GODT-1044: lite parser

pkg/message/encrypt.go

@@ -0,0 +1,244 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"bytes"
+	"encoding/base64"
+	"io"
+	"io/ioutil"
+	"mime"
+	"mime/quotedprintable"
+	"strings"
+
+	"github.com/ProtonMail/gopenpgp/v2/crypto"
+	pmmime "github.com/ProtonMail/proton-bridge/pkg/mime"
+	"github.com/emersion/go-message/textproto"
+)
+
+func EncryptRFC822(kr *crypto.KeyRing, r io.Reader) ([]byte, error) {
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		return nil, err
+	}
+
+	header, body, err := readHeaderBody(b)
+	if err != nil {
+		return nil, err
+	}
+
+	buf := new(bytes.Buffer)
+
+	result, err := writeEncryptedPart(kr, header, bytes.NewReader(body))
+	if err != nil {
+		return nil, err
+	}
+
+	if err := textproto.WriteHeader(buf, *header); err != nil {
+		return nil, err
+	}
+
+	if _, err := result.WriteTo(buf); err != nil {
+		return nil, err
+	}
+
+	return buf.Bytes(), nil
+}
+
+func writeEncryptedPart(kr *crypto.KeyRing, header *textproto.Header, r io.Reader) (io.WriterTo, error) {
+	decoder := getTransferDecoder(r, header.Get("Content-Transfer-Encoding"))
+	encoded := new(bytes.Buffer)
+
+	contentType, contentParams, err := parseContentType(header.Get("Content-Type"))
+	if err != nil {
+		return nil, err
+	}
+
+	switch {
+	case contentType == "", strings.HasPrefix(contentType, "text/"), strings.HasPrefix(contentType, "message/"):
+		header.Del("Content-Transfer-Encoding")
+
+		if charset, ok := contentParams["charset"]; ok {
+			if reader, err := pmmime.CharsetReader(charset, decoder); err == nil {
+				decoder = reader
+
+				// We can decode the charset to utf-8 so let's set that as the content type charset parameter.
+				contentParams["charset"] = "utf-8"
+
+				header.Set("Content-Type", mime.FormatMediaType(contentType, contentParams))
+			}
+		}
+
+		if err := encode(&writeCloser{encoded}, func(w io.Writer) error {
+			return writeEncryptedTextPart(w, decoder, kr)
+		}); err != nil {
+			return nil, err
+		}
+
+	case contentType == "multipart/encrypted":
+		if _, err := encoded.ReadFrom(decoder); err != nil {
+			return nil, err
+		}
+
+	case strings.HasPrefix(contentType, "multipart/"):
+		if err := encode(&writeCloser{encoded}, func(w io.Writer) error {
+			return writeEncryptedMultiPart(kr, w, header, decoder)
+		}); err != nil {
+			return nil, err
+		}
+
+	default:
+		header.Set("Content-Transfer-Encoding", "base64")
+
+		if err := encode(base64.NewEncoder(base64.StdEncoding, encoded), func(w io.Writer) error {
+			return writeEncryptedAttachmentPart(w, decoder, kr)
+		}); err != nil {
+			return nil, err
+		}
+	}
+
+	return encoded, nil
+}
+
+func writeEncryptedTextPart(w io.Writer, r io.Reader, kr *crypto.KeyRing) error {
+	dec, err := ioutil.ReadAll(r)
+	if err != nil {
+		return err
+	}
+
+	var arm string
+
+	if msg, err := crypto.NewPGPMessageFromArmored(string(dec)); err != nil {
+		enc, err := kr.Encrypt(crypto.NewPlainMessage(dec), kr)
+		if err != nil {
+			return err
+		}
+
+		if arm, err = enc.GetArmored(); err != nil {
+			return err
+		}
+	} else if arm, err = msg.GetArmored(); err != nil {
+		return err
+	}
+
+	if _, err := io.WriteString(w, arm); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func writeEncryptedAttachmentPart(w io.Writer, r io.Reader, kr *crypto.KeyRing) error {
+	dec, err := ioutil.ReadAll(r)
+	if err != nil {
+		return err
+	}
+
+	enc, err := kr.Encrypt(crypto.NewPlainMessage(dec), kr)
+	if err != nil {
+		return err
+	}
+
+	if _, err := w.Write(enc.GetBinary()); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func writeEncryptedMultiPart(kr *crypto.KeyRing, w io.Writer, header *textproto.Header, r io.Reader) error {
+	_, contentParams, err := parseContentType(header.Get("Content-Type"))
+	if err != nil {
+		return err
+	}
+
+	scanner, err := newPartScanner(r, contentParams["boundary"])
+	if err != nil {
+		return err
+	}
+
+	parts, err := scanner.scanAll()
+	if err != nil {
+		return err
+	}
+
+	writer := newPartWriter(w, contentParams["boundary"])
+
+	for _, part := range parts {
+		header, body, err := readHeaderBody(part.b)
+		if err != nil {
+			return err
+		}
+
+		result, err := writeEncryptedPart(kr, header, bytes.NewReader(body))
+		if err != nil {
+			return err
+		}
+
+		if err := writer.createPart(func(w io.Writer) error {
+			if err := textproto.WriteHeader(w, *header); err != nil {
+				return err
+			}
+
+			if _, err := result.WriteTo(w); err != nil {
+				return err
+			}
+
+			return nil
+		}); err != nil {
+			return err
+		}
+	}
+
+	return writer.done()
+}
+
+func getTransferDecoder(r io.Reader, encoding string) io.Reader {
+	switch strings.ToLower(encoding) {
+	case "base64":
+		return base64.NewDecoder(base64.StdEncoding, r)
+
+	case "quoted-printable":
+		return quotedprintable.NewReader(r)
+
+	default:
+		return r
+	}
+}
+
+func encode(wc io.WriteCloser, fn func(io.Writer) error) error {
+	if err := fn(wc); err != nil {
+		return err
+	}
+
+	return wc.Close()
+}
+
+type writeCloser struct {
+	io.Writer
+}
+
+func (writeCloser) Close() error { return nil }
+
+func parseContentType(val string) (string, map[string]string, error) {
+	if val == "" {
+		val = "text/plain"
+	}
+
+	return pmmime.ParseMediaType(val)
+}

