Fix leaking searcher when shards are removed or relocated (#52099)

We might leak a searcher if the target shard is removed (i.e., its index
is deleted) or relocated while we are creating a SearchContext from a
SearchRewriteContext.

Relates #51708
Closes #52021

I labelled this non-issue for an unreleased bug introduced in #51708.

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

@@ -694,14 +694,14 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
     }
 
     private DefaultSearchContext createSearchContext(SearchRewriteContext rewriteContext, TimeValue timeout) {
-        final ShardSearchRequest request = rewriteContext.request;
-        final Engine.Searcher searcher = rewriteContext.searcher;
-        IndexService indexService = indicesService.indexServiceSafe(request.shardId().getIndex());
-        IndexShard indexShard = indexService.getShard(request.shardId().getId());
-        SearchShardTarget shardTarget = new SearchShardTarget(clusterService.localNode().getId(),
-                indexShard.shardId(), request.getClusterAlias(), OriginalIndices.NONE);
         boolean success = false;
         try {
+            final ShardSearchRequest request = rewriteContext.request;
+            final Engine.Searcher searcher = rewriteContext.searcher;
+            IndexService indexService = indicesService.indexServiceSafe(request.shardId().getIndex());
+            IndexShard indexShard = indexService.getShard(request.shardId().getId());
+            SearchShardTarget shardTarget = new SearchShardTarget(clusterService.localNode().getId(),
+                indexShard.shardId(), request.getClusterAlias(), OriginalIndices.NONE);
             DefaultSearchContext searchContext = new DefaultSearchContext(idGenerator.incrementAndGet(), request, shardTarget,
                 searcher, clusterService, indexService, indexShard, bigArrays, threadPool::relativeTimeInMillis, timeout,
                 fetchPhase, clusterService.state().nodes().getMinNodeVersion());
@@ -709,8 +709,9 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
             return searchContext;
         } finally {
             if (success == false) {
-                // we handle the case where the DefaultSearchContext constructor throws an exception since we would otherwise
-                // leak a searcher and this can have severe implications (unable to obtain shard lock exceptions).
+                // we handle the case where `IndicesService#indexServiceSafe`or `IndexService#getShard`, or the DefaultSearchContext
+                // constructor throws an exception since we would otherwise leak a searcher and this can have severe implications
+                // (unable to obtain shard lock exceptions).
                 IOUtils.closeWhileHandlingException(rewriteContext.searcher);
             }
         }

server/src/test/java/org/elasticsearch/recovery/RelocationIT.java

@@ -87,6 +87,7 @@ import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -457,7 +458,7 @@ public class RelocationIT extends ESIntegTestCase {
         }
     }
 
-    public void testIndexAndRelocateConcurrently() throws Exception {
+    public void testIndexSearchAndRelocateConcurrently() throws Exception {
         int halfNodes = randomIntBetween(1, 3);
         Settings[] nodeSettings = Stream.concat(
             Stream.generate(() -> Settings.builder().put("node.attr.color", "blue").build()).limit(halfNodes),
@@ -474,8 +475,21 @@ public class RelocationIT extends ESIntegTestCase {
                 .put("index.routing.allocation.exclude.color", "blue")
                 .put(indexSettings())
                 .put(IndexMetaData.SETTING_NUMBER_OF_REPLICAS, randomInt(halfNodes - 1));
+        if (randomBoolean()) {
+            settings.put(IndexSettings.INDEX_REFRESH_INTERVAL_SETTING.getKey(), randomIntBetween(1, 10) + "s");
+        }
         assertAcked(prepareCreate("test", settings));
         assertAllShardsOnNodes("test", redNodes);
+        AtomicBoolean stopped = new AtomicBoolean(false);
+        Thread[] searchThreads = randomBoolean() ? new Thread[0] : new Thread[randomIntBetween(1, 4)];
+        for (int i = 0; i < searchThreads.length; i++) {
+            searchThreads[i] = new Thread(() -> {
+                while (stopped.get() == false) {
+                    assertNoFailures(client().prepareSearch("test").setRequestCache(false).get());
+                }
+            });
+            searchThreads[i].start();
+        }
         int numDocs = randomIntBetween(100, 150);
         ArrayList<String> ids = new ArrayList<>();
         logger.info(" --> indexing [{}] docs", numDocs);
@@ -513,7 +527,10 @@ public class RelocationIT extends ESIntegTestCase {
             assertNoFailures(afterRelocation);
             assertSearchHits(afterRelocation, ids.toArray(new String[ids.size()]));
         }
-
+        stopped.set(true);
+        for (Thread searchThread : searchThreads) {
+            searchThread.join();
+        }
     }
 
     public void testRelocateWhileWaitingForRefresh() {

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

@@ -898,4 +898,35 @@ public class SearchServiceTests extends ESSingleNodeTestCase {
             latch.await();
         }
     }
+
+    public void testDeleteIndexWhileSearch() throws Exception {
+        createIndex("test");
+        int numDocs = randomIntBetween(1, 20);
+        for (int i = 0; i < numDocs; i++) {
+            client().prepareIndex("test", "_doc").setSource("f", "v").get();
+        }
+        client().admin().indices().prepareRefresh("test").get();
+        AtomicBoolean stopped = new AtomicBoolean(false);
+        Thread[] searchers = new Thread[randomIntBetween(1, 4)];
+        CountDownLatch latch = new CountDownLatch(searchers.length);
+        for (int i = 0; i < searchers.length; i++) {
+            searchers[i] = new Thread(() -> {
+                latch.countDown();
+                while (stopped.get() == false) {
+                    try {
+                        client().prepareSearch("test").setRequestCache(false).get();
+                    } catch (Exception ignored) {
+                        return;
+                    }
+                }
+            });
+            searchers[i].start();
+        }
+        latch.await();
+        client().admin().indices().prepareDelete("test").get();
+        stopped.set(true);
+        for (Thread searcher : searchers) {
+            searcher.join();
+        }
+    }
 }