Allow for plugins to register REST pre processor, closes #1658.

src/main/java/org/elasticsearch/http/HttpServer.java

@@ -27,10 +27,7 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.env.Environment;
 import org.elasticsearch.node.service.NodeService;
-import org.elasticsearch.rest.BytesRestResponse;
-import org.elasticsearch.rest.RestController;
-import org.elasticsearch.rest.RestRequest;
-import org.elasticsearch.rest.StringRestResponse;
+import org.elasticsearch.rest.*;
 
 import java.io.File;
 import java.io.IOException;
@@ -114,6 +111,14 @@ public HttpStats stats() {
 
     public void internalDispatchRequest(final HttpRequest request, final HttpChannel channel) {
         if (request.rawPath().startsWith("/_plugin/")) {
+            for (RestPreProcessor preProcessor : restController.preProcessors()) {
+                if (!preProcessor.handleExternal()) {
+                    continue;
+                }
+                if (!preProcessor.process(request, channel)) {
+                    return;
+                }
+            }
             handlePluginSite(request, channel);
             return;
         }

src/main/java/org/elasticsearch/rest/RestController.java

@@ -28,6 +28,8 @@
 import org.elasticsearch.rest.support.RestUtils;
 
 import java.io.IOException;
+import java.util.Arrays;
+import java.util.Comparator;
 
 /**
  *
@@ -41,6 +43,9 @@ public class RestController extends AbstractLifecycleComponent<RestController> {
     private final PathTrie<RestHandler> headHandlers = new PathTrie<RestHandler>(RestUtils.REST_DECODER);
     private final PathTrie<RestHandler> optionsHandlers = new PathTrie<RestHandler>(RestUtils.REST_DECODER);
 
+    // non volatile since the assumption is that pre processors are registered on startup
+    private RestPreProcessor[] preProcessors = new RestPreProcessor[0];
+
     @Inject
     public RestController(Settings settings) {
         super(settings);
@@ -58,6 +63,25 @@ protected void doStop() throws ElasticSearchException {
     protected void doClose() throws ElasticSearchException {
     }
 
+    /**
+     * Registers a pre processor to be executed before the rest request is actually handled.
+     */
+    public synchronized void registerPreProcessor(RestPreProcessor preProcessor) {
+        RestPreProcessor[] copy = new RestPreProcessor[preProcessors.length + 1];
+        System.arraycopy(preProcessors, 0, copy, 0, preProcessors.length);
+        copy[preProcessors.length] = preProcessor;
+        Arrays.sort(copy, new Comparator<RestPreProcessor>() {
+            @Override
+            public int compare(RestPreProcessor o1, RestPreProcessor o2) {
+                return o2.order() - o1.order();
+            }
+        });
+        preProcessors = copy;
+    }
+
+    /**
+     * Registers a rest handler to be execute when the provided method and path match the request.
+     */
     public void registerHandler(RestRequest.Method method, String path, RestHandler handler) {
         switch (method) {
             case GET:
@@ -83,12 +107,21 @@ public void registerHandler(RestRequest.Method method, String path, RestHandler
         }
     }
 
+    public RestPreProcessor[] preProcessors() {
+        return preProcessors;
+    }
+
     public boolean dispatchRequest(final RestRequest request, final RestChannel channel) {
-        final RestHandler handler = getHandler(request);
-        if (handler == null) {
-            return false;
-        }
         try {
+            for (RestPreProcessor preProcessor : preProcessors) {
+                if (!preProcessor.process(request, channel)) {
+                    return true;
+                }
+            }
+            final RestHandler handler = getHandler(request);
+            if (handler == null) {
+                return false;
+            }
             handler.handleRequest(request, channel);
         } catch (Exception e) {
             try {

src/main/java/org/elasticsearch/rest/RestPreProcessor.java

@@ -0,0 +1,45 @@
+/*
+ * Licensed to ElasticSearch and Shay Banon 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.elasticsearch.rest;
+
+/**
+ * Rest pre processor allowing to pre process REST requests.
+ * <p/>
+ * Experimental interface.
+ */
+public interface RestPreProcessor {
+
+    /**
+     * Optionally, the order the processor will work on. Execution is done from lowest value to highest.
+     * It is a good practice to allow to configure this for the relevant processor.
+     */
+    int order();
+
+    /**
+     * Should this processor also process external (non REST) requests, like plugin site requests.
+     */
+    boolean handleExternal();
+
+    /**
+     * Process the request, returning <tt>false</tt> if no further processing should be done. Note,
+     * make sure to send a response if returning <tt>false</tt>, otherwise, no response will be sent.
+     */
+    boolean process(RestRequest request, RestChannel channel);
+}