overlay: move supportsMultipleLowerDir to utils

This moves supportsMultipleLowerDir() to overlayutils
so it can be used from both overlay and overlay2.

The only changes made were:
 * replace logger with logrus
 * don't use workDirName mergedDirName constants
 * add mnt var to improve readability a bit

This is a preparation for the next commit.

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

daemon/graphdriver/overlay2/check.go

@@ -99,35 +99,3 @@ func doesSupportNativeDiff(d string) error {
 
 	return nil
 }
-
-// supportsMultipleLowerDir checks if the system supports multiple lowerdirs,
-// which is required for the overlay2 driver. On 4.x kernels, multiple lowerdirs
-// are always available (so this check isn't needed), and backported to RHEL and
-// CentOS 3.x kernels (3.10.0-693.el7.x86_64 and up). This function is to detect
-// support on those kernels, without doing a kernel version compare.
-func supportsMultipleLowerDir(d string) error {
-	td, err := ioutil.TempDir(d, "multiple-lowerdir-check")
-	if err != nil {
-		return err
-	}
-	defer func() {
-		if err := os.RemoveAll(td); err != nil {
-			logger.Warnf("Failed to remove check directory %v: %v", td, err)
-		}
-	}()
-
-	for _, dir := range []string{"lower1", "lower2", "upper", workDirName, mergedDirName} {
-		if err := os.Mkdir(filepath.Join(td, dir), 0755); err != nil {
-			return err
-		}
-	}
-
-	opts := fmt.Sprintf("lowerdir=%s:%s,upperdir=%s,workdir=%s", path.Join(td, "lower2"), path.Join(td, "lower1"), path.Join(td, "upper"), path.Join(td, workDirName))
-	if err := unix.Mount("overlay", filepath.Join(td, mergedDirName), "overlay", 0, opts); err != nil {
-		return errors.Wrap(err, "failed to mount overlay")
-	}
-	if err := unix.Unmount(filepath.Join(td, mergedDirName), 0); err != nil {
-		logger.Warnf("Failed to unmount check directory %v: %v", filepath.Join(td, mergedDirName), err)
-	}
-	return nil
-}

daemon/graphdriver/overlay2/overlay.go

@@ -180,7 +180,7 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		if opts.overrideKernelCheck {
 			logger.Warn("Using pre-4.0.0 kernel for overlay2, mount failures may require kernel update")
 		} else {
-			if err := supportsMultipleLowerDir(testdir); err != nil {
+			if err := overlayutils.SupportsMultipleLowerDir(testdir); err != nil {
 				logger.Debugf("Multiple lower dirs not supported: %v", err)
 				return nil, graphdriver.ErrNotSupported
 			}

daemon/graphdriver/overlayutils/overlayutils.go

@@ -4,8 +4,15 @@ package overlayutils // import "github.com/docker/docker/daemon/graphdriver/over
 
 import (
 	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"path/filepath"
 
 	"github.com/docker/docker/daemon/graphdriver"
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
 )
 
 // ErrDTypeNotSupported denotes that the backing filesystem doesn't support d_type.
@@ -23,3 +30,36 @@ func ErrDTypeNotSupported(driver, backingFs string) error {
 
 	return graphdriver.NotSupportedError(msg)
 }
+
+// SupportsMultipleLowerDir checks if the system supports multiple lowerdirs,
+// which is required for the overlay2 driver. On 4.x kernels, multiple lowerdirs
+// are always available (so this check isn't needed), and backported to RHEL and
+// CentOS 3.x kernels (3.10.0-693.el7.x86_64 and up). This function is to detect
+// support on those kernels, without doing a kernel version compare.
+func SupportsMultipleLowerDir(d string) error {
+	td, err := ioutil.TempDir(d, "multiple-lowerdir-check")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := os.RemoveAll(td); err != nil {
+			logrus.Warnf("Failed to remove check directory %v: %v", td, err)
+		}
+	}()
+
+	for _, dir := range []string{"lower1", "lower2", "upper", "work", "merged"} {
+		if err := os.Mkdir(filepath.Join(td, dir), 0755); err != nil {
+			return err
+		}
+	}
+
+	mnt := filepath.Join(td, "merged")
+	opts := fmt.Sprintf("lowerdir=%s:%s,upperdir=%s,workdir=%s", path.Join(td, "lower2"), path.Join(td, "lower1"), path.Join(td, "upper"), path.Join(td, "work"))
+	if err := unix.Mount("overlay", mnt, "overlay", 0, opts); err != nil {
+		return errors.Wrap(err, "failed to mount overlay")
+	}
+	if err := unix.Unmount(mnt, 0); err != nil {
+		logrus.Warnf("Failed to unmount check directory %v: %v", mnt, err)
+	}
+	return nil
+}