dnswatcher/internal/watcher/watcher_test.go

package watcher_test

import (
	"context"
	"fmt"
	"log/slog"
	"os"
	"slices"
	"strings"
	"sync"
	"testing"
	"time"

	"go.uber.org/fx/fxtest"

	"sneak.berlin/go/dnswatcher/internal/config"
	"sneak.berlin/go/dnswatcher/internal/globals"
	"sneak.berlin/go/dnswatcher/internal/livednstest"
	"sneak.berlin/go/dnswatcher/internal/logger"
	"sneak.berlin/go/dnswatcher/internal/portcheck"
	"sneak.berlin/go/dnswatcher/internal/resolver"
	"sneak.berlin/go/dnswatcher/internal/state"
	"sneak.berlin/go/dnswatcher/internal/tlscheck"
	"sneak.berlin/go/dnswatcher/internal/watcher"
)

// The watcher looks these names up in live DNS with the real resolver,
// so tests assert on what the watcher does with the answers, never on
// the records these zones publish. testHost's nameservers and addresses
// stay the same from one check to the next, which the tests that check
// it twice rely on, and testSmallDomain's nameservers stay the same
// between a test looking them up and its check. A domain check looks up
// each nameserver's addresses, about a second per nameserver, so the
// tests that check a domain use testSmallDomain, which has two
// nameservers, and check it once. The tests that query testDomain's
// nameservers directly do no domain check.
const (
	testDomain      = "google.com"
	testSmallDomain = "example.com"
	testHost        = "cloudflare.com"
	testIssuer      = "DigiCert"
)

// Saved-state values that live DNS never returns: nameserver names
// under .invalid and a documentation address.
const (
	oldNS1 = "ns1.example.invalid."
	oldNS2 = "ns2.example.invalid."
	oldIP  = "192.0.2.1"
)

// --- Stand-ins for the port checker, TLS checker and notifier ---
//
// DNS has none: the watchers built here use the real resolver (see
// TESTING.md).

// mockPortChecker reports every port open until closed is set.
type mockPortChecker struct {
	mu     sync.Mutex
	closed bool
	calls  int
}

func (m *mockPortChecker) CheckPort(
	_ context.Context,
	_ string,
	_ int,
) (*portcheck.PortResult, error) {
	m.mu.Lock()
	defer m.mu.Unlock()

	m.calls++

	return &portcheck.PortResult{Open: !m.closed}, nil
}

// mockTLSChecker returns a certificate for the requested hostname that
// expires at notAfter.
type mockTLSChecker struct {
	mu       sync.Mutex
	notAfter time.Time
	calls    int
}

func (m *mockTLSChecker) CheckCertificate(
	_ context.Context,
	_ string,
	hostname string,
) (*tlscheck.CertificateInfo, error) {
	m.mu.Lock()
	defer m.mu.Unlock()

	m.calls++

	return &tlscheck.CertificateInfo{
		CommonName:              hostname,
		Issuer:                  testIssuer,
		NotAfter:                m.notAfter,
		SubjectAlternativeNames: []string{hostname},
	}, nil
}

type notification struct {
	Title    string
	Message  string
	Priority string
}

type mockNotifier struct {
	mu            sync.Mutex
	notifications []notification
}

func (m *mockNotifier) SendNotification(
	_ context.Context,
	title, message, priority string,
) {
	m.mu.Lock()
	defer m.mu.Unlock()

	m.notifications = append(m.notifications, notification{
		Title:    title,
		Message:  message,
		Priority: priority,
	})
}

func (m *mockNotifier) getNotifications() []notification {
	m.mu.Lock()
	defer m.mu.Unlock()

	result := make([]notification, len(m.notifications))
	copy(result, m.notifications)

	return result
}

// --- Helpers to build a Watcher and run its checks against live DNS ---

type testDeps struct {
	portChecker *mockPortChecker
	tlsChecker  *mockTLSChecker
	notifier    *mockNotifier
	state       *state.State
	config      *config.Config
	log         *logger.Logger
}

func newTestWatcher(
	t *testing.T,
	cfg *config.Config,
) (*watcher.Watcher, *testDeps) {
	t.Helper()

