Fix deadlock in ThreadPoolMergeScheduler when a failing merge closes the IndexWriter (#134656)

This change fixes a bug that causes a deadlock in the thread pool merge scheduler when a merge fails due to a tragic event.

The deadlock occurs because Lucene aborts running merges when failing with a tragic event and then waits for them to complete. But those "running" merges might in fact be waiting in the Elasticsearch's thread pool merge scheduler tasks queue, or they might be waiting in the backlogged merge tasks queue because the per-shard concurrent merges count limit has been reached, or they might simply be waiting for enough disk space to be executed. In which cases the merge thread that is failing waits indefinitely.

The proposed fix in this change uses the merge thread that is failing due to a tragic event to abort all other enqueued and backlogged merge tasks of the same shard, before pursuing with the closing of the IndexWriter. This way Lucene won't have to wait for any running merges as they would have all be aborted upfront.

Relates ES-12664

Author: tlrx

docs/changelog/134656.yaml

@@ -0,0 +1,6 @@
+pr: 134656
+summary: Fix deadlock in `ThreadPoolMergeScheduler` when a failing merge closes the
+  `IndexWriter`
+area: Engine
+type: bug
+issues: []

server/src/internalClusterTest/java/org/elasticsearch/index/engine/MergeWithFailureIT.java

@@ -2756,11 +2756,7 @@ private IndexWriter createWriter() throws IOException {
 
     // protected for testing
     protected IndexWriter createWriter(Directory directory, IndexWriterConfig iwc) throws IOException {
-        if (Assertions.ENABLED) {
-            return new AssertingIndexWriter(directory, iwc);
-        } else {
-            return new IndexWriter(directory, iwc);
-        }
+        return new ElasticsearchIndexWriter(directory, iwc, logger);
     }
 
     // with tests.verbose, lucene sets this up: plumb to align with filesystem stream
@@ -2920,8 +2916,10 @@ LiveIndexWriterConfig getCurrentIndexWriterConfig() {
         return indexWriter.getConfig();
     }
 
-    private void maybeFlushAfterMerge(OnGoingMerge merge) {
-        if (indexWriter.hasPendingMerges() == false && System.nanoTime() - lastWriteNanos >= engineConfig.getFlushMergesAfter().nanos()) {
+    protected void maybeFlushAfterMerge(OnGoingMerge merge) {
+        if (indexWriter.getTragicException() == null
+            && indexWriter.hasPendingMerges() == false
+            && System.nanoTime() - lastWriteNanos >= engineConfig.getFlushMergesAfter().nanos()) {
             // NEVER do this on a merge thread since we acquire some locks blocking here and if we concurrently rollback the
             // writer
             // we deadlock on engine#close for instance.
@@ -3377,19 +3375,49 @@ private static Map<String, String> commitDataAsMap(final IndexWriter indexWriter
         return commitData;
     }
 
-    private static class AssertingIndexWriter extends IndexWriter {
-        AssertingIndexWriter(Directory d, IndexWriterConfig conf) throws IOException {
-            super(d, conf);
+    private static class ElasticsearchIndexWriter extends IndexWriter {
+
+        private final Logger logger;
+
+        ElasticsearchIndexWriter(Directory directory, IndexWriterConfig indexWriterConfig, Logger logger) throws IOException {
+            super(directory, indexWriterConfig);
+            this.logger = logger;
+        }
+
+        @Override
+        public void onTragicEvent(Throwable tragedy, String location) {
+            assert tragedy != null;
+            try {
+                if (getConfig().getMergeScheduler() instanceof ThreadPoolMergeScheduler mergeScheduler) {
+                    try {
+                        // Must be executed before calling IndexWriter#onTragicEvent
+                        mergeScheduler.onTragicEvent(tragedy);
+                    } catch (Exception e) {
+                        logger.warn("Exception thrown when notifying the merge scheduler of a tragic event", e);
+                        if (tragedy != e) {
+                            tragedy.addSuppressed(e);
+                        }
+                    }
+                }
+            } finally {
+                super.onTragicEvent(tragedy, location);
+            }
         }
 
