watcher: a removed nameserver is an NS Change, not an NS Failure (closes #262)

A nameserver that left a domain's delegation was reported twice: as the
domain's NS Change, and by the check of the domain's own records as an
error-level NS Failure saying it disappeared. The check of a name's
records now sends one NS Change at warning level, starting Hostname: or
Domain:, listing the nameservers it was asked at before and not now.
For a domain it leaves out those of its saved delegation, which the
domain's own NS Change lists, so a domain's new delegation is now
notified and saved only after, and only together with, its records.
Nothing is sent for a nameserver that joins.

A nameserver that was failing before it left is now listed too; the
NS Failure skipped it.

Model: opus-5-5

internal/watcher/watcher_test.go

@@ -139,6 +139,22 @@ func (m *mockNotifier) getNotifications() []notification {
 	return result
 }
 
+// cancelOnNotify is a mockNotifier that calls cancel at each
+// notification, as if shutdown cut the check short there.
+type cancelOnNotify struct {
+	*mockNotifier
+
+	cancel context.CancelFunc
+}
+
+func (n cancelOnNotify) SendNotification(
+	ctx context.Context,
+	title, message, priority string,
+) {
+	n.mockNotifier.SendNotification(ctx, title, message, priority)
+	n.cancel()
+}
+
 // --- Helpers to build a Watcher and run its checks against live DNS ---
 
 type testDeps struct {
@@ -558,6 +574,11 @@ func TestDomainThatDoesNotExist(t *testing.T) {
 // nameservers and without nxdomain, and its records, asked at the
 // codeberg.page servers, are saved. Those are testSmallDomain's two
 // nameservers; github.io, the zone of the README's example, has eight.
+// The previous check saved a delegation to oldNS1, now withdrawn, and
+// records asked at oldNS1. A first check is cut short at its first
+// notification, as shutdown would, and a second runs in full: across
+// them the NS Change of its delegation removes oldNS1, and no other NS
+// Change lists it again.
 func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
 	t.Parallel()
 
@@ -569,9 +590,30 @@ func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
 	var deps *testDeps
 
 	livednstest.Retry(t, "watcher checks", func(ctx context.Context) error {
-		var w *watcher.Watcher
+		deps = newTestDeps(t, cfg)
 
-		w, deps = newTestWatcher(t, cfg)
+		firstCtx, cancel := context.WithCancel(ctx)
+		defer cancel()
+
+		w := watcher.NewForTest(
+			cfg,
+			deps.state,
+			resolver.NewFromLogger(slog.Default()),
+			deps.portChecker,
+			deps.tlsChecker,
+			cancelOnNotify{mockNotifier: deps.notifier, cancel: cancel},
+		)
+
+		deps.state.SetDomainState(domain, &state.DomainState{
+			Nameservers: []string{oldNS1},
+		})
+		deps.state.SetHostnameState(domain, saved(
+			map[string]*state.NameserverRecordState{
+				oldNS1: answered(map[string][]string{"A": {oldIP}}),
+			},
+		))
+
+		w.RunOnce(firstCtx)
 
 		err := checkOnce(ctx, w, deps)
 
@@ -593,6 +635,22 @@ func TestDomainWithNoDelegationOfItsOwn(t *testing.T) {
 	if _, ok := deps.state.GetHostnameState(domain); !ok {
 		t.Errorf("no records saved for %s", domain)
 	}
+
+	title := "NS Change: " + domain
+	want := "Domain: " + domain + "\nAdded: \nRemoved: " + oldNS1
+
+	var changes []notification
+
+	for _, n := range deps.notifier.getNotifications() {
+		if n.Title == title {
+			changes = append(changes, n)
+		}
+	}
+
+	if len(changes) != 1 || changes[0].Message != want {
+		t.Errorf("sent %v, want one %q with message:\n%s",
+			changes, title, want)
+	}
 }
 
 func TestNSAddressChangeDetection(t *testing.T) {
@@ -1049,7 +1107,7 @@ func TestSendTestNotification_Disabled(t *testing.T) {
 	}
 }
 
-func TestNSFailureAndRecovery(t *testing.T) {
+func TestNSChangeAndRecovery(t *testing.T) {
 	t.Parallel()
 
 	cfg := defaultTestConfig(t)
@@ -1059,7 +1117,7 @@ func TestNSFailureAndRecovery(t *testing.T) {
 
 	// The saved state has every nameserver live DNS lists as one that
 	// did not answer, and, as answering, one that live DNS does not
-	// list, which then disappears.
+	// list, which the check then finds removed.
 	_, deps := runChecks(t, cfg, func(deps *testDeps) {
 		byNameserver := map[string]*state.NameserverRecordState{
 			oldNS1: answered(map[string][]string{"A": {oldIP}}),
@@ -1071,7 +1129,7 @@ func TestNSFailureAndRecovery(t *testing.T) {
 		deps.state.SetHostnameState(testHost, saved(byNameserver))
 	})
 
-	assertNotified(t, deps, "NS Failure: "+testHost, "error")
+	assertNotified(t, deps, "NS Change: "+testHost, "warning")
 	assertNotified(t, deps, "NS Recovery: "+testHost, "success")
 
 	// A nameserver that did not answer has no records to compare, so