RANGER-5174: fix for script engine instantiation failure (apache#546)

Author: mneethiraj

agents-common/src/main/java/org/apache/ranger/plugin/util/GraalScriptEngineCreator.java

@@ -72,7 +72,7 @@ public ScriptEngine getScriptEngine(ClassLoader clsLoader) {
         ScriptEngine ret = null;
 
         if (clsLoader == null) {
-            clsLoader = Thread.currentThread().getContextClassLoader();
+            clsLoader = getDefaultClassLoader();
         }
 
         try {

agents-common/src/main/java/org/apache/ranger/plugin/util/JavaScriptEngineCreator.java

@@ -33,7 +33,7 @@ public ScriptEngine getScriptEngine(ClassLoader clsLoader) {
         ScriptEngine ret = null;
 
         if (clsLoader == null) {
-            clsLoader = Thread.currentThread().getContextClassLoader();
+            clsLoader = getDefaultClassLoader();
         }
 
         try {

agents-common/src/main/java/org/apache/ranger/plugin/util/NashornScriptEngineCreator.java

@@ -37,7 +37,7 @@ public ScriptEngine getScriptEngine(ClassLoader clsLoader) {
         ScriptEngine ret = null;
 
         if (clsLoader == null) {
-            clsLoader = Thread.currentThread().getContextClassLoader();
+            clsLoader = getDefaultClassLoader();
         }
 
         try {

agents-common/src/main/java/org/apache/ranger/plugin/util/ScriptEngineCreator.java

@@ -19,8 +19,22 @@
 
 package org.apache.ranger.plugin.util;
 
+import org.apache.ranger.plugin.classloader.RangerPluginClassLoader;
+
 import javax.script.ScriptEngine;
 
 public interface ScriptEngineCreator {
     ScriptEngine getScriptEngine(ClassLoader clsLoader);
+
+    default ClassLoader getDefaultClassLoader() {
+        ClassLoader ret = Thread.currentThread().getContextClassLoader();
+
+        // Most Ranger plugins use a shim layer and RangerPluginClassLoader for isolation of libraries
+        // loaded by the plugin implementation. The shim ensures that all calls to the plugin would have
+        // RangerPluginClassLoader as current thread classloader.
+        // Some plugins, like Trino, use their own isolation mechanism. In these plugins, current thread's
+        // classloader may not load libraries in the plugin directory, which can result in failure in
+        // creation of the script engine. Using the classloader of current class to resolve this issue.
+        return ret instanceof RangerPluginClassLoader ? ret : this.getClass().getClassLoader();
+    }
 }