mime/multipart: add (*Reader).NextRawPart to bypass automatic quoted-printable handling

[buchanae]

What version of Go are you using (go version)?

$ go version
go version go1.11.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"

GOBIN=""

GOCACHE="/Users/abuchanan/Library/Caches/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="darwin"

GOOS="darwin"

GOPATH="/Users/abuchanan/go"

GOPROXY=""

GORACE=""

GOROOT="/usr/local/go"

GOTMPDIR=""

GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"

GCCGO="gccgo"

CC="clang"

CXX="clang++"

CGO_ENABLED="1"

GOMOD="/Users/abuchanan/projects/mailer/go.mod"

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/r8/1_92bxy977zcfcj2jpp5j2dc0000gn/T/go-build702584477=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://play.golang.org/p/7x85EhvIbZZ

What did you expect to see?

I expected the part headers to include the Content-Transfer-Encoding as-is, and for the part body reader to return content as-is.

What did you see instead?

I found that Part has special handling for parts with Content-Transfer-Encoding: quoted-printable, which as introduced in issue #4411.

GitHub has also just led me to issue #26803 (which I failed to find before) which also proposes a way to bypass this via Part.NextRawPart. Unfortunately, although that proposal was accepted, the issue was closed by the submitter before being implemented.

Having access to the raw part body and unmodified headers is useful. In particular, for an IMAP server, I need to be able to inspect the part headers and body, including the Content-Transfer-Encoding header, in order to build the BODYSTRUCTURE response.

I can think of a couple workarounds for my particular server but they're not great, and I think this deserves a fix in the std library.

[buchanae]

To be clear, the signature would be func (r *Reader) NextRawPart() (*Part, error) and it would still decode the headers for you, it just wouldn't decode the body nor check the encoding header.

[bradfitz]

Still SGTM.

[gopherbot]

Change https://golang.org/cl/152877 mentions this issue: mime/multipart: add Part.NextRawPart() to avoid QP decoding

[abuchanan-nr]

As far as I can tell, this is ready to go still. The issue has the labels "help wanted" and "needs fix". Might want to double check whether those are still valid.

Would love to take this off my todo list.