Fix cluster state persistence for single-node discovery (#36825)

Single-node discovery is not persisting cluster states, which was caused by a recent 7.0-only
refactoring. This commit ensures that the cluster state is properly persisted when using single-node
discovery and adds a corresponding test.

server/src/main/java/org/elasticsearch/discovery/DiscoveryModule.java

@@ -132,7 +132,8 @@ public class DiscoveryModule {
             transportService, namedWriteableRegistry, allocationService, masterService,
             () -> gatewayMetaState.getPersistedState(settings, (ClusterApplierService) clusterApplier), hostsProvider, clusterApplier,
             Randomness.get()));
-        discoveryTypes.put("single-node", () -> new SingleNodeDiscovery(settings, transportService, masterService, clusterApplier));
+        discoveryTypes.put("single-node", () -> new SingleNodeDiscovery(settings, transportService, masterService, clusterApplier,
+            gatewayMetaState));
         for (DiscoveryPlugin plugin : plugins) {
             plugin.getDiscoveryTypes(threadPool, transportService, namedWriteableRegistry, masterService, clusterApplier, clusterSettings,
                 hostsProvider, allocationService, gatewayMetaState).forEach((key, value) -> {

server/src/main/java/org/elasticsearch/discovery/single/SingleNodeDiscovery.java

@@ -31,12 +31,14 @@ import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.node.DiscoveryNodes;
 import org.elasticsearch.cluster.service.ClusterApplier;
 import org.elasticsearch.cluster.service.ClusterApplier.ClusterApplyListener;
+import org.elasticsearch.cluster.service.ClusterApplierService;
 import org.elasticsearch.cluster.service.MasterService;
 import org.elasticsearch.common.component.AbstractLifecycleComponent;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.discovery.Discovery;
 import org.elasticsearch.discovery.DiscoveryStats;
+import org.elasticsearch.gateway.GatewayMetaState;
 import org.elasticsearch.transport.TransportService;
 
 import java.util.Objects;
@@ -55,12 +57,17 @@ public class SingleNodeDiscovery extends AbstractLifecycleComponent implements D
     private volatile ClusterState clusterState;
 
     public SingleNodeDiscovery(final Settings settings, final TransportService transportService,
-                               final MasterService masterService, final ClusterApplier clusterApplier) {
+                               final MasterService masterService, final ClusterApplier clusterApplier,
+                               final GatewayMetaState gatewayMetaState) {
         super(Objects.requireNonNull(settings));
         this.clusterName = ClusterName.CLUSTER_NAME_SETTING.get(settings);
         this.transportService = Objects.requireNonNull(transportService);
         masterService.setClusterStateSupplier(() -> clusterState);
         this.clusterApplier = clusterApplier;
+
+        if (clusterApplier instanceof ClusterApplierService) {
+            ((ClusterApplierService) clusterApplier).addLowPriorityApplier(gatewayMetaState);
+        }
     }
 
     @Override

server/src/test/java/org/elasticsearch/discovery/single/SingleNodeDiscoveryIT.java

@@ -167,4 +167,10 @@ public class SingleNodeDiscoveryIT extends ESIntegTestCase {
         }
     }
 
+    public void testStatePersistence() throws Exception {
+        createIndex("test");
+        internalCluster().fullRestart();
+        assertTrue(client().admin().indices().prepareExists("test").get().isExists());
+    }
+
 }

server/src/test/java/org/elasticsearch/discovery/single/SingleNodeDiscoveryTests.java

@@ -69,7 +69,7 @@ public class SingleNodeDiscoveryTests extends ESTestCase {
                                 clusterState.set(clusterStateSupplier.get());
                                 listener.onSuccess(source);
                             }
-                    });
+                    }, null);
             discovery.start();
             discovery.startInitialJoin();
             final DiscoveryNodes nodes = clusterState.get().nodes();

test/framework/src/main/java/org/elasticsearch/test/InternalTestCluster.java

@@ -961,7 +961,8 @@ public final class InternalTestCluster extends TestCluster {
                     .put(newSettings)
                     .put(NodeEnvironment.NODE_ID_SEED_SETTING.getKey(), newIdSeed)
                     .build();
-            if (DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.exists(finalSettings) == false) {
+            final boolean usingSingleNodeDiscovery = DiscoveryModule.DISCOVERY_TYPE_SETTING.get(finalSettings).equals("single-node");
+            if (usingSingleNodeDiscovery == false && DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.exists(finalSettings) == false) {
                 throw new IllegalStateException(DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey() +
                     " is not configured after restart of [" + name + "]");
             }