grorge123 commented on code in PR #5526:
URL: https://github.com/apache/datafusion-comet/pull/5526#discussion_r3886564490


##########
spark/src/main/scala/org/apache/comet/codegen/CometBatchKernelCodegen.scala:
##########
@@ -117,7 +126,7 @@ object CometBatchKernelCodegen extends Logging with 
CometExprTraitShim with Come
    * nested-field count on `spark.sql.codegen.maxFields`.
    */
   def canHandle(boundExpr: Expression): Option[String] = {
-    if (!isSupportedDataType(boundExpr.dataType)) {
+    if (!isSupportedDataType(boundExpr.dataType, allowNullType = true)) {

Review Comment:
   Fixed: `coalesceBroadcastBatches` now uses `hasNestedNullType` to check the 
schema, and ships the batches uncoalesced when a `NullType` sits inside a 
struct or map.
   
   Tests: `UtilsSuite` ("coalesceBroadcastBatches ships maps with nested 
NullType uncoalesced") checks that such batches come back uncoalesced, and 
`CometJoinSuite` ("Broadcast HashJoin with NullType map columns on the build 
side") runs a hinted broadcast hash join with `map(_1, NULL)` and 
`transform_values(map(), (k, v) -> _1)` on the build side, with AQE on and off.



##########
spark/src/main/scala/org/apache/comet/codegen/CometBatchKernelCodegen.scala:
##########
@@ -81,18 +81,27 @@ object CometBatchKernelCodegen extends Logging with 
CometExprTraitShim with Come
   /**
    * Type surface the kernel covers on both input and output sides. Recursive: 
complex types are
    * supported when their children are.
+   *
+   * `NullType` is output-only: [[CometBatchKernelCodegenOutput]] can write an 
all-null Arrow
+   * `NullVector`, but `CometScalaUDFCodegen.specFor` cannot build an 
[[ArrowColumnSpec]] for one,
+   * so a `NullType` input (nested or not) has to keep falling back to Spark.
    */
-  def isSupportedDataType(dt: DataType): Boolean = dt match {
+  def isSupportedDataType(dt: DataType): Boolean = isSupportedDataType(dt, 
allowNullType = false)
+
+  private def isSupportedDataType(dt: DataType, allowNullType: Boolean): 
Boolean = dt match {
+    case NullType => allowNullType
     case BooleanType | ByteType | ShortType | IntegerType | LongType => true
     case FloatType | DoubleType => true
     case _: DecimalType => true
     case _: StringType | _: BinaryType => true
     case DateType | TimestampType | TimestampNTZType => true
     case dt if isTimeType(dt) => true
     case _: YearMonthIntervalType | _: DayTimeIntervalType | 
CalendarIntervalType => true
-    case ArrayType(inner, _) => isSupportedDataType(inner)
-    case st: StructType => st.fields.forall(f => 
isSupportedDataType(f.dataType))
-    case mt: MapType => isSupportedDataType(mt.keyType) && 
isSupportedDataType(mt.valueType)
+    case ArrayType(inner, _) => isSupportedDataType(inner, allowNullType)
+    case st: StructType => st.fields.forall(f => 
isSupportedDataType(f.dataType, allowNullType))
+    case mt: MapType =>
+      isSupportedDataType(mt.keyType, allowNullType) &&

Review Comment:
   Since Arrow 18.3.0's `MinorType.NULL.getNewVector` is `new 
NullVector(field.getName())`, every `NullType` map key comes back nullable 
after any vector is created for it. We added `Utils.withNonNullableMapKeys`, 
which repairs the key flag, route all IPC writers through 
`Utils.newArrowStreamWriter` so the repair is applied in one place, and added a 
scalastyle rule as a lint check for `new ArrowStreamWriter`. The same repair is 
applied in `CometArrowStream.actualFieldOf` (the stream schema handed to 
native) and in `CometArrowPythonRunnerBase` before it builds its destination 
struct.
   
   Tests: `UtilsSuite` pins the Arrow behaviour (asserts the key comes back 
nullable, so the workaround can be dropped once Arrow fixes it), checks 
`withNonNullableMapKeys` restores the flag, round-trips a `NullType` map key 
through `serializeBatches`, and checks `newArrowStreamWriter` keeps/returns the 
root a later row count must be set on. `CometJoinSuite` covers the broadcast 
path, `CometColumnarShuffleSuite` covers `Map[NullType, _]` and `Map[_, 
NullType]` through the JVM columnar shuffle, and `test_pyarrow_udf.py` covers 
`mapInArrow` with `Map[NullType, _]` input on Spark 4.1.3 in both accelerated 
and fallback modes. Also verified under the `spark-3.5` profile.



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