Add note about synapse-janitor and require explicit confirmation for usage

Fixes #465 (Github Issue).

Related to https://github.com/xwiki-labs/synapse_scripts/pull/12.

docs/maintenance-synapse.md

@@ -8,11 +8,17 @@ Table of contents:
 
 - [Purging old data with the Purge History API](#purging-old-data-with-the-purge-history-api), for when you wish to delete in-use (but old) data from the Synapse database
 
-- [Compressing state with rust-synapse-compress-state](#compressing-state-with-rust-synapse-compress-state), for when you wish to compress some Synapse state tables using the [rust-synapse-compress-state](https://github.com/matrix-org/rust-synapse-compress-state) tool
+- [Synapse maintenance](#synapse-maintenance)
+	- [Purging unused data with synapse-janitor](#purging-unused-data-with-synapse-janitor)
+		- [Vacuuming Postgres](#vacuuming-postgres)
+	- [Purging old data with the Purge History API](#purging-old-data-with-the-purge-history-api)
+	- [Compressing state with rust-synapse-compress-state](#compressing-state-with-rust-synapse-compress-state)
 
 
 ## Purging unused data with synapse-janitor
 
+**NOTE**: There are [reports](https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/465) that **synapse-janitor is dangerous to use and causes database corruption**. You may wish to refrain from using it.
+
 When you **leave** and **forget** a room, Synapse can clean up its data, but currently doesn't.
 This **unused and unreachable data** remains in your database forever.
 

roles/matrix-postgres/tasks/run_synapse_janitor.yml

@@ -7,6 +7,13 @@
     msg: "Postgres via the matrix-postgres role is not enabled (`matrix_postgres_enabled`). Cannot run synapse-janitor."
   when: "not matrix_postgres_enabled|bool"
 
+- name: Fail if not aware of the risks
+  fail:
+    msg: >-
+      Using Synapse Janitor is considered dangerous and may break your database.
+      See https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/465.
+      If you'd like to run it anyway, add `--extra-vars='i_know_synapse_janitor_is_dangerous=1'` to your command.
+  when: "i_know_synapse_janitor_is_dangerous|default('') == ''"
 
 # Defaults