community.general/test/integration/roles/test_handlers/tasks/main.yml

# test code for the async keyword
# (c) 2014, James Tanner <tanner.jc@gmail.com>

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- name: reset handler_called variable to false for all hosts
  set_fact:
    handler_called: False

- name: notify the handler for host A only
  shell: echo
  notify: 
    - set handler fact
  when: inventory_hostname == 'A'

- name: force handler execution now
  meta: "flush_handlers"

- debug: var=handler_called

- name: validate the handler only ran on one host
  assert:
    that:
    - "inventory_hostname == 'A' and handler_called == True or handler_called == False"