[SPARK-31422][CORE] Fix NPE when BlockManagerSource is used after BlockManagerMaster stops

Author: dongjoon-hyun

core/src/main/scala/org/apache/spark/storage/BlockManagerMaster.scala

@@ -167,10 +167,12 @@ class BlockManagerMaster(
    * amount of remaining memory.
    */
   def getMemoryStatus: Map[BlockManagerId, (Long, Long)] = {
+    if (driverEndpoint == null) return Map.empty
     driverEndpoint.askSync[Map[BlockManagerId, (Long, Long)]](GetMemoryStatus)
   }
 
   def getStorageStatus: Array[StorageStatus] = {
+    if (driverEndpoint == null) return Array.empty
     driverEndpoint.askSync[Array[StorageStatus]](GetStorageStatus)
   }
 

core/src/test/scala/org/apache/spark/storage/BlockManagerMasterSuite.scala

@@ -0,0 +1,35 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.storage
+
+import org.junit.Assert.assertTrue
+
+import org.apache.spark.{SparkConf, SparkFunSuite}
+
+class BlockManagerMasterSuite extends SparkFunSuite {
+
+  test("getMemoryStatus should not fail after BlockManagerMaster stops") {
+    val bmm = new BlockManagerMaster(null, null, new SparkConf, true)
+    assertTrue(bmm.getMemoryStatus.isEmpty)
+  }
+
+  test("getStorageStatus should not fail after BlockManagerMaster stops") {
+    val bmm = new BlockManagerMaster(null, null, new SparkConf, true)
+    assertTrue(bmm.getStorageStatus.isEmpty)
+  }
+}