[SPARK-3032][Shuffle] Fix key comparison integer overflow introduced sorting exception

Previous key comparison in `ExternalSorter` will get wrong sorting result or exception when key comparison overflows, details can be seen in [SPARK-3032](https://issues.apache.org/jira/browse/SPARK-3032). Here fix this and add a unit test to prove it.

Author: jerryshao <saisai.shao@intel.com>

Closes #2514 from jerryshao/SPARK-3032 and squashes the following commits:

6f3c302 [jerryshao] Improve the unit test according to comments
01911e6 [jerryshao] Change the test to show the contract violate exception
83acb38 [jerryshao] Minor changes according to comments
fa2a08f [jerryshao] Fix key comparison integer overflow introduced sorting exception

core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala

@@ -144,7 +144,7 @@ private[spark] class ExternalSorter[K, V, C](
     override def compare(a: K, b: K): Int = {
       val h1 = if (a == null) 0 else a.hashCode()
       val h2 = if (b == null) 0 else b.hashCode()
-      h1 - h2
+      if (h1 < h2) -1 else if (h1 == h2) 0 else 1
     }
   })
 

core/src/test/scala/org/apache/spark/util/collection/ExternalSorterSuite.scala

@@ -24,6 +24,8 @@ import org.scalatest.{PrivateMethodTester, FunSuite}
 import org.apache.spark._
 import org.apache.spark.SparkContext._
 
+import scala.util.Random
+
 class ExternalSorterSuite extends FunSuite with LocalSparkContext with PrivateMethodTester {
   private def createSparkConf(loadDefaults: Boolean): SparkConf = {
     val conf = new SparkConf(loadDefaults)
@@ -707,4 +709,57 @@ class ExternalSorterSuite extends FunSuite with LocalSparkContext with PrivateMe
       Some(agg), Some(new HashPartitioner(FEW_PARTITIONS)), None, None)
     assertDidNotBypassMergeSort(sorter4)
   }
+
+  test("sort without breaking sorting contracts") {
+    val conf = createSparkConf(true)
+    conf.set("spark.shuffle.memoryFraction", "0.01")
+    conf.set("spark.shuffle.manager", "sort")
+    sc = new SparkContext("local-cluster[1,1,512]", "test", conf)
+
+    // Using wrongOrdering to show integer overflow introduced exception.
+    val rand = new Random(100L)
+    val wrongOrdering = new Ordering[String] {
+      override def compare(a: String, b: String) = {
+        val h1 = if (a == null) 0 else a.hashCode()
+        val h2 = if (b == null) 0 else b.hashCode()
+        h1 - h2
+      }
+    }
+
+    val testData = Array.tabulate(100000) { _ => rand.nextInt().toString }
+
+    val sorter1 = new ExternalSorter[String, String, String](
+      None, None, Some(wrongOrdering), None)
+    val thrown = intercept[IllegalArgumentException] {
+      sorter1.insertAll(testData.iterator.map(i => (i, i)))
+      sorter1.iterator
+    }
+
+    assert(thrown.getClass() === classOf[IllegalArgumentException])
+    assert(thrown.getMessage().contains("Comparison method violates its general contract"))
+    sorter1.stop()
+
+    // Using aggregation and external spill to make sure ExternalSorter using
+    // partitionKeyComparator.
+    def createCombiner(i: String) = ArrayBuffer(i)
+    def mergeValue(c: ArrayBuffer[String], i: String) = c += i
+    def mergeCombiners(c1: ArrayBuffer[String], c2: ArrayBuffer[String]) = c1 ++= c2
+
+    val agg = new Aggregator[String, String, ArrayBuffer[String]](
+      createCombiner, mergeValue, mergeCombiners)
+
+    val sorter2 = new ExternalSorter[String, String, ArrayBuffer[String]](
+      Some(agg), None, None, None)
+    sorter2.insertAll(testData.iterator.map(i => (i, i)))
+
+    // To validate the hash ordering of key
+    var minKey = Int.MinValue
+    sorter2.iterator.foreach { case (k, v) =>
+      val h = k.hashCode()
+      assert(h >= minKey)
+      minKey = h
+    }
+
+    sorter2.stop()
+ }
 }