[SPARK-16995][SQL] TreeNodeException when flat mapping...

[SPARK-16995][SQL] TreeNodeException when flat mapping RelationalGroupedDataset created from DataFrame containing a column created with lit/expr

## What changes were proposed in this pull request?

A TreeNodeException is thrown when executing the following minimal example in Spark 2.0.

    import spark.implicits._
    case class test (x: Int, q: Int)

    val d = Seq(1).toDF("x")
    d.withColumn("q", lit(0)).as[test].groupByKey(_.x).flatMapGroups{case (x, iter) => List[Int]()}.show
    d.withColumn("q", expr("0")).as[test].groupByKey(_.x).flatMapGroups{case (x, iter) => List[Int]()}.show

The problem is at `FoldablePropagation`. The rule will do `transformExpressions` on `LogicalPlan`. The query above contains a `MapGroups` which has a parameter `dataAttributes:Seq[Attribute]`. One attributes in `dataAttributes` will be transformed to an `Alias(literal(0), _)` in `FoldablePropagation`. `Alias` is not an `Attribute` and causes the error.

We can't easily detect such type inconsistency during transforming expressions. A direct approach to this problem is to skip doing `FoldablePropagation` on object operators as they should not contain such expressions.

## How was this patch tested?

Jenkins tests.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #14648 from viirya/flat-mapping.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -727,6 +727,19 @@ object FoldablePropagation extends Rule[LogicalPlan] {
         case j @ Join(_, _, LeftOuter | RightOuter | FullOuter, _) =>
           stop = true
           j
+
+        // These 3 operators take attributes as constructor parameters, and these attributes
+        // can't be replaced by alias.
+        case m: MapGroups =>
+          stop = true
+          m
+        case f: FlatMapGroupsInR =>
+          stop = true
+          f
+        case c: CoGroup =>
+          stop = true
+          c
+
         case p: LogicalPlan if !stop => p.transformExpressions {
           case a: AttributeReference if foldableMap.contains(a) =>
             foldableMap(a)

sql/core/src/test/scala/org/apache/spark/sql/DatasetSuite.scala

@@ -878,6 +878,19 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
     val ds = spark.createDataset(data)(enc)
     checkDataset(ds, (("a", "b"), "c"), (null, "d"))
   }
+
+  test("SPARK-16995: flat mapping on Dataset containing a column created with lit/expr") {
+    val df = Seq("1").toDF("a")
+
+    import df.sparkSession.implicits._
+
+    checkDataset(
+      df.withColumn("b", lit(0)).as[ClassData]
+        .groupByKey(_.a).flatMapGroups { case (x, iter) => List[Int]() })
+    checkDataset(
+      df.withColumn("b", expr("0")).as[ClassData]
+        .groupByKey(_.a).flatMapGroups { case (x, iter) => List[Int]() })
+  }
 }
 
 case class Generic[T](id: T, value: Double)