Rely on codec to pass down default lucene postings format

Change PerFieldFormatSupplier to get default postings from current codec being used. This way the default codec for standard index mode isn't tied to Lucene101PostingsFormat specifically.

And rename the feature flag that controls when stock Lucene posting format is used.

Addressed comment from #126080

Author: martijnvg

server/src/main/java/org/elasticsearch/index/codec/LegacyPerFieldMapperCodec.java

@@ -28,7 +28,7 @@ public final class LegacyPerFieldMapperCodec extends Lucene101Codec {
 
     public LegacyPerFieldMapperCodec(Lucene101Codec.Mode compressionMode, MapperService mapperService, BigArrays bigArrays) {
         super(compressionMode);
-        this.formatSupplier = new PerFieldFormatSupplier(mapperService, bigArrays);
+        this.formatSupplier = new PerFieldFormatSupplier(mapperService, bigArrays, super.postingsFormat());
         // If the below assertion fails, it is a sign that Lucene released a new codec. You must create a copy of the current Elasticsearch
         // codec that delegates to this new Lucene codec, and make PerFieldMapperCodec extend this new Elasticsearch codec.
         assert Codec.forName(Lucene.LATEST_CODEC).getClass() == getClass().getSuperclass()

server/src/main/java/org/elasticsearch/index/codec/PerFieldFormatSupplier.java

@@ -12,7 +12,6 @@
 import org.apache.lucene.codecs.DocValuesFormat;
 import org.apache.lucene.codecs.KnnVectorsFormat;
 import org.apache.lucene.codecs.PostingsFormat;
-import org.apache.lucene.codecs.lucene101.Lucene101PostingsFormat;
 import org.apache.lucene.codecs.lucene90.Lucene90DocValuesFormat;
 import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsFormat;
 import org.elasticsearch.common.util.BigArrays;
@@ -34,32 +33,32 @@
  * vectors.
  */
 public class PerFieldFormatSupplier {
-    public static final FeatureFlag USE_LUCENE101_POSTINGS_FORMAT = new FeatureFlag("use_lucene101_postings_format");
+    public static final FeatureFlag USE_DEFAULT_LUCENE_POSTINGS_FORMAT = new FeatureFlag("use_default_lucene_postings_format");
 
     private static final DocValuesFormat docValuesFormat = new Lucene90DocValuesFormat();
     private static final KnnVectorsFormat knnVectorsFormat = new Lucene99HnswVectorsFormat();
     private static final ES819TSDBDocValuesFormat tsdbDocValuesFormat = new ES819TSDBDocValuesFormat();
     private static final ES812PostingsFormat es812PostingsFormat = new ES812PostingsFormat();
-    private static final Lucene101PostingsFormat lucene101PostingsFormat = new Lucene101PostingsFormat();
     private static final PostingsFormat completionPostingsFormat = PostingsFormat.forName("Completion101");
 
     private final ES87BloomFilterPostingsFormat bloomFilterPostingsFormat;
     private final MapperService mapperService;
 
     private final PostingsFormat defaultPostingsFormat;
 
-    public PerFieldFormatSupplier(MapperService mapperService, BigArrays bigArrays) {
+    public PerFieldFormatSupplier(MapperService mapperService, BigArrays bigArrays, PostingsFormat defaultPostingsFormat) {
         this.mapperService = mapperService;
         this.bloomFilterPostingsFormat = new ES87BloomFilterPostingsFormat(bigArrays, this::internalGetPostingsFormatForField);
 
         if (mapperService != null
-            && USE_LUCENE101_POSTINGS_FORMAT.isEnabled()
+            && USE_DEFAULT_LUCENE_POSTINGS_FORMAT.isEnabled()
             && mapperService.getIndexSettings().getIndexVersionCreated().onOrAfter(IndexVersions.USE_LUCENE101_POSTINGS_FORMAT)
             && mapperService.getIndexSettings().getMode() == IndexMode.STANDARD) {
-            defaultPostingsFormat = lucene101PostingsFormat;
+            assert defaultPostingsFormat.getName().startsWith("Lucene") : "defaultPostingsFormat must be a Lucene codec";
+            this.defaultPostingsFormat = defaultPostingsFormat;
         } else {
             // our own posting format using PFOR
-            defaultPostingsFormat = es812PostingsFormat;
+            this.defaultPostingsFormat = es812PostingsFormat;
         }
     }
 

