Fix (unreported) liboverride resync creating garbage data in some cases.

Regression caused by the introduction of partial resync in February 2022
(rB1695d38989fd482d3c). Code was missing adding some existing overrides
to the mapping in some specific cases, causing resync to create 'new'
ones instead of re-using existing ones.

This commit also adds a basic resync testcase that illustrates this
issue.

tests/python/bl_blendfile_library_overrides.py

@@ -181,9 +181,118 @@ class TestLibraryTemplate(TestHelper, unittest.TestCase):
         assert(operation.operation == 'NOOP')
 
 
+class TestLibraryOverridesResync(TestHelper, unittest.TestCase):
+    DATA_NAME_CONTAINER = "LibCollection"
+    DATA_NAME_RIGGED = "LibRigged"
+    DATA_NAME_RIG = "LibRig"
+    DATA_NAME_CONTROLLER_1 = "LibController1"
+    DATA_NAME_CONTROLLER_2 = "LibController2"
+
+    def __init__(self, args):
+        self.args = args
+
+        output_dir = pathlib.Path(self.args.output_dir)
+        self.ensure_path(str(output_dir))
+        self.output_path = output_dir / "blendlib_overrides.blend"
+        self.test_output_path = output_dir / "blendlib_overrides_test.blend"
+
+        bpy.ops.wm.read_homefile(use_empty=True, use_factory_startup=True)
+
+        collection_container = bpy.data.collections.new(TestLibraryOverridesResync.DATA_NAME_CONTAINER)
+        bpy.context.collection.children.link(collection_container)
+
+        mesh = bpy.data.meshes.new(TestLibraryOverridesResync.DATA_NAME_RIGGED)
+        obj_child = bpy.data.objects.new(TestLibraryOverridesResync.DATA_NAME_RIGGED, object_data=mesh)
+        collection_container.objects.link(obj_child)
+        armature = bpy.data.armatures.new(TestLibraryOverridesResync.DATA_NAME_RIG)
+        obj_armature = bpy.data.objects.new(TestLibraryOverridesResync.DATA_NAME_RIG, object_data=armature)
+        obj_child.parent = obj_armature
+        collection_container.objects.link(obj_armature)
+
+        obj_child_modifier = obj_child.modifiers.new("", 'ARMATURE')
+        obj_child_modifier.object = obj_armature
+
+        obj_ctrl1 = bpy.data.objects.new(TestLibraryOverridesResync.DATA_NAME_CONTROLLER_1, object_data=None)
+        collection_container.objects.link(obj_ctrl1)
+
+        obj_armature_constraint = obj_armature.constraints.new('COPY_LOCATION')
+        obj_armature_constraint.target = obj_ctrl1
+
+        collection_sub = bpy.data.collections.new(TestLibraryOverridesResync.DATA_NAME_CONTROLLER_2)
+        collection_container.children.link(collection_sub)
+        obj_ctrl2 = bpy.data.objects.new(TestLibraryOverridesResync.DATA_NAME_CONTROLLER_2, object_data=None)
+        collection_sub.objects.link(obj_ctrl2)
+
+        bpy.ops.wm.save_as_mainfile(filepath=str(self.output_path), check_existing=False, compress=False)
+
+    def test_link_and_override_resync(self):
+        bpy.ops.wm.read_homefile(use_empty=True, use_factory_startup=True)
+        bpy.data.orphans_purge()
+
+        link_dir = self.output_path / "Collection"
+        bpy.ops.wm.link(directory=str(link_dir), filename=TestLibraryOverridesResync.DATA_NAME_CONTAINER, instance_collections=False)
+
+        linked_collection_container = bpy.data.collections[TestLibraryOverridesResync.DATA_NAME_CONTAINER]
+        assert(linked_collection_container.library is not None)
+        assert(linked_collection_container.override_library is None)
+        assert(len(bpy.data.collections) == 2)
+        assert(all(id_.library is not None for id_ in bpy.data.collections))
+        assert(len(bpy.data.objects) == 4)
+        assert(all(id_.library is not None for id_ in bpy.data.objects))
+        assert(len(bpy.data.meshes) == 1)
+        assert(all(id_.library is not None for id_ in bpy.data.meshes))
+        assert(len(bpy.data.armatures) == 1)
+        assert(all(id_.library is not None for id_ in bpy.data.armatures))
+
+        override_collection_container = linked_collection_container.override_hierarchy_create(bpy.context.scene, bpy.context.view_layer)
+        assert(override_collection_container.library is None)
+        assert(override_collection_container.override_library is not None)
+        # Objects and collections are duplicated as overrides, but meshes and armatures remain only linked data.
+        assert(len(bpy.data.collections) == 4)
+        assert(all((id_.library is None and id_.override_library is not None) for id_ in bpy.data.collections[:2]))
+        assert(len(bpy.data.objects) == 8)
+        assert(all((id_.library is None and id_.override_library is not None) for id_ in bpy.data.objects[:4]))
+        assert(len(bpy.data.meshes) == 1)
+        assert(len(bpy.data.armatures) == 1)
+
+        bpy.ops.wm.save_as_mainfile(filepath=str(self.test_output_path), check_existing=False, compress=False)
+
+        # Re-open the lib file, and change its ID relationships.
+        bpy.ops.wm.open_mainfile(filepath=str(self.output_path))
+
+        obj_armature = bpy.data.objects[TestLibraryOverridesResync.DATA_NAME_RIG]
+        obj_armature_constraint = obj_armature.constraints[0]
+        obj_ctrl2 = bpy.data.objects[TestLibraryOverridesResync.DATA_NAME_CONTROLLER_2]
+        obj_armature_constraint.target = obj_ctrl2
+
+        bpy.ops.wm.save_as_mainfile(filepath=str(self.output_path), check_existing=False, compress=False)
+
+        # Re-open the main file, and check that automatic resync did its work correctly, remapping the target of the
+        # armature constraint to controller 2, without creating unexpected garbage IDs along the line.
+        bpy.ops.wm.open_mainfile(filepath=str(self.test_output_path))
+
+        override_collection_container = bpy.data.collections[TestLibraryOverridesResync.DATA_NAME_CONTAINER]
+        assert(override_collection_container.library is None)
+        assert(override_collection_container.override_library is not None)
+        # Objects and collections are duplicated as overrides, but meshes and armatures remain only linked data.
+        assert(len(bpy.data.collections) == 4)
+        assert(all((id_.library is None and id_.override_library is not None) for id_ in bpy.data.collections[:2]))
+        assert(len(bpy.data.objects) == 8)
+        assert(all((id_.library is None and id_.override_library is not None) for id_ in bpy.data.objects[:4]))
+        assert(len(bpy.data.meshes) == 1)
+        assert(len(bpy.data.armatures) == 1)
+
+        obj_armature = bpy.data.objects[TestLibraryOverridesResync.DATA_NAME_RIG]
+        obj_ctrl2 = bpy.data.objects[TestLibraryOverridesResync.DATA_NAME_CONTROLLER_2]
+        assert(obj_armature.library is None and obj_armature.override_library is not None)
+        assert(obj_ctrl2.library is None and obj_ctrl2.override_library is not None)
+        assert(obj_armature.constraints[0].target == obj_ctrl2)
+
+
 TESTS = (
     TestLibraryOverrides,
     TestLibraryTemplate,
+    TestLibraryOverridesResync,
 )