Merge pull request #1529 from tonistiigi/migrate-fix

cache: avoid snapshot migration on shallow refs

cache/migrate_v2.go

@@ -205,6 +205,8 @@ func MigrateV2(ctx context.Context, from, to string, cs content.Store, s snapsho
 
 	// remove old root labels
 	for _, item := range byID {
+		em := getEqualMutable(item)
+		if em == "" {
 			if _, err := s.Update(ctx, snapshots.Info{
 				Name: getSnapshotID(item),
 			}, "labels.containerd.io/gc.root"); err != nil {
@@ -221,6 +223,7 @@ func MigrateV2(ctx context.Context, from, to string, cs content.Store, s snapsho
 				}
 			}
 		}
+	}
 
 	// previous implementation can leak views, just clean up all views
 	err = s.Walk(ctx, func(ctx context.Context, info snapshots.Info) error {