[CCR] Fixed follow and unfollow api url path according to design.

The TODOs in the rest actions was incorrect. The problem was that
these rest actions used `follow_index` as first named variable in the path
under which the rest actions were registered. Other candidate rest actions that
also have a named variable as first element in the path (but with a different
name) get resolved as rest parameters too and passed down to the rest
action that actually ends up getting executed.

In the case of the follow index api, a `index` parameter got passed down
to `RestFollowExistingAction`, but that param was never used. This caused the
follow index api call to fail, because of unused http parameters.

This change doesn't fixes that problem, but works around it by using
`index` as named variable for the follow index (instead of `follow_index`).

Relates to #30102

Author: martijnvg

x-pack/plugin/ccr/qa/multi-cluster-with-security/src/test/java/org/elasticsearch/xpack/ccr/FollowIndexSecurityIT.java

@@ -85,7 +85,7 @@ public void testFollowIndex() throws Exception {
             followIndex("leader_cluster:" + indexName1, indexName1);
             assertBusy(() -> verifyDocuments(client(), indexName1, numDocs));
             assertThat(countCcrNodeTasks(), equalTo(1));
-            assertOK(client().performRequest("POST", "/_xpack/ccr/" + indexName1 + "/_unfollow"));
+            assertOK(client().performRequest("POST", "/" + indexName1 + "/_xpack/ccr/_unfollow"));
             // Make sure that there are no other ccr relates operations running:
             assertBusy(() -> {
                 Map<String, Object> clusterState = toMap(adminClient().performRequest("GET", "/_cluster/state"));
@@ -140,7 +140,7 @@ private static void refresh(String index) throws IOException {
 
     private static void followIndex(String leaderIndex, String followIndex) throws IOException {
         Map<String, String> params = Collections.singletonMap("leader_index", leaderIndex);
-        assertOK(client().performRequest("POST", "/_xpack/ccr/" + followIndex + "/_follow", params));
+        assertOK(client().performRequest("POST", "/" + followIndex + "/_xpack/ccr/_follow", params));
     }
 
     void verifyDocuments(RestClient client, String index, int expectedNumDocs) throws IOException {

x-pack/plugin/ccr/qa/multi-cluster/src/test/java/org/elasticsearch/xpack/ccr/FollowIndexIT.java

@@ -94,7 +94,7 @@ private static void refresh(String index) throws IOException {
 
     private static void followIndex(String leaderIndex, String followIndex) throws IOException {
         Map<String, String> params = Collections.singletonMap("leader_index", leaderIndex);
-        assertOK(client().performRequest("POST", "/_xpack/ccr/" + followIndex + "/_follow", params));
+        assertOK(client().performRequest("POST", "/" + followIndex + "/_xpack/ccr/_follow", params));
     }
 
     private static void verifyDocuments(String index, int expectedNumDocs) throws IOException {

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

@@ -130,6 +130,11 @@ public TransportAction(Settings settings, ThreadPool threadPool, TransportServic
         protected void doExecute(Request request, ActionListener<Response> listener) {
             client.admin().cluster().state(new ClusterStateRequest(), ActionListener.wrap(r -> {
                 IndexMetaData followIndexMetadata = r.getState().getMetaData().index(request.followIndex);
+                if (followIndexMetadata == null) {
+                    listener.onFailure(new IllegalArgumentException("follow index [" + request.followIndex + "] does not exist"));
+                    return;
+                }
+
                 final int numShards = followIndexMetadata.getNumberOfShards();
                 final AtomicInteger counter = new AtomicInteger(numShards);
                 final AtomicReferenceArray<Object> responses = new AtomicReferenceArray<>(followIndexMetadata.getNumberOfShards());

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/rest/RestFollowExistingIndexAction.java

@@ -18,13 +18,11 @@
 import static org.elasticsearch.xpack.ccr.action.FollowExistingIndexAction.INSTANCE;
 import static org.elasticsearch.xpack.ccr.action.FollowExistingIndexAction.Request;
 
-// TODO: change to confirm with API design
 public class RestFollowExistingIndexAction extends BaseRestHandler {
 
     public RestFollowExistingIndexAction(Settings settings, RestController controller) {
         super(settings);
-        // TODO: figure out why: '/{follow_index}/_xpack/ccr/_follow' path clashes with create index api.
-        controller.registerHandler(RestRequest.Method.POST, "/_xpack/ccr/{follow_index}/_follow", this);
+        controller.registerHandler(RestRequest.Method.POST, "/{index}/_xpack/ccr/_follow", this);
     }
 
