Improve error message when pausing index (#48915)

Throw an appropriate error message when the follower index is not found 
or is a regular index.

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

@@ -13,15 +13,18 @@ import org.elasticsearch.action.support.master.TransportMasterNodeAction;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.block.ClusterBlockException;
 import org.elasticsearch.cluster.block.ClusterBlockLevel;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.persistent.PersistentTasksCustomMetaData;
 import org.elasticsearch.persistent.PersistentTasksService;
 import org.elasticsearch.tasks.Task;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.TransportService;
+import org.elasticsearch.xpack.ccr.Ccr;
 import org.elasticsearch.xpack.core.ccr.action.PauseFollowAction;
 
 import java.io.IOException;
@@ -57,11 +60,19 @@ public class TransportPauseFollowAction extends TransportMasterNodeAction<PauseF
 
     @Override
     protected void masterOperation(Task task, PauseFollowAction.Request request,
-                                   ClusterState state,
-                                   ActionListener<AcknowledgedResponse> listener) throws Exception {
+                                   ClusterState state, ActionListener<AcknowledgedResponse> listener) {
+        final IndexMetaData followerIMD = state.metaData().index(request.getFollowIndex());
+        if (followerIMD == null) {
+            listener.onFailure(new IndexNotFoundException(request.getFollowIndex()));
+            return;
+        }
+        if (followerIMD.getCustomData(Ccr.CCR_CUSTOM_METADATA_KEY) == null) {
+            listener.onFailure(new IllegalArgumentException("index [" + request.getFollowIndex() + "] is not a follower index"));
+            return;
+        }
         PersistentTasksCustomMetaData persistentTasksMetaData = state.metaData().custom(PersistentTasksCustomMetaData.TYPE);
         if (persistentTasksMetaData == null) {
-            listener.onFailure(new IllegalArgumentException("no shard follow tasks for [" + request.getFollowIndex() + "]"));
+            listener.onFailure(new IllegalArgumentException("no shard follow tasks found"));
             return;
         }
 

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/IndexFollowingIT.java

@@ -576,7 +576,7 @@ public class IndexFollowingIT extends CcrIntegTestCase {
 
     public void testUnfollowNonExistingIndex() {
         PauseFollowAction.Request unfollowRequest = new PauseFollowAction.Request("non-existing-index");
-        expectThrows(IllegalArgumentException.class,
+        expectThrows(IndexNotFoundException.class,
             () -> followerClient().execute(PauseFollowAction.INSTANCE, unfollowRequest).actionGet());
     }
 
@@ -817,6 +817,27 @@ public class IndexFollowingIT extends CcrIntegTestCase {
         ensureNoCcrTasks();
     }
 
+    public void testPauseIndex() throws Exception {
+        assertAcked(leaderClient().admin().indices().prepareCreate("leader")
+            .setSettings(Settings.builder()
+                .put(IndexSettings.INDEX_SOFT_DELETES_SETTING.getKey(), true)
+                .put(IndexMetaData.SETTING_NUMBER_OF_SHARDS, 1)
+                .put(IndexMetaData.SETTING_NUMBER_OF_REPLICAS, 0)
+                .build()));
+        followerClient().execute(PutFollowAction.INSTANCE, putFollow("leader", "follower")).get();
+        assertAcked(followerClient().admin().indices().prepareCreate("regular-index"));
+        assertAcked(followerClient().execute(PauseFollowAction.INSTANCE, new PauseFollowAction.Request("follower")).actionGet());
+        assertThat(expectThrows(IllegalArgumentException.class, () -> followerClient().execute(
+            PauseFollowAction.INSTANCE, new PauseFollowAction.Request("follower")).actionGet()).getMessage(),
+            equalTo("no shard follow tasks for [follower]"));
+        assertThat(expectThrows(IllegalArgumentException.class, () -> followerClient().execute(
+            PauseFollowAction.INSTANCE, new PauseFollowAction.Request("regular-index")).actionGet()).getMessage(),
+            equalTo("index [regular-index] is not a follower index"));
+        assertThat(expectThrows(IndexNotFoundException.class, () -> followerClient().execute(
+            PauseFollowAction.INSTANCE, new PauseFollowAction.Request("xyz")).actionGet()).getMessage(),
+            equalTo("no such index [xyz]"));
+    }
+
     public void testUnfollowIndex() throws Exception {
         String leaderIndexSettings = getIndexSettings(1, 0, singletonMap(IndexSettings.INDEX_SOFT_DELETES_SETTING.getKey(), "true"));
         assertAcked(leaderClient().admin().indices().prepareCreate("index1").setSource(leaderIndexSettings, XContentType.JSON).get());