[SPARK-19512][SQL] codegen for compare structs fails

## What changes were proposed in this pull request?

Set currentVars to null in GenerateOrdering.genComparisons before genCode is called. genCode ignores INPUT_ROW if currentVars is not null and in genComparisons we want it to use INPUT_ROW.

## How was this patch tested?

Added test with 2 queries in WholeStageCodegenSuite

Author: Bogdan Raducanu <[email protected]>

Closes apache#16852 from bogdanrdc/SPARK-19512.

Author: Bogdan Raducanu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -555,7 +555,6 @@ class CodegenContext {
       addNewFunction(compareFunc, funcCode)
       s"this.$compareFunc($c1, $c2)"
     case schema: StructType =>
-      INPUT_ROW = "i"
       val comparisons = GenerateOrdering.genComparisons(this, schema)
       val compareFunc = freshName("compareStruct")
       val funcCode: String =
@@ -566,7 +565,6 @@ class CodegenContext {
             if (a instanceof UnsafeRow && b instanceof UnsafeRow && a.equals(b)) {
               return 0;
             }
-            InternalRow i = null;
             $comparisons
             return 0;
           }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/GenerateOrdering.scala

@@ -73,7 +73,12 @@ object GenerateOrdering extends CodeGenerator[Seq[SortOrder], Ordering[InternalR
    */
   def genComparisons(ctx: CodegenContext, ordering: Seq[SortOrder]): String = {
     val comparisons = ordering.map { order =>
+      val oldCurrentVars = ctx.currentVars
+      ctx.INPUT_ROW = "i"
+      // to use INPUT_ROW we must make sure currentVars is null
+      ctx.currentVars = null
       val eval = order.child.genCode(ctx)
+      ctx.currentVars = oldCurrentVars
       val asc = order.isAscending
       val isNullA = ctx.freshName("isNullA")
       val primitiveA = ctx.freshName("primitiveA")
@@ -119,7 +124,7 @@ object GenerateOrdering extends CodeGenerator[Seq[SortOrder], Ordering[InternalR
       """
     }
 
-    ctx.splitExpressions(
+    val code = ctx.splitExpressions(
       expressions = comparisons,
       funcName = "compare",
       arguments = Seq(("InternalRow", "a"), ("InternalRow", "b")),
@@ -142,6 +147,12 @@ object GenerateOrdering extends CodeGenerator[Seq[SortOrder], Ordering[InternalR
           """
         }.mkString
       })
+    // make sure INPUT_ROW is declared even if splitExpressions
+    // returns an inlined block
+    s"""
+       |InternalRow ${ctx.INPUT_ROW} = null;
+       |$code
+     """.stripMargin
   }
 
   protected def create(ordering: Seq[SortOrder]): BaseOrdering = {
@@ -165,7 +176,6 @@ object GenerateOrdering extends CodeGenerator[Seq[SortOrder], Ordering[InternalR
         ${ctx.declareAddedFunctions()}
 
         public int compare(InternalRow a, InternalRow b) {
-          InternalRow ${ctx.INPUT_ROW} = null;  // Holds current row being evaluated.
           $comparisons
           return 0;
         }

sql/core/src/test/scala/org/apache/spark/sql/execution/WholeStageCodegenSuite.scala

@@ -143,4 +143,16 @@ class WholeStageCodegenSuite extends SparkPlanTest with SharedSQLContext {
     assert(createStackGenerator(50).find(isCodeGenerated).isDefined)
     assert(createStackGenerator(100).find(isCodeGenerated).isEmpty)
   }
+
+  test("SPARK-19512 codegen for comparing structs is incorrect") {
+    // this would raise CompileException before the fix
+    spark.range(10)
+      .selectExpr("named_struct('a', id) as col1", "named_struct('a', id+2) as col2")
+      .filter("col1 = col2").count()
+    // this would raise java.lang.IndexOutOfBoundsException before the fix
+    spark.range(10)
+      .selectExpr("named_struct('a', id, 'b', id) as col1",
+        "named_struct('a',id+2, 'b',id+2) as col2")
+      .filter("col1 = col2").count()
+  }
 }