[go-nuts] Efficiently switch io.Reader to another decoder on error

[Rory Campbell-Lange]

I'm looking to develop an alternative to an existing piece of code that reads 
email parts into byte slices and then returns these after decoding.

As library users may not wish to use these email parts and because there a 
multiple byte slice copies being used, I'm attempting to rationalise the 
process by simply wrapping the provided io.Reader with the necessary decoders 
to reduce memory usage and unnecessary processing.

The wrapping strategy seems to work ok. However there is a particular issue in 
detecting base64.StdEncoding versus base64.RawStdEncoding, which requires 
draining the io.Reader using base64.StdEncoding and (based on the current 
implementation) switching to base64.RawStdEncoding if an io.ErrUnexpectedEOF is 
found.

I'd be grateful for any thoughts on the most efficient way of dealing with this 
type of issue, avoiding the need for lots of in-memory copies of -- say -- a 
50MB email attachment. Unfortunately neither net/mail.Message.Body or 
mime/multipart.Part, which provide the input to this func, provide ReadSeekers.

Code snippet below.

Thanks!
Rory


    // decodeContent wraps the content io.Reader in either a base64 or
    // quoted printable decoder if applicable. It further wraps the reader
    // in a transform character decoder if an encoding is supplied.
    func decodeContent(content io.Reader, e encoding.Encoding, cte 
ContentTransferEncoding) io.Reader {

        var contentReader io.Reader

        switch cte {
        case cteBase64:

            contentReader = base64.NewDecoder(base64.StdEncoding, content)
            // ideally check for errors.Is(err, io.ErrUnexpectedEOF); switch 
decoder to
            // contentReader = base64.NewDecoder(base64.RawStdEncoding, content)

        case cteQuotedPrintable:
            contentReader = quotedprintable.NewReader(content)
        default:
            contentReader = content
        }

        if e == nil {
            return contentReader
        }
        return transform.NewReader(contentReader, e.NewDecoder())
    }

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