Throw AssertionError when no master (#38432)

Today we throw a fatal `RuntimeException` if an exception occurs in
`getMasterName()`, and this includes the case where there is currently no
master. However, sometimes we call this method inside an `assertBusy()` in
order to allow for a cluster that is in the process of stabilising and electing
a master. The trouble is that `assertBusy()` only retries on an
`AssertionError` and not on a general `RuntimeException`, so the lack of a
master is immediately fatal.

This commit fixes the issue by asserting there is a master, triggering a retry
if there is not.

Fixes #38331

server/src/test/java/org/elasticsearch/cluster/SpecificMasterNodesIT.java

@@ -84,7 +84,6 @@ public void testSimpleOnlyMasterNodeElection() throws IOException {
             .execute().actionGet().getState().nodes().getMasterNode().getName(), equalTo(nextMasterEligibleNodeName));
     }
 
-    @AwaitsFix(bugUrl="https://github.com/elastic/elasticsearch/issues/38331")
     public void testElectOnlyBetweenMasterNodes() throws Exception {
         internalCluster().setBootstrapMasterNodeIndex(0);
         logger.info("--> start data node / non master node");

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

@@ -167,6 +167,7 @@
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.nullValue;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
@@ -1909,8 +1910,9 @@ public String getMasterName() {
     public String getMasterName(@Nullable String viaNode) {
         try {
             Client client = viaNode != null ? client(viaNode) : client();
-            ClusterState state = client.admin().cluster().prepareState().execute().actionGet().getState();
-            return state.nodes().getMasterNode().getName();
+            final DiscoveryNode masterNode = client.admin().cluster().prepareState().get().getState().nodes().getMasterNode();
+            assertNotNull(masterNode);
+            return masterNode.getName();
         } catch (Exception e) {
             logger.warn("Can't fetch cluster state", e);
             throw new RuntimeException("Can't get master node " + e.getMessage(), e);