Use the New Driver Walk method for catalog enumeration

This changes the Walk Method used for catalog enumeration. Just to show
how much an effect this has on our s3 storage:
Original:
List calls: 6839

real    3m16.636s
user    0m0.000s
sys    0m0.016s

New:
ListObjectsV2 Calls: 1805

real    0m49.970s
user    0m0.008s
sys    0m0.000s

This is because it no longer performs a list and stat per item, and instead
is able to use the metadata gained from the list as a replacement to stat.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>

registry/storage/driver/s3-aws/s3.go

@@ -34,6 +34,7 @@ import (
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/s3"
 
+	dcontext "github.com/docker/distribution/context"
 	"github.com/docker/distribution/registry/client/transport"
 	storagedriver "github.com/docker/distribution/registry/storage/driver"
 	"github.com/docker/distribution/registry/storage/driver/base"
@@ -876,8 +877,132 @@ func (d *driver) URLFor(ctx context.Context, path string, options map[string]int
 
 // Walk traverses a filesystem defined within driver, starting
 // from the given path, calling f on each file
-func (d *driver) Walk(ctx context.Context, path string, f storagedriver.WalkFn) error {
-	return storagedriver.WalkFallback(ctx, d, path, f)
+func (d *driver) Walk(ctx context.Context, from string, f storagedriver.WalkFn) error {
+	path := from
+	if !strings.HasSuffix(path, "/") {
+		path = path + "/"
+	}
+
+	prefix := ""
+	if d.s3Path("") == "" {
+		prefix = "/"
+	}
+
+	var objectCount int64
+	if err := d.doWalk(ctx, &objectCount, d.s3Path(path), prefix, f); err != nil {
+		return err
+	}
+
+	// S3 doesn't have the concept of empty directories, so it'll return path not found if there are no objects
+	if objectCount == 0 {
+		return storagedriver.PathNotFoundError{Path: from}
+	}
+
+	return nil
+}
+
+type walkInfoContainer struct {
+	storagedriver.FileInfoFields
+	prefix *string
+}
+
+// Path provides the full path of the target of this file info.
+func (wi walkInfoContainer) Path() string {
+	return wi.FileInfoFields.Path
+}
+
+// Size returns current length in bytes of the file. The return value can
+// be used to write to the end of the file at path. The value is
+// meaningless if IsDir returns true.
+func (wi walkInfoContainer) Size() int64 {
+	return wi.FileInfoFields.Size
+}
+
+// ModTime returns the modification time for the file. For backends that
+// don't have a modification time, the creation time should be returned.
+func (wi walkInfoContainer) ModTime() time.Time {
+	return wi.FileInfoFields.ModTime
+}
+
+// IsDir returns true if the path is a directory.
+func (wi walkInfoContainer) IsDir() bool {
+	return wi.FileInfoFields.IsDir
+}
+
+func (d *driver) doWalk(parentCtx context.Context, objectCount *int64, path, prefix string, f storagedriver.WalkFn) error {
+	var retError error
+
+	listObjectsInput := &s3.ListObjectsV2Input{
+		Bucket:    aws.String(d.Bucket),
+		Prefix:    aws.String(path),
+		Delimiter: aws.String("/"),
+		MaxKeys:   aws.Int64(listMax),
+	}
+
+	ctx, done := dcontext.WithTrace(parentCtx)
+	defer done("s3aws.ListObjectsV2Pages(%s)", path)
+	listObjectErr := d.S3.ListObjectsV2PagesWithContext(ctx, listObjectsInput, func(objects *s3.ListObjectsV2Output, lastPage bool) bool {
+
+		*objectCount += *objects.KeyCount
+		walkInfos := make([]walkInfoContainer, 0, *objects.KeyCount)
+
+		for _, dir := range objects.CommonPrefixes {
+			commonPrefix := *dir.Prefix
+			walkInfos = append(walkInfos, walkInfoContainer{
+				prefix: dir.Prefix,
+				FileInfoFields: storagedriver.FileInfoFields{
+					IsDir: true,
+					Path:  strings.Replace(commonPrefix[:len(commonPrefix)-1], d.s3Path(""), prefix, 1),
+				},
+			})
+		}
+
+		for _, file := range objects.Contents {
+			walkInfos = append(walkInfos, walkInfoContainer{
+				FileInfoFields: storagedriver.FileInfoFields{
+					IsDir:   false,
+					Size:    *file.Size,
+					ModTime: *file.LastModified,
+					Path:    strings.Replace(*file.Key, d.s3Path(""), prefix, 1),
+				},
+			})
+		}
+
+		sort.SliceStable(walkInfos, func(i, j int) bool { return walkInfos[i].FileInfoFields.Path < walkInfos[j].FileInfoFields.Path })
+
+		for _, walkInfo := range walkInfos {
+			err := f(walkInfo)
+
+			if err == storagedriver.ErrSkipDir {
+				if walkInfo.IsDir() {
+					continue
+				} else {
+					break
+				}
+			} else if err != nil {
+				retError = err
+				return false
+			}
+
+			if walkInfo.IsDir() {
+				if err := d.doWalk(ctx, objectCount, *walkInfo.prefix, prefix, f); err != nil {
+					retError = err
+					return false
+				}
+			}
+		}
+		return true
+	})
+
+	if retError != nil {
+		return retError
+	}
+
+	if listObjectErr != nil {
+		return listObjectErr
+	}
+
+	return nil
 }
 
 func (d *driver) s3Path(path string) string {