From e20198ff94efbce22ea0a26e7710e7e99fc648ea Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Michael?= <b.michael@gmx.de>
Date: Tue, 9 Sep 2025 14:52:47 +0200
Subject: [PATCH] ListAppender: Synchronize on list while iterating

* Returned snapshots no longer require an unmodifiable view
---
 .../log4j/core/test/appender/ListAppender.java | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/appender/ListAppender.java b/log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/appender/ListAppender.java
index f941db12674..f36d4082cb8 100644
--- a/log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/appender/ListAppender.java
+++ b/log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/appender/ListAppender.java
@@ -195,14 +195,18 @@ public ListAppender clear() {
         return this;
     }
 
-    /** Returns an immutable snapshot of captured log events */
+    /** Returns a snapshot of captured log events */
     public List<LogEvent> getEvents() {
-        return Collections.<LogEvent>unmodifiableList(new ArrayList<>(events));
+        synchronized (events) {
+            return new ArrayList<>(events);
+        }
     }
 
-    /** Returns an immutable snapshot of captured messages */
+    /** Returns a snapshot of captured messages */
     public List<String> getMessages() {
-        return Collections.<String>unmodifiableList(new ArrayList<>(messages));
+        synchronized (messages) {
+            return new ArrayList<>(messages);
+        }
     }
 
     /**
@@ -215,9 +219,11 @@ public List<String> getMessages(final int minSize, final long timeout, final Tim
         return getMessages();
     }
 
-    /** Returns an immutable snapshot of captured data */
+    /** Returns a snapshot of captured data */
     public List<byte[]> getData() {
-        return Collections.<byte[]>unmodifiableList(new ArrayList<>(data));
+        synchronized (data) {
+            return new ArrayList<>(data);
+        }
     }
 
     public static ListAppender createAppender(