[SPARK-23697][CORE] LegacyAccumulatorWrapper should define isZero correctly

## What changes were proposed in this pull request?

It's possible that Accumulators of Spark 1.x may no longer work with Spark 2.x. This is because `LegacyAccumulatorWrapper.isZero` may return wrong answer if `AccumulableParam` doesn't define equals/hashCode.

This PR fixes this by using reference equality check in `LegacyAccumulatorWrapper.isZero`.

## How was this patch tested?

a new test

Author: Wenchen Fan <wenchen@databricks.com>

Closes #21229 from cloud-fan/accumulator.

core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala

@@ -486,7 +486,9 @@ class LegacyAccumulatorWrapper[R, T](
     param: org.apache.spark.AccumulableParam[R, T]) extends AccumulatorV2[T, R] {
   private[spark] var _value = initialValue  // Current value on driver
 
-  override def isZero: Boolean = _value == param.zero(initialValue)
+  @transient private lazy val _zero = param.zero(initialValue)
+
+  override def isZero: Boolean = _value.asInstanceOf[AnyRef].eq(_zero.asInstanceOf[AnyRef])
 
   override def copy(): LegacyAccumulatorWrapper[R, T] = {
     val acc = new LegacyAccumulatorWrapper(initialValue, param)
@@ -495,7 +497,7 @@ class LegacyAccumulatorWrapper[R, T](
   }
 
   override def reset(): Unit = {
-    _value = param.zero(initialValue)
+    _value = _zero
   }
 
   override def add(v: T): Unit = _value = param.addAccumulator(_value, v)

core/src/test/scala/org/apache/spark/util/AccumulatorV2Suite.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.util
 
 import org.apache.spark._
+import org.apache.spark.serializer.JavaSerializer
 
 class AccumulatorV2Suite extends SparkFunSuite {
 
@@ -162,4 +163,22 @@ class AccumulatorV2Suite extends SparkFunSuite {
     assert(acc3.isZero)
     assert(acc3.value === "")
   }
+
+  test("LegacyAccumulatorWrapper with AccumulatorParam that has no equals/hashCode") {
+    class MyData(val i: Int) extends Serializable
+    val param = new AccumulatorParam[MyData] {
+      override def zero(initialValue: MyData): MyData = new MyData(0)
+      override def addInPlace(r1: MyData, r2: MyData): MyData = new MyData(r1.i + r2.i)
+    }
+
+    val acc = new LegacyAccumulatorWrapper(new MyData(0), param)
+    acc.metadata = AccumulatorMetadata(
+      AccumulatorContext.newId(),
+      Some("test"),
+      countFailedValues = false)
+    AccumulatorContext.register(acc)
+
+    val ser = new JavaSerializer(new SparkConf).newInstance()
+    ser.serialize(acc)
+  }
 }