cgroups: systemd: attempt to stop test scope, if any

As reported in #477 the test scope may not be cleaned up between runs.
In order to fix this we must be polite and remove the scope after we
have done our test and attempt to remove an existing scope if it exists.
This way we can guarantee our test will run.

Signed-off-by: Brandon Philips <brandon.philips@coreos.com>

cgroups/systemd/apply_systemd.go

@@ -43,6 +43,10 @@ var subsystems = map[string]subsystem{
 	"freezer":    &fs.FreezerGroup{},
 }
 
+const (
+	testScopeWait = 4
+)
+
 var (
 	connLock                        sync.Mutex
 	theConn                         *systemd.Conn
@@ -86,16 +90,41 @@ func UseSystemd() bool {
 			}
 		}
 
+		// Ensure the scope name we use doesn't exist. Use the Pid to
+		// avoid collisions between multiple libcontainer users on a
+		// single host.
+		scope := fmt.Sprintf("libcontainer-%d-systemd-test-default-dependencies.scope", os.Getpid())
+		testScopeExists := true
+		for i := 0; i <= testScopeWait; i++ {
+			if _, err := theConn.StopUnit(scope, "replace"); err != nil {
+				if dbusError, ok := err.(dbus.Error); ok {
+					if strings.Contains(dbusError.Name, "org.freedesktop.systemd1.NoSuchUnit") {
+						testScopeExists = false
+						break
+					}
+				}
+			}
+			time.Sleep(time.Millisecond)
+		}
+
+		// Bail out if we can't kill this scope without testing for DefaultDependencies
+		if testScopeExists {
+			return hasStartTransientUnit
+		}
+
 		// Assume StartTransientUnit on a scope allows DefaultDependencies
 		hasTransientDefaultDependencies = true
 		ddf := newProp("DefaultDependencies", false)
-		if _, err := theConn.StartTransientUnit("docker-systemd-test-default-dependencies.scope", "replace", ddf); err != nil {
+		if _, err := theConn.StartTransientUnit(scope, "replace", ddf); err != nil {
 			if dbusError, ok := err.(dbus.Error); ok {
 				if strings.Contains(dbusError.Name, "org.freedesktop.DBus.Error.PropertyReadOnly") {
 					hasTransientDefaultDependencies = false
 				}
 			}
 		}
+
+		// Not critical because of the stop unit logic above.
+		theConn.StopUnit(scope, "replace")
 	}
 	return hasStartTransientUnit
 }