Fix merging of _meta field (#27352)

core/src/main/java/org/elasticsearch/index/mapper/DocumentMapper.java

@@ -56,7 +56,7 @@ public class DocumentMapper implements ToXContentFragment {
 
         private final RootObjectMapper rootObjectMapper;
 
-        private Map<String, Object> meta = emptyMap();
+        private Map<String, Object> meta;
 
         private final Mapper.BuilderContext builderContext;
 

core/src/main/java/org/elasticsearch/index/mapper/Mapping.java

@@ -98,7 +98,8 @@ public final class Mapping implements ToXContentFragment {
             }
             mergedMetaDataMappers.put(merged.getClass(), merged);
         }
-        return new Mapping(indexCreated, mergedRoot, mergedMetaDataMappers.values().toArray(new MetadataFieldMapper[0]), mergeWith.meta);
+        Map<String, Object> mergedMeta = mergeWith.meta == null ? meta : mergeWith.meta;
+        return new Mapping(indexCreated, mergedRoot, mergedMetaDataMappers.values().toArray(new MetadataFieldMapper[0]), mergedMeta);
     }
 
     /**
@@ -128,7 +129,7 @@ public final class Mapping implements ToXContentFragment {
         root.toXContent(builder, params, new ToXContent() {
             @Override
             public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
-                if (meta != null && !meta.isEmpty()) {
+                if (meta != null) {
                     builder.field("_meta", meta);
                 }
                 for (Mapper mapper : metadataMappers) {

core/src/test/java/org/elasticsearch/index/mapper/DocumentMapperMergeTests.java

@@ -289,4 +289,47 @@ public class DocumentMapperMergeTests extends ESSingleNodeTestCase {
         Exception e = expectThrows(IllegalArgumentException.class, () -> initMapper.merge(updatedMapper.mapping(), false));
         assertThat(e.getMessage(), containsString("The _parent field's type option can't be changed: [null]->[parent]"));
     }
+
+    public void testMergeMeta() throws IOException {
+        DocumentMapperParser parser = createIndex("test").mapperService().documentMapperParser();
+
+        String initMapping = XContentFactory.jsonBuilder()
+            .startObject()
+                .startObject("test")
+                    .startObject("_meta")
+                        .field("foo").value("bar")
+                    .endObject()
+                .endObject()
+            .endObject()
+            .string();
+        DocumentMapper initMapper = parser.parse("test", new CompressedXContent(initMapping));
+
+        assertThat(initMapper.meta().get("foo"), equalTo("bar"));
+
+        String updateMapping = XContentFactory.jsonBuilder()
+            .startObject()
+                .startObject("test")
+                    .startObject("properties")
+                        .startObject("name").field("type", "text").endObject()
+                    .endObject()
+                .endObject()
+            .endObject()
+            .string();
+        DocumentMapper updatedMapper = parser.parse("test", new CompressedXContent(updateMapping));
+
+        assertThat(initMapper.merge(updatedMapper.mapping(), true).meta().get("foo"), equalTo("bar"));
+
+        updateMapping = XContentFactory.jsonBuilder()
+            .startObject()
+                .startObject("test")
+                    .startObject("_meta")
+                        .field("foo").value("new_bar")
+                    .endObject()
+                .endObject()
+            .endObject()
+            .string();
+        updatedMapper = parser.parse("test", new CompressedXContent(updateMapping));
+
+        assertThat(initMapper.merge(updatedMapper.mapping(), true).meta().get("foo"), equalTo("new_bar"));
+    }
 }