net/textproto: can`t be used for rfc3030 (SMTP extensions for Large and Binary MIME Messages)

[norguhtar]

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

go version go1.15.7 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/root/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="direct"
GOROOT="/usr/lib/golang"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build209411982=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I try implement email client with support rfc3030, and got problem with writer.
Now net/textproto return me only DotWriter but rfc3030 don`t use dot (.) in close command.

What did you expect to see?

Got other writerCloser from net/textproto like DotWriter can be used for implement rfc3030

What did you see instead?

Only DotWriter. This writeCloser not compatible with rfc3030. When email sended like described in rfc3030 DATA command and dot not used for this process

[ianlancetaylor]

What would have to change to make this work?

[norguhtar]

For example we can add bdatWriter

@@ -13,8 +13,9 @@
 // A Writer implements convenience methods for writing
 // requests or responses to a text protocol network connection.
 type Writer struct {
-       W   *bufio.Writer
-       dot *dotWriter
+       W    *bufio.Writer
+       dot  *dotWriter
+       bdat *bdatWriter
 }
 
 // NewWriter returns a new Writer writing to w.
@@ -46,6 +47,11 @@
        return w.dot
 }
 
+func (w *Writer) BdatWriter() io.WriteCloser {
+       w.bdat = &bdatWriter{w: w}
+       return w.bdat
+}
+
 func (w *Writer) closeDot() {
        if w.dot != nil {
                w.dot.Close() // sets w.dot = nil
@@ -57,6 +63,10 @@
        state int
 }
 
+type bdatWriter struct {
+       w *Writer
+}
+
 const (
        wstateBegin     = iota // initial state; must be zero
        wstateBeginLine        // beginning of line
@@ -117,3 +127,26 @@
        }
        return bw.Flush()
 }
+
+func (r *bdatWriter) Write(b []byte) (n int, err error) {
+       bw := r.w.W
+       bLen := len(b)
+       if bLen > 0 {
+               bdatHeader := fmt.Sprintf("BDAT %d \r\n", bLen)
+               bw.Write([]byte(bdatHeader))
+               n, err = bw.Write(b)
+               if err == nil {
+                       err = bw.Flush()
+               }
+       }
+       return
+}
+
+func (r *bdatWriter) Close() error {
+       bw := r.w.W
+       _, err := bw.Write([]byte("BDAT 0 LAST\r\n"))
+       if err != nil {
+               return err
+       }
+       return bw.Flush()
+}

Or just create RawWriter

@@ -13,8 +13,9 @@
 // A Writer implements convenience methods for writing
 // requests or responses to a text protocol network connection.
 type Writer struct {
-       W   *bufio.Writer
-       dot *dotWriter
+       W    *bufio.Writer
+       dot  *dotWriter
+       raw *rawWriter
 }
 
 // NewWriter returns a new Writer writing to w.
@@ -46,6 +47,11 @@
        return w.dot
 }
 
+func (w *Writer) RawWriter() io.WriteCloser {
+       w.raw = &RawWriter{w: w}
+       return w.raw
+}
+
 func (w *Writer) closeDot() {
        if w.dot != nil {
                w.dot.Close() // sets w.dot = nil
@@ -57,6 +63,10 @@
        state int
 }
 
+type rawWriter struct {
+       w *Writer
+}
+
 const (
        wstateBegin     = iota // initial state; must be zero
        wstateBeginLine        // beginning of line
@@ -117,3 +127,26 @@
        }
        return bw.Flush()
 }
+
+func (r *rawWriter) Write(b []byte) (n int, err error) {
+       bw := r.w.W
+       n, err = bw.Write(b)
+       if err == nil {
+           bw.Flush()
+       }
+       return
+}
+
+func (r *bdatWriter) Close() error {
+       bw := r.w.W
+       return bw.Flush()
+}

And then create an external interface for transmission using the BDAT command.

I tried the first option, it worked as it should. But there is no control over how the result is sent. I think the second option is more general and useful

[ianlancetaylor]

OK, this sounds like we need new API, which should go through the proposal process (https://golang.org/s/proposal).

[norguhtar]

Ok. I need create new issue?

[ianlancetaylor]

Creating a new issue will be simpler. Thanks.