fix `GET _ilm` response with uninitialized ILM metadata (#34881)

ILM would return a resource-not-found exception when requesting policies
while the IndexLifecycleMetaData is not initialized. The behavior here
should not be as extreme since it is not the user's fault.

This commit changes the behavior so that it succeeds and returns no policies
when no policy names are explicitely specified, otherwise keep the same behavior
of throwing an exception

x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/indexlifecycle/action/TransportGetLifecycleAction.java

@@ -28,6 +28,7 @@ import org.elasticsearch.xpack.core.indexlifecycle.action.GetLifecycleAction.Res
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 
 public class TransportGetLifecycleAction extends TransportMasterNodeAction<Request, Response> {
@@ -53,7 +54,12 @@ public class TransportGetLifecycleAction extends TransportMasterNodeAction<Reque
     protected void masterOperation(Request request, ClusterState state, ActionListener<Response> listener) {
         IndexLifecycleMetadata metadata = clusterService.state().metaData().custom(IndexLifecycleMetadata.TYPE);
         if (metadata == null) {
-            listener.onFailure(new ResourceNotFoundException("Lifecycle policy not found: {}", Arrays.toString(request.getPolicyNames())));
+            if (request.getPolicyNames().length == 0) {
+                listener.onResponse(new Response(Collections.emptyList()));
+            } else {
+                listener.onFailure(new ResourceNotFoundException("Lifecycle policy not found: {}",
+                    Arrays.toString(request.getPolicyNames())));
+            }
         } else {
             List<LifecyclePolicyResponseItem> requestedPolicies;
             // if no policies explicitly provided, behave as if `*` was specified

x-pack/plugin/ilm/src/test/java/org/elasticsearch/xpack/indexlifecycle/IndexLifecycleInitialisationTests.java

@@ -54,6 +54,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ExecutionException;
 import java.util.stream.Collectors;
 
 import static org.elasticsearch.client.Requests.clusterHealthRequest;
@@ -64,6 +65,7 @@ import static org.elasticsearch.cluster.routing.ShardRoutingState.STARTED;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.xpack.core.indexlifecycle.LifecyclePolicyTestsUtils.newLockableLifecyclePolicy;
 import static org.hamcrest.CoreMatchers.not;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
 import static org.hamcrest.Matchers.lessThanOrEqualTo;
@@ -145,6 +147,15 @@ public class IndexLifecycleInitialisationTests extends ESIntegTestCase {
         logger.info("Starting server1");
         final String server_1 = internalCluster().startNode();
         final String node1 = getLocalNodeId(server_1);
+
+        // test get-lifecycle behavior when IndexLifecycleMetaData is null
+        GetLifecycleAction.Response getUninitializedLifecycleResponse = client().execute(GetLifecycleAction.INSTANCE,
+            new GetLifecycleAction.Request()).get();
+        assertThat(getUninitializedLifecycleResponse.getPolicies().size(), equalTo(0));
+        ExecutionException exception = expectThrows(ExecutionException.class,() -> client()
+            .execute(GetLifecycleAction.INSTANCE, new GetLifecycleAction.Request("non-existent-policy")).get());
+        assertThat(exception.getMessage(), containsString("Lifecycle policy not found: [non-existent-policy]"));
+
         logger.info("Creating lifecycle [test_lifecycle]");
         PutLifecycleAction.Request putLifecycleRequest = new PutLifecycleAction.Request(lifecyclePolicy);
         long lowerBoundModifiedDate = Instant.now().toEpochMilli();