pkg/message/encrypt_test.go

@@ -0,0 +1,101 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"bytes"
+	"io/ioutil"
+	"testing"
+
+	"github.com/ProtonMail/gopenpgp/v2/crypto"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEncryptRFC822(t *testing.T) {
+	literal, err := ioutil.ReadFile("testdata/text_plain_latin1.eml")
+	require.NoError(t, err)
+
+	key, err := crypto.GenerateKey("name", "email", "rsa", 2048)
+	require.NoError(t, err)
+
+	kr, err := crypto.NewKeyRing(key)
+	require.NoError(t, err)
+
+	enc, err := EncryptRFC822(kr, bytes.NewReader(literal))
+	require.NoError(t, err)
+
+	section(t, enc).
+		expectContentType(is(`text/plain`)).
+		expectContentTypeParam(`charset`, is(`utf-8`)).
+		expectBody(decryptsTo(kr, `ééééééé`))
+}
+
+func TestEncryptRFC822Multipart(t *testing.T) {
+	literal, err := ioutil.ReadFile("testdata/multipart_alternative_nested.eml")
+	require.NoError(t, err)
+
+	key, err := crypto.GenerateKey("name", "email", "rsa", 2048)
+	require.NoError(t, err)
+
+	kr, err := crypto.NewKeyRing(key)
+	require.NoError(t, err)
+
+	enc, err := EncryptRFC822(kr, bytes.NewReader(literal))
+	require.NoError(t, err)
+
+	section(t, enc).
+		expectContentType(is(`multipart/alternative`))
+
+	section(t, enc, 1).
+		expectContentType(is(`multipart/alternative`))
+
+	section(t, enc, 1, 1).
+		expectContentType(is(`text/plain`)).
+		expectBody(decryptsTo(kr, "*multipart 1.1*\n\n"))
+
+	section(t, enc, 1, 2).
+		expectContentType(is(`text/html`)).
+		expectBody(decryptsTo(kr, `<html>
+  <head>
+    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
+  </head>
+  <body>
+    <b>multipart 1.2</b>
+  </body>
+</html>
+`))
+
+	section(t, enc, 2).
+		expectContentType(is(`multipart/alternative`))
+
+	section(t, enc, 2, 1).
+		expectContentType(is(`text/plain`)).
+		expectBody(decryptsTo(kr, "*multipart 2.1*\n\n"))
+
+	section(t, enc, 2, 2).
+		expectContentType(is(`text/html`)).
+		expectBody(decryptsTo(kr, `<html>
+  <head>
+    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
+  </head>
+  <body>
+    <b>multipart 2.2</b>
+  </body>
+</html>
+`))
+}

