[SPARK-4264] Completion iterator should only invoke callback once

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

@@ -25,10 +25,13 @@ private[spark]
 // scalastyle:off
 abstract class CompletionIterator[ +A, +I <: Iterator[A]](sub: I) extends Iterator[A] {
 // scalastyle:on
+
+  private[this] var completed = false
   def next() = sub.next()
   def hasNext = {
     val r = sub.hasNext
-    if (!r) {
+    if (!r && !completed) {
+      completed = true
       completion()
     }
     r

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

@@ -0,0 +1,47 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.util
+
+import org.scalatest.FunSuite
+
+class CompletionIteratorSuite extends FunSuite {
+  test("basic test") {
+    var numTimesCompleted = 0
+    val iter = List(1, 2, 3).iterator
+    val completionIter = CompletionIterator[Int, Iterator[Int]](iter, { numTimesCompleted += 1 })
+
+    assert(completionIter.hasNext)
+    assert(completionIter.next() === 1)
+    assert(numTimesCompleted === 0)
+
+    assert(completionIter.hasNext)
+    assert(completionIter.next() === 2)
+    assert(numTimesCompleted === 0)
+
+    assert(completionIter.hasNext)
+    assert(completionIter.next() === 3)
+    assert(numTimesCompleted === 0)
+
+    assert(!completionIter.hasNext)
+    assert(numTimesCompleted === 1)
+
+    // SPARK-4264: Calling hasNext should not trigger the completion callback again.
+    assert(!completionIter.hasNext)
+    assert(numTimesCompleted === 1)
+  }
+}