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

…ffers

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);
 // }
@@ -385,7 +388,13 @@ 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 {
+					// 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 +795,29 @@ 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 TestRecordReaderExport(t *testing.T) {
 	// Regression test for apache/arrow#33767
 	reclist := arrdata.Records["primitives"]