Add SkipOpts for handling how subsystems are registered

This adds a flexible way for subsystems to be skipped or required within
the cgroups package.

Signed-off-by: Michael Crosby <[email protected]>

cgroup.go

@@ -30,7 +30,13 @@ import (
 )
 
 // New returns a new control via the cgroup cgroups interface
-func New(hierarchy Hierarchy, path Path, resources *specs.LinuxResources) (Cgroup, error) {
+func New(hierarchy Hierarchy, path Path, resources *specs.LinuxResources, opts ...InitOpts) (Cgroup, error) {
+	config := newInitConfig()
+	for _, o := range opts {
+		if err := o(config); err != nil {
+			return nil, err
+		}
+	}
 	subsystems, err := hierarchy()
 	if err != nil {
 		return nil, err
@@ -40,6 +46,13 @@ func New(hierarchy Hierarchy, path Path, resources *specs.LinuxResources) (Cgrou
 		// check if subsystem exists
 		if err := initializeSubsystem(s, path, resources); err != nil {
 			if err == ErrControllerNotActive {
+				if config.InitCheck != nil {
+					if skerr := config.InitCheck(s, path, err); skerr != nil {
+						if skerr != ErrIgnoreSubsystem {
+							return nil, skerr
+						}
+					}
+				}
 				continue
 			}
 			return nil, err
@@ -53,7 +66,13 @@ func New(hierarchy Hierarchy, path Path, resources *specs.LinuxResources) (Cgrou
 }
 
 // Load will load an existing cgroup and allow it to be controlled
-func Load(hierarchy Hierarchy, path Path) (Cgroup, error) {
+func Load(hierarchy Hierarchy, path Path, opts ...InitOpts) (Cgroup, error) {
+	config := newInitConfig()
+	for _, o := range opts {
+		if err := o(config); err != nil {
+			return nil, err
+		}
+	}
 	var activeSubsystems []Subsystem
 	subsystems, err := hierarchy()
 	if err != nil {
@@ -67,6 +86,13 @@ func Load(hierarchy Hierarchy, path Path) (Cgroup, error) {
 				return nil, ErrCgroupDeleted
 			}
 			if err == ErrControllerNotActive {
+				if config.InitCheck != nil {
+					if skerr := config.InitCheck(s, path, err); skerr != nil {
+						if skerr != ErrIgnoreSubsystem {
+							return nil, skerr
+						}
+					}
+				}
 				continue
 			}
 			return nil, err

opts.go

@@ -0,0 +1,61 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package cgroups
+
+import (
+	"github.com/pkg/errors"
+)
+
+var (
+	// ErrIgnoreSubsystem allows the specific subsystem to be skipped
+	ErrIgnoreSubsystem = errors.New("skip subsystem")
+	// ErrDevicesRequired is returned when the devices subsystem is required but
+	// does not exist or is not active
+	ErrDevicesRequired = errors.New("devices subsystem is required")
+)
+
+// InitOpts allows configuration for the creation or loading of a cgroup
+type InitOpts func(*InitConfig) error
+
+// InitConfig provides configuration options for the creation
+// or loading of a cgroup and its subsystems
+type InitConfig struct {
+	// InitCheck can be used to check initialization errors from the subsystem
+	InitCheck InitCheck
+}
+
+func newInitConfig() *InitConfig {
+	return &InitConfig{
+		InitCheck: RequireDevices,
+	}
+}
+
+// InitCheck allows subsystems errors to be checked when initialized or loaded
+type InitCheck func(Subsystem, Path, error) error
+
+// AllowAny allows any subsystem errors to be skipped
+func AllowAny(s Subsystem, p Path, err error) error {
+	return ErrIgnoreSubsystem
+}
+
+// RequireDevices requires the device subsystem but no others
+func RequireDevices(s Subsystem, p Path, err error) error {
+	if s.Name() == Devices {
+		return ErrDevicesRequired
+	}
+	return ErrIgnoreSubsystem
+}

paths.go

@@ -57,7 +57,7 @@ func PidPath(pid int) Path {
 	return existingPath(paths, "")
 }
 
-// ErrControllerNotActive is returned when a controller is not supported or enalbed
+// ErrControllerNotActive is returned when a controller is not supported or enabled
 var ErrControllerNotActive = errors.New("controller is not supported")
 
 func existingPath(paths map[string]string, suffix string) Path {