Add settings assertion in internal index reindexer

When performing an internal reindex, we add a setting marking the source
as read-only. We also check that this index is not already
read-only. This means that when we add the read-only setting, we expect
that it is already not there. This commit adds an assertion before we
increment the settings version validating that this is indeed the case.

Author: jasontedor

x-pack/plugin/upgrade/src/main/java/org/elasticsearch/xpack/upgrade/InternalIndexReindexer.java

@@ -151,9 +151,8 @@ public ClusterState execute(ClusterState currentState) throws Exception {
                                 .put(IndexMetaData.INDEX_READ_ONLY_SETTING.getKey(), true)
                                 .build();
                 final IndexMetaData.Builder builder = IndexMetaData.builder(indexMetaData).settings(indexSettingsBuilder);
-                if (same(indexMetaData.getSettings(), indexSettingsBuilder) == false) {
-                    builder.settingsVersion(1 + builder.settingsVersion());
-                }
+                assert same(indexMetaData.getSettings(), indexSettingsBuilder) == false;
+                builder.settingsVersion(1 + builder.settingsVersion());
 
                 MetaData.Builder metaDataBuilder = MetaData.builder(currentState.metaData()).put(builder);