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.go

@@ -169,7 +169,17 @@ func (c *linuxContainer) OCIState() (*specs.State, error) {
 }
 
 func (c *linuxContainer) Processes() ([]int, error) {
-	pids, err := c.cgroupManager.GetAllPids()
+	var pids []int
+	status, err := c.currentStatus()
+	if err != nil {
+		return pids, err
+	}
+	// for systemd cgroup, the unit's cgroup path will be auto removed if container's all processes exited
+	if status == Stopped && !c.cgroupManager.Exists() {
+		return pids, nil
+	}
+
+	pids, err = c.cgroupManager.GetAllPids()
 	if err != nil {
 		return nil, newSystemErrorWithCause(err, "getting all container pids from cgroups")
 	}
@@ -385,13 +395,17 @@ func (c *linuxContainer) start(process *Process) error {
 func (c *linuxContainer) Signal(s os.Signal, all bool) error {
 	c.m.Lock()
 	defer c.m.Unlock()
-	if all {
-		return signalAllProcesses(c.cgroupManager, s)
-	}
 	status, err := c.currentStatus()
 	if err != nil {
 		return err
 	}
+	if all {
+		// for systemd cgroup, the unit's cgroup path will be auto removed if container's all processes exited
+		if status == Stopped && !c.cgroupManager.Exists() {
+			return nil
+		}
+		return signalAllProcesses(c.cgroupManager, s)
+	}
 	// to avoid a PID reuse attack
 	if status == Running || status == Created || status == Paused {
 		if err := c.initProcess.signal(s); err != nil {