Document that upgrading to Synapse v1.57 may be dangerous in some ins…

…tances

Related to #1766

CHANGELOG.md

@@ -1,3 +1,24 @@
+# 2022-04-19
+
+## (Compatibility Break) Upgrading to Synapse v1.57 on setups using workers may require manual action
+
+If you're running a worker setup for Synapse (`matrix_synapse_workers_enabled: true`), the [Synapse v1.57 upgrade notes](https://github.com/matrix-org/synapse/blob/v1.57.0rc1/docs/upgrade.md#changes-to-database-schema-for-application-services) say that you may need to take special care when upgrading:
+
+> Synapse v1.57.0 includes a change to the way transaction IDs are managed for application services. If your deployment uses a dedicated worker for application service traffic, **it must be stopped** when the database is upgraded (which normally happens when the main process is upgraded), to ensure the change is made safely without any risk of reusing transaction IDs.
+
+If you're not running an `appservice` worker (`matrix_synapse_workers_preset: little-federation-helper` or `matrix_synapse_workers_appservice_workers_count: 0`), you are probably safe to upgrade as per normal, without taking any special care.
+
+If you are running a setup with an `appservice` worker, or otherwise want to be on the safe side, we recommend the following upgrade path:
+
+0. Pull the latest playbook changes
+1. Stop all services (`ansible-playbook -i inventory/hosts setup.yml --tags=stop`)
+2. Re-run the playbook (`ansible-playbook -i inventory/hosts setup.yml --tags=setup-all`)
+3. Start Postgres (`systemctl start matrix-postgres` on the server)
+4. Start the main Synapse process (`systemctl start matrix-synapse` on the server)
+5. Wait a while so that Synapse can start and complete the database migrations. You can use `journalctl -fu matrix-synapse` on the server to get a clue. Waiting a few minutes should also be enough.
+6. It should now be safe to start all other services. `ansible-playbook -i inventory/hosts setup.yml --tags=start` will do it for you
+
+
 # 2022-04-14
 
 ## (Compatibility Break) Changes to `docker-src` permissions necessitating manual action