Ensure parameters are updated when merging flattened mappings. (#48971) (#49014)

This PR makes the following two fixes around updating flattened fields:

* Make sure that the new value for ignore_above is immediately taken into
  affect. Previously we recorded the new value but did not use it when parsing
  documents.
* Allow depth_limit to be updated dynamically. It seems plausible that a user
  might want to tweak this setting as they encounter more data.

docs/reference/mapping/types/flattened.asciidoc

@@ -140,7 +140,8 @@ The following mapping parameters are accepted:
 
     The maximum allowed depth of the flattened object field, in terms of nested
     inner objects. If a flattened object field exceeds this limit, then an
-    error will be thrown. Defaults to `20`.
+    error will be thrown. Defaults to `20`. Note that `depth_limit` can be
+    updated dynamically through the <<indices-put-mapping, put mapping>> API.
 
 <<doc-values,`doc_values`>>::
 

x-pack/plugin/mapper-flattened/src/main/java/org/elasticsearch/xpack/flattened/mapper/FlatObjectFieldMapper.java

@@ -525,7 +525,7 @@ public final class FlatObjectFieldMapper extends DynamicKeyFieldMapper {
         }
     }
 
-    private final FlatObjectFieldParser fieldParser;
+    private FlatObjectFieldParser fieldParser;
     private int depthLimit;
     private int ignoreAbove;
 
@@ -552,7 +552,12 @@ public final class FlatObjectFieldMapper extends DynamicKeyFieldMapper {
     @Override
     protected void doMerge(Mapper mergeWith) {
         super.doMerge(mergeWith);
-        this.ignoreAbove = ((FlatObjectFieldMapper) mergeWith).ignoreAbove;
+
+        FlatObjectFieldMapper other = ((FlatObjectFieldMapper) mergeWith);
+        this.depthLimit = other.depthLimit;
+        this.ignoreAbove = other.ignoreAbove;
+        this.fieldParser = new FlatObjectFieldParser(fieldType.name(), keyedFieldName(),
+            fieldType, depthLimit, ignoreAbove);
     }
 
     @Override

x-pack/plugin/mapper-flattened/src/test/java/org/elasticsearch/xpack/flattened/mapper/FlatObjectFieldMapperTests.java

@@ -316,12 +316,12 @@ public class FlatObjectFieldMapperTests extends ESSingleNodeTestCase {
     }
 
     public void testDepthLimit() throws IOException {
-         String mapping = Strings.toString(XContentFactory.jsonBuilder().startObject()
+        // First verify the default behavior when depth_limit is not set.
+        String mapping = Strings.toString(XContentFactory.jsonBuilder().startObject()
             .startObject("type")
                 .startObject("properties")
                     .startObject("field")
                         .field("type", "flattened")
-                        .field("depth_limit", 2)
                     .endObject()
                 .endObject()
             .endObject()
@@ -340,8 +340,25 @@ public class FlatObjectFieldMapperTests extends ESSingleNodeTestCase {
             .endObject()
         .endObject());
 
+        mapper.parse(new SourceToParse("test", "type", "1", doc, XContentType.JSON));
+
+        // Set a lower value for depth_limit and check that the field is rejected.
+        String newMapping = Strings.toString(XContentFactory.jsonBuilder().startObject()
+            .startObject("type")
+                .startObject("properties")
+                    .startObject("field")
+                        .field("type", "flattened")
+                        .field("depth_limit", 2)
+                    .endObject()
+                .endObject()
+            .endObject()
+        .endObject());
+
+        DocumentMapper newMapper = mapper.merge(
+            parser.parse("type", new CompressedXContent(newMapping)).mapping());
+
         expectThrows(MapperParsingException.class, () ->
-            mapper.parse(new SourceToParse("test", "type", "1", doc, XContentType.JSON)));
+            newMapper.parse(new SourceToParse("test", "type", "1", doc, XContentType.JSON)));
     }
 
     public void testEagerGlobalOrdinals() throws IOException {
@@ -362,12 +379,12 @@ public class FlatObjectFieldMapperTests extends ESSingleNodeTestCase {
     }
 
     public void testIgnoreAbove() throws IOException {
-         String mapping = Strings.toString(XContentFactory.jsonBuilder().startObject()
+        // First verify the default behavior when ignore_above is not set.
+        String mapping = Strings.toString(XContentFactory.jsonBuilder().startObject()
             .startObject("type")
                 .startObject("properties")
                     .startObject("field")
                         .field("type", "flattened")
-                        .field("ignore_above", 10)
                     .endObject()
                 .endObject()
             .endObject()
@@ -386,7 +403,26 @@ public class FlatObjectFieldMapperTests extends ESSingleNodeTestCase {
 
         ParsedDocument parsedDoc = mapper.parse(new SourceToParse("test", "type", "1", doc, XContentType.JSON));
         IndexableField[] fields = parsedDoc.rootDoc().getFields("field");
-        assertEquals(0, fields.length);
+        assertEquals(2, fields.length);
+
+        // Set a lower value for ignore_above and check that the field is skipped.
+        String newMapping = Strings.toString(XContentFactory.jsonBuilder().startObject()
+            .startObject("type")
+                .startObject("properties")
+                    .startObject("field")
+                        .field("type", "flattened")
+                        .field("ignore_above", "10")
+                    .endObject()
+                .endObject()
+            .endObject()
+        .endObject());
+
+        DocumentMapper newMapper = mapper.merge(
+            parser.parse("type", new CompressedXContent(newMapping)).mapping());
+
+        ParsedDocument newParsedDoc = newMapper.parse(new SourceToParse("test", "type", "1", doc, XContentType.JSON));
+        IndexableField[] newFields = newParsedDoc.rootDoc().getFields("field");
+        assertEquals(0, newFields.length);
     }
 
     public void testNullValues() throws Exception {