filer: materialize remote-only rename paths before the transaction

With the lazy fetch off inside a store transaction, a source or target the
filer holds only on the remote would read as absent to AtomicRenameEntry
and StreamRenameEntry: a rename with FailIfTargetExists could overwrite it,
and a remote-only source would skip the deferred copy. PrefetchRemoteEntries
runs the fetch for both paths before BeginTransaction, under the path locks.

(cherry picked from commit e61f9718018669ef59768cd5eda25b6466d69597)

weed/server/filer_grpc_server_rename.go

@@ -47,6 +47,10 @@ func (fs *FilerServer) AtomicRenameEntry(ctx context.Context, req *filer_pb.Atom
 
 	defer fs.acquireRenamePathLocks("AtomicRenameEntry", oldParent.Child(req.OldName), newParent.Child(req.NewName))()
 
+	// the lazy remote fetch is off inside the transaction: a source or
+	// target the filer only holds on the remote is materialized first
+	fs.filer.PrefetchRemoteEntries(ctx, oldParent.Child(req.OldName), newParent.Child(req.NewName))
+
 	// a SQL store keeps the transaction in the context and refuses calls
 	// through it once committed; the work after the commit uses this one
 	afterCommitCtx := ctx
@@ -118,6 +122,7 @@ func (fs *FilerServer) StreamRenameEntry(req *filer_pb.StreamRenameEntryRequest,
 	defer fs.acquireRenamePathLocks("StreamRenameEntry", oldParent.Child(req.OldName), newParent.Child(req.NewName))()
 
 	ctx := context.Background()
+	fs.filer.PrefetchRemoteEntries(ctx, oldParent.Child(req.OldName), newParent.Child(req.NewName))
 
 	afterCommitCtx := ctx
 	ctx, err = fs.filer.BeginTransaction(ctx)