presto-main/src/main/java/io/prestosql/operator/aggregation/builder/SpillableHashAggregationBuilder.java/SpillableHashAggregationBuilder/startMemoryRevoke
class SpillableHashAggregationBuilder:    @Override
    public ListenableFuture<?> startMemoryRevoke()
    {
        if (producingOutput) {
            // all revocable memory has been released in buildResult method
            verify(localRevocableMemoryContext.getBytes() == 0);
            return NOT_BLOCKED;
        }

        return spillToDisk();
    }