Add GET /_cluster/master endpoint

docs/reference/cluster.asciidoc

@@ -81,6 +81,8 @@ include::cluster/health.asciidoc[]
 
 include::cluster/state.asciidoc[]
 
+include::cluster/master.asciidoc[]
+
 include::cluster/stats.asciidoc[]
 
 include::cluster/pending.asciidoc[]

docs/reference/cluster/master.asciidoc

@@ -0,0 +1,16 @@
+[[cluster-master]]
+== Cluster master
+
+The cluster master API returns the identity of the currently elected master
+node.
+
+[source,js]
+--------------------------------------------------
+GET /_cluster/master
+--------------------------------------------------
+// CONSOLE
+
+By default the coordinating node asks the master for its own identity to verify
+that it is still the master. The timeout for this can be set with the
+`?master_timeout` <<time-units,time value>> query parameter, which defaults to
+`30s`. This check can be skipped by setting the `?local` query parameter.

rest-api-spec/src/main/resources/rest-api-spec/api/cluster.master.json

@@ -0,0 +1,21 @@
+{
+  "cluster.master": {
+    "documentation": "http://www.elastic.co/guide/en/elasticsearch/reference/master/cluster-master.html",
+    "methods": ["GET"],
+    "url": {
+      "path": "/_cluster/master",
+      "paths": ["/_cluster/master"],
+      "params": {
+        "local": {
+          "type": "boolean",
+          "description": "Return local information, do not retrieve the state from master node (default: false)"
+        },
+        "master_timeout": {
+          "type": "time",
+          "description": "Specify timeout for connection to master"
+        }
+      }
+    },
+    "body": null
+  }
+}

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.master/10_basic.yml

@@ -0,0 +1,10 @@
+---
+"get cluster master":
+  - skip:
+      version: " - 7.99.99"
+      reason: Get cluster master API was introduced in 8.0
+
+  - do:
+      cluster.master: {}
+
+  - is_true: master_node_id

server/src/main/java/org/elasticsearch/action/ActionModule.java

@@ -227,6 +227,7 @@
 import org.elasticsearch.rest.action.admin.cluster.RestClusterAllocationExplainAction;
 import org.elasticsearch.rest.action.admin.cluster.RestClusterGetSettingsAction;
 import org.elasticsearch.rest.action.admin.cluster.RestClusterHealthAction;
+import org.elasticsearch.rest.action.admin.cluster.RestClusterMasterAction;
 import org.elasticsearch.rest.action.admin.cluster.RestClusterRerouteAction;
 import org.elasticsearch.rest.action.admin.cluster.RestClusterSearchShardsAction;
 import org.elasticsearch.rest.action.admin.cluster.RestClusterStateAction;
@@ -561,6 +562,7 @@ public void initRestHandlers(Supplier<DiscoveryNodes> nodesInCluster) {
         registerHandler.accept(new RestClusterAllocationExplainAction(settings, restController));
         registerHandler.accept(new RestClusterStatsAction(settings, restController));
         registerHandler.accept(new RestClusterStateAction(settings, restController, settingsFilter));
+        registerHandler.accept(new RestClusterMasterAction(settings, restController));
         registerHandler.accept(new RestClusterHealthAction(settings, restController));
         registerHandler.accept(new RestClusterUpdateSettingsAction(settings, restController));
         registerHandler.accept(new RestClusterGetSettingsAction(settings, restController, clusterSettings, settingsFilter));

server/src/main/java/org/elasticsearch/rest/action/admin/cluster/RestClusterMasterAction.java

@@ -0,0 +1,72 @@
+/*
+ * Licensed to Elasticsearch 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.admin.cluster;
+
+import org.elasticsearch.action.admin.cluster.state.ClusterStateRequest;
+import org.elasticsearch.action.admin.cluster.state.ClusterStateResponse;
+import org.elasticsearch.client.Requests;
+import org.elasticsearch.client.node.NodeClient;
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.discovery.MasterNotDiscoveredException;
+import org.elasticsearch.rest.BaseRestHandler;
+import org.elasticsearch.rest.BytesRestResponse;
+import org.elasticsearch.rest.RestController;
+import org.elasticsearch.rest.RestRequest;
+import org.elasticsearch.rest.RestResponse;
+import org.elasticsearch.rest.RestStatus;
+import org.elasticsearch.rest.action.RestBuilderListener;
+
+import java.io.IOException;
+
+public class RestClusterMasterAction extends BaseRestHandler {
+
+    public RestClusterMasterAction(Settings settings, RestController controller) {
+        super(settings);
+        controller.registerHandler(RestRequest.Method.GET, "/_cluster/master", this);
+    }
+
+    @Override
+    public String getName() {
+        return "cluster_master_action";
+    }
+
+    @Override
+    public RestChannelConsumer prepareRequest(final RestRequest request, final NodeClient client) throws IOException {
+        final ClusterStateRequest clusterStateRequest = Requests.clusterStateRequest();
+        clusterStateRequest.clear();
+        clusterStateRequest.nodes(true);
+        clusterStateRequest.local(request.paramAsBoolean("local", clusterStateRequest.local()));
+        clusterStateRequest.masterNodeTimeout(request.paramAsTime("master_timeout", clusterStateRequest.masterNodeTimeout()));
+        return channel -> client.admin().cluster().state(clusterStateRequest, new RestBuilderListener<ClusterStateResponse>(channel) {
+            @Override
+            public RestResponse buildResponse(ClusterStateResponse response, XContentBuilder builder) throws Exception {
+                final DiscoveryNode masterNode = response.getState().nodes().getMasterNode();
+                if (masterNode == null) {
+                    throw new MasterNotDiscoveredException();
+                }
+                builder.startObject();
+                builder.field("master_node_id", masterNode.getId());
+                builder.endObject();
+                return new BytesRestResponse(RestStatus.OK, builder);
+            }
+        });
+    }
+}