fix: return announcement refusals as prepare-phase send errors, test the List-Unsubscribe pair on the wire

mail/announcement.go

@@ -50,20 +50,31 @@ func RenderAnnouncement(a Announcement, publicURL string) (subject, html, text s
 // SendAnnouncement sends one announcement to one address, with the
 // List-Unsubscribe pair Gmail and Yahoo require of bulk senders.
 func (s Sender) SendAnnouncement(to string, a Announcement) error {
+	m, err := s.announcementFor(to, a)
+	if err != nil {
+		return err
+	}
+	return s.send(m)
+}
+
+// announcementFor builds the one-recipient message SendAnnouncement puts on
+// the wire. Every refusal is a *SendError at PhasePrepare: it is about this
+// one message, never the mail server, so a caller moves on to the next row.
+func (s Sender) announcementFor(to string, a Announcement) (message, error) {
 	if strings.Contains(to, ",") {
-		return fmt.Errorf("mail: an announcement goes to exactly one address")
+		return message{}, &SendError{Phase: PhasePrepare, Err: fmt.Errorf("mail: an announcement goes to exactly one address")}
 	}
 	if a.UnsubscribeURL == "" {
-		return fmt.Errorf("mail: announcement without an unsubscribe URL")
+		return message{}, &SendError{Phase: PhasePrepare, Err: fmt.Errorf("mail: announcement without an unsubscribe URL")}
 	}
 	m, err := announcementMessage(a, s.PublicURL)
 	if err != nil {
-		return err
+		return message{}, &SendError{Phase: PhasePrepare, Err: err}
 	}
 	m.To = to
 	m.Headers = map[string]string{
 		"List-Unsubscribe":      "<" + a.UnsubscribeURL + ">",
 		"List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
 	}
-	return s.send(m)
+	return m, nil
 }

mail/announcement_test.go

@@ -1,6 +1,9 @@
 package mail
 
 import (
+	"bytes"
+	"errors"
+	"net/mail"
 	"strings"
 	"testing"
 )
@@ -36,7 +39,7 @@ func TestRenderAnnouncement(t *testing.T) {
 				t.Errorf("%s body missing %q", label, want)
 			}
 		}
-		if strings.Contains(body, "—") {
+		if strings.Contains(body, "\u2014") {
 			t.Errorf("%s body contains an em dash", label)
 		}
 	}
@@ -59,12 +62,35 @@ func TestRenderAnnouncementWithoutPostalAddress(t *testing.T) {
 
 func TestSendAnnouncementRefusesListsAndMissingLink(t *testing.T) {
 	s := Sender{Host: "smtp.invalid", From: "updates@example.com"}
-	if err := s.SendAnnouncement("a@example.com, b@example.com", sampleAnnouncement()); err == nil {
-		t.Error("a comma-separated To must be refused")
+	prepare := func(what string, err error) {
+		t.Helper()
+		var se *SendError
+		if !errors.As(err, &se) || se.Phase != PhasePrepare {
+			t.Errorf("%s: err = %v, want a *SendError at PhasePrepare", what, err)
+		}
 	}
+	prepare("a comma-separated To", s.SendAnnouncement("a@example.com, b@example.com", sampleAnnouncement()))
 	a := sampleAnnouncement()
 	a.UnsubscribeURL = ""
-	if err := s.SendAnnouncement("a@example.com", a); err == nil {
-		t.Error("an announcement without an unsubscribe URL must be refused")
+	prepare("no unsubscribe URL", s.SendAnnouncement("a@example.com", a))
+}
+
+// Gmail and Yahoo require both headers of the RFC 8058 pair on bulk mail.
+func TestSendAnnouncementPutsListUnsubscribeOnTheWire(t *testing.T) {
+	port, got := fakeSMTPData(t, nil)
+	s := Sender{Host: "127.0.0.1", Port: port, From: "updates@example.com"}
+	a := sampleAnnouncement()
+	if err := s.SendAnnouncement("a@example.com", a); err != nil {
+		t.Fatal(err)
+	}
+	msg, err := mail.ReadMessage(bytes.NewReader(<-got))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if h := msg.Header.Get("List-Unsubscribe"); h != "<"+a.UnsubscribeURL+">" {
+		t.Errorf("List-Unsubscribe = %q", h)
+	}
+	if h := msg.Header.Get("List-Unsubscribe-Post"); h != "List-Unsubscribe=One-Click" {
+		t.Errorf("List-Unsubscribe-Post = %q", h)
 	}
 }