How would you unit test this?

backup.go

func (c *backupController) run(backup *v1.Backup) error {
  var errs []error
  
  // shallow copy to backup so we can generate a patch
  var original := backup
  // deep copy so we don't mutate original
  backup = backup.DeepCopy()
  
  file, _ := ioutil.TempFile("", "")
  defer file.Close()
  
  // May mutate backup.Status.Phase, backup.Status.VolumeInfo, backup.Status.ValidationErrors, ...
  // Current thinking is to use a mock for c.backupper in the unit tests. But for the mocking we're doing,
  // we usually just control what the mocked method returns, which in this case is just an error. But
  // this run() function depends on c.backupper.Backup() mutating the backup, to determine if we should
  // proceed with the upload below. While you could argue (or not) that that's ok for the actual code,
  // it seems a bit "wrong" since the mocking code only deals with the return value (an error) and not the mutations
  // to the input objects. Does it make more sense have multiple return arguments, or is there some
  // other better way to do this?
  if err := c.backupper.Backup(backup, file); err != nil {
    errs = append(errs, err)
  }
  
  if backup.Status.Phase == v1.BackupPhaseCompleted {
    // Current thinking is to use a mock for c.uploader in the unit tests.
    errs = append(errs, c.uploader.Upload(backup, file))
  }
  
  if err := patchBackup(original, backup); err != nil {
    errs = append(errs, err)
  }
  
  return kubeerrors.NewAggregate(errs)
}