Ensure undertow session cookie path is correct

Explicitly set a ServletSessionConfig when the context path is "". This
is a workaround for UNDERTOW-350 to ensure that the session cookie path
is correctly set.

Fixes gh-2065

spring-boot/src/main/java/org/springframework/boot/context/embedded/undertow/UndertowEmbeddedServletContainerFactory.java

@@ -31,6 +31,7 @@ import io.undertow.servlet.api.DeploymentInfo;
 import io.undertow.servlet.api.DeploymentManager;
 import io.undertow.servlet.api.ListenerInfo;
 import io.undertow.servlet.api.MimeMapping;
+import io.undertow.servlet.api.ServletSessionConfig;
 import io.undertow.servlet.api.ServletStackTraces;
 import io.undertow.servlet.handlers.DefaultServlet;
 import io.undertow.servlet.util.ImmediateInstanceFactory;
@@ -68,6 +69,7 @@ import org.springframework.core.io.ResourceLoader;
 import org.springframework.util.Assert;
 import org.springframework.util.ResourceUtils;
 import org.springframework.util.SocketUtils;
+import org.springframework.util.StringUtils;
 import org.xnio.Options;
 import org.xnio.SslClientAuthMode;
 
@@ -329,6 +331,12 @@ public class UndertowEmbeddedServletContainerFactory extends
 		deployment.setServletStackTraces(ServletStackTraces.NONE);
 		deployment.setResourceManager(getDocumentRootResourceManager());
 		configureMimeMappings(deployment);
+		if (StringUtils.isEmpty(getContextPath())) {
+			// Work around UNDERTOW-350
+			ServletSessionConfig servletSessionConfig = new ServletSessionConfig();
+			servletSessionConfig.setPath("/");
+			deployment.setServletSessionConfig(servletSessionConfig);
+		}
 		for (UndertowDeploymentInfoCustomizer customizer : this.deploymentInfoCustomizers) {
 			customizer.customize(deployment);
 		}