Filter out upgraded version index settings when starting index following

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/TransportResumeFollowAction.java

@@ -243,7 +243,8 @@ static void validate(
         Settings leaderSettings = filter(leaderIndex.getSettings());
         Settings followerSettings = filter(followIndex.getSettings());
         if (leaderSettings.equals(followerSettings) == false) {
-            throw new IllegalArgumentException("the leader and follower index settings must be identical");
+            throw new IllegalArgumentException("the leader index setting[" + leaderSettings + "] and follower index settings [" +
+                followerSettings + "] must be identical");
         }
 
         // Validates if the current follower mapping is mergable with the leader mapping.
@@ -456,6 +457,11 @@ static Settings filter(Settings originalSettings) {
         settings.remove(IndexMetaData.SETTING_INDEX_PROVIDED_NAME);
         settings.remove(IndexMetaData.SETTING_CREATION_DATE);
 
+        // Follower index may be upgraded, while the leader index hasn't been upgraded, so it is expected
+        // that these settings are different:
+        settings.remove(IndexMetaData.SETTING_VERSION_UPGRADED);
+        settings.remove(IndexMetaData.SETTING_VERSION_UPGRADED_STRING);
+
         Iterator<String> iterator = settings.keys().iterator();
         while (iterator.hasNext()) {
             String key = iterator.next();

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/action/TransportResumeFollowActionTests.java

@@ -150,7 +150,10 @@ public void testValidation() throws IOException {
                 .put("index.analysis.analyzer.my_analyzer.type", "custom")
                 .put("index.analysis.analyzer.my_analyzer.tokenizer", "standard").build(), customMetaData);
             Exception e = expectThrows(IllegalArgumentException.class, () -> validate(request, leaderIMD, followIMD, UUIDs, null));
-            assertThat(e.getMessage(), equalTo("the leader and follower index settings must be identical"));
+            assertThat(e.getMessage(), equalTo("the leader index setting[{\"index.analysis.analyzer.my_analyzer.tokenizer\"" +
+                ":\"whitespace\",\"index.analysis.analyzer.my_analyzer.type\":\"custom\",\"index.number_of_shards\":\"5\"}] " +
+                "and follower index settings [{\"index.analysis.analyzer.my_analyzer.tokenizer\":\"standard\"," +
+                "\"index.analysis.analyzer.my_analyzer.type\":\"custom\",\"index.number_of_shards\":\"5\"}] must be identical"));
         }
         {
             // should fail because the following index does not have the following_index settings
@@ -242,6 +245,21 @@ public void testDynamicIndexSettingsAreClassified() {
         }
     }
 
+    public void testFilter() {
+        Settings.Builder settings = Settings.builder();
+        settings.put(CcrSettings.CCR_FOLLOWING_INDEX_SETTING.getKey(), "");
+        settings.put(IndexSettings.INDEX_SOFT_DELETES_SETTING.getKey(), "");
+        settings.put(IndexMetaData.SETTING_INDEX_VERSION_CREATED.getKey(), "");
+        settings.put(IndexMetaData.SETTING_INDEX_UUID, "");
+        settings.put(IndexMetaData.SETTING_INDEX_PROVIDED_NAME, "");
+        settings.put(IndexMetaData.SETTING_CREATION_DATE, "");
+        settings.put(IndexMetaData.SETTING_VERSION_UPGRADED, "");
+        settings.put(IndexMetaData.SETTING_VERSION_UPGRADED_STRING, "");
+
+        Settings result = TransportResumeFollowAction.filter(settings.build());
+        assertThat(result.size(), equalTo(0));
+    }
+
     private static IndexMetaData createIMD(String index,
                                            int numberOfShards,
                                            Settings settings,

x-pack/qa/rolling-upgrade-multi-cluster/src/test/java/org/elasticsearch/upgrades/CcrRollingUpgradeIT.java

@@ -21,11 +21,6 @@
 
 public class CcrRollingUpgradeIT extends AbstractMultiClusterUpgradeTestCase {
 
-    public void test() {
-        // dummy test otherwise test run fails when both tests are ignored.
-    }
-
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/38835")
     public void testIndexFollowing() throws Exception {
         logger.info("clusterName={}, upgradeState={}", clusterName, upgradeState);