Ignore error when starting transient unit that already exists

Signed-off-by: Derek Carr <decarr@redhat.com>

libcontainer/cgroups/systemd/apply_systemd.go

@@ -282,7 +282,7 @@ func (m *Manager) Apply(pid int) error {
 		}
 	}
 
-	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, nil); err != nil {
+	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, nil); err != nil && !isUnitExists(err) {
 		return err
 	}
 
@@ -546,3 +546,13 @@ func setKernelMemory(c *configs.Cgroup) error {
 	}
 	return fs.EnableKernelMemoryAccounting(path)
 }
+
+// isUnitExists returns true if the error is that a systemd unit already exists.
+func isUnitExists(err error) bool {
+	if err != nil {
+		if dbusError, ok := err.(dbus.Error); ok {
+			return strings.Contains(dbusError.Name, "org.freedesktop.systemd1.UnitExists")
+		}
+	}
+	return false
+}