How to write an Ansible role task that only runs when any of the previous other tasks in the task file have been changed?

I am working on a role where I want one task to be run at the end of the tasks file if and only if any of the previous tasks in that task file have changed.

For example, I have:

- name: install package
  apt: name=mypackage state=latest

- name: modify a file
  lineinfile: do stuff

- name: modify a second file
  lineinfile: other stuff

- name: restart if anything changed
  service: name=mypackage state=restarted

... and I want to only restart the service if an update has been installed or any of the config files have been changed.

How can I do this?

Solution

Best practice here is to use handlers.

In your role create a file handlers/main.yml with the content:

- name: restart mypackage
  service: name=mypackage state=restarted

Then notify this handler from all tasks. The handler will be notified only if a task reports a changed state (=yellow output)

- name: install package
  apt: name=mypackage state=latest
  notify: restart mypackage

- name: modify a file
  lineinfile: do stuff
  notify: restart mypackage

- name: modify a second file
  lineinfile: other stuff
  notify: restart mypackage

Handlers will be executed at the very end of your play. If you have other roles involved which depend on the restarted mypackage service, you might want to flush all handlers at the end of the role:

- meta: flush_handlers

Additionally have a look at the force_handlers setting. In case an error happens in any other role processed after your mypackge role, the handler would not be triggered. Set force_handlers=True in your ansible.cfg to still force your handlers to be executed after errors. This is a very important topic since when you run your playbook the next time the files will not be changed and therefore the handler not get notified, hence your service never restarted.

You can also do this without handlers but this is very ugly. You need to register the output of every single task so you can later check the state in the condition applied to the restart task.

- name: install package
  apt: name=mypackage state=latest
  register: mypackage_1

- name: modify a file
  lineinfile: do stuff
  register: mypackage_2

- name: modify a second file
  lineinfile: other stuff
  register: mypackage_3

- name: restart if anything changed
  service: name=mypackage state=restarted
  when: mypackage_1 is changed or mypackage_2 is changed or mypackage_3 is changed

It was possible to use mypackage_1 | changed till ansible 2.9

See also the answer to Ansible Handler notify vs register.