Fix hang caused by race condition in test for reset of kept-alive connection

Previously, a race between the server starting to reject requests
on a kept-alive connection and the request reaching the blocking
servlet could result in a response never being sent.

This commit updates the test to disable blocking once graceful
shutdown with an in-flight request has being. Awaitility is then used
to make a request on an idle kept-alive connection until it fails
due to the connection reset. This may not happen immediately due to
the aforementioned race.

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

@@ -66,6 +66,7 @@ import org.apache.jasper.servlet.JspServlet;
 import org.apache.tomcat.JarScanFilter;
 import org.apache.tomcat.JarScanType;
 import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
+import org.awaitility.Awaitility;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 import org.mockito.ArgumentCaptor;
@@ -599,17 +600,21 @@ class TomcatServletWebServerFactoryTests extends AbstractServletWebServerFactory
 		assertThat(keepAliveRequest.get()).isInstanceOf(HttpResponse.class);
 		Future<Object> request = initiateGetRequest(port, "/blocking");
 		blockingServlet.awaitQueue();
+		blockingServlet.setBlocking(false);
 		this.webServer.shutDownGracefully((result) -> {
 		});
-		Future<Object> idleConnectionRequest = initiateGetRequest(httpClient, port, "/blocking");
-		blockingServlet.admitOne();
-		Object response = request.get();
-		assertThat(response).isInstanceOf(HttpResponse.class);
-		Object idleConnectionRequestResult = idleConnectionRequest.get();
+		Object idleConnectionRequestResult = Awaitility.await().until(() -> {
+			Future<Object> idleConnectionRequest = initiateGetRequest(httpClient, port, "/blocking");
+			Object result = idleConnectionRequest.get();
+			return result;
+		}, (result) -> result instanceof Exception);
 		assertThat(idleConnectionRequestResult).isInstanceOfAny(SocketException.class, NoHttpResponseException.class);
 		if (idleConnectionRequestResult instanceof SocketException) {
 			assertThat((SocketException) idleConnectionRequestResult).hasMessage("Connection reset");
 		}
+		blockingServlet.admitOne();
+		Object response = request.get();
+		assertThat(response).isInstanceOf(HttpResponse.class);
 		this.webServer.stop();
 	}
 

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

@@ -1445,22 +1445,26 @@ public abstract class AbstractServletWebServerFactoryTests {
 
 		private final BlockingQueue<CyclicBarrier> barriers = new ArrayBlockingQueue<>(10);
 
+		protected volatile boolean blocking = true;
+
 		public BlockingServlet() {
 
 		}
 
 		@Override
 		protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
-			CyclicBarrier barrier = new CyclicBarrier(2);
-			this.barriers.add(barrier);
-			try {
-				barrier.await();
-			}
-			catch (InterruptedException ex) {
-				Thread.currentThread().interrupt();
-			}
-			catch (BrokenBarrierException ex) {
-				throw new ServletException(ex);
+			if (this.blocking) {
+				CyclicBarrier barrier = new CyclicBarrier(2);
+				this.barriers.add(barrier);
+				try {
+					barrier.await();
+				}
+				catch (InterruptedException ex) {
+					Thread.currentThread().interrupt();
+				}
+				catch (BrokenBarrierException ex) {
+					throw new ServletException(ex);
+				}
 			}
 		}
 
@@ -1491,6 +1495,10 @@ public abstract class AbstractServletWebServerFactoryTests {
 			}
 		}
 
+		public void setBlocking(boolean blocking) {
+			this.blocking = blocking;
+		}
+
 	}
 
 	static class BlockingAsyncServlet extends HttpServlet {