	deps := newTestDeps(t, cfg)

	w := watcher.NewForTest(
		deps.config,
		deps.state,
		resolver.NewFromLogger(slog.Default()),
		deps.portChecker,
		deps.tlsChecker,
		deps.notifier,
	)

	return w, deps
}

func newTestDeps(t *testing.T, cfg *config.Config) *testDeps {
	t.Helper()

	deps := &testDeps{
		portChecker: &mockPortChecker{},
		tlsChecker: &mockTLSChecker{
			notAfter: time.Now().Add(90 * 24 * time.Hour),
		},
		notifier: &mockNotifier{},
		config:   cfg,
	}

	g, err := globals.New(nil)
	if err != nil {
		t.Fatalf("globals.New: %v", err)
	}

	deps.log, err = logger.New(nil, logger.Params{Globals: g})
	if err != nil {
		t.Fatalf("logger.New: %v", err)
	}

	// The watcher saves state after every check, into cfg.DataDir.
	deps.state, err = state.New(fxtest.NewLifecycle(t), state.Params{
		Logger: deps.log,
		Config: cfg,
	})
	if err != nil {
		t.Fatalf("state.New: %v", err)
	}

	return deps
}

func defaultTestConfig(t *testing.T) *config.Config {
	t.Helper()

	return &config.Config{
		DNSInterval:      time.Hour,
		TLSInterval:      12 * time.Hour,
		TLSExpiryWarning: 7,
		DataDir:          t.TempDir(),
	}
}

// checkOnce runs the watcher's checks once and returns an error when a
// configured name has no hostname state saved by this check, or that
// state holds no address, or a configured domain's nameserver has no
// address saved or still has oldIP, which the tests save and live DNS
// never returns. Either live DNS gave no answer for the name, or the
// watcher saved no fresh result for it.
func checkOnce(
	ctx context.Context,
	w *watcher.Watcher,
	deps *testDeps,
) error {
	started := time.Now()

	w.RunOnce(ctx)

	names := slices.Concat(deps.config.Domains, deps.config.Hostnames)

	for _, name := range names {
		hs, ok := deps.state.GetHostnameState(name)
		if !ok || hs.LastChecked.Before(started) ||
			len(addresses(hs)) == 0 {
			return fmt.Errorf(
				"%s: %w, or the watcher saved no fresh "+
					"result for it",
				name, livednstest.ErrNoAnswer,
			)
		}
	}

	for _, name := range deps.config.Domains {
		ds, _ := deps.state.GetDomainState(name)
		for _, ns := range ds.Nameservers {
			ips := ds.NameserverAddresses[ns]
			if len(ips) == 0 || slices.Contains(ips, oldIP) {
				return fmt.Errorf(
					"%s: nameserver %s: %w, or the watcher saved "+
						"no fresh addresses for it",
					name, ns, livednstest.ErrNoAnswer,
				)
			}
		}
	}

	return nil
}

// runChecks builds a watcher, lets prepare set up the saved state and
// stand-ins it starts from, and runs its checks once against live DNS.
// If change is not nil, change then alters the saved state or stand-ins
// and the checks run a second time. When either check finds no fresh
// address for a name (see checkOnce), the watcher is thrown away and
// all of this runs again on a new one, so a failed attempt leaves
// nothing behind in the saved state, the stand-ins or the notifications.
func runChecks(
	t *testing.T,
	cfg *config.Config,
	prepare, change func(deps *testDeps),
) *testDeps {
	t.Helper()

	var deps *testDeps

	livednstest.Retry(t, "watcher checks", func(ctx context.Context) error {
		var w *watcher.Watcher

		w, deps = newTestWatcher(t, cfg)

		if prepare != nil {
			prepare(deps)
		}

		err := checkOnce(ctx, w, deps)
		if err != nil || change == nil {
			return err
		}

		change(deps)

		return checkOnce(ctx, w, deps)
	})

	return deps
}

// lookupNameservers returns the nameservers live DNS lists for domain,
// for a test to save in the state its check starts from.
func lookupNameservers(t *testing.T, domain string) []string {
	t.Helper()

