[SPARK-22699][SQL] GenerateSafeProjection should not use global variables for struct

## What changes were proposed in this pull request?

GenerateSafeProjection is defining a mutable state for each struct, which is not needed. This is bad for the well known issues related to constant pool limits.
The PR replace the global variable with a local one.

## How was this patch tested?

added UT

Author: Marco Gaido <[email protected]>

Closes #19914 from mgaido91/SPARK-22699.

Author: mgaido91

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

@@ -49,8 +49,6 @@ object GenerateSafeProjection extends CodeGenerator[Seq[Expression], Projection]
     val tmpInput = ctx.freshName("tmpInput")
     val output = ctx.freshName("safeRow")
     val values = ctx.freshName("values")
-    // These expressions could be split into multiple functions
-    ctx.addMutableState("Object[]", values, s"$values = null;")
 
     val rowClass = classOf[GenericInternalRow].getName
 
@@ -66,15 +64,15 @@ object GenerateSafeProjection extends CodeGenerator[Seq[Expression], Projection]
     val allFields = ctx.splitExpressions(
       expressions = fieldWriters,
       funcName = "writeFields",
-      arguments = Seq("InternalRow" -> tmpInput)
+      arguments = Seq("InternalRow" -> tmpInput, "Object[]" -> values)
     )
-    val code = s"""
-      final InternalRow $tmpInput = $input;
-      $values = new Object[${schema.length}];
-      $allFields
-      final InternalRow $output = new $rowClass($values);
-      $values = null;
-    """
+    val code =
+      s"""
+         |final InternalRow $tmpInput = $input;
+         |final Object[] $values = new Object[${schema.length}];
+         |$allFields
+         |final InternalRow $output = new $rowClass($values);
+       """.stripMargin
 
     ExprCode(code, "false", output)
   }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/codegen/GeneratedProjectionSuite.scala

@@ -208,4 +208,15 @@ class GeneratedProjectionSuite extends SparkFunSuite {
     unsafeProj.apply(InternalRow(InternalRow(UTF8String.fromString("b"))))
     assert(row.getStruct(0, 1).getString(0).toString == "a")
   }
+
+  test("SPARK-22699: GenerateSafeProjection should not use global variables for struct") {
+    val safeProj = GenerateSafeProjection.generate(
+      Seq(BoundReference(0, new StructType().add("i", IntegerType), true)))
+    val globalVariables = safeProj.getClass.getDeclaredFields
+    // We need always 3 variables:
+    // - one is a reference to this
+    // - one is the references object
+    // - one is the mutableRow
+    assert(globalVariables.length == 3)
+  }
 }