From 3bbce154979d2dc8785448a7b1d49adc738af4ec Mon Sep 17 00:00:00 2001
From: Weizhen Huang <weizhen@blender.org>
Date: Thu, 28 Aug 2025 10:40:37 +0200
Subject: [PATCH] Fix: Cycles: Some volume octree nodes randomly disappearing

The function `recursive_build()` can change the `shared_ptr` by making
it an internal node. If `root_` is modified, it could happen that when
the children are accessing `root_->bbox.min` that memory is already
freed.

Storing `bbox_min` separately seems to fix the issue. No error is seen
after running the tests repeatedly for 2000 times.

Pull Request: https://projects.blender.org/blender/blender/pulls/145239
---
 intern/cycles/bvh/octree.cpp | 5 +++--
 intern/cycles/bvh/octree.h   | 3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/intern/cycles/bvh/octree.cpp b/intern/cycles/bvh/octree.cpp
index be51f1cd071..4e2e7f75cd2 100644
--- a/intern/cycles/bvh/octree.cpp
+++ b/intern/cycles/bvh/octree.cpp
@@ -50,7 +50,7 @@ Extrema<float> Octree::get_extrema(const int3 index_min, const int3 index_max) c
 
 __forceinline float3 Octree::position_to_index(const float3 p) const
 {
-  return (p - root_->bbox.min) * position_to_index_scale_;
+  return (p - bbox_min) * position_to_index_scale_;
 }
 
 int3 Octree::position_to_floor_index(const float3 p) const
@@ -71,7 +71,7 @@ int3 Octree::position_to_ceil_index(const float3 p) const
 
 __forceinline float3 Octree::index_to_position(int x, int y, int z) const
 {
-  return root_->bbox.min + make_float3(x, y, z) * index_to_position_scale_;
+  return bbox_min + make_float3(x, y, z) * index_to_position_scale_;
 }
 
 __forceinline float3 Octree::voxel_size() const
@@ -380,6 +380,7 @@ void Octree::build(Device *device,
 
 Octree::Octree(const BoundBox &bbox)
 {
+  bbox_min = bbox.min;
   root_ = std::make_shared<OctreeNode>(bbox, 0);
   is_built_ = false;
   is_flattened_ = false;
diff --git a/intern/cycles/bvh/octree.h b/intern/cycles/bvh/octree.h
index 7826c4ff008..993468b95f9 100644
--- a/intern/cycles/bvh/octree.h
+++ b/intern/cycles/bvh/octree.h
@@ -123,6 +123,9 @@ class Octree {
   /* Root node. */
   std::shared_ptr<OctreeNode> root_;
 
+  /* Bounding box min of the octree, used for computing the indices. */
+  float3 bbox_min;
+
   /* Whether the octree is already built. */
   bool is_built_;