Add a backwards-compatibility system for GDExtension method

This adds a way to ensure that methods that were modified in the Godot API will continue working in older builds of GDExtension even if the new signature is different.

```C++
// New version (changed)
ClassDB::bind_method(D_METHOD("add_sphere","radius","position"),&MyShapes::add_sphere);
// Compatibility version (still available to extensions).
ClassDB::bind_compatibility_method(D_METHOD("add_sphere","radius"),&MyShapes::_compat_add_sphere);
```

**Q**: If I add an extra argument and provide a default value (hence can still be called the same), do I still have to provide the compatibility version?
**A**: Yes, you must still provide a compatibility method. Most language bindings use the raw method pointer to do the call and process the default parameters in the binding language, hence if the actual method signature changes it will no longer work.

**Q**: If I removed a method, can I still bind a compatibility version even though the main method no longer exists?
**A**: Yes, for methods that were removed or renamed, compatibility versions can still be provided.

**Q**: Would it be possible to automate checking that methods were removed by mistake?
**A**: Yes, as part of a future PR, the idea is to add a a command line option to Godot that can be run like : `$ godot --test-api-compatibility older_api_dump.json`, which will also be integrated to the CI runs.

core/extension/extension_api_dump.cpp

@@ -880,6 +880,15 @@ Dictionary GDExtensionAPIDump::generate_extension_api() {
 						d2["is_virtual"] = false;
 						d2["hash"] = method->get_hash();
 
+						Vector<uint32_t> compat_hashes = ClassDB::get_method_compatibility_hashes(class_name, method_name);
+						if (compat_hashes.size()) {
+							Array compatibility;
+							for (int i = 0; i < compat_hashes.size(); i++) {
+								compatibility.push_back(compat_hashes[i]);
+							}
+							d2["hash_compatibility"] = compatibility;
+						}
+
 						Vector<Variant> default_args = method->get_default_arguments();
 
 						Array arguments;
@@ -1056,4 +1065,275 @@ void GDExtensionAPIDump::generate_extension_json_file(const String &p_path) {
 	fa->store_string(text);
 }
 
