[SPARK-46899][CORE][FOLLOWUP] Enable /workers/kill if `spark.decomm…

…ission.enabled=true`

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

This PR aims to re-enable `/workers/kill` API if `spark.decommission.enabled=true` as a follow-up of
- apache#44926

### Why are the changes needed?

To address this review comment in order to prevent a regression.
- apache#44926 (review)

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

No, this will recover the previous feature.

### How was this patch tested?

Manual review.

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

No.

Closes apache#45010 from dongjoon-hyun/SPARK-46899-2.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Kent Yao <yao@apache.org>

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

@@ -43,7 +43,7 @@ class MasterWebUI(
 
   val masterEndpointRef = master.self
   val killEnabled = master.conf.get(UI_KILL_ENABLED)
-  val decommissionDisabled = !master.conf.get(DECOMMISSION_ENABLED)
+  val decommissionEnabled = master.conf.get(DECOMMISSION_ENABLED)
   val decommissionAllowMode = master.conf.get(MASTER_UI_DECOMMISSION_ALLOW_MODE)
 
   initialize()
@@ -61,11 +61,13 @@ class MasterWebUI(
         "/app/kill", "/", masterPage.handleAppKillRequest, httpMethods = Set("POST")))
       attachHandler(createRedirectHandler(
         "/driver/kill", "/", masterPage.handleDriverKillRequest, httpMethods = Set("POST")))
+    }
+    if (decommissionEnabled) {
       attachHandler(createServletHandler("/workers/kill", new HttpServlet {
         override def doPost(req: HttpServletRequest, resp: HttpServletResponse): Unit = {
           val hostnames: Seq[String] = Option(req.getParameterValues("host"))
             .getOrElse(Array[String]()).toImmutableArraySeq
-          if (decommissionDisabled || !isDecommissioningRequestAllowed(req)) {
+          if (!isDecommissioningRequestAllowed(req)) {
             resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED)
           } else {
             val removedWorkers = masterEndpointRef.askSync[Integer](

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

@@ -24,13 +24,16 @@ import org.mockito.Mockito.{mock, when}
 import org.apache.spark.{SecurityManager, SparkConf, SparkFunSuite}
 import org.apache.spark.deploy.master._
 import org.apache.spark.deploy.master.ui.MasterWebUISuite._
+import org.apache.spark.internal.config.DECOMMISSION_ENABLED
 import org.apache.spark.internal.config.UI.UI_KILL_ENABLED
 import org.apache.spark.rpc.{RpcEndpointRef, RpcEnv}
 import org.apache.spark.util.Utils
 
 class ReadOnlyMasterWebUISuite extends SparkFunSuite {
 
-  val conf = new SparkConf().set(UI_KILL_ENABLED, false)
+  val conf = new SparkConf()
+    .set(UI_KILL_ENABLED, false)
+    .set(DECOMMISSION_ENABLED, false)
   val securityMgr = new SecurityManager(conf)
   val rpcEnv = mock(classOf[RpcEnv])
   val master = mock(classOf[Master])