Add _cat/pending_tasks.

Closes #4251.

docs/reference/cat.asciidoc

@@ -70,6 +70,8 @@ include::cat/master.asciidoc[]
 
 include::cat/nodes.asciidoc[]
 
+include::cat/pending_tasks.asciidoc[]
+
 include::cat/recovery.asciidoc[]
 
 include::cat/shards.asciidoc[]

docs/reference/cat/pending_tasks.asciidoc

@@ -0,0 +1,18 @@
+[[cat-pending-tasks]]
+== Cluster Pending Tasks
+
+`pending_tasks` provides the same information as the
+`/_cluster/pending_tasks` API in a convenient tabular format.
+
+[source,shell]
+--------------------------------------------------
+% curl 'localhost:9200/_cat/pending_tasks?v'
+insertOrder timeInQueue priority source
+       1685       855ms HIGH     update-mapping [foo][t]
+       1686       843ms HIGH     update-mapping [foo][t]
+       1693       753ms HIGH     refresh-mapping [foo][[t]]
+       1688       816ms HIGH     update-mapping [foo][t]
+       1689       802ms HIGH     update-mapping [foo][t]
+       1690       787ms HIGH     update-mapping [foo][t]
+       1691       773ms HIGH     update-mapping [foo][t]
+--------------------------------------------------

src/main/java/org/elasticsearch/rest/action/RestActionModule.java

@@ -218,5 +218,6 @@ public class RestActionModule extends AbstractModule {
         bind(RestHealthAction.class).asEagerSingleton();
         bind(RestCatAction.class).asEagerSingleton();
         bind(RestHelpAction.class).asEagerSingleton();
+        bind(org.elasticsearch.rest.action.cat.RestPendingClusterTasksAction.class).asEagerSingleton();
     }
 }

src/main/java/org/elasticsearch/rest/action/cat/RestHelpAction.java

@@ -52,6 +52,7 @@ public class RestHelpAction extends BaseRestHandler {
         s.append("/_cat/indices/{index}\n");
         s.append("/_cat/master\n");
         s.append("/_cat/nodes\n");
+        s.append("/_cat/pending_tasks\n");
         s.append("/_cat/recovery\n");
         s.append("/_cat/shards\n");
         s.append("/_cat/shards/{index}\n");

src/main/java/org/elasticsearch/rest/action/cat/RestPendingClusterTasksAction.java

@@ -0,0 +1,90 @@
+/*
+ * 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.action.cat;
+
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.admin.cluster.tasks.PendingClusterTasksRequest;
+import org.elasticsearch.action.admin.cluster.tasks.PendingClusterTasksResponse;
+import org.elasticsearch.client.Client;
+import org.elasticsearch.cluster.service.PendingClusterTask;
+import org.elasticsearch.common.Table;
+import org.elasticsearch.common.inject.Inject;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.rest.*;
+import org.elasticsearch.rest.action.support.RestTable;
+
+import java.io.IOException;
+
+import static org.elasticsearch.rest.RestRequest.Method.GET;
+
+public class RestPendingClusterTasksAction extends BaseRestHandler {
+    @Inject
+    public RestPendingClusterTasksAction(Settings settings, Client client, RestController controller) {
+        super(settings, client);
+        controller.registerHandler(GET, "/_cat/pending_tasks", this);
+    }
+
+    @Override
+    public void handleRequest(final RestRequest request, final RestChannel channel) {
+        PendingClusterTasksRequest pendingClusterTasksRequest = new PendingClusterTasksRequest();
+        client.admin().cluster().pendingClusterTasks(pendingClusterTasksRequest, new ActionListener<PendingClusterTasksResponse>() {
+            @Override
+            public void onResponse(PendingClusterTasksResponse pendingClusterTasks) {
+                try {
+                    Table tab = buildTable(pendingClusterTasks);
+                    channel.sendResponse(RestTable.buildResponse(tab, request, channel));
+                } catch (Throwable e) {
+                    onFailure(e);
+                }
+            }
+
+            @Override
+            public void onFailure(Throwable e) {
+                try {
+                    channel.sendResponse(new XContentThrowableRestResponse(request, e));
+                } catch (IOException e1) {
+                    logger.error("Failed to send failure response", e1);
+                }
+            }
+        });
+    }
+
+    private Table buildTable(PendingClusterTasksResponse tasks) {
+        Table t = new Table();
+
+        t.startHeaders();
+        t.addCell("insertOrder", "text-align:right;desc:Task insertion order");
+        t.addCell("timeInQueue", "text-align:right;desc:How long task has been in queue");
+        t.addCell("priority", "desc:Task priority");
+        t.addCell("source", "desc:Task source");
+        t.endHeaders();
+
+        for (PendingClusterTask task : tasks) {
+            t.startRow();
+            t.addCell(task.getInsertOrder());
+            t.addCell(task.getTimeInQueue());
+            t.addCell(task.getPriority());
+            t.addCell(task.getSource());
+            t.endRow();
+        }
+
+        return t;
+    }
+}