apacheGH-33936: [Go] C Data Interface: export dummy buffer for nil bu…

…ffers (apache#33951)

### Rationale for this change

While apache#14805 clarified that NULL buffers are valid in the C Data Interface, not all implementations handle this correctly (or were fixed, but earlier versions still exist).

### What changes are included in this PR?

Export a non-NULL dummy buffer for nil/empty buffers to ensure compatibility.

### Are these changes tested?

A regression test is included.
* Closes: apache#33936

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>

go/arrow/cdata/cdata_exports.go

@@ -17,13 +17,16 @@
 package cdata
 
 // #include <errno.h>
+// #include <stdint.h>
 // #include <stdlib.h>
 // #include "arrow/c/abi.h"
 // #include "arrow/c/helpers.h"
 //
 // extern void releaseExportedSchema(struct ArrowSchema* schema);
 // extern void releaseExportedArray(struct ArrowArray* array);
 //
+// const uint8_t kGoCdataZeroRegion[8] = {0};
+//
 // void goReleaseArray(struct ArrowArray* array) {
 //	releaseExportedArray(array);
 // }
@@ -45,6 +48,7 @@ import (
 	"github.com/apache/arrow/go/v12/arrow"
 	"github.com/apache/arrow/go/v12/arrow/array"
 	"github.com/apache/arrow/go/v12/arrow/endian"
+	"github.com/apache/arrow/go/v12/arrow/internal"
 	"github.com/apache/arrow/go/v12/arrow/ipc"
 )
 
@@ -376,7 +380,7 @@ func exportArray(arr arrow.Array, out *CArrowArray, outSchema *CArrowSchema) {
 		// unions don't have validity bitmaps, but we keep them shifted
 		// to make processing easier in other contexts. This means that
 		// we have to adjust for union arrays
-		if arr.DataType().ID() == arrow.DENSE_UNION || arr.DataType().ID() == arrow.SPARSE_UNION {
+		if !internal.DefaultHasValidityBitmap(arr.DataType().ID()) {
 			out.n_buffers--
 			nbuffers--
 			bufs = bufs[1:]
@@ -385,7 +389,13 @@ func exportArray(arr arrow.Array, out *CArrowArray, outSchema *CArrowSchema) {
 		for i := range bufs {
 			buf := bufs[i]
 			if buf == nil || buf.Len() == 0 {
-				buffers[i] = nil
+				if i > 0 || !internal.DefaultHasValidityBitmap(arr.DataType().ID()) {
+					// apache/arrow#33936: export a dummy buffer to be friendly to
+					// implementations that don't import NULL properly
+					buffers[i] = (*C.void)(unsafe.Pointer(&C.kGoCdataZeroRegion))
+				} else {
+					buffers[i] = nil
+				}
 				continue
 			}
 

go/arrow/cdata/cdata_test.go

@@ -795,6 +795,51 @@ func TestEmptyDictExport(t *testing.T) {
 	assert.Nil(t, out.dictionary.dictionary)
 }
 
+func TestEmptyStringExport(t *testing.T) {
+	// apache/arrow#33936: regression test
+	bldr := array.NewBuilder(memory.DefaultAllocator, &arrow.StringType{})
+	defer bldr.Release()
+
+	arr := bldr.NewArray()
+	defer arr.Release()
+
+	var out CArrowArray
+	var sc CArrowSchema
+	ExportArrowArray(arr, &out, &sc)
+
+	assert.EqualValues(t, 'u', *sc.format)
+	assert.Zero(t, sc.n_children)
+	assert.Nil(t, sc.dictionary)
+
+	assert.EqualValues(t, 3, out.n_buffers)
+	buffers := (*[3]unsafe.Pointer)(unsafe.Pointer(out.buffers))
+	assert.EqualValues(t, unsafe.Pointer(nil), buffers[0])
+	assert.NotEqualValues(t, unsafe.Pointer(nil), buffers[1])
+	assert.NotEqualValues(t, unsafe.Pointer(nil), buffers[2])
+}
+
+func TestEmptyUnionExport(t *testing.T) {
+	// apache/arrow#33936: regression test
+	bldr := array.NewBuilder(memory.DefaultAllocator, arrow.SparseUnionOf([]arrow.Field{
+		{Name: "child", Type: &arrow.Int64Type{}},
+	}, []arrow.UnionTypeCode{0}))
+	defer bldr.Release()
+
+	arr := bldr.NewArray()
+	defer arr.Release()
+
+	var out CArrowArray
+	var sc CArrowSchema
+	ExportArrowArray(arr, &out, &sc)
+
+	assert.EqualValues(t, 1, sc.n_children)
+	assert.Nil(t, sc.dictionary)
+
+	assert.EqualValues(t, 1, out.n_buffers)
+	buffers := (*[1]unsafe.Pointer)(unsafe.Pointer(out.buffers))
+	assert.NotEqualValues(t, unsafe.Pointer(nil), buffers[0])
+}
+
 func TestRecordReaderExport(t *testing.T) {
 	// Regression test for apache/arrow#33767
 	reclist := arrdata.Records["primitives"]