Save heap when dynamically mapping arrays

docs/changelog/127439.yaml

@@ -0,0 +1,6 @@
+pr: 127439
+summary: Save heap when dynamically mapping arrays
+area: Mapping
+type: bug
+issues:
+ - 117593

server/src/internalClusterTest/java/org/elasticsearch/index/mapper/DynamicMappingIT.java

@@ -122,9 +122,9 @@ public void testArrayWithDifferentTypes() {
             .get();
 
         assertTrue(bulkResponse.hasFailures());
-        assertEquals(
-            "mapper [foo] cannot be changed from type [long] to [text]",
-            bulkResponse.getItems()[0].getFailure().getCause().getMessage()
+        assertThat(
+            bulkResponse.getItems()[0].getFailure().getCause().getMessage(),
+            containsString("mapper [foo] cannot be changed from type [long] to [text]")
         );
     }
 

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

@@ -21,6 +21,8 @@
 
 import java.io.IOException;
 import java.time.DateTimeException;
+import java.util.Collections;
+import java.util.List;
 import java.util.Map;
 
 /**
@@ -312,6 +314,11 @@ private static final class Concrete implements Strategy {
         boolean createDynamicField(Mapper.Builder builder, DocumentParserContext context, MapperBuilderContext mapperBuilderContext)
             throws IOException {
             Mapper mapper = builder.build(mapperBuilderContext);
+            List<Mapper> existing;
+            if ((existing = context.getDynamicMappers(mapper.fullPath())) != null && existing.isEmpty() == false) {
+                maybeMergeWithExistingField(context, mapperBuilderContext, mapper, existing);
+                return true;
+            }
             if (context.addDynamicMapper(mapper)) {
                 parseField.accept(context, mapper);
                 return true;
@@ -320,6 +327,26 @@ boolean createDynamicField(Mapper.Builder builder, DocumentParserContext context
             }
         }
 
+        // Attempts to deduplicate arrays of mapper instances my merging with the most recently added mapper for the same path
+        // if there is one already and then replacing all entries for the field with the merge result
+        // TODO: this is only as complicated as it is to enable DocumentParser.postProcessDynamicArrayMapping, technically there is no need
+        // to duplicate the mapper other than to enabled this method's translation of multiple numeric field mappers into a vector mapper
+        // it seems?
+        private void maybeMergeWithExistingField(
+            DocumentParserContext context,
+            MapperBuilderContext mapperBuilderContext,
+            Mapper fieldMapper,
+            List<Mapper> existing
+        ) throws IOException {
+            var last = existing.getLast();
+            var merged = last.merge(fieldMapper, MapperMergeContext.from(mapperBuilderContext, Long.MAX_VALUE));
+            if (merged != last) {
+                Collections.fill(existing, merged);
+            }
+            existing.add(merged);
+            parseField.accept(context, merged);
+        }
+
         boolean createDynamicField(Mapper.Builder builder, DocumentParserContext context) throws IOException {
             return createDynamicField(builder, context, context.createDynamicMapperBuilderContext());
         }

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

@@ -37,8 +37,8 @@ public class ObjectMapperTests extends MapperServiceTestCase {
 
     public void testDifferentInnerObjectTokenFailure() throws Exception {
         DocumentMapper defaultMapper = createDocumentMapper(mapping(b -> {}));
-        IllegalArgumentException e = expectThrows(
-            IllegalArgumentException.class,
+        DocumentParsingException e = expectThrows(
+            DocumentParsingException.class,
             () -> defaultMapper.parse(new SourceToParse("1", new BytesArray("""
                 {
                      "object": {