stack update failed: port still in use

1. create a stack with template:
**********************************
heat_template_version: 2015-04-30
resources:
  s1:
    type: OS::Nova::Server
    properties:
      flavor: 1
      image: cirros-0.3.0-x86_64-disk
      networks:
      - port: {get_resource: s_port}
      - port: {get_resource: s_port2}
      name: testss
  s_port2:
    type: OS::Neutron::Port
    properties:
      network: 3447dd80-14ef-4059-8ea6-35f4fd60ba26
  s_port:
    type: OS::Neutron::Port
    properties:
      network: 3447dd80-14ef-4059-8ea6-35f4fd60ba26
**********************************
2. the stack create successful, to update the stack with template:
*******************************************
heat_template_version: 2015-04-30
resources:
  s1:
    type: OS::Nova::Server
    properties:
      flavor: 1
      image: cirros-0.3.0-x86_64-disk
      networks:
      - port: {get_resource: s_port}
      name: testss
  s2:
    type: OS::Nova::Server
    properties:
      flavor: 1
      image: cirros-0.3.0-x86_64-disk
      networks:
      - port: {get_resource: s_port2}
      name: tes2
  s_port2:
    type: OS::Neutron::Port
    properties:
      network: 3447dd80-14ef-4059-8ea6-35f4fd60ba26
  s_port:
    type: OS::Neutron::Port
    properties:
      network: 3447dd80-14ef-4059-8ea6-35f4fd60ba26
***************************************
3. the stack update failed, because the s2 create failed: port2 still in use
4. finally, we can find that port2 detached from s1

Because there is no dependence between s1 and s2, so they update in parallel. I think add the dependence to s2 will fix this problem.