server/src/main/java/org/elasticsearch/index/codec/PerFieldMapperCodec.java

@@ -32,7 +32,7 @@ public final class PerFieldMapperCodec extends Elasticsearch900Lucene101Codec {
 
     public PerFieldMapperCodec(Zstd814StoredFieldsFormat.Mode compressionMode, MapperService mapperService, BigArrays bigArrays) {
         super(compressionMode);
-        this.formatSupplier = new PerFieldFormatSupplier(mapperService, bigArrays);
+        this.formatSupplier = new PerFieldFormatSupplier(mapperService, bigArrays, this.delegate().postingsFormat());
         // If the below assertion fails, it is a sign that Lucene released a new codec. You must create a copy of the current Elasticsearch
         // codec that delegates to this new Lucene codec, and make PerFieldMapperCodec extend this new Elasticsearch codec.
         assert Codec.forName(Lucene.LATEST_CODEC).getClass() == delegate.getClass()

server/src/test/java/org/elasticsearch/index/codec/PerFieldMapperCodecTests.java

@@ -94,7 +94,7 @@ public void testUseBloomFilter() throws IOException {
         assertThat(perFieldMapperCodec.getPostingsFormatForField("_id"), instanceOf(ES87BloomFilterPostingsFormat.class));
         assertThat(perFieldMapperCodec.useBloomFilter("another_field"), is(false));
 
-        Class<? extends PostingsFormat> expectedPostingsFormat = PerFieldFormatSupplier.USE_LUCENE101_POSTINGS_FORMAT.isEnabled()
+        Class<? extends PostingsFormat> expectedPostingsFormat = PerFieldFormatSupplier.USE_DEFAULT_LUCENE_POSTINGS_FORMAT.isEnabled()
             && timeSeries == false ? Lucene101PostingsFormat.class : ES812PostingsFormat.class;
         assertThat(perFieldMapperCodec.getPostingsFormatForField("another_field"), instanceOf(expectedPostingsFormat));
     }
@@ -110,7 +110,7 @@ public void testUseBloomFilterWithTimestampFieldEnabled() throws IOException {
     public void testUseBloomFilterWithTimestampFieldEnabled_noTimeSeriesMode() throws IOException {
         PerFieldFormatSupplier perFieldMapperCodec = createFormatSupplier(true, false, false);
         assertThat(perFieldMapperCodec.useBloomFilter("_id"), is(false));
-        Class<? extends PostingsFormat> expectedPostingsFormat = PerFieldFormatSupplier.USE_LUCENE101_POSTINGS_FORMAT.isEnabled()
+        Class<? extends PostingsFormat> expectedPostingsFormat = PerFieldFormatSupplier.USE_DEFAULT_LUCENE_POSTINGS_FORMAT.isEnabled()
             ? Lucene101PostingsFormat.class
             : ES812PostingsFormat.class;
         assertThat(perFieldMapperCodec.getPostingsFormatForField("_id"), instanceOf(expectedPostingsFormat));
@@ -174,7 +174,7 @@ private PerFieldFormatSupplier createFormatSupplier(boolean timestampField, bool
                 """;
             mapperService.merge("type", new CompressedXContent(mapping), MapperService.MergeReason.MAPPING_UPDATE);
         }
-        return new PerFieldFormatSupplier(mapperService, BigArrays.NON_RECYCLING_INSTANCE);
+        return new PerFieldFormatSupplier(mapperService, BigArrays.NON_RECYCLING_INSTANCE, new Lucene101PostingsFormat());
     }
 
     public void testUseES87TSDBEncodingSettingDisabled() throws IOException {
@@ -214,7 +214,7 @@ private PerFieldFormatSupplier createFormatSupplier(boolean enableES87TSDBCodec,
         settings.put(IndexSettings.TIME_SERIES_ES87TSDB_CODEC_ENABLED_SETTING.getKey(), enableES87TSDBCodec);
         MapperService mapperService = MapperTestUtils.newMapperService(xContentRegistry(), createTempDir(), settings.build(), "test");
         mapperService.merge("type", new CompressedXContent(mapping), MapperService.MergeReason.MAPPING_UPDATE);
-        return new PerFieldFormatSupplier(mapperService, BigArrays.NON_RECYCLING_INSTANCE);
+        return new PerFieldFormatSupplier(mapperService, BigArrays.NON_RECYCLING_INSTANCE, new Lucene101PostingsFormat());
     }
 
 }