public RegionFailedOpen addToFailedOpen(final RegionStateNode regionNode) {
    final byte[] key = regionNode.getRegionInfo().getRegionName();
    return regionFailedOpen.computeIfAbsent(key, (k) -> new RegionFailedOpen(regionNode));
  }
Get the number of regions in transition.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/RegionStates.java