Follow stats api should return a 404 when requesting stats for a non existing index

x-pack/plugin/ccr/qa/rest/src/test/resources/rest-api-spec/test/ccr/follow_stats.yml

@@ -43,6 +43,12 @@
   - is_true: follow_index_shards_acked
   - is_true: index_following_started
 
+  - do:
+      ccr.follow_stats:
+        index: _all
+  - length: { indices: 1 }
+  - match: { indices.0.index: "bar" }
+
   # we can not reliably wait for replication to occur so we test the endpoint without indexing any documents
   - do:
       ccr.follow_stats:
@@ -77,3 +83,7 @@
         index: bar
   - is_true: acknowledged
 
+  - do:
+      catch: missing
+      ccr.follow_stats:
+        index: unknown

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/TransportFollowStatsAction.java

@@ -10,8 +10,10 @@
 import org.elasticsearch.action.FailedNodeException;
 import org.elasticsearch.action.TaskOperationFailure;
 import org.elasticsearch.action.support.ActionFilters;
+import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.action.support.tasks.TransportTasksAction;
 import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.license.LicenseUtils;
@@ -37,13 +39,15 @@ public class TransportFollowStatsAction extends TransportTasksAction<
         FollowStatsAction.StatsResponses, FollowStatsAction.StatsResponse> {
 
     private final CcrLicenseChecker ccrLicenseChecker;
+    private final IndexNameExpressionResolver resolver;
 
     @Inject
     public TransportFollowStatsAction(
             final ClusterService clusterService,
             final TransportService transportService,
             final ActionFilters actionFilters,
-            final CcrLicenseChecker ccrLicenseChecker) {
+            final CcrLicenseChecker ccrLicenseChecker,
+            final IndexNameExpressionResolver resolver) {
         super(
                 FollowStatsAction.NAME,
                 clusterService,
@@ -54,6 +58,7 @@ public TransportFollowStatsAction(
                 FollowStatsAction.StatsResponse::new,
                 Ccr.CCR_THREAD_POOL_NAME);
         this.ccrLicenseChecker = Objects.requireNonNull(ccrLicenseChecker);
+        this.resolver = resolver;
     }
 
     @Override
@@ -65,6 +70,10 @@ protected void doExecute(
             listener.onFailure(LicenseUtils.newComplianceException("ccr"));
             return;
         }
+
+        String[] concreteIndices =
+            resolver.concreteIndexNames(clusterService.state(), IndicesOptions.strictExpandOpenAndForbidClosed(), request.indices());
+        request.setIndices(concreteIndices);
         super.doExecute(task, request, listener);
     }