fix(auth): return correct error when trying to do a user action as a link share

Resolves https://vikunja.sentry.io/share/issue/77f7aa4cda9442439deba3cfe2cb32c6/

pkg/user/error.go

@@ -532,3 +532,30 @@ func (err *ErrUsernameMustNotContainSpaces) HTTPError() web.HTTPError {
 		Message:  "The username must not contain spaces.",
 	}
 }
+
+// ErrMustNotBeLinkShare represents a "MustNotBeLinkShare" kind of error.
+type ErrMustNotBeLinkShare struct {
+	Username string
+}
+
+// IsErrMustNotBeLinkShare checks if an error is a ErrMustNotBeLinkShare.
+func IsErrMustNotBeLinkShare(err error) bool {
+	_, ok := err.(*ErrMustNotBeLinkShare)
+	return ok
+}
+
+func (err *ErrMustNotBeLinkShare) Error() string {
+	return "user must be a *User, not a *models.LinkSharing"
+}
+
+// ErrCodeMustNotBeLinkShare holds the unique world-error code of this error
+const ErrCodeMustNotBeLinkShare = 1023
+
+// HTTPError holds the http error description
+func (err *ErrMustNotBeLinkShare) HTTPError() web.HTTPError {
+	return web.HTTPError{
+		HTTPCode: http.StatusForbidden,
+		Code:     ErrCodeMustNotBeLinkShare,
+		Message:  "You can't do that as a link share.",
+	}
+}

pkg/user/user.go

@@ -186,7 +186,11 @@ func (u *User) GetFailedPasswordAttemptsKey() string {
 func GetFromAuth(a web.Auth) (*User, error) {
 	u, is := a.(*User)
 	if !is {
-		return &User{}, fmt.Errorf("user is not user element, is %s", reflect.TypeOf(a))
+		typ := reflect.TypeOf(a)
+		if typ.String() == "*models.LinkSharing" {
+			return nil, &ErrMustNotBeLinkShare{}
+		}
+		return &User{}, fmt.Errorf("user is not user element, is %s", typ)
 	}
 	return u, nil
 }