Add concept of 'runtime' ID in Main data-base.

Such IDs are tagged with the new `LIB_TAG_RUNTIME`. They are essentially
like any other regular ID, except that they do not get written in .blend
files. They also do not make their linked data usages directly linked.
They do be written in undo steps however.

This tag should be ignored in any non-Main IDs (e.g. evaluated data,
`NO_MAIN`, etc.).

This commit also adds a new RNA ID property, `is_runtime`. This is not a
direct mapping to the DNA tag, as a non-main ID will also return True,
and the property is only editable for Main IDs.

Some basic testing for expected behavior of that new tag was also added
to `blendfile_io` py unittest.

Required for brush asset project, see T101908.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D16675

tests/python/bl_blendfile_io.py

@@ -47,8 +47,90 @@ class TestBlendFileSaveLoadBasic(TestHelper):
         assert orig_data == read_data
 
 
+# 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.
+class TestIdRuntimeTag(TestHelper):
+
+    def __init__(self, args):
+        self.args = args
+
+    def unique_blendfile_name(self, base_name):
+        return base_name + self.__class__.__name__ + ".blend"
+
+    def test_basics(self):
+        output_dir = self.args.output_dir
+        self.ensure_path(output_dir)
+        bpy.ops.wm.read_homefile(use_empty=False, use_factory_startup=True)
+
+        obj = bpy.data.objects['Cube']
+        assert obj.is_runtime_data == False
+        assert bpy.context.view_layer.depsgraph.ids['Cube'].is_runtime_data == True
+
+        output_work_path = os.path.join(output_dir, self.unique_blendfile_name("blendfile"))
+        bpy.ops.wm.save_as_mainfile(filepath=output_work_path, check_existing=False, compress=False)
+
+        bpy.ops.wm.open_mainfile(filepath=output_work_path, load_ui=False)
+        obj = bpy.data.objects['Cube']
+        assert obj.is_runtime_data == False
+
+        obj.is_runtime_data = True
+        assert obj.is_runtime_data == True
+
+        bpy.ops.wm.save_as_mainfile(filepath=output_work_path, check_existing=False, compress=False)
+        bpy.ops.wm.open_mainfile(filepath=output_work_path, load_ui=False)
+
+        assert 'Cube' not in bpy.data.objects
+        mesh = bpy.data.meshes['Cube']
+        assert mesh.is_runtime_data == False
+        assert mesh.users == 0
+
+    def test_linking(self):
+        output_dir = self.args.output_dir
+        self.ensure_path(output_dir)
+        bpy.ops.wm.read_homefile(use_empty=False, use_factory_startup=True)
+
+        material = bpy.data.materials.new("LibMaterial")
+        material.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)
+
+        bpy.ops.wm.read_homefile(use_empty=False, use_factory_startup=True)
+
+        obj = bpy.data.objects['Cube']
+        assert obj.is_runtime_data == 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
+
+        obj.material_slots[0].link = 'OBJECT'
+        obj.material_slots[0].material = linked_material
+
+        output_work_path = os.path.join(output_dir, self.unique_blendfile_name("blendfile"))
+        bpy.ops.wm.save_as_mainfile(filepath=output_work_path, check_existing=False, compress=False)
+
+        # Only usage of this linked material is a runtime ID (object),
+        # so writing .blend file will have properly reset its tag to indirectly linked data.
+        assert linked_material.is_library_indirect == True
+
+        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
+
+
 TESTS = (
     TestBlendFileSaveLoadBasic,
+
+    TestIdRuntimeTag,
 )