Fix 'newBlocks.current' getter for binary blocks. (#1388)

pkg/state/state.go

@@ -350,30 +350,47 @@ func (n *newBlocks) unmarshalBlock(block *proto.Block, blockBytes []byte) error
 	return nil
 }
 
+func getBlockDataWithOptionalSnapshot[T interface{ []byte | *proto.Block }](
+	curPos int,
+	blocks []T,
+	snapshots []*proto.BlockSnapshot,
+) (T, *proto.BlockSnapshot, error) {
+	if curPos > len(blocks) || curPos < 1 {
+		var zero T
+		return zero, nil, errors.New("bad current position")
+	}
+	var (
+		pos              = curPos - 1
+		block            = blocks[pos]
+		optionalSnapshot *proto.BlockSnapshot
+	)
+	if sl := len(snapshots); sl != 0 { // snapshots aren't empty
+		if bl := len(blocks); sl != bl { // if snapshots are present, they must have the same length as blocks
+			var zero T
+			return zero, nil, errors.Errorf("snapshots and blocks slices have different lengths %d and %d", sl, bl)
+		}
+		optionalSnapshot = snapshots[pos] // blocks and snapshots have the same length
+	}
+	return block, optionalSnapshot, nil
+}
+
 func (n *newBlocks) current() (*proto.Block, *proto.BlockSnapshot, error) {
 	if !n.binary {
-		if n.curPos > len(n.blocks) || n.curPos < 1 {
-			return nil, nil, errors.New("bad current position")
-		}
-		var (
-			pos              = n.curPos - 1
-			block            = n.blocks[pos]
-			optionalSnapshot *proto.BlockSnapshot
-		)
-		if len(n.snapshots) == len(n.blocks) { // return block with snapshot if it is set
-			optionalSnapshot = n.snapshots[pos]
+		block, optionalSnapshot, err := getBlockDataWithOptionalSnapshot(n.curPos, n.blocks, n.snapshots)
+		if err != nil {
+			return nil, nil, errors.Wrap(err, "failed to get current deserialized block")
 		}
 		return block, optionalSnapshot, nil
 	}
-	if n.curPos > len(n.binBlocks) || n.curPos < 1 {
-		return nil, nil, errors.New("bad current position")
+	blockBytes, optionalSnapshot, err := getBlockDataWithOptionalSnapshot(n.curPos, n.binBlocks, n.snapshots)
+	if err != nil {
+		return nil, nil, errors.Wrap(err, "failed to get current binary block")
 	}
-	blockBytes := n.binBlocks[n.curPos-1]
-	b := &proto.Block{}
-	if err := n.unmarshalBlock(b, blockBytes); err != nil {
-		return nil, nil, err
+	block := &proto.Block{}
+	if unmErr := n.unmarshalBlock(block, blockBytes); unmErr != nil {
+		return nil, nil, unmErr
 	}
-	return b, nil, nil
+	return block, optionalSnapshot, nil
 }
 
 func (n *newBlocks) reset() {