Remove RemoteClusterService.getConnections() method (#131948)

This method is only used in unit tests.
In an effort to reduce the API surface area and ease the analysis for
making RemoteClusterService multi-project aware, this change refactors
the test code to use other existing methods to achieve the same
functionality.

Relates: ES-11576

Author: JeremyDahlgren

server/src/main/java/org/elasticsearch/transport/RemoteClusterService.java

@@ -40,7 +40,6 @@
 import java.io.Closeable;
 import java.io.IOException;
 import java.util.Arrays;
-import java.util.Collection;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
@@ -653,10 +652,6 @@ public RemoteClusterClient getRemoteClusterClient(
         });
     }
 
-    Collection<RemoteClusterConnection> getConnections() {
-        return remoteClusters.values();
-    }
-
     static void registerRemoteClusterHandshakeRequestHandler(TransportService transportService) {
         transportService.registerRequestHandler(
             REMOTE_CLUSTER_HANDSHAKE_ACTION_NAME,

server/src/test/java/org/elasticsearch/action/search/TransportSearchActionTests.java

@@ -838,14 +838,18 @@ public void testCCSRemoteReduceWithDisconnectedRemoteClusters() throws Exception
             }
 
             CountDownLatch disconnectedLatch = new CountDownLatch(numDisconnectedClusters);
-            RemoteClusterServiceTests.addConnectionListener(remoteClusterService, new TransportConnectionListener() {
-                @Override
-                public void onNodeDisconnected(DiscoveryNode node, @Nullable Exception closeException) {
-                    if (disconnectedNodes.remove(node)) {
-                        disconnectedLatch.countDown();
+            RemoteClusterServiceTests.addConnectionListener(
+                remoteClusterService,
+                remoteIndicesByCluster.keySet(),
+                new TransportConnectionListener() {
+                    @Override
+                    public void onNodeDisconnected(DiscoveryNode node, @Nullable Exception closeException) {
+                        if (disconnectedNodes.remove(node)) {
+                            disconnectedLatch.countDown();
+                        }
                     }
                 }
-            });
+            );
             for (DiscoveryNode disconnectedNode : disconnectedNodes) {
                 service.addFailToSendNoConnectRule(disconnectedNode.getAddress());
             }
@@ -1149,14 +1153,18 @@ public void testCollectSearchShards() throws Exception {
             }
 
             CountDownLatch disconnectedLatch = new CountDownLatch(numDisconnectedClusters);
-            RemoteClusterServiceTests.addConnectionListener(remoteClusterService, new TransportConnectionListener() {
-                @Override
-                public void onNodeDisconnected(DiscoveryNode node, @Nullable Exception closeException) {
-                    if (disconnectedNodes.remove(node)) {
-                        disconnectedLatch.countDown();
+            RemoteClusterServiceTests.addConnectionListener(
+                remoteClusterService,
+                remoteIndicesByCluster.keySet(),
+                new TransportConnectionListener() {
+                    @Override
+                    public void onNodeDisconnected(DiscoveryNode node, @Nullable Exception closeException) {
+                        if (disconnectedNodes.remove(node)) {
+                            disconnectedLatch.countDown();
+                        }
                     }
                 }
-            });
+            );
             for (DiscoveryNode disconnectedNode : disconnectedNodes) {
                 service.addFailToSendNoConnectRule(disconnectedNode.getAddress());
             }

server/src/test/java/org/elasticsearch/transport/RemoteClusterServiceTests.java

@@ -1250,8 +1250,13 @@ public static void updateSkipUnavailable(RemoteClusterService service, String cl
         connection.setSkipUnavailable(skipUnavailable);
     }
 
-    public static void addConnectionListener(RemoteClusterService service, TransportConnectionListener listener) {
-        for (RemoteClusterConnection connection : service.getConnections()) {
+    public static void addConnectionListener(
+        RemoteClusterService service,
+        Set<String> clusterAliases,
+        TransportConnectionListener listener
+    ) {
+        for (final var clusterAlias : clusterAliases) {
+            final var connection = service.getRemoteClusterConnection(clusterAlias);
             ConnectionManager connectionManager = connection.getConnectionManager();
             connectionManager.addListener(listener);
         }