[SPARK-11511][STREAMING] Fix NPE when an InputDStream is not used

Just ignored `InputDStream`s that have null `rememberDuration` in `DStreamGraph.getMaxInputStreamRememberDuration`.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #9476 from zsxwing/SPARK-11511.

streaming/src/main/scala/org/apache/spark/streaming/DStreamGraph.scala

@@ -167,7 +167,8 @@ final private[streaming] class DStreamGraph extends Serializable with Logging {
    * safe remember duration which can be used to perform cleanup operations.
    */
   def getMaxInputStreamRememberDuration(): Duration = {
-    inputStreams.map { _.rememberDuration }.maxBy { _.milliseconds }
+    // If an InputDStream is not used, its `rememberDuration` will be null and we can ignore them
+    inputStreams.map(_.rememberDuration).filter(_ != null).maxBy(_.milliseconds)
   }
 
   @throws(classOf[IOException])

streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala

@@ -780,6 +780,22 @@ class StreamingContextSuite extends SparkFunSuite with BeforeAndAfter with Timeo
       "Please don't use queueStream when checkpointing is enabled."))
   }
 
+  test("Creating an InputDStream but not using it should not crash") {
+    ssc = new StreamingContext(master, appName, batchDuration)
+    val input1 = addInputStream(ssc)
+    val input2 = addInputStream(ssc)
+    val output = new TestOutputStream(input2)
+    output.register()
+    val batchCount = new BatchCounter(ssc)
+    ssc.start()
+    // Just wait for completing 2 batches to make sure it triggers
+    // `DStream.getMaxInputStreamRememberDuration`
+    batchCount.waitUntilBatchesCompleted(2, 10000)
+    // Throw the exception if crash
+    ssc.awaitTerminationOrTimeout(1)
+    ssc.stop()
+  }
+
   def addInputStream(s: StreamingContext): DStream[Int] = {
     val input = (1 to 100).map(i => 1 to i)
     val inputStream = new TestInputStream(s, input, 1)