Address #105786: How to handle linked data with 'fake user' set.

133dde41bb changed how 'fake user' flag is handled with linked data.

Previous behavior was a bug/inconsistency, in that the 'directly linked'
tag would be 'over-set' and never cleared, forcing saving references to
a lot of unused linked data.
Note that ideally, 'Fake user' flag should be ignored, and the only way
to decide whether to keep or not a linked ID should be whether it's
actually used by some local data.

However, #103867 and #105687 show that this is causing issues in some cases,
where users wrongly relied on the linked data's pre-defined 'Fake user' flag
to keep their linked data in their production files, even if said data had no
real user.

While not ideal, for now we should consider 'fake user' flag for linked data
as a real usage case. A better handling of this edge-case is related to
wider designs aboud handling of 'non used' data on file save, whether
linked IDs should keep track of being explicitly or implicitly linked by
the user, etc.

tests/python/bl_blendfile_io.py

@@ -64,7 +64,7 @@ class TestIdRuntimeTag(TestHelper):
         bpy.ops.wm.read_homefile(use_empty=False, use_factory_startup=True)
 
         obj = bpy.data.objects['Cube']
-        assert obj.is_runtime_data == False
+        assert obj.is_runtime_data is False
         assert bpy.context.view_layer.depsgraph.ids['Cube'].is_runtime_data
 
         output_work_path = os.path.join(output_dir, self.unique_blendfile_name("blendfile"))
@@ -72,7 +72,7 @@ class TestIdRuntimeTag(TestHelper):
 
         bpy.ops.wm.open_mainfile(filepath=output_work_path, load_ui=False)
         obj = bpy.data.objects['Cube']
-        assert obj.is_runtime_data == False
+        assert obj.is_runtime_data is False
 
         obj.is_runtime_data = True
         assert obj.is_runtime_data
@@ -82,7 +82,7 @@ class TestIdRuntimeTag(TestHelper):
 
         assert 'Cube' not in bpy.data.objects
         mesh = bpy.data.meshes['Cube']
-        assert mesh.is_runtime_data == False
+        assert mesh.is_runtime_data is False
         assert mesh.users == 0
 
     def test_linking(self):
@@ -91,7 +91,11 @@ class TestIdRuntimeTag(TestHelper):
         bpy.ops.wm.read_homefile(use_empty=False, use_factory_startup=True)
 
         material = bpy.data.materials.new("LibMaterial")
-        material.use_fake_user = True
+        # Use a dummy mesh as user of the material, such that the material is saved
+        # without having to use fake user on it.
+        mesh = bpy.data.meshes.new("LibMesh")
+        mesh.materials.append(material)
+        mesh.use_fake_user = True
 
         output_lib_path = os.path.join(output_dir, self.unique_blendfile_name("blendlib_runtimetag_basic"))
         bpy.ops.wm.save_as_mainfile(filepath=output_lib_path, check_existing=False, compress=False)
@@ -99,15 +103,22 @@ class TestIdRuntimeTag(TestHelper):
         bpy.ops.wm.read_homefile(use_empty=False, use_factory_startup=True)
 
         obj = bpy.data.objects['Cube']
-        assert obj.is_runtime_data == False
+        assert obj.is_runtime_data is False
         obj.is_runtime_data = True
 
         link_dir = os.path.join(output_lib_path, "Material")
         bpy.ops.wm.link(directory=link_dir, filename="LibMaterial")
 
         linked_material = bpy.data.materials['LibMaterial']
-        assert linked_material.is_library_indirect == False
+        assert linked_material.is_library_indirect is False
 
+        link_dir = os.path.join(output_lib_path, "Mesh")
+        bpy.ops.wm.link(directory=link_dir, filename="LibMesh")
+
+        linked_mesh = bpy.data.meshes['LibMesh']
+        assert linked_mesh.is_library_indirect is False
+
+        obj.data = linked_mesh
         obj.material_slots[0].link = 'OBJECT'
         obj.material_slots[0].material = linked_material
 
@@ -118,13 +129,17 @@ class TestIdRuntimeTag(TestHelper):
         # so writing .blend file will have properly reset its tag to indirectly linked data.
         assert linked_material.is_library_indirect
 
+        # Only usage of this linked mesh is a runtime ID (object), but it is flagged as 'fake user' in its library,
+        # so writing .blend file will have kept its tag to directly linked data.
+        assert not linked_mesh.is_library_indirect
+
         bpy.ops.wm.open_mainfile(filepath=output_work_path, load_ui=False)
 
         assert 'Cube' not in bpy.data.objects
-        assert 'LibMaterial' not in bpy.data.materials
-        mesh = bpy.data.meshes['Cube']
-        assert mesh.is_runtime_data == False
-        assert mesh.users == 0
+        assert 'LibMaterial' in bpy.data.materials  # Pulled-in by the linked mesh.
+        linked_mesh = bpy.data.meshes['LibMesh']
+        assert linked_mesh.use_fake_user is True
+        assert linked_mesh.is_library_indirect is False
 
 
 TESTS = (