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. 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