aufs: remove mntL

Commit 5cd62852fa added a lock around call to unix.Mount() to
avoid the race in aufs kernel code related to xino file creation
and removal. While this is going to be fixed in the kernel, we still
need to support the current aufs, so some kind of fix is required.

A think a better fix (rather than a lock) is to add a random suffix
to the file name (note it is and was a separate file per mount,
never mind the same file name -- the file is created/opened and
removed instantly, so each mount deals with its own file).

With the suffix added, the possibility to hit the race is extremely
low, and we don't have to do any locking.

Note we don't add any more characters, instead we're replacing
`xino` with four random characters in the 0-9a-z range.

See also: https://sourceforge.net/p/aufs/mailman/message/36674769/

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/graphdriver/aufs/aufs.go

@@ -28,10 +28,12 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"os"
 	"os/exec"
 	"path"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -80,7 +82,6 @@ type Driver struct {
 	pathCache     map[string]string
 	naiveDiff     graphdriver.DiffDriver
 	locker        *locker.Locker
-	mntL          sync.Mutex
 }
 
 // Init returns a new AUFS driver.
@@ -618,14 +619,14 @@ func (a *Driver) aufsMount(ro []string, rw, target, mountLabel string) (err erro
 		bp += copy(b[bp:], layer)
 	}
 
-	opts := "dio,xino=/dev/shm/aufs.xino"
+	// random 4 characters in the 0-9a-z range (e.g. "g6dz")
+	rnd := strconv.FormatInt(int64(1e9+rand.Uint32()%1e9), 36)[1:5]
+	opts := "dio,xino=/dev/shm/aufs." + rnd
 	if useDirperm() {
 		opts += ",dirperm1"
 	}
 	data := label.FormatMountLabel(fmt.Sprintf("%s,%s", string(b[:bp]), opts), mountLabel)
-	a.mntL.Lock()
 	err = unix.Mount("none", target, "aufs", 0, data)
-	a.mntL.Unlock()
 	if err != nil {
 		err = errors.Wrap(err, "mount target="+target+" data="+data)
 		return
@@ -641,9 +642,7 @@ func (a *Driver) aufsMount(ro []string, rw, target, mountLabel string) (err erro
 			bp += copy(b[bp:], layer)
 		}
 		data := label.FormatMountLabel(string(b[:bp]), mountLabel)
-		a.mntL.Lock()
 		err = unix.Mount("none", target, "aufs", unix.MS_REMOUNT, data)
-		a.mntL.Unlock()
 		if err != nil {
 			err = errors.Wrap(err, "mount target="+target+" flags=MS_REMOUNT data="+data)
 			return