always use error interface when returning errors (#8816)

* always use error interface when returning errors

Signed-off-by: Jörn Friedrich Dreyer <jfd@butonic.de>

* fix: use non pointer Error

* fix: errorcode evaluation

---------

Signed-off-by: Jörn Friedrich Dreyer <jfd@butonic.de>
Co-authored-by: Florian Schade <f.schade@icloud.com>

services/graph/pkg/errorcode/errorcode_test.go

@@ -0,0 +1,45 @@
+package errorcode_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/owncloud/ocis/v2/services/graph/pkg/errorcode"
+)
+
+type customErr struct{}
+
+func (customErr) Error() string {
+	return "some error"
+}
+
+func TestRenderError(t *testing.T) {
+	t.Parallel()
+
+	t.Run("errorcode.Error value error", func(t *testing.T) {
+		r := httptest.NewRequest("GET", "/", nil)
+		w := httptest.NewRecorder()
+		err := errorcode.New(errorcode.ItemNotFound, "test error")
+		errorcode.RenderError(w, r, err)
+		require.Equal(t, http.StatusNotFound, w.Code)
+	})
+
+	t.Run("errorcode.Error zero value error", func(t *testing.T) {
+		r := httptest.NewRequest("GET", "/", nil)
+		w := httptest.NewRecorder()
+		var err errorcode.Error
+		errorcode.RenderError(w, r, err)
+		require.Equal(t, http.StatusForbidden, w.Code)
+	})
+
+	t.Run("custom error", func(t *testing.T) {
+		r := httptest.NewRequest("GET", "/", nil)
+		w := httptest.NewRecorder()
+		var err customErr
+		errorcode.RenderError(w, r, err)
+		require.Equal(t, http.StatusInternalServerError, w.Code)
+	})
+}