fix: facts being gathered unnecessarily

Cause: The comparison of the present facts with the required facts is
being done on unsorted lists.

Consequence: The comparison may fail if the only difference is the
order.  Facts are gathered unnecessarily.

Fix: Use `difference` which works no matter what the order is.  Ensure
that the fact gathering subsets used are the absolute minimum required.

Result: The role gathers only the facts it requires, and does
not unnecessarily gather facts.

Signed-off-by: Rich Megginson <[email protected]>

tasks/set_vars.yml

@@ -1,9 +1,9 @@
 ---
 - name: Ensure ansible_facts used by role
   setup:
-    gather_subset: min
-  when: not ansible_facts.keys() | list |
-    intersect(__template_required_facts) == __template_required_facts
+    gather_subset: "{{ __systemd_required_facts_subsets }}"
+  when: __systemd_required_facts |
+    difference(ansible_facts.keys() | list) | length > 0
 
 - name: Set platform/version specific variables
   include_vars: "{{ __vars_file }}"

vars/main.yml

@@ -9,8 +9,13 @@ __template_foo_config: foo.conf
 __template_packages: []
 __template_services: []
 # ansible_facts required by the role
-__template_required_facts:
+__systemd_required_facts:
   - distribution
   - distribution_major_version
   - distribution_version
   - os_family
+# the subsets of ansible_facts that need to be gathered in case any of the
+# facts in required_facts is missing; see the documentation of
+# the 'gather_subset' parameter of the 'setup' module
+__systemd_required_facts_subsets: "{{ ['!all', '!min'] +
+  __systemd_required_facts }}"