Fix Edge-Case Threading Bug in TransportMountSearchableSnapshotAction

The callback to loading the repository-data may not run on generic in the uncached case
because of the repo data deduplication logic.
The same issue was fixed for the snapshot status API in #68023

Author: original-brownbear

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/action/TransportMountSearchableSnapshotAction.java

@@ -10,7 +10,6 @@
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionListener;
-import org.elasticsearch.action.StepListener;
 import org.elasticsearch.action.admin.cluster.snapshots.restore.RestoreSnapshotRequest;
 import org.elasticsearch.action.admin.cluster.snapshots.restore.RestoreSnapshotResponse;
 import org.elasticsearch.action.support.ActionFilters;
@@ -26,6 +25,7 @@
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.util.concurrent.ListenableFuture;
 import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.indices.ShardLimitValidator;
 import org.elasticsearch.indices.SystemIndices;
@@ -185,9 +185,9 @@ protected void masterOperation(
         final Repository repository = repositoriesService.repository(repoName);
         SearchableSnapshots.getSearchableRepository(repository); // just check it's valid
 
-        final StepListener<RepositoryData> repositoryDataListener = new StepListener<>();
+        final ListenableFuture<RepositoryData> repositoryDataListener = new ListenableFuture<>();
         repository.getRepositoryData(repositoryDataListener);
-        repositoryDataListener.whenComplete(repoData -> {
+        repositoryDataListener.addListener(ActionListener.wrap(repoData -> {
             final Map<String, IndexId> indexIds = repoData.getIndices();
             if (indexIds.containsKey(indexName) == false) {
                 throw new IndexNotFoundException("index [" + indexName + "] not found in repository [" + repoName + "]");
@@ -280,6 +280,6 @@ protected void masterOperation(
                         .snapshotUuid(snapshotId.getUUID()),
                     listener
                 );
-        }, listener::onFailure);
+        }, listener::onFailure), threadPool.generic(), null);
     }
 }