feat(sdk,ui): SlidingSync::subscribe_to_rooms has a new cancel_in_flight_request argument.

This patch adds a new `cancel_in_flight_request` argument to
`SlidingSync::subscribe_to_rooms`, which tells the method cancel the
in-flight request if any.

This patch also updates `RoomListService::subscribe_to_rooms` to turn
this new argument to `true` if the state machine isn't in a “starting”
state.

The problem it's solving is the following:

* some apps starts the room list service
* a first request is sent with `pos = None`
* the server calculates a new session (which can be expensive)
* the app subscribes to a set of rooms
* a second request is immediately sent with `pos = None` again
* the server does possibly NOT cancel its previous calculations, but
  starts a new session and its calculations

This is pretty expensive for the server. This patch makes so that the
immediate room subscriptions will be part of the second request, with
the first request not being cancelled.

crates/matrix-sdk-ui/src/notification_client.rs

@@ -380,6 +380,7 @@ impl NotificationClient {
                 required_state,
                 timeline_limit: Some(uint!(16))
             })),
+            true,
         );
 
         let mut remaining_attempts = 3;

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

@@ -396,7 +396,14 @@ impl RoomListService {
             settings.required_state.push((StateEventType::RoomCreate, "".to_owned()));
         }
 
-        self.sliding_sync.subscribe_to_rooms(room_ids, Some(settings))
+        let cancel_in_flight_request = match self.state.get() {
+            State::Init | State::Recovering | State::Error { .. } | State::Terminated { .. } => {
+                false
+            }
+            State::SettingUp | State::Running => true,
+        };
+
+        self.sliding_sync.subscribe_to_rooms(room_ids, Some(settings), cancel_in_flight_request)
     }
 
     #[cfg(test)]

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

@@ -151,12 +151,13 @@ impl SlidingSync {
         &self,
         room_ids: &[&RoomId],
         settings: Option<http::request::RoomSubscription>,
+        cancel_in_flight_request: bool,
     ) {
         let settings = settings.unwrap_or_default();
         let mut sticky = self.inner.sticky.write().unwrap();
         let room_subscriptions = &mut sticky.data_mut().room_subscriptions;
 
-        let mut skip_sync_loop = false;
+        let mut skip_over_current_sync_loop_iteration = false;
 
         for room_id in room_ids {
             // If the room subscription already exists, let's not
@@ -172,11 +173,11 @@ impl SlidingSync {
 
                 entry.insert((RoomSubscriptionState::default(), settings.clone()));
 
-                skip_sync_loop = true;
+                skip_over_current_sync_loop_iteration = true;
             }
         }
 
-        if skip_sync_loop {
+        if cancel_in_flight_request && skip_over_current_sync_loop_iteration {
             self.inner.internal_channel_send_if_possible(
                 SlidingSyncInternalMessage::SyncLoopSkipOverCurrentIteration,
             );
@@ -1219,7 +1220,7 @@ mod tests {
         // Members are now synced! We can start subscribing and see how it goes.
         assert!(room0.are_members_synced());
 
-        sliding_sync.subscribe_to_rooms(&[room_id_0, room_id_1], None);
+        sliding_sync.subscribe_to_rooms(&[room_id_0, room_id_1], None, true);
 
         // OK, we have subscribed to some rooms. Let's check on `room0` if members are
         // now marked as not synced.
@@ -1260,7 +1261,7 @@ mod tests {
         // Members are synced, good, good.
         assert!(room0.are_members_synced());
 
-        sliding_sync.subscribe_to_rooms(&[room_id_0], None);
+        sliding_sync.subscribe_to_rooms(&[room_id_0], None, false);
 
         // Members are still synced: because we have already subscribed to the
         // room, the members aren't marked as unsynced.
@@ -1280,7 +1281,7 @@ mod tests {
         let room_id_2 = room_id!("!r2:bar.org");
 
         // Subscribe to two rooms.
-        sliding_sync.subscribe_to_rooms(&[room_id_0, room_id_1], None);
+        sliding_sync.subscribe_to_rooms(&[room_id_0, room_id_1], None, false);
 
         {
             let sticky = sliding_sync.inner.sticky.read().unwrap();
@@ -1292,7 +1293,7 @@ mod tests {
         }
 
         // Subscribe to one more room.
-        sliding_sync.subscribe_to_rooms(&[room_id_2], None);
+        sliding_sync.subscribe_to_rooms(&[room_id_2], None, false);
 
         {
             let sticky = sliding_sync.inner.sticky.read().unwrap();
@@ -1314,7 +1315,7 @@ mod tests {
         }
 
         // Subscribe to one room again.
-        sliding_sync.subscribe_to_rooms(&[room_id_2], None);
+        sliding_sync.subscribe_to_rooms(&[room_id_2], None, false);
 
         {
             let sticky = sliding_sync.inner.sticky.read().unwrap();