don't perform implicit crossgrades involving M-A:same

dpkg stumbles over these (#844300) and we haven't dropped 'easier'
removes to be implicit and to be scheduled by dpkg by default so far
so we shouldn't push the decision in such cases to dpkg either.

apt-pkg/deb/dpkgpm.cc

@@ -1525,11 +1525,30 @@ bool pkgDPkgPM::Go(APT::Progress::PackageManager *progress)
 	    continue;
 
 	 auto const Grp = I->Pkg.Group();
-	 size_t installedInstances = 0;
+	 size_t installedInstances = 0, wannabeInstances = 0;
+	 bool multiArchInstances = false;
 	 for (auto Pkg = Grp.PackageList(); Pkg.end() == false; Pkg = Grp.NextPkg(Pkg))
-	    if (Pkg->CurrentVer != 0 || Cache[Pkg].Install())
+	 {
+	    if (Pkg->CurrentVer != 0)
+	    {
 	       ++installedInstances;
-	 if (installedInstances == 2)
+	       if (Cache[Pkg].Delete() == false)
+		  ++wannabeInstances;
+	    }
+	    else if (PackageOps.find(Pkg.FullName()) != PackageOps.end())
+	       ++wannabeInstances;
+	    if (multiArchInstances == false)
+	    {
+	       auto const V = Cache[Pkg].InstVerIter(Cache);
+	       if (V.end() == false && (Pkg->CurrentVer == 0 || V != Pkg.CurrentVer()))
+		  multiArchInstances = ((V->MultiArch & pkgCache::Version::Same) == pkgCache::Version::Same);
+	    }
+	 }
+	 /* theoretically the installed check would be enough as some wannabe will
+	    be first and hence be the crossgrade we were looking for, but #844300
+	    prevents this so we keep these situations explicit removes.
+	    It is also the reason why neither of them can be a M-A:same package */
+	 if (installedInstances == 1 && wannabeInstances == 1 && multiArchInstances == false)
 	 {
 	    auto const FirstInstall = std::find_if_not(I, List.end(),
 		  [](Item const &i) { return i.Op == Item::Remove || i.Op == Item::Purge; });

test/integration/test-crossgrades

@@ -26,8 +26,24 @@ singleinstance() {
 	testdpkgnotinstalled 'crosser:i386' 'unrelated'
 	testdpkginstalled 'crosser:amd64'
 
-	testsuccess apt purge crosser:amd64 -y --planner $1
-	testdpkgnotinstalled 'crosser:amd64'
+	testsuccess apt install crosser:armel=3 -y -o Debug::pkgDpkgPm=1 -o Dpkg::Use-Pty=0 --purge --planner $1
+	cp -a rootdir/tmp/testsuccess.output crosser.output
+	testsuccess grep -- '--remove.*crosser.*' crosser.output
+	testsuccess grep -- '--purge' crosser.output
+	testsuccess apt install crosser:armel=3 -y -o Debug::pkgDPkgProgressReporting=1 -o Dpkg::Use-Pty=0 --purge --planner $1
+	testdpkgnotinstalled 'crosser:i386' 'crosser:amd64' 'unrelated'
+	testdpkginstalled 'crosser:armel'
+
+	testsuccess apt install crosser=1 -y -o Debug::pkgDpkgPm=1 -o Dpkg::Use-Pty=0 --purge --planner $1 --allow-downgrades
+	cp -a rootdir/tmp/testsuccess.output crosser.output
+	testsuccess grep -- '--remove.*crosser.*' crosser.output
+	testsuccess grep -- '--purge' crosser.output
+	testsuccess apt install crosser=1 -y -o Debug::pkgDPkgProgressReporting=1 -o Dpkg::Use-Pty=0 --purge --planner $1 --allow-downgrades
+	testdpkgnotinstalled 'crosser:armel' 'crosser:amd64' 'unrelated'
+	testdpkginstalled 'crosser:i386'
+
+	testsuccess apt purge crosser:i386 -y --planner $1
+	testdpkgnotinstalled 'crosser:i386'
 }
 singleinstance 'internal'
 singleinstance 'apt'
@@ -52,7 +68,15 @@ multiinstance() {
 	testdpkgnotinstalled 'crosser:amd64' 'unrelated'
 	testdpkginstalled 'crosser:i386' 'crosser:armel'
 
-	testsuccess apt purge crosser:i386 crosser:armel -y --planner $1
-	testdpkgnotinstalled 'crosser:i386' 'crosser:armel' 'unrelated'
+	testsuccess apt install crosser:i386- crosser:armel=1 -y -o Debug::pkgDpkgPm=1 -o Dpkg::Use-Pty=0 --purge --planner $1 --allow-downgrades
+	cp -a rootdir/tmp/testsuccess.output crosser.output
+	testsuccess grep -- '--remove.*crosser.*' crosser.output
+	testsuccess grep -- '--purge' crosser.output
+	testsuccess apt install crosser:i386- crosser:armel=1 -y -o Debug::pkgDPkgProgressReporting=1 -o Dpkg::Use-Pty=0 --purge --planner $1 --allow-downgrades
+	testdpkgnotinstalled 'crosser:amd64' 'crosser:i386' 'unrelated'
+	testdpkginstalled 'crosser:armel'
+
+	testsuccess apt purge crosser:armel -y --planner $1 -o Debug::pkgDPkgProgressReporting=1
+	testdpkgnotinstalled 'crosser:i386' 'crosser:armel' 'crosser:amd64' 'unrelated'
 }
 multiinstance 'internal'