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