Make DiscoveryNodes#mastersFirstStream deterministic (elastic#94948)

Today the `CoordinatorTests` test suite is not totally deterministic
because its behaviour depends on the iteration order of the JDK's
unordered collections which are not under the control of our test seed.

This commit makes `DiscoveryNodes#mastersFirstStream` yield the nodes in
a deterministic order, which addresses one such area of
unreproducibility. It's an ugly hack to do some extra work in production
just for the sake of tests, but we're only sorting at most a few hundred
elements here so it's not a huge deal.

Relates elastic#94946

server/src/main/java/org/elasticsearch/cluster/node/DiscoveryNodes.java

@@ -24,6 +24,7 @@
 import java.util.AbstractCollection;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
@@ -185,11 +186,15 @@ public Map<String, DiscoveryNode> getCoordinatingOnlyNodes() {
         return filteredNodes(nodes, n -> n.canContainData() == false && n.isMasterNode() == false && n.isIngestNode() == false);
     }
 
+    private static final Comparator<DiscoveryNode> MASTERS_FIRST_COMPARATOR
+    // Ugly hack: when https://github.com/elastic/elasticsearch/issues/94946 is fixed, remove the sorting by ephemeral ID here
+        = Comparator.<DiscoveryNode>comparingInt(n -> n.isMasterNode() ? 0 : 1).thenComparing(DiscoveryNode::getEphemeralId);
+
     /**
      * Returns a stream of all nodes, with master nodes at the front
      */
     public Stream<DiscoveryNode> mastersFirstStream() {
-        return Stream.concat(masterNodes.values().stream(), stream().filter(n -> n.isMasterNode() == false));
+        return nodes.values().stream().sorted(MASTERS_FIRST_COMPARATOR);
     }
 
     /**

server/src/test/java/org/elasticsearch/cluster/node/DiscoveryNodesTests.java

@@ -17,7 +17,6 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
-import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -160,9 +159,16 @@ public void testMastersFirst() {
         final List<DiscoveryNode> returnedNodes = discoBuilder.build().mastersFirstStream().toList();
         assertEquals(returnedNodes.size(), inputNodes.size());
         assertEquals(new HashSet<>(returnedNodes), new HashSet<>(inputNodes));
-        final List<DiscoveryNode> sortedNodes = new ArrayList<>(returnedNodes);
-        Collections.sort(sortedNodes, Comparator.comparing(n -> n.isMasterNode() == false));
-        assertEquals(sortedNodes, returnedNodes);
+
+        boolean mastersOk = true;
+        final var message = returnedNodes.toString();
+        for (final var discoveryNode : returnedNodes) {
+            if (discoveryNode.isMasterNode()) {
+                assertTrue(message, mastersOk);
+            } else {
+                mastersOk = false;
+            }
+        }
     }
 
     public void testDeltaListsMultipleNodes() {