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.

music_server_client.gd

@@ -17,108 +17,123 @@ func _ready() -> void:
 
 ## Fetches the songs already in the server playlist, as an Array of
 ## [code]{SongNo, Soundtrack, Song}[/code] dictionaries.
-func fetch_song_list(on_done: Callable) -> void:
-    get_json("/music/list", on_done)
+func fetch_song_list(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_json("/music/list", on_done, on_failed)
 
 
 ## Downloads a random song as MP3 bytes. With [param low_played] the server prefers
 ## soundtracks that have been played the least.
-func fetch_random_song(low_played: bool, on_done: Callable) -> void:
-    get_bytes("/music/rand/low" if low_played else "/music/rand", on_done)
+func fetch_random_song(
+        low_played: bool, on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_bytes("/music/rand/low" if low_played else "/music/rand", on_done, on_failed)
 
 
 ## Adds the most recently fetched random song to the server playlist.
-func add_to_queue(on_done: Callable) -> void:
-    send("/music/addQue", on_done)
+func add_to_queue(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    send("/music/addQue", on_done, on_failed)
 
 
 ## Fetches [code]{SongNo, Soundtrack, Song}[/code] for the most recently queued song.
-func fetch_song_info(on_done: Callable) -> void:
-    get_json("/music/info", on_done)
+func fetch_song_info(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_json("/music/info", on_done, on_failed)
 
 
 ## Downloads a song from the server playlist as MP3 bytes.
-func fetch_song(song_no: int, on_done: Callable) -> void:
-    get_bytes("/music?song=" + str(song_no), on_done)
+func fetch_song(song_no: int, on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_bytes("/music?song=" + str(song_no), on_done, on_failed)
 
 
 ## Downloads the sound test song as MP3 bytes.
-func fetch_sound_test_song(on_done: Callable) -> void:
-    get_bytes("/music/soundTest", on_done)
+func fetch_sound_test_song(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_bytes("/music/soundTest", on_done, on_failed)
 
 
 ## Clears the server playlist.
-func reset_playlist(on_done: Callable) -> void:
-    send("/music/reset", on_done)
+func reset_playlist(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    send("/music/reset", on_done, on_failed)
 
 
 ## Fetches every soundtrack name as an Array of Strings.
-func fetch_all_soundtracks(on_done: Callable) -> void:
-    get_json("/music/all", on_done)
+func fetch_all_soundtracks(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_json("/music/all", on_done, on_failed)
 
 
 ## Fetches every soundtrack name, sorted, as an Array of Strings.
-func fetch_all_soundtracks_ordered(on_done: Callable) -> void:
-    get_json("/music/all/order", on_done)
+func fetch_all_soundtracks_ordered(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_json("/music/all/order", on_done, on_failed)
 
 
 ## Records in the server statistics that a song has been played.
 func mark_song_played(song_no: int) -> void:
-    send("/music/played?song=" + str(song_no), Callable(), HTTPClient.METHOD_PUT)
+    send("/music/played?song=" + str(song_no), Callable(), Callable(), HTTPClient.METHOD_PUT)
 
 
 ## Fetches the names of all character images.
-func fetch_character_list(on_done: Callable) -> void:
-    get_json("/characters", on_done)
+func fetch_character_list(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_json("/characters", on_done, on_failed)
 
 
 ## Downloads a character image as PNG bytes.
-func fetch_character_image(character_name: String, on_done: Callable) -> void:
-    get_bytes("/character?name=" + character_name.uri_encode(), on_done)
+func fetch_character_image(
+        character_name: String, on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_bytes("/character?name=" + character_name.uri_encode(), on_done, on_failed)
 
 
 ## Starts syncing the server's soundtrack library with its music folder.
-func start_sync(on_done: Callable) -> void:
-    send("/sync", on_done)
+func start_sync(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    send("/sync", on_done, on_failed)
 
 
 ## Fetches the sync state: [code]{progress, time_spent}[/code] while running, or the
 ## sync result once finished.
-func fetch_sync_progress(on_done: Callable) -> void:
-    get_json("/sync/progress", on_done)
+func fetch_sync_progress(on_done: Callable, on_failed: Callable = Callable()) -> void:
+    get_json("/sync/progress", on_done, on_failed)
 
 
 ## Requests [param path] and calls [param on_done] with the parsed JSON.
-## Nothing is called if the request fails or the body is not valid JSON.
-func get_json(path: String, on_done: Callable) -> void:
+## [param on_failed] is called with no arguments if the request fails or the body is
+## not valid JSON.
+func get_json(path: String, on_done: Callable, on_failed: Callable = Callable()) -> void:
     _request(path, HTTPClient.METHOD_GET, func(body: PackedByteArray) -> void:
         var json: JSON = JSON.new()
         if json.parse(body.get_string_from_utf8()) != OK:
             push_warning("Invalid JSON from %s: %s" % [path, json.get_error_message()])
+            _call_if_valid(on_failed)
             return
         on_done.call(json.data)
-    )
+    , on_failed)
 
 
-## Requests [param path] and calls [param on_done] with the raw response body.
-func get_bytes(path: String, on_done: Callable) -> void:
-    _request(path, HTTPClient.METHOD_GET, on_done)
+## Requests [param path] and calls [param on_done] with the raw response body, or
+## [param on_failed] with no arguments if the request fails.
+func get_bytes(path: String, on_done: Callable, on_failed: Callable = Callable()) -> void:
+    _request(path, HTTPClient.METHOD_GET, on_done, on_failed)
 
 
-## Requests [param path] and calls [param on_done] with no arguments when it succeeds.
-## [param on_done] may be an empty Callable when the result does not matter.
-func send(path: String, on_done: Callable, method: int = HTTPClient.METHOD_GET) -> void:
+## Requests [param path] and calls [param on_done] with no arguments when it succeeds,
+## or [param on_failed] when it fails. Either may be an empty Callable.
+func send(
+        path: String,
+        on_done: Callable,
+        on_failed: Callable = Callable(),
+        method: int = HTTPClient.METHOD_GET,
+) -> void:
     _request(path, method, func(_body: PackedByteArray) -> void:
-        if on_done.is_valid():
-            on_done.call()
-    )
+        _call_if_valid(on_done)
+    , on_failed)
 
 
-func _request(path: String, method: int, on_success: Callable) -> void:
+func _request(path: String, method: int, on_success: Callable, on_failed: Callable) -> void:
     var url: String = Settings.default_path + path
     transport.request(url, method, func(response_code: int, body: PackedByteArray) -> void:
         if response_code != 200:
             push_warning("Request to %s failed with response code %d" % [url, response_code])
+            _call_if_valid(on_failed)
             return
         on_success.call(body)
     )
+
+
+static func _call_if_valid(callback: Callable) -> void:
+    if callback.is_valid():
+        callback.call()