Retry getting the cgroup root at apply time.

This will allow late-binding of the cgroup hierarchy.

Fixes docker/docker#8791

Signed-off-by: Victor Marmol <vmarmol@google.com>

cgroups/fs/apply_raw.go

@@ -1,11 +1,11 @@
 package fs
 
 import (
-	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strconv"
+	"sync"
 
 	"github.com/docker/libcontainer/cgroups"
 )
@@ -25,20 +25,31 @@ var (
 )
 
 // The absolute path to the root of the cgroup hierarchies.
+var cgroupRootLock sync.Mutex
 var cgroupRoot string
 
-// TODO(vmarmol): Report error here, we'll probably need to wait for the new API.
+// Gets the cgroupRoot.
-func init() {
+func getCgroupRoot() (string, error) {
+	cgroupRootLock.Lock()
+	defer cgroupRootLock.Unlock()
+
+	if cgroupRoot != "" {
+		return cgroupRoot, nil
+	}
+
 	// we can pick any subsystem to find the root
 	cpuRoot, err := cgroups.FindCgroupMountpoint("cpu")
 	if err != nil {
-		return
+		return "", err
 	}
-	cgroupRoot = filepath.Dir(cpuRoot)
+	root := filepath.Dir(cpuRoot)
 
-	if _, err := os.Stat(cgroupRoot); err != nil {
+	if _, err := os.Stat(root); err != nil {
-		return
+		return "", err
 	}
+
+	cgroupRoot = root
+	return cgroupRoot, nil
 }
 
 type subsystem interface {
@@ -152,8 +163,9 @@ func GetPids(c *cgroups.Cgroup) ([]int, error) {
 }
 
 func getCgroupData(c *cgroups.Cgroup, pid int) (*data, error) {
-	if cgroupRoot == "" {
+	root, err := getCgroupRoot()
-		return nil, fmt.Errorf("failed to find the cgroup root")
+	if err != nil {
+		return nil, err
 	}
 
 	cgroup := c.Name
@@ -162,7 +174,7 @@ func getCgroupData(c *cgroups.Cgroup, pid int) (*data, error) {
 	}
 
 	return &data{
-		root:   cgroupRoot,
+		root:   root,
 		cgroup: cgroup,
 		c:      c,
 		pid:    pid,