podman: add new cgroup mode split

When running under systemd there is no need to create yet another
cgroup for the container.

With conmon-delegated the current cgroup will be split in two sub
cgroups:

- supervisor
- container

The supervisor cgroup will hold conmon and the podman process, while
the container cgroup is used by the OCI runtime (using the cgroupfs
backend).

Closes: https://github.com/containers/libpod/issues/6400

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/container_validate.go

@@ -34,6 +34,10 @@ func (c *Container) validate() error {
 		return errors.Wrapf(define.ErrInvalidArg, "cannot both create a network namespace and join another container's network namespace")
 	}
 
+	if c.config.CgroupsMode == cgroupSplit && c.config.CgroupParent != "" {
+		return errors.Wrapf(define.ErrInvalidArg, "cannot specify --cgroup-mode=split with a cgroup-parent")
+	}
+
 	// Not creating cgroups has a number of requirements, mostly related to
 	// the PID namespace.
 	if c.config.NoCgroups || c.config.CgroupsMode == "disabled" {