implement wait on freeze

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)

libcontainer/cgroups/fs/apply_raw.go

@@ -164,6 +164,11 @@ func writeFile(dir, file, data string) error {
 	return ioutil.WriteFile(filepath.Join(dir, file), []byte(data), 0700)
 }
 
+func readFile(dir, file string) (string, error) {
+	data, err := ioutil.ReadFile(filepath.Join(dir, file))
+	return string(data), err
+}
+
 func removePath(p string, err error) error {
 	if err != nil {
 		return err

libcontainer/cgroups/fs/freezer.go

@@ -4,6 +4,7 @@ import (
 	"io/ioutil"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"github.com/dotcloud/docker/pkg/libcontainer/cgroups"
 )
@@ -22,6 +23,17 @@ func (s *freezerGroup) Set(d *data) error {
 		if err := writeFile(dir, "freezer.state", string(d.c.Freezer)); err != nil {
 			return err
 		}
+
+		for {
+			state, err := readFile(dir, "freezer.state")
+			if err != nil {
+				return err
+			}
+			if strings.TrimSpace(state) == string(d.c.Freezer) {
+				break
+			}
+			time.Sleep(1 * time.Millisecond)
+		}
 	default:
 		if _, err := d.join("freezer"); err != nil && err != cgroups.ErrNotFound {
 			return err

libcontainer/cgroups/systemd/apply_systemd.go

@@ -3,6 +3,7 @@
 package systemd
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -10,6 +11,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	systemd1 "github.com/coreos/go-systemd/dbus"
 	"github.com/dotcloud/docker/pkg/libcontainer/cgroups"
@@ -351,7 +353,20 @@ func Freeze(c *cgroups.Cgroup, state cgroups.FreezerState) error {
 		return err
 	}
 
-	return ioutil.WriteFile(filepath.Join(path, "freezer.state"), []byte(state), 0)
+	if err := ioutil.WriteFile(filepath.Join(path, "freezer.state"), []byte(state), 0); err != nil {
+		return err
+	}
+	for {
+		state_, err := ioutil.ReadFile(filepath.Join(path, "freezer.state"))
+		if err != nil {
+			return err
+		}
+		if string(state) == string(bytes.TrimSpace(state_)) {
+			break
+		}
+		time.Sleep(1 * time.Millisecond)
+	}
+	return nil
 }
 
 func GetPids(c *cgroups.Cgroup) ([]int, error) {