Thanks very much for the playground link and thoughts.

The use case is reading base64 email parts, which could be of a very large 
size. It is unclear when processing these parts if they are base64 padded or 
not.

I'm trying to avoid reading the entire email part into memory. Consequently I 
think your earlier idea of adding padding (or removing it) in a wrapper could 
work. Perhaps wrapping the reader with another using a bufio.Reader to track 
bytes read and detect EOF. At EOF the wrapper could add padding if needed.

Rory

On 13/01/25, Axel Wagner (axel.wagner...@googlemail.com) wrote:
> Just realized: If you twist the idea around, you get something easy to
> implement and more correct.
> Instead of stripping padding if it exist, you can ensure that the body *is*
> padded to a multiple of 4 bytes: https://go.dev/play/p/SsPRXV9ZfoS
> You can then feed that to base64.StdEncoding. If the wrapped Reader returns
> padded Base64, this does nothing. If it returns unpadded Base64, it adds
> padding. If it returns incorrect Base64, it will create a padded stream,
> that will then get rejected by the Base64 decoder.
> 
> On Mon, 13 Jan 2025 at 10:31, Axel Wagner <axel.wagner...@googlemail.com>
> wrote:
> 
> > Hi,
> >
> > one way to solve your problem is to wrap the body into an io.Reader that
> > strips off everything after the first `=` it finds. That can then be fed to
> > base64.RawStdEncoding. This approach requires no extra buffering or copying
> > and is easy to implement: https://go.dev/play/p/CwcVz7oietI
> >
> > The downside is, that this will not verify that the body is *either*
> > correctly padded Base64 *or* unpadded Base64. So, it will not report an
> > error if fed something like "AAA=garbage".
> > That can be remedied by buffering up to four bytes and, when encountering
> > an EOF, check that there are at most three trailing `=` and that the total
> > length of the stream is divisible by four. It's more finicky to implement,
> > but it should also be possible without any extra copies and only requires a
> > very small extra buffer.
> >
> > On Sun, 12 Jan 2025 at 22:29, Rory Campbell-Lange <r...@campbell-lange.net>
> > wrote:
> >
> >> Thanks very much for the links, pointers and possible solution.
> >>
> >> Trying to read base64 standard (padded) encoded data with
> >> base64.RawStdEncoding can produce an error such as
> >>
> >>     illegal base64 data at input byte <n>
> >>
> >> Reading base64 raw (unpadded) encoded data produces the EOF error.
> >>
> >> I'll go with trying to read the standard encoded data up to maybe 1MB and
> >> then switch to base64.RawStdEncoding if I hit the "illegal base64 data"
> >> problem, maybe with reference to bufio.Reader which has most of the methods
> >> suggested below.
> >>
> >> Yes, the use of a "Rewind" method would be crucial. I guess this would
> >> need to:
> >> 1. error if more than one buffer of data has been read
> >> 2. else re-read from byte 0
> >>
> >> Thanks again very much for these suggestions.
> >>
> >> Rory
> >>
> >> On 12/01/25, robert engels (reng...@ix.netcom.com) wrote:
> >> > Also, see this
> >> https://stackoverflow.com/questions/69753478/use-base64-stdencoding-or-base64-rawstdencoding-to-decode-base64-string-in-go
> >> as I expected the error should be reported earlier than the end of stream
> >> if the chosen format is wrong.
> >> >
> >> > > On Jan 12, 2025, at 2:57 PM, robert engels <reng...@ix.netcom.com>
> >> wrote:
> >> > >
> >> > > Also, this is what Gemini provided which looks basically correct -
> >> but I think encapsulating it with a Rewind() method would be easier to
> >> understand.
> >> > >
> >> > >
> >> > >
> >> > > While Go doesn't have a built-in PushbackReader like some other
> >> languages (e.g., Java), you can implement similar functionality using a
> >> custom struct and a buffer.
> >> > >
> >> > > Here's an example implementation:
> >> > >
> >> > > package main
> >> > >
> >> > > import (
> >> > >     "bytes"
> >> > >     "io"
> >> > > )
> >> > >
> >> > > type PushbackReader struct {
> >> > >     reader io.Reader
> >> > >     buffer *bytes.Buffer
> >> > > }
> >> > >
> >> > > func NewPushbackReader(r io.Reader) *PushbackReader {
> >> > >     return &PushbackReader{
> >> > >         reader: r,
> >> > >         buffer: new(bytes.Buffer),
> >> > >     }
> >> > > }
> >> > >
> >> > > func (p *PushbackReader) Read(b []byte) (n int, err error) {
> >> > >     if p.buffer.Len() > 0 {
> >> > >         return p.buffer.Read(b)
> >> > >     }
> >> > >     return p.reader.Read(b)
> >> > > }
> >> > >
> >> > > func (p *PushbackReader) UnreadByte() error {
> >> > >     if p.buffer.Len() == 0 {
> >> > >         return io.EOF
> >> > >     }
> >> > >     lastByte := p.buffer.Bytes()[p.buffer.Len()-1]
> >> > >     p.buffer.Truncate(p.buffer.Len() - 1)
> >> > >     p.buffer.WriteByte(lastByte)
> >> > >     return nil
> >> > > }
> >> > >
> >> > > func (p *PushbackReader) Unread(buf []byte) error {
> >> > >     if p.buffer.Len() == 0 {
> >> > >         return io.EOF
> >> > >     }
> >> > >     p.buffer.Write(buf)
> >> > >     return nil
> >> > > }
> >> > >
> >> > > func main() {
> >> > >     // Example usage
> >> > >     r := NewPushbackReader(bytes.NewBufferString("Hello, World!"))
> >> > >     buf := make([]byte, 5)
> >> > >     r.Read(buf)
> >> > >     r.UnreadByte()
> >> > >     r.Read(buf)
> >> > > }
> >> > >
> >> > > Explanation:
> >> > > PushbackReader struct: This struct holds the underlying io.Reader and
> >> a buffer to store the pushed-back bytes.
> >> > > NewPushbackReader: This function creates a new PushbackReader from an
> >> existing io.Reader.
> >> > > Read method: This method reads bytes from either the buffer (if it
> >> contains data) or the underlying reader.
> >> > > UnreadByte method: This method pushes back a single byte into the
> >> buffer.
> >> > > Unread method: This method pushes back a slice of bytes into the
> >> buffer.
> >> > > Important Considerations:
> >> > > The buffer size is not managed automatically. You may need to adjust
> >> the buffer size based on your use case.
> >> > > This implementation does not handle pushing back beyond the initially
> >> read data. If you need to support arbitrary pushback, you'll need a more
> >> complex solution.
> >> > >
> >> > > Generative AI is experimental.
> >> > >
> >> > >> On Jan 12, 2025, at 2:53 PM, Robert Engels <reng...@ix.netcom.com>
> >> wrote:
> >> > >>
> >> > >> You can see the two pass reader here
> >> https://stackoverflow.com/questions/20666594/how-can-i-push-bytes-into-a-reader-in-go
> >> > >>
> >> > >> But yea, the basic premise is that you buffer the data so you can
> >> rewind if needed
> >> > >>
> >> > >> Are you certain it is reading to the end to return EOF? It may be
> >> returning eof once the parsing fails.
> >> > >>
> >> > >> Otherwise I would expect this is being decoded wrong - eg the mime
> >> type or encoding type should tell you the correct format before you start
> >> decoding.
> >> > >>
> >> > >>> On Jan 12, 2025, at 2:46 PM, Rory Campbell-Lange <
> >> r...@campbell-lange.net> wrote:
> >> > >>>
> >> > >>> Thanks for the suggestion of a ReadSeeker to wrap an io.Reader.
> >> > >>>
> >> > >>> My google fu must be deserting me. I can find PushbackReader
> >> implementations in Java, but the only similar thing for Go I could find was
> >> https://gitlab.com/osaki-lab/iowrapper. If you have a specific
> >> recommendation for a ReadSeeker wrapper to an io.Reader that would be great
> >> to know.
> >> > >>>
> >> > >>> Since the base64 decoding error I'm looking for is an EOF, I guess
> >> the wrapper approach will not work when the EOF byte position is > than the
> >> io.ReadSeeker buffer size.
> >> > >>>
> >> > >>> Rory
> >> > >>>
> >> > >>> On 12/01/25, robert engels (reng...@ix.netcom.com) wrote:
> >> > >>>> create a ReadSeeker that wraps the Reader providing the buffering
> >> (mark & reset) - normally the buffer only needs to be large enough to
> >> detect the format contained in the Reader.
> >> > >>>>
> >> > >>>> You can search Google for PushbackReader in Go and you’ll get a
> >> basic implementation.
> >> > >>>>
> >> > >>>>> On Jan 12, 2025, at 12:52 PM, Rory Campbell-Lange <
> >> r...@campbell-lange.net> wrote:
> >> > >>> ...
> >> > >>>>> I'm attempting to rationalise the process [of avoiding reading
> >> email parts into byte slices] 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.
> >> > >>>>>
> >> > >>
> >> > >>
> >> > >> --
> >> > >> You received this message because you are subscribed to the Google
> >> Groups "golang-nuts" group.
> >> > >> To unsubscribe from this group and stop receiving emails from it,
> >> send an email to golang-nuts+unsubscr...@googlegroups.com <mailto:
> >> golang-nuts+unsubscr...@googlegroups.com>.
> >> > >> To view this discussion visit
> >> https://groups.google.com/d/msgid/golang-nuts/DD0C1480-D237-447A-B978-78FC8951FE05%40ix.netcom.com
> >> <
> >> https://groups.google.com/d/msgid/golang-nuts/DD0C1480-D237-447A-B978-78FC8951FE05%40ix.netcom.com?utm_medium=email&utm_source=footer
> >> >.
> >> > >
> >> >
> >>
> >> --
> >> You received this message because you are subscribed to the Google Groups
> >> "golang-nuts" group.
> >> To unsubscribe from this group and stop receiving emails from it, send an
> >> email to golang-nuts+unsubscr...@googlegroups.com.
> >> To view this discussion visit
> >> https://groups.google.com/d/msgid/golang-nuts/Z4Q0AFRkkoNH52_B%40campbell-lange.net
> >> .
> >>
> >

-- 
You received this message because you are subscribed to the Google Groups 
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion visit 
https://groups.google.com/d/msgid/golang-nuts/Z4UQYJmuk7Oe6xSG%40campbell-lange.net.

Reply via email to