Implement an on_new_event callback

changelog.d/11126.feature

@@ -0,0 +1 @@
+Add an `on_new_event` third-party rules callback to allow modules to listen on new events.

docs/modules/third_party_rules_callbacks.md

@@ -111,6 +111,25 @@ callback returns `True`, Synapse falls through to the next one. The value of the
 callback that does not return `True` will be used. If this happens, Synapse will not call
 any of the subsequent implementations of this callback.
 
+### `on_new_event`
+
+```python
+async def on_new_event(
+    event: "synapse.events.EventBase",
+    state_events: "synapse.types.StateMap",
+) -> None:
+```
+
+Called after sending an event into a room. The module is passed the invite event, as well
+as the state of the room _after_ the event. This means that if the event is a state event,
+it will be included in this state.
+
+Note that this callback is called when the event has already been processed and stored
+into the room, which means this callback cannot be used to deny it. To deny an incoming
+event, see [`check_event_for_spam`](http://localhost:3000/modules/spam_checker_callbacks.html#check_event_for_spam).
+
+If multiple modules implement this callback, Synapse runs them all in order.
+
 ## Example
 
 The example below is a module that implements the third-party rules callback

synapse/api/errors.py

@@ -596,3 +596,10 @@ class ShadowBanError(Exception):
 
     This should be caught and a proper "fake" success response sent to the user.
     """
+
+
+class ModuleFailureError(Exception):
+    """
+    Raised when a module raises an exception. If this is raised in the context of an
+    HTTP(S) request, it will translate into a 500 response with a generic Matrix error.
+    """

synapse/events/third_party_rules.py

@@ -14,7 +14,7 @@
 import logging
 from typing import TYPE_CHECKING, Any, Awaitable, Callable, List, Optional, Tuple
 
-from synapse.api.errors import SynapseError
+from synapse.api.errors import ModuleFailureError, SynapseError
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
 from synapse.types import Requester, StateMap
@@ -36,6 +36,7 @@
 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 @@ def __init__(self, hs: "HomeServer"):
         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 @@ def register_third_party_rules_callbacks(
         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 @@ def register_third_party_rules_callbacks(
                 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 +327,29 @@ async def check_visibility_can_be_modified(
 
         return True
 
+    async def on_new_event(self, event: EventBase):
+        """Let modules act on events after they've been sent (e.g. auto-accepting
+        invites, etc.)
+
+        Args:
+            event: The invite event.
+
+        Raises:
+            ModuleFailureError if a callback raised any exception.
+        """
+        # Bail out early without hitting the store if we don't have any callback
+        if len(self._on_new_event_callbacks) == 0:
+            return True
+
+        state_events = 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.error("Failed to run module API callback %s: %s", callback, e)
+                raise ModuleFailureError(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.
 

synapse/handlers/federation_event.py

@@ -2019,7 +2019,7 @@ async def _notify_persisted_event(
         event_pos = PersistedEventPosition(
             self._instance_name, event.internal_metadata.stream_ordering
         )
-        self._notifier.on_new_room_event(
+        await self._notifier.on_new_room_event(
             event, event_pos, max_stream_token, extra_users=extra_users
         )
 

synapse/handlers/message.py

@@ -1537,9 +1537,9 @@ async def persist_and_notify_client_event(
             # If there's an expiry timestamp on the event, schedule its expiry.
             self._message_handler.maybe_schedule_expiry(event)
 
-        def _notify() -> None:
+        async def _notify() -> None:
             try:
-                self.notifier.on_new_room_event(
+                await self.notifier.on_new_room_event(
                     event, event_pos, max_stream_token, extra_users=extra_users
                 )
             except Exception:

synapse/notifier.py

@@ -220,6 +220,8 @@ def __init__(self, hs: "synapse.server.HomeServer"):
         # down.
         self.remote_server_up_callbacks: List[Callable[[str], None]] = []
 
+        self._third_party_rules = hs.get_third_party_event_rules()
+
         self.clock = hs.get_clock()
         self.appservice_handler = hs.get_application_service_handler()
         self._pusher_pool = hs.get_pusherpool()
@@ -267,14 +269,16 @@ def add_replication_callback(self, cb: Callable[[], None]):
         """
         self.replication_callbacks.append(cb)
 
-    def on_new_room_event(
+    async def on_new_room_event(
         self,
         event: EventBase,
         event_pos: PersistedEventPosition,
         max_room_stream_token: RoomStreamToken,
         extra_users: Optional[Collection[UserID]] = None,
     ):
-        """Unwraps event and calls `on_new_room_event_args`."""
+        """Unwraps event and calls `on_new_room_event_args`.
+        Also notifies modules listening on new events via the `on_new_event` callback.
+        """
         self.on_new_room_event_args(
             event_pos=event_pos,
             room_id=event.room_id,
@@ -285,6 +289,8 @@ def on_new_room_event(
             extra_users=extra_users or [],
         )
 
+        await self._third_party_rules.on_new_event(event)
+
     def on_new_room_event_args(
         self,
         room_id: str,

tests/rest/client/test_third_party_rules.py

@@ -15,7 +15,7 @@
 from typing import TYPE_CHECKING, Dict, Optional, Tuple
 from unittest.mock import Mock
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase
 from synapse.events.third_party_rules import load_legacy_third_party_event_rules
@@ -25,6 +25,7 @@
 from synapse.util.frozenutils import unfreeze
 
 from tests import unittest
+from tests.test_utils import make_awaitable
 
 if TYPE_CHECKING:
     from synapse.module_api import ModuleApi
@@ -89,8 +90,9 @@ def make_homeserver(self, reactor, clock):
         return hs
 
     def prepare(self, reactor, clock, homeserver):
-        # Create a user and room to play with during the tests
+        # Create some users and a room to play with during the tests
         self.user_id = self.register_user("kermit", "monkey")
+        self.invitee = self.register_user("invitee", "hackme")
         self.tok = self.login("kermit", "monkey")
 
         # Some tests might prevent room creation on purpose.
@@ -424,6 +426,32 @@ async def test_fn(event: EventBase, state_events):
             self.assertEqual(channel.code, 200)
             self.assertEqual(channel.json_body["i"], i)
 
+    def test_on_new_event(self):
+        """Test that the on_new_event callback is called on new events"""
+        on_new_event = Mock(make_awaitable(None))
+        self.hs.get_third_party_event_rules()._on_new_event_callbacks.append(
+            on_new_event
+        )
+
+        # Send a message event to the room and check that the callback is called.
+        self.helper.send(room_id=self.room_id, tok=self.tok)
+        self.assertEqual(on_new_event.call_count, 1)
+
+        # Check that the callback is also called on membership updates.
+        self.helper.invite(
+            room=self.room_id,
+            src=self.user_id,
+            targ=self.invitee,
+            tok=self.tok,
+        )
+
+        self.assertEqual(on_new_event.call_count, 2)
+
+        args, _ = on_new_event.call_args
+
+        self.assertEqual(args[0].membership, Membership.INVITE)
+        self.assertEqual(args[0].state_key, self.invitee)
+
     def _update_power_levels(self, event_default: int = 0):
         """Updates the room's power levels.