Do not lose default mapper on metadata updates (#33153)

When applying index metadata updates we run through the mappings
updating them if needed. Today if there is not an update to the default
mapper, we can lose the default mapping. This means that, for example,
if we apply a settings update to an index we will lose the default
mapper. This happens because we were not guarding updating the default
mapping with a check that the default mapping was updated in the
metadata update. When there is no update in the metadata update, we need
to continue to preserve the previous default mapping. This commit
achieves this by moving the updating of the default mapping under the
same guard that we use for updating the default mapping source. We add a
test that fails before putting the update under a guard and now passes
after moving the update under the guard.

server/src/main/java/org/elasticsearch/index/mapper/MapperService.java

@@ -468,11 +468,11 @@ private synchronized Map<String, DocumentMapper> internalMerge(@Nullable Documen
         // commit the change
         if (defaultMappingSource != null) {
             this.defaultMappingSource = defaultMappingSource;
+            this.defaultMapper = defaultMapper;
         }
         if (newMapper != null) {
             this.mapper = newMapper;
         }
-        this.defaultMapper = defaultMapper;
         this.fieldTypes = fieldTypes;
         this.hasNested = hasNested;
         this.fullPathObjectMappers = fullPathObjectMappers;

server/src/test/java/org/elasticsearch/index/mapper/MapperServiceTests.java

@@ -21,13 +21,16 @@
 
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.Version;
+import org.elasticsearch.action.admin.indices.mapping.put.PutMappingRequest;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.compress.CompressedXContent;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.common.xcontent.XContentType;
+import org.elasticsearch.common.xcontent.json.JsonXContent;
 import org.elasticsearch.index.IndexService;
 import org.elasticsearch.index.mapper.KeywordFieldMapper.KeywordFieldType;
 import org.elasticsearch.index.mapper.MapperService.MergeReason;
@@ -119,6 +122,35 @@ public void testIndexIntoDefaultMapping() throws Throwable {
         assertNull(indexService.mapperService().documentMapper(MapperService.DEFAULT_MAPPING));
     }
 
+    public void testIndexMetaDataUpdateDoesNotLoseDefaultMapper() throws IOException {
+        final IndexService indexService =
+                createIndex("test", Settings.builder().put(IndexMetaData.SETTING_VERSION_CREATED, Version.V_6_3_0).build());
+        try (XContentBuilder builder = JsonXContent.contentBuilder()) {
+            builder.startObject();
+            {
+                builder.startObject(MapperService.DEFAULT_MAPPING);
+                {
+                    builder.field("date_detection", false);
+                }
+                builder.endObject();
+            }
+            builder.endObject();
+            final PutMappingRequest putMappingRequest = new PutMappingRequest();
+            putMappingRequest.indices("test");
+            putMappingRequest.type(MapperService.DEFAULT_MAPPING);
+            putMappingRequest.source(builder);
+            client().admin().indices().preparePutMapping("test").setType(MapperService.DEFAULT_MAPPING).setSource(builder).get();
+        }
+        assertNotNull(indexService.mapperService().documentMapper(MapperService.DEFAULT_MAPPING));
+        final Settings zeroReplicasSettings = Settings.builder().put(IndexMetaData.SETTING_NUMBER_OF_REPLICAS, 0).build();
+        client().admin().indices().prepareUpdateSettings("test").setSettings(zeroReplicasSettings).get();
+        /*
+         * This assertion is a guard against a previous bug that would lose the default mapper when applying a metadata update that did not
+         * update the default mapping.
+         */
+        assertNotNull(indexService.mapperService().documentMapper(MapperService.DEFAULT_MAPPING));
+    }
+
     public void testTotalFieldsExceedsLimit() throws Throwable {
         Function<String, String> mapping = type -> {
             try {