Sorry ignore that, I hadn't checked your playground link. On Tuesday, 14 January 2025 at 10:07:53 UTC Brian Candler 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. > > 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 >> >> > > >> >. >> > > >> > > >> > > >> > >> > > >> >> > > >> -- >> > > >> 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. >> > > >> 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...@googlegroups.com. >> > To view this discussion visit >> https://groups.google.com/d/msgid/golang-nuts/Z4UQYJmuk7Oe6xSG%40campbell-lange.net. >> >> >> > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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