@Override
    public List<Pair<Cell, Cell>> postIncrementBeforeWAL(
      ObserverContext<? extends RegionCoprocessorEnvironment> ctx, Mutation mutation,
      List<Pair<Cell, Cell>> cellPairs) throws IOException {
      List<Pair<Cell, Cell>> result = super.postIncrementBeforeWAL(ctx, mutation, cellPairs);
      for (Pair<Cell, Cell> pair : result) {
        if (
          mutation.getACL() != null
            && !checkAclTag(mutation.getACL(), (ExtendedCell) pair.getSecond())
        ) {
          throw new DoNotRetryIOException("Unmatched ACL tag.");
        }
      }
      return result;
    }
Test coprocessor methods
{@link RegionObserver#postIncrementBeforeWAL(ObserverContext, Mutation, List)} and
{@link RegionObserver#postAppendBeforeWAL(ObserverContext, Mutation, List)}. These methods may
change the cells which will be applied to memstore and WAL. So add unit test for the case which
change the cell's column family and tags.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestPostIncrementAndAppendBeforeWAL.java