cg2: Add ability to set type

This adds in some simple functionality to be able to set the
cgroup type (threaded or domain).

Signed-off-by: Danny Canter <danny@dcantah.dev>

Author: dcantah

cgroup2/manager.go

@@ -43,6 +43,7 @@ const (
 	subtreeControl     = "cgroup.subtree_control"
 	controllersFile    = "cgroup.controllers"
 	killFile           = "cgroup.kill"
+	typeFile           = "cgroup.type"
 	defaultCgroup2Path = "/sys/fs/cgroup"
 	defaultSlice       = "system.slice"
 )
@@ -236,6 +237,35 @@ func setResources(path string, resources *Resources) error {
 	return nil
 }
 
+// CgroupType represents the types a cgroup can be.
+type CgroupType string
+
+const (
+	Domain   CgroupType = "domain"
+	Threaded CgroupType = "threaded"
+)
+
+func (c *Manager) GetType() (CgroupType, error) {
+	val, err := os.ReadFile(filepath.Join(c.path, typeFile))
+	if err != nil {
+		return "", err
+	}
+	trimmed := strings.TrimSpace(string(val))
+	return CgroupType(trimmed), nil
+}
+
+func (c *Manager) SetType(cgType CgroupType) error {
+	// NOTE: We could abort if cgType != Threaded here as currently
+	// it's not possible to revert back to domain, but not sure
+	// it's worth being that opinionated, especially if that may
+	// ever change.
+	v := Value{
+		filename: typeFile,
+		value:    string(cgType),
+	}
+	return writeValues(c.path, []Value{v})
+}
+
 func (c *Manager) RootControllers() ([]string, error) {
 	b, err := os.ReadFile(filepath.Join(c.unifiedMountpoint, controllersFile))
 	if err != nil {

cgroup2/manager_test.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"go.uber.org/goleak"
 )
 
@@ -221,3 +222,20 @@ func TestMoveTo(t *testing.T) {
 		return
 	}
 }
+
+func TestCgroupType(t *testing.T) {
+	checkCgroupMode(t)
+	manager, err := NewManager(defaultCgroup2Path, "/test1", ToResources(&specs.LinuxResources{}))
+	require.NoError(t, err)
+
+	cgType, err := manager.GetType()
+	require.NoError(t, err)
+	require.Equal(t, cgType, Domain)
+
+	// Swap to threaded
+	require.NoError(t, manager.SetType(Threaded))
+
+	cgType, err = manager.GetType()
+	require.NoError(t, err)
+	require.Equal(t, cgType, Threaded)
+}