@Override
  protected void handleException(Throwable t) {
    LOG.warn("Fatal error occurred while closing region {}, aborting...", encodedName, t);
    // Clear any reference in getServer().getRegionsInTransitionInRS() otherwise can hold up
    // regionserver abort on cluster shutdown. HBASE-23984.
    getServer().getRegionsInTransitionInRS().remove(Bytes.toBytes(this.encodedName));
    getServer().abort("Failed to close region " + encodedName + " and can not recover", t);
  }
Handles closing of a region on a region server.
<p/>
Just done the same thing with the old {@link CloseRegionHandler}, with some modifications on
fencing and retrying. But we need to keep the {@link CloseRegionHandler} 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/UnassignRegionHandler.java