using publish cluster-state as fallback

Author: rajiv-kv

server/src/main/java/org/opensearch/action/support/clustermanager/TransportClusterManagerNodeAction.java

@@ -381,7 +381,12 @@ public void handleResponse(GetTermVersionResponse response) {
                             if (isLatestClusterStatePresentOnLocalNode) {
                                 onLatestLocalState.accept(clusterState);
                             } else {
-                                onStaleLocalState.accept(clusterManagerNode, clusterState);
+                                ClusterState publishState = clusterService.publishState();
+                                if (publishState != null && response.matches(publishState)) {
+                                    onLatestLocalState.accept(publishState);
+                                } else {
+                                    onStaleLocalState.accept(clusterManagerNode, clusterState);
+                                }
                             }
                         }
 

server/src/main/java/org/opensearch/cluster/coordination/Coordinator.java

@@ -464,6 +464,10 @@ && getCurrentTerm() == ZEN1_BWC_TERM
             ensureTermAtLeast(sourceNode, publishRequest.getAcceptedState().term());
             final PublishResponse publishResponse = coordinationState.get().handlePublishRequest(publishRequest);
 
+            final ClusterState publishState = hideStateIfNotRecovered(coordinationState.get().getLastAcceptedState());
+            applierState = mode == Mode.CANDIDATE ? clusterStateWithNoClusterManagerBlock(publishState) : publishState;
+            clusterApplier.onPublishClusterState(publishRequest.toString(), () -> publishState);
+
             if (sourceNode.equals(getLocalNode())) {
                 preVoteCollector.update(getPreVoteResponse(), getLocalNode());
             } else {

server/src/main/java/org/opensearch/cluster/service/ClusterApplier.java

@@ -49,6 +49,8 @@ public interface ClusterApplier {
      */
     void setInitialState(ClusterState initialState);
 
+    void onPublishClusterState(String source, Supplier<ClusterState> clusterStateSupplier);
+
     /**
      * Method to invoke when a new cluster state is available to be applied
      *

server/src/main/java/org/opensearch/cluster/service/ClusterApplierService.java

@@ -118,6 +118,7 @@ public class ClusterApplierService extends AbstractLifecycleComponent implements
 
     private final Collection<ClusterStateListener> clusterStateListeners = new CopyOnWriteArrayList<>();
     private final Map<TimeoutClusterStateListener, NotifyTimeout> timeoutClusterStateListeners = new ConcurrentHashMap<>();
+    private final AtomicReference<ClusterState> publishState; // last published state
 
     private final AtomicReference<ClusterState> state; // last applied state
 
@@ -139,6 +140,7 @@ public ClusterApplierService(
     ) {
         this.clusterSettings = clusterSettings;
         this.threadPool = threadPool;
+        this.publishState = new AtomicReference<>();
         this.state = new AtomicReference<>();
         this.nodeName = nodeName;
 
@@ -232,6 +234,10 @@ public ClusterState state() {
         return clusterState;
     }
 
+    public ClusterState publishState() {
+        return publishState.get();
+    }
+
     /**
      * Returns true if the appliedClusterState is not null
      */
@@ -367,6 +373,14 @@ public ThreadPool threadPool() {
         return threadPool;
     }
 
+    @Override
+    public void onPublishClusterState(final String source, final Supplier<ClusterState> clusterStateSupplier) {
+        ClusterState nextState = clusterStateSupplier.get();
+        if (nextState != null) {
+            publishState.set(nextState);
+        }
+    }
+
     @Override
     public void onNewClusterState(
         final String source,

server/src/main/java/org/opensearch/cluster/service/ClusterService.java

@@ -183,6 +183,10 @@ public ClusterState state() {
         return clusterApplierService.state();
     }
 
+    public ClusterState publishState() {
+        return clusterApplierService.publishState();
+    }
+
     /**
      * Adds a high priority applier of updated cluster states.
      */

server/src/test/java/org/opensearch/cluster/coordination/NoOpClusterApplier.java

@@ -42,6 +42,11 @@ public void setInitialState(ClusterState initialState) {
 
     }
 
+    @Override
+    public void onPublishClusterState(String source, Supplier<ClusterState> clusterStateSupplier) {
+
+    }
+
     @Override
     public void onNewClusterState(String source, Supplier<ClusterState> clusterStateSupplier, ClusterApplyListener listener) {
         listener.onSuccess(source);