Remove completion postings format extension (elastic#125253)

A while ago we introduced a completion postings format extension to eventually
be able to customize how completion FSTs are loaded. See elastic#111494.

We have never leveraged this extension, and meanwhile Lucene is moving
to always load FSTs off-heap, and no longer allow on-heap.
See apache/lucene#14364 .

This commit removes the SPI extension as it is no longer needed.

Author: javanna

server/src/main/java/module-info.java

@@ -7,7 +7,6 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-import org.elasticsearch.internal.CompletionsPostingsFormatExtension;
 import org.elasticsearch.plugins.internal.RestExtension;
 
 /** The Elasticsearch Server Module. */
@@ -291,8 +290,7 @@
         to
             org.elasticsearch.serverless.version,
             org.elasticsearch.serverless.buildinfo,
-            org.elasticsearch.serverless.constants,
-            org.elasticsearch.serverless.codec;
+            org.elasticsearch.serverless.constants;
     exports org.elasticsearch.lucene.analysis.miscellaneous;
     exports org.elasticsearch.lucene.grouping;
     exports org.elasticsearch.lucene.queries;
@@ -401,7 +399,6 @@
             org.elasticsearch.stateless,
             org.elasticsearch.settings.secure,
             org.elasticsearch.serverless.constants,
-            org.elasticsearch.serverless.codec,
             org.elasticsearch.serverless.apifiltering,
             org.elasticsearch.internal.security;
 
@@ -422,7 +419,6 @@
     uses org.elasticsearch.node.internal.TerminationHandlerProvider;
     uses org.elasticsearch.internal.VersionExtension;
     uses org.elasticsearch.internal.BuildExtension;
-    uses CompletionsPostingsFormatExtension;
     uses org.elasticsearch.features.FeatureSpecification;
     uses org.elasticsearch.plugins.internal.LoggingDataProvider;
 

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

@@ -25,10 +25,6 @@
 import org.elasticsearch.index.mapper.Mapper;
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper;
-import org.elasticsearch.internal.CompletionsPostingsFormatExtension;
-import org.elasticsearch.plugins.ExtensionLoader;
-
-import java.util.ServiceLoader;
 
 /**
  * Class that encapsulates the logic of figuring out the most appropriate file format for a given field, across postings, doc values and
@@ -40,6 +36,7 @@ public class PerFieldFormatSupplier {
     private static final KnnVectorsFormat knnVectorsFormat = new Lucene99HnswVectorsFormat();
     private static final ES87TSDBDocValuesFormat tsdbDocValuesFormat = new ES87TSDBDocValuesFormat();
     private static final ES812PostingsFormat es812PostingsFormat = new ES812PostingsFormat();
+    private static final PostingsFormat completionPostingsFormat = PostingsFormat.forName("Completion101");
 
     private final ES87BloomFilterPostingsFormat bloomFilterPostingsFormat;
     private final MapperService mapperService;
@@ -60,26 +57,13 @@ private PostingsFormat internalGetPostingsFormatForField(String field) {
         if (mapperService != null) {
             Mapper mapper = mapperService.mappingLookup().getMapper(field);
             if (mapper instanceof CompletionFieldMapper) {
-                return CompletionPostingsFormatHolder.POSTINGS_FORMAT;
+                return completionPostingsFormat;
             }
         }
         // return our own posting format using PFOR
         return es812PostingsFormat;
     }
 
-    private static class CompletionPostingsFormatHolder {
-        private static final PostingsFormat POSTINGS_FORMAT = getCompletionPostingsFormat();
-
-        private static PostingsFormat getCompletionPostingsFormat() {
-            String defaultName = "Completion101"; // Caution: changing this name will result in exceptions if a field is created during a
-            // rolling upgrade and the new codec (specified by the name) is not available on all nodes in the cluster.
-            String codecName = ExtensionLoader.loadSingleton(ServiceLoader.load(CompletionsPostingsFormatExtension.class))
-                .map(CompletionsPostingsFormatExtension::getFormatName)
-                .orElse(defaultName);
-            return PostingsFormat.forName(codecName);
-        }
-    }
-
     boolean useBloomFilter(String field) {
         if (mapperService == null) {
             return false;