From 037121c2614826c6ba379fd447c54ed6d58d7d06 Mon Sep 17 00:00:00 2001
From: Hans Goudey <hans@blender.org>
Date: Thu, 12 Jun 2025 14:54:32 +0200
Subject: [PATCH] Mesh: Use names to store active/default UV status

Required for the transition to `AttributeStorage`, because this
information was previously stored in `CustomDataLayer` flags.
This is already the way we store the active/default color attribute status.
Migrating runtime usage to use the names isn't done here, just like
we still use `CustomData` everywhere at runtime for `Mesh`.

Pull Request: https://projects.blender.org/blender/blender/pulls/140137
---
 source/blender/blenkernel/intern/mesh.cc | 25 ++++++++++++++++++++++++
 source/blender/makesdna/DNA_mesh_types.h |  4 ++--
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 8f23e88fe95..2ce1d38f4b7 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -308,6 +308,29 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
   mesh->totface_legacy = 0;
   mesh->fdata_legacy = CustomData{};
 
+  if (U.experimental.use_attribute_storage_write) {
+    /* Convert from the format still used at runtime (flags on #CustomDataLayer) to the format
+     * reserved for future runtime use (names stored on #Mesh). */
+    if (const char *name = CustomData_get_active_layer_name(&mesh->corner_data, CD_PROP_FLOAT2)) {
+      mesh->active_uv_map_attribute = const_cast<char *>(
+          scope.allocator().copy_string(name).c_str());
+    }
+    else {
+      mesh->active_uv_map_attribute = nullptr;
+    }
+    if (const char *name = CustomData_get_render_layer_name(&mesh->corner_data, CD_PROP_FLOAT2)) {
+      mesh->default_uv_map_attribute = const_cast<char *>(
+          scope.allocator().copy_string(name).c_str());
+    }
+    else {
+      mesh->default_uv_map_attribute = nullptr;
+    }
+  }
+  else {
+    mesh->active_uv_map_attribute = nullptr;
+    mesh->default_uv_map_attribute = nullptr;
+  }
+
   /* Do not store actual geometry data in case this is a library override ID. */
   if (ID_IS_OVERRIDE_LIBRARY(mesh) && !is_undo) {
     mesh->verts_num = 0;
@@ -351,6 +374,8 @@ static void mesh_blend_write(BlendWriter *writer, ID *id, const void *id_address
   BKE_defbase_blend_write(writer, &mesh->vertex_group_names);
   BLO_write_string(writer, mesh->active_color_attribute);
   BLO_write_string(writer, mesh->default_color_attribute);
+  BLO_write_string(writer, mesh->active_uv_map_attribute);
+  BLO_write_string(writer, mesh->default_uv_map_attribute);
 
   BLO_write_pointer_array(writer, mesh->totcol, mesh->mat);
   BLO_write_struct_array(writer, MSelect, mesh->totselect, mesh->mselect);
diff --git a/source/blender/makesdna/DNA_mesh_types.h b/source/blender/makesdna/DNA_mesh_types.h
index ba0c52ed3ca..8b8ce9ca69b 100644
--- a/source/blender/makesdna/DNA_mesh_types.h
+++ b/source/blender/makesdna/DNA_mesh_types.h
@@ -180,12 +180,12 @@ typedef struct Mesh {
 
   /**
    * The UV map currently selected in the list and edited by a user.
-   * Currently only used for file forward compatibility (see #AttributeStorage).
+   * Currently only used for file reading/writing (see #AttributeStorage).
    */
   char *active_uv_map_attribute;
   /**
    * The UV map used by default (i.e. for rendering) if no name is given explicitly.
-   * Currently only used for file forward compatibility (see #AttributeStorage).
+   * Currently only used for file reading/writing (see #AttributeStorage).
    */
   char *default_uv_map_attribute;