sys: RunningInUserNS(): use sync.Once

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

archive/tar_unix.go

@@ -22,7 +22,6 @@ import (
 	"archive/tar"
 	"os"
 	"strings"
-	"sync"
 	"syscall"
 
 	"github.com/containerd/containerd/sys"
@@ -84,21 +83,11 @@ func mkdir(path string, perm os.FileMode) error {
 	return os.Chmod(path, perm)
 }
 
-var (
-	inUserNS bool
-	nsOnce   sync.Once
-)
-
-func setInUserNS() {
-	inUserNS = sys.RunningInUserNS()
-}
-
 func skipFile(hdr *tar.Header) bool {
 	switch hdr.Typeflag {
 	case tar.TypeBlock, tar.TypeChar:
 		// cannot create a device if running in user namespace
-		nsOnce.Do(setInUserNS)
-		return inUserNS
+		return sys.RunningInUserNS()
 	default:
 		return false
 	}