Move ComputedRoomInfo creation to its own long-running task

This should remove another source of Futures churn.

Plus, it makes the code simpler as computed_roominfo doesn't need to be
an Option anymore in this new design.

src/buffers/room.rs

@@ -482,12 +482,12 @@ impl SingleClientRoomBuffer {
   }
 
   async fn back_pagination_worker(self) {
+    let room_id = self.room_id.clone();
     let mut request_subscriber = self.back_pagination_request.subscribe();
     let mut num_requested_items = request_subscriber.next_now();
     loop {
       if num_requested_items != 0 {
         let inner = self.inner.read().await;
-        let room_id = self.room_id.clone();
         let timeline = inner.timeline.clone();
         let back_pagination_status = timeline.back_pagination_status();
         match back_pagination_status.get() {
@@ -532,14 +532,10 @@ pub struct RoomBuffer {
 
   room_id: Arc<OwnedRoomId>,
 
-  computed_roominfo: Option<ComputedRoomInfo>,
+  computed_roominfo: ComputedRoomInfo,
 
-  poll_updates_rx: UnboundedReceiver<(SingleClientRoomBuffer, RoomInfo)>,
-  poll_updates_tx: UnboundedSender<(SingleClientRoomBuffer, RoomInfo)>,
-
-  /// Set while we are already computing roominfo, to block other updates to the room.
-  /// Holds the next `self.computed_roominfo`.
-  update_roominfo_rx: Option<oneshot::Receiver<ComputedRoomInfo>>,
+  computed_roominfo_rx: UnboundedReceiver<ComputedRoomInfo>,
+  roominfo_tx: UnboundedSender<(SingleClientRoomBuffer, RoomInfo)>,
 
   // It's unlikely users will join the same room with more than one account;
   // avoid a useless heap allocation for the usual case.
@@ -570,16 +566,26 @@ impl RoomBuffer {
     initial_client: Client,
     room_id: Arc<OwnedRoomId>,
   ) -> Result<Self> {
-    let (poll_updates_tx, poll_updates_rx) = unbounded_channel();
+    let (roominfo_tx, roominfo_rx) = unbounded_channel();
+    let (computed_roominfo_tx, computed_roominfo_rx) = unbounded_channel();
+
+    let room = initial_client
+      .get_room(&room_id)
+      .expect("client missing room");
+    let initial_roominfo_hash = hash_roominfo(room.clone_info());
+    let computed_roominfo = compute_room_info(room, initial_roominfo_hash).await;
+
+    tokio::task::spawn(update_roominfo_worker(room_id.clone(), initial_roominfo_hash, roominfo_rx, computed_roominfo_tx));
+
     let mut self_ = RoomBuffer {
       config,
       room_id,
-      computed_roominfo: None,
-      poll_updates_tx,
-      poll_updates_rx,
-      update_roominfo_rx: None,
+      computed_roominfo_rx,
+      roominfo_tx,
+      computed_roominfo,
       buffers: SmallVec::new(),
     };
+
     self_.add_client(initial_client.clone()).await?;
     Ok(self_)
   }
@@ -630,7 +636,7 @@ impl RoomBuffer {
     };
     self.buffers.push(buffer.clone());
     tokio::task::spawn(buffer.clone().poll_updates(
-      self.poll_updates_tx.clone(),
+      self.roominfo_tx.clone(),
       write_items,
       room.subscribe_info(),
       timeline_stream,
@@ -646,8 +652,7 @@ impl Buffer for RoomBuffer {
   fn short_name(&self) -> String {
     self
       .computed_roominfo
-      .as_ref()
-      .and_then(|roominfo| roominfo.display_name.as_ref())
+      .display_name.as_ref()
       .map(|dn| dn.to_string())
       .unwrap_or_else(|| {
         self
@@ -670,15 +675,13 @@ impl Buffer for RoomBuffer {
   fn parent(&self) -> Option<BufferId> {
     self
       .computed_roominfo
-      .as_ref()
-      .and_then(|roominfo| roominfo.parent.as_ref())
+      .parent.as_ref()
       .map(|parent| BufferId::Room(parent.clone()))
   }
   fn children(&self) -> Option<SortedVec<(BufferSortKey, BufferId)>> {
     self
       .computed_roominfo
-      .as_ref()
-      .and_then(|roominfo| roominfo.children.as_ref())
+      .children.as_ref()
       .map(|children: &SortedVec<_>| {
         let children = children
           .iter()
@@ -688,35 +691,7 @@ impl Buffer for RoomBuffer {
         unsafe { SortedVec::from_sorted(children) }
       })
   }
-
   async fn poll_updates_once(&mut self) {
-    if self.update_roominfo_rx.is_none() && self.computed_roominfo.is_none() {
-      // Initialize the computed_roominfo
-      let (tx, rx) = oneshot::channel();
-      self.update_roominfo_rx = Some(rx);
-      let room = self
-        .buffers
-        .first()
-        .expect("missing sub-buffer")
-        .client
-        .get_room(&self.room_id)
-        .expect("client missing room");
-      let roominfo_hash = hash_roominfo(room.clone_info());
-      tokio::spawn(compute_room_info(room, tx, roominfo_hash));
-    }
-
-    // Wait for result of processing new updates
-    let update_roominfo_rx = async {
-      match self.update_roominfo_rx.as_mut() {
-        Some(update_roominfo_rx) => Some(
-          update_roominfo_rx
-            .await
-            .unwrap_or_else(|_| panic!("update_roominfo_rx for {} was dropped", self.room_id)),
-        ),
-        None => std::future::pending().await,
-      }
-    };
-
     let mut poll_buffers_timeline: FuturesUnordered<_> = self
       .buffers
       .iter_mut()
@@ -727,29 +702,10 @@ impl Buffer for RoomBuffer {
       .collect();
 
