presto-main/src/main/java/io/prestosql/operator/aggregation/builder/SpillableHashAggregationBuilder.java/SpillableHashAggregationBuilder/processPage
class SpillableHashAggregationBuilder:    @Override
    public Work<?> processPage(Page page)
    {
        checkState(hasPreviousSpillCompletedSuccessfully(), "Previous spill hasn't yet finished");
        // hashAggregationBuilder is constructed with non yielding UpdateMemory instance.
        // Therefore the processing of the returned Work should always be true.
        // It is not possible to spill during processing of a page.
        return hashAggregationBuilder.processPage(page);
    }