presto-main/src/main/java/io/prestosql/operator/NestedLoopJoinOperator.java/NestedLoopJoinOperator/addInput
class NestedLoopJoinOperator:    @Override
    public void addInput(Page page)
    {
        requireNonNull(page, "page is null");
        checkState(!finishing, "Operator is finishing");
        checkState(buildPages != null, "Page source has not been built yet");
        checkState(probePage == null, "Current page has not been completely processed yet");
        checkState(buildPageIterator == null || !buildPageIterator.hasNext(), "Current buildPageIterator has not been completely processed yet");

        if (page.getPositionCount() > 0) {
            probePage = page;
            buildPageIterator = buildPages.iterator();
        }
    }