Add a module callback to react to account data changes (#12327)

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

3 files changed, 108 insertions, 1 deletions

diff --git a/docs/SUMMARY.md b/docs/SUMMARY.md
index 21f80efc99..6aa48e1919 100644
--- a/docs/SUMMARY.md
+++ b/docs/SUMMARY.md
@@ -45,6 +45,7 @@
         - [Account validity callbacks](modules/account_validity_callbacks.md)
         - [Password auth provider callbacks](modules/password_auth_provider_callbacks.md)
         - [Background update controller callbacks](modules/background_update_controller_callbacks.md)
+        - [Account data callbacks](modules/account_data_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)
diff --git a/docs/modules/account_data_callbacks.md b/docs/modules/account_data_callbacks.md
new file mode 100644
index 0000000000..25de911627
--- /dev/null
+++ b/docs/modules/account_data_callbacks.md
@@ -0,0 +1,106 @@
+# Account data callbacks
+
+Account data callbacks allow module developers to react to changes of the account data
+of local users. Account data callbacks can be registered using the module API's
+`register_account_data_callbacks` method.
+
+## Callbacks
+
+The available account data callbacks are:
+
+### `on_account_data_updated`
+
+_First introduced in Synapse v1.57.0_
+
+```python
+async def on_account_data_updated(
+    user_id: str,
+    room_id: Optional[str],
+    account_data_type: str,
+    content: "synapse.module_api.JsonDict",
+) -> None:
+```
+
+Called after user's account data has been updated. The module is given the
+Matrix ID of the user whose account data is changing, the room ID the data is associated
+with, the type associated with the change, as well as the new content. If the account
+data is not associated with a specific room, then the room ID is `None`.
+
+This callback is triggered when new account data is added or when the data associated with
+a given type (and optionally room) changes. This includes deletion, since in Matrix,
+deleting account data consists of replacing the data associated with a given type
+(and optionally room) with an empty dictionary (`{}`).
+
+Note that this doesn't trigger when changing the tags associated with a room, as these are
+processed separately by Synapse.
+
+If multiple modules implement this callback, Synapse runs them all in order.
+
+## Example
+
+The example below is a module that implements the `on_account_data_updated` callback, and
+sends an event to an audit room when a user changes their account data.
+
+```python
+import json
+import attr
+from typing import Any, Dict, Optional
+
+from synapse.module_api import JsonDict, ModuleApi
+from synapse.module_api.errors import ConfigError
+
+
+@attr.s(auto_attribs=True)
+class CustomAccountDataConfig:
+    audit_room: str
+    sender: str
+
+
+class CustomAccountDataModule:
+    def __init__(self, config: CustomAccountDataConfig, api: ModuleApi):
+        self.api = api
+        self.config = config
+
+        self.api.register_account_data_callbacks(
+            on_account_data_updated=self.log_new_account_data,
+        )
+
+    @staticmethod
+    def parse_config(config: Dict[str, Any]) -> CustomAccountDataConfig:
+        def check_in_config(param: str):
+            if param not in config:
+                raise ConfigError(f"'{param}' is required")
+
+        check_in_config("audit_room")
+        check_in_config("sender")
+
+        return CustomAccountDataConfig(
+            audit_room=config["audit_room"],
+            sender=config["sender"],
+        )
+
+    async def log_new_account_data(
+        self,
+        user_id: str,
+        room_id: Optional[str],
+        account_data_type: str,
+        content: JsonDict,
+    ) -> None:
+        content_raw = json.dumps(content)
+        msg_content = f"{user_id} has changed their account data for type {account_data_type} to: {content_raw}"
+
+        if room_id is not None:
+            msg_content += f" (in room {room_id})"
+
+        await self.api.create_and_send_event_into_room(
+            {
+                "room_id": self.config.audit_room,
+                "sender": self.config.sender,
+                "type": "m.room.message",
+                "content": {
+                    "msgtype": "m.text",
+                    "body": msg_content
+                }
+            }
+        )
+```
diff --git a/docs/modules/writing_a_module.md b/docs/modules/writing_a_module.md
index e7c0ffad58..e6303b739e 100644
--- a/docs/modules/writing_a_module.md
+++ b/docs/modules/writing_a_module.md
@@ -33,7 +33,7 @@ A module can implement the following static method:
 
 ```python
 @staticmethod
-def parse_config(config: dict) -> dict
+def parse_config(config: dict) -> Any
 ```
 
 This method is given a dictionary resulting from parsing the YAML configuration for the