presto-main/src/main/java/io/prestosql/operator/WorkProcessorOperatorAdapter.java/WorkProcessorOperatorAdapter/getOutput
class WorkProcessorOperatorAdapter:    @Override
    public Page getOutput()
    {
        if (!pages.process()) {
            return null;
        }

        if (pages.isFinished()) {
            return null;
        }

        return pages.getResult();
    }