Force java9 log4j hack in testing

core/src/main/java/org/apache/log4j/Java9Hack.java

@@ -27,6 +27,10 @@ import org.apache.log4j.helpers.ThreadLocalMap;
 public class Java9Hack {
 
     public static void fixLog4j() {
-        MDC.mdc.tlm = new ThreadLocalMap();
+        System.out.println("Fixing log4j");
+        if (MDC.mdc.tlm == null) {
+            System.out.println("FOR REAL");
+            MDC.mdc.tlm = new ThreadLocalMap();
+        }
     }
 }

core/src/test/java/org/apache/log4j/Java9HackTests.java

@@ -0,0 +1,28 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.log4j;
+
+import org.elasticsearch.test.ESTestCase;
+
+public class Java9HackTests extends ESTestCase {
+    public void testJava9Hack() {
+        assertNotNull(MDC.mdc.tlm != null);
+    }
+}

core/src/test/java/org/elasticsearch/common/logging/LoggingConfigurationTests.java

@@ -27,6 +27,7 @@ import java.util.Arrays;
 
 import org.apache.log4j.Appender;
 import org.apache.log4j.Logger;
+import org.apache.log4j.MDC;
 import org.elasticsearch.cli.MockTerminal;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.env.Environment;

test/framework/src/main/java/org/elasticsearch/bootstrap/BootstrapForTesting.java

@@ -20,6 +20,8 @@
 package org.elasticsearch.bootstrap;
 
 import com.carrotsearch.randomizedtesting.RandomizedRunner;
+import org.apache.log4j.Java9Hack;
+import org.apache.lucene.util.Constants;
 import org.apache.lucene.util.LuceneTestCase;
 import org.elasticsearch.SecureSM;
 import org.elasticsearch.common.Strings;
@@ -89,6 +91,10 @@ public class BootstrapForTesting {
             throw new RuntimeException("found jar hell in test classpath", e);
         }
 
+        if (Constants.JRE_IS_MINIMUM_JAVA9) {
+            Java9Hack.fixLog4j();
+        }
+
         // install security manager if requested
         if (systemPropertyAsBoolean("tests.security.manager", true)) {
             try {