Fix stage not advancing if users leave during beatmap download stage

osu.Server.Spectator.Tests/Matchmaking/MatchmakingMatchControllerTests.cs

@@ -251,6 +251,31 @@ public async Task CanNotInvitePlayersToRoom()
             await Assert.ThrowsAsync<InvalidStateException>(() => Hub.InvitePlayer(USER_ID_2));
         }
 
+        /// <summary>
+        /// Tests that the stage advances when users leave, particularly in the case where the server is waiting on one to download the beatmap.
+        /// </summary>
+        [Fact]
+        public async Task StageAdvancesWhenUsersLeaveDuringDownload()
+        {
+            await Hub.JoinRoom(ROOM_ID);
+
+            SetUserContext(ContextUser2);
+            await Hub.JoinRoom(ROOM_ID);
+
+            await gotoStage(MatchmakingStage.WaitingForClientsBeatmapDownload);
+
+            // Ready up the first user.
+            SetUserContext(ContextUser);
+            await Hub.ChangeState(MultiplayerUserState.Ready);
+            await Hub.ChangeBeatmapAvailability(BeatmapAvailability.LocallyAvailable());
+
+            // Quit the second user.
+            SetUserContext(ContextUser2);
+            await Hub.LeaveRoom();
+
+            await verifyStage(MatchmakingStage.GameplayWarmupTime);
+        }
+
         private async Task verifyStage(MatchmakingStage stage)
         {
             using (var room = await Rooms.GetForUse(ROOM_ID))

osu.Server.Spectator/Hubs/Multiplayer/Matchmaking/MatchmakingMatchController.cs

@@ -154,9 +154,9 @@ public async Task HandleUserJoined(MultiplayerRoomUser user)
             }
         }
 
-        public Task HandleUserLeft(MultiplayerRoomUser user)
+        public async Task HandleUserLeft(MultiplayerRoomUser user)
         {
-            return Task.CompletedTask;
+            await updateStageFromUserStateChange();
         }
 
         public Task AddPlaylistItem(MultiplayerPlaylistItem item, MultiplayerRoomUser user)
@@ -176,13 +176,7 @@ public Task RemovePlaylistItem(long playlistItemId, MultiplayerRoomUser user)
 
         public async Task HandleUserStateChanged(MultiplayerRoomUser user)
         {
-            switch (state.Stage)
-            {
-                case MatchmakingStage.WaitingForClientsBeatmapDownload:
-                    if (allUsersReady())
-                        await stageGameplayWarmupTime(room);
-                    break;
-            }
+            await updateStageFromUserStateChange();
         }
 
         public void SkipToNextStage(out Task countdownTask)
@@ -367,6 +361,17 @@ await room.StartCountdown(new MatchmakingStageCountdown
             }, continuation);
         }
 
+        private async Task updateStageFromUserStateChange()
+        {
+            switch (state.Stage)
+            {
+                case MatchmakingStage.WaitingForClientsBeatmapDownload:
+                    if (allUsersReady())
+                        await stageGameplayWarmupTime(room);
+                    break;
+            }
+        }
+
         private bool allUsersReady()
         {
             return room.Users.All(u => u.State == MultiplayerUserState.Ready);