Ignore missing paths during enumeration

It's possible to run into a race condition in which the enumerator lists
lots of repositories and then starts the long process of enumerating through
them. In that time if someone deletes a repo, the enumerator may error out.

Signed-off-by: Ryan Abrams <rdabrams@gmail.com>

registry/storage/driver/walk.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"errors"
 	"sort"
+
+	"github.com/sirupsen/logrus"
 )
 
 // ErrSkipDir is used as a return value from onFileFunc to indicate that
@@ -32,7 +34,14 @@ func WalkFallback(ctx context.Context, driver StorageDriver, from string, f Walk
 		// performance bottleneck.
 		fileInfo, err := driver.Stat(ctx, child)
 		if err != nil {
-			return err
+			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() {

registry/storage/driver/walk_test.go

@@ -0,0 +1,47 @@
+package driver
+
+import (
+	"context"
+	"fmt"
+	"testing"
+)
+
+type changingFileSystem struct {
+	StorageDriver
+	fileset   []string
+	keptFiles map[string]bool
+}
+
+func (cfs *changingFileSystem) List(ctx context.Context, path string) ([]string, error) {
+	return cfs.fileset, nil
+}
+func (cfs *changingFileSystem) Stat(ctx context.Context, path string) (FileInfo, error) {
+	kept, ok := cfs.keptFiles[path]
+	if ok && kept {
+		return &FileInfoInternal{
+			FileInfoFields: FileInfoFields{
+				Path: path,
+			},
+		}, nil
+	}
+	return nil, PathNotFoundError{}
+}
+func TestWalkFileRemoved(t *testing.T) {
+	d := &changingFileSystem{
+		fileset: []string{"zoidberg", "bender"},
+		keptFiles: map[string]bool{
+			"zoidberg": true,
+		},
+	}
+	infos := []FileInfo{}
+	err := WalkFallback(context.Background(), d, "", func(fileInfo FileInfo) error {
+		infos = append(infos, fileInfo)
+		return nil
+	})
+	if len(infos) != 1 || infos[0].Path() != "zoidberg" {
+		t.Errorf(fmt.Sprintf("unexpected path set during walk: %s", infos))
+	}
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+}