triedb/pathdb: extract cached block reader helper

Deduplicate the load-and-cache logic in newBlockIter into a blockReader
method on indexReader, for reuse by upcoming ordinal accessors.

triedb/pathdb/history_index.go

@@ -124,6 +124,20 @@ func (r *indexReader) refresh() error {
 	return nil
 }
 
+// blockReader returns a cached block reader for the given block id, loading it
+// from disk on first access.
+func (r *indexReader) blockReader(id uint32) (*blockReader, error) {
+	if br, ok := r.readers[id]; ok {
+		return br, nil
+	}
+	br, err := newBlockReader(readStateIndexBlock(r.state, r.db, id), r.bitmapSize != 0)
+	if err != nil {
+		return nil, err
+	}
+	r.readers[id] = br
+	return br, nil
+}
+
 // readGreaterThan locates the first element that is greater than the specified
 // id. If no such element is found, MaxUint64 is returned.
 func (r *indexReader) readGreaterThan(id uint64) (uint64, error) {

triedb/pathdb/history_index_iterator.go

@@ -446,14 +446,9 @@ type indexIterator struct {
 
 // newBlockIter initializes the block iterator with the specified block ID.
 func (r *indexReader) newBlockIter(id uint32, filter *extFilter) (*blockIterator, error) {
-	br, ok := r.readers[id]
+	br, err := r.blockReader(id)
-	if !ok {
+	if err != nil {
-		var err error
+		return nil, err
-		br, err = newBlockReader(readStateIndexBlock(r.state, r.db, id), r.bitmapSize != 0)
-		if err != nil {
-			return nil, err
-		}
-		r.readers[id] = br
 	}
 	return br.newIterator(filter), nil
 }