Add support for configuring Postgres auto-upgrade-backup path

docs/maintenance-upgrading-postgres.md

@@ -3,15 +3,17 @@
 If you're not using an external Postgres server, this playbook initially installs Postgres for you.
 
 Once installed like that, this playbook attempts to preserve the Postgres version it starts with.
-This is because newer Postgres versions cannot start with data generated by an older Postgres version.
+This is because newer Postgres versions cannot start with data generated by older Postgres versions.
 An upgrade must be performed.
 
 This playbook can upgrade your existing Postgres setup with the following command:
 
 	ansible-playbook -i inventory/hosts setup.yml --tags=upgrade-postgres
 
-**The old Postgres data directory is backed up** (by renaming to `/matrix/postgres-auto-upgrade-backup`).
-It stays around forever, until you **manually decide to delete it**.
+**The old Postgres data directory is backed up** by renaming to `/matrix/postgres-auto-upgrade-backup`, by default.
+To rename to a different path, pass some extra flags to the command above, like this: `--extra-vars="postgres_auto_upgrade_backup_data_path=/another/disk/matrix-postgres-before-upgrade"`
+
+The auto-upgrade-backup directory stays around forever, until you **manually decide to delete it**.
 
 As part of the upgrade, the database is dumped to `/tmp`, upgraded and then restored from that dump.
 To use a different directory, pass some extra flags to the command above, like this: `--extra-vars="postgres_dump_dir=/directory/to/dump/here"`

roles/matrix-server/tasks/upgrade_postgres.yml

@@ -12,6 +12,7 @@
 
 - set_fact:
     postgres_auto_upgrade_backup_data_path: "{{ matrix_postgres_data_path }}-auto-upgrade-backup"
+  when: "postgres_auto_upgrade_backup_data_path|default('') == ''"
 
 - name: Fail, if trying to upgrade external Postgres database
   fail: