Fix error when sending message into deleted room. (#15235)

When a room is deleted in Synapse we remove the event forward
extremities in the room, so if (say a bot) tries to send a message into
the room we error out due to not being able to calculate prev events for
the new event *before* we check if the sender is in the room.

Fixes #8094

synapse/handlers/message.py

@@ -987,10 +987,11 @@ class EventCreationHandler:
         # a situation where event persistence can't keep up, causing
         # extremities to pile up, which in turn leads to state resolution
         # taking longer.
-        async with self.limiter.queue(event_dict["room_id"]):
+        room_id = event_dict["room_id"]
+        async with self.limiter.queue(room_id):
             if txn_id:
                 event = await self.get_event_from_transaction(
-                    requester, txn_id, event_dict["room_id"]
+                    requester, txn_id, room_id
                 )
                 if event:
                     # we know it was persisted, so must have a stream ordering
@@ -1000,6 +1001,18 @@ class EventCreationHandler:
                         event.internal_metadata.stream_ordering,
                     )
 
+        # If we don't have any prev event IDs specified then we need to
+        # check that the host is in the room (as otherwise populating the
+        # prev events will fail), at which point we may as well check the
+        # local user is in the room.
+        if not prev_event_ids:
+            user_id = requester.user.to_string()
+            is_user_in_room = await self.store.check_local_user_in_room(
+                user_id, room_id
+            )
+            if not is_user_in_room:
+                raise AuthError(403, f"User {user_id} not in room {room_id}")
+
         # Try several times, it could fail with PartialStateConflictError
         # in handle_new_client_event, cf comment in except block.
         max_retries = 5