Question

我正在尝试找出如何在Go中为电子邮件构建multipart/mime envelopes。以下代码生成正确嵌套的主体-但边界未正确插入。

您可以在https://play.golang.org/p/XLc4DQFObRn上观看演示

package main

import (
    "bytes"
    "fmt"
    "io"
    "log"
    "math/rand"
    "mime/multipart"
    "mime/quotedprintable"
    "net/textproto"
)

//  multipart/mixed
//  |- multipart/related
//  |  |- multipart/alternative
//  |  |  |- text/plain
//  |  |  `- text/html
//  |  `- inlines..
//  `- attachments..


func main() {

    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)

    var part io.Writer
    var err error

    // Text Content
    part, err = writer.CreatePart(textproto.MIMEHeader{"Content-Type": {"multipart/alternative"}})
    if err != nil {
        log.Fatal(err)
    }

    childWriter := multipart.NewWriter(part)

    var subpart io.Writer
    for _, contentType := range []string{"text/plain", "text/html"} {
        subpart, err = CreateQuoteTypePart(childWriter, contentType)
        if err != nil {
            log.Fatal(err)
        }
        _, err := subpart.Write([]byte("This is a line of text that needs to be wrapped by quoted-printable before it goes to far.\r\n\r\n"))
        if err != nil {
            log.Fatal(err)
        }
    }

    // Attachments
    filename := fmt.Sprintf("File_%d.jpg", rand.Int31())
    part, err = writer.CreatePart(textproto.MIMEHeader{"Content-Type": {"application/octet-stream"}, "Content-Disposition": {"attachment; filename=" + filename}})
    if err != nil {
        log.Fatal(err)
    }
    part.Write([]byte("AABBCCDDEEFF"))

    writer.Close()

    fmt.Print(`From: Bob <bob@example.com>
To: Alice <alias@example.com>
Subject: Formatted text mail
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=`)
    fmt.Println(writer.Boundary())
    fmt.Println(body.String())

}

// https://github.com/domodwyer/mailyak/blob/master/attachments.go#L142
func CreateQuoteTypePart(writer *multipart.Writer, contentType string) (part io.Writer, err error) {
    header := textproto.MIMEHeader{
        "Content-Type":              []string{contentType},
        "Content-Transfer-Encoding": []string{"quoted-printable"},
    }

    part, err = writer.CreatePart(header)
    if err != nil {
        return
    }
    part = quotedprintable.NewWriter(part)
    return
}

我想坚持使用标准库（stdlib）的答案，并避免让third一方attempts参与讨论。

Answer 1

不幸的是，用于编写多部分MIME消息的标准库支持具有错误的嵌套API。问题在于，在创建编写器之前，必须在标头中设置boundary字符串，但是在创建编写器之前，生成的边界字符串显然不可用。因此，您必须显式设置边界字符串。

这是我的解决方案（runnable in the Go Playground），为简洁起见，对其进行了简化。我选择使用外部书写器的边界来设置内部书写器的边界，并添加标签以使其在读取输出时更易于跟踪。

package main

import ("bytes"; "fmt"; "io"; "log"; "math/rand"; "mime/multipart"; "net/textproto")

//  multipart/mixed
//  |- multipart/related
//  |  |- multipart/alternative
//  |  |  |- text/plain
//  |  |  `- text/html
//  |  `- inlines..
//  `- attachments..

func main() {
    mixedContent := &bytes.Buffer{}
    mixedWriter := multipart.NewWriter(mixedContent)

    // related content, inside mixed
    var newBoundary = "RELATED-" + mixedWriter.Boundary()
    mixedWriter.SetBoundary(first70("MIXED-" + mixedWriter.Boundary()))

    relatedWriter, newBoundary := nestedMultipart(mixedWriter, "multipart/related", newBoundary)
    altWriter, newBoundary := nestedMultipart(relatedWriter, "mulitpart/alternative", "ALTERNATIVE-" + newBoundary)

    // Actual content alternatives (finally!)
    var childContent io.Writer

    childContent, _ = altWriter.CreatePart(textproto.MIMEHeader{"Content-Type": {"text/plain"}})
    childContent.Write([]byte("This is a line of text\r\n\r\n"))
    childContent, _ = altWriter.CreatePart(textproto.MIMEHeader{"Content-Type": {"text/html"}})
    childContent.Write([]byte("<html>HTML goes here\r\n</html>\r\n"))
    altWriter.Close()

    relatedWriter.Close()

    // Attachments
    filename := fmt.Sprintf("File_%d.jpg", rand.Int31())
    var fileContent io.Writer

    fileContent, _ = mixedWriter.CreatePart(textproto.MIMEHeader{"Content-Type": {"application/octet-stream"}, "Content-Disposition": {"attachment; filename=" + filename}})
    fileContent.Write([]byte("AABBCCDDEEFF"))

    mixedWriter.Close()

    fmt.Print(`From: Bob <bob@example.com>
To: Alice <alias@example.com>
Subject: Formatted text mail
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=`)
    fmt.Print(mixedWriter.Boundary(), "\n\n")
    fmt.Println(mixedContent.String())

}

func nestedMultipart(enclosingWriter *multipart.Writer, contentType, boundary string) (nestedWriter *multipart.Writer, newBoundary string) {

    var contentBuffer io.Writer
    var err error

    boundary = first70(boundary)
    contentWithBoundary := contentType + "; boundary=\"" + boundary + "\""
    contentBuffer, err = enclosingWriter.CreatePart(textproto.MIMEHeader{"Content-Type": {contentWithBoundary}})
    if err != nil {
        log.Fatal(err)
    }

    nestedWriter = multipart.NewWriter(contentBuffer)
    newBoundary = nestedWriter.Boundary()
    nestedWriter.SetBoundary(boundary)
    return
}

func first70(str string) string {
    if len(str) > 70 {
        return string(str[0:69])
    }
    return str
}

如何在Go中为电子邮件创建嵌套的多部分/ MIME信封？

1 个答案: