Fix 404 on `/sync` when the last event is a redaction of an unknown/purged event (#12905)

Currently, we try to pull the event corresponding to a sync token from the database. However, when
we fetch redaction events, we check the target of that redaction (because we aren't allowed to send
redactions to clients without validating them). So, if the sync token points to a redaction of an event
that we don't have, we have a problem.

It turns out we don't really need that event, and can just work with its ID and metadata, which
sidesteps the whole problem.

1 files changed, 19 insertions, 8 deletions

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index b5859dcb28..a1d41358d9 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -621,21 +621,32 @@ class SyncHandler:
         )
 
     async def get_state_after_event(
-        self, event: EventBase, state_filter: Optional[StateFilter] = None
+        self, event_id: str, state_filter: Optional[StateFilter] = None
     ) -> StateMap[str]:
         """
         Get the room state after the given event
 
         Args:
-            event: event of interest
+            event_id: event of interest
             state_filter: The state filter used to fetch state from the database.
         """
         state_ids = await self._state_storage_controller.get_state_ids_for_event(
-            event.event_id, state_filter=state_filter or StateFilter.all()
+            event_id, state_filter=state_filter or StateFilter.all()
         )
-        if event.is_state():
+
+        # using get_metadata_for_events here (instead of get_event) sidesteps an issue
+        # with redactions: if `event_id` is a redaction event, and we don't have the
+        # original (possibly because it got purged), get_event will refuse to return
+        # the redaction event, which isn't terribly helpful here.
+        #
+        # (To be fair, in that case we could assume it's *not* a state event, and
+        # therefore we don't need to worry about it. But still, it seems cleaner just
+        # to pull the metadata.)
+        m = (await self.store.get_metadata_for_events([event_id]))[event_id]
+        if m.state_key is not None and m.rejection_reason is None:
             state_ids = dict(state_ids)
-            state_ids[(event.type, event.state_key)] = event.event_id
+            state_ids[(m.event_type, m.state_key)] = event_id
+
         return state_ids
 
     async def get_state_at(
@@ -654,14 +665,14 @@ class SyncHandler:
         # FIXME: This gets the state at the latest event before the stream ordering,
         # which might not be the same as the "current state" of the room at the time
         # of the stream token if there were multiple forward extremities at the time.
-        last_event = await self.store.get_last_event_in_room_before_stream_ordering(
+        last_event_id = await self.store.get_last_event_in_room_before_stream_ordering(
             room_id,
             end_token=stream_position.room_key,
         )
 
-        if last_event:
+        if last_event_id:
             state = await self.get_state_after_event(
-                last_event, state_filter=state_filter or StateFilter.all()
+                last_event_id, state_filter=state_filter or StateFilter.all()
             )
 
         else: