AS I wrote earlier, I'm trying to avoid reading the entire email part into
memory to discover if I should use base64.StdEncoding or base64.RawStdEncoding.
The following seems to work reasonably well:
type B64Translator struct {
br *bufio.Reader
}
func NewB64Translator(r io.Reader) *B64Translator {
return &B64Translator{
br: bufio.NewReader(r),
}
}
// Read reads off the buffered reader expecting base64.StdEncoding bytes
// with (potentially) 1-3 '=' padding characters at the end.
// RawStdEncoding can be used for both StdEncoded and RawStdEncoded data
// if the padding is removed.
func (b *B64Translator) Read(p []byte) (n int, err error) {
h := make([]byte, len(p))
n, err = b.br.Read(h)
if err != nil {
return n, err
}
// to be optimised
c := bytes.Count(h, []byte("="))
copy(p, h[:n-c])
// fmt.Println(string(h), n, string(p), n-c)
return n - c, nil
}
https://go.dev/play/p/H6ii7Vy-8as
One odd thing is that I'm getting extraneous newlines (shown by stars in the
output), eg:
--
raw: Bonjour joyeux lion
Qm9uam91ciwgam95ZXV4IGxpb24K
ok: false
decoded: Bonjour, joyeux lion* <-------------------- e.g. here
--
std: "Bonjour, joyeux lion"
IkJvbmpvdXIsIGpveWV1eCBsaW9uIg==
ok: true
decoded: "Bonjour, joyeux lion"
--
Any thoughts on that would be gratefully received.
Rory
On 13/01/25, Rory Campbell-Lange ([email protected]) wrote:
> 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 ([email protected]) 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 <[email protected]>
> > 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
> > > <[email protected]>
> > > 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 ([email protected]) 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 <[email protected]>
> > >> 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 <[email protected]>
> > >> 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 <
> > >> [email protected]> 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 ([email protected]) 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 <
> > >> [email protected]> 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.
> > >> > >>>>>
> > >> > >>
> > >> > >>
> > >> > >> --
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> > >> Groups "golang-nuts" group.
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> > >> send an email to [email protected] <mailto:
> > >> [email protected]>.
> > >> > >> 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
> > >> >.
> > >> > >
> > >> >
> > >>
> > >> --
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> > >> .
> > >>
> > >
>
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