Update only the queue ID in settings.cfg

Saving a new queue ID wrote every setting from memory, which could also save
a setting that is in use but not saved yet, like the debug winning score.
Now it loads the file, changes only the queue ID and saves it again.

data/settings.gd

@@ -181,10 +181,19 @@ func save_settings() -> void:
 		GameLog.warning("Could not save the settings to %s: %s" % [path, error_string(error)])
 
 
-## Sets the play queue ID for [param server] and saves it.
+## Sets the play queue ID for [param server] and saves it. Only the ID is changed in
+## the file; the other settings in it stay as they were saved.
 func set_queue_id(server: String, id: String) -> void:
 	queue_ids[server] = id
-	save_settings()
+	var path: String = saved_settings_path()
+	var file: ConfigFile = ConfigFile.new()
+	# A missing file is fine: it is written with only the queue ID
+	file.load(path)
+	file.set_value("queues", server, id)
+	DirAccess.make_dir_recursive_absolute(path.get_base_dir())
+	var error: Error = file.save(path)
+	if error != OK:
+		GameLog.warning("Could not save the queue ID to %s: %s" % [path, error_string(error)])
 
 
 ## Turns fullscreen on or off and saves it.

tests/unit/test_settings_files.gd

@@ -107,6 +107,36 @@ func test_queue_ids_are_saved_and_load_back() -> void:
 	assert_dict(Settings.queue_ids).is_equal({"https://music.example.com": "queue-1"})
 
 
+func test_saving_a_queue_id_leaves_the_other_settings_as_saved() -> void:
+	Settings.winning_score = 10
+	Settings.save_settings()
+	Settings.winning_score = 12
+
+	Settings.set_queue_id("https://music.example.com", "queue-1")
+
+	var file: ConfigFile = _saved_file()
+	assert_int(file.get_value("quiz", "winning_score")).is_equal(10)
+	assert_str(file.get_value("queues", "https://music.example.com")).is_equal("queue-1")
+
+
+func test_saving_a_queue_id_without_a_file_saves_only_the_queue_id() -> void:
+	Settings.set_queue_id("https://music.example.com", "queue-1")
+
+	var file: ConfigFile = _saved_file()
+	assert_array(file.get_sections()).contains_exactly(["queues"])
+	assert_str(file.get_value("queues", "https://music.example.com")).is_equal("queue-1")
+
+
+func test_new_queue_id_replaces_only_its_server() -> void:
+	Settings.set_queue_id("https://music.example.com", "queue-1")
+	Settings.set_queue_id("http://localhost:8080", "local-1")
+	Settings.set_queue_id("https://music.example.com", "queue-2")
+
+	var file: ConfigFile = _saved_file()
+	assert_str(file.get_value("queues", "https://music.example.com")).is_equal("queue-2")
+	assert_str(file.get_value("queues", "http://localhost:8080")).is_equal("local-1")
+
+
 func test_file_without_queues_keeps_the_current_queue_ids() -> void:
 	Settings.save_settings()
 	Settings.queue_ids["https://music.example.com"] = "queue-1"