     tokio::select! {
-      new_update = self.poll_updates_rx.recv() => {
-        tracing::trace!("Got update for {}", self.room_id);
-        let (buf, roominfo) = new_update.expect("poll_updates_rx closed");
-        let roominfo_hash = hash_roominfo(roominfo);
-        if let Some(computed_roominfo) = self.computed_roominfo.as_ref() {
-          if computed_roominfo.roominfo_hash == roominfo_hash {
-            // No visible change, exit early
-            tracing::trace!("no visible change to {}", self.room_id);
-            return;
-          }
-        }
-        tracing::trace!("visible change to {}", self.room_id);
-        let (tx, rx) = oneshot::channel();
-        self.update_roominfo_rx = Some(rx); // Prevents spawning more update_room_info() calls while
-                                            // this one is still running
-        let room = buf.client.get_room(&self.room_id).expect("client missing room");
-        tokio::spawn(compute_room_info(room, tx, roominfo_hash));
-      },
-      computed_roominfo = update_roominfo_rx => {
+      computed_roominfo = self.computed_roominfo_rx.recv() => {
         // Received result of a computation
         tracing::trace!("Got computed_roominfo {}", self.room_id);
-        self.computed_roominfo = computed_roominfo;
-        self.update_roominfo_rx = None;
+        self.computed_roominfo = computed_roominfo.expect("reached end of computed_roominfo_rx");
       }
       _ = poll_buffers_timeline.next() => {
       }
@@ -819,8 +775,7 @@ impl Buffer for RoomBuffer {
   fn fully_read(&self) -> FullyReadStatus {
     match self
       .computed_roominfo
-      .as_ref()
-      .and_then(|ri| ri.fully_read_at.as_ref())
+      .fully_read_at.as_ref()
     {
       None => FullyReadStatus::All, // Unknown, assume it's read for now, we'll probably find out later
       Some(fully_read_at) => {
@@ -891,11 +846,7 @@ impl Buffer for RoomBuffer {
   }
 }
 
-async fn compute_room_info(
-  room: Room,
-  done_tx: oneshot::Sender<ComputedRoomInfo>,
-  roominfo_hash: u64,
-) {
+async fn compute_room_info(room: Room, roominfo_hash: u64) -> ComputedRoomInfo {
   let (parent, children, fully_read_at, display_name) = tokio::join!(
     async {
       // Get parent space
@@ -1014,16 +965,13 @@ async fn compute_room_info(
   } else {
     tracing::debug!("{} has no parent", room.room_id());
   }
-  let computed_roominfo = ComputedRoomInfo {
+  ComputedRoomInfo {
     display_name,
     parent,
     children,
     fully_read_at,
     roominfo_hash,
-  };
-  done_tx
-    .send(computed_roominfo)
-    .unwrap_or_else(|e| tracing::error!("update_room_info could not notify it was done: {:?}", e));
+  }
 }
 
 #[derive(Debug)]
@@ -1046,3 +994,31 @@ fn hash_roominfo(mut roominfo: RoomInfo) -> u64 {
   format!("{:?}", roominfo).hash(&mut roominfo_hasher);
   roominfo_hasher.finish()
 }
+
+/// Gets updates from [`SingleClientRoomBuffer`], and aggregates them for a
+/// [`RoomBuffer`]
+async fn update_roominfo_worker(
+  room_id: Arc<OwnedRoomId>,
+  initial_roominfo_hash: u64,
+  mut roominfo_rx: UnboundedReceiver<(SingleClientRoomBuffer, RoomInfo)>,
+  computed_roominfo_tx: UnboundedSender<ComputedRoomInfo>,
+) {
+  let mut last_roominfo_hash = initial_roominfo_hash;
+  loop {
+    tracing::trace!("Got update for {}", room_id);
+    let (buf, roominfo) = roominfo_rx.recv().await.expect("poll_updates_rx closed");
+    let roominfo_hash = hash_roominfo(roominfo);
+    if last_roominfo_hash == roominfo_hash {
+      // No visible change, exit early
+      tracing::trace!("no visible change to {}", room_id);
+      continue;
+    }
+    last_roominfo_hash = roominfo_hash;
+    tracing::trace!("visible change to {}", room_id);
+    let room = buf
+      .client
+      .get_room(&room_id)
+      .expect("client missing room");
+    computed_roominfo_tx.send(compute_room_info(room, roominfo_hash).await).expect("failed to send to computed_roominfo_tx");
+  }
+}