watcher: save state when it stops and wait for that save (closes #114)

The final save at shutdown came from the state's own stop hook, while
the watcher's stop hook only cancelled its run loop, so a check under
way could change state after that save or be cut off at exit. Run now
saves state as it returns, and the watcher's stop hook waits for Run,
bounded by the shutdown deadline. The state's own save stays; Save
holds the state lock for the whole write, so the two cannot overlap.
The start hook derives the watcher's context with WithoutCancel, so
the linter needs no exception. A new test stops a watcher built by New
and reads the change back from the state file, with no DNS.

Model: opus-5-5

README.md

@@ -618,9 +618,10 @@ repository's `Dockerfile` and runs it. The app needs:
      from a previous cycle.
 4. **On change detection**: Send notifications to all configured
    endpoints, update in-memory state, persist to disk.
-5. **Shutdown**: Persist final state to disk, wait for in-flight
+5. **Shutdown**: The watcher stops checking and saves the final state
-   notification deliveries to complete, stop gracefully. The wait is
+   to disk, and shutdown waits for that save before it goes on. Then it
-   bounded by the fx shutdown timeout (15s by default): deliveries still
+   waits for in-flight notification deliveries to complete. Both waits
+   share the fx shutdown timeout (15s by default): deliveries still
    retrying against an unreachable endpoint when that expires are
    abandoned, and the number abandoned is logged at warn level rather
    than dropped silently. Notifications generated after shutdown has

TODO.md

@@ -19,6 +19,8 @@ nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
 
 # Completed Steps
 
+- 2026-10-01: the watcher saves state when it stops, and shutdown waits for that
+  save, so it no longer relies on the state's own stop hook (closes #114).
 - 2026-10-01: `DNSWATCHER_SENTRY_DSN` reports panics in HTTP handlers to Sentry,
   and a DSN Sentry cannot parse stops startup (closes #107).
 - 2026-10-01: a port or TLS check that shutdown cuts short saves nothing and
@@ -109,7 +111,6 @@ nameserver IP address changes: https://git.eeqj.de/sneak/dnswatcher/issues/105
   https://git.eeqj.de/sneak/dnswatcher/issues/66
 - `goimports` in `make fmt-check`, Markdown formatting:
   https://git.eeqj.de/sneak/dnswatcher/issues/119
-- final state save at shutdown: https://git.eeqj.de/sneak/dnswatcher/issues/114
 - README accuracy sweep: https://git.eeqj.de/sneak/dnswatcher/issues/108
 - README sections required by policy:
   https://git.eeqj.de/sneak/dnswatcher/issues/173

internal/watcher/watcher.go

@@ -56,6 +56,7 @@ type Watcher struct {
 	tlsCheck         TLSChecker
 	notify           Notifier
 	cancel           context.CancelFunc
+	done             chan struct{} // closed when Run returns
 	firstRun         bool
 	expiryNotifiedMu sync.Mutex
 	expiryNotified   map[string]time.Time
@@ -79,31 +80,47 @@ func New(
 	}
 
 	lifecycle.Append(fx.Hook{
-		OnStart: func(_ context.Context) error {
+		OnStart: func(startCtx context.Context) error {
-			// Use context.Background() — the fx startup context
+			// The fx startup context expires after startup
-			// expires after startup completes, so deriving from it
+			// completes, so the watcher's context drops its
-			// would cancel the watcher immediately. The watcher's
+			// cancellation. The watcher's lifetime is controlled
-			// lifetime is controlled by w.cancel in OnStop.
+			// by w.cancel in OnStop.
-			ctx, cancel := context.WithCancel(context.Background())
+			ctx, cancel := context.WithCancel(
+				context.WithoutCancel(startCtx),
+			)
 			w.cancel = cancel
+			w.done = make(chan struct{})
 
-			go w.Run(ctx) //nolint:contextcheck // intentionally not derived from startCtx
+			go func() {
+				defer close(w.done)
+
+				w.Run(ctx)
+			}()
 
 			return nil
 		},
-		OnStop: func(_ context.Context) error {
+		OnStop: func(ctx context.Context) error {
-			if w.cancel != nil {
 			w.cancel()
-			}
 
+			// Run saves state as it returns. Waiting for it here
+			// means the save is done before shutdown goes on.
+			select {
+			case <-w.done:
 				return nil
+			case <-ctx.Done():
+				return fmt.Errorf(
+					"waiting for the watcher to stop: %w",
+					ctx.Err(),
+				)
+			}
 		},
 	})
 
 	return w, nil
 }
 
-// Run starts the monitoring loop with periodic scheduling.
+// Run starts the monitoring loop with periodic scheduling. When ctx
+// is cancelled, it saves state and returns.
 func (w *Watcher) Run(ctx context.Context) {
 	w.log.Info(
 		"watcher starting",
@@ -125,6 +142,7 @@ func (w *Watcher) Run(ctx context.Context) {
 	for {
 		select {
 		case <-ctx.Done():
+			w.saveState()
 			w.log.Info("watcher stopped")
 
 			return

internal/watcher/watcher_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"log/slog"
+	"os"
 	"slices"
 	"sync"
 	"testing"
@@ -135,6 +136,7 @@ type testDeps struct {
 	notifier    *mockNotifier
 	state       *state.State
 	config      *config.Config
+	log         *logger.Logger
 }
 
 func newTestWatcher(
@@ -143,6 +145,23 @@ func newTestWatcher(
 ) (*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{
@@ -157,30 +176,21 @@ func newTestWatcher(
 		t.Fatalf("globals.New: %v", err)
 	}
 
-	log, err := logger.New(nil, logger.Params{Globals: g})
+	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: log,
+		Logger: deps.log,
 		Config: cfg,
 	})
 	if err != nil {
 		t.Fatalf("state.New: %v", err)
 	}
 
-	w := watcher.NewForTest(
+	return deps
-		deps.config,
-		deps.state,
-		resolver.NewFromLogger(slog.Default()),
-		deps.portChecker,
-		deps.tlsChecker,
-		deps.notifier,
-	)
-
-	return w, deps
 }
 
 func defaultTestConfig(t *testing.T) *config.Config {
@@ -539,6 +549,85 @@ func TestGracefulShutdown(t *testing.T) {
 	}
 }
 
+// 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()