pkg/message/flags.go

@@ -59,30 +59,3 @@ func GetFlags(m *pmapi.Message) (flags []string) {
 
 	return
 }
-
-// ParseFlags sets attributes to pmapi messages based on imap flags.
-func ParseFlags(m *pmapi.Message, flags []string) {
-	if m.Header.Get("received") == "" {
-		m.Flags = pmapi.FlagSent
-	} else {
-		m.Flags = pmapi.FlagReceived
-	}
-
-	m.Unread = true
-	for _, f := range flags {
-		switch f {
-		case imap.SeenFlag:
-			m.Unread = false
-		case imap.DraftFlag:
-			m.Flags &= ^pmapi.FlagSent
-			m.Flags &= ^pmapi.FlagReceived
-			m.LabelIDs = append(m.LabelIDs, pmapi.DraftLabel)
-		case imap.FlaggedFlag:
-			m.LabelIDs = append(m.LabelIDs, pmapi.StarredLabel)
-		case imap.AnsweredFlag:
-			m.Flags |= pmapi.FlagReplied
-		case AppleMailJunkFlag, ThunderbirdJunkFlag:
-			m.LabelIDs = append(m.LabelIDs, pmapi.SpamLabel)
-		}
-	}
-}

pkg/message/header.go

@@ -0,0 +1,123 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"io/ioutil"
+
+	"github.com/emersion/go-message/textproto"
+	"github.com/pkg/errors"
+)
+
+// HeaderLines returns each line in the given header.
+func HeaderLines(header []byte) [][]byte {
+	var (
+		lines [][]byte
+		quote int
+	)
+
+	forEachLine(bufio.NewReader(bytes.NewReader(header)), func(line []byte) {
+		switch {
+		case len(bytes.TrimSpace(line)) == 0:
+			lines = append(lines, line)
+
+		case quote%2 != 0, len(bytes.SplitN(line, []byte(`: `), 2)) != 2:
+			if len(lines) > 0 {
+				lines[len(lines)-1] = append(lines[len(lines)-1], line...)
+			} else {
+				lines = append(lines, line)
+			}
+
+		default:
+			lines = append(lines, line)
+		}
+
+		quote += bytes.Count(line, []byte(`"`))
+	})
+
+	return lines
+}
+
+func forEachLine(br *bufio.Reader, fn func([]byte)) {
+	for {
+		b, err := br.ReadBytes('\n')
+		if err != nil {
+			if !errors.Is(err, io.EOF) {
+				panic(err)
+			}
+
+			if len(b) > 0 {
+				fn(b)
+			}
+
+			return
+		}
+
+		fn(b)
+	}
+}
+
+func readHeaderBody(b []byte) (*textproto.Header, []byte, error) {
+	rawHeader, body, err := splitHeaderBody(b)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	var header textproto.Header
+
+	for _, line := range HeaderLines(rawHeader) {
+		if len(bytes.TrimSpace(line)) > 0 {
+			header.AddRaw(line)
+		}
+	}
+
+	return &header, body, nil
+}
+
+func splitHeaderBody(b []byte) ([]byte, []byte, error) {
+	br := bufio.NewReader(bytes.NewReader(b))
+
+	var header []byte
+
+	for {
+		b, err := br.ReadBytes('\n')
+		if err != nil {
+			if !errors.Is(err, io.EOF) {
+				panic(err)
+			}
+
+			break
+		}
+
+		header = append(header, b...)
+
+		if len(bytes.TrimSpace(b)) == 0 {
+			break
+		}
+	}
+
+	body, err := ioutil.ReadAll(br)
+	if err != nil && !errors.Is(err, io.EOF) {
+		return nil, nil, err
+	}
+
+	return header, body, nil
+}

