Don't add runtime shutdown hook till app with hook enabled is run

Previously, the runtime shutdown hook was added as soon as a
shutdown handler was registered. This causes a memory leak in a war
deployment when the application is undeployed as
LoggingApplicationListener always registers a shutdown handler
so the runtime shutdown hook was always registered.

This commit updates the shutdown hook so that the runtime shutdown
hook is only allowed to be added once run() has been called on a
SpringApplication with the shutdown hook enabled. This approach
allows the registerShutdownHook flag on SpringApplication to be a
central point of control for the registration of the runtime shutdown
hook. When that flag is set to false, for example by
SpringBootServletInitializer, the runtime shutdown hook will not
be registered, irrespective of whether other code uses the public
API to add a shutdown handler.

An alternative approach of stopping LoggingApplicationListener from
adding its shutdown handler – for example by adding
logging.register-shutdown-hook=false to the environment – was
considered. This approach was rejected in favor of the centralized
approach described above as it would require every caller that adds
a shutdown handler to deal with the problem.

Closes gh-37096

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -296,6 +296,9 @@ private Optional<Class<?>> findMainClass(Stream<StackFrame> stack) {
 	 * @return a running {@link ApplicationContext}
 	 */
 	public ConfigurableApplicationContext run(String... args) {
+		if (this.registerShutdownHook) {
+			SpringApplication.shutdownHook.enableShutdowHookAddition();
+		}
 		long startTime = System.nanoTime();
 		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
 		ConfigurableApplicationContext context = null;

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplicationShutdownHook.java

@@ -62,12 +62,18 @@ class SpringApplicationShutdownHook implements Runnable {
 
 	private final AtomicBoolean shutdownHookAdded = new AtomicBoolean();
 
+	private volatile boolean shutdownHookAdditionEnabled = false;
+
 	private boolean inProgress;
 
 	SpringApplicationShutdownHandlers getHandlers() {
 		return this.handlers;
 	}
 
+	void enableShutdowHookAddition() {
+		this.shutdownHookAdditionEnabled = true;
+	}
+
 	void registerApplicationContext(ConfigurableApplicationContext context) {
 		addRuntimeShutdownHookIfNecessary();
 		synchronized (SpringApplicationShutdownHook.class) {
@@ -78,7 +84,7 @@ void registerApplicationContext(ConfigurableApplicationContext context) {
 	}
 
 	private void addRuntimeShutdownHookIfNecessary() {
-		if (this.shutdownHookAdded.compareAndSet(false, true)) {
+		if (this.shutdownHookAdditionEnabled && this.shutdownHookAdded.compareAndSet(false, true)) {
 			addRuntimeShutdownHook();
 		}
 	}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationShutdownHookTests.java

@@ -51,6 +51,7 @@ class SpringApplicationShutdownHookTests {
 	@Test
 	void shutdownHookIsNotAddedUntilContextIsRegistered() {
 		TestSpringApplicationShutdownHook shutdownHook = new TestSpringApplicationShutdownHook();
+		shutdownHook.enableShutdowHookAddition();
 		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isFalse();
 		ConfigurableApplicationContext context = new GenericApplicationContext();
 		shutdownHook.registerApplicationContext(context);
@@ -60,12 +61,25 @@ void shutdownHookIsNotAddedUntilContextIsRegistered() {
 	@Test
 	void shutdownHookIsNotAddedUntilHandlerIsRegistered() {
 		TestSpringApplicationShutdownHook shutdownHook = new TestSpringApplicationShutdownHook();
+		shutdownHook.enableShutdowHookAddition();
 		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isFalse();
 		shutdownHook.getHandlers().add(() -> {
 		});
 		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isTrue();
 	}
 
+	@Test
+	void shutdownHookIsNotAddedUntilAdditionIsEnabled() {
+		TestSpringApplicationShutdownHook shutdownHook = new TestSpringApplicationShutdownHook();
+		shutdownHook.getHandlers().add(() -> {
+		});
+		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isFalse();
+		shutdownHook.enableShutdowHookAddition();
+		shutdownHook.getHandlers().add(() -> {
+		});
+		assertThat(shutdownHook.isRuntimeShutdownHookAdded()).isTrue();
+	}
+
 	@Test
 	void runClosesContextsBeforeRunningHandlerActions() {
 		TestSpringApplicationShutdownHook shutdownHook = new TestSpringApplicationShutdownHook();