default void preAssign(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    final RegionInfo regionInfo) throws IOException {
  }
Called prior to assigning a specific region.
@param ctx        the environment to interact with the framework and master
@param regionInfo the regionInfo of the region
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java