Wildcard cluster names for cross cluster search

core/src/main/java/org/elasticsearch/action/search/RemoteClusterConnection.java

@@ -79,7 +79,7 @@
  * {@link RemoteClusterService#REMOTE_CONNECTIONS_PER_CLUSTER} until either all eligible nodes are exhausted or the maximum number of
  * connections per cluster has been reached.
  */
-public final class RemoteClusterConnection extends AbstractComponent implements TransportConnectionListener, Closeable {
+final class RemoteClusterConnection extends AbstractComponent implements TransportConnectionListener, Closeable {
 
     private final TransportService transportService;
     private final ConnectionProfile remoteProfile;

core/src/main/java/org/elasticsearch/action/search/RemoteClusterService.java

@@ -213,7 +213,7 @@ Map<String, List<String>> groupClusterIndices(String[] requestIndices, Predicate
             if (i >= 0) {
                 String remoteClusterName = index.substring(0, i);
                 List<String> clusters = clusterNameResolver.resolveClusterNames(remoteClusterNames, remoteClusterName);
-                if (clusters != null) {
+                if (clusters.isEmpty()) {
                     if (indexExists.test(index)) {
                         // we use : as a separator for remote clusters. might conflict if there is an index that is actually named
                         // remote_cluster_alias:index_name - for this case we fail the request. the user can easily change the cluster alias

core/src/main/java/org/elasticsearch/cluster/metadata/ClusterNameExpressionResolver.java

@@ -29,22 +29,33 @@
 import java.util.Set;
 import java.util.stream.Collectors;
 
-public class ClusterNameExpressionResolver extends AbstractComponent {
+/**
+ * Resolves cluster names from an expression. The expression must be the exact match of a cluster
+ * name or must be a wildcard expression.
+ */
+public final class ClusterNameExpressionResolver extends AbstractComponent {
 
     private final WildcardExpressionResolver wildcardResolver = new WildcardExpressionResolver();
 
     public ClusterNameExpressionResolver(Settings settings) {
         super(settings);
-        new WildcardExpressionResolver();
     }
 
+    /**
+     * Resolves the provided cluster expression to matching cluster names. This method only
+     * supports exact or wildcard matches.
+     *
+     * @param remoteClusters    the aliases for remote clusters
+     * @param clusterExpression the expressions that can be resolved to cluster names.
+     * @return the resolved cluster aliases.
+     */
     public List<String> resolveClusterNames(Set<String> remoteClusters, String clusterExpression) {
         if (remoteClusters.contains(clusterExpression)) {
             return Collections.singletonList(clusterExpression);
         } else if (Regex.isSimpleMatchPattern(clusterExpression)) {
             return wildcardResolver.resolve(remoteClusters, clusterExpression);
         } else {
-            return null;
+            return Collections.emptyList();
         }
     }
 
@@ -57,7 +68,7 @@ private List<String> resolve(Set<String> remoteClusters, String clusterExpressio
 
             Set<String> matches = matches(remoteClusters, clusterExpression);
             if (matches.isEmpty()) {
-                return null;
+                return Collections.emptyList();
             } else {
                 return new ArrayList<>(matches);
             }