queue: when shutting down, wait for pending deliveries before signaling that shutdown is complete

also fixes flaky test, which is how i found it

queue/hook.go

@@ -1006,6 +1006,10 @@ func startHookQueue(done chan struct{}) {
 	for {
 		select {
 		case <-mox.Shutdown.Done():
+			for len(busyHookURLs) > 0 {
+				url := <-hookDeliveryResults
+				delete(busyHookURLs, url)
+			}
 			done <- struct{}{}
 			return
 		case <-hookqueue:

queue/queue.go

@@ -1154,6 +1154,10 @@ func startQueue(resolver dns.Resolver, done chan struct{}) {
 	for {
 		select {
 		case <-mox.Shutdown.Done():
+			for len(busyDomains) > 0 {
+				domain := <-deliveryResults
+				delete(busyDomains, domain)
+			}
 			done <- struct{}{}
 			return
 		case <-msgqueue:

queue/queue_test.go

@@ -1183,7 +1183,7 @@ func TestQueueStart(t *testing.T) {
 	_, cleanup := setup(t)
 	defer cleanup()
 
-	done := make(chan struct{}, 4)
+	done := make(chan struct{})
 	defer func() {
 		mox.ShutdownCancel()
 		// Wait for message and hooks deliverers and cleaners.
@@ -1269,7 +1269,6 @@ func TestQueueStart(t *testing.T) {
 		t.Fatalf("kick changed %d messages, expected 1", n)
 	}
 	checkDialed(true)
-	time.Sleep(100 * time.Millisecond) // Racy... we won't get notified when work is done...
 
 	// Submit another, should be delivered immediately without HoldRule.
 	path = smtp.Path{Localpart: "mjl", IPDomain: dns.IPDomain{Domain: dns.Domain{ASCII: "mox.example"}}}
@@ -1280,8 +1279,6 @@ func TestQueueStart(t *testing.T) {
 	err = Add(ctxbg, pkglog, "mjl", mf, qm)
 	tcheck(t, err, "add message to queue for delivery")
 	checkDialed(true) // Immediate.
-
-	time.Sleep(100 * time.Millisecond) // Racy... give time to finish.
 }
 
 func TestListFilterSort(t *testing.T) {

serve.go

@@ -78,7 +78,7 @@ func start(mtastsdbRefresher, sendDMARCReports, sendTLSReports, skipForkExec boo
 		return fmt.Errorf("tlsrpt init: %s", err)
 	}
 
-	done := make(chan struct{}, 4) // Goroutines for messages and webhooks, and cleaners.
+	done := make(chan struct{}) // Goroutines for messages and webhooks, and cleaners.
 	if err := queue.Start(dns.StrictResolver{Pkg: "queue"}, done); err != nil {
 		return fmt.Errorf("queue start: %s", err)
 	}