Fix skipping items when paginating /relations forward.

changelog.d/13840.bugfix

@@ -0,0 +1,3 @@
+Fix a bug introduced in Synapse v1.53.0 where the experimental implementation of 
+[MSC3715](https://github.com/matrix-org/matrix-spec-proposals/pull/3715) would give incorrect results when paginating 
+forward.

synapse/storage/databases/main/relations.py

@@ -153,6 +153,12 @@ def _get_recent_references_for_event_txn(
             # If there are more events, generate the next pagination key.
             next_token = None
             if len(events) > limit and last_topo_id and last_stream_id:
+                # Due to how the pagination clause is generated, the stream ID is
+                # handled as an exclusive range when paginating forward. Correct
+                # for that here.
+                if direction == "f":
+                    last_stream_id -= 1
+
                 next_key = RoomStreamToken(last_topo_id, last_stream_id)
                 if from_token:
                     next_token = from_token.copy_and_replace(

tests/rest/client/test_relations.py

@@ -788,6 +788,7 @@ def test_basic_paginate_relations(self) -> None:
             channel.json_body["chunk"][0],
         )
 
+    @unittest.override_config({"experimental_features": {"msc3715_enabled": True}})
     def test_repeated_paginate_relations(self) -> None:
         """Test that if we paginate using a limit and tokens then we get the
         expected events.
@@ -809,7 +810,7 @@ def test_repeated_paginate_relations(self) -> None:
 
             channel = self.make_request(
                 "GET",
-                f"/_matrix/client/v1/rooms/{self.room}/relations/{self.parent_id}?limit=1{from_token}",
+                f"/_matrix/client/v1/rooms/{self.room}/relations/{self.parent_id}?limit=3{from_token}",
                 access_token=self.user_token,
             )
             self.assertEqual(200, channel.code, channel.json_body)
@@ -827,6 +828,32 @@ def test_repeated_paginate_relations(self) -> None:
         found_event_ids.reverse()
         self.assertEqual(found_event_ids, expected_event_ids)
 
+        # Test forward pagination.
+        prev_token = ""
+        found_event_ids = []
+        for _ in range(20):
+            from_token = ""
+            if prev_token:
+                from_token = "&from=" + prev_token
+
+            channel = self.make_request(
+                "GET",
+                f"/_matrix/client/v1/rooms/{self.room}/relations/{self.parent_id}?org.matrix.msc3715.dir=f&limit=3{from_token}",
+                access_token=self.user_token,
+            )
+            self.assertEqual(200, channel.code, channel.json_body)
+
+            found_event_ids.extend(e["event_id"] for e in channel.json_body["chunk"])
+            next_batch = channel.json_body.get("next_batch")
+
+            self.assertNotEqual(prev_token, next_batch)
+            prev_token = next_batch
+
+            if not prev_token:
+                break
+
+        self.assertEqual(found_event_ids, expected_event_ids)
+
     def test_pagination_from_sync_and_messages(self) -> None:
         """Pagination tokens from /sync and /messages can be used to paginate /relations."""
         channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "A")