Clarify that plugins can be closed

core/src/main/java/org/elasticsearch/node/Node.java

@@ -734,7 +734,7 @@ public synchronized void close() throws IOException {
             toClose.add(() -> stopWatch.stop().start("plugin(" + plugin.getClass().getName() + ")"));
             toClose.add(plugin);
         }
-        toClose.addAll(pluginsService.filterPlugins(Closeable.class));
+        toClose.addAll(pluginsService.filterPlugins(Plugin.class));
 
         toClose.add(() -> stopWatch.stop().start("script"));
         toClose.add(injector.getInstance(ScriptService.class));

core/src/main/java/org/elasticsearch/plugins/Plugin.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.plugins;
 
+import java.io.Closeable;
+import java.io.IOException;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
@@ -70,7 +72,7 @@
  * methods should cause any extensions of {@linkplain Plugin} that used the pre-5.x style extension syntax to fail to build and point the
  * plugin author at the new extension syntax. We hope that these make the process of upgrading a plugin from 2.x to 5.x only mildly painful.
  */
-public abstract class Plugin {
+public abstract class Plugin implements Closeable {
 
     /**
      * Node level guice modules.
@@ -162,6 +164,16 @@ public List<ExecutorBuilder<?>> getExecutorBuilders(Settings settings) {
         return Collections.emptyList();
     }
 
+    /**
+     * Close the resources opened by this plugin.
+     *
+     * @throws IOException if the plugin failed to close its resources
+     */
+    @Override
+    public void close() throws IOException {
+
+    }
+
     /**
      * Old-style guice index level extension point. {@code @Deprecated} and {@code final} to act as a signpost for plugin authors upgrading
      * from 2.x.