event cache: add support for running back-pagination (#3195)

This adds support for back-pagination into the event cache, supporting enough features for integrating with the timeline (which is going to happen in a separate PR).

The idea is to provide two new primitives:

- one to get (or wait, if we don't have any handy) the latest pagination token received by the sync,
- one to run a single back-pagination, given a token (or not — which will backpaginate from the end of the room's timeline)

The timeline code can then use those two primitives in a loop to replicate the current behavior it has (next PR to be open Soon™).

The representation of events in the store is changed, so that a timeline can have *entries*, which are one of two things:

- either an event, as before
- or a gap, identified by a backpagination token (at the moment)

This allows us to avoid a lot of complexity from the back-pagination code in the timeline, where we'd attach the backpagination token to an event that only had an event_id. We don't have to do this here, and I suppose we could even attach the backpagination token to the next event itself.

This doesn't do reconciliation yet; the plan is to add it as a next step.

Author: bnjbvr

crates/matrix-sdk-ui/src/room_list_service/room.rs

@@ -168,11 +168,10 @@ impl Room {
             .add_initial_events(
                 self.inner.room.room_id(),
                 self.inner.sliding_sync_room.timeline_queue().iter().cloned().collect(),
+                self.inner.sliding_sync_room.prev_batch(),
             )
             .await?;
 
-        Ok(Timeline::builder(&self.inner.room)
-            .with_pagination_token(self.inner.sliding_sync_room.prev_batch())
-            .track_read_marker_and_receipts())
+        Ok(Timeline::builder(&self.inner.room).track_read_marker_and_receipts())
     }
 }

crates/matrix-sdk-ui/src/timeline/tests/pagination.rs

@@ -22,7 +22,7 @@ use super::TestTimeline;
 use crate::timeline::{inner::HandleBackPaginatedEventsError, pagination::PaginationTokens};
 
 #[async_test]
-async fn back_pagination_token_not_updated_with_empty_chunk() {
+async fn test_back_pagination_token_not_updated_with_empty_chunk() {
     let timeline = TestTimeline::new();
 
     timeline
@@ -65,7 +65,7 @@ async fn back_pagination_token_not_updated_with_empty_chunk() {
 }
 
 #[async_test]
-async fn back_pagination_token_not_updated_invalid_event() {
+async fn test_back_pagination_token_not_updated_invalid_event() {
     let timeline = TestTimeline::new();
 
     // Invalid empty event.

crates/matrix-sdk-ui/tests/integration/timeline/sliding_sync.rs

@@ -258,14 +258,14 @@ async fn timeline_test_helper(
     // TODO: when the event cache handles its own cache, we can remove this.
     client
         .event_cache()
-        .add_initial_events(room_id, sliding_sync_room.timeline_queue().iter().cloned().collect())
+        .add_initial_events(
+            room_id,
+            sliding_sync_room.timeline_queue().iter().cloned().collect(),
+            sliding_sync_room.prev_batch(),
+        )
         .await?;
 
-    let timeline = Timeline::builder(&sdk_room)
-        .with_pagination_token(sliding_sync_room.prev_batch())
-        .track_read_marker_and_receipts()
-        .build()
-        .await?;
+    let timeline = Timeline::builder(&sdk_room).track_read_marker_and_receipts().build().await?;
 
     Ok(timeline.subscribe().await)
 }