satellite/email: add delimiter to close the last part of Multipart emails

The existing implementation doesn't send proper rfc1341 compatible mails
according to https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html as
the closing `--boundary_id--` is missing from our mails. (see 7.2.1 from
the standard).

Mailservers which are not very flexible, deny the mails.

Example mail log: (see the
missing boundary delimiter at the end).

```
Subject: Activate your email
From: "Storj DCS - EU1" <noreply@eu1.storj.io>
To: "..." <...>
...

--26d7220f6f1c9f6fb47b535319eb15dce513bb6e1d941a0efddf25e96712
Content-Type: text/html; charset=UTF-8

....

</body>
</html>
.
smtp: DATA error	{"msg_id":"6d82c9e3","reason":"unexpected EOF"}
smtp: 554 5.0.0 Internal server error (msg ID = 6d82c9e3)
```

This patch moves all the Multipart writing to a function to make sure
that the `wr.Close()` (which writes out the last part) is executed
BEFORE `body.Bytes()` (defer added it AFTER `body.Bytes()` was calculated)

Change-Id: I8f18fc81b1857b646470eab32e73d6cbdc50d2ad

private/post/message.go

@@ -66,44 +66,9 @@ func (msg *Message) Bytes() (data []byte, err error) {
 	switch {
 	// multipart upload
 	case len(msg.Parts) > 0:
-		wr := multipart.NewWriter(&body)
-		defer func() { err = errs.Combine(err, wr.Close()) }()
-
-		fmt.Fprintf(&body, "Content-Type: multipart/alternative;")
-		fmt.Fprintf(&body, "\tboundary=\"%v\"\r\n", wr.Boundary())
-		fmt.Fprintf(&body, "\r\n")
-
-		var sub io.Writer
-
-		if len(msg.PlainText) > 0 {
-			sub, err := wr.CreatePart(textproto.MIMEHeader{
-				"Content-Type":              []string{"text/plain; charset=UTF-8; format=flowed"},
-				"Content-Transfer-Encoding": []string{"quoted-printable"},
-			})
+		err = msg.writeMultipart(&body)
 		if err != nil {
-				return nil, Error.Wrap(err)
-			}
-
-			enc := quotedprintable.NewWriter(sub)
-			defer func() { err = errs.Combine(err, enc.Close()) }()
-
-			_, err = enc.Write([]byte(msg.PlainText))
-			if err != nil {
-				return nil, Error.Wrap(err)
-			}
-		}
-
-		for _, part := range msg.Parts {
-			header := textproto.MIMEHeader{"Content-Type": []string{mime.QEncoding.Encode("utf-8", part.Type)}}
-			if part.Encoding != "" {
-				header["Content-Transfer-Encoding"] = []string{mime.QEncoding.Encode("utf-8", part.Encoding)}
-			}
-			if part.Disposition != "" {
-				header["Content-Disposition"] = []string{mime.QEncoding.Encode("utf-8", part.Disposition)}
-			}
-
-			sub, _ = wr.CreatePart(header)
-			fmt.Fprint(sub, part.Content)
+			return nil, err
 		}
 
 	// fallback if there are no parts, write PlainText with appropriate Content-Type
@@ -122,6 +87,49 @@ func (msg *Message) Bytes() (data []byte, err error) {
 	return tocrlf(body.Bytes()), nil
 }
 
+func (msg *Message) writeMultipart(body *bytes.Buffer) (err error) {
+	wr := multipart.NewWriter(body)
+	defer func() { err = errs.Combine(err, wr.Close()) }()
+
+	fmt.Fprintf(body, "Content-Type: multipart/alternative;")
+	fmt.Fprintf(body, "\tboundary=\"%v\"\r\n", wr.Boundary())
+	fmt.Fprintf(body, "\r\n")
+
+	var sub io.Writer
+
+	if len(msg.PlainText) > 0 {
+		sub, err := wr.CreatePart(textproto.MIMEHeader{
+			"Content-Type":              []string{"text/plain; charset=UTF-8; format=flowed"},
+			"Content-Transfer-Encoding": []string{"quoted-printable"},
+		})
+		if err != nil {
+			return Error.Wrap(err)
+		}
+
+		enc := quotedprintable.NewWriter(sub)
+		defer func() { err = errs.Combine(err, enc.Close()) }()
+
+		_, err = enc.Write([]byte(msg.PlainText))
+		if err != nil {
+			return Error.Wrap(err)
+		}
+	}
+
+	for _, part := range msg.Parts {
+		header := textproto.MIMEHeader{"Content-Type": []string{mime.QEncoding.Encode("utf-8", part.Type)}}
+		if part.Encoding != "" {
+			header["Content-Transfer-Encoding"] = []string{mime.QEncoding.Encode("utf-8", part.Encoding)}
+		}
+		if part.Disposition != "" {
+			header["Content-Disposition"] = []string{mime.QEncoding.Encode("utf-8", part.Disposition)}
+		}
+
+		sub, _ = wr.CreatePart(header)
+		fmt.Fprint(sub, part.Content)
+	}
+	return nil
+}
+
 func tocrlf(data []byte) []byte {
 	lf := bytes.ReplaceAll(data, []byte("\r\n"), []byte("\n"))
 	crlf := bytes.ReplaceAll(lf, []byte("\n"), []byte("\r\n"))

private/post/message_test.go

@@ -0,0 +1,39 @@
+// Copyright (C) 2022 Storj Labs, Inc.
+// See LICENSE for copying information
+
+package post
+
+import (
+	"net/mail"
+	"regexp"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestMessage_ClosingLastPart(t *testing.T) {
+	from := mail.Address{Name: "No reply", Address: "noreply@eu1.storj.io"}
+
+	m := &Message{
+		From:      from,
+		To:        []mail.Address{{Name: "Foo Bar", Address: "foo@storj.io"}},
+		Subject:   "This is a proper test mail",
+		PlainText: "",
+		Parts: []Part{
+			{
+				Type:    "text/html; charset=UTF-8",
+				Content: string("<head><body><h1>ahoj</h1></body></head>"),
+			},
+		},
+	}
+
+	data, err := m.Bytes()
+	require.NoError(t, err)
+	lines := strings.Split(string(data), "\n")
+
+	// last part should be closed. see 7.2.1 of https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
+	final := regexp.MustCompile("--.+--")
+	lastNonEmptyLine := lines[len(lines)-2]
+	require.True(t, final.MatchString(lastNonEmptyLine), "Last line '%s' doesn't include RFC1341 distinguished delimiter", lastNonEmptyLine)
+}