feat(directory-watcher): add validation and test improvements

- Add validation: return error when no databases found in directory without watch enabled
- Split EmptyDirectory test to validate both watch enabled/disabled scenarios
- Add test for recursive mode detecting nested databases
- Fix race condition in Store.Close() by cloning dbs slice while holding lock

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: corylanou

cmd/litestream/main.go

@@ -594,6 +594,10 @@ func NewDBsFromDirectoryConfig(dbc *DBConfig) ([]*litestream.DB, error) {
 		return nil, fmt.Errorf("failed to scan directory %s: %w", dirPath, err)
 	}
 
+	if len(dbPaths) == 0 && !dbc.Watch {
+		return nil, fmt.Errorf("no SQLite databases found in directory %s with pattern %s", dirPath, dbc.Pattern)
+	}
+
 	// Create DB instances for each found database
 	var dbs []*litestream.DB
 	for _, dbPath := range dbPaths {

cmd/litestream/main_test.go

@@ -1809,7 +1809,7 @@ func TestNewDBsFromDirectoryConfig_ReplicasArray(t *testing.T) {
 	}
 }
 
-func TestNewDBsFromDirectoryConfig_EmptyDirectory(t *testing.T) {
+func TestNewDBsFromDirectoryConfig_EmptyDirectoryRequiresDatabases(t *testing.T) {
 	tmpDir := t.TempDir()
 	replicaDir := filepath.Join(tmpDir, "replica")
 
@@ -1819,6 +1819,22 @@ func TestNewDBsFromDirectoryConfig_EmptyDirectory(t *testing.T) {
 		Replica: &main.ReplicaConfig{Type: "file", Path: replicaDir},
 	}
 
+	if _, err := main.NewDBsFromDirectoryConfig(config); err == nil {
+		t.Fatalf("expected error for empty directory when watch disabled")
+	}
+}
+
+func TestNewDBsFromDirectoryConfig_EmptyDirectoryWithWatch(t *testing.T) {
+	tmpDir := t.TempDir()
+	replicaDir := filepath.Join(tmpDir, "replica")
+
+	config := &main.DBConfig{
+		Dir:     tmpDir,
+		Pattern: "*.db",
+		Watch:   true,
+		Replica: &main.ReplicaConfig{Type: "file", Path: replicaDir},
+	}
+
 	dbs, err := main.NewDBsFromDirectoryConfig(config)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -1883,6 +1899,49 @@ func TestDirectoryMonitor_DetectsDatabaseLifecycle(t *testing.T) {
 	}
 }
 
+func TestDirectoryMonitor_RecursiveDetectsNestedDatabases(t *testing.T) {
+	ctx := context.Background()
+	rootDir := t.TempDir()
+	replicaDir := filepath.Join(t.TempDir(), "replicas")
+
+	config := &main.DBConfig{
+		Dir:       rootDir,
+		Pattern:   "*.db",
+		Recursive: true,
+		Watch:     true,
+		Replica:   &main.ReplicaConfig{Type: "file", Path: replicaDir},
+	}
+
+	storeConfig := main.DefaultConfig()
+	store := litestream.NewStore(nil, storeConfig.CompactionLevels())
+	store.CompactionMonitorEnabled = false
+	if err := store.Open(ctx); err != nil {
+		t.Fatalf("unexpected error opening store: %v", err)
+	}
+	defer func() {
+		if err := store.Close(context.Background()); err != nil {
+			t.Fatalf("unexpected error closing store: %v", err)
+		}
+	}()
+
+	monitor, err := main.NewDirectoryMonitor(ctx, store, config, nil)
+	if err != nil {
+		t.Fatalf("failed to initialize directory monitor: %v", err)
+	}
+	defer monitor.Close()
+
+	deepDir := filepath.Join(rootDir, "tenant", "nested", "deeper")
+	if err := os.MkdirAll(deepDir, 0755); err != nil {
+		t.Fatalf("failed to create nested directories: %v", err)
+	}
+	deepDB := filepath.Join(deepDir, "deep.db")
+	createSQLiteDB(t, deepDB)
+
+	if !waitForCondition(5*time.Second, func() bool { return hasDBPath(store.DBs(), deepDB) }) {
+		t.Fatalf("expected nested database %s to be detected", deepDB)
+	}
+}
+
 // createSQLiteDB creates a minimal SQLite database file for testing
 func createSQLiteDB(t *testing.T, path string) {
 	t.Helper()

store.go

@@ -135,7 +135,11 @@ func (s *Store) Open(ctx context.Context) error {
 }
 
 func (s *Store) Close(ctx context.Context) (err error) {
-	for _, db := range s.dbs {
+	s.mu.Lock()
+	dbs := slices.Clone(s.dbs)
+	s.mu.Unlock()
+
+	for _, db := range dbs {
 		if e := db.Close(ctx); e != nil && err == nil {
 			err = e
 		}