Use valid REST version when determining capabilities

server/src/main/java/module-info.java

@@ -429,7 +429,7 @@
     provides org.elasticsearch.features.FeatureSpecification
         with
             org.elasticsearch.action.bulk.BulkFeatures,
-            org.elasticsearch.features.FeatureInfrastructureFeatures,
+            org.elasticsearch.features.InfrastructureFeatures,
             org.elasticsearch.rest.action.admin.cluster.ClusterRerouteFeatures,
             org.elasticsearch.index.mapper.MapperFeatures,
             org.elasticsearch.index.IndexFeatures,

server/src/main/java/org/elasticsearch/action/admin/cluster/node/capabilities/NodesCapabilitiesRequest.java

@@ -11,9 +11,11 @@
 
 import org.elasticsearch.action.support.nodes.BaseNodesRequest;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.RestApiVersion;
 import org.elasticsearch.rest.RestRequest;
 
+import java.util.Optional;
 import java.util.Set;
 
 public class NodesCapabilitiesRequest extends BaseNodesRequest {
@@ -22,7 +24,7 @@ public class NodesCapabilitiesRequest extends BaseNodesRequest {
     private String path = "/";
     private Set<String> parameters = Set.of();
     private Set<String> capabilities = Set.of();
-    private RestApiVersion restApiVersion = RestApiVersion.current();
+    private @Nullable RestApiVersion restApiVersion;
 
     public NodesCapabilitiesRequest() {
         // send to all nodes
@@ -75,7 +77,7 @@ public NodesCapabilitiesRequest restApiVersion(RestApiVersion restApiVersion) {
         return this;
     }
 
-    public RestApiVersion restApiVersion() {
-        return restApiVersion;
+    public Optional<RestApiVersion> restApiVersion() {
+        return Optional.ofNullable(restApiVersion);
     }
 }

server/src/main/java/org/elasticsearch/action/admin/cluster/node/capabilities/TransportNodesCapabilitiesAction.java

@@ -18,7 +18,9 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.core.RestApiVersion;
-import org.elasticsearch.core.UpdateForV9;
+import org.elasticsearch.core.UpdateForV10;
+import org.elasticsearch.features.FeatureService;
+import org.elasticsearch.features.InfrastructureFeatures;
 import org.elasticsearch.injection.guice.Inject;
 import org.elasticsearch.rest.RestController;
 import org.elasticsearch.rest.RestRequest;
@@ -41,14 +43,16 @@ public class TransportNodesCapabilitiesAction extends TransportNodesAction<
     public static final ActionType<NodesCapabilitiesResponse> TYPE = new ActionType<>("cluster:monitor/nodes/capabilities");
 
     private final RestController restController;
+    private final FeatureService featureService;
 
     @Inject
     public TransportNodesCapabilitiesAction(
         ThreadPool threadPool,
         ClusterService clusterService,
         TransportService transportService,
         ActionFilters actionFilters,
-        RestController restController
+        RestController restController,
+        FeatureService featureService
     ) {
         super(
             TYPE.name(),
@@ -59,6 +63,7 @@ public TransportNodesCapabilitiesAction(
             threadPool.executor(ThreadPool.Names.MANAGEMENT)
         );
         this.restController = restController;
+        this.featureService = featureService;
     }
 
