Implement missing Node & Resource placeholders

Implemented by request of @neikeq to advance in the GDExtension version of Mono.

* If a Resource type is missing upon load, it will be remembered together with its data (Unless manually overriden).
* If a Node type is missing upon load, it will be also be remembered together with its data (unless deleted).

This feature makes working with GDExtension much easier, as it ensures that missing types no longer cause data loss.

7 files changed, 247 insertions, 24 deletions

diff --git a/core/io/missing_resource.cpp b/core/io/missing_resource.cpp
new file mode 100644
index 0000000000..7aae6c6f0a
--- /dev/null
+++ b/core/io/missing_resource.cpp
@@ -0,0 +1,90 @@
+/*************************************************************************/
+/*  missing_resource.cpp                                                 */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2022 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2022 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#include "missing_resource.h"
+
+bool MissingResource::_set(const StringName &p_name, const Variant &p_value) {
+	if (is_recording_properties()) {
+		properties.insert(p_name, p_value);
+		return true; //always valid to set (add)
+	} else {
+		if (!properties.has(p_name)) {
+			return false;
+		}
+
+		properties[p_name] = p_value;
+		return true;
+	}
+}
+
+bool MissingResource::_get(const StringName &p_name, Variant &r_ret) const {
+	if (!properties.has(p_name)) {
+		return false;
+	}
+	r_ret = properties[p_name];
+	return true;
+}
+
+void MissingResource::_get_property_list(List<PropertyInfo> *p_list) const {
+	for (OrderedHashMap<StringName, Variant>::ConstElement E = properties.front(); E; E = E.next()) {
+		p_list->push_back(PropertyInfo(E.value().get_type(), E.key()));
+	}
+}
+
+void MissingResource::set_original_class(const String &p_class) {
+	original_class = p_class;
+}
+
+String MissingResource::get_original_class() const {
+	return original_class;
+}
+
+void MissingResource::set_recording_properties(bool p_enable) {
+	recording_properties = p_enable;
+}
+
+bool MissingResource::is_recording_properties() const {
+	return recording_properties;
+}
+
+void MissingResource::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_original_class", "name"), &MissingResource::set_original_class);
+	ClassDB::bind_method(D_METHOD("get_original_class"), &MissingResource::get_original_class);
+
+	ClassDB::bind_method(D_METHOD("set_recording_properties", "enable"), &MissingResource::set_recording_properties);
+	ClassDB::bind_method(D_METHOD("is_recording_properties"), &MissingResource::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::BOOL, "recording_properties", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_NONE), "set_recording_properties", "is_recording_properties");
+}
+
+MissingResource::MissingResource() {
+}
diff --git a/core/io/missing_resource.h b/core/io/missing_resource.h
new file mode 100644
index 0000000000..e87efe1a98
--- /dev/null
+++ b/core/io/missing_resource.h
@@ -0,0 +1,63 @@
+/*************************************************************************/
+/*  missing_resource.h                                                   */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2022 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2022 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#ifndef MISSING_RESOURCE_H
+#define MISSING_RESOURCE_H
+
+#include "core/io/resource.h"
+
+#define META_PROPERTY_MISSING_RESOURCES "metadata/_missing_resources"
+#define META_MISSING_RESOURCES "_missing_resources"
+
+class MissingResource : public Resource {
+	GDCLASS(MissingResource, Resource)
+	OrderedHashMap<StringName, Variant> properties;
+
+	String original_class;
+	bool recording_properties = false;
+
+protected:
+	bool _set(const StringName &p_name, const Variant &p_value);
+	bool _get(const StringName &p_name, Variant &r_ret) const;
+	void _get_property_list(List<PropertyInfo> *p_list) const;
+
+	static void _bind_methods();
+
+public:
+	void set_original_class(const String &p_class);
+	String get_original_class() const;
+
+	void set_recording_properties(bool p_enable);
+	bool is_recording_properties() const;
+
+	MissingResource();
+};
+
+#endif // MISSING_RESOURCE_H
diff --git a/core/io/resource_format_binary.cpp b/core/io/resource_format_binary.cpp
index c9eb0e5234..fbb4293961 100644
--- a/core/io/resource_format_binary.cpp
+++ b/core/io/resource_format_binary.cpp
@@ -35,6 +35,7 @@
 #include "core/io/file_access_compressed.h"
 #include "core/io/image.h"
 #include "core/io/marshalls.h"
+#include "core/io/missing_resource.h"
 #include "core/version.h"
 
 //#define print_bl(m_what) print_line(m_what)
@@ -728,13 +729,23 @@ Error ResourceLoaderBinary::load() {
 			}
 		}
 
+		MissingResource *missing_resource = nullptr;
+
 		if (res.is_null()) {
 			//did not replace
 
 			Object *obj = ClassDB::instantiate(t);
 			if (!obj) {
-				error = ERR_FILE_CORRUPT;
-				ERR_FAIL_V_MSG(ERR_FILE_CORRUPT, local_path + ":Resource of unrecognized type in file: " + t + ".");
+				if (ResourceLoader::is_creating_missing_resources_if_class_unavailable_enabled()) {
+					//create a missing resource
+					missing_resource = memnew(MissingResource);
+					missing_resource->set_original_class(t);
+					missing_resource->set_recording_properties(true);
+					obj = missing_resource;
+				} else {
+					error = ERR_FILE_CORRUPT;
+					ERR_FAIL_V_MSG(ERR_FILE_CORRUPT, local_path + ":Resource of unrecognized type in file: " + t + ".");
+				}
 			}
 
 			Resource *r = Object::cast_to<Resource>(obj);
@@ -760,6 +771,8 @@ Error ResourceLoaderBinary::load() {
 
 		//set properties
 
+		Dictionary missing_resource_properties;
+
 		for (int j = 0; j < pc; j++) {
 			StringName name = _get_string();
 
@@ -775,8 +788,32 @@ Error ResourceLoaderBinary::load() {
 				return error;
 			}
 
-			res->set(name, value);
+			bool set_valid = true;
+			if (value.get_type() == Variant::OBJECT && missing_resource != nullptr) {
+				// If the property being set is a missing resource (and the parent is not),
+				// then setting it will most likely not work.
+				// Instead, save it as metadata.
+
+				Ref<MissingResource> mr = value;
+				if (mr.is_valid()) {
+					missing_resource_properties[name] = mr;
+					set_valid = false;
+				}
+			}
+
+			if (set_valid) {
+				res->set(name, value);
+			}
+		}
+
+		if (missing_resource) {
+			missing_resource->set_recording_properties(false);
+		}
+
+		if (!missing_resource_properties.is_empty()) {
+			res->set_meta(META_MISSING_RESOURCES, missing_resource_properties);
 		}
+
 #ifdef TOOLS_ENABLED
 		res->set_edited(false);
 #endif
@@ -1833,6 +1870,15 @@ int ResourceFormatSaverBinaryInstance::get_string_index(const String &p_string)
 	return strings.size() - 1;
 }
 
+static String _resource_get_class(Ref<Resource> p_resource) {
+	Ref<MissingResource> missing_resource = p_resource;
+	if (missing_resource.is_valid()) {
+		return missing_resource->get_original_class();
+	} else {
+		return p_resource->get_class();
+	}
+}
+
 Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const Ref<Resource> &p_resource, uint32_t p_flags) {
 	Error err;
 	Ref<FileAccess> f;
@@ -1885,7 +1931,7 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const Ref<Re
 		return ERR_CANT_CREATE;
 	}
 
-	save_unicode_string(f, p_resource->get_class());
+	save_unicode_string(f, _resource_get_class(p_resource));
 	f->store_64(0); //offset to import metadata
 	{
 		uint32_t format_flags = FORMAT_FLAG_NAMED_SCENE_IDS | FORMAT_FLAG_UIDS;
@@ -1902,10 +1948,12 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const Ref<Re
 
 	List<ResourceData> resources;
 
+	Dictionary missing_resource_properties = p_resource->get_meta(META_MISSING_RESOURCES, Dictionary());
+
 	{
 		for (const Ref<Resource> &E : saved_resources) {
 			ResourceData &rd = resources.push_back(ResourceData())->get();
-			rd.type = E->get_class();
+			rd.type = _resource_get_class(E);
 
 			List<PropertyInfo> property_list;
 			E->get_property_list(&property_list);
@@ -1914,6 +1962,10 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const Ref<Re
 				if (skip_editor && F.name.begins_with("__editor")) {
 					continue;
 				}
+				if (F.name == META_PROPERTY_MISSING_RESOURCES) {
+					continue;
+				}
+
 				if ((F.usage & PROPERTY_USAGE_STORAGE)) {
 					Property p;
 					p.name_idx = get_string_index(F.name);
@@ -1929,6 +1981,14 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const Ref<Re
 						p.value = E->get(F.name);
 					}
 
+					if (p.pi.type == Variant::OBJECT && missing_resource_properties.has(F.name)) {
+						// Was this missing resource overriden? If so do not save the old value.
+						Ref<Resource> res = p.value;
+						if (res.is_null()) {
+							p.value = missing_resource_properties[F.name];
+						}
+					}
+
 					Variant default_value = ClassDB::class_get_default_property_value(E->get_class(), F.name);
 
 					if (default_value.get_type() != Variant::NIL && bool(Variant::evaluate(Variant::OP_EQUAL, p.value, default_value))) {
@@ -1990,7 +2050,7 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const Ref<Re
 				String new_id;
 
 				while (true) {
-					new_id = r->get_class() + "_" + Resource::generate_scene_unique_id();
+					new_id = _resource_get_class(r) + "_" + Resource::generate_scene_unique_id();
 					if (!used_unique_ids.has(new_id)) {
 						break;
 					}
diff --git a/core/io/resource_loader.cpp b/core/io/resource_loader.cpp
index c8cebd672a..d125dd4e91 100644
--- a/core/io/resource_loader.cpp
+++ b/core/io/resource_loader.cpp
@@ -979,6 +979,10 @@ void ResourceLoader::remove_custom_resource_format_loader(String script_path) {
 	}
 }
 
+void ResourceLoader::set_create_missing_resources_if_class_unavailable(bool p_enable) {
+	create_missing_resources_if_class_unavailable = p_enable;
+}
+
 void ResourceLoader::add_custom_loaders() {
 	// Custom loaders registration exploits global class names
 
@@ -1030,6 +1034,7 @@ void *ResourceLoader::err_notify_ud = nullptr;
 DependencyErrorNotify ResourceLoader::dep_err_notify = nullptr;
 void *ResourceLoader::dep_err_notify_ud = nullptr;
 
+bool ResourceLoader::create_missing_resources_if_class_unavailable = false;
 bool ResourceLoader::abort_on_missing_resource = true;
 bool ResourceLoader::timestamp_on_load = false;
 
diff --git a/core/io/resource_loader.h b/core/io/resource_loader.h
index 15ecfacf4a..ea18ac23fd 100644
--- a/core/io/resource_loader.h
+++ b/core/io/resource_loader.h
@@ -110,6 +110,7 @@ private:
 	static void *dep_err_notify_ud;
 	static DependencyErrorNotify dep_err_notify;
 	static bool abort_on_missing_resource;
+	static bool create_missing_resources_if_class_unavailable;
 	static HashMap<String, Vector<String>> translation_remaps;
 	static HashMap<String, String> path_remaps;
 
@@ -222,6 +223,9 @@ public:
 	static void add_custom_loaders();
 	static void remove_custom_loaders();
 
+	static void set_create_missing_resources_if_class_unavailable(bool p_enable);
+	_FORCE_INLINE_ static bool is_creating_missing_resources_if_class_unavailable_enabled() { return create_missing_resources_if_class_unavailable; }
+
 	static void initialize();
 	static void finalize();
 };
diff --git a/core/object/object.cpp b/core/object/object.cpp
index 1defd85a14..2b62041533 100644
--- a/core/object/object.cpp
+++ b/core/object/object.cpp
@@ -434,15 +434,6 @@ void Object::set(const StringName &p_name, const Variant &p_value, bool *r_valid
 		}
 	}
 
-	// Something inside the object... :|
-	bool success = _setv(p_name, p_value);
-	if (success) {
-		if (r_valid) {
-			*r_valid = true;
-		}
-		return;
-	}
-
 #ifdef TOOLS_ENABLED
 	if (script_instance) {
 		bool valid;
@@ -456,6 +447,15 @@ void Object::set(const StringName &p_name, const Variant &p_value, bool *r_valid
 	}
 #endif
 
+	// Something inside the object... :|
+	bool success = _setv(p_name, p_value);
+	if (success) {
+		if (r_valid) {
+			*r_valid = true;
+		}
+		return;
+	}
+
 	if (r_valid) {
 		*r_valid = false;
 	}
@@ -518,15 +518,6 @@ Variant Object::get(const StringName &p_name, bool *r_valid) const {
 		return ret;
 
 	} else {
-		// Something inside the object... :|
-		bool success = _getv(p_name, ret);
-		if (success) {
-			if (r_valid) {
-				*r_valid = true;
-			}
-			return ret;
-		}
-
 #ifdef TOOLS_ENABLED
 		if (script_instance) {
 			bool valid;
@@ -539,6 +530,14 @@ Variant Object::get(const StringName &p_name, bool *r_valid) const {
 			}
 		}
 #endif
+		// Something inside the object... :|
+		bool success = _getv(p_name, ret);
+		if (success) {
+			if (r_valid) {
+				*r_valid = true;
+			}
+			return ret;
+		}
 
 		if (r_valid) {
 			*r_valid = false;
diff --git a/core/register_core_types.cpp b/core/register_core_types.cpp
index 415b56cd83..e60d325f74 100644
--- a/core/register_core_types.cpp
+++ b/core/register_core_types.cpp
@@ -49,6 +49,7 @@
 #include "core/io/image_loader.h"
 #include "core/io/json.h"
 #include "core/io/marshalls.h"
+#include "core/io/missing_resource.h"
 #include "core/io/packed_data_container.h"
 #include "core/io/packet_peer.h"
 #include "core/io/packet_peer_dtls.h"
@@ -151,6 +152,7 @@ void register_core_types() {
 	GDREGISTER_CLASS(RefCounted);
 	GDREGISTER_CLASS(WeakRef);
 	GDREGISTER_CLASS(Resource);
+	GDREGISTER_VIRTUAL_CLASS(MissingResource);
 	GDREGISTER_CLASS(Image);
 
 	GDREGISTER_CLASS(Shortcut);