Check for non-active/supported cgroups

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cgroup.go

@@ -35,14 +35,20 @@ func New(hierarchy Hierarchy, path Path, resources *specs.LinuxResources) (Cgrou
 	if err != nil {
 		return nil, err
 	}
+	var active []Subsystem
 	for _, s := range subsystems {
+		// check if subsystem exists
 		if err := initializeSubsystem(s, path, resources); err != nil {
+			if err == ErrControllerNotActive {
+				continue
+			}
 			return nil, err
 		}
+		active = append(active, s)
 	}
 	return &cgroup{
 		path:       path,
-		subsystems: subsystems,
+		subsystems: active,
 	}, nil
 }
 
@@ -60,6 +66,9 @@ func Load(hierarchy Hierarchy, path Path) (Cgroup, error) {
 			if os.IsNotExist(errors.Cause(err)) {
 				return nil, ErrCgroupDeleted
 			}
+			if err == ErrControllerNotActive {
+				continue
+			}
 			return nil, err
 		}
 		if _, err := os.Lstat(s.Path(p)); err != nil {

paths.go

@@ -57,6 +57,9 @@ func PidPath(pid int) Path {
 	return existingPath(paths, "")
 }
 
+// ErrControllerNotActive is returned when a controller is not supported or enalbed
+var ErrControllerNotActive = errors.New("controller is not supported")
+
 func existingPath(paths map[string]string, suffix string) Path {
 	// localize the paths based on the root mount dest for nested cgroups
 	for n, p := range paths {
@@ -77,7 +80,7 @@ func existingPath(paths map[string]string, suffix string) Path {
 		root, ok := paths[string(name)]
 		if !ok {
 			if root, ok = paths[fmt.Sprintf("name=%s", name)]; !ok {
-				return "", fmt.Errorf("unable to find %q in controller set", name)
+				return "", ErrControllerNotActive
 			}
 		}
 		if suffix != "" {

paths_test.go

@@ -112,3 +112,29 @@ func TestEmptySubsystem(t *testing.T) {
 		}
 	}
 }
+
+func TestSystemd240(t *testing.T) {
+	const data = `8:net_cls:/
+	7:memory:/system.slice/docker.service
+	6:freezer:/
+	5:blkio:/system.slice/docker.service
+	4:devices:/system.slice/docker.service
+	3:cpuset:/
+	2:cpu,cpuacct:/system.slice/docker.service
+	1:name=systemd:/system.slice/docker.service
+	0::/system.slice/docker.service`
+	r := strings.NewReader(data)
+	paths, err := parseCgroupFromReader(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	path := existingPath(paths, "")
+	_, err = path("net_prio")
+	if err == nil {
+		t.Fatal("error for net_prio shoulld not be nil")
+	}
+	if err != ErrControllerNotActive {
+		t.Fatalf("expected error %q but received %q", ErrControllerNotActive, err)
+	}
+}