[SPARK-4569] Rename 'externalSorting' in Aggregator

Hi all - I've renamed the unhelpfully named variable and added a comment clarifying what's actually happening.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #3666 from ilganeli/SPARK-4569B and squashes the following commits:

1810394 [Ilya Ganelin] [SPARK-4569] Rename 'externalSorting' in Aggregator
e2d2092 [Ilya Ganelin] [SPARK-4569] Rename 'externalSorting' in Aggregator
d7cefec [Ilya Ganelin] [SPARK-4569] Rename 'externalSorting' in Aggregator
5b3f39c [Ilya Ganelin] [SPARK-4569] Rename  in Aggregator

core/src/main/scala/org/apache/spark/Aggregator.scala

@@ -34,7 +34,9 @@ case class Aggregator[K, V, C] (
     mergeValue: (C, V) => C,
     mergeCombiners: (C, C) => C) {
 
-  private val externalSorting = SparkEnv.get.conf.getBoolean("spark.shuffle.spill", true)
+  // When spilling is enabled sorting will happen externally, but not necessarily with an 
+  // ExternalSorter. 
+  private val isSpillEnabled = SparkEnv.get.conf.getBoolean("spark.shuffle.spill", true)
 
   @deprecated("use combineValuesByKey with TaskContext argument", "0.9.0")
   def combineValuesByKey(iter: Iterator[_ <: Product2[K, V]]): Iterator[(K, C)] =
@@ -42,7 +44,7 @@ case class Aggregator[K, V, C] (
 
   def combineValuesByKey(iter: Iterator[_ <: Product2[K, V]],
                          context: TaskContext): Iterator[(K, C)] = {
-    if (!externalSorting) {
+    if (!isSpillEnabled) {
       val combiners = new AppendOnlyMap[K,C]
       var kv: Product2[K, V] = null
       val update = (hadValue: Boolean, oldValue: C) => {
@@ -71,9 +73,9 @@ case class Aggregator[K, V, C] (
     combineCombinersByKey(iter, null)
 
   def combineCombinersByKey(iter: Iterator[_ <: Product2[K, C]], context: TaskContext)
-      : Iterator[(K, C)] =
+    : Iterator[(K, C)] =
   {
-    if (!externalSorting) {
+    if (!isSpillEnabled) {
       val combiners = new AppendOnlyMap[K,C]
       var kc: Product2[K, C] = null
       val update = (hadValue: Boolean, oldValue: C) => {