Support cancellation in DirWalk

from/from.go

@@ -58,6 +58,12 @@ type DirStep struct {
 func DirWalk(ctx context.Context, fsys fs.FS, root string) iter.Seq2[DirStep, error] {
 	return func(yield func(DirStep, error) bool) {
 		fs.WalkDir(fsys, root, func(path string, d fs.DirEntry, err error) error {
+			select {
+			case <-ctx.Done():
+				yield(DirStep{}, ctx.Err())
+				return ctx.Err()
+			default:
+			}
 			if !yield(DirStep{Path: path, Entry: d}, err) {
 				return errors.New("consumer stopped")
 			}

from/from_test.go

@@ -90,3 +90,26 @@ func TestDirWalk(t *testing.T) {
 		t.Errorf("DirWalk errors: got %v want none", errs)
 	}
 }
+
+func TestDirWalkCancellation(t *testing.T) {
+	fsys := fstest.MapFS(map[string]*fstest.MapFile{
+		"root/foo/bar.txt": {Data: []byte("hello bar")},
+		"root/empty":       {Mode: fs.ModeDir},
+		"root/cat.txt":     {Data: []byte("hello cat")},
+	})
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	var errs []error
+	var dirs []string
+	from.DirWalk(ctx, fsys, "root")(func(ds from.DirStep, err error) bool {
+		if err != nil {
+			errs = append(errs, err)
+		}
+		dirs = append(dirs, ds.Path)
+		return true
+	})
+	if len(errs) == 0 {
+		t.Errorf("DirWalk interruption: got err nil, wanted err")
+	}
+}