Fix concurrent requests race over scroll context limit (#53449)

Concurrent search scroll requests can lead to more scroll contexts than the limit.

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

@@ -642,7 +642,6 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
         boolean success = false;
         try {
             if (context.scrollContext() != null) {
-                openScrollContexts.incrementAndGet();
                 context.indexShard().getSearchOperationListener().onNewScrollContext(context);
             }
             context.indexShard().getSearchOperationListener().onNewContext(context);
@@ -661,14 +660,15 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
     final SearchContext createContext(SearchRewriteContext rewriteContext) throws IOException {
         final DefaultSearchContext context = createSearchContext(rewriteContext, defaultSearchTimeout);
         try {
-            if (rewriteContext.request != null && openScrollContexts.get() >= maxOpenScrollContext) {
-                throw new ElasticsearchException(
-                    "Trying to create too many scroll contexts. Must be less than or equal to: [" +
-                        maxOpenScrollContext + "]. " + "This limit can be set by changing the ["
-                        + MAX_OPEN_SCROLL_CONTEXT.getKey() + "] setting.");
-            }
             final ShardSearchRequest request = rewriteContext.request;
             if (request.scroll() != null) {
+                context.addReleasable(openScrollContexts::decrementAndGet, Lifetime.CONTEXT);
+                if (openScrollContexts.incrementAndGet() > maxOpenScrollContext) {
+                    throw new ElasticsearchException(
+                        "Trying to create too many scroll contexts. Must be less than or equal to: [" +
+                            maxOpenScrollContext + "]. " + "This limit can be set by changing the ["
+                            + MAX_OPEN_SCROLL_CONTEXT.getKey() + "] setting.");
+                }
                 context.scrollContext(new ScrollContext());
                 context.scrollContext().scroll = request.scroll();
             }
@@ -768,7 +768,6 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
         assert activeContexts.containsKey(context.id().getId()) == false;
         context.indexShard().getSearchOperationListener().onFreeContext(context);
         if (context.scrollContext() != null) {
-            openScrollContexts.decrementAndGet();
             context.indexShard().getSearchOperationListener().onFreeScrollContext(context);
         }
     }

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

@@ -539,6 +539,47 @@ public class SearchServiceTests extends ESSingleNodeTestCase {
             ex.getMessage());
     }
 
+    public void testOpenScrollContextsConcurrently() throws Exception {
+        createIndex("index");
+        final IndicesService indicesService = getInstanceFromNode(IndicesService.class);
+        final IndexShard indexShard = indicesService.indexServiceSafe(resolveIndex("index")).getShard(0);
+
+        final int maxScrollContexts = SearchService.MAX_OPEN_SCROLL_CONTEXT.get(Settings.EMPTY);
+        final SearchService searchService = getInstanceFromNode(SearchService.class);
+        Thread[] threads = new Thread[randomIntBetween(2, 8)];
+        CountDownLatch latch = new CountDownLatch(threads.length);
+        for (int i = 0; i < threads.length; i++) {
+            threads[i] = new Thread(() -> {
+                latch.countDown();
+                try {
+                    latch.await();
+                    for (; ; ) {
+                        SearchService.SearchRewriteContext rewriteContext =
+                            searchService.acquireSearcherAndRewrite(new ShardScrollRequestTest(indexShard.shardId()), indexShard);
+                        try {
+                            searchService.createAndPutContext(rewriteContext);
+                        } catch (ElasticsearchException e) {
+                            assertThat(e.getMessage(), equalTo(
+                                "Trying to create too many scroll contexts. Must be less than or equal to: " +
+                                    "[" + maxScrollContexts + "]. " +
+                                    "This limit can be set by changing the [search.max_open_scroll_context] setting."));
+                            return;
+                        }
+                    }
+                } catch (Exception e) {
+                    throw new AssertionError(e);
+                }
+            });
+            threads[i].setName("elasticsearch[node_s_0][search]");
+            threads[i].start();
+        }
+        for (Thread thread : threads) {
+            thread.join();
+        }
+        assertThat(searchService.getActiveContexts(), equalTo(maxScrollContexts));
+        searchService.freeAllScrollContexts();
+    }
+
     public static class FailOnRewriteQueryPlugin extends Plugin implements SearchPlugin {
         @Override
         public List<QuerySpec<?>> getQueries() {