public SettableFuture<Void> getBlockedDependencyDriver() {
    if (blockedDependencyDriver == null) {
      blockedDependencyDriver = SettableFuture.create();
    }
    return blockedDependencyDriver;
  }
For ExchangeOperator in pipeline, the maxReturnSize is equal to the maxReturnSize of the child
operator.

@param maxReturnSize max return size of child operator
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/ExchangeOperator.java