Add ratelimit callbacks to module API to allow dynamic ratelimiting (#18458)

docs/SUMMARY.md

@@ -49,7 +49,8 @@
         - [Background update controller callbacks](modules/background_update_controller_callbacks.md)
         - [Account data callbacks](modules/account_data_callbacks.md)
         - [Add extra fields to client events unsigned section callbacks](modules/add_extra_fields_to_client_events_unsigned.md)
-        - [Media repository](modules/media_repository_callbacks.md)
+        - [Media repository callbacks](modules/media_repository_callbacks.md)
+        - [Ratelimit callbacks](modules/ratelimit_callbacks.md)
         - [Porting a legacy module to the new interface](modules/porting_legacy_module.md)
     - [Workers](workers.md)
       - [Using `synctl` with Workers](synctl_workers.md)

docs/modules/ratelimit_callbacks.md

@@ -0,0 +1,33 @@
+# Ratelimit callbacks
+
+Ratelimit callbacks allow module developers to override ratelimit settings dynamically whilst
+Synapse is running. Ratelimit callbacks can be registered using the module API's
+`register_ratelimit_callbacks` method.
+
+The available ratelimit callbacks are:
+
+### `get_ratelimit_override_for_user`
+
+_First introduced in Synapse v1.132.0_
+
+```python
+async def get_ratelimit_override_for_user(user: str, limiter_name: str) -> Optional[RatelimitOverride]
+```
+
+Called when constructing a ratelimiter of a particular type for a user. The module can
+return a `messages_per_second` and `burst_count` to be used, or `None` if
+the default settings are adequate. The user is represented by their Matrix user ID
+(e.g. `@alice:example.com`). The limiter name is usually taken from the `RatelimitSettings` key
+value.
+
+The limiters that are currently supported are:
+
+- `rc_invites.per_room`
+- `rc_invites.per_user`
+- `rc_invites.per_issuer`
+
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `None`, Synapse falls through to the next one. The value of the first
+callback that does not return `None` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback. If no module returns a non-`None` value
+then the default settings will be used.