presto-main/src/main/java/io/prestosql/operator/unnest/UnnestOperator.java/UnnestOperator/addInput
class UnnestOperator:    @Override
    public void addInput(Page page)
    {
        checkState(!finishing, "Operator is already finishing");
        requireNonNull(page, "page is null");
        checkState(currentPage == null, "currentPage is not null");

        currentPage = page;
        currentPosition = 0;
        resetBlockBuilders();
    }