[SPARK-4882] Register PythonBroadcast with Kryo so that PySpark works with KryoSerializer

This PR fixes an issue where PySpark broadcast variables caused NullPointerExceptions if KryoSerializer was used.  The fix is to register PythonBroadcast with Kryo so that it's deserialized with a KryoJavaSerializer.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3831 from JoshRosen/SPARK-4882 and squashes the following commits:

0466c7a [Josh Rosen] Register PythonBroadcast with Kryo.
d5b409f [Josh Rosen] Enable registrationRequired, which would have caught this bug.
069d8a7 [Josh Rosen] Add failing test for SPARK-4882

core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala

@@ -26,6 +26,7 @@ import com.esotericsoftware.kryo.serializers.{JavaSerializer => KryoJavaSerializ
 import com.twitter.chill.{AllScalaRegistrar, EmptyScalaKryoInstantiator}
 
 import org.apache.spark._
+import org.apache.spark.api.python.PythonBroadcast
 import org.apache.spark.broadcast.HttpBroadcast
 import org.apache.spark.network.nio.{PutBlock, GotBlock, GetBlock}
 import org.apache.spark.scheduler.MapStatus
@@ -90,6 +91,7 @@ class KryoSerializer(conf: SparkConf)
     // Allow sending SerializableWritable
     kryo.register(classOf[SerializableWritable[_]], new KryoJavaSerializer())
     kryo.register(classOf[HttpBroadcast[_]], new KryoJavaSerializer())
+    kryo.register(classOf[PythonBroadcast], new KryoJavaSerializer())
 
     try {
       // Use the default classloader when calling the user registrator.

core/src/test/scala/org/apache/spark/api/python/PythonBroadcastSuite.scala

@@ -0,0 +1,60 @@
+/*
+ * 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.api.python
+
+import scala.io.Source
+
+import java.io.{PrintWriter, File}
+
+import org.scalatest.{Matchers, FunSuite}
+
+import org.apache.spark.{SharedSparkContext, SparkConf}
+import org.apache.spark.serializer.KryoSerializer
+import org.apache.spark.util.Utils
+
+// This test suite uses SharedSparkContext because we need a SparkEnv in order to deserialize
+// a PythonBroadcast:
+class PythonBroadcastSuite extends FunSuite with Matchers with SharedSparkContext {
+  test("PythonBroadcast can be serialized with Kryo (SPARK-4882)") {
+    val tempDir = Utils.createTempDir()
+    val broadcastedString = "Hello, world!"
+    def assertBroadcastIsValid(broadcast: PythonBroadcast): Unit = {
+      val source = Source.fromFile(broadcast.path)
+      val contents = source.mkString
+      source.close()
+      contents should be (broadcastedString)
+    }
+    try {
+      val broadcastDataFile: File = {
+        val file = new File(tempDir, "broadcastData")
+        val printWriter = new PrintWriter(file)
+        printWriter.write(broadcastedString)
+        printWriter.close()
+        file
+      }
+      val broadcast = new PythonBroadcast(broadcastDataFile.getAbsolutePath)
+      assertBroadcastIsValid(broadcast)
+      val conf = new SparkConf().set("spark.kryo.registrationRequired", "true")
+      val deserializedBroadcast =
+        Utils.clone[PythonBroadcast](broadcast, new KryoSerializer(conf).newInstance())
+      assertBroadcastIsValid(deserializedBroadcast)
+    } finally {
+      Utils.deleteRecursively(tempDir)
+    }
+  }
+}