Verify that ClusterFormationFailureHelper emits logs (#92743)

Today `ClusterFormationFailureHelperTests` check that the log-emitting
method is called but makes no assertions about logs actually being
emitted. This commit adds the missing assertions.

Relates #92741

Author: DaveCTurner

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

@@ -7,6 +7,7 @@
  */
 package org.elasticsearch.cluster.coordination;
 
+import org.apache.logging.log4j.Level;
 import org.elasticsearch.Version;
 import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.ClusterState;
@@ -25,6 +26,7 @@
 import org.elasticsearch.monitor.StatusInfo;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.test.EqualsHashCodeTestUtils;
+import org.elasticsearch.test.MockLogAppender;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -54,6 +56,9 @@ public class ClusterFormationFailureHelperTests extends ESTestCase {
 
     private static final ElectionStrategy electionStrategy = ElectionStrategy.DEFAULT_INSTANCE;
 
+    // Hard-coding the class name here because it is also mentioned in the troubleshooting docs, so should not be renamed without care.
+    private static final String LOGGER_NAME = "org.elasticsearch.cluster.coordination.ClusterFormationFailureHelper";
+
     public void testScheduling() {
         final long expectedDelayMillis;
         final Settings.Builder settingsBuilder = Settings.builder();
@@ -103,16 +108,23 @@ public void testScheduling() {
         final long startTimeMillis = deterministicTaskQueue.getCurrentTimeMillis();
         clusterFormationFailureHelper.start();
 
-        while (warningCount.get() == 0) {
-            assertTrue(clusterFormationFailureHelper.isRunning());
-            if (deterministicTaskQueue.hasRunnableTasks()) {
-                deterministicTaskQueue.runRandomTask();
-            } else {
-                deterministicTaskQueue.advanceTime();
+        var mockLogAppender = new MockLogAppender();
+        mockLogAppender.addExpectation(
+            new MockLogAppender.SeenEventExpectation("master not discovered", LOGGER_NAME, Level.WARN, "master not discovered")
+        );
+        try (var ignored = mockLogAppender.capturing(ClusterFormationFailureHelper.class)) {
+            while (warningCount.get() == 0) {
+                assertTrue(clusterFormationFailureHelper.isRunning());
+                if (deterministicTaskQueue.hasRunnableTasks()) {
+                    deterministicTaskQueue.runRandomTask();
+                } else {
+                    deterministicTaskQueue.advanceTime();
+                }
             }
         }
         assertThat(warningCount.get(), is(1L));
         assertThat(deterministicTaskQueue.getCurrentTimeMillis() - startTimeMillis, is(expectedDelayMillis));
+        mockLogAppender.assertAllExpectationsMatched();
 
         while (warningCount.get() < 5) {
             assertTrue(clusterFormationFailureHelper.isRunning());

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

@@ -8,11 +8,16 @@
 package org.elasticsearch.test;
 
 import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.appender.AbstractAppender;
 import org.apache.logging.log4j.core.config.Property;
+import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.regex.Regex;
+import org.elasticsearch.core.Releasable;
 
+import java.util.Arrays;
 import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.regex.Pattern;
@@ -208,4 +213,18 @@ private static String getLoggerName(String name) {
         }
         return COMMON_PREFIX + name;
     }
+
+    public Releasable capturing(Class<?>... classes) {
+        start();
+        final var loggers = Arrays.stream(classes).map(LogManager::getLogger).toArray(Logger[]::new);
+        for (final var logger : loggers) {
+            Loggers.addAppender(logger, this);
+        }
+        return () -> {
+            for (final var logger : loggers) {
+                Loggers.removeAppender(logger, this);
+            }
+            stop();
+        };
+    }
 }