Reinstate testLeaderDisconnectionWithDisconnectEventDetectedQuickly (#95415)

This test now works for atomic-register coordinators, with a slightly
weaker time bound.

Relates #95409

Author: DaveCTurner

server/src/test/java/org/elasticsearch/cluster/coordination/AtomicRegisterCoordinatorTests.java

@@ -45,10 +45,17 @@
 
 @TestLogging(reason = "these tests do a lot of log-worthy things but we usually don't care", value = "org.elasticsearch:FATAL")
 public class AtomicRegisterCoordinatorTests extends CoordinatorTests {
+
     @Override
-    @AwaitsFix(bugUrl = "ES-5645")
     public void testLeaderDisconnectionWithDisconnectEventDetectedQuickly() {
-        // In this test the leader still has access to the register, therefore it is still considered as a leader.
+        // must allow a little extra time for the heartbeat to expire before the election can happen
+        testLeaderDisconnectionWithDisconnectEventDetectedQuickly(
+            Settings.builder()
+                .put(MAX_MISSED_HEARTBEATS.getKey(), 1)
+                .put(HEARTBEAT_FREQUENCY.getKey(), TimeValue.timeValueSeconds(1))
+                .build(),
+            TimeValue.timeValueSeconds(1)
+        );
     }
 
     @Override

server/src/test/java/org/elasticsearch/cluster/coordination/CoordinatorTests.java

@@ -219,7 +219,11 @@ public void testNodesJoinAfterStableCluster() {
     }
 
     public void testLeaderDisconnectionWithDisconnectEventDetectedQuickly() {
-        try (Cluster cluster = new Cluster(randomIntBetween(3, 5))) {
+        testLeaderDisconnectionWithDisconnectEventDetectedQuickly(Settings.EMPTY, TimeValue.ZERO);
+    }
+
+    protected void testLeaderDisconnectionWithDisconnectEventDetectedQuickly(Settings settings, TimeValue extraStabilisationTime) {
+        try (Cluster cluster = new Cluster(randomIntBetween(3, 5), true, settings)) {
             cluster.runRandomly();
             cluster.stabilise();
 
@@ -238,6 +242,8 @@ public void testLeaderDisconnectionWithDisconnectEventDetectedQuickly() {
                     + DEFAULT_CLUSTER_STATE_UPDATE_DELAY
                     // then wait for the followup reconfiguration
                     + DEFAULT_CLUSTER_STATE_UPDATE_DELAY
+                    // plus possibly some extra time according to the config
+                    + extraStabilisationTime.millis()
             );
             assertThat(cluster.getAnyLeader().getId(), not(equalTo(originalLeader.getId())));
         }