[SPARK-7777][Streaming] Handle the case when there is no block in a batch

In the old implementation, if a batch has no block, `areWALRecordHandlesPresent` will be `true` and it will return `WriteAheadLogBackedBlockRDD`.

This PR handles this case by returning `WriteAheadLogBackedBlockRDD` or `BlockRDD` according to the configuration.

Author: zsxwing <zsxwing@gmail.com>

Closes #6372 from zsxwing/SPARK-7777 and squashes the following commits:

788f895 [zsxwing] Handle the case when there is no block in a batch

streaming/src/main/scala/org/apache/spark/streaming/dstream/ReceiverInputDStream.scala

@@ -73,27 +73,38 @@ abstract class ReceiverInputDStream[T: ClassTag](@transient ssc_ : StreamingCont
         val inputInfo = InputInfo(id, blockInfos.map(_.numRecords).sum)
         ssc.scheduler.inputInfoTracker.reportInfo(validTime, inputInfo)
 
-        // Are WAL record handles present with all the blocks
-        val areWALRecordHandlesPresent = blockInfos.forall { _.walRecordHandleOption.nonEmpty }
+        if (blockInfos.nonEmpty) {
+          // Are WAL record handles present with all the blocks
+          val areWALRecordHandlesPresent = blockInfos.forall { _.walRecordHandleOption.nonEmpty }
 
-        if (areWALRecordHandlesPresent) {
-          // If all the blocks have WAL record handle, then create a WALBackedBlockRDD
-          val isBlockIdValid = blockInfos.map { _.isBlockIdValid() }.toArray
-          val walRecordHandles = blockInfos.map { _.walRecordHandleOption.get }.toArray
-          new WriteAheadLogBackedBlockRDD[T](
-            ssc.sparkContext, blockIds, walRecordHandles, isBlockIdValid)
-        } else {
-          // Else, create a BlockRDD. However, if there are some blocks with WAL info but not others
-          // then that is unexpected and log a warning accordingly.
-          if (blockInfos.find(_.walRecordHandleOption.nonEmpty).nonEmpty) {
-            if (WriteAheadLogUtils.enableReceiverLog(ssc.conf)) {
-              logError("Some blocks do not have Write Ahead Log information; " +
-                "this is unexpected and data may not be recoverable after driver failures")
-            } else {
-              logWarning("Some blocks have Write Ahead Log information; this is unexpected")
+          if (areWALRecordHandlesPresent) {
+            // If all the blocks have WAL record handle, then create a WALBackedBlockRDD
+            val isBlockIdValid = blockInfos.map { _.isBlockIdValid() }.toArray
+            val walRecordHandles = blockInfos.map { _.walRecordHandleOption.get }.toArray
+            new WriteAheadLogBackedBlockRDD[T](
+              ssc.sparkContext, blockIds, walRecordHandles, isBlockIdValid)
+          } else {
+            // Else, create a BlockRDD. However, if there are some blocks with WAL info but not
+            // others then that is unexpected and log a warning accordingly.
+            if (blockInfos.find(_.walRecordHandleOption.nonEmpty).nonEmpty) {
+              if (WriteAheadLogUtils.enableReceiverLog(ssc.conf)) {
+                logError("Some blocks do not have Write Ahead Log information; " +
+                  "this is unexpected and data may not be recoverable after driver failures")
+              } else {
+                logWarning("Some blocks have Write Ahead Log information; this is unexpected")
+              }
             }
+            new BlockRDD[T](ssc.sc, blockIds)
+          }
+        } else {
+          // If no block is ready now, creating WriteAheadLogBackedBlockRDD or BlockRDD
+          // according to the configuration
+          if (WriteAheadLogUtils.enableReceiverLog(ssc.conf)) {
+            new WriteAheadLogBackedBlockRDD[T](
+              ssc.sparkContext, Array.empty, Array.empty, Array.empty)
+          } else {
+            new BlockRDD[T](ssc.sc, Array.empty)
           }
-          new BlockRDD[T](ssc.sc, blockIds)
         }
       }
     }

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

@@ -39,6 +39,7 @@ import org.apache.spark.storage.StorageLevel
 import org.apache.spark.streaming.scheduler.{StreamingListenerBatchCompleted, StreamingListener}
 import org.apache.spark.util.{ManualClock, Utils}
 import org.apache.spark.streaming.dstream.{InputDStream, ReceiverInputDStream}
+import org.apache.spark.streaming.rdd.WriteAheadLogBackedBlockRDD
 import org.apache.spark.streaming.receiver.Receiver
 
 class InputStreamsSuite extends TestSuiteBase with BeforeAndAfter {
@@ -105,6 +106,36 @@ class InputStreamsSuite extends TestSuiteBase with BeforeAndAfter {
     }
   }
 
+  test("socket input stream - no block in a batch") {
+    withTestServer(new TestServer()) { testServer =>
+      testServer.start()
+
+      withStreamingContext(new StreamingContext(conf, batchDuration)) { ssc =>
+        ssc.addStreamingListener(ssc.progressListener)
+
+        val batchCounter = new BatchCounter(ssc)
+        val networkStream = ssc.socketTextStream(
+          "localhost", testServer.port, StorageLevel.MEMORY_AND_DISK)
+        val outputBuffer = new ArrayBuffer[Seq[String]] with SynchronizedBuffer[Seq[String]]
+        val outputStream = new TestOutputStream(networkStream, outputBuffer)
+        outputStream.register()
+        ssc.start()
+
+        val clock = ssc.scheduler.clock.asInstanceOf[ManualClock]
+        clock.advance(batchDuration.milliseconds)
+
+        // Make sure the first batch is finished
+        if (!batchCounter.waitUntilBatchesCompleted(1, 30000)) {
+          fail("Timeout: cannot finish all batches in 30 seconds")
+        }
+
+        networkStream.generatedRDDs.foreach { case (_, rdd) =>
+          assert(!rdd.isInstanceOf[WriteAheadLogBackedBlockRDD[_]])
+        }
+      }
+    }
+  }
+
   test("binary records stream") {
     val testDir: File = null
     try {