From c4091fbea0ebbdbfb8b10914fc83d9c8e1010dc8 Mon Sep 17 00:00:00 2001
From: Pooya Salehi <pxsalehi@users.noreply.github.com>
Date: Mon, 3 Apr 2023 17:30:08 +0200
Subject: [PATCH] Return a copy of messages in MockMTAppender (#94982)

Couldn't reproduce the failure, but in principle there is an issue
there. Other options are a CopyOnWriteArrayList or just synchronizing on
`messages()` return value. Returning a copy should be enough, since this
is only used in one place for asserting some test expectations in an
`assertBusy`.

Closes https://github.com/elastic/elasticsearch/issues/94559
---
 .../org/elasticsearch/index/engine/InternalEngineTests.java     | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java b/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
index 5200388af8881..5c4cdadbfecf4 100644
--- a/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
+++ b/server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
@@ -2546,7 +2546,7 @@ private static class MockMTAppender extends AbstractAppender {
         private final List<String> messages = Collections.synchronizedList(new ArrayList<>());
 
         List<String> messages() {
-            return messages;
+            return List.copyOf(messages);
         }
 
         MockMTAppender(final String name) throws IllegalAccessException {