Make Postgres version detection logic reusable to ease maintenance

We've had that logic in 2 places so far, leading to duplication
and a maintenance burden.

In the future, we'll also have an import-postgres feature,
which will also need Postgres version detection,
leading to more benefit from that logic being reusable.

roles/matrix-server/tasks/setup/setup_postgres.yml

@@ -4,41 +4,17 @@
 # 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:
-    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_latest }}"
-
-- name: Determine Postgres version to use (use 9.x, if detected)
-  set_fact:
-    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_v9 }}"
-  when: "pg_version.startswith('9.')"
-
-- name: Determine Postgres version to use (use 10.x, if detected)
-  set_fact:
-    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_v10 }}"
-  when: "pg_version == '10' or pg_version.startswith('10.')"
-
-- debug:
+- include: tasks/util/detect_existing_postgres_version.yml
+
+# If we have found an existing version (installed from before), we use its corresponding Docker image.
+# If not, we install using the latest Postgres.
+#
+# Upgrading is supposed to be performed separately and explicitly (see `upgrade_postgres.yml`).
+- set_fact:
+    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_latest if matrix_postgres_detected_version_corresponding_docker_image == '' else matrix_postgres_detected_version_corresponding_docker_image }}"
+
+- name: Warn if on an old version of Postgres
+  debug:
     msg: "NOTE: Your setup is on an old Postgres version ({{ matrix_postgres_docker_image_to_use }}), while {{ matrix_postgres_docker_image_latest }} is supported. You can upgrade using --tags=upgrade-postgres"
   when: "matrix_postgres_docker_image_to_use != matrix_postgres_docker_image_latest"
 

roles/matrix-server/tasks/upgrade_postgres.yml

@@ -35,48 +35,18 @@
     msg: "Detected that a left-over {{ postgres_auto_upgrade_backup_data_path }} exists. You should rename it to {{ matrix_postgres_data_path }} if the previous upgrade went wrong, or delete it if it went well."
   when: "result_auto_upgrade_path.stat.exists"
 
-- name: Determine existing Postgres version (check PG_VERSION file)
-  stat:
-    path: "{{ matrix_postgres_data_path }}/PG_VERSION"
-  register: result_pg_version_stat
+- include: tasks/util/detect_existing_postgres_version.yml
 
 - name: Abort, if no existing Postgres version detected
   fail: msg="Could not find existing Postgres installation"
-  when: "not result_pg_version_stat.stat.exists"
-
-- name: Determine existing Postgres version (read PG_VERSION file)
-  slurp:
-    src: "{{ matrix_postgres_data_path }}/PG_VERSION"
-  register: result_pg_version
-
-- 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', '') }}"
-
-- name: Determine Postgres version to use (default to latest)
-  set_fact:
-    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_latest }}"
-
-- name: Determine Postgres version to use (use 9.x, if detected)
-  set_fact:
-    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_v9 }}"
-  when: "pg_version.startswith('9.')"
-
-- name: Determine Postgres version to use (use 10.x, if detected)
-  set_fact:
-    matrix_postgres_docker_image_to_use: "{{ matrix_postgres_docker_image_v10 }}"
-  when: "pg_version == '10' or pg_version.startswith('10.')"
+  when: "not matrix_postgres_detected_existing"
 
 - name: Abort, if already at latest Postgres version
   fail: msg="You are already running the latest Postgres version supported ({{ matrix_postgres_docker_image_latest }}). Nothing to do"
-  when: "matrix_postgres_docker_image_to_use == matrix_postgres_docker_image_latest"
+  when: "matrix_postgres_detected_version_corresponding_docker_image == matrix_postgres_docker_image_latest"
 
 - debug:
-    msg: "Upgrading database from {{ matrix_postgres_docker_image_to_use }} to {{ matrix_postgres_docker_image_latest }}"
+    msg: "Upgrading database from {{ matrix_postgres_detected_version_corresponding_docker_image }} to {{ matrix_postgres_docker_image_latest }}"
 
 - name: Ensure matrix-synapse is stopped
   service: name=matrix-synapse state=stopped
@@ -96,7 +66,7 @@
     --network={{ matrix_docker_network }} \
     --env-file={{ matrix_environment_variables_data_path }}/env-postgres-pgsql-docker \
     -v {{ postgres_dump_dir }}:/out \
-    {{ matrix_postgres_docker_image_to_use }} pg_dump -h matrix-postgres {{ matrix_postgres_db_name }} -f /out/{{ postgres_dump_name }}
+    {{ matrix_postgres_detected_version_corresponding_docker_image }} pg_dump -h matrix-postgres {{ matrix_postgres_db_name }} -f /out/{{ postgres_dump_name }}
 
 - name: Ensure matrix-postgres is stopped
   service: name=matrix-postgres state=stopped

roles/matrix-server/tasks/util/detect_existing_postgres_version.yml

@@ -0,0 +1,46 @@
+---
+
+# This utility aims to determine if there is some existing Postgres version in use or not.
+# If there is, it also tries to detect the Docker image that corresponds to that version.
+
+- name: Initialize Postgres version determination variables (default to empty)
+  set_fact:
+    matrix_postgres_detection_pg_version_path: "{{ matrix_postgres_data_path }}/PG_VERSION"
+    matrix_postgres_detected_existing: false
+    matrix_postgres_detected_version: ""
+    matrix_postgres_detected_version_corresponding_docker_image: ""
+
+- name: Determine existing Postgres version (check PG_VERSION file)
+  stat:
+    path: "{{ matrix_postgres_detection_pg_version_path }}"
+  register: result_pg_version_stat
+
+- set_fact:
+    matrix_postgres_detected_existing: true
+  when: "result_pg_version_stat.stat.exists"
+
+- name: Determine existing Postgres version (read PG_VERSION file)
+  slurp:
+    src: "{{ matrix_postgres_detection_pg_version_path }}"
+  register: result_pg_version
+  when: "matrix_postgres_detected_existing"
+
+- name: Determine existing Postgres version (make sense of PG_VERSION file)
+  set_fact:
+    matrix_postgres_detected_version: "{{ result_pg_version['content']|b64decode|replace('\n', '') }}"
+  when: "matrix_postgres_detected_existing"
+
+- name: Determine corresponding Docker image to detected version (assume default of latest)
+  set_fact:
+    matrix_postgres_detected_version_corresponding_docker_image: "{{ matrix_postgres_docker_image_latest }}"
+  when: "matrix_postgres_detected_version != ''"
+
+- name: Determine corresponding Docker image to detected version (use 9.x, if detected)
+  set_fact:
+    matrix_postgres_detected_version_corresponding_docker_image: "{{ matrix_postgres_docker_image_v9 }}"
+  when: "matrix_postgres_detected_version.startswith('9.')"
+
+- name: Determine corresponding Docker image to detected version (use 10.x, if detected)
+  set_fact:
+    matrix_postgres_detected_version_corresponding_docker_image: "{{ matrix_postgres_docker_image_v10 }}"
+  when: "matrix_postgres_detected_version == '10' or matrix_postgres_detected_version.startswith('10.')"