@Override
  protected void handleException(Throwable t) {
    LOG.warn("Fatal error occurred while opening region {}, aborting...",
      regionInfo.getRegionNameAsString(), t);
    // Clear any reference in getServer().getRegionsInTransitionInRS() otherwise can hold up
    // regionserver abort on cluster shutdown. HBASE-23984.
    getServer().getRegionsInTransitionInRS().remove(regionInfo.getEncodedNameAsBytes());
    getServer().abort(
      "Failed to open region " + regionInfo.getRegionNameAsString() + " and can not recover", t);
  }
Handles opening of a region on a region server.
<p/>
Just done the same thing with the old {@link OpenRegionHandler}, with some modifications on
fencing and retrying. But we need to keep the {@link OpenRegionHandler} as is to keep compatible
with the zk less assignment for 1.x, otherwise it is not possible to do rolling upgrade.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/handler/AssignRegionHandler.java