099-multiarch-conflicts

@@ -95,12 +95,24 @@ The following NEW packages will be installed:
 Inst foobar (1.0 stable [i386])
 Conf foobar (1.0 stable [i386])" aptget install foobar/stable -st testing
 
+testfailureequal "Reading package lists...
+Building dependency tree...
+Reading state information...
+Selected version '1.0' (stable [amd64]) for 'foobar:amd64'
+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.
+The following information may help to resolve the situation:
+
+The following packages have unmet dependencies:
+ foobar:amd64 : Depends: libc6:amd64 but it is not installable
+E: Unable to correct problems, you have held broken packages." aptget install foobar:amd64/stable -st testing
 testsuccessequal "Reading package lists...
 Building dependency tree...
 Reading state information...
 Selected version '1.0' (stable [amd64]) for 'foobar:amd64'
-The following additional packages will be installed:
-  libc6:amd64
+Selected version '1.0' (stable [amd64]) for 'libc6:amd64'
 The following packages will be REMOVED:
   libc6
 The following NEW packages will be installed:
@@ -110,7 +122,7 @@ Remv libc6 [2.0]
 Inst libc6:amd64 (1.0 stable [amd64])
 Inst foobar:amd64 (1.0 stable [amd64])
 Conf libc6:amd64 (1.0 stable [amd64])
-Conf foobar:amd64 (1.0 stable [amd64])" aptget install foobar:amd64/stable -st testing
+Conf foobar:amd64 (1.0 stable [amd64])" aptget install foobar:amd64/stable libc6:amd64/stable -st testing
 
 
 testsuccessequal "Reading package lists...
@@ -207,12 +219,25 @@ The following NEW packages will be installed:
 Inst foobar-same (1.0 stable [i386])
 Conf foobar-same (1.0 stable [i386])" aptget install foobar-same/stable -st testing
 
+testfailureequal "Reading package lists...
+Building dependency tree...
+Reading state information...
+Selected version '1.0' (stable [amd64]) for 'foobar-same:amd64'
+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.
+The following information may help to resolve the situation:
+
+The following packages have unmet dependencies:
+ foobar-same:amd64 : Depends: libc6-same:amd64 but it is not installable
+E: Unable to correct problems, you have held broken packages." aptget install foobar-same:amd64/stable -st testing
+
 testsuccessequal "Reading package lists...
 Building dependency tree...
 Reading state information...
 Selected version '1.0' (stable [amd64]) for 'foobar-same:amd64'
-The following additional packages will be installed:
-  libc6-same:amd64
+Selected version '1.0' (stable [amd64]) for 'libc6-same:amd64'
 The following packages will be REMOVED:
   libc6-same
 The following NEW packages will be installed:
@@ -222,4 +247,4 @@ Remv libc6-same [2.0]
 Inst libc6-same:amd64 (1.0 stable [amd64])
 Inst foobar-same:amd64 (1.0 stable [amd64])
 Conf libc6-same:amd64 (1.0 stable [amd64])
-Conf foobar-same:amd64 (1.0 stable [amd64])" aptget install foobar-same:amd64/stable -st testing
+Conf foobar-same:amd64 (1.0 stable [amd64])" aptget install foobar-same:amd64/stable libc6-same:amd64/stable -st testing