[KYUUBI #4360][FOLLOWUP] Get valid unlimited users from existing limiters instead of conf

### _Why are the changes needed?_

It is more accurate to get the unlimited users from existing limiters.
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [x] [Run test](https://kyuubi.readthedocs.io/en/master/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #4363 from turboFei/refresh_unlimit.

Closes #4360

a880b413 [fwang12] refactor
6da9f9bd [fwang12] get

Authored-by: fwang12 <fwang12@ebay.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>

kyuubi-server/src/main/scala/org/apache/kyuubi/server/KyuubiServer.scala

@@ -131,12 +131,10 @@ object KyuubiServer extends Logging {
   }
 
   private[kyuubi] def refreshUnlimitedUsers(): Unit = synchronized {
-    val existingUnlimitedUsers =
-      kyuubiServer.conf.get(KyuubiConf.SERVER_LIMIT_CONNECTIONS_USER_UNLIMITED_LIST).toSet
-    val refreshedUnlimitedUsers = KyuubiConf().loadFileDefaults().get(
-      KyuubiConf.SERVER_LIMIT_CONNECTIONS_USER_UNLIMITED_LIST).toSet
-    kyuubiServer.backendService.sessionManager.asInstanceOf[KyuubiSessionManager]
-      .refreshUnlimitedUsers(refreshedUnlimitedUsers)
+    val sessionMgr = kyuubiServer.backendService.sessionManager.asInstanceOf[KyuubiSessionManager]
+    val existingUnlimitedUsers = sessionMgr.getUnlimitedUsers()
+    sessionMgr.refreshUnlimitedUsers(KyuubiConf().loadFileDefaults())
+    val refreshedUnlimitedUsers = sessionMgr.getUnlimitedUsers()
     info(s"Refreshed unlimited users from $existingUnlimitedUsers to $refreshedUnlimitedUsers")
   }
 }

kyuubi-server/src/main/scala/org/apache/kyuubi/session/KyuubiSessionManager.scala

@@ -300,9 +300,14 @@ class KyuubiSessionManager private (name: String) extends SessionManager(name) {
       userUnlimitedList)
   }
 
-  private[kyuubi] def refreshUnlimitedUsers(userUnlimitedList: Set[String]): Unit = {
-    limiter.foreach(SessionLimiter.resetUnlimitedUsers(_, userUnlimitedList))
-    batchLimiter.foreach(SessionLimiter.resetUnlimitedUsers(_, userUnlimitedList))
+  private[kyuubi] def getUnlimitedUsers(): Set[String] = {
+    limiter.orElse(batchLimiter).map(SessionLimiter.getUnlimitedUsers).getOrElse(Set.empty)
+  }
+
+  private[kyuubi] def refreshUnlimitedUsers(conf: KyuubiConf): Unit = {
+    val unlimitedUsers = conf.get(SERVER_LIMIT_CONNECTIONS_USER_UNLIMITED_LIST).toSet
+    limiter.foreach(SessionLimiter.resetUnlimitedUsers(_, unlimitedUsers))
+    batchLimiter.foreach(SessionLimiter.resetUnlimitedUsers(_, unlimitedUsers))
   }
 
   private def applySessionLimiter(

kyuubi-server/src/main/scala/org/apache/kyuubi/session/SessionLimiter.scala

@@ -138,10 +138,14 @@ object SessionLimiter {
       unlimitedUsers)
   }
 
-  def resetUnlimitedUsers(limiter: SessionLimiter, unlimitedUsers: Set[String]): Unit = {
+  def resetUnlimitedUsers(limiter: SessionLimiter, unlimitedUsers: Set[String]): Unit =
     limiter match {
       case l: SessionLimiterWithUnlimitedUsersImpl => l.setUnlimitedUsers(unlimitedUsers)
       case _ =>
     }
+
+  def getUnlimitedUsers(limiter: SessionLimiter): Set[String] = limiter match {
+    case l: SessionLimiterWithUnlimitedUsersImpl => l.unlimitedUsers
+    case _ => Set.empty
   }
 }