HDFS-17198. RBF: fix bug of getRepresentativeQuorum when records have same dateModified

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/store/impl/MembershipStoreImpl.java

@@ -279,30 +279,30 @@ private MembershipState getRepresentativeQuorum(
       Collection<MembershipState> records) {
 
     // Collate objects by field value: field value -> order set of records
-    Map<FederationNamenodeServiceState, TreeSet<MembershipState>> occurenceMap =
+    Map<FederationNamenodeServiceState, Set<MembershipState>> occurenceMap =
         new HashMap<>();
     for (MembershipState record : records) {
       FederationNamenodeServiceState state = record.getState();
-      TreeSet<MembershipState> matchingSet = occurenceMap.get(state);
+      Set<MembershipState> matchingSet = occurenceMap.get(state);
       if (matchingSet == null) {
-        // TreeSet orders elements by descending date via comparators
-        matchingSet = new TreeSet<>();
+        matchingSet = new HashSet<>();
         occurenceMap.put(state, matchingSet);
       }
       matchingSet.add(record);
     }
 
     // Select largest group
-    TreeSet<MembershipState> largestSet = new TreeSet<>();
-    for (TreeSet<MembershipState> matchingSet : occurenceMap.values()) {
+    Set<MembershipState> largestSet = new HashSet<>();
+    for (Set<MembershipState> matchingSet : occurenceMap.values()) {
       if (largestSet.size() < matchingSet.size()) {
         largestSet = matchingSet;
       }
     }
 
     // If quorum, use the newest element here
     if (largestSet.size() > records.size() / 2) {
-      return largestSet.first();
+      TreeSet<MembershipState> sortedList = new TreeSet<>(largestSet);
+      return sortedList.first();
       // Otherwise, return most recent by class comparator
     } else if (records.size() > 0) {
       TreeSet<MembershipState> sortedList = new TreeSet<>(records);

hadoop-hdfs-project/hadoop-hdfs-rbf/src/test/java/org/apache/hadoop/hdfs/server/federation/store/TestStateStoreMembershipState.java

@@ -223,6 +223,57 @@ public void testRegistrationMajorityQuorum()
     assertEquals(quorumEntry.getRouterId(), ROUTERS[3]);
   }
 
+  /**
+   * Fix getRepresentativeQuorum when records have same date modified time.
+   */
+  @Test
+  public void testRegistrationMajorityQuorumEqDateModified()
+          throws InterruptedException, IOException {
+
+    // Populate the state store with a set of non-matching elements
+    // 1) ns0:nn0 - Standby (newest)
+    // 2) ns0:nn0 - Active
+    // 3) ns0:nn0 - Active
+    // 4) ns0:nn0 - Active
+    // (2), (3), (4) have the same date modified time
+    // Verify the selected entry is the newest majority opinion (4)
+    String ns = "ns0";
+    String nn = "nn0";
+
+    long dateModified = Time.now();
+    // Active - oldest
+    MembershipState report = createRegistration(
+            ns, nn, ROUTERS[1], FederationNamenodeServiceState.ACTIVE);
+    report.setDateModified(dateModified);
+    assertTrue(namenodeHeartbeat(report));
+
+    // Active - 2nd oldest
+    report = createRegistration(
+            ns, nn, ROUTERS[2], FederationNamenodeServiceState.ACTIVE);
+    report.setDateModified(dateModified);
+    assertTrue(namenodeHeartbeat(report));
+
+    // Active - 3rd oldest
+    report = createRegistration(
+            ns, nn, ROUTERS[3], FederationNamenodeServiceState.ACTIVE);
+    report.setDateModified(dateModified);
+    assertTrue(namenodeHeartbeat(report));
+
+    // standby - newest overall
+    report = createRegistration(
+            ns, nn, ROUTERS[0], FederationNamenodeServiceState.STANDBY);
+    assertTrue(namenodeHeartbeat(report));
+
+    // Load and calculate quorum
+    assertTrue(getStateStore().loadCache(MembershipStore.class, true));
+
+    // Verify quorum entry
+    MembershipState quorumEntry = getNamenodeRegistration(
+            report.getNameserviceId(), report.getNamenodeId());
+    assertNotNull(quorumEntry);
+    assertEquals(quorumEntry.getState(), FederationNamenodeServiceState.ACTIVE);
+  }
+
   @Test
   public void testRegistrationQuorumExcludesExpired()
       throws InterruptedException, IOException {