Allow Tomcat be destroyed regardless of exceptions

Update `TomcatWebServer` so that lifecycle exceptions are silently
swallowed when attempting shutdown. Prior to this commit it was
possible that a Tomcat instance might not be properly destroyed and
could leave non daemon threads running, which prevent the JVM from
exiting.

Fixes gh-16892

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatWebServer.java

@@ -122,6 +122,7 @@ public class TomcatWebServer implements WebServer {
 			}
 			catch (Exception ex) {
 				stopSilently();
+				destroySilently();
 				throw new WebServerException("Unable to start embedded Tomcat", ex);
 			}
 		}
@@ -242,6 +243,15 @@ public class TomcatWebServer implements WebServer {
 		}
 	}
 
+	private void destroySilently() {
+		try {
+			this.tomcat.destroy();
+		}
+		catch (LifecycleException ex) {
+			// Ignore
+		}
+	}
+
 	private void stopTomcat() throws LifecycleException {
 		if (Thread.currentThread()
 				.getContextClassLoader() instanceof TomcatEmbeddedWebappClassLoader) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactoryTests.java

@@ -523,6 +523,25 @@ public class TomcatServletWebServerFactoryTests
 		assertThat(response.getStatusCode()).isEqualTo(HttpStatus.OK);
 	}
 
+	@Test
+	public void exceptionThrownOnContextListenerDestroysServer() {
+		TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory(0) {
+			@Override
+			protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
+				try {
+					return super.getTomcatWebServer(tomcat);
+				}
+				finally {
+					assertThat(tomcat.getServer().getState())
+							.isEqualTo(LifecycleState.DESTROYED);
+				}
+			}
+		};
+		assertThatExceptionOfType(WebServerException.class)
+				.isThrownBy(() -> factory.getWebServer((context) -> context
+						.addListener(new FailingServletContextListener())));
+	}
+
 	@Override
 	protected JspServlet getJspServlet() throws ServletException {
 		Tomcat tomcat = ((TomcatWebServer) this.webServer).getTomcat();

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/server/AbstractServletWebServerFactoryTests.java

@@ -1401,6 +1401,15 @@ public abstract class AbstractServletWebServerFactoryTests {
 
 	}
 
+	public static class FailingServletContextListener implements ServletContextListener {
+
+		@Override
+		public void contextInitialized(ServletContextEvent sce) {
+			throw new FailingServletException();
+		}
+
+	}
+
 	private static class FailingServletException extends RuntimeException {
 
 		FailingServletException() {