Use Postgres 10.x by default (only for new installs)

This playbook just tries to avoid trying to setup a Postgres 10
database with existing 9.x files, as that makes Postgres complain.

Due to this, existing installs (still on 9.x) are detected
and left on Postgres 9.x.
They need to be upgraded to Postgres 10.x manually.

roles/matrix-server/tasks/setup_postgres.yml

@@ -4,10 +4,39 @@
 # Generic tasks, no matter what kind of server we're using (internal/external)
 #
 
+- name: Determine existing Postgres version (check PG_VERSION file)
+  stat:
+    path: "{{ matrix_postgres_data_path }}/PG_VERSION"
+  register: result_pg_version_stat
+
+- name: Determine existing Postgres version (read PG_VERSION file)
+  slurp:
+    src: "{{ matrix_postgres_data_path }}/PG_VERSION"
+  register: result_pg_version
+  when: "result_pg_version_stat.stat.exists"
+
+- name: Determine existing Postgres version (default to empty)
+  set_fact:
+    pg_version: ""
+
+- name: Determine existing Postgres version (make sense of PG_VERSION file)
+  set_fact:
+    pg_version: "{{ result_pg_version['content']|b64decode|replace('\n', '') }}"
+  when: "result_pg_version_stat.stat.exists"
+
+- name: Determine Postgres version to use (default to latest)
+  set_fact:
+    docker_postgres_image_to_use: "{{ docker_postgres_image_latest }}"
+
+- name: Determine Postgres version to use (use 9.x, if detected)
+  set_fact:
+    docker_postgres_image_to_use: "{{ docker_postgres_image_v9 }}"
+  when: "pg_version.startswith('9.')"
+
 # Even if we don't run the internal server, we still need this for running the CLI
 - name: Ensure postgres Docker image is pulled
   docker_image:
-    name: "{{ docker_postgres_image }}"
+    name: "{{ docker_postgres_image_to_use }}"
 
 - name: Ensure Postgres environment variables file created
   template: