Explicitly ensure when apt cache updates happen

In https://review.openstack.org/547015 we adjusted
the apt installation process to ensure that the cache
update happens after the repository changes, but left
out the explicit instruction not to do it in the task
which changes the repo config.

This patch adds that, but also seperates out the cache
update into its own task so that the package install
task remains more readable.

Partial-Bug: #1750656
Change-Id: Id70ad28caa63267c2bb17d8c56a8fa6e00cf55d4

tasks/install_apt.yml

@@ -58,6 +58,7 @@
     repo: "{{ rabbitmq_repo.repo }}"
     state: "{{ rabbitmq_repo.state }}"
     filename: "{{ rabbitmq_repo.filename | default(omit) }}"
+    update_cache: no
   register: add_rabbitmq_repos
   when:
     - rabbitmq_install_method == 'external_repo'
@@ -76,16 +77,35 @@
     repo: "{{ rabbitmq_erlang_repo.repo }}"
     state: "{{ rabbitmq_erlang_repo.state }}"
     filename: "{{ rabbitmq_erlang_repo.filename | default(omit) }}"
+    update_cache: no
   register: add_erlang_repos
   tags:
     - rabbitmq-repos
 
+# Due to our Ansible strategy, a skipped task does not
+# have a dictionary result, so we have to cater to the
+# situation where either of the apt_repository tasks
+# may not have the results dict in the register. As
+# such we validate that the register is a mapping (dict).
+- name: Update Apt cache
+  apt:
+    update_cache: yes
+  when:
+    - (add_rabbitmq_repos is mapping and add_rabbitmq_repos | changed) or
+      (add_erlang_repos is mapping and add_erlang_repos | changed)
+  register: update_apt_cache
+  until: update_apt_cache | success
+  retries: 5
+  delay: 2
+  tags:
+    - rabbitmq-repos
+
 - name: Install RabbitMQ package dependencies
   apt:
     pkg: "{{ rabbitmq_dependencies | deprecated(rabbitmq_apt_packages, 'rabbitmq_apt_packages', 'rabbitmq_dependencies', 'Ocata') }}"
     state: "{{ rabbitmq_package_state }}"
     update_cache: yes
-    cache_valid_time: "{{ (add_rabbitmq_repos | changed or add_erlang_repos | changed) | ternary('0', cache_timeout) }}"
+    cache_valid_time: "{{ cache_timeout }}"
   register: install_packages
   until: install_packages | success
   retries: 5