public Block[] getInvalidateBlocks(int maxblocks) {
    synchronized (invalidateBlocks) {
      Block[] deleteList = invalidateBlocks.pollToArray(new Block[Math.min(
          invalidateBlocks.size(), maxblocks)]);
      return deleteList.length == 0 ? null : deleteList;
    }
  }
@Test
  public void testGetInvalidateBlocks() throws Exception {
    final int MAX_BLOCKS = 10;
    final int REMAINING_BLOCKS = 2;
    final int MAX_LIMIT = MAX_BLOCKS - REMAINING_BLOCKS;
    
    DatanodeDescriptor dd = DFSTestUtil.getLocalDatanodeDescriptor();
    ArrayList<Block> blockList = new ArrayList<Block>(MAX_BLOCKS);
    for (int i=0; i<MAX_BLOCKS; i++) {
      blockList.add(new Block(i, 0, GenerationStamp.LAST_RESERVED_STAMP));
    }
    dd.addBlocksToBeInvalidated(blockList);
    Block[] bc = dd.getInvalidateBlocks(MAX_LIMIT);
    assertEquals(bc.length, MAX_LIMIT);
    bc = dd.getInvalidateBlocks(MAX_LIMIT);
    assertEquals(bc.length, REMAINING_BLOCKS);
  }