Add Data field to api errors (#6)

Closes #5

This commit adds the `Data` field to the `APIError` type and provides a helper function `ErrorData` to obtain
the slice of errors from the response. This will allow us to expose errors in ACL tests.

Signed-off-by: David Bond <[email protected]>

tailscale/client.go

@@ -25,10 +25,17 @@ type (
 
 	// The APIError type describes an error as returned by the Tailscale API.
 	APIError struct {
-		Message string `json:"message"`
+		Message string         `json:"message"`
+		Data    []APIErrorData `json:"data"`
 		status  int
 	}
 
+	// The APIErrorData type describes elements of the data field within errors returned by the Tailscale API.
+	APIErrorData struct {
+		User   string   `json:"user"`
+		Errors []string `json:"errors"`
+	}
+
 	// The ClientOption type is a function that is used to modify a Client.
 	ClientOption func(c *Client) error
 )
@@ -485,3 +492,14 @@ func IsNotFound(err error) bool {
 
 	return false
 }
+
+// ErrorData returns the contents of the APIError.Data field from the provided error if it is of type APIError. Returns
+// a nil slice if the given error is not of type APIError.
+func ErrorData(err error) []APIErrorData {
+	var apiErr APIError
+	if errors.As(err, &apiErr) {
+		return apiErr.Data
+	}
+
+	return nil
+}

tailscale/client_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	_ "embed"
 	"encoding/json"
+	"io"
 	"net/http"
 	"testing"
 	"time"
@@ -537,3 +538,27 @@ func TestClient_SetDeviceTags(t *testing.T) {
 	assert.NoError(t, json.Unmarshal(server.Body.Bytes(), &body))
 	assert.EqualValues(t, tags, body["tags"])
 }
+
+func TestErrorData(t *testing.T) {
+	t.Parallel()
+
+	t.Run("It should return the data element from a valid error", func(t *testing.T) {
+		expected := tailscale.APIError{
+			Data: []tailscale.APIErrorData{
+				{
+					User: "[email protected]",
+					Errors: []string{
+						"address \"[email protected]:400\": want: Accept, got: Drop",
+					},
+				},
+			},
+		}
+
+		actual := tailscale.ErrorData(expected)
+		assert.EqualValues(t, expected.Data, actual)
+	})
+
+	t.Run("It should return an empty slice for any other error", func(t *testing.T) {
+		assert.Empty(t, tailscale.ErrorData(io.EOF))
+	})
+}