Implement an on_new_event callback (#11126)

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

synapse/events/third_party_rules.py

@@ -36,6 +36,7 @@ CHECK_THREEPID_CAN_BE_INVITED_CALLBACK = Callable[
 CHECK_VISIBILITY_CAN_BE_MODIFIED_CALLBACK = Callable[
     [str, StateMap[EventBase], str], Awaitable[bool]
 ]
+ON_NEW_EVENT_CALLBACK = Callable[[EventBase, StateMap[EventBase]], Awaitable]
 
 
 def load_legacy_third_party_event_rules(hs: "HomeServer") -> None:
@@ -152,6 +153,7 @@ class ThirdPartyEventRules:
         self._check_visibility_can_be_modified_callbacks: List[
             CHECK_VISIBILITY_CAN_BE_MODIFIED_CALLBACK
         ] = []
+        self._on_new_event_callbacks: List[ON_NEW_EVENT_CALLBACK] = []
 
     def register_third_party_rules_callbacks(
         self,
@@ -163,6 +165,7 @@ class ThirdPartyEventRules:
         check_visibility_can_be_modified: Optional[
             CHECK_VISIBILITY_CAN_BE_MODIFIED_CALLBACK
         ] = None,
+        on_new_event: Optional[ON_NEW_EVENT_CALLBACK] = None,
     ) -> None:
         """Register callbacks from modules for each hook."""
         if check_event_allowed is not None:
@@ -181,6 +184,9 @@ class ThirdPartyEventRules:
                 check_visibility_can_be_modified,
             )
 
+        if on_new_event is not None:
+            self._on_new_event_callbacks.append(on_new_event)
+
     async def check_event_allowed(
         self, event: EventBase, context: EventContext
     ) -> Tuple[bool, Optional[dict]]:
@@ -321,6 +327,31 @@ class ThirdPartyEventRules:
 
         return True
 
+    async def on_new_event(self, event_id: str) -> None:
+        """Let modules act on events after they've been sent (e.g. auto-accepting
+        invites, etc.)
+
+        Args:
+            event_id: The ID of the event.
+
+        Raises:
+            ModuleFailureError if a callback raised any exception.
+        """
+        # Bail out early without hitting the store if we don't have any callbacks
+        if len(self._on_new_event_callbacks) == 0:
+            return
+
+        event = await self.store.get_event(event_id)
+        state_events = await self._get_state_map_for_room(event.room_id)
+
+        for callback in self._on_new_event_callbacks:
+            try:
+                await callback(event, state_events)
+            except Exception as e:
+                logger.exception(
+                    "Failed to run module API callback %s: %s", callback, e
+                )
+
     async def _get_state_map_for_room(self, room_id: str) -> StateMap[EventBase]:
         """Given a room ID, return the state events of that room.