add postgres support mx-appservice-[slack|irc] with fallback to nedb in role and migration notice

roles/matrix-bridge-appservice-irc/tasks/validate_config.yml

@@ -33,3 +33,23 @@
   when: "item.old in vars"
   with_items:
     - {'old': 'matrix_appservice_irc_container_expose_client_server_api_port', 'new': '<superseded by matrix_appservice_irc_container_http_host_bind_port>'}
+
+- block:
+  - name: Check if a neDB database already exists
+    stat:
+      path: "{{ matrix_appservice_irc_data_path }}/"
+    register: matrix_appservice_irc_nedb_stat_result
+
+  - name: Fail if an neDB database already exists when using Postgres
+    fail:
+      msg: >-2
+        matrix_appservice_irc_database_engine has been set to `postgres` (which is our new default now).
+        However, we've discovered an existing neDB database in {{ matrix_appservice_irc_data_path }}/.
+        It appears that you've been using this bridge with the neDB engine until now.
+        To continue using neDB, opt into it explicitly: add `matrix_appservice_irc_database_engine: nedb` to your vars.yml file and re-run this same command.
+        Alternatively, to migrate your existing neDB database to Postgres:
+        1. Stop all services (`ansible-playbook -i inventory/hosts setup.yml --tags=stop`)
+        2. Import the neDB database into Postgres (`ansible-playbook -v -i inventory/hosts setup.yml --tags=import-generic-sqlite-db --extra-vars='sqlite_database_path={{ matrix_appservice_irc_data_path }} postgres_connection_string_variable_name=matrix_appservice_irc_database_connString'`)
+        3. Re-run the playbook (`ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start`)
+    when: "matrix_appservice_irc_nedb_stat_result.stat.exists"
+  when: "matrix_appservice_irc_database_engine == 'postgres'"