Fix T65352: bpy.data.meshes.new_from_object() doesn't increment user count for materials referenced by the mesh.

We cannot do refcount operations in a non-Main ID, this is forbidden.

While that whole func could probably use some love and refactor, for now
sticking to minimal changes and just moving refcounting op after mesh
has been transferred to Main database.

1 files changed, 25 insertions, 9 deletions

diff --git a/source/blender/blenkernel/intern/mesh_convert.c b/source/blender/blenkernel/intern/mesh_convert.c
index fce80272738..f99dee7197f 100644
--- a/source/blender/blenkernel/intern/mesh_convert.c
+++ b/source/blender/blenkernel/intern/mesh_convert.c
@@ -1169,17 +1169,28 @@ Mesh *BKE_mesh_new_from_object(Depsgraph *depsgraph, Object *object, bool preser
   return new_mesh;
 }
 
-static int foreach_libblock_make_original_and_usercount_callback(void *user_data_v,
-                                                                 ID *id_self,
-                                                                 ID **id_p,
-                                                                 int cb_flag)
+static int foreach_libblock_make_original_callback(void *UNUSED(user_data_v),
+                                                   ID *UNUSED(id_self),
+                                                   ID **id_p,
+                                                   int UNUSED(cb_flag))
 {
-  UNUSED_VARS(user_data_v, id_self, cb_flag);
   if (*id_p == NULL) {
     return IDWALK_RET_NOP;
   }
   *id_p = DEG_get_original_id(*id_p);
 
+  return IDWALK_RET_NOP;
+}
+
+static int foreach_libblock_make_usercounts_callback(void *UNUSED(user_data_v),
+                                                     ID *UNUSED(id_self),
+                                                     ID **id_p,
+                                                     int cb_flag)
+{
+  if (*id_p == NULL) {
+    return IDWALK_RET_NOP;
+  }
+
   if (cb_flag & IDWALK_CB_USER) {
     id_us_plus(*id_p);
   }
@@ -1204,10 +1215,10 @@ Mesh *BKE_mesh_new_from_object_to_bmain(Main *bmain,
    * Going to original data-blocks is required to have bmain in a consistent state, where
    * everything is only allowed to reference original data-blocks.
    *
-   * user-count is required is because so far mesh was in a limbo, where library management does
-   * not perform any user management (i.e. copy of a mesh will not increase users of materials). */
+   * Note that user-count updates has to be done *after* mesh has been transferred to Main database
+   * (since doing refcounting on non-Main IDs is forbidden). */
   BKE_library_foreach_ID_link(
-      NULL, &mesh->id, foreach_libblock_make_original_and_usercount_callback, NULL, IDWALK_NOP);
+      NULL, &mesh->id, foreach_libblock_make_original_callback, NULL, IDWALK_NOP);
 
   /* Append the mesh to bmain.
    * We do it a bit longer way since there is no simple and clear way of adding existing datablock
@@ -1216,7 +1227,7 @@ Mesh *BKE_mesh_new_from_object_to_bmain(Main *bmain,
   Mesh *mesh_in_bmain = BKE_mesh_add(bmain, mesh->id.name + 2);
 
   /* NOTE: BKE_mesh_nomain_to_mesh() does not copy materials and instead it preserves them in the
-   * destinaion mesh .So we "steal" all related fields before calling it.
+   * destinaion mesh. So we "steal" all related fields before calling it.
    *
    * TODO(sergey): We really better have a function which gets and ID and accepts it for the bmain.
    */
@@ -1228,6 +1239,11 @@ Mesh *BKE_mesh_new_from_object_to_bmain(Main *bmain,
 
   BKE_mesh_nomain_to_mesh(mesh, mesh_in_bmain, NULL, &CD_MASK_MESH, true);
 
+  /* User-count is required because so far mesh was in a limbo, where library management does
+   * not perform any user management (i.e. copy of a mesh will not increase users of materials). */
+  BKE_library_foreach_ID_link(
+      NULL, &mesh_in_bmain->id, foreach_libblock_make_usercounts_callback, NULL, IDWALK_NOP);
+
   /* Make sure user count from BKE_mesh_add() is the one we expect here and bring it down to 0. */
   BLI_assert(mesh_in_bmain->id.us == 1);
   id_us_min(&mesh_in_bmain->id);