[Test] Allow configuring configDir for the Java test cluster

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/local/AbstractLocalClusterFactory.java

@@ -286,6 +286,32 @@ public InputStream getLog(LogType logType) {
             throw new IllegalArgumentException("Log file " + logFile + " does not exist.");
         }
 
+        public void writeAdHocConfigFile(String fileName, String content) {
+            final Path target = configDir.resolve(fileName);
+            final Path directory = target.getParent();
+            if (Files.exists(directory) == false) {
+                try {
+                    Files.createDirectories(directory);
+                } catch (IOException e) {
+                    throw new UncheckedIOException(e);
+                }
+            }
+            try {
+                Files.writeString(target, content);
+            } catch (IOException e) {
+                throw new UncheckedIOException("Could not write ad-hoc config file: " + fileName, e);
+            }
+        }
+
+        public void removeAdHocConfigFile(String fileName) {
+            final Path target = configDir.resolve(fileName);
+            try {
+                Files.deleteIfExists(target);
+            } catch (IOException e) {
+                throw new UncheckedIOException("Could not remove ad-hoc config file: " + fileName, e);
+            }
+        }
+
         public LocalNodeSpec getSpec() {
             return spec;
         }

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/local/DefaultLocalClusterHandle.java

@@ -200,6 +200,16 @@ public InputStream getNodeLog(int index, LogType logType) {
         return nodes.get(index).getLog(logType);
     }
 
+    @Override
+    public void writeAdHocConfigFile(int index, String fileName, String content) {
+        nodes.get(index).writeAdHocConfigFile(fileName, content);
+    }
+
+    @Override
+    public void deleteAdHocConfigFile(int index, String fileName) {
+        nodes.get(index).removeAdHocConfigFile(fileName);
+    }
+
     @Override
     public void updateStoredSecureSettings() {
         execute(() -> nodes.parallelStream().forEach(Node::updateStoredSecureSettings));

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/local/DefaultLocalElasticsearchCluster.java

@@ -169,6 +169,16 @@ public InputStream getNodeLog(int index, LogType logType) {
         return handle.getNodeLog(index, logType);
     }
 
+    @Override
+    public void writeAdHocConfigFile(int index, String fileName, String content) {
+        handle.writeAdHocConfigFile(index, fileName, content);
+    }
+
+    @Override
+    public void deleteAdHocConfigFile(int index, String fileName) {
+        handle.deleteAdHocConfigFile(index, fileName);
+    }
+
     @Override
     public void updateStoredSecureSettings() {
         checkHandle();

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/local/LocalClusterHandle.java

@@ -108,6 +108,16 @@ public interface LocalClusterHandle extends ClusterHandle {
      */
     InputStream getNodeLog(int index, LogType logType);
 
+    /**
+     * Write an ad-hoc file in the config directory
+     */
+    void writeAdHocConfigFile(int index, String fileName, String content);
+
+    /**
+     * Delete an ad-hoc file in the config directory
+     */
+    void deleteAdHocConfigFile(int index, String fileName);
+
     /**
      * Writes secure settings to the relevant secure config file on each node. Use this method if you are dynamically updating secure
      * settings via a {@link MutableSettingsProvider} and need the update to be written to file, without a cluster restart.