> 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.
As I asked before, why would you ever need to use RawStdEncoding? It just
means the MIME part was invalid, most likely corrupted/truncated.
> One odd thing is that I'm getting extraneous newlines (shown by stars in
the output), eg:
You are feeding two different inputs which do not differ by truncation
alone.
% echo -n "Qm9uam91ciwgam95ZXV4IGxpb24K" | base64 -D | hexdump -c
0000000 B o n j o u r , j o y e u x
0000010 l i o n \n
0000015
% echo -n "IkJvbmpvdXIsIGpveWV1eCBsaW9uIg==" | base64 -D | hexdump -c
0000000 " B o n j o u r , j o y e u x
0000010 l i o n "
0000016
The second one has encoded double-quotes before and after the content.
On Monday, 13 January 2025 at 22:43:51 UTC Rory Campbell-Lange wrote:
> 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 (ro...@campbell-lange.net) 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 (axel.wa...@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.wa...@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 <
> ro...@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 (ren...@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 <
> ren...@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 <
> ren...@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 <
> > > >> ro...@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 (ren...@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 <
> > > >> ro...@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...@googlegroups.com <mailto:
> > > >> golang-nuts...@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
> > > >> >.
> > > >> > >
> > > >> >
> > > >>
> > > >> --
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> Groups
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> > > >> To unsubscribe from this group and stop receiving emails from it,
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> > > >> To view this discussion visit
> > > >>
> https://groups.google.com/d/msgid/golang-nuts/Z4Q0AFRkkoNH52_B%40campbell-lange.net
> > > >> .
> > > >>
> > > >
> >
> > --
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