code: func (i *PendingItem) done() error {
  	tracer.CounterAdd(i.a, "itemsProcessing", -1)
  	i.a = nil
  	return nil
  }
docs: // done is called when the PendingItem processing is done.
  //
  // It can be because there was an error, it was linked to another item, it was
  // present on the server or finally it was uploaded successfully.