Omit zero-valued data from marshalled APIResponse

message/message.go

@@ -151,9 +151,10 @@ type ReauthenticateResponse struct {
 }
 
 // APIResponse is a standard format for the DN API. It does not apply to the DNClient API.
+// The API sends exactly one of data or errors.
 type APIResponse[T any] struct {
-	Data   T                 `json:"data"`
-	Errors APIResponseErrors `json:"errors"`
+	Data   T                 `json:"data,omitzero"`
+	Errors APIResponseErrors `json:"errors,omitempty"`
 }
 
 // APIResponseError represents a single error returned in an API error response.

message/message_test.go

@@ -0,0 +1,37 @@
+package message
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAPIResponseErrorOnlyRoundTrip(t *testing.T) {
+	b, err := json.Marshal(APIResponse[EnrollResponseData]{
+		Errors: APIResponseErrors{{Code: "ERR_TEST", Message: "test error"}},
+	})
+	require.NoError(t, err)
+	require.NotContains(t, string(b), `"data"`)
+
+	var r APIResponse[EnrollResponseData]
+	require.NoError(t, json.Unmarshal(b, &r))
+	require.EqualError(t, r.Errors.Err(), "test error")
+}
+
+func TestAPIResponseDataOnlyOmitsErrors(t *testing.T) {
+	b, err := json.Marshal(APIResponse[EnrollResponseData]{
+		Data: EnrollResponseData{
+			HostID: "foobar",
+			// a valid curve is required for the decode below, as on the real API
+			Network: HostNetworkMetadata{Curve: NetworkCurve25519},
+		},
+	})
+	require.NoError(t, err)
+	require.NotContains(t, string(b), `"errors"`)
+
+	var r APIResponse[EnrollResponseData]
+	require.NoError(t, json.Unmarshal(b, &r))
+	require.NoError(t, r.Errors.Err())
+	require.Equal(t, "foobar", r.Data.HostID)
+}