Add a hook to Engine to wrap the DirectoryReader provided to Searchers (#135216)

Adds a protected hook in Engine to allow wrapping the
DirectoryReader supplied to a Searcher.

Co-authored-by: Oleksandr Kolomiiets <[email protected]>

Author: bcully

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -998,6 +998,11 @@ public final SearcherSupplier acquireSearcherSupplier(Function<Searcher, Searche
     // Called before a {@link Searcher} is created, to allow subclasses to perform any stats or logging operations.
     protected void onSearcherCreation(String source, SearcherScope scope) {}
 
+    // Allows subclasses to wrap the DirectoryReader before it is used to create Searchers
+    protected DirectoryReader wrapDirectoryReader(DirectoryReader reader) throws IOException {
+        return reader;
+    }
+
     /**
      * Acquires a point-in-time reader that can be used to create {@link Engine.Searcher}s on demand.
      */
@@ -1012,14 +1017,15 @@ public SearcherSupplier acquireSearcherSupplier(Function<Searcher, Searcher> wra
         try {
             ReferenceManager<ElasticsearchDirectoryReader> referenceManager = getReferenceManager(scope);
             ElasticsearchDirectoryReader acquire = referenceManager.acquire();
+            DirectoryReader wrappedDirectoryReader = wrapDirectoryReader(acquire);
             SearcherSupplier reader = new SearcherSupplier(wrapper) {
                 @Override
                 public Searcher acquireSearcherInternal(String source) {
                     assert assertSearcherIsWarmedUp(source, scope);
                     onSearcherCreation(source, scope);
                     return new Searcher(
                         source,
-                        acquire,
+                        wrappedDirectoryReader,
                         engineConfig.getSimilarity(),
                         engineConfig.getQueryCache(),
                         engineConfig.getQueryCachingPolicy(),