     @Override
@@ -72,13 +77,21 @@ protected NodesCapabilitiesResponse newResponse(
 
     @Override
     protected NodeCapabilitiesRequest newNodeRequest(NodesCapabilitiesRequest request) {
-        return new NodeCapabilitiesRequest(
-            request.method(),
-            request.path(),
-            request.parameters(),
-            request.capabilities(),
-            request.restApiVersion()
-        );
+        @UpdateForV10(owner = UpdateForV10.Owner.CORE_INFRA)  // rather than v8/v9, this should handle v9/v10
+        RestApiVersion restVersion;
+        if (request.restApiVersion().isPresent()) {
+            // explicit version - just use it, and see what happens
+            restVersion = request.restApiVersion().get();
+        } else if (featureService.clusterHasFeature(clusterService.state(), InfrastructureFeatures.ELASTICSEARCH_V9)) {
+            restVersion = RestApiVersion.V_9; // every node is at least v9, so use that
+        } else {
+            // not all nodes are v9. v8 nodes do not understand v9 rest API, so query using v8.
+            // Capabilities can come and go, so it's ok for the response to change
+            // when the nodes change
+            restVersion = RestApiVersion.V_8;
+        }
+
+        return new NodeCapabilitiesRequest(request.method(), request.path(), request.parameters(), request.capabilities(), restVersion);
     }
 
     @Override
@@ -129,10 +142,6 @@ public NodeCapabilitiesRequest(
             this.restApiVersion = restApiVersion;
         }
 
-        @UpdateForV9(owner = UpdateForV9.Owner.CORE_INFRA) // 8.x blows up in a mixed cluster when trying to read RestApiVersion.forMajor(9)
-        // ./gradlew ":qa:mixed-cluster:v8.16.0#mixedClusterTest"
-        // -Dtests.class="org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT"
-        // -Dtests.method="test {p0=capabilities/10_basic/Capabilities API}"
         @Override
         public void writeTo(StreamOutput out) throws IOException {
             super.writeTo(out);
@@ -141,9 +150,7 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeString(path);
             out.writeCollection(parameters, StreamOutput::writeString);
             out.writeCollection(capabilities, StreamOutput::writeString);
-            // Fixme: lies! all lies!
-            out.writeVInt(8);
-            // out.writeVInt(restApiVersion.major);
+            out.writeVInt(restApiVersion.major);
         }
     }
 }

server/src/main/java/org/elasticsearch/features/FeatureInfrastructureFeatures.java → server/src/main/java/org/elasticsearch/features/InfrastructureFeatures.java

@@ -12,9 +12,16 @@
 import java.util.Set;
 
 /**
- * This class specifies features for the features functionality itself.
+ * This class specifies features for Elasticsearch infrastructure.
  */
-public class FeatureInfrastructureFeatures implements FeatureSpecification {
+public class InfrastructureFeatures implements FeatureSpecification {
+
+    public static final NodeFeature ELASTICSEARCH_V9 = new NodeFeature("es_v9");
+
+    @Override
+    public Set<NodeFeature> getFeatures() {
+        return Set.of(ELASTICSEARCH_V9);
+    }
 
     @Override
     public Set<NodeFeature> getTestFeatures() {

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

@@ -66,8 +66,10 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
         NodesCapabilitiesRequest r = requestNodes.method(RestRequest.Method.valueOf(request.param("method", "GET")))
             .path(path)
             .parameters(request.paramAsStringArray("parameters", Strings.EMPTY_ARRAY))
-            .capabilities(request.paramAsStringArray("capabilities", Strings.EMPTY_ARRAY))
-            .restApiVersion(request.getRestApiVersion());
+            .capabilities(request.paramAsStringArray("capabilities", Strings.EMPTY_ARRAY));
+        if (request.hasExplicitRestApiVersion()) {
+            r.restApiVersion(request.getRestApiVersion());
+        }
 
         return channel -> client.admin().cluster().nodesCapabilities(r, new NodesResponseRestListener<>(channel));
     }

server/src/main/resources/META-INF/services/org.elasticsearch.features.FeatureSpecification

@@ -8,7 +8,7 @@
 #
 
 org.elasticsearch.action.bulk.BulkFeatures
-org.elasticsearch.features.FeatureInfrastructureFeatures
+org.elasticsearch.features.InfrastructureFeatures
 org.elasticsearch.rest.action.admin.cluster.ClusterRerouteFeatures
 org.elasticsearch.index.IndexFeatures
 org.elasticsearch.index.mapper.MapperFeatures