diff --git a/synapse/handlers/_base.py b/synapse/handlers/_base.py
index 2333fc0c09..c6a74b0e3d 100644
--- a/synapse/handlers/_base.py
+++ b/synapse/handlers/_base.py
@@ -199,8 +199,7 @@ class BaseHandler(object):
# events in the room, because we don't know enough about the graph
# fragment we received to treat it like a graph, so the above returned
# no relevant events. It may have returned some events (if we have
- # joined and left the room), but not useful ones, like the invite. So we
- # forcibly set our context to the invite we received over federation.
+ # joined and left the room), but not useful ones, like the invite.
if (
not self.is_host_in_room(context.current_state) and
builder.type == EventTypes.Member
@@ -208,7 +207,27 @@ class BaseHandler(object):
prev_member_event = yield self.store.get_room_member(
builder.sender, builder.room_id
)
- if prev_member_event:
+
+ # The prev_member_event may already be in context.current_state,
+ # despite us not being present in the room; in particular, if
+ # inviting user, and all other local users, have already left.
+ #
+ # In that case, we have all the information we need, and we don't
+ # want to drop "context" - not least because we may need to handle
+ # the invite locally, which will require us to have the whole
+ # context (not just prev_member_event) to auth it.
+ #
+ context_event_ids = (
+ e.event_id for e in context.current_state.values()
+ )
+
+ if (
+ prev_member_event and
+ prev_member_event.event_id not in context_event_ids
+ ):
+ # The prev_member_event is missing from context, so it must
+ # have arrived over federation and is an outlier. We forcibly
+ # set our context to the invite we received over federation
builder.prev_events = (
prev_member_event.event_id,
prev_member_event.prev_events
|
