Do not allow MSC3440 threads to fork threads

synapse/handlers/message.py

@@ -1039,7 +1039,7 @@ async def _validate_event_relation(self, event: EventBase) -> None:
         else:
             # There must be some reason that the client knows the event exists,
             # see if there are existing relations. If so, assume everything is fine.
-            if not await self.store.event_related_to(relates_to):
+            if not await self.store.event_has_relation(relates_to):
                 # Otherwise, the client can't know about the parent event!
                 raise SynapseError(400, "Can't send relation to unknown event")
 
@@ -1057,7 +1057,7 @@ async def _validate_event_relation(self, event: EventBase) -> None:
 
         # Don't attempt to start a thread if the parent event is a relation.
         elif relation_type == RelationTypes.THREAD:
-            if await self.store.event_has_relations(relates_to):
+            if await self.store.event_includes_relation(relates_to):
                 raise SynapseError(
                     400, "Cannot start threads from an event with a relation"
                 )

synapse/storage/databases/main/relations.py

@@ -132,41 +132,66 @@ def _get_recent_references_for_event_txn(
             "get_recent_references_for_event", _get_recent_references_for_event_txn
         )
 
-    async def event_has_relations(self, event_id: str) -> bool:
-        """Check if a given event has relations.
+    async def event_includes_relation(self, event_id: str) -> bool:
+        """Check if the given event includes a valid relation.
+
+        An event has a relation if it has a valid m.relates_to with a rel_type
+        and event_id in the content:
+
+        {
+            "content": {
+                "m.relates_to": {
+                    "rel_type": "m.replace",
+                    "event_id": "$other_event_id"
+                }
+            }
+        }
 
         Args:
             event_id: The event to check.
 
         Returns:
-            True if the event has any relations.
+            True if the event includes a valid relation.
         """
 
         result = await self.db_pool.simple_select_one_onecol(
             table="event_relations",
             keyvalues={"event_id": event_id},
             retcol="event_id",
             allow_none=True,
-            desc="event_has_relations",
+            desc="event_includes_relation",
         )
         return result is not None
 
-    async def event_related_to(self, parent_id: str) -> bool:
-        """Check if a given event has other event relating to it.
+    async def event_has_relation(self, parent_id: str) -> bool:
+        """Check if the given event is referred to via another event's relation.
+
+        An event is the target of a relation if another event has a valid
+        m.relates_to with a rel_type and event_id pointing to parent_id in the
+        content:
+
+        {
+            "content": {
+                "m.relates_to": {
+                    "rel_type": "m.replace",
+                    "event_id": "$parent_id"
+                }
+            }
+        }
 
         Args:
             parent_id: The event to check.
 
         Returns:
-            True if the event is the target of any relations.
+            True if the event is the target of another event's relation.
         """
 
         result = await self.db_pool.simple_select_one_onecol(
             table="event_relations",
             keyvalues={"relates_to_id": parent_id},
             retcol="event_id",
             allow_none=True,
-            desc="event_has_relations",
+            desc="event_has_relation",
         )
         return result is not None
 
@@ -400,7 +425,7 @@ async def events_have_relations(
                 %s;
         """
 
-        def _get_if_event_has_relations(txn) -> List[str]:
+        def _get_if_events_have_relations(txn) -> List[str]:
             clauses: List[str] = []
             clause, args = make_in_list_sql_clause(
                 txn.database_engine, "relates_to_id", parent_ids
@@ -425,7 +450,7 @@ def _get_if_event_has_relations(txn) -> List[str]:
             return [row[0] for row in txn]
 
         return await self.db_pool.runInteraction(
-            "get_if_event_has_relations", _get_if_event_has_relations
+            "get_if_events_have_relations", _get_if_events_have_relations
         )
 
     async def has_user_annotated_event(