feat: per-message extra headers in the mail envelope

mail/sender.go

@@ -20,6 +20,7 @@ import (
 	"net/smtp"
 	"net/textproto"
 	"os"
+	"sort"
 	"strings"
 	"time"
 )
@@ -76,6 +77,11 @@ type message struct {
 	Subject string
 	HTML    string
 	Text    string
+
+	// Headers are extra header lines for this one message, such as an
+	// announcement's List-Unsubscribe pair. Keys and values are CR/LF-stripped
+	// like every other header.
+	Headers map[string]string
 }
 
 // sendTemplate renders name against data and delivers the result.
@@ -226,6 +232,14 @@ func (s Sender) envelope(m message) ([]byte, error) {
 	b.WriteString("Message-ID: " + messageID(s.From) + "\r\n")
 	b.WriteString("Subject: " + mime.QEncoding.Encode("utf-8", sanitizeHeader(m.Subject)) + "\r\n")
 	b.WriteString("MIME-Version: 1.0\r\n")
+	keys := make([]string, 0, len(m.Headers))
+	for k := range m.Headers {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, k := range keys {
+		b.WriteString(sanitizeHeader(k) + ": " + sanitizeHeader(m.Headers[k]) + "\r\n")
+	}
 	b.WriteString("Content-Type: multipart/alternative; boundary=" + w.Boundary() + "\r\n")
 	b.WriteString("\r\n")
 	b.WriteString(parts.String())

mail/sender_test.go

@@ -83,3 +83,30 @@ func TestEnvelopePartsAreQuotedPrintable(t *testing.T) {
 		}
 	}
 }
+
+func TestEnvelopeWritesExtraHeadersSanitised(t *testing.T) {
+	s := Sender{From: "Vantage <updates@example.com>"}
+	raw, err := s.envelope(message{
+		To: "a@example.com", Subject: "s", Text: "t", HTML: "<p>h</p>",
+		Headers: map[string]string{
+			"List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
+			"List-Unsubscribe":      "<https://x.example/u?t=1>\r\nBcc: evil@example.com",
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	msg, err := mail.ReadMessage(bytes.NewReader(raw))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := msg.Header.Get("List-Unsubscribe-Post"); got != "List-Unsubscribe=One-Click" {
+		t.Fatalf("List-Unsubscribe-Post = %q", got)
+	}
+	if got := msg.Header.Get("Bcc"); got != "" {
+		t.Fatalf("header injection: Bcc = %q", got)
+	}
+	if !strings.HasPrefix(msg.Header.Get("List-Unsubscribe"), "<https://x.example/u?t=1>") {
+		t.Fatalf("List-Unsubscribe = %q", msg.Header.Get("List-Unsubscribe"))
+	}
+}