WIP: Add online prewarming service extension point.

server/src/main/java/org/elasticsearch/search/OnlinePrewarmingService.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.search;
+
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.store.FilterDirectory;
+
+/**
+ * Temporary stub for index shard prewarming capabilities that is a no-op in the default build but can be extende
+ * by classes implementing the interface
+ */
+public interface OnlinePrewarmingService {
+    void prewarm();
+
+    static OnlinePrewarmingService unwrapDirectory(final Directory directory) {
+        Directory dir = directory;
+        while (dir != null) {
+            if (dir instanceof OnlinePrewarmingService searchDirectory) {
+                return searchDirectory;
+            } else if (dir instanceof FilterDirectory) {
+                dir = ((FilterDirectory) dir).getDelegate();
+            } else {
+                dir = null;
+            }
+        }
+        return () -> {
+            // no-Op by default
+        };
+    }
+
+}

server/src/main/java/org/elasticsearch/search/SearchService.java

@@ -656,6 +656,8 @@ public void executeQueryPhase(ShardSearchRequest request, CancellableTask task,
         assert request.canReturnNullResponseIfMatchNoDocs() == false || request.numberOfShards() > 1
             : "empty responses require more than one shard";
         final IndexShard shard = getShard(request);
+        // TODO this call probably needs to be somewhere else where we want to fork the online prewarming of shards
+        OnlinePrewarmingService.unwrapDirectory(shard.store().directory()).prewarm();
         rewriteAndFetchShardRequest(
             shard,
             request,