pkg/message/header_test.go

@@ -0,0 +1,76 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestHeaderLines(t *testing.T) {
+	const header = "To: somebody\r\nFrom: somebody else\r\nSubject: this is\r\n\ta multiline field\r\n\r\n"
+
+	assert.Equal(t, [][]byte{
+		[]byte("To: somebody\r\n"),
+		[]byte("From: somebody else\r\n"),
+		[]byte("Subject: this is\r\n\ta multiline field\r\n"),
+		[]byte("\r\n"),
+	}, HeaderLines([]byte(header)))
+}
+
+func TestHeaderLinesMultilineFilename(t *testing.T) {
+	const header = "Content-Type: application/msword; name=\"this is a very long\nfilename.doc\""
+
+	assert.Equal(t, [][]byte{
+		[]byte("Content-Type: application/msword; name=\"this is a very long\nfilename.doc\""),
+	}, HeaderLines([]byte(header)))
+}
+
+func TestHeaderLinesMultilineFilenameWithColon(t *testing.T) {
+	const header = "Content-Type: application/msword; name=\"this is a very long\nfilename: too long.doc\""
+
+	assert.Equal(t, [][]byte{
+		[]byte("Content-Type: application/msword; name=\"this is a very long\nfilename: too long.doc\""),
+	}, HeaderLines([]byte(header)))
+}
+
+func TestHeaderLinesMultilineFilenameWithColonAndNewline(t *testing.T) {
+	const header = "Content-Type: application/msword; name=\"this is a very long\nfilename: too long.doc\"\n"
+
+	assert.Equal(t, [][]byte{
+		[]byte("Content-Type: application/msword; name=\"this is a very long\nfilename: too long.doc\"\n"),
+	}, HeaderLines([]byte(header)))
+}
+
+func TestHeaderLinesMultipleMultilineFilenames(t *testing.T) {
+	const header = `Content-Type: application/msword; name="=E5=B8=B6=E6=9C=89=E5=A4=96=E5=9C=8B=E5=AD=97=E7=AC=A6=E7=9A=84=E9=99=84=E4=
+=BB=B6.DOC"
+Content-Transfer-Encoding: base64
+Content-Disposition: attachment; filename="=E5=B8=B6=E6=9C=89=E5=A4=96=E5=9C=8B=E5=AD=97=E7=AC=A6=E7=9A=84=E9=99=84=E4=
+=BB=B6.DOC"
+Content-ID: <>
+`
+
+	assert.Equal(t, [][]byte{
+		[]byte("Content-Type: application/msword; name=\"=E5=B8=B6=E6=9C=89=E5=A4=96=E5=9C=8B=E5=AD=97=E7=AC=A6=E7=9A=84=E9=99=84=E4=\n=BB=B6.DOC\"\n"),
+		[]byte("Content-Transfer-Encoding: base64\n"),
+		[]byte("Content-Disposition: attachment; filename=\"=E5=B8=B6=E6=9C=89=E5=A4=96=E5=9C=8B=E5=AD=97=E7=AC=A6=E7=9A=84=E9=99=84=E4=\n=BB=B6.DOC\"\n"),
+		[]byte("Content-ID: <>\n"),
+	}, HeaderLines([]byte(header)))
+}

pkg/message/scanner.go

@@ -0,0 +1,96 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+)
+
+type partScanner struct {
+	r *bufio.Reader
+
+	boundary string
+	progress int
+}
+
+type part struct {
+	b      []byte
+	offset int
+}
+
+func newPartScanner(r io.Reader, boundary string) (*partScanner, error) {
+	scanner := &partScanner{r: bufio.NewReader(r), boundary: boundary}
+
+	if _, _, err := scanner.readToBoundary(); err != nil {
+		return nil, err
+	}
+
+	return scanner, nil
+}
+
+func (s *partScanner) scanAll() ([]part, error) {
+	var parts []part
+
+	for {
+		offset := s.progress
+
+		b, more, err := s.readToBoundary()
+		if err != nil {
+			return nil, err
+		}
+
+		if !more {
+			return parts, nil
+		}
+
+		parts = append(parts, part{b: b, offset: offset})
+	}
+}
+
+func (s *partScanner) readToBoundary() ([]byte, bool, error) {
+	var res []byte
+
+	for {
+		line, err := s.r.ReadBytes('\n')
+		if err != nil {
+			if !errors.Is(err, io.EOF) {
+				return nil, false, err
+			}
+
+			if len(line) == 0 {
+				return nil, false, nil
+			}
+		}
+
+		s.progress += len(line)
+
+		switch {
+		case bytes.HasPrefix(bytes.TrimSpace(line), []byte("--"+s.boundary)):
+			return bytes.TrimSuffix(bytes.TrimSuffix(res, []byte("\n")), []byte("\r")), true, nil
+
+		case bytes.HasSuffix(bytes.TrimSpace(line), []byte(s.boundary+"--")):
+			return bytes.TrimSuffix(bytes.TrimSuffix(res, []byte("\n")), []byte("\r")), false, nil
+
+		default:
+			res = append(res, line...)
+		}
+	}
+}

