Fix: Potential use after scope in curve to mesh node

I don't think this has been visible, since I only ran into it after
changing other code that affected this. However, some attributes
can keep a reference to the source component to use when tagging
caches dirty (like the position attribute tagging the normals dirty).
Here, the component was created inside a function, then the attributes
were used afterwards.

Also add some comments warning about this in the header file.

2 files changed, 16 insertions, 7 deletions

diff --git a/source/blender/blenkernel/BKE_attribute_access.hh b/source/blender/blenkernel/BKE_attribute_access.hh
index fd30813a506..ae9969aae82 100644
--- a/source/blender/blenkernel/BKE_attribute_access.hh
+++ b/source/blender/blenkernel/BKE_attribute_access.hh
@@ -181,11 +181,14 @@ struct ReadAttributeLookup {
  * Used when looking up a "plain attribute" based on a name for reading from it and writing to it.
  */
 struct WriteAttributeLookup {
-  /* The virtual array that is used to read from and write to the attribute. */
+  /** The virtual array that is used to read from and write to the attribute. */
   GVMutableArray varray;
-  /* Domain the attributes lives on in the geometry. */
+  /** Domain the attributes lives on in the geometry component. */
   AttributeDomain domain;
-  /* Call this after changing the attribute to invalidate caches that depend on this attribute. */
+  /**
+   * Call this after changing the attribute to invalidate caches that depend on this attribute.
+   * \note Do not call this after the component the attribute is from has been destructed.
+   */
   std::function<void()> tag_modified_fn;
 
   /* Convenience function to check if the attribute has been found. */
@@ -205,6 +208,10 @@ struct WriteAttributeLookup {
  *   VMutableArray_Span in many cases).
  * - An output attribute can live side by side with an existing attribute with a different domain
  *   or data type. The old attribute will only be overwritten when the #save function is called.
+ *
+ * \note The lifetime of an output attribute should not be longer than the the lifetime of the
+ * geometry component it comes from, since it can keep a reference to the component for use in
+ * the #save method.
  */
 class OutputAttribute {
  public:
diff --git a/source/blender/blenkernel/intern/curve_to_mesh_convert.cc b/source/blender/blenkernel/intern/curve_to_mesh_convert.cc
index 5522a84d094..8c256da44cd 100644
--- a/source/blender/blenkernel/intern/curve_to_mesh_convert.cc
+++ b/source/blender/blenkernel/intern/curve_to_mesh_convert.cc
@@ -401,10 +401,8 @@ struct ResultAttributes {
 };
 static ResultAttributes create_result_attributes(const CurveEval &curve,
                                                  const CurveEval &profile,
-                                                 Mesh &mesh)
+                                                 MeshComponent &mesh_component)
 {
-  MeshComponent mesh_component;
-  mesh_component.replace(&mesh, GeometryOwnershipType::Editable);
   Set<AttributeIDRef> curve_attributes;
 
   /* In order to prefer attributes on the main curve input when there are name collisions, first
@@ -708,7 +706,11 @@ Mesh *curve_to_mesh_sweep(const CurveEval &curve, const CurveEval &profile, cons
   mesh->smoothresh = DEG2RADF(180.0f);
   BKE_mesh_normals_tag_dirty(mesh);
 
-  ResultAttributes attributes = create_result_attributes(curve, profile, *mesh);
+  /* Create the mesh component for retrieving attributes at this scope, since output attributes
+   * can keep a reference to the component for updating after retrieving write access. */
+  MeshComponent mesh_component;
+  mesh_component.replace(mesh, GeometryOwnershipType::Editable);
+  ResultAttributes attributes = create_result_attributes(curve, profile, mesh_component);
 
   threading::parallel_for(curves.index_range(), 128, [&](IndexRange curves_range) {
     for (const int i_spline : curves_range) {