fix empty session directories: add WithListDir hook

DirStore.ListDir now uses listDirFn when set.
SessionManager wires ListDir through the hook so
session subdirectory listing works via the DirStore view.

store/dirstore.go

@@ -23,6 +23,7 @@ type DirStore struct {
 	renameFn  func([]string, string, string) error
 	openFn    func([]string, string) (StoreEntry, error)
 	statFn    func([]string, string) (os.FileInfo, error)
+	listDirFn func([]string, string) ([]os.DirEntry, error)
 	children  map[string]*DirStore
 }
 
@@ -73,6 +74,11 @@ func WithStat(fn func(dirs []string, name string) (os.FileInfo, error)) DirOptio
 	return func(d *DirStore) { d.statFn = fn }
 }
 
+// WithListDir overrides subdirectory listing.
+func WithListDir(fn func(dirs []string, name string) ([]os.DirEntry, error)) DirOption {
+	return func(d *DirStore) { d.listDirFn = fn }
+}
+
 // NewDirStore creates a DirStore backed by one or more directories.
 // The first directory is the write target for unions.
 func NewDirStore(dirs []string, perm os.FileMode, opts ...DirOption) *DirStore {
@@ -338,6 +344,9 @@ func (d *DirStore) ListDir(rel string) ([]os.DirEntry, error) {
 		}
 		return child.ListDir(rest)
 	}
+	if d.listDirFn != nil {
+		return d.listDirFn(d.dirs, rel)
+	}
 	if len(d.dirs) == 0 {
 		return nil, fmt.Errorf("%s: not found", rel)
 	}

store/session.go

@@ -155,6 +155,7 @@ func NewSessionManager(cfg SessionManagerConfig) *SessionManager {
 		WithStat(func(_ []string, name string) (os.FileInfo, error) { return ss.stat(name) }),
 		WithOpener(func(_ []string, name string) (StoreEntry, error) { return ss.openEntry(name) }),
 		WithLister(func(_ []string) ([]os.DirEntry, error) { return ss.list() }),
+		WithListDir(func(_ []string, name string) ([]os.DirEntry, error) { return ss.ListDir(name) }),
 		WithDeleter(func(_ []string, name string) error { return ss.del(name) }),
 		WithCreator(func(_ []string, name string, _ os.FileMode) error { return ss.create(name) }),
 		WithRenamer(func(_ []string, old, new string) error { return ss.renameSession(old, new) }),