Move systemd.Manager initialization into a function in that module

This will permit us to extend the internals of systemd.Manager to include
further information about the system, such as whether cgroupv1, cgroupv2 or
both are in effect.

Furthermore, it allows a future refactor of moving more of UseSystemd() code
into the factory initialization function.

Signed-off-by: Filipe Brandenburger <filbranden@gmail.com>

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
 }