Fix handling empty key case in the terms aggregation (#90822)

docs/changelog/90822.yaml

@@ -0,0 +1,5 @@
+pr: 90822
+summary: Fix handling empty key case in the terms aggregation
+area: Mapping
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/index/mapper/flattened/FlattenedFieldMapper.java

@@ -703,7 +703,13 @@ public final class FlattenedFieldMapper extends FieldMapper {
         }
 
         XContentParser xContentParser = context.parser();
-        context.doc().addAll(fieldParser.parse(xContentParser));
+        try {
+            // make sure that we don't expand dots in field names while parsing
+            context.path().setWithinLeafObject(true);
+            context.doc().addAll(fieldParser.parse(xContentParser));
+        } finally {
+            context.path().setWithinLeafObject(false);
+        }
 
         if (mappedFieldType.hasDocValues() == false) {
             context.addToFieldNames(fieldType().name());

server/src/test/java/org/elasticsearch/index/mapper/flattened/FlattenedFieldMapperTests.java

@@ -176,6 +176,82 @@ public class FlattenedFieldMapperTests extends MapperTestCase {
         assertEquals(0, fields.length);
     }
 
+    public void testBlankFieldName() throws Exception {
+        DocumentMapper mapper = createDocumentMapper(fieldMapping(this::minimalMapping));
+        ParsedDocument parsedDoc = mapper.parse(source(b -> b.startObject("field").field("", "value").endObject()));
+        IndexableField[] fields = parsedDoc.rootDoc().getFields("field");
+        assertEquals(2, fields.length);
+    }
+
+    public void testDotOnlyFieldName() throws Exception {
+        DocumentMapper mapper = createDocumentMapper(fieldMapping(this::minimalMapping));
+        ParsedDocument parsedDoc = mapper.parse(
+            source(b -> b.startObject("field").field(".", "value1").field("..", "value2").field("...", "value3").endObject())
+        );
+        IndexableField[] fields = parsedDoc.rootDoc().getFields("field");
+        assertEquals(6, fields.length);
+    }
+
+    public void testMixOfOrdinaryAndFlattenedFields() throws Exception {
+        DocumentMapper mapper = createDocumentMapper(
+            topMapping(
+                b -> b.field("dynamic", "strict")
+                    .startObject("properties")
+                    .startObject("field")
+                    .field("type", "flattened")
+                    .endObject()
+                    .startObject("a")
+                    .field("type", "object")
+                    .startObject("properties")
+                    .startObject("b")
+                    .field("type", "object")
+                    .startObject("properties")
+                    .startObject("c")
+                    .field("type", "keyword")
+                    .endObject()
+                    .endObject()
+                    .endObject()
+                    .endObject()
+                    .endObject()
+                    .startObject("d")
+                    .field("type", "object")
+                    .startObject("properties")
+                    .startObject("e")
+                    .field("type", "keyword")
+                    .endObject()
+                    .endObject()
+                    .endObject()
+                    .endObject()
+            )
+        );
+        ParsedDocument parsedDoc = mapper.parse(
+            source(
+                b -> b.startObject("field")
+                    .field("", "value")
+                    .field("subfield1", "value1")
+                    .field("subfield2", "value2")
+                    .endObject()
+                    .startObject("a")
+                    .startObject("b")
+                    .field("c", "value3")
+                    .endObject()
+                    .endObject()
+                    .field("d.e", "value4")
+            )
+        );
+        assertNull(parsedDoc.dynamicMappingsUpdate());
+        IndexableField[] fields = parsedDoc.rootDoc().getFields("field");
+        assertEquals(6, fields.length);
+        fields = parsedDoc.rootDoc().getFields("a.b");
+        assertEquals(0, fields.length);
+        fields = parsedDoc.rootDoc().getFields("a.b.c");
+        assertEquals(2, fields.length);
+        fields = parsedDoc.rootDoc().getFields("d");
+        assertEquals(0, fields.length);
+        fields = parsedDoc.rootDoc().getFields("d.e");
+        assertEquals(2, fields.length);
+    }
+
     public void testMalformedJson() throws Exception {
         DocumentMapper mapper = createDocumentMapper(fieldMapping(this::minimalMapping));