Move wrapDirectoryReader to wrapExternalDirectoryReader

After reviewing the users of SearcherScope.INTERNAL none of them appear to
want filtering, so it is simpler and safer not to provide an interface for them to do so.

Author: bcully

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

@@ -999,8 +999,8 @@ 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, SplitShardCountSummary ignored) throws IOException {
+    // Allows subclasses to wrap the DirectoryReader before it is used to create external Searchers
+    protected DirectoryReader wrapExternalDirectoryReader(DirectoryReader reader, SplitShardCountSummary ignored) throws IOException {
         return reader;
     }
 
@@ -1026,15 +1026,20 @@ public SearcherSupplier acquireSearcherSupplier(
         try {
             ReferenceManager<ElasticsearchDirectoryReader> referenceManager = getReferenceManager(scope);
             ElasticsearchDirectoryReader acquire = referenceManager.acquire();
-            DirectoryReader wrappedDirectoryReader = wrapDirectoryReader(acquire, splitShardCountSummary);
+            final DirectoryReader maybeWrappedDirectoryReader;
+            if (scope == SearcherScope.EXTERNAL) {
+                maybeWrappedDirectoryReader = wrapExternalDirectoryReader(acquire, splitShardCountSummary);
+            } else {
+                maybeWrappedDirectoryReader = acquire;
+            }
             SearcherSupplier reader = new SearcherSupplier(wrapper) {
                 @Override
                 public Searcher acquireSearcherInternal(String source) {
                     assert assertSearcherIsWarmedUp(source, scope);
                     onSearcherCreation(source, scope);
                     return new Searcher(
                         source,
-                        wrappedDirectoryReader,
+                        maybeWrappedDirectoryReader,
                         engineConfig.getSimilarity(),
                         engineConfig.getQueryCache(),
                         engineConfig.getQueryCachingPolicy(),