	res := resolver.NewFromLogger(slog.Default())

	var nameservers []string

	livednstest.Retry(t, "LookupNS("+domain+")", func(ctx context.Context) error {
		var err error

		nameservers, err = res.LookupNS(ctx, domain)

		return err
	})

	return nameservers
}

// addresses returns the A and AAAA values saved for a hostname.
func addresses(hs *state.HostnameState) []string {
	var ips []string

	for _, nsState := range hs.RecordsByNameserver {
		ips = append(ips, nsState.Records["A"]...)
		ips = append(ips, nsState.Records["AAAA"]...)
	}

	return ips
}

// assertNotified checks that a notification with this title and
// priority was sent.
func assertNotified(
	t *testing.T,
	deps *testDeps,
	title, priority string,
) {
	t.Helper()

	notifications := deps.notifier.getNotifications()

	for _, n := range notifications {
		if n.Title == title && n.Priority == priority {
			return
		}
	}

	t.Errorf(
		"expected %s notification %q, got: %v",
		priority, title, notifications,
	)
}

// countNotifications counts the notifications sent with this title.
func countNotifications(deps *testDeps, title string) int {
	count := 0

	for _, n := range deps.notifier.getNotifications() {
		if n.Title == title {
			count++
		}
	}

	return count
}

func TestFirstRunBaseline(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Domains = []string{testSmallDomain}
	cfg.Hostnames = []string{testHost}

	deps := runChecks(t, cfg, nil, nil)

	assertNoNotifications(t, deps)
	assertStatePopulated(t, deps)
}

func assertNoNotifications(
	t *testing.T,
	deps *testDeps,
) {
	t.Helper()

	notifications := deps.notifier.getNotifications()
	if len(notifications) != 0 {
		t.Errorf(
			"expected 0 notifications on first run, got %d",
			len(notifications),
		)
	}
}

func assertStatePopulated(
	t *testing.T,
	deps *testDeps,
) {
	t.Helper()

	snap := deps.state.GetSnapshot()

	if len(snap.Domains) != 1 {
		t.Errorf(
			"expected 1 domain in state, got %d",
			len(snap.Domains),
		)
	}

	// Hostnames includes both explicit hostnames and domains
	// (domains now also get hostname state for port/TLS checks).
	if len(snap.Hostnames) < 1 {
		t.Errorf(
			"expected at least 1 hostname in state, got %d",
			len(snap.Hostnames),
		)
	}
}

func TestDomainPortAndTLSChecks(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Domains = []string{testSmallDomain}

	deps := runChecks(t, cfg, nil, nil)

	snap := deps.state.GetSnapshot()

	// Domain should have port state populated
	if len(snap.Ports) == 0 {
		t.Error("expected port state for domain, got none")
	}

	// Domain should have certificate state populated
	if len(snap.Certificates) == 0 {
		t.Error("expected certificate state for domain, got none")
	}

	// Verify port checker was actually called
	deps.portChecker.mu.Lock()
	calls := deps.portChecker.calls
	deps.portChecker.mu.Unlock()

	if calls == 0 {
		t.Error("expected port checker to be called for domain")
	}

	// Verify TLS checker was actually called
	deps.tlsChecker.mu.Lock()
	tlsCalls := deps.tlsChecker.calls
	deps.tlsChecker.mu.Unlock()

	if tlsCalls == 0 {
		t.Error("expected TLS checker to be called for domain")
	}
}

func TestNSChangeDetection(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Domains = []string{testSmallDomain}

	// The saved state lists nameservers that live DNS does not.
	deps := runChecks(t, cfg, func(deps *testDeps) {
		deps.state.SetDomainState(testSmallDomain, &state.DomainState{
			Nameservers: []string{oldNS1, oldNS2},
		})
	}, nil)

	assertNotified(t, deps, "NS Change: "+testSmallDomain, "warning")

	ds, _ := deps.state.GetDomainState(testSmallDomain)
	if slices.Contains(ds.Nameservers, oldNS1) {
		t.Errorf("saved nameservers not updated: %v", ds.Nameservers)
	}
}

func TestNSAddressChangeDetection(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Domains = []string{testSmallDomain}

	nameservers := lookupNameservers(t, testSmallDomain)

