fix(event cache): wait for the initial previous-batch token, if there…

… wasn't any

crates/matrix-sdk/src/event_cache/room/mod.rs

@@ -849,9 +849,16 @@ mod private {
                         new_first_chunk
                     }
                     Ok(None) => {
-                        // No previous chunk: no events to insert. Better, it means we've reached
-                        // the start of the timeline!
-                        return Ok(LoadMoreEventsBackwardsOutcome::StartOfTimeline);
+                        // No previous chunk: no events to insert. This means one of two things:
+                        // - either the linked chunk is at the start of the timeline,
+                        // - or we haven't received any back-pagination token yet, and we should
+                        //   wait for one.
+                        if self.waited_for_initial_prev_token {
+                            return Ok(LoadMoreEventsBackwardsOutcome::StartOfTimeline);
+                        }
+                        // If we haven't waited yet, we request to resolve the gap, once we get the
+                        // previous-batch token from sync.
+                        return Ok(LoadMoreEventsBackwardsOutcome::Gap);
                     }
                     Err(err) => {
                         error!("error when loading the previous chunk of a linked chunk: {err}");
@@ -1070,7 +1077,6 @@ mod private {
         #[instrument(skip_all)]
         async fn propagate_changes(&mut self) -> Result<(), EventCacheError> {
             let updates = self.events.store_updates().take();
-
             self.send_updates_to_store(updates).await
         }
 
@@ -1182,8 +1188,19 @@ mod private {
             func: F,
         ) -> Result<(O, Vec<VectorDiff<TimelineEvent>>), EventCacheError> {
             let output = func(&mut self.events);
+
             self.propagate_changes().await?;
+
+            // If we've never waited for an initial previous-batch token, and we now have at
+            // least one gap in the chunk, no need to wait for a previous-batch token later.
+            if !self.waited_for_initial_prev_token
+                && self.events.chunks().any(|chunk| chunk.is_gap())
+            {
+                self.waited_for_initial_prev_token = true;
+            }
+
             let updates_as_vector_diffs = self.events.updates_as_vector_diffs();
+
             Ok((output, updates_as_vector_diffs))
         }
     }

crates/matrix-sdk/tests/integration/event_cache.rs

@@ -832,6 +832,70 @@ async fn test_limited_timeline_resets_pagination() {
     assert!(room_stream.is_empty());
 }
 
+#[async_test]
+async fn test_persistent_storage_waits_for_pagination_token() {
+    let server = MatrixMockServer::new().await;
+    let client = server.client_builder().build().await;
+
+    let event_cache = client.event_cache();
+
+    // Immediately subscribe the event cache to sync updates.
+    event_cache.subscribe().unwrap();
+    // TODO: remove this test when persistent storage is enabled by default, as it's
+    // doing the same as the one above.
+    event_cache.enable_storage().unwrap();
+
+    // If I sync and get informed I've joined The Room, without a previous batch
+    // token,
+    let room_id = room_id!("!omelette:fromage.fr");
+    let f = EventFactory::new().room(room_id).sender(user_id!("@a:b.c"));
+    let room = server.sync_joined_room(&client, room_id).await;
+
+    let (room_event_cache, _drop_handles) = room.event_cache().await.unwrap();
+
+    let (events, mut room_stream) = room_event_cache.subscribe().await;
+
+    assert!(events.is_empty());
+    assert!(room_stream.is_empty());
+
+    server
+        .mock_room_messages()
+        .ok(RoomMessagesResponseTemplate::default()
+            .events(vec![f.text_msg("hi").event_id(event_id!("$2")).into_raw_timeline()]))
+        .mock_once()
+        .mount()
+        .await;
+
+    // At the beginning, the paginator is in the initial state.
+    let pagination = room_event_cache.pagination();
+    let mut pagination_status = pagination.status();
+    assert_eq!(pagination_status.get(), PaginatorState::Initial);
+
+    // If we try to back-paginate with a token, it will hit the end of the timeline
+    // and give us the resulting event.
+    let BackPaginationOutcome { events, reached_start } =
+        pagination.run_backwards_once(20).await.unwrap();
+
+    assert_eq!(events.len(), 1);
+    assert!(reached_start);
+
+    assert_let_timeout!(
+        Ok(RoomEventCacheUpdate::UpdateTimelineEvents { diffs, .. }) = room_stream.recv()
+    );
+    assert_eq!(diffs.len(), 1);
+    assert_matches!(&diffs[0], VectorDiff::Append { values: events } => {
+        assert_eq!(events.len(), 1);
+        assert_event_matches_msg(&events[0], "hi");
+    });
+
+    // And the paginator state delivers this as an update, and is internally
+    // consistent with it:
+    assert_next_matches_with_timeout!(pagination_status, PaginatorState::Idle);
+    assert!(pagination.hit_timeline_start());
+
+    assert!(room_stream.is_empty());
+}
+
 #[async_test]
 async fn test_limited_timeline_with_storage() {
     let server = MatrixMockServer::new().await;