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
 	}

sys/userns_linux.go

@@ -20,22 +20,29 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"sync"
+)
+
+var (
+	inUserNS bool
+	nsOnce   sync.Once
 )
 
 // RunningInUserNS detects whether we are currently running in a user namespace.
 // Originally copied from github.com/lxc/lxd/shared/util.go
 func RunningInUserNS() bool {
+	nsOnce.Do(func() {
 		file, err := os.Open("/proc/self/uid_map")
 		if err != nil {
 			// This kernel-provided file only exists if user namespaces are supported
-		return false
+			return
 		}
 		defer file.Close()
 
 		buf := bufio.NewReader(file)
 		l, _, err := buf.ReadLine()
 		if err != nil {
-		return false
+			return
 		}
 
 		line := string(l)
@@ -47,7 +54,9 @@ func RunningInUserNS() bool {
 		 * range - 4294967295 uids starting at uid 0.
 		 */
 		if a == 0 && b == 0 && c == 4294967295 {
-		return false
+			return
 		}
-	return true
+		inUserNS = true
+	})
+	return inUserNS
 }