Fix T99256: Regression: Meta balls segfaulting copy-to-selected.

Revealed by rB43167a2c251b, but actuall issue is the
`rna_MetaBall_update_data` function expecting a never-NULL `scene`
pointer, which is not guaranteed.

This lead to refactoring the duo
`rna_MetaBall_update_data`/`BKE_mball_properties_copy`, since it was
doing a very sub-optimal O(n^2) process, new code is O(2n) now
(n being the number of Objects).

Not sure why the objects were processed through the existing bases of
the existing scene in the first place, this could miss a lot of affected
objects (e.g. in case said objects are in an excluded collection, etc.).

Also noticed that both old and new implementation can fail to fully propagate
changes to all affected meta-balls in some specific corner cases, added
a comment about it in the code.

Reviewed By: sergey

Maniphest Tasks: T99256

Differential Revision: https://developer.blender.org/D15338

1 files changed, 17 insertions, 5 deletions

diff --git a/source/blender/blenkernel/BKE_mball.h b/source/blender/blenkernel/BKE_mball.h
index f40d0bb3004..5a4988c7a5f 100644
--- a/source/blender/blenkernel/BKE_mball.h
+++ b/source/blender/blenkernel/BKE_mball.h
@@ -24,14 +24,25 @@ struct MetaBall *BKE_mball_add(struct Main *bmain, const char *name);
 bool BKE_mball_is_any_selected(const struct MetaBall *mb);
 bool BKE_mball_is_any_selected_multi(struct Base **bases, int bases_len);
 bool BKE_mball_is_any_unselected(const struct MetaBall *mb);
-bool BKE_mball_is_basis_for(struct Object *ob1, struct Object *ob2);
+
+/**
+ * Return `true` if `ob1` and `ob2` are part of the same metaBall group.
+ *
+ * \note Currently checks whether their two base names (without numerical suffix) is the same.
+ */
+bool BKE_mball_is_same_group(const struct Object *ob1, const struct Object *ob2);
+/**
+ * Return `true` if `ob1` and `ob2` are part of the same metaBall group, and `ob1` is its
+ * basis.
+ */
+bool BKE_mball_is_basis_for(const struct Object *ob1, const struct Object *ob2);
 /**
  * Test, if \a ob is a basis meta-ball.
  *
  * It test last character of Object ID name.
  * If last character is digit it return 0, else it return 1.
  */
-bool BKE_mball_is_basis(struct Object *ob);
+bool BKE_mball_is_basis(const struct Object *ob);
 /**
  * This function finds the basis meta-ball.
  *
@@ -58,13 +69,14 @@ struct BoundBox *BKE_mball_boundbox_get(struct Object *ob);
 float *BKE_mball_make_orco(struct Object *ob, struct ListBase *dispbase);
 
 /**
- * Copy some properties from object to other meta-ball object with same base name.
+ * Copy some properties from a meta-ball obdata to all other meta-ball obdata belonging to the same
+ * family (i.e. object sharing the same name basis).
  *
  * When some properties (wire-size, threshold, update flags) of meta-ball are changed, then this
  * properties are copied to all meta-balls in same "group" (meta-balls with same base name:
  * `MBall`, `MBall.001`, `MBall.002`, etc). The most important is to copy properties to the base
- * meta-ball, because this meta-ball influence polygonization of meta-balls. */
-void BKE_mball_properties_copy(struct Scene *scene, struct Object *active_object);
+ * meta-ball, because this meta-ball influences polygonization of meta-balls. */
+void BKE_mball_properties_copy(struct Main *bmain, struct MetaBall *active_metaball);
 
 bool BKE_mball_minmax_ex(
     const struct MetaBall *mb, float min[3], float max[3], const float obmat[4][4], short flag);