vendor/github.com/docker/distribution/registry/storage/driver/walk.go

package driver

import (
	"context"
	"errors"
	"sort"

	"github.com/sirupsen/logrus"
)

// ErrSkipDir is used as a return value from onFileFunc to indicate that
// the directory named in the call is to be skipped. It is not returned
// as an error by any function.
var ErrSkipDir = errors.New("skip this directory")

// WalkFn is called once per file by Walk
type WalkFn func(fileInfo FileInfo) error

// WalkFallback traverses a filesystem defined within driver, starting
// from the given path, calling f on each file. It uses the List method and Stat to drive itself.
// If the returned error from the WalkFn is ErrSkipDir and fileInfo refers
// to a directory, the directory will not be entered and Walk
// will continue the traversal.  If fileInfo refers to a normal file, processing stops
func WalkFallback(ctx context.Context, driver StorageDriver, from string, f WalkFn) error {
	children, err := driver.List(ctx, from)
	if err != nil {
		return err
	}
	sort.Stable(sort.StringSlice(children))
	for _, child := range children {
		// TODO(stevvooe): Calling driver.Stat for every entry is quite
		// expensive when running against backends with a slow Stat
		// implementation, such as s3. This is very likely a serious
		// performance bottleneck.
		fileInfo, err := driver.Stat(ctx, child)
		if err != nil {
			switch err.(type) {
			case PathNotFoundError:
				// repository was removed in between listing and enumeration. Ignore it.
				logrus.WithField("path", child).Infof("ignoring deleted path")
				continue
			default:
				return err
			}
		}
		err = f(fileInfo)
		if err == nil && fileInfo.IsDir() {
			if err := WalkFallback(ctx, driver, child, f); err != nil {
				return err
			}
		} else if err == ErrSkipDir {
			// Stop iteration if it's a file, otherwise noop if it's a directory
			if !fileInfo.IsDir() {
				return nil
			}
		} else if err != nil {
			return err
		}
	}
	return nil
}