Move systemd.Manager initialization into a function in that module

libcontainer/cgroups/systemd/apply_nosystemd.go

@@ -18,6 +18,10 @@ func UseSystemd() bool {
 	return false
 }
 
+func NewSystemdCgroupsManager() (func(config *configs.Cgroup, paths map[string]string) cgroups.Manager, error) {
+	return nil, fmt.Errorf("Systemd not supported")
+}
+
 func (m *Manager) Apply(pid int) error {
 	return fmt.Errorf("Systemd not supported")
 }

libcontainer/cgroups/systemd/apply_systemd.go

@@ -163,6 +163,18 @@ func UseSystemd() bool {
 	return hasStartTransientUnit
 }
 
+func NewSystemdCgroupsManager() (func(config *configs.Cgroup, paths map[string]string) cgroups.Manager, error) {
+	if !systemdUtil.IsRunningSystemd() {
+		return nil, fmt.Errorf("systemd not running on this host, can't use systemd as a cgroups.Manager")
+	}
+	return func(config *configs.Cgroup, paths map[string]string) cgroups.Manager {
+		return &Manager{
+			Cgroups: config,
+			Paths:   paths,
+		}
+	}, nil
+}
+
 func (m *Manager) Apply(pid int) error {
 	var (
 		c          = m.Cgroups

libcontainer/factory_linux.go

@@ -51,12 +51,11 @@ func InitArgs(args ...string) func(*LinuxFactory) error {
 // SystemdCgroups is an options func to configure a LinuxFactory to return
 // containers that use systemd to create and manage cgroups.
 func SystemdCgroups(l *LinuxFactory) error {
-	l.NewCgroupsManager = func(config *configs.Cgroup, paths map[string]string) cgroups.Manager {
-		return &systemd.Manager{
-			Cgroups: config,
-			Paths:   paths,
-		}
+	systemdCgroupsManager, err := systemd.NewSystemdCgroupsManager()
+	if err != nil {
+		return err
 	}
+	l.NewCgroupsManager = systemdCgroupsManager
 	return nil
 }