Add support for just joining in apply using cgroup paths

libcontainer/cgroups/fs/apply_raw.go

@@ -106,11 +106,29 @@ func (m *Manager) Apply(pid int) (err error) {
 		return nil
 	}
 
+	var c = m.Cgroups
+
 	d, err := getCgroupData(m.Cgroups, pid)
 	if err != nil {
 		return err
 	}
 
+	if c.Paths != nil {
+		paths := make(map[string]string)
+		for name, path := range c.Paths {
+			_, err := d.path(name)
+			if err != nil {
+				if cgroups.IsNotFound(err) {
+					continue
+				}
+				return err
+			}
+			paths[name] = path
+		}
+		m.Paths = paths
+		return cgroups.EnterPid(m.Paths, pid)
+	}
+
 	paths := make(map[string]string)
 	defer func() {
 		if err != nil {
@@ -138,6 +156,9 @@ func (m *Manager) Apply(pid int) (err error) {
 }
 
 func (m *Manager) Destroy() error {
+	if m.Cgroups.Paths != nil {
+		return nil
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	if err := cgroups.RemovePaths(m.Paths); err != nil {

libcontainer/cgroups/systemd/apply_systemd.go

@@ -168,6 +168,23 @@ func (m *Manager) Apply(pid int) error {
 		properties []systemdDbus.Property
 	)
 
+	if c.Paths != nil {
+		paths := make(map[string]string)
+		for name, path := range c.Paths {
+			_, err := getSubsystemPath(m.Cgroups, name)
+			if err != nil {
+				// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
+				if cgroups.IsNotFound(err) {
+					continue
+				}
+				return err
+			}
+			paths[name] = path
+		}
+		m.Paths = paths
+		return cgroups.EnterPid(m.Paths, pid)
+	}
+
 	if c.Parent != "" {
 		slice = c.Parent
 	}
@@ -286,6 +303,9 @@ func (m *Manager) Apply(pid int) error {
 }
 
 func (m *Manager) Destroy() error {
+	if m.Cgroups.Paths != nil {
+		return nil
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	theConn.StopUnit(getUnitName(m.Cgroups), "replace", nil)

libcontainer/configs/cgroup_unix.go

@@ -19,6 +19,9 @@ type Cgroup struct {
 	// ScopePrefix decribes prefix for the scope name
 	ScopePrefix string `json:"scope_prefix"`
 
+	// Paths represent the cgroups paths to join
+	Paths map[string]string
+
 	// Resources contains various cgroups settings to apply
 	*Resources
 }