Change not found error to be typed with cgroup subsystem

Signed-off-by: Michael Crosby <michael@docker.com>

cgroups/cgroups.go

@@ -1,15 +1,11 @@
 package cgroups
 
 import (
-	"errors"
+	"fmt"
 
 	"github.com/docker/libcontainer/devices"
 )
 
-var (
-	ErrNotFound = errors.New("mountpoint not found")
-)
-
 type FreezerState string
 
 const (
@@ -18,6 +14,29 @@ const (
 	Thawed    FreezerState = "THAWED"
 )
 
+type NotFoundError struct {
+	Subsystem string
+}
+
+func (e *NotFoundError) Error() string {
+	return fmt.Sprintf("mountpoint for %s not found", e.Subsystem)
+}
+
+func NewNotFoundError(sub string) error {
+	return &NotFoundError{
+		Subsystem: sub,
+	}
+}
+
+func IsNotFound(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	_, ok := err.(*NotFoundError)
+	return ok
+}
+
 type Cgroup struct {
 	Name   string `json:"name,omitempty"`
 	Parent string `json:"parent,omitempty"` // name of parent cgroup or slice

cgroups/fs/apply_raw.go

@@ -76,7 +76,7 @@ func GetStats(c *cgroups.Cgroup) (*cgroups.Stats, error) {
 		path, err := d.path(sysname)
 		if err != nil {
 			// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
-			if err == cgroups.ErrNotFound {
+			if cgroups.IsNotFound(err) {
 				continue
 			}
 
@@ -172,7 +172,7 @@ func (raw *data) path(subsystem string) (string, error) {
 
 		if _, err := os.Stat(path); err != nil {
 			if os.IsNotExist(err) {
-				return "", cgroups.ErrNotFound
+				return "", cgroups.NewNotFoundError(subsystem)
 			}
 
 			return "", err

cgroups/fs/blkio.go

@@ -16,9 +16,10 @@ type BlkioGroup struct {
 
 func (s *BlkioGroup) Set(d *data) error {
 	// we just want to join this group even though we don't set anything
-	if _, err := d.join("blkio"); err != nil && err != cgroups.ErrNotFound {
+	if _, err := d.join("blkio"); err != nil && !cgroups.IsNotFound(err) {
 		return err
 	}
+
 	return nil
 }
 

cgroups/fs/cpuacct.go

@@ -27,9 +27,10 @@ type CpuacctGroup struct {
 
 func (s *CpuacctGroup) Set(d *data) error {
 	// we just want to join this group even though we don't set anything
-	if _, err := d.join("cpuacct"); err != nil && err != cgroups.ErrNotFound {
+	if _, err := d.join("cpuacct"); err != nil && !cgroups.IsNotFound(err) {
 		return err
 	}
+
 	return nil
 }
 

cgroups/fs/freezer.go

@@ -33,7 +33,7 @@ func (s *FreezerGroup) Set(d *data) error {
 			time.Sleep(1 * time.Millisecond)
 		}
 	default:
-		if _, err := d.join("freezer"); err != nil && err != cgroups.ErrNotFound {
+		if _, err := d.join("freezer"); err != nil && !cgroups.IsNotFound(err) {
 			return err
 		}
 	}

cgroups/fs/perf_event.go

@@ -9,7 +9,7 @@ type PerfEventGroup struct {
 
 func (s *PerfEventGroup) Set(d *data) error {
 	// we just want to join this group even though we don't set anything
-	if _, err := d.join("perf_event"); err != nil && err != cgroups.ErrNotFound {
+	if _, err := d.join("perf_event"); err != nil && !cgroups.IsNotFound(err) {
 		return err
 	}
 	return nil

cgroups/systemd/apply_systemd.go

@@ -166,7 +166,7 @@ func (c *systemdCgroup) Paths() (map[string]string, error) {
 		subsystemPath, err := getSubsystemPath(c.cgroup, sysname)
 		if err != nil {
 			// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
-			if err == cgroups.ErrNotFound {
+			if cgroups.IsNotFound(err) {
 				continue
 			}
 
@@ -274,7 +274,7 @@ func GetStats(c *cgroups.Cgroup) (*cgroups.Stats, error) {
 		subsystemPath, err := getSubsystemPath(c, sysname)
 		if err != nil {
 			// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
-			if err == cgroups.ErrNotFound {
+			if cgroups.IsNotFound(err) {
 				continue
 			}
 

cgroups/utils.go

@@ -29,7 +29,8 @@ func FindCgroupMountpoint(subsystem string) (string, error) {
 			}
 		}
 	}
-	return "", ErrNotFound
+
+	return "", NewNotFoundError(subsystem)
 }
 
 type Mount struct {
@@ -153,19 +154,23 @@ func ReadProcsFile(dir string) ([]int, error) {
 
 func parseCgroupFile(subsystem string, r io.Reader) (string, error) {
 	s := bufio.NewScanner(r)
+
 	for s.Scan() {
 		if err := s.Err(); err != nil {
 			return "", err
 		}
+
 		text := s.Text()
 		parts := strings.Split(text, ":")
+
 		for _, subs := range strings.Split(parts[1], ",") {
 			if subs == subsystem {
 				return parts[2], nil
 			}
 		}
 	}
-	return "", ErrNotFound
+
+	return "", NewNotFoundError(subsystem)
 }
 
 func pathExists(path string) bool {