pkg/message/scanner_test.go

@@ -0,0 +1,136 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestScanner(t *testing.T) {
+	const literal = `this part of the text should be ignored
+
+--longrandomstring
+
+body1
+
+--longrandomstring
+
+body2
+
+--longrandomstring--
+`
+
+	scanner, err := newPartScanner(strings.NewReader(literal), "longrandomstring")
+	require.NoError(t, err)
+
+	parts, err := scanner.scanAll()
+	require.NoError(t, err)
+
+	assert.Equal(t, "\nbody1\n", string(parts[0].b))
+	assert.Equal(t, "\nbody2\n", string(parts[1].b))
+
+	assert.Equal(t, "\nbody1\n", literal[parts[0].offset:parts[0].offset+len(parts[0].b)])
+	assert.Equal(t, "\nbody2\n", literal[parts[1].offset:parts[1].offset+len(parts[1].b)])
+}
+
+func TestScannerNested(t *testing.T) {
+	const literal = `This is the preamble.  It is to be ignored, though it 
+is a handy place for mail composers to include an 
+explanatory note to non-MIME compliant readers. 
+--simple boundary 
+Content-type: multipart/mixed; boundary="nested boundary" 
+
+This is the preamble.  It is to be ignored, though it 
+is a handy place for mail composers to include an 
+explanatory note to non-MIME compliant readers. 
+--nested boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does not end with a linebreak.
+--nested boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does end with a linebreak.
+
+--nested boundary--
+--simple boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does end with a linebreak.
+
+--simple boundary-- 
+This is the epilogue.  It is also to be ignored.
+`
+
+	scanner, err := newPartScanner(strings.NewReader(literal), "simple boundary")
+	require.NoError(t, err)
+
+	parts, err := scanner.scanAll()
+	require.NoError(t, err)
+
+	assert.Equal(t, `Content-type: multipart/mixed; boundary="nested boundary" 
+
+This is the preamble.  It is to be ignored, though it 
+is a handy place for mail composers to include an 
+explanatory note to non-MIME compliant readers. 
+--nested boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does not end with a linebreak.
+--nested boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does end with a linebreak.
+
+--nested boundary--`, string(parts[0].b))
+	assert.Equal(t, `Content-type: text/plain; charset=us-ascii
+
+This part does end with a linebreak.
+`, string(parts[1].b))
+}
+
+func TestScannerNoFinalLinebreak(t *testing.T) {
+	const literal = `--nested boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does not end with a linebreak.
+--nested boundary 
+Content-type: text/plain; charset=us-ascii
+
+This part does end with a linebreak.
+
+--nested boundary--`
+
+	scanner, err := newPartScanner(strings.NewReader(literal), "nested boundary")
+	require.NoError(t, err)
+
+	parts, err := scanner.scanAll()
+	require.NoError(t, err)
+
+	assert.Equal(t, `Content-type: text/plain; charset=us-ascii
+
+This part does not end with a linebreak.`, string(parts[0].b))
+	assert.Equal(t, `Content-type: text/plain; charset=us-ascii
+
+This part does end with a linebreak.
+`, string(parts[1].b))
+}

pkg/message/writer.go

@@ -0,0 +1,48 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package message
+
+import (
+	"fmt"
+	"io"
+)
+
+type partWriter struct {
+	w        io.Writer
+	boundary string
+}
+
+func newPartWriter(w io.Writer, boundary string) *partWriter {
+	return &partWriter{w: w, boundary: boundary}
+}
+
+func (w *partWriter) createPart(fn func(io.Writer) error) error {
+	if _, err := fmt.Fprintf(w.w, "\r\n--%v\r\n", w.boundary); err != nil {
+		return err
+	}
+
+	return fn(w.w)
+}
+
+func (w *partWriter) done() error {
+	if _, err := fmt.Fprintf(w.w, "\r\n--%v--\r\n", w.boundary); err != nil {
+		return err
+	}
+
+	return nil
+}