fix path error in systemd when stopped

When we use cgroup with systemd driver, the cgroup path will be auto removed
by systemd when all processes exited. So we should check cgroup path exists
when we access the cgroup path, for example in `kill/ps`, or else we will
got an error.

Signed-off-by: lifubang <lifubang@acmcoder.com>

libcontainer/container_linux_test.go

@@ -50,6 +50,15 @@ func (m *mockCgroupManager) Destroy() error {
 	return nil
 }
 
+func (m *mockCgroupManager) Exists() bool {
+	paths := m.GetPaths()
+	if paths != nil {
+		_, err := os.Lstat(paths["devices"])
+		return err == nil
+	}
+	return false
+}
+
 func (m *mockCgroupManager) GetPaths() map[string]string {
 	return m.paths
 }
@@ -134,11 +143,27 @@ func (m *mockProcess) forwardChildLogs() {
 }
 
 func TestGetContainerPids(t *testing.T) {
-	container := &linuxContainer{
-		id:            "myid",
-		config:        &configs.Config{},
-		cgroupManager: &mockCgroupManager{allPids: []int{1, 2, 3}},
+	pid := 1
+	stat, err := system.Stat(pid)
+	if err != nil {
+		t.Fatalf("can't stat pid %d, got %v", pid, err)
 	}
+	container := &linuxContainer{
+		id:     "myid",
+		config: &configs.Config{},
+		cgroupManager: &mockCgroupManager{
+			allPids: []int{1, 2, 3},
+			paths: map[string]string{
+				"device": "/proc/self/cgroups",
+			},
+		},
+		initProcess: &mockProcess{
+			_pid:    1,
+			started: 10,
+		},
+		initProcessStartTime: stat.StartTime,
+	}
+	container.state = &runningState{c: container}
 	pids, err := container.Processes()
 	if err != nil {
 		t.Fatal(err)