Allow users that have left a room to get the messages that happend in the room before they left

synapse/handlers/message.py

@@ -83,21 +83,44 @@ class MessageHandler(BaseHandler):
         Returns:
             dict: Pagination API results
         """
-        yield self.auth.check_joined_room(room_id, user_id)
+        member_event = yield self.auth.check_user_was_in_room(room_id, user_id)
 
         data_source = self.hs.get_event_sources().sources["room"]
 
-        if not pagin_config.from_token:
+        if pagin_config.from_token:
+            room_token = pagin_config.from_token.room_key
+        else:
             pagin_config.from_token = (
                 yield self.hs.get_event_sources().get_current_token(
                     direction='b'
                 )
             )
+            room_token = pagin_config.from_token.room_key
 
-        room_token = RoomStreamToken.parse(pagin_config.from_token.room_key)
+        room_token = RoomStreamToken.parse(room_token)
         if room_token.topological is None:
             raise SynapseError(400, "Invalid token")
 
+        pagin_config.from_token = pagin_config.from_token.copy_and_replace(
+            "room_key", str(room_token)
+        )
+
+        source_config = pagin_config.get_source_config("room")
+
+        if member_event.membership == Membership.LEAVE:
+            # If they have left the room then clamp the token to be before
+            # they left the room
+            leave_token = yield self.store.get_topological_token_for_event(
+                member_event.event_id
+            )
+            leave_token = RoomStreamToken.parse(leave_token)
+            if leave_token.topological < room_token.topological:
+                source_config.from_key = str(leave_token)
+
+            if source_config.direction == "f":
+                if source_config.to_key is None:
+                    source_config.to_key = str(leave_token)
+
         yield self.hs.get_handlers().federation_handler.maybe_backfill(
             room_id, room_token.topological
         )
@@ -105,7 +128,7 @@ class MessageHandler(BaseHandler):
         user = UserID.from_string(user_id)
 
         events, next_key = yield data_source.get_pagination_rows(
-            user, pagin_config.get_source_config("room"), room_id
+            user, source_config, room_id
         )
 
         next_token = pagin_config.from_token.copy_and_replace(

synapse/storage/stream.py

@@ -386,7 +386,24 @@ class StreamStore(SQLBaseStore):
             table="events",
             keyvalues={"event_id": event_id},
             retcol="stream_ordering",
-        ).addCallback(lambda stream_ordering: "s%d" % (stream_ordering,))
+        ).addCallback(lambda row: "s%d" % (row,))
+
+    def get_topological_token_for_event(self, event_id):
+        """The stream token for an event
+        Args:
+            event_id(str): The id of the event to look up a stream token for.
+        Raises:
+            StoreError if the event wasn't in the database.
+        Returns:
+            A deferred "t%d-%d" topological token.
+        """
+        return self._simple_select_one(
+            table="events",
+            keyvalues={"event_id": event_id},
+            retcols=("stream_ordering", "topological_ordering"),
+        ).addCallback(lambda row: "t%d-%d" % (
+            row["topological_ordering"], row["stream_ordering"],)
+        )
 
     def _get_max_topological_txn(self, txn):
         txn.execute(