+static bool compare_dict_array(const Dictionary &p_old_api, const Dictionary &p_new_api, const String &p_base_array, const String &p_name_field, const Vector<String> &p_fields_to_compare, bool p_compare_hashes, const String &p_outer_class = String(), bool p_compare_operators = false) {
+	String base_array = p_outer_class + p_base_array;
+	if (!p_old_api.has(p_base_array)) {
+		return true; // May just not have this array and its still good. Probably added recently.
+	}
+	bool failed = false;
+	ERR_FAIL_COND_V_MSG(!p_new_api.has(p_base_array), false, "New API lacks base array: " + p_base_array);
+	Array new_api = p_new_api[p_base_array];
+	HashMap<String, Dictionary> new_api_assoc;
+
+	for (int i = 0; i < new_api.size(); i++) {
+		Dictionary elem = new_api[i];
+		ERR_FAIL_COND_V_MSG(!elem.has(p_name_field), false, "Validate extension JSON: Element of base_array '" + base_array + "' is missing field '" + p_name_field + "'. This is a bug.");
+		String name = elem[p_name_field];
+		if (p_compare_operators && elem.has("right_type")) {
+			name += " " + String(elem["right_type"]);
+		}
+		new_api_assoc.insert(name, elem);
+	}
+
+	Array old_api = p_old_api[p_base_array];
+	for (int i = 0; i < old_api.size(); i++) {
+		Dictionary old_elem = old_api[i];
+		if (!old_elem.has(p_name_field)) {
+			failed = true;
+			print_error("Validate extension JSON: JSON file: element of base array '" + base_array + "' is missing the field: '" + p_name_field + "'.");
+			continue;
+		}
+		String name = old_elem[p_name_field];
+		if (p_compare_operators && old_elem.has("right_type")) {
+			name += " " + String(old_elem["right_type"]);
+		}
+		if (!new_api_assoc.has(name)) {
+			failed = true;
+			print_error("Validate extension JSON: API was removed: " + base_array + "/" + name);
+			continue;
+		}
+
+		Dictionary new_elem = new_api_assoc[name];
+
+		for (int j = 0; j < p_fields_to_compare.size(); j++) {
+			String field = p_fields_to_compare[j];
+			bool optional = field.begins_with("*");
+			if (optional) {
+				// This is an optional field, but if exists it has to exist in both.
+				field = field.substr(1, field.length());
+			}
+
+			bool added = field.begins_with("+");
+			if (added) {
+				// Meaning this field must either exist or contents may not exist.
+				field = field.substr(1, field.length());
+			}
+
+			Variant old_value;
+
+			if (!old_elem.has(field)) {
+				if (optional) {
+					if (new_elem.has(field)) {
+						failed = true;
+						print_error("Validate extension JSON: JSON file: Field was added in a way that breaks compatibility '" + base_array + "/" + name + "': " + field);
+					}
+				} else if (added && new_elem.has(field)) {
+					// Should be ok, field now exists, should not be verified in prior versions where it does not.
+				} else {
+					failed = true;
+					print_error("Validate extension JSON: JSON file: Missing filed in '" + base_array + "/" + name + "': " + field);
+				}
+				continue;
+			} else {
+				old_value = old_elem[field];
+			}
+
+			if (!new_elem.has(field)) {
+				failed = true;
+				ERR_PRINT("Validate extension JSON: Missing filed in current API '" + base_array + "/" + name + "': " + field + ". This is a bug.");
+				continue;
+			}
+
+			Variant new_value = new_elem[field];
+
+			bool equal = Variant::evaluate(Variant::OP_EQUAL, old_value, new_value);
+			if (!equal) {
+				failed = true;
+				print_error("Validate extension JSON: Error: Field '" + base_array + "/" + name + "': " + field + " changed value in new API, from " + old_value.get_construct_string() + " to " + new_value.get_construct_string() + ".");
+				continue;
+			}
+		}
+
+		if (p_compare_hashes) {
+			if (!old_elem.has("hash")) {
+				if (old_elem.has("is_virtual") && bool(old_elem["is_virtual"]) && !new_elem.has("hash")) {
+					continue; // No hash for virtual methods, go on.
+				}
+
+				failed = true;
+				print_error("Validate extension JSON: JSON file: element of base array '" + base_array + "' is missing the field: 'hash'.");
+				continue;
+			}
+
+			uint64_t old_hash = old_elem["hash"];
+
+			if (!new_elem.has("hash")) {
+				failed = true;
+				print_error("Validate extension JSON: Error: Field '" + base_array + "' is missing the field: 'hash'.");
+				continue;
+			}
+
+			uint64_t new_hash = new_elem["hash"];
+			bool hash_found = false;
+			if (old_hash == new_hash) {
+				hash_found = true;
+			} else if (new_elem.has("hash_compatibility")) {
+				Array compatibility = new_elem["hash_compatibility"];
+				for (int j = 0; j < compatibility.size(); j++) {
+					new_hash = compatibility[j];
+					if (new_hash == old_hash) {
+						hash_found = true;
+						break;
+					}
+				}
+			}
+
+			if (!hash_found) {
+				failed = true;
+				print_error("Validate extension JSON: Error: Hash mismatch for '" + base_array + "/" + name + "'. This means that the function has changed and no compatibility function was provided.");
+				continue;
+			}
+		}
+	}
+
+	return !failed;
+}
+
+static bool compare_sub_dict_array(HashSet<String> &r_removed_classes_registered, const String &p_outer, const String &p_outer_name, const Dictionary &p_old_api, const Dictionary &p_new_api, const String &p_base_array, const String &p_name_field, const Vector<String> &p_fields_to_compare, bool p_compare_hashes, bool p_compare_operators = false) {
+	if (!p_old_api.has(p_outer)) {
+		return true; // May just not have this array and its still good. Probably added recently or optional.
+	}
+	bool failed = false;
+	ERR_FAIL_COND_V_MSG(!p_new_api.has(p_outer), false, "New API lacks base array: " + p_outer);
+	Array new_api = p_new_api[p_outer];
+	HashMap<String, Dictionary> new_api_assoc;
+
+	for (int i = 0; i < new_api.size(); i++) {
+		Dictionary elem = new_api[i];
+		ERR_FAIL_COND_V_MSG(!elem.has(p_outer_name), false, "Validate extension JSON: Element of base_array '" + p_outer + "' is missing field '" + p_outer_name + "'. This is a bug.");
+		new_api_assoc.insert(elem[p_outer_name], elem);
+	}
+
+	Array old_api = p_old_api[p_outer];
+
+	for (int i = 0; i < old_api.size(); i++) {
+		Dictionary old_elem = old_api[i];
+		if (!old_elem.has(p_outer_name)) {
+			failed = true;
+			print_error("Validate extension JSON: JSON file: element of base array '" + p_outer + "' is missing the field: '" + p_outer_name + "'.");
+			continue;
+		}
+		String name = old_elem[p_outer_name];
+		if (!new_api_assoc.has(name)) {
+			failed = true;
+			if (!r_removed_classes_registered.has(name)) {
+				print_error("Validate extension JSON: API was removed: " + p_outer + "/" + name);
+				r_removed_classes_registered.insert(name);
+			}
+			continue;
+		}
+
+		Dictionary new_elem = new_api_assoc[name];
+
+		if (!compare_dict_array(old_elem, new_elem, p_base_array, p_name_field, p_fields_to_compare, p_compare_hashes, p_outer + "/" + name + "/", p_compare_operators)) {
+			failed = true;
+		}
+	}
+
+	return !failed;
+}
+
+Error GDExtensionAPIDump::validate_extension_json_file(const String &p_path) {
+	Error error;
+	String text = FileAccess::get_file_as_string(p_path, &error);
+	if (error != OK) {
+		ERR_PRINT("Validate extension JSON: Could not open file '" + p_path + "'.");
+		return error;
+	}
+
+	Ref<JSON> json;
+	json.instantiate();
+	error = json->parse(text);
+	if (error != OK) {
+		ERR_PRINT("Validate extension JSON: Error parsing '" + p_path + "' at line " + itos(json->get_error_line()) + ": " + json->get_error_message());
+		return error;
+	}
+
+	Dictionary old_api = json->get_data();
+	Dictionary new_api = generate_extension_api();
+
+	{ // Validate header:
+		Dictionary header = old_api["header"];
+		ERR_FAIL_COND_V(!header.has("version_major"), ERR_INVALID_DATA);
+		ERR_FAIL_COND_V(!header.has("version_minor"), ERR_INVALID_DATA);
+		int major = header["version_major"];
+		int minor = header["version_minor"];
+
+		ERR_FAIL_COND_V_MSG(major != VERSION_MAJOR, ERR_INVALID_DATA, "JSON API dump is for a different engine version (" + itos(major) + ") than this one (" + itos(major) + ")");
+		ERR_FAIL_COND_V_MSG(minor > VERSION_MINOR, ERR_INVALID_DATA, "JSON API dump is for a newer version of the engine: " + itos(major) + "." + itos(minor));
+	}
+
+	bool failed = false;
+
+	HashSet<String> removed_classes_registered;
+
+	if (!compare_dict_array(old_api, new_api, "constants", "name", Vector<String>({ "value", "is_bitfield" }), false)) {
+		failed = true;
+	}
+
+	if (!compare_dict_array(old_api, new_api, "utility_functions", "name", Vector<String>({ "category" }), true)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "members", "name", { "type" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "constants", "name", { "type", "value" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "operators", "name", { "return_type" }, false, true)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "methods", "name", {}, true)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "builtin_classes", "name", old_api, new_api, "constructors", "index", { "*arguments" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "constants", "name", { "value" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "methods", "name", { "is_virtual", "is_vararg", "is_static" }, true)) { // is_const sometimes can change because they are added if someone forgot, but should not be a problem for the extensions.
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "signals", "name", { "*arguments" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_sub_dict_array(removed_classes_registered, "classes", "name", old_api, new_api, "properties", "name", { "type", "*setter", "*getter", "*index" }, false)) {
+		failed = true;
+	}
+
+	if (!compare_dict_array(old_api, new_api, "singletons", "name", Vector<String>({ "type" }), false)) {
+		failed = true;
+	}
+
+	if (!compare_dict_array(old_api, new_api, "native_structures", "name", Vector<String>({ "format" }), false)) {
+		failed = true;
+	}
+
+	if (failed) {
+		return ERR_INVALID_DATA;
+	} else {
+		return OK;
+	}
+}
+
 #endif // TOOLS_ENABLED

