Fix recursion error when fetching auth chain over federation (#7817)

When fetching the state of a room over federation we receive the event
IDs of the state and auth chain. We then fetch those events that we
don't already have.

However, we used a function that recursively fetched any missing auth
events for the fetched events, which can lead to a lot of recursion if
the server is missing most of the auth chain. This work is entirely
pointless because would have queued up the missing events in the auth
chain to be fetched already.

Let's just diable the recursion, since it only gets called from one
place anyway.

changelog.d/7817.bugfix

@@ -0,0 +1 @@
+Fix bug where Synapse fails to process an incoming event over federation if the server is missing too much of the event's auth chain.

synapse/event_auth.py

@@ -65,14 +65,16 @@ def check(
 
     room_id = event.room_id
 
-    # I'm not really expecting to get auth events in the wrong room, but let's
+    # We need to ensure that the auth events are actually for the same room, to
-    # sanity-check it
+    # stop people from using powers they've been granted in other rooms for
+    # example.
     for auth_event in auth_events.values():
         if auth_event.room_id != room_id:
-            raise Exception(
+            raise AuthError(
+                403,
                 "During auth for event %s in room %s, found event %s in the state "
                 "which is in room %s"
-                % (event.event_id, room_id, auth_event.event_id, auth_event.room_id)
+                % (event.event_id, room_id, auth_event.event_id, auth_event.room_id),
             )
 
     if do_sig_check:

synapse/handlers/federation.py

@@ -618,6 +618,11 @@ class FederationHandler(BaseHandler):
         will be omitted from the result. Likewise, any events which turn out not to
         be in the given room.
 
+        This function *does not* automatically get missing auth events of the
+        newly fetched events. Callers must include the full auth chain of
+        of the missing events in the `event_ids` argument, to ensure that any
+        missing auth events are correctly fetched.
+
         Returns:
             map from event_id to event
         """
@@ -1131,12 +1136,16 @@ class FederationHandler(BaseHandler):
     ):
         """Fetch the given events from a server, and persist them as outliers.
 
+        This function *does not* recursively get missing auth events of the
+        newly fetched events. Callers must include in the `events` argument
+        any missing events from the auth chain.
+
         Logs a warning if we can't find the given event.
         """
 
         room_version = await self.store.get_room_version(room_id)
 
-        event_infos = []
+        event_map = {}  # type: Dict[str, EventBase]
 
         async def get_event(event_id: str):
             with nested_logging_context(event_id):
@@ -1150,17 +1159,7 @@ class FederationHandler(BaseHandler):
                         )
                         return
 
-                    # recursively fetch the auth events for this event
+                    event_map[event.event_id] = event
-                    auth_events = await self._get_events_from_store_or_dest(
-                        destination, room_id, event.auth_event_ids()
-                    )
-                    auth = {}
-                    for auth_event_id in event.auth_event_ids():
-                        ae = auth_events.get(auth_event_id)
-                        if ae:
-                            auth[(ae.type, ae.state_key)] = ae
-
-                    event_infos.append(_NewEventInfo(event, None, auth))
 
                 except Exception as e:
                     logger.warning(
@@ -1172,6 +1171,32 @@ class FederationHandler(BaseHandler):
 
         await concurrently_execute(get_event, events, 5)
 
+        # Make a map of auth events for each event. We do this after fetching
+        # all the events as some of the events' auth events will be in the list
+        # of requested events.
+
+        auth_events = [
+            aid
+            for event in event_map.values()
+            for aid in event.auth_event_ids()
+            if aid not in event_map
+        ]
+        persisted_events = await self.store.get_events(
+            auth_events, allow_rejected=True,
+        )
+
+        event_infos = []
+        for event in event_map.values():
+            auth = {}
+            for auth_event_id in event.auth_event_ids():
+                ae = persisted_events.get(auth_event_id) or event_map.get(auth_event_id)
+                if ae:
+                    auth[(ae.type, ae.state_key)] = ae
+                else:
+                    logger.info("Missing auth event %s", auth_event_id)
+
+            event_infos.append(_NewEventInfo(event, None, auth))
+
         await self._handle_new_events(
             destination, event_infos,
         )