@Override
  public ListenableFuture<?> isBlocked() {
    // Avoid registering a new callback in the source handle when one is already pending
    if (isBlocked.isDone()) {
      isBlocked = sourceHandle.isBlocked();
      if (isBlocked.isDone()) {
        isBlocked = NOT_BLOCKED;
      }
    }
    return isBlocked;
  }
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