backfill: add notification of disappearing messages at correct timestamp

This makes it so that the timestamp of the chat in Matrix looks correct,
even though the message is not there to be bridged since it has
disappeared.

historysync.go

@@ -158,7 +158,19 @@ func (user *User) backfillInChunks(req *database.Backfill, conv *database.Histor
 		}
 	}
 	allMsgs := user.bridge.DB.HistorySyncQuery.GetMessagesBetween(user.MXID, conv.ConversationID, req.TimeStart, req.TimeEnd, req.MaxTotalEvents)
-	if len(allMsgs) == 0 {
+
+	sendDisappearedNotice := false
+	// If expired messages are on, and a notice has not been sent to this chat
+	// about it having disappeared messages at the conversation timestamp, send
+	// a notice indicating so.
+	if len(allMsgs) == 0 && conv.EphemeralExpiration != nil && *conv.EphemeralExpiration > 0 {
+		lastMessage := portal.bridge.DB.Message.GetLastInChat(portal.Key)
+		if lastMessage == nil || !conv.LastMessageTimestamp.Equal(lastMessage.Timestamp) {
+			sendDisappearedNotice = true
+		}
+	}
+
+	if !sendDisappearedNotice && len(allMsgs) == 0 {
 		user.log.Debugfln("Not backfilling %s: no bridgeable messages found", portal.Key.JID)
 		return
 	}
@@ -172,6 +184,28 @@ func (user *User) backfillInChunks(req *database.Backfill, conv *database.Histor
 		}
 	}
 
+	if sendDisappearedNotice {
+		user.log.Debugfln("Sending notice to %s that there are disappeared messages ending at %v", portal.Key.JID, conv.LastMessageTimestamp)
+		resp, err := portal.sendMessage(portal.MainIntent(), event.EventMessage, &event.MessageEventContent{
+			MsgType: event.MsgNotice,
+			Body:    portal.formatDisappearingMessageNotice(),
+		}, nil, conv.LastMessageTimestamp.UnixMilli())
+
+		if err != nil {
+			portal.log.Errorln("Error sending disappearing messages notice event")
+			return
+		}
+
+		msg := portal.bridge.DB.Message.New()
+		msg.Chat = portal.Key
+		msg.MXID = resp.EventID
+		msg.JID = types.MessageID(resp.EventID)
+		msg.Timestamp = conv.LastMessageTimestamp
+		msg.Sent = true
+		msg.Insert()
+		return
+	}
+
 	user.log.Infofln("Backfilling %d messages in %s, %d messages at a time (queue ID: %d)", len(allMsgs), portal.Key.JID, req.MaxBatchEvents, req.QueueID)
 	toBackfill := allMsgs[0:]
 	var insertionEventIds []id.EventID