[SPARK-20567] Lazily bind in GenerateExec

It is not valid to eagerly bind with the child's output as this causes failures when we attempt to canonicalize the plan (replacing the attribute references with dummies).

Author: Michael Armbrust <michael@databricks.com>

Closes #17838 from marmbrus/fixBindExplode.

sql/core/src/main/scala/org/apache/spark/sql/execution/GenerateExec.scala

@@ -78,7 +78,7 @@ case class GenerateExec(
 
   override def outputPartitioning: Partitioning = child.outputPartitioning
 
-  val boundGenerator: Generator = BindReferences.bindReference(generator, child.output)
+  lazy val boundGenerator: Generator = BindReferences.bindReference(generator, child.output)
 
   protected override def doExecute(): RDD[InternalRow] = {
     // boundGenerator.terminate() should be triggered after all of the rows in the partition

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingAggregationSuite.scala

@@ -69,6 +69,22 @@ class StreamingAggregationSuite extends StateStoreMetricsTest with BeforeAndAfte
     )
   }
 
+  test("count distinct") {
+    val inputData = MemoryStream[(Int, Seq[Int])]
+
+    val aggregated =
+      inputData.toDF()
+        .select($"*", explode($"_2") as 'value)
+        .groupBy($"_1")
+        .agg(size(collect_set($"value")))
+        .as[(Int, Int)]
+
+    testStream(aggregated, Update)(
+      AddData(inputData, (1, Seq(1, 2))),
+      CheckLastBatch((1, 2))
+    )
+  }
+
   test("simple count, complete mode") {
     val inputData = MemoryStream[Int]