From 336583a86bdaef6339c60fc00c52d469727e262e Mon Sep 17 00:00:00 2001
From: davier <bricedavier@gmail.com>
Date: Tue, 10 Aug 2021 01:12:42 +0000
Subject: [PATCH] Update EntityMut's location in push_children() and
 insert_children() (#2604)

## Objective

This code would result in a crash:
```rust
use bevy::prelude::*;

fn main() {
    let mut world = World::new();
    let child = world.spawn().id();
    world.spawn().push_children(&[child]);
}
```

## Solution

Update the `EntityMut`'s location after inserting a component on the children entities, as it may have changed.
---
 .../bevy_transform/src/hierarchy/child_builder.rs | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/crates/bevy_transform/src/hierarchy/child_builder.rs b/crates/bevy_transform/src/hierarchy/child_builder.rs
index 9731c8c02b0e5..71fc54aeb24ce 100644
--- a/crates/bevy_transform/src/hierarchy/child_builder.rs
+++ b/crates/bevy_transform/src/hierarchy/child_builder.rs
@@ -219,7 +219,7 @@ impl<'w> BuildWorldChildren for EntityMut<'w> {
     fn push_children(&mut self, children: &[Entity]) -> &mut Self {
         let parent = self.id();
         {
-            // SAFE: parent entity is not modified
+            // SAFE: parent entity is not modified and its location is updated manually
             let world = unsafe { self.world_mut() };
             for child in children.iter() {
                 world
@@ -227,6 +227,8 @@ impl<'w> BuildWorldChildren for EntityMut<'w> {
                     // FIXME: don't erase the previous parent (see #1545)
                     .insert_bundle((Parent(parent), PreviousParent(parent)));
             }
+            // Inserting a bundle in the children entities may change the parent entity's location if they were of the same archetype
+            self.update_location();
         }
         if let Some(mut children_component) = self.get_mut::<Children>() {
             children_component.0.extend(children.iter().cloned());
@@ -239,7 +241,7 @@ impl<'w> BuildWorldChildren for EntityMut<'w> {
     fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
         let parent = self.id();
         {
-            // SAFE: parent entity is not modified
+            // SAFE: parent entity is not modified and its location is updated manually
             let world = unsafe { self.world_mut() };
             for child in children.iter() {
                 world
@@ -247,6 +249,8 @@ impl<'w> BuildWorldChildren for EntityMut<'w> {
                     // FIXME: don't erase the previous parent (see #1545)
                     .insert_bundle((Parent(parent), PreviousParent(parent)));
             }
+            // Inserting a bundle in the children entities may change the parent entity's location if they were of the same archetype
+            self.update_location();
         }
 
         if let Some(mut children_component) = self.get_mut::<Children>() {
@@ -471,4 +475,11 @@ mod tests {
             PreviousParent(parent)
         );
     }
+
+    #[test]
+    fn regression_push_children_same_archetype() {
+        let mut world = World::new();
+        let child = world.spawn().id();
+        world.spawn().push_children(&[child]);
+    }
 }