Avoid invalid song state when loading/importing broken files

Author: YuriSizov

globals/IOManager.gd

@@ -144,7 +144,7 @@ func load_ceol_song_safe() -> void:
 
 func _load_ceol_song_confirmed(path: String) -> bool:
 	var loaded_song: Song = SongLoader.load(path)
-	if not loaded_song:
+	if not loaded_song || not loaded_song.is_valid_song():
 		Controller.update_status("FAILED TO LOAD SONG", Controller.StatusLevel.ERROR)
 		return false
 
@@ -298,7 +298,7 @@ func _import_song_confirmed(import_config: ImportMasterPopup.ImportConfig) -> vo
 
 func _import_mid_song(import_config: ImportMasterPopup.ImportConfig) -> void:
 	var imported_song := MidiImporter.import(import_config)
-	if not imported_song:
+	if not imported_song || not imported_song.is_valid_song():
 		Controller.update_status("FAILED TO IMPORT SONG", Controller.StatusLevel.ERROR)
 		return
 

io/SongLoader.gd

@@ -30,8 +30,8 @@ static func load(path: String) -> Song:
 	if reader.get_version() == 3:
 		return _load_v3(reader)
 
-	printerr("SongLoader: The song file at '%s' has unsupported version %d, an empty song is created instead." % [ path, reader.get_version() ])
-	return Song.create_default_song()
+	printerr("SongLoader: The song file at '%s' has unsupported version %d." % [ path, reader.get_version() ])
+	return null
 
 
 # Original release; due to a bug it never saved the instrument volume.

objects/Song.gd

@@ -102,6 +102,17 @@ static func create_default_song() -> Song:
 	return song
 
 
+func is_valid_song() -> bool:
+	# A valid song must have at least one instrument and one pattern.
+	# We automatically create one of each whenever the last one is
+	# deleted. Loaded and imported songs are expected to have at least
+	# one as well.
+	if instruments.is_empty() || patterns.is_empty():
+		return false
+	
+	return true
+
+
 func get_safe_filename(extension: String = FILE_EXTENSION) -> String:
 	if filename.is_empty():
 		return "%s.%s" % [ FILE_DEFAULT_NAME, extension ]