@Override
  public List<Pair<Cell, Cell>> postAppendBeforeWAL(
    ObserverContext<? extends RegionCoprocessorEnvironment> ctx, Mutation mutation,
    List<Pair<Cell, Cell>> cellPairs) throws IOException {
    List<Pair<Cell, Cell>> resultPairs = new ArrayList<>(cellPairs.size());
    for (Pair<Cell, Cell> pair : cellPairs) {
      resultPairs.add(new Pair<>(pair.getFirst(),
        createNewCellWithTags(mutation, (ExtendedCell) pair.getSecond())));
    }
    return resultPairs;
  }
Verify, when servicing an RPC, that the caller is the scanner owner. If so, we assume that
access control is correctly enforced based on the checks performed in preScannerOpen()
hbase-server/src/main/java/org/apache/hadoop/hbase/security/visibility/VisibilityController.java