presto-spi/src/main/java/io/prestosql/spi/block/RowBlockBuilder.java/RowBlockBuilder/beginBlockEntry
class RowBlockBuilder:    @Override
    public SingleRowBlockWriter beginBlockEntry()
    {
        if (currentEntryOpened) {
            throw new IllegalStateException("Expected current entry to be closed but was opened");
        }
        currentEntryOpened = true;
        return new SingleRowBlockWriter(fieldBlockBuilders[0].getPositionCount(), fieldBlockBuilders);
    }