Avoid using empty string as default preferred location

This is causing the TaskSetManager to try to schedule certain
tasks on the host "" (empty string). The intended semantics here,
however, is that the partition does not have preferred location,
and the TSM should schedule the corresponding task in accordance.

core/src/main/scala/org/apache/spark/rdd/CoalescedRDD.scala

@@ -35,11 +35,10 @@ import org.apache.spark.util.Utils
  * @param preferredLocation the preferred location for this partition
  */
 private[spark] case class CoalescedRDDPartition(
-                                  index: Int,
-                                  @transient rdd: RDD[_],
-                                  parentsIndices: Array[Int],
-                                  @transient preferredLocation: String = ""
-                                  ) extends Partition {
+    index: Int,
+    @transient rdd: RDD[_],
+    parentsIndices: Array[Int],
+    @transient preferredLocation: Option[String] = None) extends Partition {
   var parents: Seq[Partition] = parentsIndices.map(rdd.partitions(_))
 
   @throws(classOf[IOException])
@@ -55,9 +54,11 @@ private[spark] case class CoalescedRDDPartition(
    * @return locality of this coalesced partition between 0 and 1
    */
   def localFraction: Double = {
-    val loc = parents.count(p =>
-      rdd.context.getPreferredLocs(rdd, p.index).map(tl => tl.host).contains(preferredLocation))
-
+    val loc = parents.count { p =>
+      preferredLocation.exists { l =>
+        rdd.context.getPreferredLocs(rdd, p.index).map(_.host).contains(l)
+      }
+    }
     if (parents.size == 0) 0.0 else (loc.toDouble / parents.size.toDouble)
   }
 }
@@ -73,9 +74,9 @@ private[spark] case class CoalescedRDDPartition(
  * @param balanceSlack used to trade-off balance and locality. 1.0 is all locality, 0 is all balance
  */
 private[spark] class CoalescedRDD[T: ClassTag](
-                                      @transient var prev: RDD[T],
-                                      maxPartitions: Int,
-                                      balanceSlack: Double = 0.10)
+    @transient var prev: RDD[T],
+    maxPartitions: Int,
+    balanceSlack: Double = 0.10)
   extends RDD[T](prev.context, Nil) {  // Nil since we implement getDependencies
 
   override def getPartitions: Array[Partition] = {
@@ -113,7 +114,7 @@ private[spark] class CoalescedRDD[T: ClassTag](
    * @return the machine most preferred by split
    */
   override def getPreferredLocations(partition: Partition): Seq[String] = {
-    List(partition.asInstanceOf[CoalescedRDDPartition].preferredLocation)
+    partition.asInstanceOf[CoalescedRDDPartition].preferredLocation.toSeq
   }
 }
 
@@ -147,7 +148,7 @@ private[spark] class CoalescedRDD[T: ClassTag](
  *
  */
 
-private[spark] class PartitionCoalescer(maxPartitions: Int, prev: RDD[_], balanceSlack: Double) {
+private class PartitionCoalescer(maxPartitions: Int, prev: RDD[_], balanceSlack: Double) {
 
   def compare(o1: PartitionGroup, o2: PartitionGroup): Boolean = o1.size < o2.size
   def compare(o1: Option[PartitionGroup], o2: Option[PartitionGroup]): Boolean =
@@ -341,8 +342,11 @@ private[spark] class PartitionCoalescer(maxPartitions: Int, prev: RDD[_], balanc
   }
 }
 
-private[spark] case class PartitionGroup(prefLoc: String = "") {
+private case class PartitionGroup(prefLoc: Option[String] = None) {
   var arr = mutable.ArrayBuffer[Partition]()
-
   def size = arr.size
 }
+
+private object PartitionGroup {
+  def apply(prefLoc: String): PartitionGroup = PartitionGroup(Some(prefLoc))
+}