docker[linux]: change docker error valiable and add cgroup error.

docker/docker.go

@@ -2,7 +2,8 @@ package docker
 
 import "errors"
 
-var ErrNotAvailable = errors.New("docker not available")
+var ErrDockerNotAvailable = errors.New("docker not available")
+var ErrCgroupNotAvailable = errors.New("cgroup not available")
 
 type CgroupMemStat struct {
 	ContainerID             string `json:"container_id"`

docker/docker_linux.go

@@ -4,7 +4,6 @@ package docker
 
 import (
 	"encoding/json"
-	"errors"
 	"os/exec"
 	"path"
 	"strconv"

docker/docker_notlinux.go

@@ -12,23 +12,23 @@ import (
 // GetDockerIDList returnes a list of DockerID.
 // This requires certain permission.
 func GetDockerIDList() ([]string, error) {
-	return nil, common.NotImplementedError
+	return nil, ErrDockerNotAvailable
 }
 
 // CgroupCPU returnes specified cgroup id CPU status.
 // containerid is same as docker id if you use docker.
 // If you use container via systemd.slice, you could use
 // containerid = docker-<container id>.scope and base=/sys/fs/cgroup/cpuacct/system.slice/
 func CgroupCPU(containerid string, base string) (*cpu.CPUTimesStat, error) {
-	return nil, common.NotImplementedError
+	return nil, ErrCgroupNotAvailable
 }
 
 func CgroupCPUDocker(containerid string) (*cpu.CPUTimesStat, error) {
 	return CgroupCPU(containerid, "/sys/fs/cgroup/cpuacct/docker")
 }
 
 func CgroupMem(containerid string, base string) (*CgroupMemStat, error) {
-	return nil, common.NotImplementedError
+	return nil, ErrCgroupNotAvailable
 }
 
 func CgroupMemDocker(containerid string) (*CgroupMemStat, error) {