Implement MSC3912: Relation-based redactions

synapse/config/experimental.py

@@ -125,3 +125,6 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.msc3886_endpoint: Optional[str] = experimental.get(
             "msc3886_endpoint", None
         )
+
+        # MSC3912: Relation-based redactions.
+        self.msc3912_enabled: bool = experimental.get("msc3912_enabled", False)

synapse/handlers/message.py

@@ -877,6 +877,36 @@ async def deduplicate_state_event(
                 return prev_event
         return None
 
+    async def get_event_from_transaction(
+        self,
+        requester: Requester,
+        txn_id: str,
+        room_id: str,
+    ) -> Optional[EventBase]:
+        """For the given transaction ID and room ID, check if there is a matching event.
+        If so, fetch it and return it.
+
+        Args:
+            requester: The requester making the request in the context of which we want
+                to fetch the event.
+            txn_id: The transaction ID.
+            room_id: The room ID.
+
+        Returns:
+            An event if one could be found, None otherwise.
+        """
+        if requester.access_token_id:
+            existing_event_id = await self.store.get_event_id_from_transaction_id(
+                room_id,
+                requester.user.to_string(),
+                requester.access_token_id,
+                txn_id,
+            )
+            if existing_event_id:
+                return await self.store.get_event(existing_event_id)
+
+        return None
+
     async def create_and_send_nonmember_event(
         self,
         requester: Requester,
@@ -956,18 +986,17 @@ async def create_and_send_nonmember_event(
         # extremities to pile up, which in turn leads to state resolution
         # taking longer.
         async with self.limiter.queue(event_dict["room_id"]):
-            if txn_id and requester.access_token_id:
-                existing_event_id = await self.store.get_event_id_from_transaction_id(
-                    event_dict["room_id"],
-                    requester.user.to_string(),
-                    requester.access_token_id,
-                    txn_id,
+            if txn_id:
+                event = await self.get_event_from_transaction(
+                    requester, txn_id, event_dict["room_id"]
                 )
-                if existing_event_id:
-                    event = await self.store.get_event(existing_event_id)
+                if event:
                     # we know it was persisted, so must have a stream ordering
                     assert event.internal_metadata.stream_ordering
-                    return event, event.internal_metadata.stream_ordering
+                    return (
+                        event,
+                        event.internal_metadata.stream_ordering,
+                    )
 
             event, context = await self.create_event(
                 requester,

synapse/handlers/relations.py

@@ -17,7 +17,7 @@
 
 import attr
 
-from synapse.api.constants import RelationTypes
+from synapse.api.constants import EventTypes, RelationTypes
 from synapse.api.errors import SynapseError
 from synapse.events import EventBase, relation_from_event
 from synapse.logging.opentracing import trace
@@ -75,6 +75,7 @@ def __init__(self, hs: "HomeServer"):
         self._clock = hs.get_clock()
         self._event_handler = hs.get_event_handler()
         self._event_serializer = hs.get_event_client_serializer()
+        self._event_creation_handler = hs.get_event_creation_handler()
 
     async def get_relations(
         self,
@@ -205,6 +206,59 @@ async def get_relations_for_event(
 
         return related_events, next_token
 
+    async def redact_events_related_to(
+        self,
+        requester: Requester,
+        event_id: str,
+        initial_redaction_event: EventBase,
+        relation_types: List[str],
+    ) -> None:
+        """Redacts all events related to the given event ID with one of the given
+        relation types.
+
+        This method is expected to be called when redacting the event referred to by
+        the given event ID.
+
+        If an event cannot be redacted (e.g. because of insufficient permissions), log
+        the error and try to redact the next one.
+
+        Args:
+            requester: The requester to redact events on behalf of.
+            event_id: The event IDs to look and redact relations of.
+            initial_redaction_event: The redaction for the event referred to by
+                event_id.
+            relation_types: The types of relations to look for.
+
+        Raises:
+            ShadowBanError if the requester is shadow-banned
+        """
+        related_event_ids = (
+            await self._main_store.get_all_relations_for_event_with_types(
+                event_id, relation_types
+            )
+        )
+
+        for related_event_id in related_event_ids:
+            try:
+                await self._event_creation_handler.create_and_send_nonmember_event(
+                    requester,
+                    {
+                        "type": EventTypes.Redaction,
+                        "content": initial_redaction_event.content,
+                        "room_id": initial_redaction_event.room_id,
+                        "sender": requester.user.to_string(),
+                        "redacts": related_event_id,
+                    },
+                    ratelimit=False,
+                )
+            except SynapseError as e:
+                logger.warning(
+                    "Failed to redact event %s (related to event %s): %s",
+                    related_event_id,
+                    event_id,
+                    e.msg,
+                )
+
     async def get_annotations_for_event(
         self,
         event_id: str,

