From 1c06153a0d3c24039a70b0c770947874bc05c246 Mon Sep 17 00:00:00 2001
From: Eric Eastwood <eric.eastwood@beta.gouv.fr>
Date: Tue, 18 Jun 2024 21:22:40 -0500
Subject: [PATCH] Determine limited before filtering

---
 synapse/handlers/sliding_sync.py | 27 ++++++++++++++++-----------
 tests/rest/client/test_sync.py   |  8 ++++++++
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index e418a6e074..fe369949c5 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -804,17 +804,9 @@ class SlidingSyncHandler:
             # most recent).
             timeline_events.reverse()
 
-            # Make sure we don't expose any events that the client shouldn't see
-            timeline_events = await filter_events_for_client(
-                self.storage_controllers,
-                user.to_string(),
-                timeline_events,
-                is_peeking=rooms_for_user_membership_at_to_token.membership
-                != Membership.JOIN,
-                filter_send_to_client=True,
-            )
-
-            # Determine our `limited` status
+            # Determine our `limited` status based on the timeline. We do this before
+            # filtering the events so we can accurately determine if there is more to
+            # paginate even if we filter out some/all events.
             if len(timeline_events) > room_sync_config.timeline_limit:
                 limited = True
                 # Get rid of that extra "+ 1" event because we only used it to determine
@@ -825,6 +817,19 @@ class SlidingSyncHandler:
                     stream=timeline_events[0].internal_metadata.stream_ordering - 1
                 )
 
+            # TODO: Does `newly_joined` affect `limited`? It does in sync v2 but I fail
+            # to understand why.
+
+            # Make sure we don't expose any events that the client shouldn't see
+            timeline_events = await filter_events_for_client(
+                self.storage_controllers,
+                user.to_string(),
+                timeline_events,
+                is_peeking=rooms_for_user_membership_at_to_token.membership
+                != Membership.JOIN,
+                filter_send_to_client=True,
+            )
+
             # Determine how many "live" events we have (events within the given token range).
             #
             # This is mostly useful to determine whether a given @mention event should
diff --git a/tests/rest/client/test_sync.py b/tests/rest/client/test_sync.py
index 838ff6e2b4..df85c94bd5 100644
--- a/tests/rest/client/test_sync.py
+++ b/tests/rest/client/test_sync.py
@@ -1874,6 +1874,13 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
             [],
             channel.json_body["rooms"][room_id1]["timeline"],
         )
+        # Even though we don't get any timeline events because they are filtered out,
+        # there is still more to paginate
+        self.assertEqual(
+            channel.json_body["rooms"][room_id1]["limited"],
+            True,
+            channel.json_body["rooms"][room_id1],
+        )
         # We should have some `stripped_state` so the potential joiner can identify the
         # room (we don't care about the order).
         self.assertCountEqual(
@@ -1906,3 +1913,4 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
             ],
             channel.json_body["rooms"][room_id1]["invite_state"],
         )
+