code: public BlockHeartbeatReport generateReport() {
      synchronized (mLock) {
        BlockHeartbeatReport report
            = new BlockHeartbeatReport(mAddedBlocks, mRemovedBlocks, mLostStorage);
        // Clear added and removed blocks
        mAddedBlocks.clear();
        mRemovedBlocks.clear();
        mLostStorage.clear();
        return report;
      }
    }
docs: Generates the report of the block store delta in the last heartbeat period. Calling this method
  marks the end of a period and the start of a new heartbeat period.
  
  @return the block store delta report for the last heartbeat period