CgroupsPath in runC

This sets up the cgroupsPath. If cgroupsPath is passed in, runC will
check if the cgroup exists. If it exists, runC will just join the
cgroup, and will not make any modifications to the cgroups.
If it does not exist, runC will create the cgroup and join it.
Signed-off-by: Abin Shahab <[email protected]>

Author: ashahab-altiscale

libcontainer/cgroups/fs/apply_raw.go

@@ -122,6 +122,7 @@ func (m *Manager) Apply(pid int) (err error) {
 		if err := sys.Apply(d); err != nil {
 			return err
 		}
+
 		// TODO: Apply should, ideally, be reentrant or be broken up into a separate
 		// create and join phase so that the cgroup hierarchy for a container can be
 		// created then join consists of writing the process pids to cgroup.procs
@@ -289,8 +290,14 @@ func (raw *cgroupData) join(subsystem string) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if err := os.MkdirAll(path, 0755); err != nil {
-		return "", err
+	_, err = os.Stat(path)
+	if err != nil && os.IsNotExist(err) {
+		if err := os.MkdirAll(path, 0755); err != nil {
+			return "", err
+		}
+	} else if err == nil {
+		//cgroup exists, therefore join only
+		raw.config.ExternalCgroup = true
 	}
 	if err := writeFile(path, CgroupProcesses, strconv.Itoa(raw.pid)); err != nil {
 		return "", err

libcontainer/cgroups/fs/devices.go

@@ -30,6 +30,9 @@ func (s *DevicesGroup) Apply(d *cgroupData) error {
 }
 
 func (s *DevicesGroup) Set(path string, cgroup *configs.Cgroup) error {
+	if cgroup.ExternalCgroup {
+		return nil
+	}
 	if !cgroup.AllowAllDevices {
 		if err := writeFile(path, "devices.deny", "a"); err != nil {
 			return err

libcontainer/configs/cgroup_unix.go

@@ -12,6 +12,8 @@ const (
 
 type Cgroup struct {
 	Name string `json:"name"`
+	// indicates that this is an externally passed, fully created cgroup
+	ExternalCgroup bool `json:"external_cgroup"`
 
 	// name of parent cgroup or slice
 	Parent string `json:"parent"`

spec.go

@@ -437,6 +437,9 @@ func createCgroupConfig(name string, spec *specs.LinuxRuntimeSpec, devices []*co
 	if err != nil {
 		return nil, err
 	}
+	if spec.Linux.CgroupsPath != "" {
+		name = spec.Linux.CgroupsPath
+	}
 	c := &configs.Cgroup{
 		Name:           name,
 		Parent:         myCgroupPath,