[SPARK-21677][SQL] json_tuple throws NullPointException when column is null as string type

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/jsonExpressions.scala

@@ -362,9 +362,9 @@ case class JsonTuple(children: Seq[Expression])
   @transient private lazy val fieldExpressions: Seq[Expression] = children.tail
 
   // eagerly evaluate any foldable the field names
-  @transient private lazy val foldableFieldNames: IndexedSeq[String] = {
+  @transient private lazy val foldableFieldNames: IndexedSeq[Option[String]] = {
     fieldExpressions.map {
-      case expr if expr.foldable => expr.eval().asInstanceOf[UTF8String].toString
+      case expr if expr.foldable => Option(expr.eval()).map(_.asInstanceOf[UTF8String].toString)
       case _ => null
     }.toIndexedSeq
   }
@@ -417,7 +417,7 @@ case class JsonTuple(children: Seq[Expression])
     val fieldNames = if (constantFields == fieldExpressions.length) {
       // typically the user will provide the field names as foldable expressions
       // so we can use the cached copy
-      foldableFieldNames
+      foldableFieldNames.map(_.orNull)
     } else if (constantFields == 0) {
       // none are foldable so all field names need to be evaluated from the input row
       fieldExpressions.map(_.eval(input).asInstanceOf[UTF8String].toString)
@@ -426,7 +426,7 @@ case class JsonTuple(children: Seq[Expression])
       // prefer the cached copy when available
       foldableFieldNames.zip(fieldExpressions).map {
         case (null, expr) => expr.eval(input).asInstanceOf[UTF8String].toString
-        case (fieldName, _) => fieldName
+        case (fieldName, _) => fieldName.orNull
       }
     }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/JsonExpressionsSuite.scala

@@ -363,6 +363,16 @@ class JsonExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
       InternalRow(UTF8String.fromString("b\nc")))
   }
 
+  test("SPARK-21677: json_tuple throws NullPointException when column is null as string type") {
+    checkJsonTuple(
+      JsonTuple(Literal("""{"f1": 1, "f2": 2}""") ::
+        NonFoldableLiteral("f1") ::
+        NonFoldableLiteral("cast(NULL AS STRING)") ::
+        NonFoldableLiteral("f2") ::
+        Nil),
+      InternalRow(UTF8String.fromString("1"), null, UTF8String.fromString("2")))
+  }
+
   val gmtId = Option(DateTimeUtils.TimeZoneGMT.getID)
 
   test("from_json") {

sql/core/src/test/resources/sql-tests/inputs/json-functions.sql

@@ -20,3 +20,9 @@ select from_json('{"a":1}', 'a InvalidType');
 select from_json('{"a":1}', 'a INT', named_struct('mode', 'PERMISSIVE'));
 select from_json('{"a":1}', 'a INT', map('mode', 1));
 select from_json();
+-- json_tuple
+describe function json_tuple;
+describe function extended json_tuple;
+select json_tuple('{"a" : 1, "b" : 2}', cast(NULL AS STRING), 'b', cast(NULL AS STRING), 'a')
+create temporary view jsonTable(jsonField, a, b) as select * from values '{"a": 1, "b": 2}', 'a', 'b';
+SELECT json_tuple(jsonField, b, cast(NULL AS STRING), 'a') FROM jsonTable