presto-main/src/main/java/io/prestosql/operator/exchange/LocalExchangeSink.java/LocalExchangeSink/waitForWriting
class LocalExchangeSink:    public ListenableFuture<?> waitForWriting()
    {
        if (isFinished()) {
            return NOT_BLOCKED;
        }
        return exchanger.waitForWriting();
    }