Watcher: Fix NPE in watcher index template registry (elastic/x-pack-elasticsearch#3571)

The current code throws an NPE, when there is no master node available
in the cluster state. This commit adds an additional check for an
existing master node.

relates elastic/x-pack-elasticsearch#3569

Original commit: elastic/x-pack-elasticsearch@e0024b5170

plugin/src/main/java/org/elasticsearch/xpack/watcher/support/WatcherIndexTemplateRegistry.java

@@ -81,10 +81,15 @@ public class WatcherIndexTemplateRegistry extends AbstractComponent implements C
             return;
         }
 
+        // no master node, exit immediately
+        DiscoveryNode masterNode = event.state().getNodes().getMasterNode();
+        if (masterNode == null) {
+            return;
+        }
+
         // if this node is newer than the master node, we probably need to add the history template, which might be newer than the
         // history template the master node has, so we need potentially add new templates despite being not the master node
         DiscoveryNode localNode = event.state().getNodes().getLocalNode();
-        DiscoveryNode masterNode = event.state().getNodes().getMasterNode();
         boolean localNodeVersionAfterMaster = localNode.getVersion().after(masterNode.getVersion());
 
         if (event.localNodeMaster() || localNodeVersionAfterMaster) {

plugin/src/test/java/org/elasticsearch/xpack/watcher/support/WatcherIndexTemplateRegistryTests.java

@@ -128,6 +128,17 @@ public class WatcherIndexTemplateRegistryTests extends ESTestCase {
         verifyZeroInteractions(client);
     }
 
+    public void testThatMissingMasterNodeDoesNothing() {
+        DiscoveryNode localNode = new DiscoveryNode("node", ESTestCase.buildNewFakeTransportAddress(), Version.CURRENT);
+        DiscoveryNodes nodes = DiscoveryNodes.builder().localNodeId("node").add(localNode).build();
+
+        ClusterChangedEvent event = createClusterChangedEvent(Arrays.asList(WatcherIndexTemplateRegistry.TRIGGERED_TEMPLATE_NAME,
+                WatcherIndexTemplateRegistry.WATCHES_TEMPLATE_NAME, ".watch-history-6"), nodes);
+        registry.clusterChanged(event);
+
+        verifyZeroInteractions(client);
+    }
+
     private ClusterChangedEvent createClusterChangedEvent(List<String> existingTemplateNames, DiscoveryNodes nodes) {
         ClusterChangedEvent event = mock(ClusterChangedEvent.class);
         when(event.localNodeMaster()).thenReturn(nodes.isLocalNodeElectedMaster());