fix(arrow/cdata): handle export struct with no fields

Author: zeroshade

arrow/cdata/cdata_exports.go

@@ -401,6 +401,9 @@ func exportArray(arr arrow.Array, out *CArrowArray, outSchema *CArrowSchema) {
 		out.children = (**CArrowArray)(unsafe.Pointer(&childPtrs[0]))
 	case *array.Struct:
 		out.n_children = C.int64_t(arr.NumField())
+		if arr.NumField() == 0 {
+			return
+		}
 		childPtrs := allocateArrowArrayPtrArr(arr.NumField())
 		children := allocateArrowArrayArr(arr.NumField())
 		for i := 0; i < arr.NumField(); i++ {

arrow/cdata/cdata_test.go

@@ -598,6 +598,14 @@ func createTestStructArr() arrow.Array {
 	return bld.NewArray()
 }
 
+func createTestEmptyStructArr() arrow.Array {
+	bld := array.NewStructBuilder(memory.DefaultAllocator, arrow.StructOf())
+	defer bld.Release()
+
+	bld.AppendNull()
+	return bld.NewArray()
+}
+
 func createTestRunEndsArr() arrow.Array {
 	bld := array.NewRunEndEncodedBuilder(memory.DefaultAllocator,
 		arrow.PrimitiveTypes.Int32, arrow.PrimitiveTypes.Int8)
@@ -687,6 +695,7 @@ func TestNestedArrays(t *testing.T) {
 		{"sparse union", createTestSparseUnion},
 		{"dense union", createTestDenseUnion},
 		{"run-end encoded", createTestRunEndsArr},
+		{"empty struct", createTestEmptyStructArr},
 	}
 
 	for _, tt := range tests {

arrow/cdata/cdata_test_framework.go

@@ -309,6 +309,9 @@ func createCArr(arr arrow.Array, alloc *mallocator.Mallocator) *CArrowArray {
 		children = (**CArrowArray)(unsafe.Pointer(&clist[0]))
 		nchildren += 1
 	case *array.Struct:
+		if arr.NumField() == 0 {
+			break
+		}
 		clist := allocateChildrenPtrArr(alloc, arr.NumField())
 		for i := 0; i < arr.NumField(); i++ {
 			clist[i] = createCArr(arr.Field(i), alloc)