default void preGrant(ObserverContext<MasterCoprocessorEnvironment> ctx,
    UserPermission userPermission, boolean mergeExistingPermissions) throws IOException {
  }
Called before granting user permissions.
@param ctx                      the coprocessor instance's environment
@param userPermission           the user and permissions
@param mergeExistingPermissions True if merge with previous granted permissions
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java