Retry failed song downloads instead of getting stuck

MusicServerClient functions now take an optional on_failed callback,
called when a request fails or returns invalid JSON. Before, a failed
request was silently dropped, so a failure while preloading a song left
Playlist.is_song_downloading set forever and no more songs were fetched.

Song fetching retries the step that failed (download, queue or info)
every song_fetch_retry_seconds, so the server and the game keep the same
playlist.

MainWindow.gd

@@ -140,6 +140,9 @@ var songs: Array= []
 var soundtracks: Array = []
 var current_player: Node
 
+## Seconds to wait before retrying a song download step that failed.
+var song_fetch_retry_seconds: float = 5.0
+
 # Called when the node enters the scene tree for the first time.
 func _ready() -> void:
 	var debug: bool = OS.has_feature("debug")
@@ -277,36 +280,46 @@ func play_next_song() -> void:
 func fetch_song() -> void:
 	print("fetch_song")
 	Playlist.is_song_downloading = true
-	var new_song_object: SongObject = SongObject.new()
-	var fetch_song_done: Callable = func fetch_song_done(body: PackedByteArray) -> void:
-		print("fetch_song_done")
+	_download_random_song(SongObject.new())
+
+# Fetching a song takes three requests. If one fails, only that step is retried, so
+# the server and the game keep the same songs in their playlists.
+func _download_random_song(new_song_object: SongObject) -> void:
+	var song_downloaded: Callable = func song_downloaded(body: PackedByteArray) -> void:
+		print("song_downloaded")
 		var sound: AudioStream = AudioStreamMP3.new()
 		sound.data = body
 		new_song_object.song = sound
-		
-		var added_to_que_done: Callable = func added_to_que_done() -> void:
-			print("added_to_que_done")
-			var fetch_song_info_done: Callable = func fetch_song_info_done(data_received: Dictionary) -> void:
-				print("fetch_song_info_done")
-				new_song_object.song_number = data_received.SongNo
-				new_song_object.soundtrack_title = data_received.Soundtrack
-				new_song_object.song_title = data_received.Song
-			
-				print("new_song_object: ", new_song_object.to_string())
-				Playlist.add_to_playlist(new_song_object)
-				next_button.disabled = false
-				if Playlist.should_preload_more_songs():
-					fetch_song()
-				else:
-					Playlist.is_song_downloading = false
-				if Settings.is_debug:
-					update_song_list()
-			MusicServerClient.fetch_song_info(fetch_song_info_done)
+		_add_song_to_queue(new_song_object)
+	MusicServerClient.fetch_random_song(Settings.use_low_played_mode, song_downloaded,
+			_retry_later.bind(_download_random_song.bind(new_song_object)))
 
-		MusicServerClient.add_to_queue(added_to_que_done)
+func _add_song_to_queue(new_song_object: SongObject) -> void:
+	MusicServerClient.add_to_queue(_fetch_song_info.bind(new_song_object),
+			_retry_later.bind(_add_song_to_queue.bind(new_song_object)))
+
+func _fetch_song_info(new_song_object: SongObject) -> void:
+	var song_info_fetched: Callable = func song_info_fetched(data_received: Dictionary) -> void:
+		print("song_info_fetched")
+		new_song_object.song_number = data_received.SongNo
+		new_song_object.soundtrack_title = data_received.Soundtrack
+		new_song_object.song_title = data_received.Song
+		print("new_song_object: ", new_song_object.to_string())
+		Playlist.add_to_playlist(new_song_object)
+		next_button.disabled = false
+		if Playlist.should_preload_more_songs():
+			fetch_song()
+		else:
+			Playlist.is_song_downloading = false
+		if Settings.is_debug:
+			update_song_list()
+	MusicServerClient.fetch_song_info(song_info_fetched,
+			_retry_later.bind(_fetch_song_info.bind(new_song_object)))
+
+func _retry_later(step: Callable) -> void:
+	print("Song download failed, retrying in %s seconds" % song_fetch_retry_seconds)
+	get_tree().create_timer(song_fetch_retry_seconds).timeout.connect(step)
 
-	MusicServerClient.fetch_random_song(Settings.use_low_played_mode, fetch_song_done)
-	
 func update_song_list() -> void:
 	print("update_song_list")
 	if Playlist.currently_playing_song >= 0: