presto-main/src/main/java/io/prestosql/operator/GroupIdOperator.java/GroupIdOperator/addInput
class GroupIdOperator:    @Override
    public void addInput(Page page)
    {
        checkState(!finishing, "Operator is already finishing");
        checkState(currentPage == null, "currentPage must be null to add a new page");

        currentPage = requireNonNull(page, "page is null");
    }