bevy_ecs: flush entities after running observers and hooks in despawn

Author: jrobsonchase

crates/bevy_ecs/src/observer/mod.rs

@@ -1160,4 +1160,22 @@ mod tests {
         world.flush();
         assert_eq!(vec!["event", "event"], world.resource::<Order>().0);
     }
+
+    #[test]
+    fn observer_on_remove_during_despawn_spawn_empty() {
+        let mut world = World::new();
+
+        let ent = world
+            .spawn(A)
+            .observe(|trigger: Trigger<OnRemove, A>, mut cmd: Commands| {
+                cmd.spawn_empty();
+            })
+            .id();
+
+        world.flush();
+
+        world.entity_mut(ent).despawn();
+
+        world.flush();
+    }
 }

crates/bevy_ecs/src/world/entity_ref.rs

@@ -1319,6 +1319,10 @@ impl<'w> EntityWorldMut<'w> {
             }
         }
 
+        // Observers and on_remove hooks may reserve new entities, which
+        // requires a flush before Entities::free may be called.
+        world.flush_entities();
+
         for component_id in archetype.components() {
             world.removed_components.send(component_id, self.entity);
         }