Make roles more independent of one another

With this change, the following roles are now only dependent
on the minimal `matrix-base` role:
- `matrix-corporal`
- `matrix-coturn`
- `matrix-mailer`
- `matrix-mxisd`
- `matrix-postgres`
- `matrix-riot-web`
- `matrix-synapse`

The `matrix-nginx-proxy` role still does too much and remains
dependent on the others.

Wiring up the various (now-independent) roles happens
via a glue variables file (`group_vars/matrix-servers`).
It's triggered for all hosts in the `matrix-servers` group.

According to Ansible's rules of priority, we have the following
chain of inclusion/overriding now:
- role defaults (mostly empty or good for independent usage)
- playbook glue variables (`group_vars/matrix-servers`)
- inventory host variables (`inventory/host_vars/matrix.<your-domain>`)

All roles default to enabling their main component
(e.g. `matrix_mxisd_enabled: true`, `matrix_riot_web_enabled: true`).
Reasoning: if a role is included in a playbook (especially separately,
in another playbook), it should "work" by default.

Our playbook disables some of those if they are not generally useful
(e.g. `matrix_corporal_enabled: false`).

roles/matrix-coturn/defaults/main.yml

@@ -1,8 +1,13 @@
+matrix_coturn_enabled: true
+
 matrix_coturn_docker_image: "instrumentisto/coturn:4.5.0.8"
 
 matrix_coturn_base_path: "{{ matrix_base_data_path }}/coturn"
 matrix_coturn_config_path: "{{ matrix_coturn_base_path }}/turnserver.conf"
 
+# List of systemd services that matrix-coturn.service depends on
+matrix_coturn_systemd_required_services_list: ['docker.service']
+
 # A shared secret (between Synapse and Coturn) used for authentication.
 # You can put any string here, but generating a strong one is preferred (e.g. `pwgen -s 64 1`).
 matrix_coturn_turn_static_auth_secret: ""

roles/matrix-coturn/tasks/main.yml

@@ -2,8 +2,14 @@
   tags:
     - always
 
+- import_tasks: "{{ role_path }}/tasks/validate_config.yml"
+  when: "run_setup and matrix_coturn_enabled"
+  tags:
+    - setup-all
+    - setup-coturn
+
 - import_tasks: "{{ role_path }}/tasks/setup_coturn.yml"
   when: run_setup
   tags:
-    - setup-coturn
-    - setup-all
+    - setup-all
+    - setup-coturn

roles/matrix-coturn/tasks/setup_coturn.yml

@@ -1,13 +1,13 @@
 ---
 
-- name: Fail if Coturn secret is missing
-  fail:
-    msg: "You need to set a secret in the matrix_coturn_turn_static_auth_secret variable"
-  when: "matrix_coturn_turn_static_auth_secret == ''"
+#
+# Tasks related to setting up Coturn
+#
 
 - name: Ensure Coturn image is pulled
   docker_image:
     name: "{{ matrix_coturn_docker_image }}"
+  when: matrix_coturn_enabled
 
 - name: Ensure Coturn configuration path exists
   file:
@@ -16,18 +16,21 @@
     mode: 0750
     owner: "{{ matrix_user_username }}"
     group: "{{ matrix_user_username }}"
+  when: matrix_coturn_enabled
 
 - name: Ensure turnserver.conf installed
   template:
     src: "{{ role_path }}/templates/turnserver.conf.j2"
     dest: "{{ matrix_coturn_config_path }}"
     mode: 0644
+  when: matrix_coturn_enabled
 
 - name: Ensure matrix-coturn.service installed
   template:
     src: "{{ role_path }}/templates/systemd/matrix-coturn.service.j2"
     dest: "/etc/systemd/system/matrix-coturn.service"
     mode: 0644
+  when: matrix_coturn_enabled
 
 - name: Allow access to Coturn ports in firewalld
   firewalld:
@@ -39,4 +42,39 @@
     - '3478/tcp' # STUN
     - '3478/udp' # STUN
     - "{{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}/udp" # TURN
-  when: ansible_os_family == 'RedHat'
+  when: "matrix_coturn_enabled and ansible_os_family == 'RedHat'"
+
+#
+# Tasks related to getting rid of Coturn (if it was previously enabled)
+#
+
+- name: Check existence of matrix-coturn service
+  stat:
+    path: "/etc/systemd/system/matrix-coturn.service"
+  register: matrix_coturn_service_stat
+
+- name: Ensure matrix-coturn is stopped
+  service:
+    name: matrix-coturn
+    state: stopped
+    daemon_reload: yes
+  register: stopping_result
+  when: "not matrix_coturn_enabled and matrix_coturn_service_stat.stat.exists"
+
+- name: Ensure matrix-coturn.service doesn't exist
+  file:
+    path: "/etc/systemd/system/matrix-coturn.service"
+    state: absent
+  when: "not matrix_coturn_enabled and matrix_coturn_service_stat.stat.exists"
+
+- name: Ensure Matrix coturn paths don't exist
+  file:
+    path: "{{ matrix_coturn_base_path }}"
+    state: absent
+  when: "not matrix_coturn_enabled"
+
+- name: Ensure coturn Docker image doesn't exist
+  docker_image:
+    name: "{{ matrix_coturn_docker_image }}"
+    state: absent
+  when: "not matrix_coturn_enabled"

roles/matrix-coturn/tasks/validate_config.yml

@@ -0,0 +1,9 @@
+---
+
+- name: Fail if required Coturn settings not defined
+  fail:
+    msg: >
+      You need to define a required configuration setting (`{{ item }}`) for using Coturn.
+  when: "vars[item] == ''"
+  with_items:
+    - "matrix_coturn_turn_static_auth_secret"

roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2

@@ -1,7 +1,9 @@
 [Unit]
 Description=Matrix Coturn server
-After=docker.service
-Requires=docker.service
+{% for service in matrix_coturn_systemd_required_services_list %}
+Requires={{ service }}
+After={{ service }}
+{% endfor %}
 
 [Service]
 Type=simple