Move get pid into cgroup implementation

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

libcontainer/cgroups/systemd/apply_systemd.go

@@ -3,6 +3,7 @@
 package systemd
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -78,7 +79,7 @@ type cgroupArg struct {
 
 func Apply(c *cgroups.Cgroup, pid int) (cgroups.ActiveCgroup, error) {
 	var (
-		unitName   = c.Parent + "-" + c.Name + ".scope"
+		unitName   = getUnitName(c)
 		slice      = "system.slice"
 		properties []systemd1.Property
 		cpuArgs    []cgroupArg
@@ -303,3 +304,24 @@ func (c *systemdCgroup) Cleanup() error {
 
 	return nil
 }
+
+func GetPids(c *cgroups.Cgroup) ([]int, error) {
+	unitName := getUnitName(c)
+
+	mountpoint, err := cgroups.FindCgroupMountpoint("cpu")
+	if err != nil {
+		return nil, err
+	}
+
+	props, err := theConn.GetUnitTypeProperties(unitName, getIfaceForUnit(unitName))
+	if err != nil {
+		return nil, err
+	}
+	cgroup := props["ControlGroup"].(string)
+
+	return cgroups.ReadProcsFile(filepath.Join(mountpoint, cgroup))
+}
+
+func getUnitName(c *cgroups.Cgroup) string {
+	return fmt.Sprintf("%s-%s.scope", c.Parent, c.Name)
+}