[SPARK-26891][YARN] Fixing flaky test in YarnSchedulerBackendSuite

The test "RequestExecutors reflects node blacklist and is serializable" is flaky because of multi threaded access of the mock task scheduler. For details check [Mockito FAQ (occasional exceptions like: WrongTypeOfReturnValue)](https://github.com/mockito/mockito/wiki/FAQ#is-mockito-thread-safe). So instead of mocking the task scheduler in the test TaskSchedulerImpl is simply subclassed.

This multithreaded access of the `nodeBlacklist()` method is coming from:
1) the unit test thread via calling of the method `prepareRequestExecutors()`
2) the `DriverEndpoint.onStart` which runs a periodic task that ends up calling this method

Existing unittest.

Closes #23801 from attilapiros/SPARK-26891.

Authored-by: “attilapiros” <piros.attila.zsolt@gmail.com>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>

resource-managers/yarn/src/test/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackendSuite.scala

@@ -17,6 +17,7 @@
 package org.apache.spark.scheduler.cluster
 
 import java.net.URL
+import java.util.concurrent.atomic.AtomicReference
 import javax.servlet.http.{HttpServlet, HttpServletRequest, HttpServletResponse}
 
 import scala.language.reflectiveCalls
@@ -32,15 +33,35 @@ import org.apache.spark.ui.TestFilter
 
 class YarnSchedulerBackendSuite extends SparkFunSuite with MockitoSugar with LocalSparkContext {
 
+  private var yarnSchedulerBackend: YarnSchedulerBackend = _
+
+  override def afterEach(): Unit = {
+    try {
+      if (yarnSchedulerBackend != null) {
+        yarnSchedulerBackend.stop()
+      }
+    } finally {
+      super.afterEach()
+    }
+  }
+
   test("RequestExecutors reflects node blacklist and is serializable") {
     sc = new SparkContext("local", "YarnSchedulerBackendSuite")
-    val sched = mock[TaskSchedulerImpl]
-    when(sched.sc).thenReturn(sc)
-    val yarnSchedulerBackend = new YarnSchedulerBackend(sched, sc) {
+    // Subclassing the TaskSchedulerImpl here instead of using Mockito. For details see SPARK-26891.
+    val sched = new TaskSchedulerImpl(sc) {
+      val blacklistedNodes = new AtomicReference[Set[String]]()
+
+      def setNodeBlacklist(nodeBlacklist: Set[String]): Unit = blacklistedNodes.set(nodeBlacklist)
+
+      override def nodeBlacklist(): Set[String] = blacklistedNodes.get()
+    }
+
+    val yarnSchedulerBackendExtended = new YarnSchedulerBackend(sched, sc) {
       def setHostToLocalTaskCount(hostToLocalTaskCount: Map[String, Int]): Unit = {
         this.hostToLocalTaskCount = hostToLocalTaskCount
       }
     }
+    yarnSchedulerBackend = yarnSchedulerBackendExtended
     val ser = new JavaSerializer(sc.conf).newInstance()
     for {
       blacklist <- IndexedSeq(Set[String](), Set("a", "b", "c"))
@@ -50,9 +71,9 @@ class YarnSchedulerBackendSuite extends SparkFunSuite with MockitoSugar with Loc
         Map("a" -> 1, "b" -> 2)
       )
     } {
-      yarnSchedulerBackend.setHostToLocalTaskCount(hostToLocalCount)
-      when(sched.nodeBlacklist()).thenReturn(blacklist)
-      val req = yarnSchedulerBackend.prepareRequestExecutors(numRequested)
+      yarnSchedulerBackendExtended.setHostToLocalTaskCount(hostToLocalCount)
+      sched.setNodeBlacklist(blacklist)
+      val req = yarnSchedulerBackendExtended.prepareRequestExecutors(numRequested)
       assert(req.requestedTotal === numRequested)
       assert(req.nodeBlacklist === blacklist)
       assert(req.hostToLocalTaskCount.keySet.intersect(blacklist).isEmpty)
@@ -75,9 +96,9 @@ class YarnSchedulerBackendSuite extends SparkFunSuite with MockitoSugar with Loc
     // Before adding the "YARN" filter, should get the code from the filter in SparkConf.
     assert(TestUtils.httpResponseCode(url) === HttpServletResponse.SC_BAD_GATEWAY)
 
-    val backend = new YarnSchedulerBackend(sched, sc) { }
+    yarnSchedulerBackend = new YarnSchedulerBackend(sched, sc) { }
 
-    backend.addWebUIFilter(classOf[TestFilter2].getName(),
+    yarnSchedulerBackend.addWebUIFilter(classOf[TestFilter2].getName(),
       Map("responseCode" -> HttpServletResponse.SC_NOT_ACCEPTABLE.toString), "")
 
     sc.ui.get.getHandlers.foreach { h =>