presto-spi/src/main/java/io/prestosql/spi/block/VariableWidthBlockBuilder.java/VariableWidthBlockBuilder/build
class VariableWidthBlockBuilder:    @Override
    public Block build()
    {
        if (currentEntrySize > 0) {
            throw new IllegalStateException("Current entry must be closed before the block can be built");
        }
        return new VariableWidthBlock(0, positions, sliceOutput.slice(), offsets, hasNullValue ? valueIsNull : null);
    }