Fix #111488: Copying object crashes Blender after recent refactor.

Caused by 8bb5916183.

Trivial mistake, would have been caught by a basic test, so also added
said test for the 'write partial' feature.

source/blender/blenloader/BLO_readfile.h

@@ -497,7 +497,7 @@ void BLO_main_expander(BLOExpandDoitCallback expand_doit_func);
  * Loop over all ID data in Main to mark relations.
  * Set (id->tag & LIB_TAG_NEED_EXPAND) to mark expanding. Flags get cleared after expanding.
  *
- * \param fdhandle: usually file-data, or own handle.
+ * \param fdhandle: usually file-data, or own handle. May be nullptr.
  * \param mainvar: the Main database to expand.
  */
 void BLO_expand_main(void *fdhandle, struct Main *mainvar);

source/blender/blenloader/intern/readfile.cc

@@ -4148,7 +4148,9 @@ void BLO_expand_main(void *fdhandle, Main *mainvar)
        * Expanding should never modify ID pointers themselves.
        * Handling of DNA deprecated data should never be needed in undo case. */
       const int flag = IDWALK_READONLY | IDWALK_NO_ORIG_POINTERS_ACCESS |
-                       ((fd->flags & FD_FLAGS_IS_MEMFILE) ? 0 : IDWALK_DO_DEPRECATED_POINTERS);
+                       ((!fd || (fd->flags & FD_FLAGS_IS_MEMFILE)) ?
+                            0 :
+                            IDWALK_DO_DEPRECATED_POINTERS);
       BKE_library_foreach_ID_link(nullptr, id_iter, expand_cb, &expander, flag);
 
       do_it = true;

tests/python/bl_blendfile_io.py

@@ -49,6 +49,52 @@ class TestBlendFileSaveLoadBasic(TestHelper):
         assert orig_data == read_data
 
 
+class TestBlendFileSavePartial(TestHelper):
+    OBJECT_MESH_NAME = "ObjectMesh"
+    OBJECT_MATERIAL_NAME = "ObjectMaterial"
+    OBJECT_NAME = "Object"
+    UNUSED_MESH_NAME = "UnusedMesh"
+
+    def __init__(self, args):
+        self.args = args
+
+    def test_save_load(self):
+        bpy.ops.wm.read_homefile(use_empty=True, use_factory_startup=True)
+
+        ob_mesh = bpy.data.meshes.new(self.OBJECT_MESH_NAME)
+        ob_material = bpy.data.materials.new(self.OBJECT_MATERIAL_NAME)
+        ob_mesh.materials.append(ob_material)
+        ob = bpy.data.objects.new(self.OBJECT_NAME, object_data=ob_mesh)
+        bpy.context.collection.objects.link(ob)
+
+        unused_mesh = bpy.data.meshes.new(self.UNUSED_MESH_NAME)
+        unused_mesh.materials.append(ob_material)
+
+        assert ob_mesh.users == 1
+        assert ob_material.users == 2
+        assert ob.users == 1
+        assert unused_mesh.users == 0
+
+        output_dir = self.args.output_dir
+        self.ensure_path(output_dir)
+
+        # Take care to keep the name unique so multiple test jobs can run at once.
+        output_path = os.path.join(output_dir, "blendfile_io_partial.blend")
+
+        bpy.data.libraries.write(filepath=output_path, datablocks={ob, unused_mesh}, fake_user=False)
+        bpy.ops.wm.open_mainfile(filepath=output_path, load_ui=False)
+
+        assert self.OBJECT_MESH_NAME in bpy.data.meshes
+        assert self.OBJECT_MATERIAL_NAME in bpy.data.materials
+        assert self.OBJECT_NAME in bpy.data.objects
+        assert self.UNUSED_MESH_NAME in bpy.data.meshes
+
+        assert bpy.data.meshes[self.OBJECT_MESH_NAME].users == 1
+        assert bpy.data.materials[self.OBJECT_MATERIAL_NAME].users == 2
+        assert bpy.data.objects[self.OBJECT_NAME].users == 0
+        assert bpy.data.meshes[self.UNUSED_MESH_NAME].users == 0
+
+
 # NOTE: Technically this should rather be in `bl_id_management.py` test, but that file uses `unittest` module,
 #       which makes mixing it with tests system used here and passing extra parameters complicated.
 #       Since the main effect of 'RUNTIME' ID tag is on file save, it can as well be here for now.
@@ -145,6 +191,7 @@ class TestIdRuntimeTag(TestHelper):
 
 TESTS = (
     TestBlendFileSaveLoadBasic,
+    TestBlendFileSavePartial,
 
     TestIdRuntimeTag,
 )