Fix - Requesting _inference_fields when using legacy format causes shard failure

docs/changelog/121720.yaml

@@ -0,0 +1,5 @@
+pr: 121720
+summary: Skip fetching _inference_fields field in legacy semantic_text format
+area: Search
+type: bug
+issues: []

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

@@ -801,7 +801,8 @@ public static boolean isMetadataFieldStatic(String fieldName) {
      * this method considers all mapper plugins
      */
     public boolean isMetadataField(String field) {
-        return mapperRegistry.getMetadataMapperParsers(indexVersionCreated).containsKey(field);
+        var mapper = mappingLookup().getMapper(field);
+        return mapper instanceof MetadataFieldMapper;
     }
 
     /**

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/InferenceFeatures.java

@@ -36,6 +36,7 @@ public Set<NodeFeature> getTestFeatures() {
             SemanticTextFieldMapper.SEMANTIC_TEXT_DELETE_FIX,
             SemanticTextFieldMapper.SEMANTIC_TEXT_ZERO_SIZE_FIX,
             SemanticTextFieldMapper.SEMANTIC_TEXT_ALWAYS_EMIT_INFERENCE_ID_FIX,
+            SemanticTextFieldMapper.SEMANTIC_TEXT_SKIP_INFERENCE_FIELDS,
             SEMANTIC_TEXT_HIGHLIGHTER,
             SEMANTIC_MATCH_QUERY_REWRITE_INTERCEPTION_SUPPORTED,
             SEMANTIC_SPARSE_VECTOR_QUERY_REWRITE_INTERCEPTION_SUPPORTED,

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/mapper/SemanticTextFieldMapper.java

@@ -117,6 +117,7 @@ public class SemanticTextFieldMapper extends FieldMapper implements InferenceFie
     public static final NodeFeature SEMANTIC_TEXT_ALWAYS_EMIT_INFERENCE_ID_FIX = new NodeFeature(
         "semantic_text.always_emit_inference_id_fix"
     );
+    public static final NodeFeature SEMANTIC_TEXT_SKIP_INFERENCE_FIELDS = new NodeFeature("semantic_text.skip_inference_fields");
 
     public static final String CONTENT_TYPE = "semantic_text";
     public static final String DEFAULT_ELSER_2_INFERENCE_ID = DEFAULT_ELSER_ID;

x-pack/plugin/inference/src/yamlRestTest/resources/rest-api-spec/test/inference/30_semantic_text_inference_bwc.yml

@@ -650,3 +650,27 @@ setup:
   - match: { hits.total.value: 1 }
   - match: { hits.total.relation: eq }
   - match: { hits.hits.0._source.dense_field.text: "updated text" }
+
+---
+"Skip fetching _inference_fields":
+  - requires:
+      cluster_features: semantic_text.skip_inference_fields
+      reason: Skip _inference_fields when search is performed on legacy semantic_text format.
+
+  - do:
+      index:
+        index: test-index
+        id: doc_1
+        body:
+          sparse_field: "test value"
+        refresh: true
+
+  - do:
+      search:
+        index: test-index
+        body:
+          fields: [ _inference_fields ]
+          query:
+            match_all: { }
+
+  - not_exists: hits.hits.0._source._inference_fields