fix: provide remote servers a way to find out about an event created during the remote join handshake

changelog.d/19390.bugfix

@@ -0,0 +1 @@
+Provide remote servers a way to find out about an event created during the remote join handshake. Contributed by @FrenchGithubUser and @jason-famedly @ Famedly.

synapse/federation/federation_server.py

@@ -758,6 +758,10 @@ async def on_send_join_request(
         event, context = await self._on_send_membership_event(
             origin, content, Membership.JOIN, room_id
         )
+        # Collect this now, the internal metadata of event(which should have it) doesn't
+        stream_ordering_of_join = (
+            await self.store.get_current_room_stream_token_for_room_id(room_id)
+        )
 
         prev_state_ids = await context.get_prev_state_ids()
 
@@ -798,6 +802,27 @@ async def on_send_join_request(
             "members_omitted": caller_supports_partial_state,
         }
 
+        # Check the forward extremities for the room here. If there is more than one, it
+        # is likely that another event was created in the room during the
+        # make_join/send_join handshake. Without being able to determine how long until
+        # the next event will be created that references this 'missing event',
+        # proactively send a dummy extensible event that ties these forward extremities
+        # together. The remote server should search out this missing event on its own.
+        #
+        # By not sending the 'missing event' directly, the stream ordering for it will
+        # be consistent between servers(in that it technically was created before the
+        # join itself).
+
+        forward_extremities = await self.store._get_forward_extremeties_for_room(
+            room_id, stream_ordering_of_join.get_max_stream_pos()
+        )
+
+        if len(forward_extremities) > 1:
+            # I do not feel it is necessary to set this onto the FederationServer class
+            # itself. Its likelihood of being used is extremely low. Make it on-demand
+            _creation_handler = self.hs.get_event_creation_handler()
+            await _creation_handler._send_dummy_events_to_patch_room(room_id)
+
         if servers_in_room is not None:
             resp["servers_in_room"] = list(servers_in_room)
 

synapse/handlers/message.py

@@ -2220,7 +2220,36 @@ async def _send_dummy_events_to_fill_extremities(self) -> None:
                 now = self.clock.time_msec()
                 self._rooms_to_exclude_from_dummy_event_insertion[room_id] = now
 
-    async def _send_dummy_event_for_room(self, room_id: str) -> bool:
+    async def _send_dummy_events_to_patch_room(self, room_id: str) -> None:
+        """
+        Send a dummy event into this room to patch in a missed forward extremity.
+        This should only be triggered during a remote join if there was a forward
+        extremity that occurred during the make_join/send_join handshake.
+        """
+        async with self._worker_lock_handler.acquire_read_write_lock(
+            NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
+        ):
+            dummy_event_sent = await self._send_dummy_event_for_room(
+                room_id, proactively_send=True
+            )
+
+        if not dummy_event_sent:
+            # Did not find a valid user in the room, so remove from future attempts
+            # Exclusion is time limited, so the room will be rechecked in the future
+            # dependent on _DUMMY_EVENT_ROOM_EXCLUSION_EXPIRY
+            logger.info(
+                "Failed to send dummy event into room %s. Will exclude it from "
+                "future attempts until cache expires",
+                room_id,
+            )
+            # This mapping is room_id -> time of last attempt(in ms)
+            self._rooms_to_exclude_from_dummy_event_insertion[room_id] = (
+                self.clock.time_msec()
+            )
+
+    async def _send_dummy_event_for_room(
+        self, room_id: str, proactively_send: bool = False
+    ) -> bool:
         """Attempt to send a dummy event for the given room.
 
         Args:
@@ -2252,8 +2281,7 @@ async def _send_dummy_event_for_room(self, room_id: str) -> bool:
                             },
                         )
                         context = await unpersisted_context.persist(event)
-
-                        event.internal_metadata.proactively_send = False
+                        event.internal_metadata.proactively_send = proactively_send
 
                         # Since this is a dummy-event it is OK if it is sent by a
                         # shadow-banned user.