check for an existing postgres database in a data directory, and do not initdb if found

check_existing_postgresdb.yml

# assumes you have postgres_data_directory, postgres_user, and postgres_bin_directory set
- name: Check for existing database
  become: yes
  become_user: postgres
  stat:
    path: "{{ postgres_data_directory }}/PG_VERSION"
  register: pgdata_directory_version

- name: Initialize PostgreSQL DB
  become: yes
  become_user: "{{ postgres_user }}"
  command: "{{ postgres_bin_directory }}/initdb -D {{ postgres_data_directory }}"
  when: not pgdata_directory_version.stat.exists
  # https://github.com/ansible/ansible/issues/16048#issuecomment-229012509
  vars:
    ansible_ssh_pipelining: true