test(sdk): Improve m.fully_read test coverage and documentation

test(sdk): Improve `m.fully_read` test coverage and documentation

crates/matrix-sdk/src/room/timeline/event_handler.rs

@@ -646,6 +646,7 @@ pub(crate) fn update_read_marker(
     let Some(fully_read_event) = fully_read_event else { return };
     let read_marker_idx = find_read_marker(items_lock);
     let fully_read_event_idx = find_event_by_id(items_lock, fully_read_event).map(|(idx, _)| idx);
+
     match (read_marker_idx, fully_read_event_idx) {
         (None, None) => {}
         (None, Some(idx)) => {

crates/matrix-sdk/src/room/timeline/inner.rs

@@ -45,6 +45,8 @@ pub(super) struct TimelineInnerMetadata {
     // Reaction event / txn ID => sender and reaction data
     pub(super) reaction_map: HashMap<TimelineKey, (OwnedUserId, Annotation)>,
     pub(super) fully_read_event: Option<OwnedEventId>,
+    /// Whether the event that the fully-ready event _refers to_ is part of the
+    /// timeline.
     pub(super) fully_read_event_in_timeline: bool,
 }
 
@@ -184,7 +186,7 @@ impl<P: ProfileProvider> TimelineInner<P> {
     }
 
     pub(super) async fn handle_fully_read(&self, raw: Raw<FullyReadEvent>) {
-        let fully_read_event = match raw.deserialize() {
+        let fully_read_event_id = match raw.deserialize() {
             Ok(ev) => ev.content.event_id,
             Err(error) => {
                 error!(?error, "Failed to deserialize `m.fully_read` account data");
@@ -192,12 +194,13 @@ impl<P: ProfileProvider> TimelineInner<P> {
             }
         };
 
-        self.set_fully_read_event(fully_read_event).await;
+        self.set_fully_read_event(fully_read_event_id).await;
     }
 
     pub(super) async fn set_fully_read_event(&self, fully_read_event_id: OwnedEventId) {
         let mut metadata_lock = self.metadata.lock().await;
 
+        // A similar event has been handled already. We can ignore it.
         if metadata_lock.fully_read_event.as_ref().map_or(false, |id| *id == fully_read_event_id) {
             return;
         }

crates/matrix-sdk/src/room/timeline/tests.rs

@@ -273,11 +273,15 @@ async fn update_read_marker() {
     timeline.inner.set_fully_read_event(event_id.clone()).await;
     assert_matches!(stream.next().await, Some(VecDiff::Move { old_index: 2, new_index: 3 }));
 
+    // Nothing should happen if the fully read event is set back to the same event
+    // as before.
+    timeline.inner.set_fully_read_event(event_id.clone()).await;
+
     // Nothing should happen if the fully read event isn't found.
     timeline.inner.set_fully_read_event(event_id!("$fake_event_id").to_owned()).await;
 
-    // Nothing should happen if the fully read event is set back to the same event
-    // as before.
+    // Nothing should happen if the fully read event is referring to an old event
+    // that has already been marked as fully read.
     timeline.inner.set_fully_read_event(event_id).await;
 
     timeline.handle_live_message_event(&ALICE, RoomMessageEventContent::text_plain("C")).await;