GH-14883: [Go] Fix IPC encoding empty maps

go/arrow/ipc/ipc_test.go

@@ -589,3 +589,34 @@ func Example_listSlice() {
 	//   rows: 1
 	//   col[0][list]: [["index3" "tag_int"]]
 }
+
+func TestIpcEmptyMap(t *testing.T) {
+	mem := memory.NewCheckedAllocator(memory.DefaultAllocator)
+	defer mem.AssertSize(t, 0)
+
+	dt := arrow.MapOf(arrow.BinaryTypes.String, arrow.BinaryTypes.String)
+	schema := arrow.NewSchema([]arrow.Field{{
+		Name: "map",
+		Type: dt,
+	}}, nil)
+
+	arr, _, err := array.FromJSON(mem, dt, strings.NewReader(`[]`))
+	require.NoError(t, err)
+	defer arr.Release()
+
+	rec := array.NewRecord(schema, []arrow.Array{arr}, int64(arr.Len()))
+	defer rec.Release()
+
+	var buf bytes.Buffer
+	w := ipc.NewWriter(&buf, ipc.WithSchema(rec.Schema()))
+	require.NoError(t, w.Write(rec))
+	assert.NoError(t, w.Close())
+
+	r, err := ipc.NewReader(&buf)
+	require.NoError(t, err)
+	defer r.Release()
+
+	assert.True(t, r.Next())
+	assert.Zero(t, r.Record().NumRows())
+	assert.True(t, arrow.TypeEqual(dt, r.Record().Column(0).DataType()))
+}

go/arrow/ipc/writer.go

@@ -649,44 +649,7 @@ func (w *recordEncoder) visit(p *Payload, arr arrow.Array) error {
 			}
 		}
 		w.depth++
-	case *arrow.MapType:
-		arr := arr.(*array.Map)
-		voffsets, err := w.getZeroBasedValueOffsets(arr)
-		if err != nil {
-			return fmt.Errorf("could not retrieve zero-based value offsets for array %T: %w", arr, err)
-		}
-		p.body = append(p.body, voffsets)
-
-		w.depth--
-		var (
-			values        = arr.ListValues()
-			mustRelease   = false
-			values_offset int64
-			values_end    int64
-		)
-		defer func() {
-			if mustRelease {
-				values.Release()
-			}
-		}()
-
-		if voffsets != nil {
-			values_offset, _ = arr.ValueOffsets(0)
-			_, values_end = arr.ValueOffsets(arr.Len() - 1)
-		}
-
-		if arr.Len() != 0 || values_end < int64(values.Len()) {
-			// must also slice the values
-			values = array.NewSlice(values, values_offset, values_end)
-			mustRelease = true
-		}
-		err = w.visit(p, values)
-
-		if err != nil {
-			return fmt.Errorf("could not visit list element for array %T: %w", arr, err)
-		}
-		w.depth++
-	case *arrow.ListType, *arrow.LargeListType:
+	case *arrow.MapType, *arrow.ListType, *arrow.LargeListType:
 		arr := arr.(array.ListLike)
 		voffsets, err := w.getZeroBasedValueOffsets(arr)
 		if err != nil {