[CCR] Fix follow stats API's follower index filtering feature (#36647)

Currently always all follow stats for all follower indices are being
returned even if follow stats for only specific indices are requested.

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

@@ -22,6 +22,9 @@ import org.elasticsearch.xpack.ccr.Ccr;
 import org.elasticsearch.xpack.ccr.CcrLicenseChecker;
 import org.elasticsearch.xpack.core.ccr.action.FollowStatsAction;
 
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
 import java.util.Set;
@@ -82,12 +85,15 @@ public class TransportFollowStatsAction extends TransportTasksAction<
             return;
         }
 
+        final Set<String> requestedFollowerIndices = request.indices() != null ?
+            new HashSet<>(Arrays.asList(request.indices())) : Collections.emptySet();
         final Set<String> followerIndices = persistentTasksMetaData.tasks().stream()
             .filter(persistentTask -> persistentTask.getTaskName().equals(ShardFollowTask.NAME))
             .map(persistentTask -> {
                 ShardFollowTask shardFollowTask = (ShardFollowTask) persistentTask.getParams();
                 return shardFollowTask.getFollowShardId().getIndexName();
             })
+            .filter(followerIndex -> requestedFollowerIndices.isEmpty() || requestedFollowerIndices.contains(followerIndex))
             .collect(Collectors.toSet());
 
         for (final Task task : taskManager.getTasks().values()) {

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

@@ -67,19 +67,19 @@ public abstract class CcrSingleNodeTestCase extends ESSingleNodeTestCase {
         assertAcked(client().admin().cluster().updateSettings(updateSettingsRequest).actionGet());
     }
 
-    protected ResumeFollowAction.Request getResumeFollowRequest() {
+    protected ResumeFollowAction.Request getResumeFollowRequest(String followerIndex) {
         ResumeFollowAction.Request request = new ResumeFollowAction.Request();
-        request.setFollowerIndex("follower");
+        request.setFollowerIndex(followerIndex);
         request.setMaxRetryDelay(TimeValue.timeValueMillis(10));
         request.setReadPollTimeout(TimeValue.timeValueMillis(10));
         return request;
     }
 
-    protected PutFollowAction.Request getPutFollowRequest() {
+    protected PutFollowAction.Request getPutFollowRequest(String leaderIndex, String followerIndex) {
         PutFollowAction.Request request = new PutFollowAction.Request();
         request.setRemoteCluster("local");
-        request.setLeaderIndex("leader");
-        request.setFollowRequest(getResumeFollowRequest());
+        request.setLeaderIndex(leaderIndex);
+        request.setFollowRequest(getResumeFollowRequest(followerIndex));
         return request;
     }
 

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

@@ -49,7 +49,7 @@ public class CcrLicenseIT extends CcrSingleNodeTestCase {
     }
 
     public void testThatFollowingIndexIsUnavailableWithNonCompliantLicense() throws InterruptedException {
-        final ResumeFollowAction.Request followRequest = getResumeFollowRequest();
+        final ResumeFollowAction.Request followRequest = getResumeFollowRequest("follower");
         final CountDownLatch latch = new CountDownLatch(1);
         client().execute(
                 ResumeFollowAction.INSTANCE,
@@ -71,7 +71,7 @@ public class CcrLicenseIT extends CcrSingleNodeTestCase {
     }
 
     public void testThatCreateAndFollowingIndexIsUnavailableWithNonCompliantLicense() throws InterruptedException {
-        final PutFollowAction.Request createAndFollowRequest = getPutFollowRequest();
+        final PutFollowAction.Request createAndFollowRequest = getPutFollowRequest("leader", "follower");
         final CountDownLatch latch = new CountDownLatch(1);
         client().execute(
                 PutFollowAction.INSTANCE,

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

@@ -11,11 +11,13 @@ import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentType;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.xpack.CcrSingleNodeTestCase;
+import org.elasticsearch.xpack.core.ccr.action.FollowStatsAction;
 import org.elasticsearch.xpack.core.ccr.action.PauseFollowAction;
 import org.elasticsearch.xpack.core.ccr.action.PutFollowAction;
 import org.elasticsearch.xpack.core.ccr.action.ResumeFollowAction;
 
 import java.io.IOException;
+import java.util.Comparator;
 import java.util.Map;
 
 import static java.util.Collections.singletonMap;
@@ -31,7 +33,7 @@ public class LocalIndexFollowingIT extends CcrSingleNodeTestCase {
         assertAcked(client().admin().indices().prepareCreate("leader").setSource(leaderIndexSettings, XContentType.JSON));
         ensureGreen("leader");
 
