TransportUnfollowAction should increase settings version (#37859)

The TransportUnfollowAction updates the index settings but does not 
increase the settings version to reflect that change.

This issue has been caught while working on the replication of closed 
indices (#33888). The IndexFollowingIT.testUnfollowIndex() started to 
fail and this specific assertion tripped. It does not happen on master 
branch today because index metadata for closed indices are never 
updated in IndexService instances, but this is something that is going 
to change with the replication of closed indices.

Author: tlrx

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

@@ -105,18 +105,19 @@ static ClusterState unfollow(String followerIndex, ClusterState current) {
             }
         }
 
-        IndexMetaData.Builder newIMD = IndexMetaData.builder(followerIMD);
         // Remove index.xpack.ccr.following_index setting
         Settings.Builder builder = Settings.builder();
         builder.put(followerIMD.getSettings());
         builder.remove(CcrSettings.CCR_FOLLOWING_INDEX_SETTING.getKey());
 
-        newIMD.settings(builder);
+        final IndexMetaData.Builder newIndexMetaData = IndexMetaData.builder(followerIMD);
+        newIndexMetaData.settings(builder);
+        newIndexMetaData.settingsVersion(followerIMD.getSettingsVersion() + 1);
         // Remove ccr custom metadata
-        newIMD.removeCustom(Ccr.CCR_CUSTOM_METADATA_KEY);
+        newIndexMetaData.removeCustom(Ccr.CCR_CUSTOM_METADATA_KEY);
 
         MetaData newMetaData = MetaData.builder(current.metaData())
-            .put(newIMD)
+            .put(newIndexMetaData)
             .build();
         return ClusterState.builder(current)
             .metaData(newMetaData)

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

@@ -30,8 +30,10 @@
 public class TransportUnfollowActionTests extends ESTestCase {
 
     public void testUnfollow() {
+        final long settingsVersion = randomNonNegativeLong();
         IndexMetaData.Builder followerIndex = IndexMetaData.builder("follow_index")
             .settings(settings(Version.CURRENT).put(CcrSettings.CCR_FOLLOWING_INDEX_SETTING.getKey(), true))
+            .settingsVersion(settingsVersion)
             .numberOfShards(1)
             .numberOfReplicas(0)
             .state(IndexMetaData.State.CLOSE)
@@ -47,6 +49,7 @@ public void testUnfollow() {
         IndexMetaData resultIMD = result.metaData().index("follow_index");
         assertThat(resultIMD.getSettings().get(CcrSettings.CCR_FOLLOWING_INDEX_SETTING.getKey()), nullValue());
         assertThat(resultIMD.getCustomData(Ccr.CCR_CUSTOM_METADATA_KEY), nullValue());
+        assertThat(resultIMD.getSettingsVersion(), equalTo(settingsVersion + 1));
     }
 
     public void testUnfollowIndexOpen() {