Add automatic purge after all users forget a room (#15488)

 Also add restore of purge/shutdown rooms after a synapse restart.

Co-authored-by:  Eric Eastwood <erice@matrix.org>
Co-authored-by: Erik Johnston <erikj@matrix.org>

changelog.d/15488.feature

@@ -0,0 +1 @@
+Add automatic purge after all users forgotten a room. Also add restore of purge/shutdown rooms after a synapse restart.

docs/usage/configuration/config_documentation.md

@@ -936,6 +936,17 @@ Example configuration:
 redaction_retention_period: 28d
 ```
 ---
+### `forgotten_room_retention_period`
+
+How long to keep locally forgotten rooms before purging them from the DB.
+
+Defaults to `null`, meaning it's disabled.
+
+Example configuration:
+```yaml
+forgotten_room_retention_period: 28d
+```
+---
 ### `user_ips_max_age`
 
 How long to track users' last seen time and IPs in the database.

synapse/app/generic_worker.py

@@ -77,6 +77,7 @@
 )
 from synapse.storage.databases.main.presence import PresenceStore
 from synapse.storage.databases.main.profile import ProfileWorkerStore
+from synapse.storage.databases.main.purge_events import PurgeEventsStore
 from synapse.storage.databases.main.push_rule import PushRulesWorkerStore
 from synapse.storage.databases.main.pusher import PusherWorkerStore
 from synapse.storage.databases.main.receipts import ReceiptsWorkerStore
@@ -134,6 +135,7 @@ class GenericWorkerStore(
     RelationsWorkerStore,
     EventFederationWorkerStore,
     EventPushActionsWorkerStore,
+    PurgeEventsStore,
     StateGroupWorkerStore,
     SignatureWorkerStore,
     UserErasureWorkerStore,

synapse/config/server.py

@@ -486,6 +486,17 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         else:
             self.redaction_retention_period = None
 
+        # How long to keep locally forgotten rooms before purging them from the DB.
+        forgotten_room_retention_period = config.get(
+            "forgotten_room_retention_period", None
+        )
+        if forgotten_room_retention_period is not None:
+            self.forgotten_room_retention_period: Optional[int] = self.parse_duration(
+                forgotten_room_retention_period
+            )
+        else:
+            self.forgotten_room_retention_period = None
+
         # How long to keep entries in the `users_ips` table.
         user_ips_max_age = config.get("user_ips_max_age", "28d")
         if user_ips_max_age is not None: