code: void notifyNamenodeReceivedBlock(Block block, String delHint) {
      if (block==null) {
        throw new IllegalArgumentException("Block is null");
      }
      if (delHint != null && !delHint.isEmpty()) {
        block = new ReceivedBlockInfo(block, delHint);
      }
      synchronized (receivedAndDeletedBlockList) {
        receivedAndDeletedBlockList.add(block);
        pendingReceivedRequests++;
        if (!shouldBackoff) {
          receivedAndDeletedBlockList.notifyAll();
        }
      }
    }
docs: Add a block to the pending received/deleted ACKs.
  to inform the namenode that we have received a block.