implement Fallback-Of for IndexTargets

Sometimes index files are in different locations in a repository as it
is currently the case for Contents files which are per-component in
Debian, but aren't in Ubuntu. This has historic reasons and is perhaps
changed soon, but such cases of transitions can always happen in the
future again, so we should prepare:

Introduced is a new field declaring that the current item should only be
downloaded if the mentioned item wasn't allowing for transitions without
a flagday in clients and archives.

This isn't implemented 'simpler' with multiple MetaKeys as items (could)
change their descriptions and perhaps also other configuration bits with
their location.

1 files changed, 13 insertions, 1 deletions

diff --git a/apt-pkg/acquire-item.cc b/apt-pkg/acquire-item.cc
index 959627a01..8c45acddd 100644
--- a/apt-pkg/acquire-item.cc
+++ b/apt-pkg/acquire-item.cc
@@ -1196,9 +1196,18 @@ void pkgAcqMetaClearSig::QueueIndexes(bool const verify)			/*{{{*/
    // at this point the real Items are loaded in the fetcher
    ExpectedAdditionalItems = 0;
 
+   std::set<std::string> targetsSeen;
    bool const metaBaseSupportsByHash = TransactionManager->MetaIndexParser->GetSupportsAcquireByHash();
-   for (auto Target : TransactionManager->MetaIndexParser->GetIndexTargets())
+   for (auto &Target: TransactionManager->MetaIndexParser->GetIndexTargets())
    {
+      // if we have seen a target which is created-by a target this one here is declared a
+      // fallback to, we skip acquiring the fallback (but we make sure we clean up)
+      if (targetsSeen.find(Target.Option(IndexTarget::FALLBACK_OF)) != targetsSeen.end())
+      {
+	 targetsSeen.emplace(Target.Option(IndexTarget::CREATED_BY));
+	 new CleanupItem(Owner, TransactionManager, Target);
+	 continue;
+      }
       // all is an implementation detail. Users shouldn't use this as arch
       // We need this support trickery here as e.g. Debian has binary-all files already,
       // but arch:all packages are still in the arch:any files, so we would waste precious
@@ -1267,6 +1276,7 @@ void pkgAcqMetaClearSig::QueueIndexes(bool const verify)			/*{{{*/
 	       else if (hashes.FileSize() == 0)
 	       {
 		  new CleanupItem(Owner, TransactionManager, Target);
+		  targetsSeen.emplace(Target.Option(IndexTarget::CREATED_BY));
 		  continue;
 	       }
 	    }
@@ -1327,6 +1337,7 @@ void pkgAcqMetaClearSig::QueueIndexes(bool const verify)			/*{{{*/
 	    std::string const idxfilename = GetFinalFileNameFromURI(GetDiffIndexURI(Target));
 	    if (FileExists(idxfilename))
 	       new NoActionItem(Owner, Target, idxfilename);
+	    targetsSeen.emplace(Target.Option(IndexTarget::CREATED_BY));
 	    continue;
 	 }
 
@@ -1348,6 +1359,7 @@ void pkgAcqMetaClearSig::QueueIndexes(bool const verify)			/*{{{*/
       }
 
       // Queue the Index file (Packages, Sources, Translation-$foo, …)
+      targetsSeen.emplace(Target.Option(IndexTarget::CREATED_BY));
       if (trypdiff)
          new pkgAcqDiffIndex(Owner, TransactionManager, Target);
       else