[SPARK-4313][WebUI][Yarn] Fix link issue of the executor thread dump page in yarn-cluster mode

In yarn-cluster mode, the Web UI is running behind a yarn proxy server. Some features(or bugs?) of yarn proxy server will break the links for thread dump.

1. Yarn proxy server will do http redirect internally, so if opening `http://example.com:8088/cluster/app/application_1415344371838_0012/executors`, it will fetch `http://example.com:8088/cluster/app/application_1415344371838_0012/executors/` and return the content but won't change the link in the browser. Then when a user clicks `Thread Dump`, it will jump to `http://example.com:8088/proxy/application_1415344371838_0012/threadDump/?executorId=2`. This is a wrong link. The correct link should be `http://example.com:8088/proxy/application_1415344371838_0012/executors/threadDump/?executorId=2`.

Adding "/" to the tab links will fix it.

2. Yarn proxy server has a bug about the URL encode/decode. When a user accesses `http://example.com:8088/proxy/application_1415344371838_0006/executors/threadDump/?executorId=%3Cdriver%3E`, the yarn proxy server will require `http://example.com:36429/executors/threadDump/?executorId=%25253Cdriver%25253E`. But Spark web server expects `http://example.com:36429/executors/threadDump/?executorId=%3Cdriver%3E`. Related to [YARN-2844](https://issues.apache.org/jira/browse/YARN-2844).

For now, it's a tricky approach to bypass the yarn bug.

![threaddump](https://cloud.githubusercontent.com/assets/1000778/4972567/d1ccba64-68ad-11e4-983e-257530cef35a.png)

Author: zsxwing <zsxwing@gmail.com>

Closes #3183 from zsxwing/SPARK-4313 and squashes the following commits:

3379ca8 [zsxwing] Encode the executor id in the thread dump link and update the comment
abfa063 [zsxwing] Fix link issue of the executor thread dump page in yarn-cluster mode

3 files changed, 18 insertions, 3 deletions

diff --git a/core/src/main/scala/org/apache/spark/ui/UIUtils.scala b/core/src/main/scala/org/apache/spark/ui/UIUtils.scala
index 3312671b6f..7bc1e24d58 100644
--- a/core/src/main/scala/org/apache/spark/ui/UIUtils.scala
+++ b/core/src/main/scala/org/apache/spark/ui/UIUtils.scala
@@ -175,7 +175,7 @@ private[spark] object UIUtils extends Logging {
     val shortAppName = if (appName.length < 36) appName else appName.take(32) + "..."
     val header = activeTab.headerTabs.map { tab =>
       <li class={if (tab == activeTab) "active" else ""}>
-        <a href={prependBaseUri(activeTab.basePath, "/" + tab.prefix)}>{tab.name}</a>
+        <a href={prependBaseUri(activeTab.basePath, "/" + tab.prefix + "/")}>{tab.name}</a>
       </li>
     }
 
diff --git a/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala b/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala
index e9c755e36f..c82730f524 100644
--- a/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala
+++ b/core/src/main/scala/org/apache/spark/ui/exec/ExecutorThreadDumpPage.scala
@@ -17,6 +17,7 @@
 
 package org.apache.spark.ui.exec
 
+import java.net.URLDecoder
 import javax.servlet.http.HttpServletRequest
 
 import scala.util.Try
@@ -29,7 +30,19 @@ private[ui] class ExecutorThreadDumpPage(parent: ExecutorsTab) extends WebUIPage
   private val sc = parent.sc
 
   def render(request: HttpServletRequest): Seq[Node] = {
-    val executorId = Option(request.getParameter("executorId")).getOrElse {
+    val executorId = Option(request.getParameter("executorId")).map {
+      executorId =>
+        // Due to YARN-2844, "<driver>" in the url will be encoded to "%25253Cdriver%25253E" when
+        // running in yarn-cluster mode. `request.getParameter("executorId")` will return
+        // "%253Cdriver%253E". Therefore we need to decode it until we get the real id.
+        var id = executorId
+        var decodedId = URLDecoder.decode(id, "UTF-8")
+        while (id != decodedId) {
+          id = decodedId
+          decodedId = URLDecoder.decode(id, "UTF-8")
+        }
+        id
+    }.getOrElse {
       return Text(s"Missing executorId parameter")
     }
     val time = System.currentTimeMillis()
diff --git a/core/src/main/scala/org/apache/spark/ui/exec/ExecutorsPage.scala b/core/src/main/scala/org/apache/spark/ui/exec/ExecutorsPage.scala
index 048fee3ce1..71b59b1d07 100644
--- a/core/src/main/scala/org/apache/spark/ui/exec/ExecutorsPage.scala
+++ b/core/src/main/scala/org/apache/spark/ui/exec/ExecutorsPage.scala
@@ -17,6 +17,7 @@
 
 package org.apache.spark.ui.exec
 
+import java.net.URLEncoder
 import javax.servlet.http.HttpServletRequest
 
 import scala.xml.Node
@@ -139,8 +140,9 @@ private[ui] class ExecutorsPage(
       </td>
       {
         if (threadDumpEnabled) {
+          val encodedId = URLEncoder.encode(info.id, "UTF-8")
           <td>
-            <a href={s"threadDump/?executorId=${info.id}"}>Thread Dump</a>
+            <a href={s"threadDump/?executorId=${encodedId}"}>Thread Dump</a>
           </td>
         } else {
           Seq.empty