[SPARK-21319][SQL] Fix memory leak in UnsafeExternalRowSorter.RowComparator (apache-spark-on-k8s#222)

* SPARK-21319: Fix memory leak in UnsafeExternalRowSorter.RowComparator

UnsafeExternalRowSorter.RowComparator contains references to the objects
backing the last arrays sorted. This causes memory leaks, since those
objects become deallocated but are still live.

We make sure to unset them.

* Symmetric fix for UnsafeKVExternalSorter

Author: j-baker

sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeExternalRowSorter.java

@@ -211,7 +211,10 @@ public int compare(Object baseObj1, long baseOff1, Object baseObj2, long baseOff
       // TODO: Why are the sizes -1?
       row1.pointTo(baseObj1, baseOff1, -1);
       row2.pointTo(baseObj2, baseOff2, -1);
-      return ordering.compare(row1, row2);
+      int comparison = ordering.compare(row1, row2);
+      row1.pointTo(null, 0L, -1);
+      row2.pointTo(null, 0L, -1);
+      return comparison;
     }
   }
 }

sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java

@@ -242,7 +242,10 @@ public int compare(Object baseObj1, long baseOff1, Object baseObj2, long baseOff
       // into the row.
       row1.pointTo(baseObj1, baseOff1 + 4, -1);
       row2.pointTo(baseObj2, baseOff2 + 4, -1);
-      return ordering.compare(row1, row2);
+      int comparison = ordering.compare(row1, row2);
+      row1.pointTo(null, 0L, -1);
+      row2.pointTo(null, 0L, -1);
+      return comparison;
     }
   }