Avoid creating IndexSearcher in Engine.refreshNeeded (#123218)

Checking whether we need to refresh does not require a searcher
so we can simplify this to just work based on the reader
and avoid lots of contention etc. for setting up the searcher.

relates #122374

Author: original-brownbear

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

@@ -1214,25 +1214,29 @@ private void fillSegmentInfo(
     public abstract List<Segment> segments(boolean includeVectorFormatsInfo);
 
     public boolean refreshNeeded() {
-        if (store.tryIncRef()) {
-            /*
-              we need to inc the store here since we acquire a searcher and that might keep a file open on the
-              store. this violates the assumption that all files are closed when
-              the store is closed so we need to make sure we increment it here
-             */
+        if (store.tryIncRef() == false) {
+            return false;
+        }
+        /*
+          we need to inc the store here since we acquire a directory reader and that might open a file on the store.
+          This violates the assumption that all files are closed when the store is closed so we need to make
+          sure we increment it here.
+         */
+        try {
+            var refManager = getReferenceManager(SearcherScope.EXTERNAL);
+            var reader = refManager.acquire();
             try {
-                try (Searcher searcher = acquireSearcher("refresh_needed", SearcherScope.EXTERNAL)) {
-                    return searcher.getDirectoryReader().isCurrent() == false;
-                }
-            } catch (IOException e) {
-                logger.error("failed to access searcher manager", e);
-                failEngine("failed to access searcher manager", e);
-                throw new EngineException(shardId, "failed to access searcher manager", e);
+                return reader.isCurrent() == false;
             } finally {
-                store.decRef();
+                refManager.release(reader);
             }
+        } catch (IOException e) {
+            logger.error("failed to access directory reader", e);
+            failEngine("failed to access directory reader", e);
+            throw new EngineException(shardId, "failed to access directory reader", e);
+        } finally {
+            store.decRef();
         }
-        return false;
     }
 
     /**