presto-main/src/main/java/io/prestosql/operator/ExchangeClient.java/ExchangeClient/isBlocked
class ExchangeClient:    public synchronized ListenableFuture<?> isBlocked()
    {
        if (isClosed() || isFailed() || pageBuffer.peek() != null) {
            return Futures.immediateFuture(true);
        }
        SettableFuture<?> future = SettableFuture.create();
        blockedCallers.add(future);
        return future;
    }