queue: Send DSN for messages that time out in the queue

The queue currently only considers failed recipients when deciding
whether to send a DSN or not. This is a bug, as recipients that time out
are not taken into account.

This patch fixes that issue by including both failed and pending
recipients in the DSN.

It also adds more comprehensive tests for this case, both in the queue
and in the dsn generation code.

internal/queue/queue_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"reflect"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -113,6 +114,45 @@ func TestBasic(t *testing.T) {
 	}
 }
 
+func TestDSNOnTimeout(t *testing.T) {
+	localC := newTestCourier()
+	remoteC := newTestCourier()
+	q := New("/tmp/queue_test", set.NewString("loco"), aliases.NewResolver(),
+		localC, remoteC, "dsndomain")
+
+	// Insert an expired item in the queue.
+	item := &Item{
+		Message: Message{
+			ID:   <-newID,
+			From: fmt.Sprintf("from@loco"),
+			Rcpt: []*Recipient{
+				{"to@to", Recipient_EMAIL, Recipient_PENDING, "err", "to@to"}},
+			Data: []byte("data"),
+		},
+		CreatedAt: time.Now().Add(-24 * time.Hour),
+	}
+	q.q[item.ID] = item
+	err := item.WriteTo(q.path)
+	if err != nil {
+		t.Errorf("failed to write item: %v", err)
+	}
+
+	// Launch the sending loop, expect 1 local delivery (the DSN).
+	localC.wg.Add(1)
+	go item.SendLoop(q)
+	localC.wg.Wait()
+
+	req := localC.reqFor["from@loco"]
+	if req == nil {
+		t.Fatal("missing DSN")
+	}
+
+	if req.from != "<>" || req.to != "from@loco" ||
+		!strings.Contains(string(req.data), "X-Failed-Recipients: to@to,") {
+		t.Errorf("wrong DSN: %q", string(req.data))
+	}
+}
+
 func TestFullQueue(t *testing.T) {
 	q := New("/tmp/queue_test", set.NewString(), aliases.NewResolver(),
 		dumbCourier, dumbCourier, "dsndomain")