Fix T100863, T100875: Vertex group reading broken for recent files

This patch consists of two related fixes. The first is a simple fix for
forward compatibility, setting the Mesh.dvert pointer when writing a
file allows old Blender versions to read vertex groups from newly saved
files.

The second part is a bit uglier and more complex. Normally mesh vertex
group data is read in mesh_blend_read_data, for backward compatibility
with very old files. However, after 05952aa94d33eeb50 the mesh.dvert
pointer was not set, so the data was not read. Reading vertex group
layers when reading custom data is enough to fix that issue. We need to
read the data from *both* places, but BKE_defvert_blend_read cannot run
twice without memory leaks, so first try reading from custom data, then
read the pointer if that fails.

Differential Revision: https://developer.blender.org/D15905

2 files changed, 9 insertions, 4 deletions

diff --git a/source/blender/blenkernel/intern/customdata.cc b/source/blender/blenkernel/intern/customdata.cc
index 12425a67e70..cfb8416b0b4 100644
--- a/source/blender/blenkernel/intern/customdata.cc
+++ b/source/blender/blenkernel/intern/customdata.cc
@@ -5404,6 +5404,9 @@ void CustomData_blend_read(BlendDataReader *reader, CustomData *data, const int
       else if (layer->type == CD_GRID_PAINT_MASK) {
         blend_read_paint_mask(reader, count, static_cast<GridPaintMask *>(layer->data));
       }
+      else if (layer->type == CD_MDEFORMVERT) {
+        BKE_defvert_blend_read(reader, count, static_cast<MDeformVert *>(layer->data));
+      }
       i++;
     }
   }
diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 1bf068fcd45..84fc9005d53 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -249,7 +249,6 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
   else {
     Set<std::string> names_to_skip;
     if (!BLO_write_is_undo(writer)) {
-
       BKE_mesh_legacy_convert_hide_layers_to_flags(mesh);
       BKE_mesh_legacy_convert_material_indices_to_mpoly(mesh);
       /* When converting to the old mesh format, don't save redundant attributes. */
@@ -260,6 +259,7 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
       mesh->medge = const_cast<MEdge *>(mesh->edges().data());
       mesh->mpoly = const_cast<MPoly *>(mesh->polys().data());
       mesh->mloop = const_cast<MLoop *>(mesh->loops().data());
+      mesh->dvert = const_cast<MDeformVert *>(mesh->deform_verts().data());
     }
 
     CustomData_blend_write_prepare(mesh->vdata, vert_layers, names_to_skip);
@@ -314,9 +314,6 @@ static void mesh_blend_read_data(BlendDataReader *reader, ID *id)
   BLO_read_data_address(reader, &mesh->adt);
   BKE_animdata_blend_read_data(reader, mesh->adt);
 
-  /* Normally BKE_defvert_blend_read should be called in CustomData_blend_read,
-   * but for backwards compatibility in do_versions to work we do it here. */
-  BKE_defvert_blend_read(reader, mesh->totvert, mesh->dvert);
   BLO_read_list(reader, &mesh->vertex_group_names);
 
   CustomData_blend_read(reader, &mesh->vdata, mesh->totvert);
@@ -324,6 +321,11 @@ static void mesh_blend_read_data(BlendDataReader *reader, ID *id)
   CustomData_blend_read(reader, &mesh->fdata, mesh->totface);
   CustomData_blend_read(reader, &mesh->ldata, mesh->totloop);
   CustomData_blend_read(reader, &mesh->pdata, mesh->totpoly);
+  if (mesh->deform_verts().is_empty()) {
+    /* Vertex group data was also an owning pointer in old Blender versions.
+     * Don't read them again if they were read as part of #CustomData. */
+    BKE_defvert_blend_read(reader, mesh->totvert, mesh->dvert);
+  }
 
   mesh->texflag &= ~ME_AUTOSPACE_EVALUATED;
   mesh->edit_mesh = nullptr;