fix: correctly detect Disruptor major version

Ensure the Disruptor version is detected using the classloader that loaded `DisruptorUtil`, rather than the thread-context classloader. The previous implementation relied on `LoaderUtil.isClassAvailable`, which may fail in environments where the Disruptor classes aren't visible to the thread-context classloader.

Author: ppkarwasz

log4j-core/src/main/java/org/apache/logging/log4j/core/async/DisruptorUtil.java

@@ -50,8 +50,19 @@ final class DisruptorUtil {
     static final boolean ASYNC_CONFIG_SYNCHRONIZE_ENQUEUE_WHEN_QUEUE_FULL = PropertiesUtil.getProperties()
             .getBooleanProperty("AsyncLoggerConfig.SynchronizeEnqueueWhenQueueFull", true);
 
-    static final int DISRUPTOR_MAJOR_VERSION =
-            LoaderUtil.isClassAvailable("com.lmax.disruptor.SequenceReportingEventHandler") ? 3 : 4;
+    static final int DISRUPTOR_MAJOR_VERSION = detectDisruptorMajorVersion();
+
+    // TODO: replace with LoaderUtil.isClassAvailable() when TCCL is removed
+    // See: https://github.com/apache/logging-log4j2/issues/3706
+    private static int detectDisruptorMajorVersion() {
+        try {
+            Class.forName(
+                    "com.lmax.disruptor.SequenceReportingEventHandler", true, DisruptorUtil.class.getClassLoader());
+            return 3;
+        } catch (final ClassNotFoundException e) {
+            return 4;
+        }
+    }
 
     private DisruptorUtil() {}
 

src/changelog/.2.x.x/3706_disruptor-tccl.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns="https://logging.apache.org/xml/ns"
+       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xsi:schemaLocation="
+           https://logging.apache.org/xml/ns
+           https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="3706" link="https://github.com/apache/logging-log4j2/issues/3706"/>
+  <description format="asciidoc">
+    Fix detection of the Disruptor major version in some environments.
+  </description>
+</entry>