	// The saved state lists the nameservers live DNS lists, each at an
	// address live DNS never returns.
	deps := runChecks(t, cfg, func(deps *testDeps) {
		nsAddresses := make(map[string][]string, len(nameservers))
		for _, ns := range nameservers {
			nsAddresses[ns] = []string{oldIP}
		}

		deps.state.SetDomainState(testSmallDomain, &state.DomainState{
			Nameservers:         nameservers,
			NameserverAddresses: nsAddresses,
		})
	}, nil)

	title := "NS Address Change: " + testSmallDomain
	ds, _ := deps.state.GetDomainState(testSmallDomain)

	// One alert per nameserver, naming it and the address it had.
	for _, ns := range ds.Nameservers {
		prefix := "Domain: " + testSmallDomain + "\nNameserver: " + ns +
			"\nOld: " + oldIP + "\nNew: "

		sent := 0

		for _, n := range deps.notifier.getNotifications() {
			if n.Title == title && strings.HasPrefix(n.Message, prefix) {
				sent++
			}
		}

		if sent != 1 {
			t.Errorf("sent %d address changes for %s, want 1", sent, ns)
		}
	}

	if n := countNotifications(deps, title); n != len(ds.Nameservers) {
		t.Errorf(
			"sent %d address changes for %d nameservers",
			n, len(ds.Nameservers),
		)
	}

	if n := countNotifications(deps, "NS Change: "+testSmallDomain); n != 0 {
		t.Errorf("sent %d NS changes, want 0", n)
	}
}

func TestNSAddedAndRemovedIsNoAddressChange(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Domains = []string{testSmallDomain}

	nameservers := lookupNameservers(t, testSmallDomain)

	// The saved state lists oldNS1, which live DNS does not, in place of
	// the first nameserver live DNS lists, so that the check finds that
	// one added and oldNS1 removed. Only oldNS1 has addresses saved.
	deps := runChecks(t, cfg, func(deps *testDeps) {
		deps.state.SetDomainState(testSmallDomain, &state.DomainState{
			Nameservers:         append([]string{oldNS1}, nameservers[1:]...),
			NameserverAddresses: map[string][]string{oldNS1: {oldIP}},
		})
	}, nil)

	if n := countNotifications(deps, "NS Change: "+testSmallDomain); n != 1 {
		t.Errorf("sent %d NS changes, want 1", n)
	}

	title := "NS Address Change: " + testSmallDomain
	if n := countNotifications(deps, title); n != 0 {
		t.Errorf("sent %d address changes, want 0", n)
	}
}

func TestRecordChangeDetection(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Hostnames = []string{testHost}

	// Between the checks, save for every nameserver an address live DNS
	// never returns.
	deps := runChecks(t, cfg, nil, func(deps *testDeps) {
		hs, _ := deps.state.GetHostnameState(testHost)
		for _, nsState := range hs.RecordsByNameserver {
			nsState.Records = map[string][]string{"A": {oldIP}}
		}

		deps.state.SetHostnameState(testHost, hs)
	})

	assertNotified(t, deps, "Record Change: "+testHost, "warning")
}

func TestPortStateChange(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Hostnames = []string{testHost}

	// Between the checks, every port closes.
	deps := runChecks(t, cfg, nil, func(deps *testDeps) {
		deps.portChecker.mu.Lock()
		deps.portChecker.closed = true
		deps.portChecker.mu.Unlock()
	})

	hs, _ := deps.state.GetHostnameState(testHost)
	assertNotified(
		t, deps, "Port Change: "+addresses(hs)[0]+":443", "warning",
	)
}

