KAFKA-15271: Historicalterator can exposes elements that are too new (#…

…14125)

A HistoricalIterator at epoch N is supposed to only reveal elements at epoch N or earlier. However,
due to a bug, we sometimes will reveal elements which are at a newer epoch than N. The bug does
not affect elements that are in the latest epoch (aka topTier). It only affects elements that are
newer than N, but which do not persist until the latest epoch.  This PR fixes the bug and adds a
unit test for this case.

Reviewers: David Arthur <mumrah@gmail.com>

server-common/src/main/java/org/apache/kafka/timeline/SnapshottableHashTable.java

@@ -261,8 +261,10 @@ public boolean hasNext() {
                         int tierSlot = slot >>> shift;
                         BaseHashTable.unpackSlot(temp, deltaTable.baseElements(), tierSlot);
                         for (T object : temp) {
-                            if (BaseHashTable.findSlot(object, topTier.length) == slot) {
-                                ready.add(object);
+                            if (object.startEpoch() <= snapshot.epoch()) {
+                                if (BaseHashTable.findSlot(object, topTier.length) == slot) {
+                                    ready.add(object);
+                                }
                             }
                         }
                         temp.clear();

server-common/src/test/java/org/apache/kafka/timeline/SnapshottableHashTableTest.java

@@ -273,6 +273,21 @@ public void testReset() {
         assertIteratorYields(table.snapshottableIterator(Long.MAX_VALUE));
     }
 
+    @Test
+    public void testIteratorAtOlderEpoch() {
+        SnapshotRegistry registry = new SnapshotRegistry(new LogContext());
+        SnapshottableHashTable<TestElement> table =
+                new SnapshottableHashTable<>(registry, 4);
+        assertNull(table.snapshottableAddOrReplace(E_3B));
+        registry.getOrCreateSnapshot(0);
+        assertNull(table.snapshottableAddOrReplace(E_1A));
+        registry.getOrCreateSnapshot(1);
+        assertEquals(E_1A, table.snapshottableAddOrReplace(E_1B));
+        registry.getOrCreateSnapshot(2);
+        assertEquals(E_1B, table.snapshottableRemove(E_1B));
+        assertIteratorYields(table.snapshottableIterator(1), E_3B, E_1A);
+    }
+
     /**
      * Assert that the given iterator contains the given elements, in any order.
      * We compare using reference equality here, rather than object equality.