Merge pull request #615 from matrix-org/rav/SYN-642

Handle rejections of invites from local users locally

synapse/handlers/_base.py

@@ -199,16 +199,35 @@ def _create_new_client_event(self, builder):
         # 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
         ):
             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

synapse/handlers/room.py

@@ -500,6 +500,8 @@ def send_membership_event(
         Raises:
             SynapseError if there was a problem changing the membership.
         """
+        remote_room_hosts = remote_room_hosts or []
+
         target_user = UserID.from_string(event.state_key)
         room_id = event.room_id
 
@@ -534,8 +536,24 @@ def send_membership_event(
                 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 self.hs.is_mine(inviter):
+                    # 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 = remote_room_hosts + [inviter.domain]
+                    action = "remote_reject"
 
         federation_handler = self.hs.get_handlers().federation_handler
 
@@ -554,11 +572,8 @@ def send_membership_event(
                 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
             )