KAFKA-19163: Avoid deleting groups with pending transactional offsets (#19496)

When a group has pending transactional offsets but no committed offsets,
we can accidentally delete it while cleaning up expired offsets. Add a
check to avoid this case.

Reviewers: David Jacot <djacot@confluent.io>

Author: squah-confluent

group-coordinator/src/main/java/org/apache/kafka/coordinator/group/OffsetMetadataManager.java

@@ -1000,13 +1000,14 @@ public OffsetFetchResponseData.OffsetFetchResponseGroup fetchAllOffsets(
      * @param groupId The group id.
      * @param records The list of records to populate with offset commit tombstone records.
      *
-     * @return True if no offsets exist or if all offsets expired, false otherwise.
+     * @return True if no offsets exist after expiry and no pending transactional offsets exist,
+     *         false otherwise.
      */
     public boolean cleanupExpiredOffsets(String groupId, List<CoordinatorRecord> records) {
         TimelineHashMap<String, TimelineHashMap<Integer, OffsetAndMetadata>> offsetsByTopic =
             offsets.offsetsByGroup.get(groupId);
         if (offsetsByTopic == null) {
-            return true;
+            return !openTransactions.contains(groupId);
         }
 
         // We expect the group to exist.

group-coordinator/src/test/java/org/apache/kafka/coordinator/group/OffsetMetadataManagerTest.java

@@ -2662,6 +2662,42 @@ public void testCleanupExpiredOffsetsWithPendingTransactionalOffsets() {
         assertEquals(List.of(), records);
     }
 
+    @Test
+    public void testCleanupExpiredOffsetsWithPendingTransactionalOffsetsOnly() {
+        GroupMetadataManager groupMetadataManager = mock(GroupMetadataManager.class);
+        Group group = mock(Group.class);
+
+        OffsetMetadataManagerTestContext context = new OffsetMetadataManagerTestContext.Builder()
+            .withGroupMetadataManager(groupMetadataManager)
+            .withOffsetsRetentionMinutes(1)
+            .build();
+
+        long commitTimestamp = context.time.milliseconds();
+
+        context.commitOffset("group-id", "foo", 0, 100L, 0, commitTimestamp);
+        context.commitOffset(10L, "group-id", "foo", 1, 101L, 0, commitTimestamp + 500);
+
+        context.time.sleep(Duration.ofMinutes(1).toMillis());
+
+        when(groupMetadataManager.group("group-id")).thenReturn(group);
+        when(group.offsetExpirationCondition()).thenReturn(Optional.of(
+            new OffsetExpirationConditionImpl(offsetAndMetadata -> offsetAndMetadata.commitTimestampMs)));
+        when(group.isSubscribedToTopic("foo")).thenReturn(false);
+
+        // foo-0 is expired, but the group is not deleted beacuse it has pending transactional offset commits.
+        List<CoordinatorRecord> expectedRecords = List.of(
+            GroupCoordinatorRecordHelpers.newOffsetCommitTombstoneRecord("group-id", "foo", 0)
+        );
+        List<CoordinatorRecord> records = new ArrayList<>();
+        assertFalse(context.cleanupExpiredOffsets("group-id", records));
+        assertEquals(expectedRecords, records);
+
+        // No offsets are expired, and the group is still not deleted because it has pending transactional offset commits.
+        records = new ArrayList<>();
+        assertFalse(context.cleanupExpiredOffsets("group-id", records));
+        assertEquals(List.of(), records);
+    }
+
     private static OffsetFetchResponseData.OffsetFetchResponsePartitions mkOffsetPartitionResponse(
         int partition,
         long offset,