// expiresInThreeDays makes the TLS checker return certificates that
// expire within the seven-day warning period.
func expiresInThreeDays(deps *testDeps) {
	deps.tlsChecker.notAfter = time.Now().Add(3 * 24 * time.Hour)
}

func TestTLSExpiryWarning(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Hostnames = []string{testHost}

	deps := runChecks(t, cfg, expiresInThreeDays, nil)

	assertNotified(t, deps, "TLS Expiry Warning: "+testHost, "warning")
}

func TestTLSExpiryWarningDedup(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Hostnames = []string{testHost}
	cfg.TLSInterval = 24 * time.Hour

	title := "TLS Expiry Warning: " + testHost

	// The second check comes within the TLS interval of the first,
	// so it must not warn again.
	var warnings int

	deps := runChecks(t, cfg, expiresInThreeDays, func(deps *testDeps) {
		warnings = countNotifications(deps, title)
	})

	if warnings == 0 {
		t.Fatal("expected expiry warnings from the first check")
	}

	got := countNotifications(deps, title)
	if got != warnings {
		t.Errorf(
			"expected %d expiry warnings (dedup), got %d",
			warnings, got,
		)
	}
}

func TestGracefulShutdown(t *testing.T) {
	t.Parallel()

	// No domains or hostnames: stopping does not involve DNS.
	cfg := defaultTestConfig(t)
	cfg.DNSInterval = 100 * time.Millisecond
	cfg.TLSInterval = 100 * time.Millisecond

	w, _ := newTestWatcher(t, cfg)

	ctx, cancel := context.WithCancel(t.Context())

	done := make(chan struct{})

	go func() {
		w.Run(ctx)
		close(done)
	}()

	time.Sleep(250 * time.Millisecond)
	cancel()

	select {
	case <-done:
		// Shut down cleanly
	case <-time.After(5 * time.Second):
		t.Error("watcher did not shut down within timeout")
	}
}

// TestStopSavesState stops a watcher built by New the way fx stops it,
// and checks that a change made to the state after the last check is in
// the state file afterwards. The state's own stop hook never runs here,
// so only the watcher can have saved it. Nothing is configured to
// check, so no DNS is involved.
func TestStopSavesState(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	deps := newTestDeps(t, cfg)
	lc := fxtest.NewLifecycle(t)

	_, err := watcher.New(lc, watcher.Params{
		Logger:    deps.log,
		Config:    cfg,
		State:     deps.state,
		Resolver:  resolver.NewFromLogger(slog.Default()),
		PortCheck: deps.portChecker,
		TLSCheck:  deps.tlsChecker,
		Notify:    deps.notifier,
	})
	if err != nil {
		t.Fatalf("watcher.New: %v", err)
	}

	lc.RequireStart()

	// The first check saves state once. Wait for that save before
	// changing the state, so the change can reach the file only
	// through the save made at stop.
	deadline := time.Now().Add(5 * time.Second)

	for {
		_, err = os.Stat(cfg.StatePath())
		if err == nil {
			break
		}

		if time.Now().After(deadline) {
			t.Fatalf("the first check saved no state: %v", err)
		}

		time.Sleep(10 * time.Millisecond)
	}

	deps.state.SetDomainState(testDomain, &state.DomainState{
		Nameservers: []string{oldNS1},
	})

	ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
	defer cancel()

	err = lc.Stop(ctx)
	if err != nil {
		t.Fatalf("stopping the watcher: %v", err)
	}

	saved, err := state.New(fxtest.NewLifecycle(t), state.Params{
		Logger: deps.log,
		Config: cfg,
	})
	if err != nil {
		t.Fatalf("state.New: %v", err)
	}

	err = saved.Load()
	if err != nil {
		t.Fatalf("loading the state file: %v", err)
	}

	ds, ok := saved.GetDomainState(testDomain)
	if !ok || !slices.Equal(ds.Nameservers, []string{oldNS1}) {
		t.Errorf(
			"state file after stop has %+v for %s, want nameservers %v",
			ds, testDomain, []string{oldNS1},
		)
	}
}

func TestDNSRunsBeforePortAndTLSChecks(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Hostnames = []string{testHost}

	// The saved state says the last check found testHost at oldIP.
	deps := runChecks(t, cfg, func(deps *testDeps) {
		deps.state.SetHostnameState(testHost, &state.HostnameState{
			RecordsByNameserver: map[string]*state.NameserverRecordState{
				oldNS1: {
					Records: map[string][]string{"A": {oldIP}},
					Status:  "ok",
				},
			},
		})
	}, nil)

	snap := deps.state.GetSnapshot()

	if _, ok := snap.Ports[oldIP+":80"]; ok {
		t.Error("port check used stale DNS: found " + oldIP + ":80")
	}

