presto-main/src/main/java/io/prestosql/operator/SpilledLookupSourceHandle.java/SpilledLookupSourceHandle/getLookupSource
class SpilledLookupSourceHandle:    public synchronized ListenableFuture<Supplier<LookupSource>> getLookupSource()
    {
        assertState(State.SPILLED);
        unspillingRequested.set(null);
        setState(State.UNSPILLING);
        checkState(unspilledLookupSource == null, "unspilledLookupSource already set");
        unspilledLookupSource = SettableFuture.create();
        return unspilledLookupSource;
    }