Enable New Semantic Text Format Only On Newly Created Indices (elastic#121556)

(cherry picked from commit d23699f)

Author: Mikep86

docs/changelog/121556.yaml

@@ -0,0 +1,5 @@
+pr: 121556
+summary: Enable New Semantic Text Format Only On Newly Created Indices
+area: Mapping
+type: bug
+issues: []

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

@@ -15,6 +15,7 @@
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.query.SearchExecutionContext;
 
@@ -41,6 +42,10 @@ public abstract class InferenceMetadataFieldsMapper extends MetadataFieldMapper
         Setting.Property.InternalIndex
     );
 
+    // Check index version SOURCE_MAPPER_MODE_ATTRIBUTE_NOOP because that index version was added in the same serverless promotion
+    // where the new format was enabled by default
+    public static final IndexVersion USE_NEW_SEMANTIC_TEXT_FORMAT_BY_DEFAULT = IndexVersions.SOURCE_MAPPER_MODE_ATTRIBUTE_NOOP;
+
     public static final String NAME = "_inference_fields";
     public static final String CONTENT_TYPE = "_inference_fields";
 
@@ -86,10 +91,12 @@ public abstract ValueFetcher valueFetcher(
      */
     public static boolean isEnabled(Settings settings) {
         var version = IndexMetadata.SETTING_INDEX_VERSION_CREATED.get(settings);
-        if (version.before(IndexVersions.INFERENCE_METADATA_FIELDS)
-            && version.between(IndexVersions.INFERENCE_METADATA_FIELDS_BACKPORT, IndexVersions.UPGRADE_TO_LUCENE_10_0_0) == false) {
+        if ((version.before(IndexVersions.INFERENCE_METADATA_FIELDS)
+            && version.between(IndexVersions.INFERENCE_METADATA_FIELDS_BACKPORT, IndexVersions.UPGRADE_TO_LUCENE_10_0_0) == false)
+            || (version.before(USE_NEW_SEMANTIC_TEXT_FORMAT_BY_DEFAULT) && USE_LEGACY_SEMANTIC_TEXT_FORMAT.exists(settings) == false)) {
             return false;
         }
+
         return USE_LEGACY_SEMANTIC_TEXT_FORMAT.get(settings) == false;
     }
 

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/mapper/SemanticInferenceMetadataFieldsMapperTests.java

@@ -55,6 +55,24 @@ public void testIsEnabled() {
         assertTrue(InferenceMetadataFieldsMapper.isEnabled(settings));
     }
 
+    public void testIsEnabledByDefault() {
+        var settings = Settings.builder()
+            .put(
+                IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey(),
+                IndexVersionUtils.getPreviousVersion(InferenceMetadataFieldsMapper.USE_NEW_SEMANTIC_TEXT_FORMAT_BY_DEFAULT)
+            )
+            .build();
+        assertFalse(InferenceMetadataFieldsMapper.isEnabled(settings));
+
+        settings = Settings.builder()
+            .put(
+                IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey(),
+                InferenceMetadataFieldsMapper.USE_NEW_SEMANTIC_TEXT_FORMAT_BY_DEFAULT
+            )
+            .build();
+        assertTrue(InferenceMetadataFieldsMapper.isEnabled(settings));
+    }
+
     @Override
     public void testFieldHasValue() {
         assertTrue(