return mutex to package-private, replace with test infra helper metho…

…d; slight comment improvement

server/src/main/java/org/elasticsearch/cluster/coordination/Coordinator.java

@@ -147,7 +147,7 @@ public class Coordinator extends AbstractLifecycleComponent implements ClusterSt
     private final MasterServiceTaskQueue<NodeLeftExecutor.Task> nodeLeftQueue;
     private final Supplier<CoordinationState.PersistedState> persistedStateSupplier;
     private final NoMasterBlockService noMasterBlockService;
-    public final Object mutex = new Object(); // public to allow tests to call methods that assert that the mutex is held
+    final Object mutex = new Object(); // package-private to allow tests to call methods that assert that the mutex is held
     private final SetOnce<CoordinationState> coordinationState = new SetOnce<>(); // initialized on start-up (see doStart)
     private volatile ClusterState applierState; // the state that should be exposed to the cluster state applier
 

server/src/main/java/org/elasticsearch/cluster/coordination/ElectionStrategy.java

@@ -118,6 +118,6 @@ public NodeEligibility nodeMayWinElection(ClusterState lastAcceptedState, Discov
             return new NodeEligibility(true, "");
         }
 
-        return new NodeEligibility(false, "node is ineligible for election, not part of the voting configuration");
+        return new NodeEligibility(false, "node is ineligible for election, not a voting node in the voting configuration");
     }
 }

test/framework/src/main/java/org/elasticsearch/cluster/coordination/AbstractCoordinatorTestCase.java

@@ -364,6 +364,12 @@ List<ClusterNode> addNodes(int newNodesCount) {
             return addedNodes;
         }
 
+        public static void becomeCandidate(ClusterNode node, String reason) {
+            synchronized (node.coordinator.mutex) {
+                node.coordinator.becomeCandidate(reason);
+            }
+        }
+
         public int size() {
             return clusterNodes.size();
         }