如何返回一个空的io.Reader？

Question

I have a function which returns an io.Reader:

func getData(bucket string, key string) (io.Reader, error) {
  svc := s3.New(cfg)

  req := svc.GetObjectRequest(&s3.GetObjectInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(key),
  })

  res, err := req.Send()
  if err != nil {
    // How to return a empty io.Reader here?
    return bytes.NewReader([]byte), err
  }
  return res.Body, nil
}

And I would like to know, how to return a empty io.Reader from the err block.

Thank you.

Answer 1

In case of a non-nil error return value, usually other parameters are left to the zero value of their type, which in case of io.Reader (which is an interface type) should be nil:

return nil, err

Callers are responsible to first always check the returned error, and only proceed to use other return values if the error is nil. This is the general "rule". There may be exceptions of course, in which case you should document the deviant behavior.

Actually there's nothing wrong with providing other values along with non-nil errors, and often it's useful. Typical example is the io.Reader.Read() method, which documents how implementations should (must) work:

It returns the number of bytes read (0 <= n <= len(p)) and any error encountered.

... When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read.

So even if Reader.Read() returns a non-nil error, it also reports how many bytes it was able to read successfully before the error was encountered, so you may process / use up to n bytes from the slice.