cgroup2: v2 subsystems should not call WriteCgroupProc()

[AkihiroSuda]

runc/libcontainer/cgroups/fs/cpu_v2.go

Lines 32 to 42 in c4d8e16

	func (s *CpuGroupV2) ApplyDir(path string, cgroup *configs.Cgroup, pid int) error {
	// This might happen if we have no cpu cgroup mounted.
	// Just do nothing and don't fail.
	if path == "" {
	return nil
	}
	if err := os.MkdirAll(path, 0755); err != nil {
	return err
	}
	return cgroups.WriteCgroupProc(path, pid)
	}

v2 subsystems should not call WriteCgroupProc(), because it should be managed by Manager rather than each of subsystems for unified mode

#2148 (comment)

The subsystem interface should be modified like this:

diff --git a/libcontainer/cgroups/fs/apply_raw.go b/libcontainer/cgroups/fs/apply_raw.go
index 512fd700..66ab43ec 100644
--- a/libcontainer/cgroups/fs/apply_raw.go
+++ b/libcontainer/cgroups/fs/apply_raw.go
@@ -58,20 +58,24 @@ func (s subsystemSet) Get(name string) (subsystem, error) {
 }
 
 type subsystem interface {
        // Name returns the name of the subsystem.
        Name() string
        // Returns the stats, as 'stats', corresponding to the cgroup under 'path'.
        GetStats(path string, stats *cgroups.Stats) error
+       // Set the cgroup represented by cgroup.
+       Set(path string, cgroup *configs.Cgroup) error
+}
+
+type subsystemV1 interface {
+       subsystem
        // Removes the cgroup represented by 'cgroupData'.
        Remove(*cgroupData) error
        // Creates and joins the cgroup represented by 'cgroupData'.
        Apply(*cgroupData) error
-       // Set the cgroup represented by cgroup.
-       Set(path string, cgroup *configs.Cgroup) error
 }
 
 type Manager struct {
        mu       sync.Mutex
        Cgroups  *configs.Cgroup
        Rootless bool // ignore permission-related errors
        Paths    map[string]string