code: void copyOngoingCreates(Block block) throws CloneNotSupportedException {
      ActiveFile af = ongoingCreates.get(block);
      if (af == null) {
        return;
      }
  
      ongoingCreates.put(block, af.getClone());
    }
docs: If there is an ActiveFile object for the block, create a copy of the
  old one and replace the old one. This is to make sure that the VisibleLength
  applied to the old object will have no impact to the local map. In
  that way, BlockReceiver can directly update visible length without
  holding the lock.
  
  @param block
  @throws CloneNotSupportedException