[8.16] Fallback to field-caps (#115977)

server/src/main/java/org/elasticsearch/action/fieldcaps/TransportFieldCapabilitiesAction.java

@@ -22,6 +22,7 @@
 import org.elasticsearch.action.support.ChannelActionListener;
 import org.elasticsearch.action.support.HandledTransportAction;
 import org.elasticsearch.action.support.RefCountingRunnable;
+import org.elasticsearch.client.internal.RemoteClusterClient;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.block.ClusterBlockLevel;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
@@ -113,23 +114,28 @@ public TransportFieldCapabilitiesAction(
 
     @Override
     protected void doExecute(Task task, FieldCapabilitiesRequest request, final ActionListener<FieldCapabilitiesResponse> listener) {
-        executeRequest(task, request, REMOTE_TYPE, listener);
+        executeRequest(
+            task,
+            request,
+            (remoteClient, remoteRequest, remoteListener) -> remoteClient.execute(REMOTE_TYPE, remoteRequest, remoteListener),
+            listener
+        );
     }
 
     public void executeRequest(
         Task task,
         FieldCapabilitiesRequest request,
-        RemoteClusterActionType<FieldCapabilitiesResponse> remoteAction,
+        RemoteRequestExecutor remoteRequestExecutor,
         ActionListener<FieldCapabilitiesResponse> listener
     ) {
         // workaround for https://github.com/elastic/elasticsearch/issues/97916 - TODO remove this when we can
-        searchCoordinationExecutor.execute(ActionRunnable.wrap(listener, l -> doExecuteForked(task, request, remoteAction, l)));
+        searchCoordinationExecutor.execute(ActionRunnable.wrap(listener, l -> doExecuteForked(task, request, remoteRequestExecutor, l)));
     }
 
     private void doExecuteForked(
         Task task,
         FieldCapabilitiesRequest request,
-        RemoteClusterActionType<FieldCapabilitiesResponse> remoteAction,
+        RemoteRequestExecutor remoteRequestExecutor,
         ActionListener<FieldCapabilitiesResponse> listener
     ) {
         if (ccsCheckCompatibility) {
@@ -282,8 +288,8 @@ private void doExecuteForked(
                         handleIndexFailure.accept(RemoteClusterAware.buildRemoteIndexName(clusterAlias, index), ex);
                     }
                 });
-                remoteClusterClient.execute(
-                    remoteAction,
+                remoteRequestExecutor.executeRemoteRequest(
+                    remoteClusterClient,
                     remoteRequest,
                     // The underlying transport service may call onFailure with a thread pool other than search_coordinator.
                     // This fork is a workaround to ensure that the merging of field-caps always occurs on the search_coordinator.
@@ -298,6 +304,14 @@ private void doExecuteForked(
         }
     }
 
+    public interface RemoteRequestExecutor {
+        void executeRemoteRequest(
+            RemoteClusterClient remoteClient,
+            FieldCapabilitiesRequest remoteRequest,
+            ActionListener<FieldCapabilitiesResponse> remoteListener
+        );
+    }
+
     private static void checkIndexBlocks(ClusterState clusterState, String[] concreteIndices) {
         var blocks = clusterState.blocks();
         if (blocks.global().isEmpty() && blocks.indices().isEmpty()) {

x-pack/plugin/esql/qa/server/multi-clusters/build.gradle

@@ -18,8 +18,8 @@ dependencies {
 }
 
 def supportedVersion = bwcVersion -> {
-  // ESQL requires its own resolve_fields API
-  return bwcVersion.onOrAfter(Version.fromString("8.16.0"));
+  // CCS in ES|QL available in 8.13
+  return bwcVersion.onOrAfter(Version.fromString("8.13.0"));
 }
 
 BuildParams.bwcVersions.withWireCompatible(supportedVersion) { bwcVersion, baseName ->

x-pack/plugin/esql/qa/server/multi-clusters/src/javaRestTest/java/org/elasticsearch/xpack/esql/ccq/MultiClusterSpecIT.java

@@ -104,7 +104,6 @@ public MultiClusterSpecIT(
     protected void shouldSkipTest(String testName) throws IOException {
         super.shouldSkipTest(testName);
         checkCapabilities(remoteClusterClient(), remoteFeaturesService(), testName, testCase);
-        assumeTrue("CCS requires its own resolve_fields API", remoteFeaturesService().clusterHasFeature("esql.resolve_fields_api"));
         assumeFalse("can't test with _index metadata", hasIndexMetadata(testCase.query));
         assumeTrue(
             "Test " + testName + " is skipped on " + Clusters.oldVersion(),

x-pack/plugin/esql/qa/server/multi-clusters/src/javaRestTest/java/org/elasticsearch/xpack/esql/ccq/MultiClustersIT.java

@@ -67,7 +67,6 @@ record Doc(int id, String color, long data) {
 
     @Before
     public void setUpIndices() throws Exception {
-        assumeTrue("CCS requires its own resolve_fields API", remoteFeaturesService().clusterHasFeature("esql.resolve_fields_api"));
         final String mapping = """
              "properties": {
                "data": { "type": "long" },

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlResolveFieldsAction.java

@@ -6,6 +6,7 @@
  */
 package org.elasticsearch.xpack.esql.action;
 
+import org.elasticsearch.TransportVersions;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.ActionType;
 import org.elasticsearch.action.RemoteClusterActionType;
@@ -14,6 +15,7 @@
 import org.elasticsearch.action.fieldcaps.TransportFieldCapabilitiesAction;
 import org.elasticsearch.action.support.ActionFilters;
 import org.elasticsearch.action.support.HandledTransportAction;
+import org.elasticsearch.client.internal.RemoteClusterClient;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.injection.guice.Inject;
 import org.elasticsearch.tasks.Task;
@@ -27,7 +29,7 @@
 public class EsqlResolveFieldsAction extends HandledTransportAction<FieldCapabilitiesRequest, FieldCapabilitiesResponse> {
     public static final String NAME = "indices:data/read/esql/resolve_fields";
     public static final ActionType<FieldCapabilitiesResponse> TYPE = new ActionType<>(NAME);
-    public static final RemoteClusterActionType<FieldCapabilitiesResponse> REMOTE_TYPE = new RemoteClusterActionType<>(
+    public static final RemoteClusterActionType<FieldCapabilitiesResponse> RESOLVE_REMOTE_TYPE = new RemoteClusterActionType<>(
         NAME,
         FieldCapabilitiesResponse::new
     );
@@ -47,6 +49,19 @@ public EsqlResolveFieldsAction(
 
     @Override
     protected void doExecute(Task task, FieldCapabilitiesRequest request, final ActionListener<FieldCapabilitiesResponse> listener) {
-        fieldCapsAction.executeRequest(task, request, REMOTE_TYPE, listener);
+        fieldCapsAction.executeRequest(task, request, this::executeRemoteRequest, listener);
+    }
+
+    void executeRemoteRequest(
+        RemoteClusterClient remoteClient,
+        FieldCapabilitiesRequest remoteRequest,
+        ActionListener<FieldCapabilitiesResponse> remoteListener
+    ) {
+        remoteClient.getConnection(remoteRequest, remoteListener.delegateFailure((l, conn) -> {
+            var remoteAction = conn.getTransportVersion().onOrAfter(TransportVersions.ESQL_ORIGINAL_INDICES)
+                ? RESOLVE_REMOTE_TYPE
+                : TransportFieldCapabilitiesAction.REMOTE_TYPE;
+            remoteClient.execute(conn, remoteAction, remoteRequest, l);
+        }));
     }
 }