     @Override
@@ -36,7 +34,7 @@ public String getName() {
     protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
         Request request = new Request();
         request.setLeaderIndex(restRequest.param("leader_index"));
-        request.setFollowIndex(restRequest.param("follow_index"));
+        request.setFollowIndex(restRequest.param("index"));
         if (restRequest.hasParam(ShardFollowTask.MAX_CHUNK_SIZE.getPreferredName())) {
             request.setBatchSize(Long.valueOf(restRequest.param(ShardFollowTask.MAX_CHUNK_SIZE.getPreferredName())));
         }

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/rest/RestUnfollowIndexAction.java

@@ -17,13 +17,11 @@
 import static org.elasticsearch.xpack.ccr.action.UnfollowIndexAction.INSTANCE;
 import static org.elasticsearch.xpack.ccr.action.UnfollowIndexAction.Request;
 
-// TODO: change to confirm with API design
 public class RestUnfollowIndexAction extends BaseRestHandler {
 
     public RestUnfollowIndexAction(Settings settings, RestController controller) {
         super(settings);
-        // TODO: figure out why: '/{follow_index}/_xpack/ccr/_unfollow' path clashes with create index api.
-        controller.registerHandler(RestRequest.Method.POST, "/_xpack/ccr/{follow_index}/_unfollow", this);
+        controller.registerHandler(RestRequest.Method.POST, "/{index}/_xpack/ccr/_unfollow", this);
     }
 
     @Override
@@ -34,7 +32,7 @@ public String getName() {
     @Override
     protected RestChannelConsumer prepareRequest(RestRequest restRequest, NodeClient client) throws IOException {
         Request request = new Request();
-        request.setFollowIndex(restRequest.param("follow_index"));
+        request.setFollowIndex(restRequest.param("index"));
         return channel -> client.execute(INSTANCE, request, new RestToXContentListener<>(channel));
     }
 }

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

@@ -276,6 +276,12 @@ public void testFollowIndexWithNestedField() throws Exception {
         });
     }
 
+    public void testUnfollowNonExistingIndex() {
+        UnfollowIndexAction.Request unfollowRequest = new UnfollowIndexAction.Request();
+        unfollowRequest.setFollowIndex("non-existing-index");
+        expectThrows(IllegalArgumentException.class, () -> client().execute(UnfollowIndexAction.INSTANCE, unfollowRequest).actionGet());
+    }
+
     public void testFollowNonExistentIndex() throws Exception {
         assertAcked(client().admin().indices().prepareCreate("test-leader").get());
         assertAcked(client().admin().indices().prepareCreate("test-follower").get());

x-pack/plugin/src/test/resources/rest-api-spec/api/xpack.ccr.follow_existing_index.json

@@ -3,10 +3,10 @@
     "documentation": "http://www.elastic.co/guide/en/elasticsearch/reference/current",
     "methods": [ "POST" ],
     "url": {
-      "path": "/_xpack/ccr/{follow_index}/_follow",
-      "paths": [ "/_xpack/ccr/{follow_index}/_follow" ],
+      "path": "/{index}/_xpack/ccr/_follow",
+      "paths": [ "/{index}/_xpack/ccr/_follow" ],
       "parts": {
-        "follow_index": {
+        "index": {
           "type": "string",
           "required": true,
           "description": "The name of the index that follows to leader index."

x-pack/plugin/src/test/resources/rest-api-spec/api/xpack.ccr.unfollow_index.json

@@ -3,10 +3,10 @@
     "documentation": "http://www.elastic.co/guide/en/elasticsearch/reference/current",
     "methods": [ "POST" ],
     "url": {
-      "path": "/_xpack/ccr/{follow_index}/_unfollow",
-      "paths": [ "/_xpack/ccr/{follow_index}/_unfollow" ],
+      "path": "/{index}/_xpack/ccr/_unfollow",
+      "paths": [ "/{index}/_xpack/ccr/_unfollow" ],
       "parts": {
-        "follow_index": {
+        "index": {
           "type": "string",
           "required": true,
           "description": "The name of the follow index that should stop following its leader index."

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

@@ -25,10 +25,10 @@
   - do:
       xpack.ccr.follow_existing_index:
         leader_index: foo
-        follow_index: bar
+        index: bar
   - is_true: acknowledged
 
   - do:
       xpack.ccr.unfollow_index:
-        follow_index: bar
+        index: bar
   - is_true: acknowledged