show distribution mismatch for changed codenames

We have the last Release file around for other checks, so its trivial to
look if the new Release file contains a new codename (e.g. the user has
"testing" in the sources and it flipped from stretch to buster).

Such a change can be okay and expected, but also be a hint of problems,
so a warning if we see it happen seems okay. We can only print it once
anyhow and frontends and co are likely to ignore/hide it.

apt-pkg/acquire-item.cc

@@ -1608,6 +1608,14 @@ bool pkgAcqMetaBase::VerifyVendor(string const &)			/*{{{*/
    if (TransactionManager->MetaIndexParser->CheckDist(ExpectedDist) == false)
       _error->Warning(_("Conflicting distribution: %s (expected %s but got %s)"),
 	    Desc.Description.c_str(), ExpectedDist.c_str(), NowCodename.c_str());
+   // might be okay, might be not
+   if (TransactionManager->LastMetaIndexParser != nullptr)
+   {
+      auto const LastCodename = TransactionManager->LastMetaIndexParser->GetCodename();
+      if (LastCodename.empty() == false && NowCodename.empty() == false && LastCodename != NowCodename)
+	 _error->Warning(_("Conflicting distribution: %s (expected %s but got %s)"),
+	       Desc.Description.c_str(), LastCodename.c_str(), NowCodename.c_str());
+   }
    return true;
 }
 									/*}}}*/

test/integration/test-bug-841874-warning-for-mismatching-distribution

@@ -47,3 +47,15 @@ testfailure apt show foo
 ln -s "${APTARCHIVE}/dists/testing" "${APTARCHIVE}/dists/buster"
 testsuccess apt update
 testsuccess apt show foo
+
+# changing codenames gets a warning, too
+rm -rf rootdir/var/lib/apt/lists
+sed -i -e 's#buster#testing#g' rootdir/etc/apt/sources.list.d/*
+testsuccess apt update
+testsuccess apt show foo
+sed -i -e 's#^Codename: buster#Codename: zurg#g' $(find ./aptarchive -name 'Release')
+signreleasefiles
+testwarningmsg "W: Conflicting distribution: file:$APTARCHIVE testing/updates InRelease (expected buster/updates but got zurg/updates)" apt update
+testsuccess apt show foo
+testsuccess apt update
+testsuccess apt show foo