[SPARK-50022][CORE][UI] Fix MasterPage to hide App UI links when UI…

… is disabled

### What changes were proposed in this pull request?

This PR aims to fix `MasterPage` to hide App UI links when UI is disabled.

Previously, the link leads the following errors if a user clicks it.
<img width="997" alt="Screenshot 2024-10-17 at 22 06 22" src="https://github.com/user-attachments/assets/e53ba01f-533f-4d42-a488-830afaf40efa">

### Why are the changes needed?

**1. PREPARATION**

```
$ cat conf/spark-defaults.conf
spark.ui.reverseProxy true
spark.ui.reverseProxyUrl http://localhost:8080

$ sbin/start-master.sh

$ sbin/start-worker.sh spark://$(hostname):7077

$ bin/spark-shell --master spark://$(hostname):7077 -c spark.ui.enabled=false
```

**2. BEFORE**

<img width="340" alt="Screenshot 2024-10-17 at 22 01 16" src="https://github.com/user-attachments/assets/3069e43d-ba8c-4d36-8101-65e10b420879">

**3. AFTER**

<img width="345" alt="Screenshot 2024-10-17 at 22 04 12" src="https://github.com/user-attachments/assets/b9feba47-90fb-4557-803c-94eaa8ce62e1">

### Does this PR introduce _any_ user-facing change?

The previous behavior shows HTTP 500 error.

### How was this patch tested?

Pass the CIs with a newly added test case.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #48535 from dongjoon-hyun/SPARK-50022.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

core/src/main/scala/org/apache/spark/deploy/master/ui/MasterPage.scala

@@ -316,7 +316,7 @@ private[ui] class MasterPage(parent: MasterWebUI) extends WebUIPage("") {
       </td>
       <td>
         {
-          if (app.isFinished) {
+          if (app.isFinished || app.desc.appUiUrl.isBlank()) {
             app.desc.name
           } else {
             <a href={UIUtils.makeHref(parent.master.reverseProxy,

core/src/test/scala/org/apache/spark/deploy/master/ui/ReadOnlyMasterWebUISuite.scala

@@ -17,12 +17,15 @@
 
 package org.apache.spark.deploy.master.ui
 
+import java.util.Date
+
 import scala.io.Source
 
 import jakarta.servlet.http.HttpServletResponse.{SC_METHOD_NOT_ALLOWED, SC_OK}
 import org.mockito.Mockito.{mock, when}
 
 import org.apache.spark.{SecurityManager, SparkConf, SparkFunSuite}
+import org.apache.spark.deploy.DeployMessages.{MasterStateResponse, RequestMasterState}
 import org.apache.spark.deploy.master._
 import org.apache.spark.deploy.master.ui.MasterWebUISuite._
 import org.apache.spark.internal.config.DECOMMISSION_ENABLED
@@ -33,6 +36,8 @@ import org.apache.spark.util.Utils
 
 class ReadOnlyMasterWebUISuite extends SparkFunSuite {
 
+  import org.apache.spark.deploy.DeployTestUtils._
+
   val conf = new SparkConf()
     .set(UI_KILL_ENABLED, false)
     .set(DECOMMISSION_ENABLED, false)
@@ -45,6 +50,14 @@ class ReadOnlyMasterWebUISuite extends SparkFunSuite {
   when(master.conf).thenReturn(conf)
   when(master.rpcEnv).thenReturn(rpcEnv)
   when(master.self).thenReturn(masterEndpointRef)
+  val desc1 = createAppDesc().copy(name = "WithUI")
+  val desc2 = desc1.copy(name = "WithoutUI", appUiUrl = "")
+  val app1 = new ApplicationInfo(new Date().getTime, "app1", desc1, new Date(), null, Int.MaxValue)
+  val app2 = new ApplicationInfo(new Date().getTime, "app2", desc2, new Date(), null, Int.MaxValue)
+  val state = new MasterStateResponse(
+    "host", 8080, None, Array.empty, Array(app1, app2), Array.empty,
+    Array.empty, Array.empty, RecoveryState.ALIVE)
+  when(masterEndpointRef.askSync[MasterStateResponse](RequestMasterState)).thenReturn(state)
   val masterWebUI = new MasterWebUI(master, 0)
 
   override def beforeAll(): Unit = {
@@ -97,4 +110,13 @@ class ReadOnlyMasterWebUISuite extends SparkFunSuite {
     assert(result.contains("Environment Variables"))
     assert(result.contains("<tr><td>SPARK_SCALA_VERSION</td><td>2.1"))
   }
+
+  test("SPARK-50022: Fix 'MasterPage' to hide App UI links when UI is disabled") {
+    val url = s"http://${Utils.localHostNameForURI()}:${masterWebUI.boundPort}/"
+    val conn = sendHttpRequest(url, "GET")
+    assert(conn.getResponseCode === SC_OK)
+    val result = Source.fromInputStream(conn.getInputStream).mkString
+    assert(result.contains("<a href=\"appUiUrl\">WithUI</a>"))
+    assert(result.contains("  WithoutUI\n"))
+  }
 }