synapse/rest/client/room.py

@@ -1029,6 +1029,8 @@ def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
         self.event_creation_handler = hs.get_event_creation_handler()
         self.auth = hs.get_auth()
+        self._relation_handler = hs.get_relations_handler()
+        self._msc3912_enabled = hs.config.experimental.msc3912_enabled
 
     def register(self, http_server: HttpServer) -> None:
         PATTERNS = "/rooms/(?P<room_id>[^/]*)/redact/(?P<event_id>[^/]*)"
@@ -1045,20 +1047,45 @@ async def on_POST(
         content = parse_json_object_from_request(request)
 
         try:
-            (
-                event,
-                _,
-            ) = await self.event_creation_handler.create_and_send_nonmember_event(
-                requester,
-                {
-                    "type": EventTypes.Redaction,
-                    "content": content,
-                    "room_id": room_id,
-                    "sender": requester.user.to_string(),
-                    "redacts": event_id,
-                },
-                txn_id=txn_id,
-            )
+            with_relations = None
+            if self._msc3912_enabled and "org.matrix.msc3912.with_relations" in content:
+                with_relations = content["org.matrix.msc3912.with_relations"]
+                del content["org.matrix.msc3912.with_relations"]
+
+            # Check if there's an existing event for this transaction now (even though
+            # create_and_send_nonmember_event also does it) because, if there's one,
+            # then we want to skip the call to redact_events_related_to.
+            event = None
+            if txn_id:
+                event = await self.event_creation_handler.get_event_from_transaction(
+                    requester, txn_id, room_id
+                )
+
+            if event is None:
+                (
+                    event,
+                    _,
+                ) = await self.event_creation_handler.create_and_send_nonmember_event(
+                    requester,
+                    {
+                        "type": EventTypes.Redaction,
+                        "content": content,
+                        "room_id": room_id,
+                        "sender": requester.user.to_string(),
+                        "redacts": event_id,
+                    },
+                    txn_id=txn_id,
+                )
+
+                if with_relations:
+                    run_in_background(
+                        self._relation_handler.redact_events_related_to,
+                        requester=requester,
+                        event_id=event_id,
+                        initial_redaction_event=event,
+                        relation_types=with_relations,
+                    )
+
             event_id = event.event_id
         except ShadowBanError:
             event_id = "$" + random_string(43)

synapse/storage/databases/main/relations.py

@@ -295,6 +295,44 @@ def _get_recent_references_for_event_txn(
             "get_recent_references_for_event", _get_recent_references_for_event_txn
         )
 
+    async def get_all_relations_for_event_with_types(
+        self,
+        event_id: str,
+        relation_types: List[str],
+    ) -> List[str]:
+        """Get the event IDs of all events that have a relation to the given event with
+        one of the given relation types.
+
+        Args:
+            event_id: The event for which to look for related events.
+            relation_types: The types of relations to look for.
+
+        Returns:
+            A list of the IDs of the events that relate to the given event with one of
+            the given relation types.
+        """
+
+        def get_all_relation_ids_for_event_with_types_txn(
+            txn: LoggingTransaction,
+        ) -> List[str]:
+            sql = """
+            SELECT event_id FROM event_relations
+            WHERE
+                relates_to_id = ?
+                AND relation_type IN (%s)
+            """ % (
+                ",".join(["?" for _ in relation_types]),
+            )
+
+            txn.execute(sql, [event_id] + relation_types)
+
+            return [row[0] for row in txn]
+
+        return await self.db_pool.runInteraction(
+            desc="get_all_relation_ids_for_event_with_types",
+            func=get_all_relation_ids_for_event_with_types_txn,
+        )
+
     async def event_includes_relation(self, event_id: str) -> bool:
         """Check if the given event relates to another event.