Cleanup stale device lists for users we don't share a room with anymore (#2857)

The stale device lists table might contain entries for users we don't
share a room with anymore. This now asks the roomserver about left users
and removes those entries from the table.

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

roomserver/api/api.go

@@ -17,6 +17,7 @@ type RoomserverInternalAPI interface {
 	ClientRoomserverAPI
 	UserRoomserverAPI
 	FederationRoomserverAPI
+	KeyserverRoomserverAPI
 
 	// needed to avoid chicken and egg scenario when setting up the
 	// interdependencies between the roomserver and other input APIs
@@ -199,3 +200,7 @@ type FederationRoomserverAPI interface {
 	// Query a given amount (or less) of events prior to a given set of events.
 	PerformBackfill(ctx context.Context, req *PerformBackfillRequest, res *PerformBackfillResponse) error
 }
+
+type KeyserverRoomserverAPI interface {
+	QueryLeftUsers(ctx context.Context, req *QueryLeftUsersRequest, res *QueryLeftUsersResponse) error
+}

roomserver/api/api_trace.go

@@ -19,6 +19,12 @@ type RoomserverInternalAPITrace struct {
 	Impl RoomserverInternalAPI
 }
 
+func (t *RoomserverInternalAPITrace) QueryLeftUsers(ctx context.Context, req *QueryLeftUsersRequest, res *QueryLeftUsersResponse) error {
+	err := t.Impl.QueryLeftUsers(ctx, req, res)
+	util.GetLogger(ctx).WithError(err).Infof("QueryLeftUsers req=%+v res=%+v", js(req), js(res))
+	return err
+}
+
 func (t *RoomserverInternalAPITrace) SetFederationAPI(fsAPI fsAPI.RoomserverFederationAPI, keyRing *gomatrixserverlib.KeyRing) {
 	t.Impl.SetFederationAPI(fsAPI, keyRing)
 }

roomserver/api/query.go

@@ -447,3 +447,15 @@ type QueryMembershipAtEventResponse struct {
 	// do not have known state will return an empty array here.
 	Memberships map[string][]*gomatrixserverlib.HeaderedEvent `json:"memberships"`
 }
+
+// QueryLeftUsersRequest is a request to calculate users that we (the server) don't share a
+// a room with anymore. This is used to cleanup stale device list entries, where we would
+// otherwise keep on trying to get device lists.
+type QueryLeftUsersRequest struct {
+	StaleDeviceListUsers []string `json:"user_ids"`
+}
+
+// QueryLeftUsersResponse is the response to QueryLeftUsersRequest.
+type QueryLeftUsersResponse struct {
+	LeftUsers []string `json:"user_ids"`
+}