         @Override
-        public long deleteDocuments(Term... terms) {
-            throw new AssertionError("must not hard delete documents");
+        public long deleteDocuments(Term... terms) throws IOException {
+            if (Assertions.ENABLED) {
+                throw new AssertionError("must not hard delete documents");
+            }
+            return super.deleteDocuments(terms);
         }
 
         @Override
-        public long tryDeleteDocument(IndexReader readerIn, int docID) {
-            throw new AssertionError("tryDeleteDocument is not supported. See Lucene#DirectoryReaderWithAllLiveDocs");
+        public long tryDeleteDocument(IndexReader readerIn, int docID) throws IOException {
+            if (Assertions.ENABLED) {
+                throw new AssertionError("tryDeleteDocument is not supported. See Lucene#DirectoryReaderWithAllLiveDocs");
+            }
+            return super.tryDeleteDocument(readerIn, docID);
         }
     }
 

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

@@ -31,7 +31,9 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Comparator;
+import java.util.HashSet;
 import java.util.IdentityHashMap;
 import java.util.Iterator;
 import java.util.List;
@@ -48,6 +50,7 @@
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Consumer;
 import java.util.function.LongUnaryOperator;
+import java.util.function.Predicate;
 import java.util.function.ToLongFunction;
 
 import static org.elasticsearch.cluster.routing.allocation.DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_DISK_FLOOD_STAGE_MAX_HEADROOM_SETTING;
@@ -372,7 +375,7 @@ private void runMergeTask(MergeTask mergeTask) {
         }
     }
 
-    private void abortMergeTask(MergeTask mergeTask) {
+    void abortMergeTask(MergeTask mergeTask) {
         assert mergeTask.hasStartedRunning() == false;
         assert runningMergeTasks.contains(mergeTask) == false;
         try {
@@ -385,6 +388,25 @@ private void abortMergeTask(MergeTask mergeTask) {
         }
     }
 
+    private void abortMergeTasks(Collection<MergeTask> mergeTasks) {
+        if (mergeTasks != null && mergeTasks.isEmpty() == false) {
+            for (var mergeTask : mergeTasks) {
+                abortMergeTask(mergeTask);
+            }
+        }
+    }
+
+    /**
+     * Removes all {@link MergeTask} that match the predicate and aborts them.
+     * @param predicate             the predicate to filter merge tasks to be aborted
+     */
+    void abortQueuedMergeTasks(Predicate<MergeTask> predicate) {
+        final var queuedMergesToAbort = new HashSet<MergeTask>();
+        if (queuedMergeTasks.drainMatchingElementsTo(predicate, queuedMergesToAbort) > 0) {
+            abortMergeTasks(queuedMergesToAbort);
+        }
+    }
+
     /**
      * Start monitoring the available disk space, and update the available budget for running merge tasks
      * Note: this doesn't work correctly for nodes with multiple data paths, as it only considers the data path with the MOST
@@ -675,6 +697,25 @@ ElementWithReleasableBudget take() throws InterruptedException {
             }
         }
 
+        int drainMatchingElementsTo(Predicate<E> predicate, Collection<? super E> c) {
+            int removed = 0;
+            final ReentrantLock lock = this.lock;
+            lock.lock();
+            try {
+                for (Iterator<Tuple<E, Long>> iterator = enqueuedByBudget.iterator(); iterator.hasNext();) {
+                    E item = iterator.next().v1();
+                    if (predicate.test(item)) {
+                        iterator.remove();
+                        c.add(item);
+                        removed++;
+                    }
+                }
+                return removed;
+            } finally {
+                lock.unlock();
+            }
+        }
+
         /**
          * Updates the available budged given the passed-in argument, from which it deducts the budget hold up by taken elements
          * that are still in use. The elements budget is also updated by re-applying the budget function.
@@ -704,7 +745,7 @@ void updateBudget(long availableBudget) {
 
         void postBudgetUpdate() {
             assert lock.isHeldByCurrentThread();
-        };
+        }
 
         private void updateBudgetOfEnqueuedElementsAndReorderQueue() {
             assert this.lock.isHeldByCurrentThread();