multipart: NextPart: EOF error for multipart/mixed content type

[swagftw]

I was trying the read the mail using go-imap and go-message libraries, but for a particular mail I am getting
multipart: NextPart: EOF error on part, err := reader.NextPart() where reader is the mail.CreateReader.

content type of the email is as following. I got it from gmail client show original mail

Content-Type: multipart/mixed; boundary="-----SECBOUND"

-------SECBOUND
Content-Type: text/html
Content-Transfer-Encoding: 8bit

html content only

-------SECBOUND--

[emersion]

What is the code reading this message?

NextPart will return EOF when there are not more parts to read.

[swagftw]

part, err := reader.NextPart()
if err == io.EOF {
	break
} else if err != nil && !message.IsUnknownCharset(err) {
	break
}

From so long I was thinking that this is a bug because I was already handling the io.EOF error, but as you said it should be just another EOF error, I dug deeper.

The code is pretty straight forward and I am not doing anything fancy here. The error is getting populated through

go-message/textproto/multipart.go

Line 262 in 8138ff9

return nil, fmt.Errorf("multipart: NextPart: %v", err)

But I am not sure why it is not just another io.EOF even though the whole body itself is of multipart/mixed type. Also we don't have a concrete error type to check/match the error if this occurs.

How do you think is the right way to handle this error for now?

[emersion]

I've tried running your code with your source message, but I don't get the error: https://go.dev/play/p/cSU0T0XgRL7

[rexxDigital]

This still happens by the way, got me confused. when just returning an io.EOF would be sufficient.

[emersion]

Please post a playground link with a reproducer, if you can reproduce.

[rexxDigital]

Please post a playground link with a reproducer, if you can reproduce.

Cant post with the actual mail since its private info. But this happens rarely with mails received from certain platforms. Wouldnt returning an io.EOF be just fine here? or create an error type like MultipartEOF?

Can create a pr for this if you want me to

[emersion]

No, the error happens when EOF is unexpected in the middle of a multipart message, so it doesn't make sense to ignore it or treat it like a regular EOF.

[rexxDigital]

No, the error happens when EOF is unexpected in the middle of a multipart message, so it doesn't make sense to ignore it or treat it like a regular EOF.

I see, from my own testing when this happens we have already gotten the body in both html and plain text. Letting the loop run ends in it never breaking as it does not reach the io.EOF return. Having an error type for this would good so we can atleast check for it.

Edit: While debugging i see that it does indeed reach EOF, the mail is wrongly formatted and does not have the --boundary-- after the html part.

[emersion]

If you don't care about errors after the parts you're interested in, you can stop the loop as soon as you've seen these parts.