Mark chat as fully read on Telegram if read receipt target is unknown

mautrix_telegram/portal/matrix.py

@@ -112,11 +112,19 @@ class PortalMatrix(BasePortal, ABC):
         space = self.tgid if self.peer_type == "channel" else user.tgid
         message = DBMessage.get_by_mxid(event_id, self.mxid, space)
         if not message:
-            self.log.debug(f"Dropping Matrix read receipt from {user.mxid}: "
-                           f"target message {event_id} not known")
-            return
-        self.log.debug(f"Marking messages up to {message.mxid}/{message.tgid} "
-                       f"as read by {user.mxid}/{user.tgid}")
+            message = DBMessage.find_last(self.mxid, space)
+            if not message:
+                self.log.debug(f"Dropping Matrix read receipt from {user.mxid}: "
+                               f"target message {event_id} not known and last message"
+                               " in chat not found")
+                return
+            else:
+                self.log.debug(f"Matrix read receipt target {event_id} not known, marking "
+                               f"messages up to most recent ({message.mxid}/{message.tgid}) "
+                               f"as read by {user.mxid}/{user.tgid}")
+        else:
+            self.log.debug("Handling Matrix read receipt: marking messages up to "
+                           f"{message.mxid}/{message.tgid} as read by {user.mxid}/{user.tgid}")
         await user.client.send_read_acknowledge(self.peer, max_id=message.tgid,
                                                 clear_mentions=True)