golang: parsing emails with mime, mime/multipart and net/mail packages

msg_test.go

pacakge msgtest

import (
	"io"
	"mime"
	"mime/multipart"
	"net/mail"
	"net/textproto"
	"os"
	"path/filepath"
	"strings"
	"testing"

	"github.com/pkg/errors"
)

func TestMsgParse(t *testing.T) {
	r, _ := os.Open(filepath.Join("testdata", "yourfile.eml"))

	readMsg(t, r)
}

func getMediaType(t *testing.T, h textproto.MIMEHeader) (mediaType string, params map[string]string) {
	cth := h.Get(hnContentType)
	mediaType, params, err := mime.ParseMediaType(cth)
	if err != nil {
		t.Fatalf("Failed to parse content type header %q: %v", cth, err)
	}

	return mediaType, params
}

func isMultipart(mediaType string) bool {
	return strings.HasPrefix(mediaType, "multipart/")
}

func readMultipart(t *testing.T, r *multipart.Reader) {
	for {
		p, err := r.NextPart()
		switch err {
		case nil:
			// Carry on
		case io.EOF:
			return
		default:
			t.Fatalf("%+v", errors.WithStack(err))
		}

		readPart(t, p)
	}
}

func readPart(t *testing.T, p *multipart.Part) {
	mediaType, params := getMediaType(t, p.Header)

	if isMultipart(mediaType) {
		boundary := params[hpBoundary]

		t.Logf("Reading multipart with boundary %q", boundary)

		pr := multipart.NewReader(p, boundary)
		readMultipart(t, pr)
		return
	} else if mediaType == "message/rfc822" {
		t.Log("Part is attached email")
		readMsg(t, p)
	}

	t.Logf("Not multipart: %s", p.Header.Get(hnContentType))
}

func readMsg(t *testing.T, r io.Reader) {
	t.Log("Reading message")

	m, err := mail.ReadMessage(r)
	if err != nil {
		t.Fatalf("%+v", errors.WithStack(err))
	}

	mediaType, params := getMediaType(t, textproto.MIMEHeader(m.Header))

	if !isMultipart(mediaType) {
		t.Logf("Not multipart: %s", m.Header.Get(hnContentType))
		return
	}

	t.Logf("Reading multipart with boundary %q", params[hpBoundary])

	mr := multipart.NewReader(m.Body, params[hpBoundary])
	readMultipart(t, mr)
}