Allow opening scenes with missing scene dependency

doc/classes/MissingNode.xml

@@ -13,8 +13,11 @@
 		<member name="original_class" type="String" setter="set_original_class" getter="get_original_class">
 			The name of the class this node was supposed to be (see [method Object.get_class]).
 		</member>
+		<member name="original_scene" type="String" setter="set_original_scene" getter="get_original_scene">
+			Returns the path of the scene this node was instance of originally.
+		</member>
 		<member name="recording_properties" type="bool" setter="set_recording_properties" getter="is_recording_properties">
-			If set to [code]true[/code], allows new properties to be added on top of the existing ones with [method Object.set].
+			If [code]true[/code], allows new properties to be set along with existing ones. If [code]false[/code], only existing properties' values can be set, and new properties cannot be added.
 		</member>
 	</members>
 </class>

scene/main/missing_node.cpp

@@ -66,6 +66,14 @@ String MissingNode::get_original_class() const {
 	return original_class;
 }
 
+void MissingNode::set_original_scene(const String &p_scene) {
+	original_scene = p_scene;
+}
+
+String MissingNode::get_original_scene() const {
+	return original_scene;
+}
+
 void MissingNode::set_recording_properties(bool p_enable) {
 	recording_properties = p_enable;
 }
@@ -77,8 +85,15 @@ bool MissingNode::is_recording_properties() const {
 Array MissingNode::get_configuration_warnings() const {
 	// The mere existence of this node is warning.
 	Array ret;
-	ret.push_back(vformat(RTR("This node was saved as class type '%s', which was no longer available when this scene was loaded."), original_class));
+	if (!original_scene.is_empty()) {
-	ret.push_back(RTR("Data from the original node is kept as a placeholder until this type of node is available again. It can hence be safely re-saved without risk of data loss."));
+		ret.push_back(vformat(RTR("This node was an instance of scene '%s', which was no longer available when this scene was loaded."), original_scene));
+		ret.push_back(vformat(RTR("Saving current scene will discard instance and all its properties, including editable children edits (if existing).")));
+	} else if (!original_class.is_empty()) {
+		ret.push_back(vformat(RTR("This node was saved as class type '%s', which was no longer available when this scene was loaded."), original_class));
+		ret.push_back(RTR("Data from the original node is kept as a placeholder until this type of node is available again. It can hence be safely re-saved without risk of data loss."));
+	} else {
+		ret.push_back(RTR("Unrecognized missing node. Check scene dependency errors for details."));
+	}
 	return ret;
 }
 
@@ -86,11 +101,15 @@ void MissingNode::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_original_class", "name"), &MissingNode::set_original_class);
 	ClassDB::bind_method(D_METHOD("get_original_class"), &MissingNode::get_original_class);
 
+	ClassDB::bind_method(D_METHOD("set_original_scene", "name"), &MissingNode::set_original_class);
+	ClassDB::bind_method(D_METHOD("get_original_scene"), &MissingNode::get_original_class);
+
 	ClassDB::bind_method(D_METHOD("set_recording_properties", "enable"), &MissingNode::set_recording_properties);
 	ClassDB::bind_method(D_METHOD("is_recording_properties"), &MissingNode::is_recording_properties);
 
 	// Expose, but not save.
 	ADD_PROPERTY(PropertyInfo(Variant::STRING, "original_class", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_NONE), "set_original_class", "get_original_class");
+	ADD_PROPERTY(PropertyInfo(Variant::STRING, "original_scene", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_NONE), "set_original_scene", "get_original_scene");
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "recording_properties", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_NONE), "set_recording_properties", "is_recording_properties");
 }
 

scene/main/missing_node.h

@@ -39,6 +39,7 @@ class MissingNode : public Node {
 	HashMap<StringName, Variant> properties;
 
 	String original_class;
+	String original_scene;
 	bool recording_properties = false;
 
 protected:
@@ -52,6 +53,9 @@ public:
 	void set_original_class(const String &p_class);
 	String get_original_class() const;
 
+	void set_original_scene(const String &p_scene);
+	String get_original_scene() const;
+
 	void set_recording_properties(bool p_enable);
 	bool is_recording_properties() const;
 

scene/resources/packed_scene.cpp

@@ -191,7 +191,7 @@ Node *SceneState::instantiate(GenEditState p_edit_state) const {
 		MissingNode *missing_node = nullptr;
 
 		if (i == 0 && base_scene_idx >= 0) {
-			//scene inheritance on root node
+			// Scene inheritance on root node.
 			Ref<PackedScene> sdata = props[base_scene_idx];
 			ERR_FAIL_COND_V(!sdata.is_valid(), nullptr);
 			node = sdata->instantiate(p_edit_state == GEN_EDIT_STATE_DISABLED ? PackedScene::GEN_EDIT_STATE_DISABLED : PackedScene::GEN_EDIT_STATE_INSTANCE); //only main gets main edit state
@@ -201,14 +201,22 @@ Node *SceneState::instantiate(GenEditState p_edit_state) const {
 			}
 
 		} else if (n.instance >= 0) {
-			//instance a scene into this node
+			// Instance a scene into this node.
 			if (n.instance & FLAG_INSTANCE_IS_PLACEHOLDER) {
-				String scene_path = props[n.instance & FLAG_MASK];
+				const String scene_path = props[n.instance & FLAG_MASK];
 				if (disable_placeholders) {
 					Ref<PackedScene> sdata = ResourceLoader::load(scene_path, "PackedScene");
-					ERR_FAIL_COND_V(!sdata.is_valid(), nullptr);
+					if (sdata.is_valid()) {
-					node = sdata->instantiate(p_edit_state == GEN_EDIT_STATE_DISABLED ? PackedScene::GEN_EDIT_STATE_DISABLED : PackedScene::GEN_EDIT_STATE_INSTANCE);
+						node = sdata->instantiate(p_edit_state == GEN_EDIT_STATE_DISABLED ? PackedScene::GEN_EDIT_STATE_DISABLED : PackedScene::GEN_EDIT_STATE_INSTANCE);
-					ERR_FAIL_NULL_V(node, nullptr);
+						ERR_FAIL_NULL_V(node, nullptr);
+					} else if (ResourceLoader::is_creating_missing_resources_if_class_unavailable_enabled()) {
+						missing_node = memnew(MissingNode);
+						missing_node->set_original_scene(scene_path);
+						missing_node->set_recording_properties(true);
+						node = missing_node;
+					} else {
+						ERR_FAIL_V_MSG(nullptr, "Placeholder scene is missing.");
+					}
 				} else {
 					InstancePlaceholder *ip = memnew(InstancePlaceholder);
 					ip->set_instance_path(scene_path);
@@ -216,14 +224,27 @@ Node *SceneState::instantiate(GenEditState p_edit_state) const {
 				}
 				node->set_scene_instance_load_placeholder(true);
 			} else {
-				Ref<PackedScene> sdata = props[n.instance & FLAG_MASK];
+				Ref<Resource> res = props[n.instance & FLAG_MASK];
-				ERR_FAIL_COND_V(!sdata.is_valid(), nullptr);
+				Ref<PackedScene> sdata = res;
-				node = sdata->instantiate(p_edit_state == GEN_EDIT_STATE_DISABLED ? PackedScene::GEN_EDIT_STATE_DISABLED : PackedScene::GEN_EDIT_STATE_INSTANCE);
+				if (sdata.is_valid()) {
-				ERR_FAIL_NULL_V_MSG(node, nullptr, vformat("Failed to load scene dependency: \"%s\". Make sure the required scene is valid.", sdata->get_path()));
+					node = sdata->instantiate(p_edit_state == GEN_EDIT_STATE_DISABLED ? PackedScene::GEN_EDIT_STATE_DISABLED : PackedScene::GEN_EDIT_STATE_INSTANCE);
+					ERR_FAIL_NULL_V_MSG(node, nullptr, vformat("Failed to load scene dependency: \"%s\". Make sure the required scene is valid.", sdata->get_path()));
+				} else if (ResourceLoader::is_creating_missing_resources_if_class_unavailable_enabled()) {
+					missing_node = memnew(MissingNode);
+#ifdef TOOLS_ENABLED
+					if (res.is_valid()) {
+						missing_node->set_original_scene(res->get_meta("__load_path__", ""));
+					}
+#endif
+					missing_node->set_recording_properties(true);
+					node = missing_node;
+				} else {
+					ERR_FAIL_V_MSG(nullptr, "Scene instance is missing.");
+				}
 			}
 
 		} else if (n.type == TYPE_INSTANTIATED) {
-			//get the node from somewhere, it likely already exists from another instance
+			// Get the node from somewhere, it likely already exists from another instance.
 			if (parent) {
 				node = parent->_get_child_by_name(snames[n.name]);
 #ifdef DEBUG_ENABLED
@@ -233,7 +254,7 @@ Node *SceneState::instantiate(GenEditState p_edit_state) const {
 #endif
 			}
 		} else {
-			//node belongs to this scene and must be created
+			// Node belongs to this scene and must be created.
 			Object *obj = ClassDB::instantiate(snames[n.type]);
 
 			node = Object::cast_to<Node>(obj);

scene/resources/resource_format_text.cpp

@@ -176,6 +176,13 @@ Error ResourceLoaderText::_parse_ext_resource(VariantParser::Stream *p_stream, R
 		} else {
 			r_res = Ref<Resource>();
 		}
+#ifdef TOOLS_ENABLED
+		if (r_res.is_null()) {
+			// Hack to allow checking original path.
+			r_res.instantiate();
+			r_res->set_meta("__load_path__", ext_resources[id].path);
+		}
+#endif
 	}
 
 	VariantParser::get_token(p_stream, token, line, r_err_str);