Implement message forward pagination from start when no from is given, fixes #12383 (#14149)

Fixes https://github.com/matrix-org/synapse/issues/12383

changelog.d/14149.bugfix

@@ -0,0 +1 @@
+Fix #12383: paginate room messages from the start if no from is given. Contributed by @gnunicorn .

synapse/handlers/pagination.py

@@ -448,6 +448,12 @@ class PaginationHandler:
 
         if pagin_config.from_token:
             from_token = pagin_config.from_token
+        elif pagin_config.direction == "f":
+            from_token = (
+                await self.hs.get_event_sources().get_start_token_for_pagination(
+                    room_id
+                )
+            )
         else:
             from_token = (
                 await self.hs.get_event_sources().get_current_token_for_pagination(

synapse/streams/events.py

@@ -73,6 +73,19 @@ class EventSources:
         )
         return token
 
+    @trace
+    async def get_start_token_for_pagination(self, room_id: str) -> StreamToken:
+        """Get the start token for a given room to be used to paginate
+        events.
+
+        The returned token does not have the current values for fields other
+        than `room`, since they are not used during pagination.
+
+        Returns:
+            The start token for pagination.
+        """
+        return StreamToken.START
+
     @trace
     async def get_current_token_for_pagination(self, room_id: str) -> StreamToken:
         """Get the current token for a given room to be used to paginate

tests/rest/admin/test_room.py

@@ -1857,6 +1857,46 @@ class RoomMessagesTestCase(unittest.HomeserverTestCase):
         self.assertIn("chunk", channel.json_body)
         self.assertIn("end", channel.json_body)
 
+    def test_room_messages_backward(self) -> None:
+        """Test room messages can be retrieved by an admin that isn't in the room."""
+        latest_event_id = self.helper.send(
+            self.room_id, body="message 1", tok=self.user_tok
+        )["event_id"]
+
+        # Check that we get the first and second message when querying /messages.
+        channel = self.make_request(
+            "GET",
+            "/_synapse/admin/v1/rooms/%s/messages?dir=b" % (self.room_id,),
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        chunk = channel.json_body["chunk"]
+        self.assertEqual(len(chunk), 6, [event["content"] for event in chunk])
+
+        # in backwards, this is the first event
+        self.assertEqual(chunk[0]["event_id"], latest_event_id)
+
+    def test_room_messages_forward(self) -> None:
+        """Test room messages can be retrieved by an admin that isn't in the room."""
+        latest_event_id = self.helper.send(
+            self.room_id, body="message 1", tok=self.user_tok
+        )["event_id"]
+
+        # Check that we get the first and second message when querying /messages.
+        channel = self.make_request(
+            "GET",
+            "/_synapse/admin/v1/rooms/%s/messages?dir=f" % (self.room_id,),
+            access_token=self.admin_user_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        chunk = channel.json_body["chunk"]
+        self.assertEqual(len(chunk), 6, [event["content"] for event in chunk])
+
+        # in forward, this is the last event
+        self.assertEqual(chunk[5]["event_id"], latest_event_id)
+
     def test_room_messages_purge(self) -> None:
         """Test room messages can be retrieved by an admin that isn't in the room."""
         store = self.hs.get_datastores().main