[WICKET-7074] write AJAX responses to a temporary buffer so that is A…

…JAX fails in rendering phase response is not polluted with an invalid XML

wicket-core/src/main/java/org/apache/wicket/ajax/AjaxRequestHandler.java

@@ -273,14 +273,16 @@ public final void respond(final IRequestCycle requestCycle)
 		final List<IResponseFilter> filters = Application.get()
 			.getRequestCycleSettings()
 			.getResponseFilters();
+		// KP-7074 we need to write to a temporary buffer, otherwise, if an exception is produced,
+		// and a redirect is done we will end up with a malformed XML
+		final StringResponse bodyResponse = new StringResponse();
+		update.writeTo(bodyResponse, encoding);
 		if (filters == null || filters.isEmpty())
 		{
-			update.writeTo(response, encoding);
+			response.write(bodyResponse.getBuffer());
 		}
 		else
 		{
-			final StringResponse bodyResponse = new StringResponse();
-			update.writeTo(bodyResponse, encoding);
 			CharSequence filteredResponse = invokeResponseFilters(bodyResponse, filters);
 			response.write(filteredResponse);
 		}

wicket-core/src/test/java/org/apache/wicket/ajax/AjaxRequestHandlerTest.java

@@ -34,11 +34,15 @@
 import org.apache.wicket.MarkupContainer;
 import org.apache.wicket.MockPageWithLink;
 import org.apache.wicket.MockPageWithLinkAndComponent;
+import org.apache.wicket.Page;
+import org.apache.wicket.RuntimeConfigurationType;
 import org.apache.wicket.ajax.markup.html.AjaxLink;
 import org.apache.wicket.event.IEvent;
 import org.apache.wicket.markup.IMarkupResourceStreamProvider;
 import org.apache.wicket.markup.html.WebComponent;
 import org.apache.wicket.markup.html.WebPage;
+import org.apache.wicket.markup.html.basic.Label;
+import org.apache.wicket.mock.MockApplication;
 import org.apache.wicket.request.IRequestHandler;
 import org.apache.wicket.request.cycle.IRequestCycleListener;
 import org.apache.wicket.request.cycle.RequestCycle;
@@ -47,7 +51,9 @@
 import org.apache.wicket.util.resource.StringResourceStream;
 import org.apache.wicket.util.tester.DiffUtil;
 import org.apache.wicket.util.tester.WicketTestCase;
+import org.apache.wicket.util.tester.WicketTester;
 import org.apache.wicket.util.time.Instants;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -306,6 +312,94 @@ void ajaxRedirectSetsNoCachingHeaders()
 		assertEquals("no-cache, no-store", tester.getLastResponse().getHeader("Cache-Control"));
 	}
 
+	private static class Wicket7074Application extends MockApplication
+	{
+
+		@Override
+		public Class<? extends Page> getHomePage()
+		{
+			return Wicket7074.class;
+		}
+
+
+		@Override
+		public RuntimeConfigurationType getConfigurationType() {
+			// this ise needed so that no filters are added
+			return RuntimeConfigurationType.DEPLOYMENT;
+		}
+	}
+
+	private static class WeirdException extends IllegalStateException {
+
+	}
+
+	private static class Wicket7074 extends WebPage implements IMarkupResourceStreamProvider
+	{
+		private static final long serialVersionUID = 1L;
+
+		private boolean generateError = false;
+
+		private final Component label;
+		/**
+		 * Construct.
+		 */
+		private Wicket7074()
+		{
+			setOutputMarkupId(true);
+
+			add(new AjaxLink<Void>("action")
+			{
+				private static final long serialVersionUID = 1L;
+
+				@Override
+				public void onClick(AjaxRequestTarget target)
+				{
+					// we simulate producing an exception in rendering phase
+					generateError = true;
+					target.add(label);
+				}
+			});
+
+			add(label = new Label("label", "error")
+			{
+				private static final long serialVersionUID = 1L;
+
+				@Override
+				protected void onBeforeRender()
+				{
+					super.onBeforeRender();
+					// we simulate producing an exception in rendering phase
+					if (generateError)
+					{
+						throw new WeirdException();
+					}
+				}
+			}.setOutputMarkupId(true));
+		}
+
+		@Override
+		public IResourceStream getMarkupResourceStream(MarkupContainer container,
+													   Class<?> containerClass)
+		{
+			return new StringResourceStream(
+					"<html><body><a wicket:id='action'>link1</a><br/><br/><br/><span wicket:id='label'>Link2</span></body></html>");
+		}
+	}
+
+	@Test
+	void ajaxResposeIsEmptyIfExceptionIsProducedDuringRenderingAndNoFiltersAreSet() {
+		Wicket7074Application application = new Wicket7074Application();
+		WicketTester tester = newWicketTester(application);
+		// page renders normally first time
+		tester.startPage(new Wicket7074());
+		// click on action to produce render phase error
+		assertThrows(WeirdException.class, () -> {
+			tester.clickLink("action");
+		});
+		// AJAX response is written now into a buffer => response should empty
+		assertTrue(tester.getLastResponseAsString().isEmpty());
+	}
+
 	/**
 	 * https://issues.apache.org/jira/browse/WICKET-6808
 	 */