Fix crash when saving resources with circular references

When saving resources, marking of already seen resources was
done too late, causing infinite loop traversing referenced resources
and eventual stack overflow. The change marks traversed resource
before descending to it's children, thus when this resource is
encountered again, it is already marked as seen and traversal stops.

core/io/resource_format_binary.cpp

@@ -1960,6 +1960,8 @@ void ResourceFormatSaverBinaryInstance::_find_resources(const Variant &p_variant
 				return;
 			}
 
+			resource_set.insert(res);
+
 			List<PropertyInfo> property_list;
 
 			res->get_property_list(&property_list);
@@ -1983,7 +1985,6 @@ void ResourceFormatSaverBinaryInstance::_find_resources(const Variant &p_variant
 				}
 			}
 
-			resource_set.insert(res);
 			saved_resources.push_back(res);
 
 		} break;

scene/resources/resource_format_text.cpp

@@ -1877,6 +1877,8 @@ void ResourceFormatSaverTextInstance::_find_resources(const Variant &p_variant,
 				return;
 			}
 
+			resource_set.insert(res);
+
 			List<PropertyInfo> property_list;
 
 			res->get_property_list(&property_list);
@@ -1908,8 +1910,7 @@ void ResourceFormatSaverTextInstance::_find_resources(const Variant &p_variant,
 				I = I->next();
 			}
 
-			resource_set.insert(res); //saved after, so the children it needs are available when loaded
+			saved_resources.push_back(res); // Saved after, so the children it needs are available when loaded
-			saved_resources.push_back(res);
 
 		} break;
 		case Variant::ARRAY: {

tests/core/io/test_resource.h

@@ -109,6 +109,58 @@ TEST_CASE("[Resource] Saving and loading") {
 			loaded_child_resource_text->get_name() == "I'm a child resource",
 			"The loaded child resource name should be equal to the expected value.");
 }
+
+TEST_CASE("[Resource] Breaking circular references on save") {
+	Ref<Resource> resource_a = memnew(Resource);
+	resource_a->set_name("A");
+	Ref<Resource> resource_b = memnew(Resource);
+	resource_b->set_name("B");
+	Ref<Resource> resource_c = memnew(Resource);
+	resource_c->set_name("C");
+	resource_a->set_meta("next", resource_b);
+	resource_b->set_meta("next", resource_c);
+	resource_c->set_meta("next", resource_b);
+
+	const String save_path_binary = OS::get_singleton()->get_cache_path().path_join("resource.res");
+	const String save_path_text = OS::get_singleton()->get_cache_path().path_join("resource.tres");
+	ResourceSaver::save(resource_a, save_path_binary);
+	ResourceSaver::save(resource_a, save_path_text);
+
+	const Ref<Resource> &loaded_resource_a_binary = ResourceLoader::load(save_path_binary);
+	CHECK_MESSAGE(
+			loaded_resource_a_binary->get_name() == "A",
+			"The loaded resource name should be equal to the expected value.");
+	const Ref<Resource> &loaded_resource_b_binary = loaded_resource_a_binary->get_meta("next");
+	CHECK_MESSAGE(
+			loaded_resource_b_binary->get_name() == "B",
+			"The loaded child resource name should be equal to the expected value.");
+	const Ref<Resource> &loaded_resource_c_binary = loaded_resource_b_binary->get_meta("next");
+	CHECK_MESSAGE(
+			loaded_resource_c_binary->get_name() == "C",
+			"The loaded child resource name should be equal to the expected value.");
+	CHECK_MESSAGE(
+			!loaded_resource_c_binary->get_meta("next"),
+			"The loaded child resource circular reference should be NULL.");
+
+	const Ref<Resource> &loaded_resource_a_text = ResourceLoader::load(save_path_text);
+	CHECK_MESSAGE(
+			loaded_resource_a_text->get_name() == "A",
+			"The loaded resource name should be equal to the expected value.");
+	const Ref<Resource> &loaded_resource_b_text = loaded_resource_a_text->get_meta("next");
+	CHECK_MESSAGE(
+			loaded_resource_b_text->get_name() == "B",
+			"The loaded child resource name should be equal to the expected value.");
+	const Ref<Resource> &loaded_resource_c_text = loaded_resource_b_text->get_meta("next");
+	CHECK_MESSAGE(
+			loaded_resource_c_text->get_name() == "C",
+			"The loaded child resource name should be equal to the expected value.");
+	CHECK_MESSAGE(
+			!loaded_resource_c_text->get_meta("next"),
+			"The loaded child resource circular reference should be NULL.");
+
+	// Break circular reference to avoid memory leak
+	resource_c->remove_meta("next");
+}
 } // namespace TestResource
 
 #endif // TEST_RESOURCE_H