Reject make_join for empty rooms (#1439)

* Sanity-check room version on RS event input

* Update gomatrixserverlib

* Reject make_join when no room members are left

* Revert some changes from wrong branch

* Distinguish between room not existing and room being abandoned on this server

* nolint

roomserver/api/api.go

@@ -89,6 +89,13 @@ type RoomserverInternalAPI interface {
 		response *QueryMembershipsForRoomResponse,
 	) error
 
+	// Query if we think we're still in a room.
+	QueryServerJoinedToRoom(
+		ctx context.Context,
+		request *QueryServerJoinedToRoomRequest,
+		response *QueryServerJoinedToRoomResponse,
+	) error
+
 	// Query whether a server is allowed to see an event
 	QueryServerAllowedToSeeEvent(
 		ctx context.Context,

roomserver/api/api_trace.go

@@ -134,6 +134,16 @@ func (t *RoomserverInternalAPITrace) QueryMembershipsForRoom(
 	return err
 }
 
+func (t *RoomserverInternalAPITrace) QueryServerJoinedToRoom(
+	ctx context.Context,
+	req *QueryServerJoinedToRoomRequest,
+	res *QueryServerJoinedToRoomResponse,
+) error {
+	err := t.Impl.QueryServerJoinedToRoom(ctx, req, res)
+	util.GetLogger(ctx).WithError(err).Infof("QueryServerJoinedToRoom req=%+v res=%+v", js(req), js(res))
+	return err
+}
+
 func (t *RoomserverInternalAPITrace) QueryServerAllowedToSeeEvent(
 	ctx context.Context,
 	req *QueryServerAllowedToSeeEventRequest,

roomserver/api/query.go

@@ -140,6 +140,22 @@ type QueryMembershipsForRoomResponse struct {
 	HasBeenInRoom bool `json:"has_been_in_room"`
 }
 
+// QueryServerJoinedToRoomRequest is a request to QueryServerJoinedToRoom
+type QueryServerJoinedToRoomRequest struct {
+	// Server name of the server to find
+	ServerName gomatrixserverlib.ServerName `json:"server_name"`
+	// ID of the room to see if we are still joined to
+	RoomID string `json:"room_id"`
+}
+
+// QueryMembershipsForRoomResponse is a response to QueryServerJoinedToRoom
+type QueryServerJoinedToRoomResponse struct {
+	// True if the room exists on the server
+	RoomExists bool `json:"room_exists"`
+	// True if we still believe that we are participating in the room
+	IsInRoom bool `json:"is_in_room"`
+}
+
 // QueryServerAllowedToSeeEventRequest is a request to QueryServerAllowedToSeeEvent
 type QueryServerAllowedToSeeEventRequest struct {
 	// The event ID to look up invites in.