presto-spi/src/main/java/io/prestosql/spi/block/SingleMapBlockWriter.java/SingleMapBlockWriter/closeEntry
class SingleMapBlockWriter:    @Override
    public BlockBuilder closeEntry()
    {
        if (writeToValueNext) {
            valueBlockBuilder.closeEntry();
        }
        else {
            keyBlockBuilder.closeEntry();
        }
        entryAdded();
        return this;
    }