[SPARK-9724] [WEB UI] Avoid unnecessary redirects in the Spark Web UI.

Author: Rohit Agarwal <rohita@qubole.com>

Closes #8014 from mindprince/SPARK-9724 and squashes the following commits:

a7af5ff [Rohit Agarwal] [SPARK-9724] [WEB UI] Inline attachPrefix and attachPrefixForRedirect. Fix logic of attachPrefix
8a977cd [Rohit Agarwal] [SPARK-9724] [WEB UI] Address review comments: Remove unneeded code, update scaladoc.
b257844 [Rohit Agarwal] [SPARK-9724] [WEB UI] Avoid unnecessary redirects in the Spark Web UI.

core/src/main/scala/org/apache/spark/ui/JettyUtils.scala

@@ -106,7 +106,11 @@ private[spark] object JettyUtils extends Logging {
       path: String,
       servlet: HttpServlet,
       basePath: String): ServletContextHandler = {
-    val prefixedPath = attachPrefix(basePath, path)
+    val prefixedPath = if (basePath == "" && path == "/") {
+      path
+    } else {
+      (basePath + path).stripSuffix("/")
+    }
     val contextHandler = new ServletContextHandler
     val holder = new ServletHolder(servlet)
     contextHandler.setContextPath(prefixedPath)
@@ -121,7 +125,7 @@ private[spark] object JettyUtils extends Logging {
       beforeRedirect: HttpServletRequest => Unit = x => (),
       basePath: String = "",
       httpMethods: Set[String] = Set("GET")): ServletContextHandler = {
-    val prefixedDestPath = attachPrefix(basePath, destPath)
+    val prefixedDestPath = basePath + destPath
     val servlet = new HttpServlet {
       override def doGet(request: HttpServletRequest, response: HttpServletResponse): Unit = {
         if (httpMethods.contains("GET")) {
@@ -246,11 +250,6 @@ private[spark] object JettyUtils extends Logging {
     val (server, boundPort) = Utils.startServiceOnPort[Server](port, connect, conf, serverName)
     ServerInfo(server, boundPort, collection)
   }
-
-  /** Attach a prefix to the given path, but avoid returning an empty path */
-  private def attachPrefix(basePath: String, relativePath: String): String = {
-    if (basePath == "") relativePath else (basePath + relativePath).stripSuffix("/")
-  }
 }
 
 private[spark] case class ServerInfo(

core/src/main/scala/org/apache/spark/ui/SparkUI.scala

@@ -64,11 +64,11 @@ private[spark] class SparkUI private (
     attachTab(new EnvironmentTab(this))
     attachTab(new ExecutorsTab(this))
     attachHandler(createStaticHandler(SparkUI.STATIC_RESOURCE_DIR, "/static"))
-    attachHandler(createRedirectHandler("/", "/jobs", basePath = basePath))
+    attachHandler(createRedirectHandler("/", "/jobs/", basePath = basePath))
     attachHandler(ApiRootResource.getServletHandler(this))
     // This should be POST only, but, the YARN AM proxy won't proxy POSTs
     attachHandler(createRedirectHandler(
-      "/stages/stage/kill", "/stages", stagesTab.handleKillRequest,
+      "/stages/stage/kill", "/stages/", stagesTab.handleKillRequest,
       httpMethods = Set("GET", "POST")))
   }
   initialize()