code: void notifyNamenodeDeletedBlock(Block block) {
      if (block==null) {
        throw new IllegalArgumentException("Block is null");
      }
      // mark it as a deleted block
      DFSUtil.markAsDeleted(block);
      synchronized (receivedAndDeletedBlockList) {
        receivedAndDeletedBlockList.add(block);
      }
    }
docs: Add a block to the pending received/deleted ACKs.
  to inform the namenode that we have deleted a block.