presto-main/src/main/java/io/prestosql/operator/EnforceSingleRowOperator.java/EnforceSingleRowOperator/getOutput
class EnforceSingleRowOperator:    @Override
    public Page getOutput()
    {
        if (!finishing) {
            return null;
        }
        checkState(page != null, "Operator is already done");

        Page pageToReturn = page;
        page = null;
        return pageToReturn;
    }