[SPARK-15140][SQL] make the semantics of null input object for encoder clear

## What changes were proposed in this pull request?

For input object of non-flat type, we can't encode it to row if it's null, as Spark SQL doesn't allow row to be null, only its columns can be null.

This PR explicitly add this constraint and throw exception if users break it.

## How was this patch tested?

several new tests

Author: Wenchen Fan <wenchen@databricks.com>

Closes #13469 from cloud-fan/null-object.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoder.scala

@@ -25,7 +25,7 @@ import org.apache.spark.sql.catalyst.{InternalRow, JavaTypeInference, ScalaRefle
 import org.apache.spark.sql.catalyst.analysis.{Analyzer, GetColumnByOrdinal, SimpleAnalyzer, UnresolvedAttribute, UnresolvedExtractValue}
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.expressions.codegen.{GenerateSafeProjection, GenerateUnsafeProjection}
-import org.apache.spark.sql.catalyst.expressions.objects.{Invoke, NewInstance}
+import org.apache.spark.sql.catalyst.expressions.objects.{AssertNotNull, Invoke, NewInstance}
 import org.apache.spark.sql.catalyst.optimizer.SimplifyCasts
 import org.apache.spark.sql.catalyst.plans.logical.{CatalystSerde, DeserializeToObject, LocalRelation}
 import org.apache.spark.sql.types.{ObjectType, StructField, StructType}
@@ -50,8 +50,15 @@ object ExpressionEncoder {
     val cls = mirror.runtimeClass(tpe)
     val flat = !ScalaReflection.definedByConstructorParams(tpe)
 
-    val inputObject = BoundReference(0, ScalaReflection.dataTypeFor[T], nullable = false)
-    val serializer = ScalaReflection.serializerFor[T](inputObject)
+    val inputObject = BoundReference(0, ScalaReflection.dataTypeFor[T], nullable = true)
+    val nullSafeInput = if (flat) {
+      inputObject
+    } else {
+      // For input object of non-flat type, we can't encode it to row if it's null, as Spark SQL
+      // doesn't allow top-level row to be null, only its columns can be null.
+      AssertNotNull(inputObject, Seq("top level non-flat input object"))
+    }
+    val serializer = ScalaReflection.serializerFor[T](nullSafeInput)
     val deserializer = ScalaReflection.deserializerFor[T]
 
     val schema = ScalaReflection.schemaFor[T] match {

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/RowEncoder.scala

@@ -57,8 +57,8 @@ import org.apache.spark.unsafe.types.UTF8String
 object RowEncoder {
   def apply(schema: StructType): ExpressionEncoder[Row] = {
     val cls = classOf[Row]
-    val inputObject = BoundReference(0, ObjectType(cls), nullable = false)
-    val serializer = serializerFor(inputObject, schema)
+    val inputObject = BoundReference(0, ObjectType(cls), nullable = true)
+    val serializer = serializerFor(AssertNotNull(inputObject, Seq("top level row object")), schema)
     val deserializer = deserializerFor(schema)
     new ExpressionEncoder[Row](
       schema,
@@ -153,8 +153,7 @@ object RowEncoder {
         val fieldValue = serializerFor(
           GetExternalRowField(
             inputObject, index, field.name, externalDataTypeForInput(field.dataType)),
-          field.dataType
-        )
+          field.dataType)
         val convertedField = if (field.nullable) {
           If(
             Invoke(inputObject, "isNullAt", BooleanType, Literal(index) :: Nil),

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

@@ -519,7 +519,7 @@ case class CreateExternalRow(children: Seq[Expression], schema: StructType)
     val code = s"""
       $values = new Object[${children.size}];
       $childrenCode
-      final ${classOf[Row].getName} ${ev.value} = new $rowClass($values, this.$schemaField);
+      final ${classOf[Row].getName} ${ev.value} = new $rowClass($values, $schemaField);
       """
     ev.copy(code = code, isNull = "false")
   }
@@ -675,7 +675,7 @@ case class AssertNotNull(child: Expression, walkedTypePath: Seq[String])
       ${childGen.code}
 
       if (${childGen.isNull}) {
-        throw new RuntimeException(this.$errMsgField);
+        throw new RuntimeException($errMsgField);
       }
      """
     ev.copy(code = code, isNull = "false", value = childGen.value)

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/RowEncoderSuite.scala

@@ -224,6 +224,14 @@ class RowEncoderSuite extends SparkFunSuite {
     assert(convertedBack.getSeq(2) == Seq(Seq(Seq(0L, null), null), null))
   }
 
+  test("RowEncoder should throw RuntimeException if input row object is null") {
+    val schema = new StructType().add("int", IntegerType)
+    val encoder = RowEncoder(schema)
+    val e = intercept[RuntimeException](encoder.toRow(null))
+    assert(e.getMessage.contains("Null value appeared in non-nullable field"))
+    assert(e.getMessage.contains("top level row object"))
+  }
+
   private def encodeDecodeTest(schema: StructType): Unit = {
     test(s"encode/decode: ${schema.simpleString}") {
       val encoder = RowEncoder(schema).resolveAndBind()

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

@@ -790,6 +790,16 @@ class DatasetSuite extends QueryTest with SharedSQLContext {
     assert(e.getMessage.contains(
       "`abstract` is a reserved keyword and cannot be used as field name"))
   }
+
+  test("Dataset should support flat input object to be null") {
+    checkDataset(Seq("a", null).toDS(), "a", null)
+  }
+
+  test("Dataset should throw RuntimeException if non-flat input object is null") {
+    val e = intercept[RuntimeException](Seq(ClassData("a", 1), null).toDS())
+    assert(e.getMessage.contains("Null value appeared in non-nullable field"))
+    assert(e.getMessage.contains("top level non-flat input object"))
+  }
 }
 
 case class Generic[T](id: T, value: Double)