[SPARK-31422][CORE] Fix NPE when BlockManagerSource is used after Blo…

…ckManagerMaster stops

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

This PR (SPARK-31422) aims to return empty result in order to avoid `NullPointerException` at `getStorageStatus` and `getMemoryStatus` which happens after `BlockManagerMaster` stops. The empty result is consistent with the current status of `SparkContext` because `BlockManager` and `BlockManagerMaster` is already stopped.

### Why are the changes needed?

In `SparkEnv.stop`, the following stop sequence is used and `metricsSystem.stop` invokes `sink.stop`.
```
blockManager.master.stop()
metricsSystem.stop() --> sinks.foreach(_.stop)
```

However, some sink can invoke `BlockManagerSource` and ends up with `NullPointerException` because `BlockManagerMaster` is already stopped and `driverEndpoint` became `null`.
```
java.lang.NullPointerException
at org.apache.spark.storage.BlockManagerMaster.getStorageStatus(BlockManagerMaster.scala:170)
at org.apache.spark.storage.BlockManagerSource$$anonfun$10.apply(BlockManagerSource.scala:63)
at org.apache.spark.storage.BlockManagerSource$$anonfun$10.apply(BlockManagerSource.scala:63)
at org.apache.spark.storage.BlockManagerSource$$anon$1.getValue(BlockManagerSource.scala:31)
at org.apache.spark.storage.BlockManagerSource$$anon$1.getValue(BlockManagerSource.scala:30)
```

Since `SparkContext` registers and forgets `BlockManagerSource` without deregistering, we had better avoid `NullPointerException` inside `BlockManagerMaster` preventively.
```scala
_env.metricsSystem.registerSource(new BlockManagerSource(_env.blockManager))
```

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

Yes. This will remove NPE for the users who uses `BlockManagerSource`.

### How was this patch tested?

Pass the Jenkins with the newly added test cases.

Closes apache#28187 from dongjoon-hyun/SPARK-31422.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

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("SPARK-31422: getMemoryStatus should not fail after BlockManagerMaster stops") {
+    val bmm = new BlockManagerMaster(null, null, new SparkConf, true)
+    assertTrue(bmm.getMemoryStatus.isEmpty)
+  }
+
+  test("SPARK-31422: getStorageStatus should not fail after BlockManagerMaster stops") {
+    val bmm = new BlockManagerMaster(null, null, new SparkConf, true)
+    assertTrue(bmm.getStorageStatus.isEmpty)
+  }
+}