Add a test for ScatterGather showing failed deferred access with map

tasks/src/test/scala/dagr/tasks/ScatterGatherTests.scala

@@ -395,4 +395,88 @@ class ScatterGatherTests extends UnitSpec with LazyLogging with BeforeAndAfterAl
       (wordLength.toInt, count.toInt)
     }.sortBy(_._1) should contain theSameElementsInOrderAs Seq((3, 9), (4, 3), (5, 3))
   }
+
+  private case class SumLines(input: Path) extends SimpleInJvmTask {
+    private var _result: Option[Int] = None
+    def result: Int = this._result.get
+    def run(): Unit = _result = Some(Io.readLines(input).map(_.toInt).sum)
+  }
+
+  it should "map twice in a row" in {
+    val lines = Seq("one", "one two", "one two three", "one two three four", "one two three four five")
+
+    // setup the input and output
+    val input = tmp()
+    val resultPath = tmp()
+    Io.writeLines(input, lines)
+
+    val pipeline = new Pipeline() {
+      name = "Pipeline"
+
+      override def build(): Unit = {
+        // the initial scatter: scatters across lines
+        val scatter: Scatter[Path] = Scatter(SplitByLine(input=input))
+        scatter
+          .map { path: Path => CountWords(input=path, output=tmp()) }
+          .map { countWords: CountWords => SumLines(input=countWords.output) }
+          .gather { sumLines: Seq[SumLines] => WriteNumber(sumLines.map(_.result).sum, resultPath) }
+        root ==> scatter
+      }
+    }
+
+    val taskManager = buildTaskManager
+    taskManager.addTask(pipeline)
+    val taskMap = taskManager.runToCompletion(true)
+
+    taskMap.foreach { case (_, info) =>
+      info.status shouldBe TaskStatus.SUCCEEDED
+    }
+
+    val outputLines = Io.readLines(resultPath).toList
+    outputLines should have size 1
+    outputLines.head.toInt shouldBe 15
+  }
+
+  /** Splits a file into one file per line. */
+  private case class IdentityPartitioner[Result](inputs: Seq[Result]) extends SimpleInJvmTask with Partitioner[Result] {
+    name = s"Partition to $inputs"
+    var partitions: Option[Seq[Result]] = None
+    def run(): Unit = this.partitions = Some(inputs)
+  }
+
+  private case class AddOne(input: Int) extends SimpleInJvmTask {
+    name = s"AddOne to $input"
+    var _result: Option[Int] = None
+    def result: Int = _result.getOrElse {
+      throw new IllegalArgumentException(s"Accessing result before $name has completed")
+    }
+    override def run(): Unit = this._result = Some(input + 1)
+  }
+
+  it should "map twice in a row requiring deferred access" in {
+    val output = tmp()
+    val pipeline = new Pipeline() {
+      name = "Pipeline"
+      override def build(): Unit = {
+        val scatter: Scatter[Int] = Scatter(IdentityPartitioner(Seq(1, 2, 3, 4, 5)))
+        scatter
+          .map { int => AddOne(int) }
+          .map { addOne => AddOne(addOne.result) } // FIXME: addOne.result should only be accessed **after** addOne has completed
+          .gather { addOnes => WriteNumber(addOnes.map(_.result).sum, output) }
+        root ==> scatter
+      }
+    }
+
+    val taskManager = buildTaskManager
+    taskManager.addTask(pipeline)
+    val taskMap = taskManager.runToCompletion(true)
+
+    taskMap.foreach { case (_, info) =>
+      info.status shouldBe TaskStatus.SUCCEEDED
+    }
+
+    val lines = Io.readLines(output).toList
+    lines should have size 1
+    lines.head.toInt shouldBe 25
+  }
 }