Add purge_retention_period config

docs/usage/configuration/config_documentation.md

@@ -893,7 +893,7 @@ allowed_avatar_mimetypes: ["image/png", "image/jpeg", "image/gif"]
 How long to keep redacted events in unredacted form in the database. After
 this period redacted events get replaced with their redacted form in the DB.
 
-Synapse will check whether the rentention period has concluded for redacted
+Synapse will check whether the retention period has concluded for redacted
 events every 5 minutes. Thus, even if this option is set to `0`, Synapse may
 still take up to 5 minutes to purge redacted events from the database.
 
@@ -904,6 +904,23 @@ Example configuration:
 redaction_retention_period: 28d
 ```
 ---
+---
+### `purge_retention_period`
+
+How long to keep locally forgotten room in the DB. After this period the room
+will be fully purged from the DB.
+
+Synapse will check whether the retention period has concluded for room
+purges every hour. Thus, even if this option is set to `0`, Synapse may
+still take up to one hour to purge forgotten rooms from the database.
+
+Defaults to `7d`. Set to `null` to disable.
+
+Example configuration:
+```yaml
+purge_retention_period: 28d
+```
+---
 ### `user_ips_max_age`
 
 How long to track users' last seen time and IPs in the database.

synapse/config/server.py

@@ -486,6 +486,15 @@ class ServerConfig(Config):
         else:
             self.redaction_retention_period = None
 
+        # How long to keep locally forgotten rooms before purging them.
+        purge_retention_period = config.get("purge_retention_period", "7d")
+        if purge_retention_period is not None:
+            self.purge_retention_period: Optional[int] = self.parse_duration(
+                purge_retention_period
+            )
+        else:
+            self.purge_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: