presto-main/src/main/java/io/prestosql/execution/buffer/OutputBufferMemoryManager.java/OutputBufferMemoryManager/getBufferBlockedFuture
class OutputBufferMemoryManager:    public synchronized ListenableFuture<?> getBufferBlockedFuture()
    {
        if ((isBufferFull() || isBlockedOnMemory()) && bufferBlockedFuture.isDone()) {
            bufferBlockedFuture = SettableFuture.create();
        }
        return bufferBlockedFuture;
    }