gate new behaviour behind an index version

Author: martijnvg

modules/mapper-extras/src/main/java/org/elasticsearch/index/mapper/extras/MatchOnlyTextFieldMapper.java

@@ -33,6 +33,7 @@
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.unit.Fuzziness;
 import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.analysis.IndexAnalyzers;
 import org.elasticsearch.index.analysis.NamedAnalyzer;
 import org.elasticsearch.index.fielddata.FieldDataContext;
@@ -136,9 +137,14 @@ private MatchOnlyTextFieldType buildFieldType(MapperBuilderContext context) {
         @Override
         public MatchOnlyTextFieldMapper build(MapperBuilderContext context) {
             MatchOnlyTextFieldType tft = buildFieldType(context);
-            boolean storeSource = context.isSourceSynthetic()
-                && withinMultiField == false
-                && multiFieldsBuilder.hasSyntheticSourceCompatibleKeywordField() == false;
+            final boolean storeSource;
+            if (indexCreatedVersion.onOrAfter(IndexVersions.MAPPER_TEXT_MATCH_ONLY_MULTI_FIELDS_DEFAULT_NOT_STORED)) {
+                storeSource = context.isSourceSynthetic()
+                    && withinMultiField == false
+                    && multiFieldsBuilder.hasSyntheticSourceCompatibleKeywordField() == false;
+            } else {
+                storeSource = context.isSourceSynthetic();
+            }
             return new MatchOnlyTextFieldMapper(leafName(), Defaults.FIELD_TYPE, tft, builderParams(this, context), storeSource, this);
         }
     }

server/src/main/java/org/elasticsearch/index/IndexVersions.java

@@ -171,6 +171,7 @@ private static Version parseUnchecked(String version) {
     public static final IndexVersion DEFAULT_TO_ACORN_HNSW_FILTER_HEURISTIC = def(9_026_0_00, Version.LUCENE_10_2_1);
     public static final IndexVersion SEQ_NO_WITHOUT_POINTS = def(9_027_0_00, Version.LUCENE_10_2_1);
     public static final IndexVersion INDEX_INT_SORT_INT_TYPE = def(9_028_0_00, Version.LUCENE_10_2_1);
+    public static final IndexVersion MAPPER_TEXT_MATCH_ONLY_MULTI_FIELDS_DEFAULT_NOT_STORED = def(9_029_0_00, Version.LUCENE_10_2_1);
 
     /*
      * STOP! READ THIS FIRST! No, really,

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

@@ -307,12 +307,15 @@ public Builder(
             //
             // If 'store' parameter was explicitly provided we'll reject the request.
             // Note that if current builder is a multi field, then we don't need to store, given that responsibility lies with parent field
-            this.store = Parameter.storeParam(
-                m -> ((TextFieldMapper) m).store,
-                () -> isSyntheticSourceEnabled
-                    && this.withinMultiField == false
-                    && multiFieldsBuilder.hasSyntheticSourceCompatibleKeywordField() == false
-            );
+            this.store = Parameter.storeParam(m -> ((TextFieldMapper) m).store, () -> {
+                if (indexCreatedVersion.onOrAfter(IndexVersions.MAPPER_TEXT_MATCH_ONLY_MULTI_FIELDS_DEFAULT_NOT_STORED)) {
+                    return isSyntheticSourceEnabled
+                        && this.withinMultiField == false
+                        && multiFieldsBuilder.hasSyntheticSourceCompatibleKeywordField() == false;
+                } else {
+                    return isSyntheticSourceEnabled;
+                }
+            });
             this.indexCreatedVersion = indexCreatedVersion;
             this.analyzers = new TextParams.Analyzers(
                 indexAnalyzers,