Try to handle multiple invite rejections better

federationapi/routing/leave.go

@@ -76,6 +76,23 @@ func MakeLeave(
 		return jsonerror.InternalServerError()
 	}
 
+	// If the user has already left then just return their last leave
+	// event. This means that /send_leave will be a no-op, which helps
+	// to reject invites multiple times - hopefully.
+	for _, state := range queryRes.StateEvents {
+		if state.Type() == gomatrixserverlib.MRoomMember && state.StateKeyEquals(userID) {
+			if mem, merr := state.Membership(); merr == nil && mem == gomatrixserverlib.Leave {
+				return util.JSONResponse{
+					Code: http.StatusOK,
+					JSON: map[string]interface{}{
+						"room_version": event.RoomVersion,
+						"event":        state,
+					},
+				}
+			}
+		}
+	}
+
 	// Check that the leave is allowed or not
 	stateEvents := make([]*gomatrixserverlib.Event, len(queryRes.StateEvents))
 	for i := range queryRes.StateEvents {
@@ -99,6 +116,7 @@ func MakeLeave(
 }
 
 // SendLeave implements the /send_leave API
+// nolint:gocyclo
 func SendLeave(
 	httpReq *http.Request,
 	request *gomatrixserverlib.FederationRequest,
@@ -149,6 +167,48 @@ func SendLeave(
 		}
 	}
 
+	// Check if the user has already left. If so, no-op!
+	queryReq := &api.QueryLatestEventsAndStateRequest{
+		RoomID: roomID,
+		StateToFetch: []gomatrixserverlib.StateKeyTuple{
+			{
+				EventType: gomatrixserverlib.MRoomMember,
+				StateKey:  *event.StateKey(),
+			},
+		},
+	}
+	queryRes := &api.QueryLatestEventsAndStateResponse{}
+	err = rsAPI.QueryLatestEventsAndState(httpReq.Context(), queryReq, queryRes)
+	if err != nil {
+		util.GetLogger(httpReq.Context()).WithError(err).Error("rsAPI.QueryLatestEventsAndState failed")
+		return jsonerror.InternalServerError()
+	}
+	// The room doesn't exist or we weren't ever joined to it. Might as well
+	// no-op here.
+	if !queryRes.RoomExists || len(queryRes.StateEvents) == 0 {
+		return util.JSONResponse{
+			Code: http.StatusOK,
+			JSON: struct{}{},
+		}
+	}
+	// Check if we're recycling a previous leave event.
+	if event.EventID() == queryRes.StateEvents[0].EventID() {
+		return util.JSONResponse{
+			Code: http.StatusOK,
+			JSON: struct{}{},
+		}
+	}
+	// We are/were joined/invited/banned or something. Check if
+	// we can no-op here.
+	if len(queryRes.StateEvents) == 1 {
+		if mem, merr := queryRes.StateEvents[0].Membership(); merr == nil && mem == gomatrixserverlib.Leave {
+			return util.JSONResponse{
+				Code: http.StatusOK,
+				JSON: struct{}{},
+			}
+		}
+	}
+
 	// Check that the event is signed by the server sending the request.
 	redacted := event.Redact()
 	verifyRequests := []gomatrixserverlib.VerifyJSONRequest{{
@@ -174,7 +234,8 @@ func SendLeave(
 	if err != nil {
 		util.GetLogger(httpReq.Context()).WithError(err).Error("event.Membership failed")
 		return jsonerror.InternalServerError()
-	} else if mem != gomatrixserverlib.Leave {
+	}
+	if mem != gomatrixserverlib.Leave {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: jsonerror.BadJSON("The membership in the event content must be set to leave"),