Fix concurrent search and index delete (#42621)

Changed order of listener invocation so that we notify before
registering search context and notify after unregistering same.

This ensures that count up/down like what we do in ShardSearchStats
works. Otherwise, we risk notifying onFreeScrollContext before notifying
onNewScrollContext (same for onFreeContext/onNewContext, but we
currently have no assertions failing in those).

Closes #28053

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

@@ -548,19 +548,35 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
         }
 
         SearchContext context = createContext(request);
+        onNewContext(context);
         boolean success = false;
         try {
             putContext(context);
-            if (request.scroll() != null) {
+            success = true;
+            return context;
+        } finally {
+            if (success == false) {
+                freeContext(context.id());
+            }
+        }
+    }
+
+    private void onNewContext(SearchContext context) {
+        boolean success = false;
+        try {
+            if (context.scrollContext() != null) {
                 openScrollContexts.incrementAndGet();
                 context.indexShard().getSearchOperationListener().onNewScrollContext(context);
             }
             context.indexShard().getSearchOperationListener().onNewContext(context);
             success = true;
-            return context;
         } finally {
-            if (!success) {
-                freeContext(context.id());
+            // currently, the concrete listener is CompositeListener, which swallows exceptions, but here we anyway try to do the
+            // right thing by closing and notifying onFreeXXX in case one of the listeners fails with an exception in the future.
+            if (success == false) {
+                try (SearchContext dummy = context) {
+                    onFreeContext(context);
+                }
             }
         }
     }
@@ -648,16 +664,21 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
     public boolean freeContext(long id) {
         try (SearchContext context = removeContext(id)) {
             if (context != null) {
+                onFreeContext(context);
+                return true;
+            }
+            return false;
+        }
+    }
+
+    private void onFreeContext(SearchContext context) {
         assert context.refCount() > 0 : " refCount must be > 0: " + context.refCount();
+        assert activeContexts.containsKey(context.id()) == false;
         context.indexShard().getSearchOperationListener().onFreeContext(context);
         if (context.scrollContext() != null) {
             openScrollContexts.decrementAndGet();
             context.indexShard().getSearchOperationListener().onFreeScrollContext(context);
         }
-                return true;
-            }
-            return false;
-        }
     }
 
     public void freeAllScrollContexts() {

server/src/test/java/org/elasticsearch/search/SearchServiceTests.java

@@ -19,7 +19,6 @@
 package org.elasticsearch.search;
 
 import com.carrotsearch.hppc.IntArrayList;
-
 import org.apache.lucene.search.Query;
 import org.apache.lucene.store.AlreadyClosedException;
 import org.elasticsearch.ElasticsearchException;
@@ -51,6 +50,7 @@ import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryRewriteContext;
 import org.elasticsearch.index.query.QueryShardContext;
 import org.elasticsearch.index.query.TermQueryBuilder;
+import org.elasticsearch.index.search.stats.SearchStats;
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.index.shard.SearchOperationListener;
 import org.elasticsearch.index.shard.ShardId;
@@ -226,6 +226,7 @@ public class SearchServiceTests extends ESSingleNodeTestCase {
         AtomicBoolean running = new AtomicBoolean(true);
         CountDownLatch startGun = new CountDownLatch(1);
         Semaphore semaphore = new Semaphore(Integer.MAX_VALUE);
+
         final Thread thread = new Thread() {
             @Override
             public void run() {
@@ -261,12 +262,16 @@ public class SearchServiceTests extends ESSingleNodeTestCase {
         try {
             final int rounds = scaledRandomIntBetween(100, 10000);
             SearchRequest searchRequest = new SearchRequest().allowPartialSearchResults(true);
+            SearchRequest scrollSearchRequest = new SearchRequest().allowPartialSearchResults(true)
+                .scroll(new Scroll(TimeValue.timeValueMinutes(1)));
             for (int i = 0; i < rounds; i++) {
                 try {
                     try {
                         PlainActionFuture<SearchPhaseResult> result = new PlainActionFuture<>();
+                        final boolean useScroll = randomBoolean();
                         service.executeQueryPhase(
-                            new ShardSearchLocalRequest(searchRequest, indexShard.shardId(), 1,
+                            new ShardSearchLocalRequest(useScroll ? scrollSearchRequest : searchRequest,
+                                indexShard.shardId(), 1,
                                 new AliasFilter(null, Strings.EMPTY_ARRAY), 1.0f, -1, null, null),
                             new SearchTask(123L, "", "", "", null, Collections.emptyMap()), result);
                         SearchPhaseResult searchPhaseResult = result.get();
@@ -276,6 +281,9 @@ public class SearchServiceTests extends ESSingleNodeTestCase {
                         PlainActionFuture<FetchSearchResult> listener = new PlainActionFuture<>();
                         service.executeFetchPhase(req, new SearchTask(123L, "", "", "", null, Collections.emptyMap()), listener);
                         listener.get();
+                        if (useScroll) {
+                            service.freeContext(searchPhaseResult.getRequestId());
+                        }
                     } catch (ExecutionException ex) {
                         assertThat(ex.getCause(), instanceOf(RuntimeException.class));
                         throw ((RuntimeException)ex.getCause());
@@ -293,6 +301,13 @@ public class SearchServiceTests extends ESSingleNodeTestCase {
             thread.join();
             semaphore.acquire(Integer.MAX_VALUE);
         }
+
+        assertEquals(0, service.getActiveContexts());
+
+        SearchStats.Stats totalStats = indexShard.searchStats().getTotal();
+        assertEquals(0, totalStats.getQueryCurrent());
+        assertEquals(0, totalStats.getScrollCurrent());
+        assertEquals(0, totalStats.getFetchCurrent());
     }
 
     public void testTimeout() throws IOException {