feat(event cache): auto-shrink a room event cache's chunk after all listeners are left

Author: bnjbvr

crates/matrix-sdk-ui/src/timeline/builder.rs

@@ -277,6 +277,7 @@ impl TimelineBuilder {
                                     match origin {
                                         EventsOrigin::Sync => RemoteEventOrigin::Sync,
                                         EventsOrigin::Pagination => RemoteEventOrigin::Pagination,
+                                        EventsOrigin::Cache => RemoteEventOrigin::Cache,
                                     },
                                 )
                                 .await;

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

@@ -57,7 +57,7 @@ use ruma::{
 };
 use tokio::sync::{
     broadcast::{error::RecvError, Receiver},
-    Mutex, RwLock,
+    mpsc, Mutex, RwLock,
 };
 use tracing::{error, info, info_span, instrument, trace, warn, Instrument as _, Span};
 
@@ -129,6 +129,9 @@ pub struct EventCacheDropHandles {
 
     /// Task that listens to updates to the user's ignored list.
     ignore_user_list_update_task: JoinHandle<()>,
+
+    /// The task used to automatically shrink the linked chunks.
+    auto_shrink_linked_chunk_task: JoinHandle<()>,
 }
 
 impl Debug for EventCacheDropHandles {
@@ -141,6 +144,7 @@ impl Drop for EventCacheDropHandles {
     fn drop(&mut self) {
         self.listen_updates_task.abort();
         self.ignore_user_list_update_task.abort();
+        self.auto_shrink_linked_chunk_task.abort();
     }
 }
 
@@ -172,6 +176,7 @@ impl EventCache {
                 by_room: Default::default(),
                 drop_handles: Default::default(),
                 all_events: Default::default(),
+                auto_shrink_sender: Default::default(),
             }),
         }
     }
@@ -213,7 +218,19 @@ impl EventCache {
                 client.subscribe_to_ignore_user_list_changes(),
             ));
 
-            Arc::new(EventCacheDropHandles { listen_updates_task, ignore_user_list_update_task })
+            let (tx, rx) = mpsc::channel(32);
+
+            // Force-initialize the sender in the [`RoomEventCacheInner`].
+            self.inner.auto_shrink_sender.get_or_init(|| tx);
+
+            let auto_shrink_linked_chunk_tasks =
+                spawn(Self::auto_shrink_linked_chunk_task(self.inner.clone(), rx));
+
+            Arc::new(EventCacheDropHandles {
+                listen_updates_task,
+                ignore_user_list_update_task,
+                auto_shrink_linked_chunk_task: auto_shrink_linked_chunk_tasks,
+            })
         });
 
         Ok(())
@@ -309,6 +326,60 @@ impl EventCache {
         }
     }
 
+    /// Spawns the task that will listen to auto-shrink notifications.
+    ///
+    /// The auto-shrink mechanism works this way:
+    ///
+    /// - Each time there's a new subscriber to a [`RoomEventCache`], it will
+    ///   increment the active number of listeners to that room, aka
+    ///   [`RoomEventCacheState::listener_count`].
+    /// - When that subscriber is dropped, it will decrement that count; and
+    ///   notify the task below if it reached 0.
+    /// - The task spawned here, owned by the [`EventCacheInner`], will listen
+    ///   to such notifications that a room may be shrunk. It will attempt an
+    ///   auto-shrink, by letting the inner state decide whether this is a good
+    ///   time to do so (new listeners might have spawned in the meanwhile).
+    #[instrument(skip_all)]
+    async fn auto_shrink_linked_chunk_task(
+        inner: Arc<EventCacheInner>,
+        mut rx: mpsc::Receiver<AutoShrinkChannelPayload>,
+    ) {
+        while let Some(room_id) = rx.recv().await {
+            let room = match inner.for_room(&room_id).await {
+                Ok(room) => room,
+                Err(err) => {
+                    warn!(for_room = %room_id, "error when getting a RoomEventCache: {err}");
+                    continue;
+                }
+            };
+
+            let mut state = room.inner.state.write().await;
+
+            match state.auto_shrink_if_no_listeners().await {
+                Ok(diffs) => {
+                    if let Some(diffs) = diffs {
+                        // Hey, fun stuff: we shrunk the linked chunk, so there shouldn't be any
+                        // listeners, right? RIGHT? Especially because the state is guarded behind
+                        // a lock.
+                        //
+                        // However, better safe than sorry, and it's cheap to send an update here,
+                        // so let's do it!
+                        let _ =
+                            room.inner.sender.send(RoomEventCacheUpdate::UpdateTimelineEvents {
+                                diffs,
+                                origin: EventsOrigin::Cache,
+                            });
+                    }
+                }
+
+                Err(err) => {
+                    // There's not much we can do here, unfortunately.
+                    warn!(for_room = %room_id, "error when attempting to shrink linked chunk: {err}");
+                }
+            }
+        }
+    }
+
     /// Return a room-specific view over the [`EventCache`].
     pub(crate) async fn for_room(
         &self,
@@ -534,8 +605,18 @@ struct EventCacheInner {
 
     /// Handles to keep alive the task listening to updates.
     drop_handles: OnceLock<Arc<EventCacheDropHandles>>,
+
+    /// A sender for notifications that a room *may* need to be auto-shrunk.
+    ///
+    /// Needs to live here, so it may be passed to each [`RoomEventCache`]
+    /// instance.
+    ///
+    /// See doc comment of [`EventCache::auto_shrink_linked_chunk_task`].
+    auto_shrink_sender: OnceLock<mpsc::Sender<AutoShrinkChannelPayload>>,
 }
 
+type AutoShrinkChannelPayload = OwnedRoomId;
+
 impl EventCacheInner {
     fn client(&self) -> Result<Client> {
         self.client.get().ok_or(EventCacheError::ClientDropped)
@@ -644,12 +725,20 @@ impl EventCacheInner {
                         RoomVersionId::V1
                     });
 
+                // SAFETY: we must have subscribed before reaching this coed, otherwise
+                // something is very wrong.
+                let auto_shrink_sender =
+                    self.auto_shrink_sender.get().cloned().expect(
+                        "we must have called `EventCache::subscribe()` before calling here.",
+                    );
+
                 let room_event_cache = RoomEventCache::new(
                     self.client.clone(),
                     room_state,
                     room_id.to_owned(),
                     room_version,
                     self.all_events.clone(),
+                    auto_shrink_sender,
                 );
 
                 by_room_guard.insert(room_id.to_owned(), room_event_cache.clone());
@@ -718,6 +807,9 @@ pub enum EventsOrigin {
 
     /// Events are coming from pagination.
     Pagination,
+
+    /// The cause of the change is purely internal to the cache.
+    Cache,
 }
 
 #[cfg(test)]