core/extension/extension_api_dump.h

@@ -39,6 +39,7 @@ class GDExtensionAPIDump {
 public:
 	static Dictionary generate_extension_api();
 	static void generate_extension_json_file(const String &p_path);
+	static Error validate_extension_json_file(const String &p_path);
 };
 #endif
 

core/extension/gdextension_interface.cpp

@@ -984,7 +984,12 @@ static GDExtensionScriptInstancePtr gdextension_script_instance_create(const GDE
 static GDExtensionMethodBindPtr gdextension_classdb_get_method_bind(GDExtensionConstStringNamePtr p_classname, GDExtensionConstStringNamePtr p_methodname, GDExtensionInt p_hash) {
 	const StringName classname = *reinterpret_cast<const StringName *>(p_classname);
 	const StringName methodname = *reinterpret_cast<const StringName *>(p_methodname);
-	MethodBind *mb = ClassDB::get_method(classname, methodname);
+	bool exists = false;
+	MethodBind *mb = ClassDB::get_method_with_compatibility(classname, methodname, p_hash, &exists);
+	if (!mb && exists) {
+		ERR_PRINT("Method '" + classname + "." + methodname + "' has changed and no compatibility fallback has been provided. Please open an issue.");
+		return nullptr;
+	}
 	ERR_FAIL_COND_V(!mb, nullptr);
 	if (mb->get_hash() != p_hash) {
 		ERR_PRINT("Hash mismatch for method '" + classname + "." + methodname + "'.");