code: Collection<DatanodeDescriptor> getNodes(Block blk) {
      if (corruptReplicasMap.size() == 0)
        return null;
      return corruptReplicasMap.get(blk);
    }
docs: Get Nodes which have corrupt replicas of Block
  
  @param blk Block for which nodes are requested
  @return collection of nodes. Null if does not exists