repository: Move cache preparation into function

internal/repository/repository.go

@@ -413,9 +413,23 @@ func (r *Repository) LoadIndex(ctx context.Context) error {
 		r.idx.Insert(idx)
 	}
 
-	if r.Cache != nil {
+	err := r.PrepareCache(validIndex)
+	if err != nil {
+		return err
+	}
+
+	return <-errCh
+}
+
+// PrepareCache initializes the local cache. indexIDs is the list of IDs of
+// index files still present in the repo.
+func (r *Repository) PrepareCache(indexIDs restic.IDSet) error {
+	if r.Cache == nil {
+		return nil
+	}
+
 	// clear old index files
-		err := r.Cache.Clear(restic.IndexFile, validIndex)
+	err := r.Cache.Clear(restic.IndexFile, indexIDs)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "error clearing index files in cache: %v\n", err)
 	}
@@ -441,22 +455,29 @@ func (r *Repository) LoadIndex(ctx context.Context) error {
 	}
 
 	// use readahead
+	debug.Log("using readahead")
 	cache := r.Cache.(*cache.Cache)
 	cache.PerformReadahead = func(h restic.Handle) bool {
 		if h.Type != restic.DataFile {
+			debug.Log("no readahead for %v, is not data file", h)
 			return false
 		}
 
 		id, err := restic.ParseID(h.Name)
 		if err != nil {
+			debug.Log("no readahead for %v, invalid ID", h)
 			return false
 		}
 
-			return treePacks.Has(id)
+		if treePacks.Has(id) {
+			debug.Log("perform readahead for %v", h)
+			return true
 		}
+		debug.Log("no readahead for %v, not tree file", h)
+		return false
 	}
 
-	return <-errCh
+	return nil
 }
 
 // LoadIndex loads the index id from backend and returns it.