Fix race condition in InternalTestCluster.getMasterName()

Prevents assertion errors when the master node is null after obtaining
the cluster state in the second call.  See PRs 127213 and 127891 for
details about recent changes to this method, and where the code used in
this commit was proposed.

Author: JeremyDahlgren

test/framework/src/main/java/org/elasticsearch/test/InternalTestCluster.java

@@ -28,6 +28,7 @@
 import org.elasticsearch.action.admin.indices.stats.CommonStatsFlags.Flag;
 import org.elasticsearch.action.support.DestructiveOperations;
 import org.elasticsearch.action.support.RefCountingRunnable;
+import org.elasticsearch.action.support.SubscribableListener;
 import org.elasticsearch.action.support.replication.TransportReplicationAction;
 import org.elasticsearch.client.internal.Client;
 import org.elasticsearch.cluster.ClusterName;
@@ -2038,13 +2039,12 @@ public String getMasterName(@Nullable String viaNode) {
             throw new AssertionError("Unable to get master name, no node found");
         }
         try {
-            ClusterServiceUtils.awaitClusterState(logger, state -> state.nodes().getMasterNode() != null, clusterService(viaNode));
-            final ClusterState state = client(viaNode).admin().cluster().prepareState(TEST_REQUEST_TIMEOUT).setLocal(true).get().getState();
-            final DiscoveryNode masterNode = state.nodes().getMasterNode();
-            if (masterNode == null) {
-                throw new AssertionError("Master is not stable but the method expects a stable master node");
-            }
-            return masterNode.getName();
+            final var masterNameListener = new SubscribableListener<String>();
+            return safeAwait(ClusterServiceUtils.addTemporaryStateListener(clusterService(viaNode), cs -> {
+                Optional.ofNullable(cs.nodes().getMasterNode())
+                    .ifPresent(masterNode -> masterNameListener.onResponse(masterNode.getName()));
+                return masterNameListener.isDone();
+            }).andThen(masterNameListener::addListener));
         } catch (Exception e) {
             logger.warn("Can't fetch cluster state", e);
             throw new RuntimeException("Can't get master node " + e.getMessage(), e);