Comments

eduserver/input/input.go

@@ -119,6 +119,11 @@ func (t *EDUServerInputAPI) sendToDeviceEvent(ise *api.InputSendToDeviceEvent) e
 		return err
 	}
 
+	// If the event is targeted locally then we want to expand the wildcard
+	// out into individual device IDs so that we can send them to each respective
+	// device. If the event isn't targeted locally then we can't expand the
+	// wildcard as we don't know about the remote devices, so instead we leave it
+	// as-is, so that the federation sender can send it on with the wildcard intact.
 	if domain == t.ServerName && ise.DeviceID == "*" {
 		devs, err := t.DeviceDB.GetDevicesByLocalpart(context.TODO(), localpart)
 		if err != nil {

syncapi/storage/tables/interface.go

@@ -109,6 +109,23 @@ type BackwardsExtremities interface {
 	DeleteBackwardExtremity(ctx context.Context, txn *sql.Tx, roomID, knownEventID string) (err error)
 }
 
+// SendToDevice tracks send-to-device messages which are sent to individual
+// clients. Each message gets inserted into this table at the point that we
+// receive it from the EDU server.
+//
+// We're supposed to try and do our best to deliver send-to-device messages
+// once, but the only way that we can really guarantee that they have been
+// delivered is if the client successfully requests the next sync as given
+// in the next_batch. Each time the device syncs, we will request all of the
+// updates that either haven't been sent yet, along with all updates that we
+// *have* sent but we haven't confirmed to have been received yet. If it's the
+// first time we're sending a given update then we update the table to say
+// what the "since" parameter was when we tried to send it.
+//
+// When the client syncs again, if their "since" parameter is *later* than
+// the recorded one, we drop the entry from the DB as it's "sent". If the
+// sync parameter isn't later then we will keep including the updates in the
+// sync response, as the client is seemingly trying to repeat the same /sync.
 type SendToDevice interface {
 	InsertSendToDeviceMessage(ctx context.Context, txn *sql.Tx, userID, deviceID, content string) (err error)
 	SelectSendToDeviceMessages(ctx context.Context, txn *sql.Tx, userID, deviceID string) (events []types.SendToDeviceEvent, err error)