code: func (fm *filesManager) setBeforeCloseCb(file *file, bccb beforeFileClose) {
  	fm.Lock()
  	file.beforeClose = bccb
  	fm.Unlock()
  }
docs: // setBeforeCloseCb sets the beforeFileClose callback for this file.
  // When this callback is set, and the files manager closes a file,
  // the callback is invoked prior to actual closing of the file handle.
  // This allows the caller to perfom some work before the file is
  // asynchronously (form its perspective) closed.