	// Port and TLS checks must use the addresses this check found.
	for _, ip := range addresses(snap.Hostnames[testHost]) {
		if _, ok := snap.Ports[ip+":80"]; !ok {
			t.Error("port check used stale DNS: missing " + ip + ":80")
		}

		certKey := ip + ":443:" + testHost
		if _, ok := snap.Certificates[certKey]; !ok {
			t.Error("TLS check used stale DNS: missing " + certKey)
		}
	}
}

func TestSendTestNotification_Enabled(t *testing.T) {
	t.Parallel()

	// No domains or hostnames: the startup notification does not
	// involve DNS.
	cfg := defaultTestConfig(t)
	cfg.SendTestNotification = true

	w, deps := newTestWatcher(t, cfg)

	w.RunOnce(t.Context())

	// RunOnce does not send the test notification — it is
	// sent by Run after RunOnce completes. Call the exported
	// RunOnce then check that no test notification was sent
	// (only Run triggers it). We test the full path via Run.
	notifications := deps.notifier.getNotifications()
	if len(notifications) != 0 {
		t.Errorf(
			"RunOnce should not send test notification, got %d",
			len(notifications),
		)
	}
}

func TestSendTestNotification_ViaRun(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.SendTestNotification = true
	cfg.DNSInterval = 24 * time.Hour
	cfg.TLSInterval = 24 * time.Hour

	w, deps := newTestWatcher(t, cfg)

	ctx, cancel := context.WithCancel(t.Context())

	done := make(chan struct{})

	go func() {
		w.Run(ctx)
		close(done)
	}()

	// Wait for the initial scan and test notification.
	time.Sleep(500 * time.Millisecond)
	cancel()

	<-done

	notifications := deps.notifier.getNotifications()

	found := false

	for _, n := range notifications {
		if n.Priority == "success" &&
			n.Title == "✅ dnswatcher startup complete" {
			found = true
		}
	}

	if !found {
		t.Errorf(
			"expected startup test notification, got: %v",
			notifications,
		)
	}
}

func TestSendTestNotification_Disabled(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.SendTestNotification = false
	cfg.DNSInterval = 24 * time.Hour
	cfg.TLSInterval = 24 * time.Hour

	w, deps := newTestWatcher(t, cfg)

	ctx, cancel := context.WithCancel(t.Context())

	done := make(chan struct{})

	go func() {
		w.Run(ctx)
		close(done)
	}()

	time.Sleep(500 * time.Millisecond)
	cancel()

	<-done

	notifications := deps.notifier.getNotifications()

	for _, n := range notifications {
		if n.Title == "✅ dnswatcher startup complete" {
			t.Error(
				"test notification should not be sent when disabled",
			)
		}
	}
}

func TestNSFailureAndRecovery(t *testing.T) {
	t.Parallel()

	cfg := defaultTestConfig(t)
	cfg.Hostnames = []string{testHost}

	// Between the checks, save every nameserver the first check found
	// as one that did not answer, and add, as answering, one that live
	// DNS does not list, which then disappears.
	deps := runChecks(t, cfg, nil, func(deps *testDeps) {
		hs, _ := deps.state.GetHostnameState(testHost)
		for ns := range hs.RecordsByNameserver {
			hs.RecordsByNameserver[ns] = failed()
		}

		hs.RecordsByNameserver[oldNS1] = &state.NameserverRecordState{
			Records: map[string][]string{"A": {oldIP}},
			Status:  "ok",
		}

		deps.state.SetHostnameState(testHost, hs)
	})

	assertNotified(t, deps, "NS Failure: "+testHost, "error")
	assertNotified(t, deps, "NS Recovery: "+testHost, "success")

	// A nameserver that did not answer has no records to compare, so
	// its recovery is not also a record change.
	if n := countNotifications(deps, "Record Change: "+testHost); n != 0 {
		t.Errorf("sent %d record changes on recovery, want 0", n)
	}
}