[SPARK-6405] Limiting the maximum Kryo buffer size to be 2GB.

Kryo buffers are backed by byte arrays, but primitive arrays can only be
up to 2GB in size. It is misleading to allow users to set buffers past
this size.

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

@@ -49,10 +49,20 @@ class KryoSerializer(conf: SparkConf)
   with Logging
   with Serializable {
 
-  private val bufferSize =
-    (conf.getDouble("spark.kryoserializer.buffer.mb", 0.064) * 1024 * 1024).toInt
+  private val bufferSizeMb: Double = conf.getDouble("spark.kryoserializer.buffer.mb", 0.064)
+  if (bufferSizeMb > 2048) {
+    throw new IllegalArgumentException("spark.kryoserializer.buffer.mb must be less than " +
+      s"2048 megabytes, got: + $bufferSizeMb mb.")
+  }
+  private val bufferSize = (bufferSizeMb * 1024 * 1024).toInt
+
+  val maxBufferSizeMb: Int = conf.getInt("spark.kryoserializer.buffer.max.mb", 64)
+  if (maxBufferSizeMb > 2048) {
+    throw new IllegalArgumentException("spark.kryoserializer.buffer.max.mb must be less than " +
+      s"2048 megabytes, got: + $maxBufferSizeMb mb.")
+  }
+  private val maxBufferSize = maxBufferSizeMb * 1024 * 1024
 
-  private val maxBufferSize = conf.getInt("spark.kryoserializer.buffer.max.mb", 64) * 1024 * 1024
   private val referenceTracking = conf.getBoolean("spark.kryo.referenceTracking", true)
   private val registrationRequired = conf.getBoolean("spark.kryo.registrationRequired", false)
   private val userRegistrator = conf.getOption("spark.kryo.registrator")