Test that the song cache fills up to the cache size setting

Covers a cache size above 1: songs are preloaded until the setting is
reached, playing a song tops the cache up again, and the cache keeps
filling after a failed download once the server answers again.

tests/fakes/fake_http_transport.gd

@@ -12,6 +12,7 @@ var requested_paths: Array[String] = []
 var requested_methods: Array[int] = []
 
 var _responses: Dictionary = {}
+var _successes_left: Dictionary = {}
 var _previous_transport: HttpTransport
 
 
@@ -39,6 +40,13 @@ func respond_json(path: String, data: Variant, response_code: int = 200) -> void
 ## Answers requests to [param path] with [param body] as-is.
 func respond_bytes(path: String, body: PackedByteArray, response_code: int = 200) -> void:
     _responses[path] = {"code": response_code, "body": body}
+    _successes_left.erase(path)
+
+
+## Lets [param successes] more requests to [param path] succeed, then answers 503 until
+## a new response is registered for the path.
+func fail_after(path: String, successes: int) -> void:
+    _successes_left[path] = successes
 
 
 ## Returns how many times [param path] has been requested.
@@ -50,6 +58,11 @@ func request(url: String, method: int, on_complete: Callable) -> void:
     var path: String = url.trim_prefix(Settings.default_path)
     requested_paths.append(path)
     requested_methods.append(method)
+    if _successes_left.has(path):
+        if _successes_left[path] == 0:
+            on_complete.call(503, PackedByteArray())
+            return
+        _successes_left[path] -= 1
     if _responses.has(path):
         on_complete.call(_responses[path].code, _responses[path].body)
     else:

tests/integration/test_main_window.gd

@@ -119,3 +119,42 @@ func test_failed_song_info_retries_only_that_step() -> void:
     assert_int(Playlist.song_object_array.size()).is_equal(songs_before + 1)
     assert_str(Playlist.song_object_array.back().song_title).is_equal("Dancing Mad")
     assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func test_preloads_as_many_songs_as_the_cache_setting() -> void:
+    Playlist.number_of_tracks_to_preload = 3
+
+    _runner.invoke("fetch_song")
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+    assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func test_playing_a_song_tops_the_cache_up_again() -> void:
+    Playlist.number_of_tracks_to_preload = 3
+    _runner.invoke("fetch_song")
+
+    await _play_first_song()
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+    assert_int(Playlist.song_object_array.size()).is_equal(4)
+
+
+func test_cache_keeps_filling_after_a_failed_download() -> void:
+    _window.song_fetch_retry_seconds = 0.05
+    Playlist.number_of_tracks_to_preload = 3
+    var mp3: PackedByteArray = FileAccess.get_file_as_bytes("res://tests/data/short.mp3")
+    # One more download works, then the server stops answering
+    _server.fail_after("/music/rand", 1)
+
+    _runner.invoke("fetch_song")
+    await await_millis(150)
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(2)
+    assert_bool(Playlist.is_song_downloading).is_true()
+
+    _server.respond_bytes("/music/rand", mp3)
+    await await_millis(150)
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+    assert_bool(Playlist.is_song_downloading).is_false()