code: synchronized Lease addLease(String holder, String src, long timestamp) {
      Lease lease = getLease(holder);
      if (lease == null) {
        lease = new Lease(holder);
        leases.put(holder, lease);
        sortedLeases.add(lease);
      } else {
        renewLease(lease);
      }
      sortedLeasesByPath.put(src, new LeaseOpenTime(lease, timestamp));
      lease.paths.add(src);
  
      return lease;
    }
docs: Adds (or re-adds) the lease for the specified file.
  @param client that will hold the lease
  @param src file path to associated with the lease.
  @param timestamp time that the file was opened. (could be in the
  past if loaded from FsImage