Don't panic on race condition in client info updating

CHANGELOG.md

@@ -16,6 +16,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Make `NetworkChannels` channel-creation methods public (`create_client_channel()` and `create_server_channel()`).
 - Implement `Eq` and `PartialEq` on `EventType`.
 
+### Fixed
+
+- Don't panic when handling client acks if the ack references a despawned entity.
+
 ### Removed
 
 - `LastChangeTick` resource, `ClientsInfo` should be used instead.

src/server.rs

@@ -1,4 +1,4 @@
-pub(super) mod clients_info;
+pub mod clients_info;
 pub(super) mod despawn_buffer;
 pub(super) mod removal_buffer;
 pub(super) mod replicated_archetypes_info;
@@ -336,9 +336,8 @@ fn collect_changes(
                             component,
                         )?;
                     } else {
-                        let tick = *client_info
-                            .ticks
-                            .get(&entity.entity())
+                        let tick = client_info
+                            .get_change_limit(entity.entity())
                             .expect("entity should be present after adding component");
                         if ticks.is_changed(tick, change_tick.this_run()) {
                             update_message.write_component(
@@ -358,9 +357,7 @@ fn collect_changes(
                     // If there is any insertion or we must initialize, include all updates into init message
                     // and bump the last acknowledged tick to keep entity updates atomic.
                     init_message.take_entity_data(update_message)?;
-                    client_info
-                        .ticks
-                        .insert(entity.entity(), change_tick.this_run());
+                    client_info.set_change_limit(entity.entity(), change_tick.this_run());
                 } else {
                     update_message.end_entity_data()?;
                 }
@@ -420,7 +417,7 @@ fn collect_despawns(
     for entity in despawn_buffer.drain(..) {
         let mut shared_bytes = None;
         for (message, _, client_info) in messages.iter_mut_with_info() {
-            client_info.ticks.remove(&entity);
+            client_info.remove_despawned(entity);
             message.write_entity(&mut shared_bytes, entity)?;
         }
     }
@@ -446,7 +443,7 @@ fn collect_removals(
         for (message, _, client_info) in messages.iter_mut_with_info() {
             message.start_entity_data(entity);
             for &replication_id in components {
-                client_info.ticks.insert(entity, tick);
+                client_info.set_change_limit(entity, tick);
                 message.write_replication_id(replication_id)?;
             }
             message.end_entity_data(false)?;

src/server/clients_info.rs

@@ -89,10 +89,11 @@ pub(crate) struct ClientInfo {
     /// Indicates whether the client connected in this tick.
     pub(super) just_connected: bool,
 
-    /// Last acknowledged tick for each entity.
-    pub(super) ticks: EntityHashMap<Entity, Tick>,
+    /// Lowest tick for use in change detection for each entity.
+    ticks: EntityHashMap<Entity, Tick>,
 
-    /// The last tick in which a replicated entity was spawned, despawned, or gained/lost a component.
+    /// The last tick in which a replicated entity was spawned, despawned, or gained/lost a component from the perspective
+    /// of the client.
     ///
     /// It should be included in update messages and server events to avoid needless waiting for the next init message to arrive.
     pub(crate) change_tick: RepliconTick,
@@ -172,9 +173,22 @@ impl ClientInfo {
         (update_index, &mut update_info.entities)
     }
 
+    /// Sets the change limit for an entity that is replicated to this client.
+    ///
+    /// The change limit is the reference point for determining if components on an entity have changed and
+    /// need to be replicated. Component changes older than the change limit are assumed to be acked by the client.
+    pub(super) fn set_change_limit(&mut self, entity: Entity, tick: Tick) {
+        self.ticks.insert(entity, tick);
+    }
+
+    /// Gets the change limit for an entity that is replicated to this client.
+    pub(super) fn get_change_limit(&mut self, entity: Entity) -> Option<Tick> {
+        self.ticks.get(&entity).copied()
+    }
+
     /// Marks update with the specified index as acknowledged.
     ///
-    /// Ticks for all entities from this update will be set to the update's tick if it's higher.
+    /// Change limits for all entities from this update will be set to the update's tick if it's higher.
     ///
     /// Keeps allocated memory in the buffers for reuse.
     pub(super) fn acknowledge(
@@ -192,10 +206,10 @@ impl ClientInfo {
         };
 
         for entity in &update_info.entities {
-            let last_tick = self
-                .ticks
-                .get_mut(entity)
-                .expect("tick should be inserted on any component insertion");
+            let Some(last_tick) = self.ticks.get_mut(entity) else {
+                // We ignore missing entities, since they were probably despawned.
+                continue;
+            };
 
             // Received tick could be outdated because we bump it
             // if we detect any insertion on the entity in `collect_changes`.
@@ -212,6 +226,13 @@ impl ClientInfo {
         );
     }
 
+    /// Removes a despawned entity tracked by this client.
+    pub fn remove_despawned(&mut self, entity: Entity) {
+        self.ticks.remove(&entity);
+        // We don't clean up `self.updates` for efficiency reasons.
+        // `Self::acknowledge()` will properly ignore despawned entities.
+    }
+
     /// Removes all updates older then `min_timestamp`.
     ///
     /// Keeps allocated memory in the buffers for reuse.

tests/replication.rs

@@ -605,6 +605,7 @@ fn despawn_update_replication() {
 
     server_app.update();
     client_app.update();
+    server_app.update(); // Let server receive an update to trigger acknowledgment.
 
     assert!(client_app.world.entities().is_empty());
 }