presto-main/src/main/java/io/prestosql/operator/AssignUniqueIdOperator.java/AssignUniqueIdOperator/addInput
class AssignUniqueIdOperator:    @Override
    public void addInput(Page page)
    {
        checkState(!finishing, "Operator is already finishing");
        requireNonNull(page, "page is null");
        checkState(inputPage == null);
        inputPage = page;
    }