-        final PutFollowAction.Request followRequest = getPutFollowRequest();
+        final PutFollowAction.Request followRequest = getPutFollowRequest("leader", "follower");
         client().execute(PutFollowAction.INSTANCE, followRequest).get();
 
         final long firstBatchNumDocs = randomIntBetween(2, 64);
@@ -61,7 +63,7 @@ public class LocalIndexFollowingIT extends CcrSingleNodeTestCase {
             client().prepareIndex("leader", "doc").setSource("{}", XContentType.JSON).get();
         }
 
-        client().execute(ResumeFollowAction.INSTANCE, getResumeFollowRequest()).get();
+        client().execute(ResumeFollowAction.INSTANCE, getResumeFollowRequest("follower")).get();
         assertBusy(() -> {
             assertThat(client().prepareSearch("follower").get().getHits().getTotalHits().value,
                 equalTo(firstBatchNumDocs + secondBatchNumDocs + thirdBatchNumDocs));
@@ -69,13 +71,46 @@ public class LocalIndexFollowingIT extends CcrSingleNodeTestCase {
         ensureEmptyWriteBuffers();
     }
 
+    public void testFollowStatsApiFollowerIndexFiltering() throws Exception {
+        final String leaderIndexSettings = getIndexSettings(1, 0,
+            singletonMap(IndexSettings.INDEX_SOFT_DELETES_SETTING.getKey(), "true"));
+        assertAcked(client().admin().indices().prepareCreate("leader1").setSource(leaderIndexSettings, XContentType.JSON));
+        ensureGreen("leader1");
+        assertAcked(client().admin().indices().prepareCreate("leader2").setSource(leaderIndexSettings, XContentType.JSON));
+        ensureGreen("leader2");
+
+        PutFollowAction.Request followRequest = getPutFollowRequest("leader1", "follower1");
+        client().execute(PutFollowAction.INSTANCE, followRequest).get();
+
+        followRequest = getPutFollowRequest("leader2", "follower2");
+        client().execute(PutFollowAction.INSTANCE, followRequest).get();
+
+        FollowStatsAction.StatsRequest statsRequest = new FollowStatsAction.StatsRequest();
+        statsRequest.setIndices(new String[] {"follower1"});
+        FollowStatsAction.StatsResponses response = client().execute(FollowStatsAction.INSTANCE, statsRequest).actionGet();
+        assertThat(response.getStatsResponses().size(), equalTo(1));
+        assertThat(response.getStatsResponses().get(0).status().followerIndex(), equalTo("follower1"));
+
+        statsRequest = new FollowStatsAction.StatsRequest();
+        statsRequest.setIndices(new String[] {"follower2"});
+        response = client().execute(FollowStatsAction.INSTANCE, statsRequest).actionGet();
+        assertThat(response.getStatsResponses().size(), equalTo(1));
+        assertThat(response.getStatsResponses().get(0).status().followerIndex(), equalTo("follower2"));
+
+        response = client().execute(FollowStatsAction.INSTANCE,  new FollowStatsAction.StatsRequest()).actionGet();
+        assertThat(response.getStatsResponses().size(), equalTo(2));
+        response.getStatsResponses().sort(Comparator.comparing(o -> o.status().followerIndex()));
+        assertThat(response.getStatsResponses().get(0).status().followerIndex(), equalTo("follower1"));
+        assertThat(response.getStatsResponses().get(1).status().followerIndex(), equalTo("follower2"));
+    }
+
     public void testDoNotCreateFollowerIfLeaderDoesNotHaveSoftDeletes() throws Exception {
         final String leaderIndexSettings = getIndexSettings(2, 0,
             singletonMap(IndexSettings.INDEX_SOFT_DELETES_SETTING.getKey(), "false"));
         assertAcked(client().admin().indices().prepareCreate("leader-index").setSource(leaderIndexSettings, XContentType.JSON));
-        ResumeFollowAction.Request followRequest = getResumeFollowRequest();
+        ResumeFollowAction.Request followRequest = getResumeFollowRequest("follower");
         followRequest.setFollowerIndex("follower-index");
-        PutFollowAction.Request putFollowRequest = getPutFollowRequest();
+        PutFollowAction.Request putFollowRequest = getPutFollowRequest("leader", "follower");
         putFollowRequest.setLeaderIndex("leader-index");
         putFollowRequest.setFollowRequest(followRequest);
         IllegalArgumentException error = expectThrows(IllegalArgumentException.class,