Ensure we mark sent knocks as outliers

changelog.d/10873.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.37.0 which caused `knock` events which we sent to remote servers to be incorrectly stored in the local database.

synapse/handlers/federation.py

@@ -820,9 +820,6 @@ async def do_remotely_reject_invite(
         origin, event, room_version = await self._make_and_verify_event(
             target_hosts, room_id, user_id, "leave", content=content
         )
-        # Mark as outlier as we don't have any state for this event; we're not
-        # even in the room.
-        event.internal_metadata.outlier = True
         event.internal_metadata.out_of_band_membership = True
 
         # Try the host that we successfully called /make_leave/ on first for
@@ -868,6 +865,17 @@ async def _make_and_verify_event(
         assert event.user_id == user_id
         assert event.state_key == user_id
         assert event.room_id == room_id
+
+        # if this is a proto-join, it will never get peristed in its current form
+        # (instead we'll end up persisting the event that comes back from /send_join),
+        # so it doesn't really matter if we mark it as an outlier or not, but let's
+        # do so for consistency.
+        #
+        # Otherwise (ie, if it's an invite-rejecting leave or a knock), we need
+        # to mark it as an outlier because we don't yet know the state at this point
+        # in the DAG.
+        event.internal_metadata.outlier = True
+
         return origin, event, room_version
 
     async def on_make_leave_request(