From 42c270dd9ffc5ac1cca89591b31f0172c7aa4123 Mon Sep 17 00:00:00 2001
From: Tanguy Leroux <tlrx.dev@gmail.com>
Date: Wed, 6 Sep 2023 15:31:08 +0200
Subject: [PATCH] Assert commits order in IndexCommitListener

---
 .../index/engine/InternalEngine.java          | 38 +++++++++++++++++--
 1 file changed, 35 insertions(+), 3 deletions(-)

diff --git a/server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java b/server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java
index 363a71719efbe..94e9bbdc22917 100644
--- a/server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java
+++ b/server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java
@@ -88,6 +88,7 @@
 import org.elasticsearch.index.seqno.SequenceNumbers;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.index.shard.ShardLongFieldRange;
+import org.elasticsearch.index.store.Store;
 import org.elasticsearch.index.translog.Translog;
 import org.elasticsearch.index.translog.TranslogConfig;
 import org.elasticsearch.index.translog.TranslogCorruptedException;
@@ -360,26 +361,57 @@ private SoftDeletesPolicy newSoftDeletesPolicy() throws IOException {
 
     @Nullable
     private CombinedDeletionPolicy.CommitsListener newCommitsListener() {
-        final Engine.IndexCommitListener listener = engineConfig.getIndexCommitListener();
+        Engine.IndexCommitListener listener = engineConfig.getIndexCommitListener();
         if (listener != null) {
+            final IndexCommitListener wrappedListener = Assertions.ENABLED ? assertingCommitsOrderListener(listener) : listener;
             return new CombinedDeletionPolicy.CommitsListener() {
                 @Override
                 public void onNewAcquiredCommit(final IndexCommit commit, final Set<String> additionalFiles) {
                     final IndexCommitRef indexCommitRef = acquireIndexCommitRef(() -> commit);
                     var primaryTerm = config().getPrimaryTermSupplier().getAsLong();
                     assert indexCommitRef.getIndexCommit() == commit;
-                    listener.onNewCommit(shardId, store, primaryTerm, indexCommitRef, additionalFiles);
+                    wrappedListener.onNewCommit(shardId, store, primaryTerm, indexCommitRef, additionalFiles);
                 }
 
                 @Override
                 public void onDeletedCommit(IndexCommit commit) {
-                    listener.onIndexCommitDelete(shardId, commit);
+                    wrappedListener.onIndexCommitDelete(shardId, commit);
                 }
             };
         }
         return null;
     }
 
+    private IndexCommitListener assertingCommitsOrderListener(final IndexCommitListener listener) {
+        final AtomicLong generation = new AtomicLong(0L);
+        return new IndexCommitListener() {
+            @Override
+            public void onNewCommit(
+                ShardId shardId,
+                Store store,
+                long primaryTerm,
+                IndexCommitRef indexCommitRef,
+                Set<String> additionalFiles
+            ) {
+                final long nextGen = indexCommitRef.getIndexCommit().getGeneration();
+                final long prevGen = generation.getAndSet(nextGen);
+                assert prevGen < nextGen
+                    : "Expect new commit generation "
+                        + nextGen
+                        + " to be greater than previous commit generation "
+                        + prevGen
+                        + " for shard "
+                        + shardId;
+                listener.onNewCommit(shardId, store, primaryTerm, indexCommitRef, additionalFiles);
+            }
+
+            @Override
+            public void onIndexCommitDelete(ShardId shardId, IndexCommit deletedCommit) {
+                listener.onIndexCommitDelete(shardId, deletedCommit);
+            }
+        };
+    }
+
     @Override
     public CompletionStats completionStats(String... fieldNamePatterns) {
         return completionStatsCache.get(fieldNamePatterns);