Stop agents before exiting test cases, and reliably close all_*.log when agents exit

pom.xml

@@ -70,6 +70,8 @@
     <!-- https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ -->
     <jenkins.baseline>2.479</jenkins.baseline>
     <jenkins.version>${jenkins.baseline}.1</jenkins.version>
+    <!-- TODO until in parent -->
+    <jenkins-test-harness.version>2403.v256947ecb_c8a_</jenkins-test-harness.version>
     <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>
     <spotless.check.skip>false</spotless.check.skip>
   </properties>

src/main/java/com/cloudbees/jenkins/support/SupportLogHandler.java

@@ -234,6 +234,7 @@ private void setFile(File file) throws FileNotFoundException {
                 parentFile.mkdirs();
             }
 
+            StreamUtils.closeQuietly(null); // ensure class is loaded so close() can succeed
             boolean success = false;
             FileOutputStream fos = null;
             BufferedOutputStream bos = null;

src/main/java/com/cloudbees/jenkins/support/SupportPlugin.java

@@ -862,6 +862,15 @@
             // avoid double installation of the handler. JNLP agents can reconnect to the controller multiple times
             // and each connection gets a different RemoteClassLoader, so we need to evict them by class name,
             // not by their identity.
+            closeAll();
+            Runtime.getRuntime().addShutdownHook(new Thread(LogInitializer::closeAll, "close log handlers"));
+            LogHolder.AGENT_LOG_HANDLER.setLevel(level);
+            LogHolder.AGENT_LOG_HANDLER.setDirectory(new File(rootPath.getRemote(), SUPPORT_DIRECTORY_NAME), "all");
+            ROOT_LOGGER.addHandler(LogHolder.AGENT_LOG_HANDLER);
+            return null;
+        }
+
+        private static void closeAll() {
             for (Handler h : ROOT_LOGGER.getHandlers()) {
                 if (h.getClass()
                         .getName()
@@ -874,10 +883,6 @@
                     }
                 }
             }
-            LogHolder.AGENT_LOG_HANDLER.setLevel(level);
-            LogHolder.AGENT_LOG_HANDLER.setDirectory(new File(rootPath.getRemote(), SUPPORT_DIRECTORY_NAME), "all");
-            ROOT_LOGGER.addHandler(LogHolder.AGENT_LOG_HANDLER);
-            return null;
         }
     }