Handle rejections of invites from local users locally

Slightly hacky fix to SYN-642, which avoids the federation codepath when trying
to reject invites from local users.

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 +207,18 @@ class BaseHandler(object):
             prev_member_event = yield self.store.get_room_member(
                 builder.sender, builder.room_id
             )
-            if prev_member_event:
+
+            # if we have the prev_member_event in context, we didn't receive
+            # the invite over federation. (More likely is that the inviting
+            # user, and all other local users, have left, making
+            # is_host_in_room return false).
+            #
+            context_events = (e.event_id for e in context.current_state.values())
+
+            if prev_member_event and not prev_member_event.event_id in context_events:
+                # 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

synapse/handlers/room.py

@@ -521,8 +521,24 @@ class RoomMemberHandler(BaseHandler):
                 action = "remote_join"
         elif event.membership == Membership.LEAVE:
             is_host_in_room = self.is_host_in_room(context.current_state)
+
             if not is_host_in_room:
-                action = "remote_reject"
+                # perhaps we've been invited
+                inviter = self.get_inviter(target_user.to_string(), context.current_state)
+                if not inviter:
+                    raise SynapseError(404, "Not a known room")
+
+                if inviter.domain == self.server_name:
+                    # the inviter was on our server, but has now left. Carry on
+                    # with the normal rejection codepath.
+                    #
+                    # This is a bit of a hack, because the room might still be
+                    # active on other servers.
+                    pass
+                else:
+                    # send the rejection to the inviter's HS.
+                    remote_room_hosts = [inviter.domain]
+                    action = "remote_reject"
 
         federation_handler = self.hs.get_handlers().federation_handler
 
@@ -541,11 +557,8 @@ class RoomMemberHandler(BaseHandler):
                 event.content,
             )
         elif action == "remote_reject":
-            inviter = self.get_inviter(target_user.to_string(), context.current_state)
-            if not inviter:
-                raise SynapseError(404, "No known servers")
             yield federation_handler.do_remotely_reject_invite(
-                [inviter.domain],
+                remote_room_hosts,
                 room_id,
                 event.user_id
             )