Fix collection syncing when creating new collections from the outliner

We were not passing a scene collection parent to the BKE_collection_add
function, which in turn made syncing not work.

Right now we:
* Explicitly pass the master collection in this case
* Fallback to the master collection in other cases

With unittest.

source/blender/blenkernel/intern/collection.c

@@ -111,6 +111,10 @@ static SceneCollection *collection_add(ID *owner_id, SceneCollection *sc_parent,
  */
 SceneCollection *BKE_collection_add(ID *owner_id, SceneCollection *sc_parent, const int type, const char *name_custom)
 {
+	if (sc_parent == NULL) {
+		sc_parent = BKE_collection_master(owner_id);
+	}
+
 	SceneCollection *scene_collection = collection_add(owner_id, sc_parent, type, name_custom);
 	BKE_layer_sync_new_scene_collection(owner_id, sc_parent, scene_collection);
 	return scene_collection;

source/blender/blenkernel/intern/layer.c

@@ -375,6 +375,7 @@ static void layer_collection_sync_flags(
 
 static void layer_collections_sync_flags(ListBase *layer_collections_dst, const ListBase *layer_collections_src)
 {
+	BLI_assert(BLI_listbase_count(layer_collections_dst) == BLI_listbase_count(layer_collections_src));
 	LayerCollection *layer_collection_dst = (LayerCollection *)layer_collections_dst->first;
 	const LayerCollection *layer_collection_src = (const LayerCollection *)layer_collections_src->first;
 	while (layer_collection_dst != NULL) {

source/blender/editors/space_outliner/outliner_collections.c

@@ -342,7 +342,8 @@ static int collection_new_exec(bContext *C, wmOperator *UNUSED(op))
 	Main *bmain = CTX_data_main(C);
 	Scene *scene = CTX_data_scene(C);
 	ViewLayer *view_layer = CTX_data_view_layer(C);
-	SceneCollection *scene_collection = BKE_collection_add(&scene->id, NULL, COLLECTION_TYPE_NONE, NULL);
+	SceneCollection *scene_collection_parent = BKE_collection_master(&scene->id);
+	SceneCollection *scene_collection = BKE_collection_add(&scene->id, scene_collection_parent, COLLECTION_TYPE_NONE, NULL);
 	BKE_collection_link(view_layer, scene_collection);
 
 	DEG_relations_tag_update(bmain);

tests/python/view_layer/CMakeLists.txt

@@ -62,6 +62,7 @@ endmacro()
 
 VIEW_LAYER_TEST(active_collection)
 VIEW_LAYER_TEST(background_set)
+VIEW_LAYER_TEST(collection_new_sync)
 VIEW_LAYER_TEST(collection_rename_a)
 VIEW_LAYER_TEST(collection_rename_b)
 VIEW_LAYER_TEST(evaluation_render_settings_a)

tests/python/view_layer/test_collection_new_sync.py

@@ -0,0 +1,47 @@
+# ############################################################
+# Importing - Same For All Render Layer Tests
+# ############################################################
+
+import unittest
+import os
+import sys
+
+from view_layer_common import *
+
+
+# ############################################################
+# Testing
+# ############################################################
+
+class UnitTesting(ViewLayerTesting):
+    def test_view_layer_syncing(self):
+        """
+        See if we can copy view layers.
+        """
+        import bpy
+        scene = bpy.context.scene
+        view_layer = scene.view_layers.new("All")
+
+        self.assertEqual(len(view_layer.collections), 1)
+        self.assertEqual(view_layer.collections[0].collection, scene.master_collection)
+
+        self.assertEqual(
+                {collection.name for collection in view_layer.collections[0].collections},
+                {'Collection 1'})
+
+        self.assertEqual(
+                bpy.ops.outliner.collection_new(),
+                {'FINISHED'})
+
+        self.assertEqual(
+                {collection.name for collection in view_layer.collections[0].collections},
+                {'Collection 1', 'Collection 2'})
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    UnitTesting._extra_arguments = setup_extra_arguments(__file__)
+    unittest.main()