Update IOContext.READONCE to use the new ReadOnceHint.INSTANCE

Author: ChrisHegarty

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/MergeReaderWrapper.java

@@ -86,7 +86,9 @@ public Collection<Accountable> getChildResources() {
 
     @Override
     public Map<String, Long> getOffHeapByteSize(FieldInfo fieldInfo) {
-        return mainReader.getOffHeapByteSize(fieldInfo);
+        // TODO: https://github.com/elastic/elasticsearch/issues/128672
+        // return mainReader.getOffHeapByteSize(fieldInfo);
+        return Map.of(); // no off-heap when using direct IO
     }
 
     @Override

server/src/main/java/org/elasticsearch/index/store/Store.java

@@ -34,6 +34,7 @@
 import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.store.Lock;
 import org.apache.lucene.store.NIOFSDirectory;
+import org.apache.lucene.store.ReadOnceHint;
 import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.Version;
@@ -153,7 +154,7 @@ public class Store extends AbstractIndexShardComponent implements Closeable, Ref
 
     // while equivalent, these different read once contexts are checked by identity in directory implementations
     private static IOContext createReadOnceContext() {
-        var context = IOContext.READONCE.withHints(DataAccessHint.SEQUENTIAL);
+        var context = IOContext.READONCE.withHints(DataAccessHint.SEQUENTIAL, ReadOnceHint.INSTANCE);
         assert context != IOContext.READONCE;
         assert context.equals(IOContext.READONCE);
         return context;