Fix concurrency issue in TestDispatcherServlet

This change fixes a timing issue in tests using Spring MVC Tests where
assertions on an async result may not wait long enough.

The fix involves the use of a new callback in  MockAsyncContext that
allows tests to detect when an async dispatch has been invoked.

Issue: SPR-10838

Author: rstoyanchev

spring-test/src/main/java/org/springframework/mock/web/MockAsyncContext.java

@@ -28,7 +28,9 @@
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
+import org.apache.commons.logging.Log;
 import org.springframework.beans.BeanUtils;
+import org.springframework.util.Assert;
 import org.springframework.web.util.WebUtils;
 
 /**
@@ -49,13 +51,20 @@ public class MockAsyncContext implements AsyncContext {
 
 	private long timeout = 10 * 1000L;	// 10 seconds is Tomcat's default
 
+	private final List<Runnable> dispatchHandlers = new ArrayList<Runnable>();
+
 
 	public MockAsyncContext(ServletRequest request, ServletResponse response) {
 		this.request = (HttpServletRequest) request;
 		this.response = (HttpServletResponse) response;
 	}
 
 
+	public void addDispatchHandler(Runnable handler) {
+		Assert.notNull(handler);
+		this.dispatchHandlers.add(handler);
+	}
+
 	@Override
 	public ServletRequest getRequest() {
 		return this.request;
@@ -84,6 +93,9 @@ public void dispatch(String path) {
 	@Override
 	public void dispatch(ServletContext context, String path) {
 		this.dispatchedPath = path;
+		for (Runnable r : this.dispatchHandlers) {
+			r.run();
+		}
 	}
 
 	public String getDispatchedPath() {

spring-test/src/main/java/org/springframework/test/web/servlet/TestDispatcherServlet.java

@@ -25,13 +25,10 @@
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
+import org.springframework.mock.web.MockAsyncContext;
 import org.springframework.web.context.WebApplicationContext;
 import org.springframework.web.context.request.NativeWebRequest;
-import org.springframework.web.context.request.async.CallableProcessingInterceptorAdapter;
-import org.springframework.web.context.request.async.DeferredResult;
-import org.springframework.web.context.request.async.DeferredResultProcessingInterceptorAdapter;
-import org.springframework.web.context.request.async.WebAsyncManager;
-import org.springframework.web.context.request.async.WebAsyncUtils;
+import org.springframework.web.context.request.async.*;
 import org.springframework.web.servlet.DispatcherServlet;
 import org.springframework.web.servlet.HandlerExecutionChain;
 import org.springframework.web.servlet.ModelAndView;
@@ -50,44 +47,52 @@ final class TestDispatcherServlet extends DispatcherServlet {
 
 	private static final String KEY = TestDispatcherServlet.class.getName() + ".interceptor";
 
+
 	/**
 	 * Create a new instance with the given web application context.
 	 */
 	public TestDispatcherServlet(WebApplicationContext webApplicationContext) {
 		super(webApplicationContext);
 	}
 
+
 	@Override
 	protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 
-		CountDownLatch latch = registerAsyncInterceptors(request);
-		getMvcResult(request).setAsyncResultLatch(latch);
+		registerAsyncResultInterceptors(request);
 
 		super.service(request, response);
-	}
 
-	private CountDownLatch registerAsyncInterceptors(final HttpServletRequest servletRequest) {
-
-		final CountDownLatch asyncResultLatch = new CountDownLatch(1);
-
-		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(servletRequest);
+		if (request.isAsyncStarted()) {
+			addAsyncResultLatch(request);
+		}
+	}
 
+	private void registerAsyncResultInterceptors(final HttpServletRequest request) {
+		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
 		asyncManager.registerCallableInterceptor(KEY, new CallableProcessingInterceptorAdapter() {
 			@Override
-			public <T> void postProcess(NativeWebRequest request, Callable<T> task, Object value) throws Exception {
-				getMvcResult(servletRequest).setAsyncResult(value);
-				asyncResultLatch.countDown();
+			public <T> void postProcess(NativeWebRequest r, Callable<T> task, Object value) throws Exception {
+				getMvcResult(request).setAsyncResult(value);
 			}
 		});
 		asyncManager.registerDeferredResultInterceptor(KEY, new DeferredResultProcessingInterceptorAdapter() {
 			@Override
-			public <T> void postProcess(NativeWebRequest request, DeferredResult<T> result, Object value) throws Exception {
-				getMvcResult(servletRequest).setAsyncResult(value);
-				asyncResultLatch.countDown();
+			public <T> void postProcess(NativeWebRequest r, DeferredResult<T> result, Object value) throws Exception {
+				getMvcResult(request).setAsyncResult(value);
 			}
 		});
+	}
 
-		return asyncResultLatch;
+	private void addAsyncResultLatch(HttpServletRequest request) {
+		final CountDownLatch latch = new CountDownLatch(1);
+		((MockAsyncContext) request.getAsyncContext()).addDispatchHandler(new Runnable() {
+			@Override
+			public void run() {
+				latch.countDown();
+			}
+		});
+		getMvcResult(request).setAsyncResultLatch(latch);
 	}
 
 	protected DefaultMvcResult getMvcResult(ServletRequest request) {