Prepare matrix-registation for (SQLite + Postgres) support

Auto-migration and everything seems to work. It's just that
matrix-registration cannot load the Python modules required
for talking to a Postgres database.

Tracked here: https://github.com/ZerataX/matrix-registration/issues/44

Until this gets fixed, we'll continue default to 'sqlite'.

roles/matrix-registration/tasks/setup_install.yml

@@ -1,5 +1,31 @@
 ---
 
+- set_fact:
+    matrix_registration_requires_restart: false
+
+- block:
+    - name: Check if an SQLite database already exists
+      stat:
+        path: "{{ matrix_registration_sqlite_database_path_local }}"
+      register: matrix_registration_sqlite_database_path_local_stat_result
+
+    - block:
+        - set_fact:
+            matrix_postgres_db_migration_request:
+              src: "{{ matrix_registration_sqlite_database_path_local }}"
+              dst: "{{ matrix_registration_database_connection_string }}"
+              caller: "{{ role_path|basename }}"
+              engine_variable_name: 'matrix_registration_database_engine'
+              engine_old: 'sqlite'
+              systemd_services_to_stop: ['matrix-registration.service']
+
+        - import_tasks: "roles/matrix-postgres/tasks/util/migrate_db_to_postgres.yml"
+
+        - set_fact:
+            matrix_registration_requires_restart: true
+      when: "matrix_registration_sqlite_database_path_local_stat_result.stat.exists|bool"
+  when: "matrix_registration_database_engine == 'postgres'"
+
 - name: Ensure matrix-registration paths exist
   file:
     path: "{{ item.path }}"
@@ -61,3 +87,9 @@
   service:
     daemon_reload: yes
   when: "matrix_registration_systemd_service_result.changed|bool"
+
+- name: Ensure matrix-registration.service restarted, if necessary
+  service:
+    name: "matrix-registration.service"
+    state: restarted
+  when: "matrix_registration_requires_restart|bool"

roles/matrix-registration/tasks/validate_config.yml

@@ -18,3 +18,10 @@
   when: "item.old in vars"
   with_items:
     - {'old': 'matrix_registration_docker_repo', 'new': 'matrix_registration_container_image_self_build_repo'}
+
+- name: Fail if Postgres usage attempted
+  fail:
+    msg: >
+      matrix-registration doesn't support using Postgres just yet.
+      Learn more here: https://github.com/ZerataX/matrix-registration/issues/44
+  when: "matrix_registration_database_engine == 'postgres'"